Grupo de Química Atmosférica y Clima
  • Español
  • English

   Listado completo de publicaciones

 

Investigating differences in DOAS retrieval codes using MAD-CAT campaign data

Enno Peters , Gaia Pinardi, André Seyler, Andreas Richter, Folkard Wittrock, Tim Bösch, Michel Van Roozendael, François Hendrick, Theano Drosoglou, Alkiviadis F. Bais, Yugo Kanaya, Xiaoyi Zhao, Kimberly Strong, Johannes Lampel, Rainer Volkamer, Theodore Koenig, Ivan Ortega, Olga Puentedura, Mónica Navarro-Comas, Laura Gómez, Margarita Yela González, Ankie Piters, Julia Remmers, Yang Wang, ThomasWagner, ShanshanWang,  Alfonso Saiz-Lopez, David García-Nieto, Carlos A. Cuevas, Nuria Benavent, Richard Querel, Paul Johnston, Oleg Postylyakov, Alexander Borovski, Alexander Elokhov, Ilya Bruchkouski, Haoran Liu, Cheng Liu, Qianqian Hong, Claudia Rivera, Michel Grutter, Wolfgang Stremme, M. Fahim Khokhar, Junaid Khayyam, John P. Burrows

Atmospheric Measurement Techniques, Volume 10, Issue 3, pp  955-978, doi:10.5194/amt-10-955-2017, 2017.Atmospheric Environment, Volume 155, pp 97-107; doi.org/10.1016/j.atmosenv, 2017.02.018

 

Abstract:

The differential optical absorption spectroscopy (DOAS) method is a well-known remote sensing technique that is nowadays widely used for measurements of atmospheric trace gases, creating the need for harmonization and characterization efforts. In this study, an intercomparison exercise of DOAS retrieval codes from 17 international groups is presented, focusing on NO2 slant columns. The study is based on data collected by one instrument during the Multi-Axis DOAS Comparison campaign for Aerosols and Trace gases (MAD-CAT) in Mainz, Germany, in summer 2013. As data from the same instrument are used by all groups, the results are free of biases due to instrumental differences, which is in contrast to previous intercomparison exercises. While in general an excellent correlation of NO2 slant columns between groups of >99.98% (noon reference fits) and >99.2% (sequential reference fits) for all elevation angles is found, differences between individual retrievals are as large as 8% for NO2 slant columns and 100% for rms residuals
in small elevation angles above the horizon. Comprehensive sensitivity studies revealed that absolute slant column differences result predominantly from the choice of the reference spectrum while relative differences originate from the numerical approach for solving the DOAS equation as well as the treatment of the slit function. Furthermore, differences in the implementation of the intensity offset correction were found to produce disagreements for measurements close to sunrise (8–10% for NO2, 80% for rms residual). The largest effect of 8% difference in NO2 was found to arise from the reference treatment; in particular
for fits using a sequential reference. In terms of rms fit residual, the reference treatment has only a minor impact. In contrast, the wavelength calibration as well as the intensity offset correction were found to have the largest impact (up to 80 %) on rms residual while having only a minor impact on retrieved NO2 slant columns.


700 years reconstruction of mercury and lead atmospheric deposition in the Pyrenees (NE Spain)

J.P. Corella, B.L. Valero-Garcés,  F. Wang , A. Martínez-Cortizas, C.A. Cuevas, A. Saiz-Lopez

Atmospheric Environment, Volume 155, pp 97-107; doi.org/10.1016/j.atmosenv, 2017.02.018

 

Abstract:

Geochemical analyses in varved lake sediment cores (Lake Montcortes, Pre-Pyrenees) allowed reconstruction of mercury (Hg) and lead (Pb) atmospheric deposition over the past seven centuries in the Pyrenees (NE Spain). Accumulation Rates (AR) from the Middle Ages to the Industrial Period ranged from 2500 to 26130 mg m2.y1 and 15e152 mg m2.y1 for Pb and Hg respectively. Significant metal pollution started ca CE 1550 during a period of increased exploitation of ore resources in Spain. Colder and humid conditions in the Pyrenees during the Little Ice Age may have also favoured Hg and Pb atmospheric deposition in the lake. Therefore, the interplay between increased rainfall (wet deposition) and mining activities in the Iberian Peninsula has driven Hg and Pb AR during the Pre-industrial Period. More recently, the use of leaded gasoline in Europe in the mid-20th century may explain the highest Pb AR between CE 1953 and 1971. The highest Hg AR occurred in CE 1940 synchronous with the highest Hg production peak in Almaden mining district (southern Spain) and the Second World War. The record of Hg enrichment in Lake Montcortes shows a decrease during the last decades in Western Europe similar to other regional records and global emission models. This study highlights the exceptional quality of varved sequences to tease apart pollutants depositional mechanisms, identify historical periods of increased atmospheric pollution and provide a historical context for pollutant baseline values to make correct assessments of recent (atmospheric) pollution in lake ecosystems.


Sea-ice-related halogen enrichment at Law Dome, coastal East Antarctica

Paul Vallelong, Niccolo Maffezzoli, Andrew D. Moy, Mark A. J. Curran, Tessa R. Vance, Ross Edwards, Gwyn Hughes, Emily Barker, Gunnar Spreen, Alfonso Saiz-Lopez, J. Pablo Corella6, Carlos A. Cuevas, and Andrea Spolaor.

Climate of the Past, Vol 13, Issue 2, 27 February 2017, Pages 171-184 ; doi:10.5194/cp-13-171-2017, 2017

 

Abstract:

The Law Dome site is ideal for the evaluation of sea ice proxies due to its location near to the Antarctic coast, regular and high accumulation throughout the year, an absence of surface melting or remobilization, and minimal multiyear sea ice.We present records of bromine and iodine concentrations and their enrichment beyond seawater compositions and compare these to satellite observations of first-year sea ice area in the 90–130 E sector of the Wilkes coast. Our findings support the results of previous studies of sea ice variability from Law Dome, indicating that Wilkes coast sea ice area is currently at its lowest level since the start of the 20th century. From the Law Dome DSS1213 firn core, 26 years of monthly deposition data indicate that the period of peak bromine enrichment is during austral spring–summer, from November to February. Results from a traverse along the lee (western) side of Law Dome show low levels of sodium and bromine deposition, with the greatest fluxes in the vicinity of the Law Dome summit. Finally, multidecadal variability in iodine enrichment appears well correlated to bromine enrichment, suggesting a common source of variability that may be related to the Interdecadal Pacific Oscillation (IPO).


On the summertime air quality and related photochemical processes in the megacity Shanghai, China

Ka Lok Chan, ShanshanWang, Cheng Liu, Bin Zhou, Mark O.Wenig, Alfonso Saiz-Lopez.

Science of The Total Environment, Vol. 580, 15 February 2017, Pages 974–983;dx.doi.org/10.1016/j.scitotenv.2016.12.052

 

Abstract:

Summertime surface ozone (O3) and related secondary formation of fine particles are the major air quality concerns in the megacity of Shanghai. We performed mobile Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) measurements to investigate the spatial distribution of on-road nitrogen dioxide (NO2) concentrations along the Inner Ring Elevated Road (IRER) in Shanghai. The observations show a ratio of 4 between CE-DOAS averaged on-road NO2 and the in-situ ambient measurements, illustrating the strong impact of vehicle emissions over the urban area. The air mass transport analysis suggests that the observed episodic ozone events arise from both the abundance of volatile organic compounds (VOCs) precursors in the sampled plume and the regional transport of ozone-rich air masses. Analysis of the sources of PM2.5 shows that the secondary heterogeneous gas-to-particle conversion of sulfate and nitrate from sulfur dioxide (SO2) and nitrogen oxides (NOx) is the largest source of PM2.5 contributing 44.8 ± 9.2% of the total PM2.5. Ozone-related photochemical formation of fine particles is estimated to contribute about 22.5 ± 11.9% of the total PM2.5, which is strongly facilitated by solar radiation in summer. According to our results, nitrous acid (HONO) is the major precursor of hydroxyl radicals (OH) accounting for 40% to 80% of the total OH production during daytime. A significant correlation is found between the HONO levels in the early morning and the daily O3 and PM2.5 levels. The summertime measurements indicate that the photolytic reaction of HONO after sunrise increased the abundance of daytime OH and oxidative capacity, resulting in an enhancement of ground level ozone and secondary organic aerosol formation. This study provides quantitative information to better understand photochemical formation of ozone and fine particles in Shanghai during summertime, which is useful for designing collaborative strategies to mitigate emissions of precursor pollutants.


Impact of biogenic very short-lived bromine on the Antarctic ozone hole during the 21st century

Rafael P. Fernandez, Douglas E. Kinnison, Jean-Francois Lamarque, Simone Tilmes, and Alfonso Saiz-Lopez.

Atmos. Chem. Phys., 17, 1673-1688, 2017; doi:10.5194/acp-17-1673-2017

 

Abstract:

Active bromine released from the photochemical decomposition of biogenic very short-lived bromocarbons (VSLBr) enhances stratospheric ozone depletion. Based on a dual set of 1960–2100 coupled chemistry–climate simulations (i.e. with and without VSLBr), we show that the maximum Antarctic ozone hole depletion increases by up to 14 % when natural VSLBr are considered, which is in better agreement with ozone observations. The impact of the additional 5 pptv VSLBr on Antarctic ozone is most evident in the periphery of the ozone hole, producing an expansion of the ozone hole area of ∼ 5 million km2, which is equivalent in magnitude to the recently estimated Antarctic ozone healing due to the implementation of the Montreal Protocol. We find that the inclusion of VSLBr in CAM-Chem (Community Atmosphere Model with Chemistry, version 4.0) does not introduce a significant delay of the modelled ozone return date to 1980 October levels, but instead affects the depth and duration of the simulated ozone hole. Our analysis further shows that total bromine-catalysed ozone destruction in the lower stratosphere surpasses that of chlorine by the year 2070 and indicates that natural VSLBr chemistry would dominate Antarctic ozone seasonality before the end of the 21st century. This work suggests a large influence of biogenic bromine on the future Antarctic ozone layer.

The Impact of Iodide-Mediated Ozone Deposition and Halogen Chemistry on Surface Ozone Concentrations Across the Continental United States

Brett Gantt, Golam Sarwar, Jia Xing, Heather Simon, Donna Schwede, William T. Hutzell, Rohit Mathur,  Alfonso Saiz-Lopez

Environ. Sci. Technol., 2017, 51 (3), pp 1458–1466; doi: 10.1021/acs.est.6b03556.

 

Abstract:

The air quality of many large coastal areas in the United States is affected by the confluence of polluted urban and relatively clean marine airmasses, each with distinct atmospheric chemistry. In this context, the role of iodide-mediated ozone (O3) deposition over seawater and marine halogen chemistry accounted for in both the lateral boundary conditions and coastal waters surrounding the continental U.S. is examined using the Community Multiscale Air Quality (CMAQ) model. Several nested simulations are conducted in which these halogen processes are implemented separately in the continental U.S. and hemispheric CMAQ domains, the latter providing lateral boundary conditions for the former. Overall, it is the combination of these processes within both the continental U.S. domain and from lateral boundary conditions that lead to the largest reductions in modeled surface O3 concentrations. Predicted reductions in surface O3 concentrations occur mainly along the coast where CMAQ typically has large overpredictions. These results suggest that a  realistic representation of halogen processes in marine regions can improve model prediction of O3 concentrations near the coast.

Thermochemistry of halogen-containing organic compounds with influence on atmospheric chemistry.

Juan Z. Dávalos, Rafael Notario, Carlos A. Cuevas, Josep M. Oliva, Alfonso Saiz-Lopez.

Computational and Theoretical Chemistry, Volume 1099, 1 January 2017, Pages 36–44; DOI:org/10.1016/j.comptc.2016.11.009

 

Abstract:

We report a study on the thermochemical properties of a wide variety of halogen-containing organic compounds with relevance on several atmospheric chemical processes, such as catalytic ozone destruction. In particular, we have computationally determined the standard molar enthalpies of formation, Δf Hm° (g), and the carbon-halogen bond dissociation enthalpies, BDE, in the gas phase at 298.15 K. A reliable estimation of these thermodynamic magnitudes was deduced, using atomization and isodesmic reactions methodologies, from ab initio computational methods. The enthalpies of formation of the radicals formed through bond dissociations have also been computed.

The Convective Transport of Active Species in the Tropics (CONTRAST) Experiment

L.L.Pan, E.L.Atlas, R.J.Salawitch, S.B.Honomichl, J.F.Bresch, W.J.Randel, E.C.Apel, R.S.Hornbrook, A.J.Weinheimer, D.C.Anderson, S.J.Andrews, S.Baidar, S.P.Beatn, T.L.Campos, L.J.Carpenter, D.Chen, B.Dix, V.Donets, S.R.Hall, T.F.Hanisco, C.R.Homyer, L.G.Huey, J.B.Jensen, L.Kaser, D.E.Kinnison, T.K.Koenig, J-­FLamarque, C.Liu, J.Luo, Z.J.Luo, D.D.Montzka, J.M.Nicely, R.B.Pierce, D.D.Riemer, T.Robinson, P.Romashkin, A.Saiz­Lopez, S.Schauffler, O.Shieh, M.H.Stell1, G.Vaughan, K.Ullmann, R.Volkamer, G.Wlfe.

Bull. Amer. Meteor. Soc., 0, DOI: 10.1175/BAMS-D-14-00272.1,2016

 

Abstract:

Airborne observations over the tropical western Pacific warm pool characterize the role of tropical convection in linking oceanic processes to ozone chemistry in the upper troposphere and lower stratosphere

The Convective Transport of Active Species in the Tropics (CONTRAST) experiment was conducted from Guam (13.5° N, 144.8° E) during January-February 2014. Using the NSF/NCAR Gulfstream V research aircraft, the experiment investigated the photochemical environment over the tropical western Pacific (TWP) warm pool, a region of massive deep convection and the major pathway for air to enter the stratosphere during Northern Hemisphere (NH) winter. The new observations provide a wealth of information for quantifying the influence of convection on the vertical distributions of active species. The airborne in situ measurements up to 15 km altitude fill a significant gap by characterizing the abundance and altitude variation of a wide suite of trace gases. These measurements, together with observations of dynamical and microphysical parameters, provide significant new data for constraining and evaluating global chemistry climate models. Measurements include precursor and product gas species of reactive halogen compounds that impact ozone in the upper troposphere/lower stratosphere. High accuracy, in-situ measurements of ozone obtained during CONTRAST quantify ozone concentration profiles in the UT, where previous observations from balloon-borne ozonesondes were often near or below the limit of detection. CONTRAST was one of the three coordinated experiments to observe the TWP during January-February 2014. Together, CONTRAST, ATTREX and CAST, using complementary capabilities of the three aircraft platforms as well as ground-based instrumentation, provide a comprehensive quantification of the regional distribution and vertical structure of natural and pollutant trace gases in the TWP during NH winter, from the oceanic boundary to the lower stratosphere.

# Now at Lanzhou University, Lanzhou, China

Contact information of the corresponding author: Laura L. Pan, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000, Email: Esta dirección de correo electrónico está siendo protegida contra los robots de spam. Necesita tener JavaScript habilitado para poder verlo., Phone: 303-497-1467

Tropospheric emissions: Monitoring of pollution (TEMPO)

P.Zoogman; X.Liu; R.M.Suleiman; W.F.Pennington; D.E.Flittner; J.A.Al-Saadi; B.B.Hilton; D.K.Nicks; M.J.Newchurch; L.Carr; S.J.Janz; M.R. Andraschko; A.Arola; B.D.Baker; B.P.Canova; C.ChanMiller; R.C. Cohen; J.E.Davis; M.E.Dussault; D.P.Edwards; J.Fishman; A.Ghulam; G. GonzálezAbad; M.Grutter; J.R.Hermanm; J.Houck; D.J.Jacob; J.Joiner; B.J. Kerridge; J.Kim; N.A.Krotkov; L.Lamsal; C.Li; A.Lindfors; R.V. Martin; C.T.M. Elroy; C.McLinden; V.Natraj; D.O.Neil; C.R.Nowlan; E.J. O'Sullivan; P.I.Palmer; R.B.Pierce; M.R.Pippin; A.Saiz-Lopez; R.J.D. Spurr; J.J.Szykman; O.Torres; J.P.Veefkind; B.Veihelmann; H. Wang; J.Wang; K.Chance.

J. Quant. Spectrosc. Radiat. Transfer., doi.org/10.1016/j.jqsrt.2016.05.008 ,2016

 

Abstract:

TEMPO was selected in 2012 by NASA as the first Earth Venture Instrument, for launch between 2018 and 2021. It will measure atmospheric pollution for greater North America from space using ultraviolet and visible spectroscopy. TEMPO observes from Mexico City, Cuba, and the Bahamas to the Canadian oil sands, and from the Atlantic to the Pacific, hourly and at high spatial resolution (~2.1 km N/S×4.4 km E/W at 36.5°N, 100°W). TEMPO provides a tropospheric measurement suite that includes the key elements of tropospheric air pollution chemistry, as well as contributing to carbon cycle knowledge. Measurements are made hourly from geostationary (GEO) orbit, to capture the high variability present in the diurnal cycle of emissions and chemistry that are unobservable from current low-Earth orbit (LEO) satellites that measure once per day. The small product spatial footprint resolves pollution sources at sub-urban scale. Together, this temporal and spatial resolution improves emission inventories, monitors population exposure, and enables effective emission-control strategies.

TEMPO takes advantage of a commercial GEO host spacecraft to provide a modest cost mission that measures the spectra required to retrieve ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), formaldehyde (H2CO), glyoxal (C2H2O2), bromine monoxide (BrO), IO (iodine monoxide), water vapor, aerosols, cloud parameters, ultraviolet radiation, and foliage properties. TEMPO thus measures the major elements, directly or by proxy, in the tropospheric O3 chemistry cycle. Multi-spectral observations provide sensitivity to O3 in the lowermost troposphere, substantially reducing uncertainty in air quality predictions. TEMPO quantifies and tracks the evolution of aerosol loading. It provides these near-real-time air quality products that will be made publicly available. TEMPO will launch at a prime time to be the North American component of the global geostationary constellation of pollution monitoring together with the European Sentinel-4 (S4) and Korean Geostationary Environment Monitoring Spectrometer (GEMS) instruments.

Ab initio quantum–chemical computations of the electronic states in HgBr2 and IBr: Molecules of interest on the Earth’s atmosphere

Sebastian P. Sitkiewicz, Josep M. Oliva, Juan Z. Dávalos, Rafael Notario, Alfonso Saiz–Lopez, Diego R. Alcoba, Ofelia B. Oña, and Daniel Roca-Sanjuán

J. Chem. Phys. Volum.145, 244304 (2016), doi: 10.1063/1.4971856

 

Abstract

The electronic states of atmospheric relevant molecules IBr and HgBr2 are reported, within the UV-Vis spectrum range (170 nm ≤ λphoton ≤ 600 nm) by means of the complete–active–space self–consistent field/multi–state complete–active–space second–order perturbation theory/spin–orbit restricted–active–space state–interaction (CASSCF/MS–CASPT2/SO–RASSI) quantum–chemical approach and atomic–natural–orbital relativistic–correlation–consistent (ANO–RCC) basis sets. Several analyses of the methodology were carried out in order to reach converged results and therefore to establish a highly accurate level of theory. Good agreement is found with the experimental data with errors not higher than around 0.1 eV. The presented analyses shall allow upcoming studies aimed to accurately determine the absorption cross sections of interhalogen molecules and compounds with Hg that are relevant to better comprehend the photochemical processes taking place in the atmosphere.

Nighttime atmospheric chemistry of iodine

Alfonso Saiz-Lopez, John M. C. Plane, Carlos A. Cuevas, Anoop S. Mahajan, Jean-François Lamarque, and Douglas E. Kinnison

J. Chem. Phys. Volum.145, 244304 (2016), doi: 10.1063/1.4971856

 

Abstract:

Little attention has so far been paid to the nighttime atmospheric chemistry of iodine species. Current atmospheric models predict a buildup of HOI and I2 during the night that leads to a spike of IO at sunrise, which is not observed by measurements. In this work, electronic structure calculations are used to survey possible reactions that HOI and I2 could undergo at night in the lower troposphere, and hence reduce their nighttime accumulation. The new reaction NO3+ HOI  →  IO + HNO3 is proposed, with a rate coefficient calculated from statistical rate theory over the temperature range 260–300 K and at a pressure of 1000 hPa to be k(T)  =  2.7  ×  10−12(300 K/T)2.66 cm3 molecule−1 s−1. This reaction is included in two atmospheric models, along with the known reaction between I2 and NO3, to explore a new nocturnal iodine radical activation mechanism. The results show that this iodine scheme leads to a considerable reduction of nighttime HOI and I2, which results in the enhancement of more than 25 % of nighttime ocean emissions of HOI + I2and the removal of the anomalous spike of IO at sunrise. We suggest that active nighttime iodine can also have a considerable, so far unrecognized, impact on the reduction of the NO3 radical levels in the marine boundary layer (MBL) and hence upon the nocturnal oxidizing capacity of the marine atmosphere. The effect of this is exemplified by the indirect effect on dimethyl sulfide (DMS) oxidation.

A global model of tropospheric chlorine chemistry: Organic versus inorganic sources and impact on methane oxidation

Ryan Hossaini, Martyn P. Chipperfield, Alfonso Saiz-Lopez, Rafael Fernández, Sarah Monks, Wufu Feng, Peter Brauer, Roland von Glasow

J. Geophys. Res. Atmos. Volum.121, Pag. 14271-14297, doi: 10.1002/2016JD025756

 

Abstract:

Chlorine atoms (Cl) are highly reactive toward hydrocarbons in the Earth's troposphere, including the greenhouse gas methane (CH4). However, the regional and global CH4 sink from Cl is poorly quantified as tropospheric Cl concentrations ([Cl]) are uncertain by ~2 orders of magnitude. Here we describe the addition of a detailed tropospheric chlorine scheme to the TOMCAT chemical transport model. The model includes several sources of tropospheric inorganic chlorine (Cly), including (i) the oxidation of chlorocarbons of natural (CH3Cl, CHBr2Cl, CH2BrCl, and CHBrCl2) and anthropogenic (CH2Cl2, CHCl3, C2Cl4, C2HCl3, and CH2ClCH2Cl) origin and (ii) sea-salt aerosol dechlorination. Simulations were performed to quantify tropospheric [Cl], with a focus on the marine boundary layer, and quantify the global significance of Cl atom CH4 oxidation. In agreement with observations, simulated surface levels of hydrogen chloride (HCl), the most abundant Cly reservoir, reach several parts per billion (ppb) over polluted coastal/continental regions, with sub-ppb levels typical in more remote regions. Modeled annual mean surface [Cl] exhibits large spatial variability with the largest levels, typically in the range of 1–5 × 104 atoms cm−3, in the polluted northern hemisphere. Chlorocarbon oxidation provides a tropospheric Cly source of up to ~4320 Gg Cl/yr, sustaining a background surface [Cl] of <0.1 to 0.5 × 103 atoms cm−3 over large areas. Globally, we estimate a tropospheric methane sink of ~12–13 Tg CH4/yr due the CH4 + Cl reaction (~2.5% of total CH4 oxidation). Larger regional effects are predicted, with Cl accounting for ~10 to >20% of total boundary layer CH4 oxidation in some locations.

Photolysis of frozen iodate salts as a source of active iodine in the polar environment.

Oscar Gálvez, M. Teresa Baeza-Romero, Mikel Sanz and Alfonso Saiz-Lopez.

Atmos. Chem. Phys., 16, doi:10.5194/acp-16-12703-2016

 

Abstract:

Reactive halogens play a key role in the oxidation capacity of the polar troposphere. However, sources and mechanisms, particularly those involving active iodine, are still poorly understood. In this paper, the photolysis of an atmospherically relevant frozen iodate salt has been experimentally studied using infrared (IR) spectroscopy. The samples were generated at low temperatures in the presence of different amounts of water. The IR spectra have confirmed that, under near-ultraviolet–visible (UV–Vis) radiation, iodate is efficiently photolysed. The integrated IR absorption coefficient of the iodate anion on the band at 750 cm−1 has been measured to be A  =  9.8 ± 0.5  ×  10−17 cm molecule−1. The photolysis rate of the ammonium iodate salt was measured by monitoring the decay of ammonium or iodate IR bands (1430 and 750 cm−1 respectively) in the presence of a solar simulator. The absorption cross section of the liquid solutions of ammonium iodate at wavelengths relevant for the troposphere (250 to 400 nm) has been obtained and used to estimate the photolytic quantum yield for the frozen salt. Finally, using an atmospheric model, constrained with the experimental data, we suggest that the photolysis of iodate in frozen salt can potentially provide a pathway for the release of active iodine to the polar atmosphere.

MAX-DOAS retrieval of aerosol extinction properties in Madrid (Spain).

Shanshan Wang, Carlos A. Cuevas, Udo FrieB, and Alfonso Saiz-Lopez.

Atmos. Meas. Tech., 9, DOI:10.5194/amt-9-5089-2016

 

Abstract:

Multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements were performed in the urban environment of Madrid, Spain, from March to September 2015. The O4 absorption in the ultraviolet (UV) spectral region was used to retrieve the aerosol extinction profile using an inversion algorithm. The results show a good agreement between the hourly retrieved aerosol optical depth (AOD) and the correlative Aerosol Robotic Network (AERONET) product, with a correlation coefficient of R =  0.87. Higher AODs are found in the summer season due to the more frequent occurrence of Saharan dust intrusions. The surface aerosol extinction coefficient as retrieved by the MAX-DOAS measurements was also compared to in situ PM2.5 concentrations. The level of agreement between both measurements indicates that the MAX-DOAS retrieval has the ability to characterize the extinction of aerosol particles near the surface. The retrieval algorithm was also used to study a case of severe dust intrusion on 12 May 2015. The capability of the MAX-DOAS retrieval to recognize the dust event including an elevated particle layer is investigated along with air mass back-trajectory analysis.

The polar iodine paradox.

Alfonso Saiz-Lopez; Christopher Shawn Blaszczak-Boxe. J.

Atmos. Environ, 140, pp 72-73; doi.org/10.1016/j.atmosenv, 2016

 

Abstract:

The uneven presence of iodine in the polar regions presents a scientific challenge, which connects marine algae, ice and atmosphere.

Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem.

Tomás Sherwen; Johan A. Schmidt; Mat J. Evans; Lucy J. Carpenter; Katja Großmann; Sebastian D. Eastham; Daniel J. Jacob; Barbara Dix; Theodore K. Koenig; Roman Sinreich, Ivan Ortega; Rainer Volkamer; Alfonso Saiz-Lopez; Cristina Prados-Roman; Anoop S. Mahajan; Carlos Ordóñez.

Atmos. Chem. Phys., 16, 12239-12271, DOI:10.5194/acp-16-12239-2016, 2016.

 

Abstract:

We present a simulation of the global present-day composition of the troposphere which includes the chemistry of halogens (Cl, Br, I). Building on previous work within the GEOS-Chem model we include emissions of inorganic iodine from the oceans, anthropogenic and biogenic sources of halogenated gases, gas phase chemistry, and a parameterised approach to heterogeneous halogen chemistry. Consistent with Schmidt et al. (2016) we do not include sea-salt debromination. Observations of halogen radicals (BrO, IO) are sparse but the model has some skill in reproducing these. Modelled IO shows both high and low biases when compared to different datasets, but BrO concentrations appear to be modelled low. Comparisons to the very sparse observations dataset of reactive Cl species suggest the model represents a lower limit of the impacts of these species, likely due to underestimates in emissions and therefore burdens. Inclusion of Cl, Br, and I results in a general improvement in simulation of ozone (O3) concentrations, except in polar regions where the model now underestimates O3 concentrations. Halogen chemistry reduces the global tropospheric O3 burden by 18.6 %, with the O3 lifetime reducing from 26 to 22 days. Global mean OH concentrations of 1.28  ×  106 molecules cm−3 are 8.2 % lower than in a simulation without halogens, leading to an increase in the CH4 lifetime (10.8 %) due to OH oxidation from 7.47 to 8.28 years. Oxidation of CH4 by Cl is small (∼  2 %) but Cl oxidation of other VOCs (ethane, acetone, and propane) can be significant (∼  15–27 %). Oxidation of VOCs by Br is smaller, representing 3.9 % of the loss of acetaldehyde and 0.9 % of the loss of formaldehyde.

On the variability of ozone in the equatorial eastern Pacific boundary layer.

J. C. Gómez Martín; H. Vömel; T. D. Hay; A. S. Mahajan; C. Ordóñez; M. C. Parrondo Sempere; M. Gil-Ojeda; A. Saiz-Lopez.

J. Geophys. Res. Atmos., 121, DOI:10.1002/2016JD025392 (2016)

 

Abstract:

Observations of surface ozone (O3) mixing ratios carried out during two ground-based field campaigns in the Galápagos Islands are reported. The first campaign, Primera Investigación sobre la Química, Evolución y Reparto de Ozono, was carried out from September 2000 to July 2002. The second study, Climate and HAlogen Reactivity tropicaL EXperiment, was conducted from September 2010 to March 2012. These measurements complement the Southern Hemisphere ADditional OZonesonde observations made with weekly to monthly frequency at Galápagos. In this work, the daily, intraseasonal, seasonal and interannual variability of O3 in the marine boundary layer are described and compared to those observed in other tropical locations. The O3 diurnal cycle shows two regimes: (i) photochemical destruction followed by nighttime recovery in the cold season (July to November) and (ii) daytime advection and photochemical loss followed by nighttime depositional loss associated to windless conditions in the warm season (February to April). Wavelet spectral analysis of the intraseasonal variability of O3 reveals components with periods characteristic of tropical instability waves. The O3 seasonal variation in Galápagos is typical of the Southern Hemisphere, with a maximum in August and a minimum in February–March. Comparison with other measurements in remote tropical ocean locations shows that the change of the surface O3 seasonal cycle across the equator is explained by the position of the Intertropical Convergence Zone and the O3 levels upwind.

Canadian Arctic sea ice reconstructed from bromine in the Greenland NEEM ice core.

Andrea Spolaor; Paul Vallelonga; Clara Turetta; Niccolò Maffezzoli; Giulio Cozzi; Jacopo Gabrieli; Carlo Barbante; Kumiko Goto-Azuma; Alfonso Saiz-Lopez; Carlos A. Cuevas; Dorthe Dahl-Jensen.

Sci. Rep. 6, 33925; DOI: 10.1038/srep33925 (2016)

 

Abstract:

Reconstructing the past variability of Arctic sea ice provides an essential context for recent multi-year sea ice decline, although few quantitative reconstructions cover the Holocene period prior to the earliest historical records 1,200 years ago. Photochemical recycling of bromine is observed over first-year, or seasonal, sea ice in so-called “bromine explosions” and we employ a 1-D chemistry transport model to quantify processes of bromine enrichment over first-year sea ice and depositional transport over multi-year sea ice and land ice. We report bromine enrichment in the Northwest Greenland Eemian NEEM ice core since the end of the Eemian interglacial 120,000 years ago, finding the maximum extension of first-year sea ice occurred approximately 9,000 years ago during the Holocene climate optimum, when Greenland temperatures were 2 to 3 °C above present values. First-year sea ice extent was lowest during the glacial stadials suggesting complete coverage of the Arctic Ocean by multi-year sea ice. These findings demonstrate a clear relationship between temperature and first-year sea ice extent in the Arctic and suggest multi-year sea ice will continue to decline as polar amplification drives Arctic temperatures beyond the 2 °C global average warming target of the recent COP21 Paris climate agreement.

A multi-model intercomparison of halogenated very short-lived substances (TransCom-VSLS): linking oceanic emissions and tropospheric transport for a reconciled estimate of the stratospheric source gas injection of bromine.

R. Hossaini, P. K. Patra, A. A. Leeson, G. Krysztofiak, N. L. Abraham, S. J. Andrews, A. T. Archibald, J. Aschmann, E. L. Atlas, D. A. Belikov, H. Bönisch, R. Butler, L. J. Carpenter, S. Dhomse, M. Dorf, A. Engel, L. Feng, W. Feng, S. Fuhlbrügge, P. T. Griffiths, N. R. P. Harris, R. Hommel, T. Keber, K. Krüger, S. T. Lennartz, S. Maksyutov, H. Mantle, G. P. Mills, B. Miller, S. A. Montzka, F. Moore, M. A. Navarro, D. E. Oram, P. I. Palmer, K. Pfeilsticker, J. A. Pyle, B. Quack, A. D. Robinson, E. Saikawa, A. Saiz-Lopez, S. Sala, B.-M. Sinnhuber, S. Taguchi, S. Tegtmeier, R. T. Lidster, C. Wilson, and F. Ziska.

Atmos. Chem. Phys., DOI:10.5194/acp-16-9163-2016

 

Abstract:

The first concerted multi-model intercomparison of halogenated very short-lived substances (VSLS) has been performed, within the framework of the ongoing Atmospheric Tracer Transport Model Intercomparison Project (TransCom). Eleven global models or model variants participated (nine chemical transport models and two chemistry–climate models) by simulating the major natural bromine VSLS, bromoform (CHBr3) and dibromomethane (CH2Br2), over a 20-year period (1993–2012). Except for three model simulations, all others were driven offline by (or nudged to) reanalysed meteorology. The overarching goal of TransCom-VSLS was to provide a reconciled model estimate of the stratospheric source gas injection (SGI) of bromine from these gases, to constrain the current measurement-derived range, and to investigate inter-model differences due to emissions and transport processes. Models ran with standardised idealised chemistry, to isolate differences due to transport, and we investigated the sensitivity of results to a range of VSLS emission inventories. Models were tested in their ability to reproduce the observed seasonal and spatial distribution of VSLS at the surface, using measurements from NOAA's long-term global monitoring network, and in the tropical troposphere, using recent aircraft measurements – including high-altitude observations from the NASA Global Hawk platform.

The models generally capture the observed seasonal cycle of surface CHBr3 and CH2Br2 well, with a strong model–measurement correlation (r  ≥  0.7) at most sites. In a given model, the absolute model–measurement agreement at the surface is highly sensitive to the choice of emissions. Large inter-model differences are apparent when using the same emission inventory, highlighting the challenges faced in evaluating such inventories at the global scale. Across the ensemble, most consistency is found within the tropics where most of the models (8 out of 11) achieve best agreement to surface CHBr3 observations using the lowest of the three CHBr3 emission inventories tested (similarly, 8 out of 11 models for CH2Br2). In general, the models reproduce observations of CHBr3 and CH2Br2 obtained in the tropical tropopause layer (TTL) at various locations throughout the Pacific well. Zonal variability in VSLS loading in the TTL is generally consistent among models, with CHBr3 (and to a lesser extent CH2Br2) most elevated over the tropical western Pacific during boreal winter. The models also indicate the Asian monsoon during boreal summer to be an important pathway for VSLS reaching the stratosphere, though the strength of this signal varies considerably among models.

We derive an ensemble climatological mean estimate of the stratospheric bromine SGI from CHBr3 and CH2Br2 of 2.0 (1.2–2.5) ppt,  ∼  57 % larger than the best estimate from the most recent World Meteorological Organization (WMO) Ozone Assessment Report. We find no evidence for a long-term, transport-driven trend in the stratospheric SGI of bromine over the simulation period. The transport-driven interannual variability in the annual mean bromine SGI is of the order of ±5 %, with SGI exhibiting a strong positive correlation with the El Niño–Southern Oscillation (ENSO) in the eastern Pacific. Overall, our results do not show systematic differences between models specific to the choice of reanalysis meteorology, rather clear differences are seen related to differences in the implementation of transport processes in the models.

An observationally constrained evaluation of the oxidative capacity in the tropical western Pacific troposphere

Julie M. Nicely, Daniel C. Anderson, Timothy P. Canty, Ross J. Salawitch, Glenn M. Wolfe, Eric C. Apel, Steve R. Arnold, Elliot L. Atlas, Nicola J. Blake, James F. Bresch, Teresa L. Campos, Russell R. Dickerson, Bryan Duncan, Louisa K. Emmons, Mathew J. Evans, Rafael P. Fernandez, Johannes Flemming, Samuel R. Hall, Thomas F. Hanisco, Shawn B. Honomichl, Rebecca S. Hornbrook, Vincent Huijnen, Lisa Kaser, Douglas E. Kinnison, Jean-Francois Lamarque, Jingqiu Mao, Sarah A. Monks, Denise D. Montzka, Laura L. Pan, Daniel D. Riemer, Alfonso Saiz-Lopez, Stephen D. Steenrod, Meghan H. Stell, Simone Tilmes, Solene Turquety, Kirk Ullmann, Andrew J. Weinheimer.

J. Geophys. Res. Atmos., 121, 7461–7488, DOI:10.1002/2016JD025067, 2016

 

Abstract:

Hydroxyl radical (OH) is the main daytime oxidant in the troposphere and determines the atmospheric lifetimes of many compounds. We use aircraft measurements of O3, H2O, NO, and other species from the Convective Transport of Active Species in the Tropics (CONTRAST) field campaign, which occurred in the tropical western Pacific (TWP) during January–February 2014, to constrain a photochemical box model and estimate concentrations of OH throughout the troposphere. We find that tropospheric column OH (OHCOL) inferred from CONTRAST observations is 12 to 40% higher than found in chemical transport models (CTMs), including CAM-chem-SD run with 2014 meteorology as well as eight models that participated in POLMIP (2008 meteorology). Part of this discrepancy is due to a clear-sky sampling bias that affects CONTRAST observations; accounting for this bias and also for a small difference in chemical mechanism results in our empirically based value of OHCOL being 0 to 20% larger than found within global models. While these global models simulate observed O3 reasonably well, they underestimate NOx (NO + NO2) by a factor of 2, resulting in OHCOL ~30% lower than box model simulations constrained by observed NO. Underestimations by CTMs of observed CH3CHO throughout the troposphere and of HCHO in the upper troposphere further contribute to differences between our constrained estimates of OH and those calculated by CTMs. Finally, our calculations do not support the prior suggestion of the existence of a tropospheric OH minimum in the TWP, because during January–February 2014 observed levels of O3 and NO were considerably larger than previously reported values in the TWP.

Evidence of atmospheric nanoparticle formation from emissions of marine microorganisms

K. Sellegri1, J. Pey; C. Rose; A. Culot; H. L. DeWitt; S. Mas; A. N. Schwier; B. Temime-Roussel; B. Charriere; A. Saiz-Lopez; A. S. Mahajan; D. Parin; A. Kukui;R. Sempere; B. D’Anna; N. Marchand.

Geophys. Res. Lett.,43, DOI: 10.1002/2016GL069389, 2016

 

Abstract:

Earth, as a whole, can be considered as a living organism emitting gases and particles into its atmosphere, in order to regulate its own temperature. In particular, oceans may respond to climate change by emitting particles that ultimately will influence cloud coverage. At the global scale, a large fraction of the aerosol number concentration is formed by nucleation of gas-phase species, but this process has never been directly observed above oceans. Here we present, using semicontrolled seawater-air enclosures, evidence that nucleation may occur from marine biological emissions in the atmosphere of the open ocean. We identify iodine-containing species as major precursors for new particle clusters' formation, while questioning the role of the commonly accepted dimethyl sulfide oxidation products, in forming new particle clusters in the region investigated and within a time scale on the order of an hour. We further show that amines would sustain the new particle formation process by growing the new clusters to larger sizes. Our results suggest that iodine-containing species and amines are correlated to different biological tracers. These observations, if generalized, would call for a substantial change of modeling approaches of the sea-to-air interactions.

On the formation of tropical rings of atomic halogens: Causes and implications

Alfonso Saiz-Lopez; Rafael P. Fernandez

Geophys. Res. Lett., 43, DOI:10.1002/2015GL067608, 2016

 

Abstract:

Halogens produced by ocean biological and photochemical processes reach the tropical tropopause layer (TTL), where cold temperatures and the prevailing low ozone abundances favor the diurnal photochemical enhancement of halogen atoms. Under these conditions atomic bromine and iodine are modeled to be the dominant inorganic halogen species in the sunlit TTL, surpassing the abundance of the commonly targeted IO and BrO radicals. We suggest that due to the rapid photochemical equilibrium between halogen oxides and halogen atoms a natural atmospheric phenomenon evolves, which we have collectively termed “tropical rings of atomic halogens.” We describe the main causes controlling the modeled appearance and variability of these superposed rings of bare bromine and iodine atoms that circle the tropics following the Sun. Some potential implications for atmospheric oxidizing capacity are also explored. Our model results suggest that if experimentally confirmed, the extent and intensity of the halogen rings would directly respond to changes in oceanic halocarbon emissions, their atmospheric transport, and photochemistry.

Iodine’s impact on tropospheric oxidants: a global model study in GEOS-Chem

A. Spolaor; T. Opel; J. R. McConnell; O. J. Maselli; G. Spreen; C. Varin; T. Kirchgeorg; D. Fritzsche; A. Saiz-Lopez; P. Vallelonga.

Atmos. Chem. Phys., 16, 1161–1186, 2016, DOI:10.5194/acp-16-1161-2016

 

Abstract:

We present a global simulation of tropospheric iodine chemistry within the GEOS-Chem chemical transport model. This includes organic and inorganic iodine sources, standard gas-phase iodine chemistry, and simplified higher iodine oxide (I2OX, X  = 2, 3, 4) chemistry, photolysis, deposition, and parametrized heterogeneous reactions. In comparisons with recent iodine oxide (IO) observations, the simulation shows an average bias of  ∼ +90 % with available surface observations in the marine boundary layer (outside of polar regions), and of  ∼ +73 % within the free troposphere (350 hPa  <  p  <  900 hPa) over the eastern Pacific. Iodine emissions (3.8 Tg yr−1) are overwhelmingly dominated by the inorganic ocean source, with 76 % of this emission from hypoiodous acid (HOI). HOI is also found to be the dominant iodine species in terms of global tropospheric IY burden (contributing up to 70 %). The iodine chemistry leads to a significant global tropospheric O3 burden decrease (9.0 %) compared to standard GEOS-Chem (v9-2). The iodine-driven OX loss rate1 (748 Tg OX yr−1) is due to photolysis of HOI (78 %), photolysis of OIO (21 %), and reaction between IO and BrO (1 %). Increases in global mean OH concentrations (1.8 %) by increased conversion of hydroperoxy radicals exceeds the decrease in OH primary production from the reduced O3 concentration. We perform sensitivity studies on a range of parameters and conclude that the simulation is sensitive to choices in parametrization of heterogeneous uptake, ocean surface iodide, and I2OX (X  = 2, 3, 4) photolysis. The new iodine chemistry combines with previously implemented bromine chemistry to yield a total bromine- and iodine-driven tropospheric O3 burden decrease of 14.4 % compared to a simulation without iodine and bromine chemistry in the model, and a small increase in OH (1.8 %). This is a significant impact and so halogen chemistry needs to be considered in both climate and air quality models.

1 Here OX is defined as O3 + NO2 + 2NO3 + PAN + PMN+PPN + HNO4 + 3N2O5 + HNO3 + BrO + HOBr + BrNO2+2BrNO3 + MPN + IO + HOI + INO2 + 2INO3 + 2OIO+2I2O2 + 3I2O3 + 4I2O4, where PAN  =  peroxyacetyl nitrate, PPN  =  peroxypropionyl nitrate, MPN  =  methyl peroxy nitrate, and MPN  =  peroxymethacryloyl nitrate.

Halogen-based reconstruction of Russian Arctic sea ice area from the Akademii Nauk ice core (Severnaya Zemlya)

A. Spolaor; T. Opel; J. R. McConnell; O. J. Maselli; G. Spreen; C. Varin; T. Kirchgeorg; D. Fritzsche; A. Saiz-Lopez; P. Vallelonga.

The Cryosphere, 10, 245-256, DOI:10.5194/tc-10-245-2016, 2016.

Abstract:

The role of sea ice in the Earth climate system is still under debate, although it is known to influence albedo, ocean circulation, and atmosphere–ocean heat and gas exchange. Here we present a reconstruction of 1950 to 1998 AD sea ice in the Laptev Sea based on the Akademii Nauk ice core (Severnaya Zemlya, Russian Arctic). The chemistry of halogens bromine (Br) and iodine (I) is strongly active and influenced by sea ice dynamics, in terms of physical, chemical and biological process. Bromine reacts on the sea ice surface in autocatalyzing "bromine explosion" events, causing an enrichment of the Br / Na ratio and hence a bromine excess (Brexc) in snow compared to that in seawater. Iodine is suggested to be emitted from algal communities growing under sea ice. The results suggest a connection between Brexc and spring sea ice area, as well as a connection between iodine concentration and summer sea ice area. The correlation coefficients obtained between Brexc and spring sea ice (r  =  0.44) as well as between iodine and summer sea ice (r   =  0.50) for the Laptev Sea suggest that these two halogens could become good candidates for extended reconstructions of past sea ice changes in the Arctic.

A pervasive role for biomass burning in tropical high ozone/low water structures

Daniel C. Anderson; Julie M. Nicely; Ross J. Salawitch; Timothy P. Canty; Russell R. Dickerson; Thomas F. Hanisco; Glenn M. Wolfe; Eric C. Apel; Elliot Atlas; Thomas Bannan; Stephane Bauguitte; Nicola J. Blake; James F. Bresch; Teresa L. Campos; Lucy J. Carpenter; Mark D. Cohen; Mathew Evans; Rafael P. Fernandez; Brian H. Kahn; Douglas E. Kinnison; Samuel R. Hall; Neil R.P. Harris;Rebecca S. Hornbrook; Jean-Francois Lamarque; Michael Le Breton; James D. Lee; Carl Percival; Leonhard Pfister; R. Bradley Pierce; Daniel D. Riemer; Alfonso Saiz-Lopez; Barbara J.B. Stunder; Anne M. Thompson; Kirk Ullmann; Adam Vaughan; Andrew J. Weinheimer.

Nature Communications, 7, Article number: 10267. DOI: 10.1038/ncomms10267

Abstract:

Air parcels with mixing ratios of high O3 and low H2O (HOLW) are common features in the tropical western Pacific (TWP) mid-troposphere (300–700 hPa). Here, using data collected during aircraft sampling of the TWP in winter 2014, we find strong, positive correlations of O3 with multiple biomass burning tracers in these HOLW structures. Ozone levels in these structures are about a factor of three larger than background. Models, satellite data and aircraft observations are used to show fires in tropical Africa and Southeast Asia are the dominant source of high O3 and that low H2O results from large-scale descent within the tropical troposphere. Previous explanations that attribute HOLW structures to transport from the stratosphere or mid-latitude troposphere are inconsistent with our observations. This study suggest a larger role for biomass burning in the radiative forcing of climate in the remote TWP than is commonly appreciated.

Fuction of Molecular Iodine and Tri-iodide in the Frozen Solution of Iodide: Implication for Polar Atmosphere

Kitae Kim; Akihiro Yabushita; Masanori Okumura; Alfonso Saiz-Lopez; Carlos A. Cuevas; Christopher S. Blaszczak-Boxe; Dae Wi Min; Ho-Il Yoon; Wonyong Choi.

Environ. Sci. Technol., 2016, 50 (3), pp 1280–1287. DOI: 10.1021/acs.est.5b05148

 

Abstract:

The chemistry of reactive halogens in the polar atmosphere plays important roles in ozone and mercury depletion events, oxidizing capacity, and dimethylsulfide oxidation to form cloud-condensation nuclei. Among halogen species, the sources and emission mechanisms of inorganic iodine compounds in the polar boundary layer remain unknown. Here, we demonstrate that the production of tri-iodide (I3) via iodide oxidation, which is negligible in aqueous solution, is significantly accelerated in frozen solution, both in the presence and the absence of solar irradiation. Field experiments carried out in the Antarctic region (King George Island, 62°13′S, 58°47′W) also showed that the generation of tri-iodide via solar photo-oxidation was enhanced when iodide was added to various ice media. The emission of gaseous I2 from the irradiated frozen solution of iodide to the gas phase was detected by using cavity ring-down spectroscopy, which was observed both in the frozen state at 253 K and after thawing the ice at 298 K. The accelerated (photo-)oxidation of iodide and the subsequent formation of tri-iodide and I2 in ice appear to be related with the freeze concentration of iodide and dissolved O2 trapped in the ice crystal grain boundaries. We propose that an accelerated abiotic transformation of iodide to gaseous I2 in ice media provides a previously unrecognized formation pathway of active iodine species in the polar atmosphere.

Bimodal distribution of free tropospheric ozone over the tropical western Pacific revealed by airborne observations

L. L. Pan; S. B. Honomichl; W. J. Randel; E. C. Apel; E. L. Atlas; S. P. Beaton; J. F. Bresch; R. Hornbrook; D. E. Kinnison; J.-F. Lamarque; A. Saiz-Lopez; R. J. Salawitch; A. J. Weinheimer.

Geophysical Research Letters Volume 42, Issue 18 Pages 7844–7851. DOI: 10.1002/2015GL065562

Abstract:

A recent airborne field campaign over the remote western Pacific obtained the first intensive in situ ozone sampling over the warm pool region from oceanic surface to 15 km altitude (near 360 K potential temperature level). The new data set quantifies ozone in the tropical tropopause layer under significant influence of convective outflow. The analysis further reveals a bimodal distribution of free tropospheric ozone mixing ratio. A primary mode, narrowly distributed around 20 ppbv, dominates the troposphere from the surface to 15 km. A secondary mode, broadly distributed with a 60 ppbv modal value, is prominent between 3 and 8 km (320 K to 340 K potential temperature levels). The latter mode occurs as persistent layers of ozone-rich drier air and is characterized by relative humidity under 45%. Possible controlling mechanisms are discussed. These findings provide new insight into the physical interpretation of the “S”-shaped mean ozone profiles in the tropics.

Atmospheric ammonia and its impacts on regional air quality over the megacity of Shanghai, China

Shanshan Wang; Jialiang Nan; Chanzhen Shi; Qingyan Fu; Song Gao; Dongfang Wang; Huxiong Cui; Alfonso Saiz-Lopez; Bin Zhou.

Scientific Reports 5, Article number: 15842 (2015) doi:10.1038/srep15842

Abstract:

Atmospheric ammonia (NH3) has great environmental implications due to its important role in ecosystem and global nitrogen cycle, as well as contribution to secondary particle formation. Here, we report long-term continuous measurements of NH3 at different locations (i.e. urban, industrial and rural) in Shanghai, China, which provide an unprecedented portrait of temporal and spatial characteristics of atmospheric NH3 in and around this megacity. In addition to point emission sources, air masses originated from or that have passed over ammonia rich areas, e.g. rural and industrial sites, increase the observed NH3 concentrations inside the urban area of Shanghai. Remarkable high-frequency NH3 variations were measured at the industrial site, indicating instantaneous nearby industrial emission peaks. Additionally, we observed strong positive exponential correlations between NH4+/(NH4++NH3) and sulfate-nitrate-ammonium (SNA) aerosols, PM2.5 mass concentrations, implying a considerable contribution of gas-to-particle conversion of ammonia to SNA aerosol formation. Lower temperature and higher humidity conditions were found to favor the conversion of gaseous ammonia to particle ammonium, particularly in autumn. Although NH3 is currently not included in China’s emission control policies of air pollution precursors, our results highlight the urgency and importance of monitoring gaseous ammonia and improving its emission inventory in and around Shanghai.

On the concentration and size distribution of sub-micron aerosol in the Galápagos Islands

M. Sorribas, J.C. Gómez Martín; T.D. Hay; A.S. Mahajan; C.A. Cuevas; M.V. Agama Reyes; Paredes Mora; M. Gil-Ojeda; A. Saiz-Lopez.

Atmospheric Environment Volume 123, Part A, December 2015, Pages 39–48 doi:10.1016/j.atmosenv.2015.10.028

Abstract:

During the CHARLEX campaign in the Galápagos Islands, a Scanning Particle Mobility Sizer was deployed on San Cristobal Island in July–August 2011 to carry out size-resolved measurements of the concentration of submicron aerosols. To our knowledge these are the first measurements of aerosol concentrations in this unique environment. The particles with marine origin displayed a tri-modal number size distribution with peak diameters of 0.016 μm, 0.050 μm and 0.174 μm and a cloud-processed intermodal minimum at 0.093 μm. The mean total aerosol number concentration for the marine contribution was 470 ± 160 cm−3. A low particle concentration of 70 ± 50 cm−3 for the nucleation size range was measured, but no evidence of new particle production in the atmospheric marine boundary layer (MBL) was observed. The concentration of the Aitken size mode was found to be related to aerosol entrainment from the free troposphere off the coast of Chile followed by transport within the MBL to the Galápagos Islands. Cloud processing may activate the particles in the Aitken size range, growing through ‘in-cloud’ sulphate production and increasing the particle concentration in the accumulation size range. The 0.093 μm cloud processed minima suggests that the critical supersaturation at which the particle is activated to a cloud droplet is in the 0.14–0.21% range. The daytime marine particle background concentration was influenced by human activity around the sampling site, as well as by new particle formation triggered by biogenic emissions from the vegetation cover of the island's semiarid lowlands. Effective CCN formation may play a role in the formation and properties of the stratus clouds, which permanently cover the top of the windward side of the islands and establish one of their characteristic climatic bands.

Airborne measurements of organic bromine compounds in the Pacific tropical tropopause layer

Maria A. Navarro; Elliot L. Atlasa; Alfonso Saiz-Lopez; Xavier Rodriguez-Lloveras; Douglas E. Kinnison; Jean-Francois Lamarque; Simone Tilmes; Michal Filus; Neil R. P. Harris; Elena Meneguz; Matthew J. Ashfold; Alistair J. Manning; Carlos A. Cuevas; Sue M. Schauffler; and Valeria Donets.

PNAS November 10, 2015 vol. 112 no. 45 13789-13793,

Abstract:

Very short-lived brominated substances (VSLBr) are an important source of stratospheric bromine, an effective ozone destruction catalyst. However, the accurate estimation of the organic and inorganic partitioning of bromine and the input to the stratosphere remains uncertain. Here, we report near-tropopause measurements of organic brominated substances found over the tropical Pacific during the NASA Airborne Tropical Tropopause Experiment campaigns. We combine aircraft observations and a chemistry−climate model to quantify the total bromine loading injected to the stratosphere. Surprisingly, despite differences in vertical transport between the Eastern and Western Pacific, VSLBr (organic + inorganic) contribute approximately similar amounts of bromine [∼6 (4−9) parts per thousand] to the stratospheric input at the tropical tropopause. These levels of bromine cause substantial ozone depletion in the lower stratosphere, and any increases in future abundances (e.g., as a result of aquaculture) will lead to larger depletions.

 

A nocturnal atmospheric loss of CH2I2 in the remote marine boundary layer

Lucy J. Carpenter; Stephen J. Andrews; Richard T. Lidster; Alfonso Saiz-Lopez; Miguel Fernandez-Sanchez; William J. Bloss; Bin Ouyang; Roderic L. Jones.

J Atmos Chem., 1-12, 10.1007/s10874-015-9320-6

Abstract:

Ocean emissions of inorganic and organic iodine compounds drive the biogeochemical cycle of iodine and produce reactive ozone-destroying iodine radicals that influence the oxidizing capacity of the atmosphere. Di-iodomethane (CH2I2) and chloro-iodomethane (CH2ICl) are the two most important organic iodine precursors in the marine boundary layer. Ship-borne measurements made during the TORERO (Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated VOC) field campaign in the east tropical Pacific Ocean in January/February 2012 revealed strong diurnal cycles of CH2I2 and CH2ICl in air and of CH2I2 in seawater. Both compounds are known to undergo rapid photolysis during the day, but models assume no night-time atmospheric losses. Surprisingly, the diurnal cycle of CH2I2 was lower in amplitude than that of CH2ICl, despite its faster photolysis rate. We speculate that night-time loss of CH2I2 occurs due to reaction with NO3 radicals. Indirect results from a laboratory study under ambient atmospheric boundary layer conditions indicate a k CH2I2+NO3 of ≤4 × 10−13 cm3 molecule−1 s−1; a previous kinetic study carried out at ≤100 Torr found k CH2I2+NO3 of 4 × 10−13 cm3 molecule−1 s−1. Using the 1-dimensional atmospheric THAMO model driven by sea-air fluxes calculated from the seawater and air measurements (averaging 1.8 +/− 0.8 nmol m−2 d−1 for CH2I2 and 3.7 +/− 0.8 nmol m−2 d−1 for CH2ICl), we show that the model overestimates night-time CH2I2 by >60 % but reaches good agreement with the measurements when the CH2I2 + NO3 reaction is included at 2–4 × 10−13 cm3 molecule−1 s−1. We conclude that the reaction has a significant effect on CH2I2 and helps reconcile observed and modeled concentrations. We recommend further direct measurements of this reaction under atmospheric conditions, including of product branching ratios.

NO2 seasonal evolution in the subtropical free troposphere

M. Gil-Ojeda, M. Navarro-Comas, L. Gómez-Martin, J.A. Adame, A. Saiz-Lopez, C.A. Cuevas, Y. González, O. Puentedura, E. Cuevas, J.-F. Lamarque, D. Kinnison, and S. Tilmes

Atmos. Chem. Phys., 15, 10567-10579, 2015

Abstract:three years of multi-axis differential optical absorption spectroscopy (MAXDOAS) measurements (2011–2013) have been used for estimating the NO2 mixing ratio along a horizontal line of sight from the high mountain subtropical observatory of Izaña, at 2370 m a.s.l. (NDACC station, 28.3° N, 16.5° W). The method is based on horizontal path calculation from the O2–O2 collisional complex at the 477 nm absorption band which is measured simultaneously to the NO2 column density, and is applicable under low aerosol-loading conditions.

The MAXDOAS technique, applied in horizontal mode in the free troposphere, minimizes the impact of the NO2 contamination resulting from the arrival of marine boundary layer (MBL) air masses from thermally forced upwelling breeze during middle hours of the day. Comparisons with in situ observations show that during most of the measuring period, the MAXDOAS is insensitive or very slightly sensitive to the upwelling breeze. Exceptions are found for pollution events during southern wind conditions. On these occasions, evidence of fast, efficient and irreversible transport from the surface to the free troposphere is found.

Background NO2 volume mixing ratio (vmr), representative of the remote free troposphere, is in the range of 20–45 pptv. The observed seasonal evolution shows an annual wave where the peak is in phase with the solar radiation. Model simulations with the chemistry–climate CAM-Chem model are in good agreement with the NO2 measurements, and are used to further investigate the possible drivers of the NO2 seasonality observed at Izaña.

Injection of iodine to the stratosphere

A. Saiz-Lopez, S. Baidar, C.A. Cuevas, T.K. Koening, R.P. Fernandez, B. Dix, D.E. Kinnison, J.-F. Lamarque, X. Rodriguez-Lloveras, T.L. Campos, R. Volkamer

Geophys. Res. Lett., 42, doi:10.1002/2015GL064796

Abstract: We report a new estimation of the injection of iodine into the stratosphere based on novel daytime (solar zenith angle < 45°) aircraft observations in the tropical tropopause layer and a global atmospheric model with the most recent knowledge about iodine photochemistry. The results indicate that significant levels of total reactive iodine (0.25–0.7 parts per trillion by volume), between 2 and 5 times larger than the accepted upper limits, can be injected into the stratosphere via tropical convective outflow. At these iodine levels, modeled iodine catalytic cycles account for up to 30% of the contemporary ozone loss in the tropical lower stratosphere and can exert a stratospheric ozone depletion potential equivalent to, or even larger than, that of very short-lived bromocarbons. Therefore, we suggest that iodine sources and chemistry need to be considered in assessments of the historical and future evolution of the stratospheric ozone layer.

A mechanism for biologically induced iodine emissions from sea ice

A. Saiz-Lopez, C.S. Blaszczak-Boxe and L.J. Carpenter

Atmos. Chem. Phys., 15, 9731-9746, 2015

Abstract: Ground- and satellite-based measurements have reported high concentrations of iodine monoxide (IO) in coastal Antarctica. The sources of such a large iodine burden in the coastal Antarctic atmosphere remain unknown. We propose a mechanism for iodine release from sea ice based on the premise that micro-algae are the primary source of iodine emissions in this environment. The emissions are triggered by the biological production of iodide (I) and hypoiodous acid (HOI) from micro-algae (contained within and underneath sea ice) and their diffusion through sea-ice brine channels, ultimately accumulating in a thin brine layer (BL) on the surface of sea ice. Prior to reaching the BL, the diffusion timescale of iodine within sea ice is depth-dependent. The BL is also a vital component of the proposed mechanism as it enhances the chemical kinetics of iodine-related reactions, which allows for the efficient release of iodine to the polar boundary layer. We suggest that iodine is released to the atmosphere via three possible pathways: (1) emitted from the BL and then transported throughout snow atop sea ice, from where it is released to the atmosphere; (2) released directly from the BL to the atmosphere in regions of sea ice that are not covered with snowpack; or (3) emitted to the atmosphere directly through fractures in the sea-ice pack. To investigate the proposed biology–ice–atmosphere coupling at coastal Antarctica we use a multiphase model that incorporates the transport of iodine species, via diffusion, at variable depths, within brine channels of sea ice. Model simulations were conducted to interpret observations of elevated springtime IO in the coastal Antarctic, around the Weddell Sea. While a lack of experimental and observational data adds uncertainty to the model predictions, the results nevertheless show that the levels of inorganic iodine (i.e. I2, IBr, ICl) released from sea ice through this mechanism could account for the observed IO concentrations during this timeframe. The model results also indicate that iodine may trigger the catalytic release of bromine from sea ice through phase equilibration of IBr. Considering the extent of sea ice around the Antarctic continent, we suggest that the resulting high levels of iodine may have widespread impacts on catalytic ozone destruction and aerosol formation in the Antarctic lower troposphere.

Particles and iodine compounds in coastal Antarctica

Howard K. Roscoe, Anna E. Jones, Neil Brough, Rolf Weller, Alfonso Saiz-Lopez, Anoop S. Mahajan, Anja Schönhardt, John P. Burrows and Zoe L. Fleming

J. Geophys. Res. Atmos., 120, doi:10.1002/2015JD023301

Abstract. Aerosol particle number concentrations have been measured at Halley and Neumayer on the Antarctic coast, since 2004 and 1984, respectively. Sulphur compounds known to be implicated in particle formation and growth were independently measured: sulphate ions and methane sulphonic acid in filtered aerosol samples and gas phase dimethyl sulphide for limited periods. Iodine oxide, IO, was determined by a satellite sensor from 2003 to 2009 and by different ground-based sensors at Halley in 2004 and 2007. Previous model results and midlatitude observations show that iodine compounds consistent with the large values of IO observed may be responsible for an increase in number concentrations of small particles. Coastal Antarctica is useful for investigating correlations between particles, sulphur, and iodine compounds, because of their large annual cycles and the source of iodine compounds in sea ice. After smoothing all the measured data by several days, the shapes of the annual cycles in particle concentration at Halley and Neumayer are approximated by linear combinations of the shapes of sulphur compounds and IO but not by sulphur compounds alone. However, there is no short-term correlation between IO and particle concentration. The apparent correlation by eye after smoothing but not in the short term suggests that iodine compounds and particles are sourced some distance offshore. This suggests that new particles formed from iodine compounds are viable, i.e., they can last long enough to grow to the larger particles that contribute to cloud condensation nuclei, rather than being simply collected by existing particles. If so, there is significant potential for climate feedback near the sea ice zone via the aerosol indirect effect.

Impact of Enhanced Ozone Deposition and Halogen Chemistry on Tropospheric Ozone over the Northen Hemisphere

Golam Sarwar, Brett Gantt, Donna Schewede, Kristen Foley, Robit Mathur and Alfonso Saiz-Lopez

Environ. Sci. Technol. DOI_10.1021/acs.est.5b01657

Abstract. Fate of ozone in marine environments has been receiving increased attention due to the tightening of ambient air quality standards. The role of deposition and halogen chemistry is examined through incorporation of an enhanced ozone deposition algorithm and inclusion of halogen chemistry in a comprehensive atmospheric modeling system. The enhanced ozone deposition treatment accounts for the interaction of iodide in seawater with ozone and increases deposition velocities by 1 order of magnitude. Halogen chemistry includes detailed chemical reactions of organic and inorganic bromine and iodine species. Two different simulations are completed with the halogen chemistry: without and with photochemical reactions of higher iodine oxides. Enhanced deposition reduces mean summer-time surface ozone by ∼3% over marine regions in the Northern Hemisphere. Halogen chemistry without the photochemical reactions of higher iodine oxides reduces surface ozone by ∼15% whereas simulations with the photochemical reactions of higher iodine oxides indicate ozone reductions of ∼48%. The model without these processes overpredicts ozone compared to observations whereas the inclusion of these processes improves predictions. The inclusion of photochemical reactions for higher iodine oxides leads to ozone predictions that are lower than observations, underscoring the need for further refinement of the halogen emissions and chemistry scheme in the model.

Growth in stratospheric chlorine from short-lived chemicals not controlled by the Montreal Protocol

R. Hossaini, M.P. Chipperfield, A. Saiz-Lopez, J.J. Harrison, R. von Glasow, R. Sommariva, E. Atlas, M. Navarro, S.A. Montzka, W. Feng, S. Dhomse, C. Harth, J. Mühle, C. Lunder, S. O'Doherty, D. Young, S. Reimann, M.K. Vollmer, P.B. Krummel, P.F. Bernath.

Geophys. Res. Lett., 42, doi:10.1002/2015GL063783.

Abstract. We have developed a chemical mechanism describing the tropospheric degradation of chlorine containing very short-lived substances (VSLS). The scheme was included in a global atmospheric model and used to quantify the stratospheric injection of chlorine from anthropogenic VSLS (ClyVSLS) between 2005 and 2013. By constraining the model with surface measurements of chloroform (CHCl3), dichloromethane (CH2Cl2), tetrachloroethene (C2Cl4), trichloroethene (C2HCl3), and 1,2-dichloroethane (CH2ClCH2Cl), we infer a 2013 (ClyVSLS) mixing ratio of 123 parts per trillion (ppt). Stratospheric injection of source gases dominates this supply, accounting for ∼83% of the total. The remainder comes from VSLS-derived organic products, phosgene (COCl2, 7%) and formyl chloride (CHClO, 2%), and also hydrogen chloride (HCl, 8%). Stratospheric (ClyVSLS)  increased by ∼52% between 2005 and 2013, with a mean growth rate of 3.7 ppt Cl/yr. This increase is due to recent and ongoing growth in anthropogenic CH2Cl2—the most abundant chlorinated VSLS not controlled by the Montreal Protocol.

Quantifying the impacts of an updated global dimethyl sulfide climatology on cloud microphysics and aerosol radiative forcing

Anoop S. Mahajan, Suvarna Fadnavis, Manu A. Thomas, Luca Pozzoli, Smrati Gupta, Sarah-Jeanne Royer, Alfonso Saiz-Lopez, Rafel Simó.

J. Geophys. Res. 120, 6, 2524-2536. DOI: 10.1002/2014JD022687

Abstract. One of the critical parameters in assessing the global impacts of dimethyl sulfide (DMS) on cloud properties and the radiation budget is the estimation of phytoplankton-induced ocean emissions, which are derived from prescribed, climatological surface seawater DMS concentrations. The most widely used global ocean DMS climatology was published 15 years ago and has recently been updated using a much larger database of observations. The updated climatology displays significant differences in terms of the global distribution and regional monthly averages of sea surface DMS. In this study, we use the ECHAM5-HAMMOZ aerosol-chemistry-climate general circulation model to quantify the influence of the updated DMS climatology in computed atmospheric properties, namely, the spatial and temporal distributions of atmospheric DMS concentration, sulfuric acid concentration, sulfate aerosols, number of activated aerosols, cloud droplet number concentration, and the aerosol radiative forcing at the top of the atmosphere. Significant differences are observed for all the modeled variables. Comparison with observations of atmospheric DMS and total sulfate also shows that in places with large DMS emissions, the updated climatology shows a better match with the observations. This highlights the importance of using the updated climatology for projecting future impacts of oceanic DMS emissions, especially considering that the relative importance of the natural sulfur fluxes is likely to increase due to legislation to “clean up” anthropogenic emissions. The largest estimated differences are in the Southern Ocean, Indian Ocean, and parts of the Pacific Ocean, where the climatologies differ in seasonal concentrations over large geographical areas. The model results also indicate that the former DMS climatology underestimated the effect of DMS on the globally averaged annual aerosol radiative forcing at the top of the atmosphere by about 20%.

Tropospheric Halogen Chemistry: Sources, Cycling, and Impacts

William R. Simpson, Steven S. Brown, Alfonso Saiz-Lopez, Joel A. Thornton, and Roland von Glasow

Chem. Rev., Article. DOI: 10.1021/cr5006638

A negative feedback between anthropogenic ozone pollution and enhanced ocean emissions of iodine

C. Prados-Roman, C.A. Cuevas, R.P. Fernandez, D.E. Kinnison, J.-F. Lamarque, and A. Saiz-Lopez

Atmos. Chem. Phys., 15, 2215-2224, 2015

Abstract. Naturally emitted from the oceans, iodine compounds efficiently destroy atmospheric ozone and reduce its positive radiative forcing effects in the troposphere. Emissions of inorganic iodine have been experimentally shown to depend on the deposition to the oceans of tropospheric ozone, whose concentrations have significantly increased since 1850 as a result of human activities. A chemistry–climate model is used herein to quantify the current ocean emissions of inorganic iodine and assess the impact that the anthropogenic increase in tropospheric ozone has had on the natural cycle of iodine in the marine environment since pre-industrial times. Our results indicate that the human-driven enhancement of tropospheric ozone has doubled the oceanic inorganic iodine emissions following the reaction of ozone with iodide at the sea surface. The consequent build-up of atmospheric iodine, with maximum enhancements of up to 70% with respect to pre-industrial times in continental pollution outflow regions, has in turn accelerated the ozone chemical loss over the oceans with strong spatial patterns. We suggest that this ocean–atmosphere interaction represents a negative geochemical feedback loop by which current ocean emissions of iodine act as a natural buffer for ozone pollution and its radiative forcing in the global marine environment.

Iodine oxide in the global marine boundary layer

C. Prados-Roman, C.A. Cuevas, T. Hay, R.P. Fernandez, A.S. Mahajan, S.-J. Royer, M. Gali, R. Simó, J. Dachs, K. Groβmann, D.E. Kinnison, J.-F. Lamarque, and A. Saiz-Lopez

Atmos. Chem. Phys., 15, 583-593, 2015

Abstract. Emitted mainly by the oceans, iodine is a halogen compound important for atmospheric chemistry due to its high ozone depletion potential and effect on the oxidizing capacity of the atmosphere. Here we present a comprehensive dataset of iodine oxide (IO) measurements in the open marine boundary layer (MBL) made during the Malaspina 2010 circumnavigation. Results show IO mixing ratios ranging from 0.4 to 1 pmol mol−1 and, complemented with additional field campaigns, this dataset confirms through observations the ubiquitous presence of reactive iodine chemistry in the global marine environment. We use a global model with organic (CH3I, CH2ICl, CH2I2 and CH2IBr) and inorganic (HOI and I2) iodine ocean emissions to investigate the contribution of the different iodine source gases to the budget of IO in the global MBL. In agreement with previous estimates, our results indicate that, globally averaged, the abiotic precursors contribute about 75% to the iodine oxide budget. However, this work reveals a strong geographical pattern in the contribution of organic vs. inorganic precursors to reactive iodine in the global MBL.

New insights into martian atmospheric chemistry

C.S. Boxe, J.S. Francisco, R.-L. Shia, Y.L. Yung, H. Nair, M.-C. Liang, A. Saiz-Lopez

Icarus 242 (2014) 97-104

Abstract. HOx radicals are produced in the martian atmosphere by the photolysis of water vapor and subsequently participate in catalytic cycles that recycle carbon dioxide (CO2) from its photolysis product carbon monoxide (CO), providing a qualitative explanation for the stability of its atmosphere. Balancing CO2 production and loss based on our current understanding of martian gas-phase chemistry has, however, proven to be difficult. The photolysis of O3 produces O(1D), while oxidation of CO produces HOCO radicals, a new member of the HOx family. The O(1D) quantum yield has recently been updated, which quantifies nonzero quantum yields in the Huggins bands. In Earth’s atmosphere HOCO is considered to be unimportant since it is quickly removed by abundant oxygen molecules. The smaller amount of O2 in the Mars’ atmosphere causes HOCO’s lifetime to be longer in Mars’ atmosphere than Earth’s (3 × 10−5 s to 1.2 days from Mars’s surface to 240 km, respectively). Limited kinetic data on reactions involving HOCO prevented consideration of its reactions directly in atmospheric models. Therefore, the impact of HOCO reactions on martian chemistry is currently unknown. Here, we incorporate new literature rate constants for HOCO chemistry and an updated representation of the O(1D) quantum yield in the Caltech/JPL 1-D photochemical model for Mars’ atmosphere. Our simulations exemplify perturbations to NOy, HOx, and COx species, ranging from 5% to 50%. The modified O(1D) quantum yield and new HOCO chemistry cause a 10% decrease and a 50% increase in OH and H2O2 total column abundances, respectively. At low altitudes, HOCO production contributes 5% towards CO2 production. Given recent experimentally-obtained branching ratios for the oxidation of CO, HOCO may contribute up to 70% toward the production of NOy, where HOx and NOy species are enhanced up to a factor 3, which has implications for rethinking the fundamental understanding of NOy, HOx, and CO/CO2 cycling on Mars. Two new reaction mechanisms for converting CO to CO2 using HOCO reactions are proposed, which reveal that H2O2 is more intimately coupled to COx chemistry. Our simulations are in good agreement with satellite/spacecraft measurements of CO and H2O2 on Mars.

Iodine chemistry in the troposphere and its effect on ozone

A. Saiz-Lopez, R.P. Fernandez, C. Ordoñez, D.E. Kinnison, J.C. Gómez Martín, J.-F. Lamarque and S. Tilmes.

Atmos. Chem. Phys., 14, 13119-13143, 2014

Abstract. Despite the potential influence of iodine chemistry on the oxidizing capacity of the troposphere, reactive iodine distributions and their impact on tropospheric ozone remain almost unexplored aspects of the global atmosphere. Here we present a comprehensive global modelling experiment aimed at estimating lower and upper limits of the inorganic iodine burden and its impact on tropospheric ozone. Two sets of simulations without and with the photolysis of IxOy oxides (i.e. I2O2, I2O3 and I2O4) were conducted to define the range of inorganic iodine loading, partitioning and impact in the troposphere. Our results show that the most abundant daytime iodine species throughout the middle to upper troposphere is atomic iodine, with an annual average tropical abundance of (0.15–0.55) pptv. We propose the existence of a "tropical ring of atomic iodine" that peaks in the tropical upper troposphere (~11–14 km) at the equator and extends to the sub-tropics (30° N–30° S). Annual average daytime I / IO ratios larger than 3 are modelled within the tropics, reaching ratios up to ~20 during vigorous uplift events within strong convective regions. We calculate that the integrated contribution of catalytic iodine reactions to the total rate of tropospheric ozone loss (IOx Loss) is 2–5 times larger than the combined bromine and chlorine cycles. When IxOy photolysis is included, IOx Loss represents an upper limit of approximately 27, 14 and 27% of the tropical annual ozone loss for the marine boundary layer (MBL), free troposphere (FT) and upper troposphere (UT), respectively, while the lower limit throughout the tropical troposphere is ~9%. Our results indicate that iodine is the second strongest ozone-depleting family throughout the global marine UT and in the tropical MBL. We suggest that (i) iodine sources and its chemistry need to be included in global tropospheric chemistry models, (ii) experimental programs designed to quantify the iodine budget in the troposphere should include a strategy for the measurement of atomic I, and (iii) laboratory programs are needed to characterize the photochemistry of higher iodine oxides to determine their atmospheric fate since they can potentially dominate halogen-catalysed ozone destruction in the troposphere.

Bromine partitioning in the tropical tropopause layer: implications for stratospheric injection

R.P. Fernandez, R.J. Salawitch, D.E. Kinnison, J.-F. Lamarque and A. Saiz-Lopez

Atmos. Chem. Phys., 14, 13391-13410, 2014

Abstract. Very short-lived (VSL) bromocarbons are produced at a prodigious rate by ocean biology and these source compounds (SGVSL), together with their inorganic degradation products (PGVSL), are lofted by vigorous convection to the tropical tropopause layer (TTL). Using a state-of-the-art photochemical mechanism within a global model, we calculate annual average stratospheric injection of total bromine due to VSL sources to be 5 pptv (parts per trillion by volume), with ~ 3 pptv entering the stratosphere as PGVSL and ~ 2 pptv as SGVSL. The geographic distribution and partitioning of VSL bromine within the TTL, and its consequent stratospheric injection, is highly dependent on the oceanic flux, the strength of convection and the occurrence of heterogeneous recycling reactions. Our calculations indicate atomic Br should be the dominant inorganic species in large regions of the TTL during daytime, due to the low ozone and cold conditions of this region. We propose the existence of a "tropical ring of atomic bromine" located approximately between 15 and 19 km and between 30° N and 30° S. Daytime Br / BrO ratios of up to ~ 4 are predicted within this inhomogeneous ring in regions of highly convective transport, such as the tropical Western Pacific. Therefore, we suggest that experimental programs designed to quantify the bromine budget of the TTL and the stratospheric injection of VSL biogenic bromocarbons should include a strategy for the measurement of atomic Br during daytime as well as HOBr and BrCl during nighttime.

Evolution of NO2 levels in Spain from 1996 to 2012

C.A. Cuevas, A. Notario, J.A. Adame, A. Hilboll, A. Richter, J.P. Burrows and A. Saiz-Lopez

Sci. Rep. 4, 5887; DOI:10.1038/srep05887 (2014).

Abstract. We report on the evolution of tropospheric nitrogen dioxide (NO2) over Spain, focusing on the densely populated cities of Barcelona, Bilbao, Madrid, Sevilla and Valencia, during 17 years, from 1996 to 2012. This data series combines observations from in-situ air quality monitoring networks and the satellite-based instruments GOME and SCIAMACHY. The results in these five cities show a smooth decrease in the NO2 concentrations of ~2% per year in the period 1996–2008, due to the implementation of emissions control environmental legislation, and a more abrupt descend of ~7% per year from 2008 to 2012 as a consequence of the economic recession. In the whole Spanish territory the NO2 levels have decreased by ~22% from 1996 to 2012. Statistical analysis of several economic indicators is used to investigate the different factors driving the NO2 concentration trends over Spain during the last two decades.

A laboratory characterisation of inorganic iodine emissions from the sea surface: dependence on oceanic variables and parameterisation for global modelling

S.M. MacDonald, J.C. Gómez Martín, R. Chance, S.Warriner, A. Saiz-Lopez, L.J. Carpenter, and J.M.C. Plane

Atmos. Chem. Phys., 14, 5841-5852, 2014

Abstract. Reactive iodine compounds play a significant role in the atmospheric chemistry of the oceanic boundary layer by influencing the oxidising capacity through catalytically removing O3 and altering the HOx and NOx balance. The sea-to-air flux of iodine over the open ocean is therefore an important quantity in assessing these impacts on a global scale. This paper examines the effect of a number of relevant environmental parameters, including water temperature, salinity and organic compounds, on the magnitude of the HOI and I2 fluxes produced from the uptake of O3 and its reaction with iodide ions in aqueous solution. The results of these laboratory experiments and those reported previously (Carpenter et al., 2013), along with sea surface iodide concentrations measured or inferred from measurements of dissolved total iodine and iodate reported in the literature, were then used to produce parameterised expressions for the HOI and I2 fluxes as a function of wind speed, sea-surface temperature and O3. These expressions were used in the Tropospheric HAlogen chemistry MOdel (THAMO) to compare with MAX-DOAS measurements of iodine monoxide (IO) performed during the HaloCAST-P cruise in the eastern Pacific ocean (Mahajan et al., 2012). The modelled IO agrees reasonably with the field observations, although significant discrepancies are found during a period of low wind speeds (< 3 m s−1), when the model overpredicts IO by up to a factor of 3. The inorganic iodine flux contributions to IO are found to be comparable to, or even greater than, the contribution of organo-iodine compounds and therefore its inclusion in atmospheric models is important to improve predictions of the influence of halogen chemistry in the marine boundary layer.

Glyoxal observations in the marine boundary layer

Anoop S. Mahajan, Cristina Prados-Roman, Timothy D. Hay, Johannes Lampel, Denis Pöhler, Katia Grossmann, Jens Tschritter, Udo Friess, Ulrich Platt, Paul Johnston, Karin Kreher, Folkard Wittrock, John P. Burrows, John M.C. Plane and Alfonso Saiz-Lopez.

J. of Geophys. Res.: atmos. DOI: 10.1002/2013JD021388

Abstract: Glyoxal is an important intermediate species formed by the oxidation of common biogenic and anthropogenic volatile organic compounds such as isoprene, toluene and acetylene. Although glyoxal has been shown to play an important role in urban and forested environments, its role in the open ocean environment is still not well understood, with only a few observations showing evidence for its presence in the open ocean marine boundary layer (MBL). In this study, we report observations of glyoxal from ten field campaigns in different parts of the world's oceans. These observations together represent the largest database of glyoxal in the MBL. The measurements are made with similar instruments that have been used in the past, although the open ocean values reported here, average of about 25 pptv with an upper limit of 40 pptv, are much lower than previously reported observations that were consistently higher than 40 pptv and had an upper limit of 140 pptv, highlighting the uncertainties in the Differential Optical Absorption Spectroscopy (DOAS) method for the retrieval of glyoxal. Despite retrieval uncertainties, the results reported in this work support previous suggestions that the currently known sources of glyoxal are insufficient to explain the average MBL concentrations. This suggests that there is an additional missing source, more than a magnitude larger than currently known sources, which is necessary to account for the observed atmospheric levels of glyoxal. Therefore it could play a more important role in the MBL than previously considered.

Observations of I2 at a remote marine site

M. J. Lawler, A. S. Mahajan, A. Saiz-Lopez, and E. S. Saltzman

Atmos. Chem. Phys., 14, 2669-2678, 2014

Abstract. Inorganic iodine plays a significant role in the photochemistry of the marine boundary layer, but the sources and cycling of iodine are not well understood. We report the first I2 observations in marine air that is not impacted by coastal macroalgal emissions or sea ice chemistry. The data clearly demonstrate that the very high I2 levels previously reported for coastal air are not representative of open ocean conditions. In this study, gas phase I2 was measured at the Cape Verde Atmospheric Observatory, a semi-remote site in the eastern tropical Atlantic, using atmospheric pressure chemical ionization tandem mass spectrometry. Atmospheric I2 levels typically increased beginning at sunset, leveled off after midnight, and then rapidly decreased at sunrise. There was also a smaller midday maximum in I2 that was probably caused by a measurement artifact. Ambient I2 mixing ratios ranged from <0.02–0.6 pmol mol−1 in May 2007 and <0.03–1.67 pmol mol−1 in May 2009. The sea-air flux implied by the nighttime buildup of I2 is too small to explain the observed daytime IO levels at this site. Iodocarbon measurements made in this region previously are also insufficient to explain the observed 1–2 pmol mol−1 of daytime IO. The observations imply the existence of an unknown daytime source of gas phase inorganic iodine. Carpenter et al. (2013) recently proposed that sea surface emissions of HOI are several times larger than the flux of I2. Such a flux could account for both the nighttime I2 and the daytime IO observations.

Enhanced production of oxidised mercury over the tropical Pacific Ocean: a key missing oxidation pathway

F. Wang, A. Saiz-Lopez, A. S. Mahajan, J. C. Gómez Martín, D. Armstrong, M. Lemes, T. Hay, and C. Prados-Roman

Atmos. Chem. Phys., 14, 1323-1335, 2014

Abstract. Mercury is a contaminant of global concern. It is transported in the atmosphere primarily as gaseous elemental mercury, but its reactivity and deposition to the surface environment, through which it enters the aquatic food chain, is greatly enhanced following oxidation. Measurements and modelling studies of oxidised mercury in the polar to sub-tropical marine boundary layer (MBL) have suggested that photolytically produced bromine atoms are the primary oxidant of mercury. We report year-round measurements of elemental and oxidised mercury, along with ozone, halogen oxides (IO and BrO) and nitrogen oxides (NO2), in the MBL over the Galápagos Islands in the equatorial Pacific. Elemental mercury concentration remained low throughout the year, while higher than expected levels of oxidised mercury occurred around midday. Our results show that the production of oxidised mercury in the tropical MBL cannot be accounted for by bromine oxidation only, or by the inclusion of ozone and hydroxyl. As a two-step oxidation mechanism, where the HgBr intermediate is further oxidised to Hg(II), depends critically on the stability of HgBr, an additional oxidant is needed to react with HgBr to explain more than 50% of the observed oxidised mercury. Based on best available thermodynamic data, we show that atomic iodine, NO2, or HO2 could all play the potential role of the missing oxidant, though their relative importance cannot be determined explicitly at this time due to the uncertainties associated with mercury oxidation kinetics. We conclude that the key pathway that significantly enhances atmospheric mercury oxidation and deposition to the tropical oceans is missing from the current understanding of atmospheric mercury oxidation.

 

Evaluating global emission inventories of biogenic bromocarbons

R. Hossaini, H. Mantle, M.P. Chipperfield, S.A. Montzka, P. Hamer, F. Ziska, B. Quack, K. Krüger, S. Tegtmeier, E. Atlas, S. Sala, A. Engel, H. Bönisch, T. Keber, D. Oram, G. Mills, C. Ordoñez, A. Saiz-Lopez, N. Warnick, Q.Liang, W. Feng, F. Moore, B.R. Miller, V. Marécal, N.A. Richards, M. Dorf, and K. Pfeilsticker.

Atmos. Chem. Phys., 13, 11819-11838, 2013

Abstract. Emissions of halogenated very short-lived substances (VSLS) are poorly constrained. However, their inclusion in global models is required to simulate a realistic inorganic bromine (Bry) loading in both the troposphere, where bromine chemistry perturbs global oxidising capacity, and in the stratosphere, where it is a major sink for ozone (O3). We have performed simulations using a 3-D chemical transport model (CTM) including three top-down and a single bottom-up derived emission inventory of the major brominated VSLS bromoform (CHBr3) and dibromomethane (CH2Br2). We perform the first concerted evaluation of these inventories, comparing both the magnitude and spatial distribution of emissions. For a quantitative evaluation of each inventory, model output is compared with independent long-term observations at National Oceanic and Atmospheric Administration (NOAA) ground-based stations and with aircraft observations made during the NSF (National Science Foundation) HIAPER Pole-to-Pole Observations (HIPPO) project. For CHBr3, the mean absolute deviation between model and surface observation ranges from 0.22 (38%) to 0.78 (115%) parts per trillion (ppt) in the tropics, depending on emission inventory. For CH2Br2, the range is 0.17 (24%) to 1.25 (167%) ppt. We also use aircraft observations made during the 2011 Stratospheric Ozone: Halogen Impacts in a Varying Atmosphere (SHIVA) campaign, in the tropical western Pacific. Here, the performance of the various inventories also varies significantly, but overall the CTM is able to reproduce observed CHBr3 well in the free troposphere using an inventory based on observed sea-to-air fluxes. Finally, we identify the range of uncertainty associated with these VSLS emission inventories on stratospheric bromine loading due to VSLS (BryVSLS). Our simulations show BryVSLS ranges from ~4.0 to 8.0 ppt depending on the inventory. We report an optimised estimate at the lower end of this range (~4 ppt) based on combining the CHBr3 and CH2Br2 inventories which give best agreement with the compilation of observations in the tropics.

 

 

A theoretical study on the formation of iodine oxides aggregates and monohydrates

Óscar Gálvez, Juan Carlos Gomez Martin, Pedro C Gomez, Alfonso Saiz-Lopez and Luis F Palacios

Phys. Chem. Chem. Phys., 2013, 15, 15572. DOI: 10.1039/c3cp51219c

Abstract. Biotic and abiotic emissions of molecular iodine and iodocarbons from the sea or ice surface and the intertidal zone to the coastal/polar marine boundary layer lead to the formation of iodine oxides, which subsequently nucleate forming iodine oxide particles (IOPs). Although the link between coastal iodine emissions and ultrafine aerosol bursts is well established, the details of the nucleation mechanism have not yet been elucidated. In this paper, results of a theoretical study of a range of potentially relevant aggregation reactions of different iodine oxides, as well as complexation with water molecules, are reported. Thermochemical properties for these reactions are obtained from high level ab initio correlated calculations including spin-orbit corrections. The results show that the nucleation path most likely proceeds through dimerisation of I2O4. It is also shown that water can hinder gas-to-particle conversion to some extent, although complexation with key iodine oxides does not remove enough of these to stop IOP formation. A consistent picture of this process emerges from the theoretical study presented here and the findings of a new laboratory study reported in the accompanying paper (Gomez Martin, et al., 2013).

 

 

Iodine chemistry in the eastern Pacific marine boundary layer

Juan C. Gómez Martín, Anoop S. Mahajan, Timothy D. Hay, Cristina Prados-Román, Carlos Ordóñez, Samantha M . McDonals, John M.C. Plane, Mar Sorribas, Manuel Gil, J. Francisco Paredes Mora, Mario V. Agama Reyes, David E. Oram, Emma Leedham, Alfonso Saiz-Lopez.

Journal of Geophysical Research: Atmospheres, vol, 118, 1-14, doi:10.1002/jgrd.50550, 2013

 

Abstract. Observations of gas-phase iodine species were made during a field campaign in the eastern Pacific marine boundary layer (MBL). The Climate and Halogen Reactivity Tropical Experiment (CHARLEX) in the Galápagos Islands, running from September 2010 to present, is the first long-term ground-based study of trace gases in this region. Observations of gas-phase iodine species were made using long-path differential optical absorption spectroscopy (LP-DOAS), multi-axis DOAS (MAX-DOAS), and resonance and off-resonance fluorescence by lamp excitation (ROFLEX). These measurements were supported by ancillary measurements of ozone, nitrogen oxides, and meteorological variables. Selective halocarbon and ultrafine aerosol concentration measurements were also made. MAX-DOAS observations of iodine monoxide (IO) display a weak seasonal variation. The maximum differential slant column density was 3.8 × 1013 molecule cm−2 (detection limit ~7 × 1012 molecule cm−2). The seasonal variation of reactive iodine IOx (= I + IO) is stronger, peaking at 1.6 pptv during the warm season (February–April). This suggests a dependence of the iodine sources on the annual cycle in sea surface temperature, although perturbations by changes in ocean surface iodide concentration and solar radiation are also possible. An observed negative correlation of IOx with chlorophyll-a indicates a predominance of abiotic sources. The low IO mixing ratios measured (below the LP-DOAS detection limit of 0.9 pptv) are not consistent with satellite observations if IO is confined to the MBL. The IOx loading is consistent with the observed absence of strong ozone depletion and nucleation events, indicating a small impact of iodine chemistry on these climatically relevant factors in the eastern Pacific MBL.

 

 

Modelling future changes to the stratosphere source gas injection of biogenic bromocarbons

R. Hossaini, M.P. Chipperfield, S. Dhomse, C. Ordoñez, A. Saiz-Lopez, N.H. Abraham, A. Archibald, P. Braesicke, P. Telford, N. Warwick, X. Yang and J. Pyle

Geophysical Research Letters Vol 39 issue 20, 2012, DOI: 10.1029/2012GL053401

Abstract. [1] Simulations with a chemistry-climate model (CCM) show a future increase in the stratospheric source gas injection (SGI) of biogenic very short-lived substances (VSLS). For 2000, the modelled SGI of bromine from VSLS is ∼1.7 parts per trillion (pptv) and largest over the tropical West Pacific. For 2100, this increases to ∼2.0 and ∼2.7 pptv when the model is forced with Intergovernmental Panel on Climate Change (IPCC) representative concentration pathways (RCPs) 4.5 and 8.5. The increase is largely due to stronger tropical deep convection transporting more CHBr3 to the lower stratosphere. For CH2Br2, CHBr2Cl, CH2BrCl and CHBrCl2, changes to primary oxidant OH determines their SGI contribution. Under RCP 4.5 (moderate warming), OH increases in a warmer, more humid troposphere. Under RCP 8.5 (extreme warming) OH decreases significantly due to a large methane increase, allowing greater SGI of bromine from these VSLS. Potentially enhanced VSLS emissions in the future would further increase these estimates.

 

 

Summertime NOx measurements during the CHABLIS campaign: can source and sink estimates unravel observed diurnal cycles?

S. J.-B. Bauguitte, W. J. Bloss, M. J. Evans, R. A. Salmon, P. S. Anderson, A. E. Jones, J. D. Lee, A. Saiz-Lopez, H. K. Roscoe, E. W. Wolff, and J. M. C. Plane

Atmos. Chem. Phys., 12, 989-1002, 2012

Abstract. NOx measurements were conducted at the Halley Research Station, coastal Antarctica, during the austral summer period 1 January–10 February 2005. A clear NOx diurnal cycle was observed with minimum concentrations close to instrumental detection limit (5 pptv) measured between 04:00–05:00 GMT. NOx concentrations peaked (24 pptv) between 19:00–20:00 GMT, approximately 5 h after local solar noon. An optimised box model of NOx concentrations based on production from in-snow nitrate photolysis and chemical loss derives a mean noon emission rate of 3.48 × 108 molec cm−2 s−1, assuming a 100 m boundary layer mixing height, and a relatively short NOx lifetime of ~6.4 h. This emission rate compares to directly measured values ranging from 2.1 to 12.6 × 108 molec cm−2 s−1 made on 3 days at the end of the study period. Calculations of the maximum rate of NO2 loss via a variety of conventional HOx and halogen oxidation processes show that the lifetime of NOx is predominantly controlled by halogen processing, namely BrNO3 and INO3 gas-phase formation and their subsequent heterogeneous uptake. Furthermore the presence of halogen oxides is shown to significantly perturb NOx concentrations by decreasing the NO/NO2 ratio. We conclude that in coastal Antarctica, the potential ozone production efficiency of NOx emitted from the snowpack is mitigated by the more rapid NOx loss due to halogen nitrate hydrolysis.

 

 

Reactive halogen chemistry in the troposphere

Alfonso Saiz-Lopez, Roland von Glasow

Chem. Soc. Rev., 2012, 41, 6448-6472, DOI:10.1039/C2CS35208G

Abstract. Halogen chemistry is well known for ozone destruction in the stratosphere, however reactive halogens also play an important role in the chemistry of the troposphere. In the last two decades, an increasing number of reactive halogen species have been detected in a wide range of environmental conditions from the polar to the tropical troposphere. Growing observational evidence suggests a regional to global relevance of reactive halogens for the oxidising capacity of the troposphere. This critical review summarises our current understanding and uncertainties of the main halogen photochemistry processes, including the current knowledge of the atmospheric impact of halogen chemistry as well as open questions and future research needs.

 

 

Atmospheric chemistry

Dwayne E. Heard and Alfonso Saiz-Lopez

Chem. Soc. Rev., 2012, 41, 6229-6230. DOI: 10.1039/C2CS90076A

 

 

On the sodium D line emission in the terrestrial nightglow

J.M.C. Plane, H. Oetjen, M. de Miranda, A. Saiz-Lopez, M. Gausa, B. Williams

J. Atmos. and Solar-Terrestrial Physics, 74, 181-188, 2012

Abstract. Emission from atomic Na, consisting of a doublet of lines at 589.0 and 589.6 nm, is a prominent feature of the earth’s nightglow. A large data-base of measurements of the relative intensities of the D lines (RD) was gathered at three locations: the ALOMAR observatory, Andenes (Norway, 69°N), Kuujjuarapik (Canada, 55°N) and the Danum Valley (Borneo, 8°N). RD varies between 1.5 and 2.0, with an average value of 1.67. These results were interpreted using a theoretical model of the Na nightglow which involves initial production of electronically excited NaO(A2Σ) from the reaction between Na and O3, followed either by reaction with O to generate Na(2PJ) with a branching ratio of 1/6 and a J=3/2 to 1/2 propensity of 2.0, or quenching of NaO(A) to NaO(X2Π) by O2. The resulting NaO(X) then reacts with O to generate Na(2PJ) with a branching ratio of 1/6 and a J=3/2 to 1/2 propensity of 1.5. These branching ratios and spin-orbit propensities are derived from statistical correlation of the electronic potential energy surfaces connecting the reactants NaO(A)+O and NaO(X)+O with the products Na+O2, through the Na+O2 ion-pair intermediate. A fit of this statistical model to the results of an earlier laboratory study (Slanger et al., 2005), where RD was measured as a function of the ratio [O]/[O2], indicates that the rate coefficient for the quenching of NaO(A) by O2 is around 1×10−11 cm3 molecule−1 s−1. The statistical model is also in good accord with recent high resolution observations of the Na D line widths (Harrell et al., 2010). An atmospheric model is then used to show that gravity wave-driven perturbations to the Na layer can account for the observed variability of RD.

 

 

Latitudinal distribution of reactive iodine in the Eastern Pacific and its link to open ocean sources

A. S. Mahajan, J. C. Gómez Martín, T. D. Hay, S.-J. Royer, S. Yvon-Lewis, Y. Liu, L. Hu, C. Prados-Roman, C. Ordóñez, J. M. C. Plane, and A. Saiz-Lopez.

Atmos. Chem. Phys., 12, 11609-11617, 2012

Abstract. Ship-based Multi-Axis Differential Optical Absorption Spectroscopy measurements of iodine monoxide (IO) and atmospheric and seawater Gas Chromatography-Mass Spectrometer observations of methyl iodide (CH3I) were made in the Eastern Pacific marine boundary layer during April 2010 as a part of the HaloCarbon Air Sea Transect-Pacific (HaloCAST-P) scientific cruise. The presence of IO in the open ocean environment was confirmed, with a maximum differential slant column density of 5 × 1013 molecules cm−2 along the 1° elevation angle (corresponding to approximately 1 pptv) measured in the oligotrophic region of the Southeastern Pacific. Such low IO mixing ratios and their observed geographical distribution are inconsistent with satellite estimates and with previous understanding of oceanic sources of iodine. A strong correlation was observed between reactive iodine (defined as IO + I) and CH3I, suggesting common sources. In situ measurements of meteorological parameters and physical ocean variables, along with satellite-based observations of Chlorophyll a (Chl a) and Chromophoric Dissolved Organic Matter (CDOM) were used to gain insight into the possible sources of iodine in this remote environment. Surprisingly, reactive iodine showed a negative correlation (> 99% confidence) to Chl a and CDOM across the cruise transect. However, a significant positive correlation (> 99% confidence) with sea surface temperature (SST) and salinity instead suggests a widespread abiotic source related to the availability of aqueous iodine and to temperature. 

 

 

Iodine monoxide in the north subtropical free troposphere

O. Puentedura, M. Gil, A. Saiz-Lopez, T. Hay, M. Navarro-Comas, A. Gómez-Pelaez, E. Cuevas, J. Iglesias, and L. Gomez

Atmos. Chem. Phys., 12, 4909-4921, 2012

Abstract. Iodine monoxide (IO) differential slant column densities (DSCD) have been retrieved from a new multi-axis differential optical absorption spectroscopy (MAX-DOAS) instrument deployed at the Izaña subtropical observatory as part of the Network for the Detection of Atmospheric Composition Change (NDACC) programme. The station is located at 2370 m a.s.l., well above the trade wind inversion that limits the top of the marine boundary layer, and hence is representative of the free troposphere. We report daily observations from May to August 2010 at different viewing angles. During this period, the spectral signature of IO was unequivocally detected on every day of measurement. A mean IO DSCD of 1.52×1013 molecules cm−2 was observed at the 5° instrument elevation angle (IEA) on clear days using a single zenith reference for the reported period, with a day-to-day variability of 33% at one standard deviation. Based on the simulation of the DSCDs using radiative transfer calculations with five different hypothesized IO profiles, the IO mixing ratio is estimated to range between 0.2 and 0.4 pptv in the free troposphere. Episodes of Saharan dust outbreaks were also observed, with large increases in the DSCDs at higher IEA, suggesting an enhancement of IO inside the dust cloud.

 

Iodine emissions from the sea ice of the Weddell Sea

H. M. Atkinson, R.-J. Huang, R. Chance, H. K. Roscoe, C. Hughes, B. Davison, A. Schönhardt, A. S. Mahajan, A. Saiz-Lopez, T. Hoffmann, and P. S. Liss

Atmos. Chem. Phys., 12, 11229-11244, 2012

Abstract. Iodine compounds were measured above, below and within the sea ice of the Weddell Sea during a cruise in 2009, to make progress in elucidating the mechanism of local enhancement and volatilisation of iodine. I2 mixing ratios of up to 12.4 pptv were measured 10 m above the sea ice, and up to 31 pptv was observed above surface snow on the nearby Brunt Ice Shelf – large amounts. Atmospheric IO of up to 7 pptv was measured from the ship, and the average sum of HOI and ICl was 1.9 pptv. These measurements confirm the Weddell Sea as an iodine hotspot. Average atmospheric concentrations of CH3I, C2H5I, CH2ICl, 2-C3H7I, CH2IBr and 1-C3H7I were each 0.2 pptv or less. On the Brunt Ice Shelf, enhanced concentrations of CH3I and C2H5I (up to 0.5 and 1 pptv respectively) were observed in firn air, with a diurnal profile that suggests the snow may be a source. In the sea ice brine, iodocarbons concentrations were over 10 times those of the sea water below. The sum of iodide + iodate was depleted in sea ice samples, suggesting some missing iodine chemistry. Flux calculations suggest I2 dominates the iodine atom flux to the atmosphere, but models cannot reconcile the observations and suggest either a missing iodine source or other deficiencies in our understanding of iodine chemistry. The observation of new particle formation, consistent with the model predictions, strongly suggests an iodine source. This combined study of iodine compounds is the first of its kind in this unique region of sea ice rich in biology and rich in iodine chemistry.

 

 

Halogen activation via interactions with environmental ice and snow in the polar lower troposphere and other regions

J. P. D. Abbatt, J. L. Thomas, K. Abrahamsson, C. Boxe, A. Granfors, A. E. Jones, M. D. King, A. Saiz-Lopez, P. B. Shepson, J. Sodeau, D. W. Toohey, C. Toubin, R. von Glasow, S. N. Wren, and X. Yang

Atmos. Chem. Phys., 12, 6237-6271, 2012

Abstract. The role of ice in the formation of chemically active halogens in the environment requires a full understanding because of its role in atmospheric chemistry, including controlling the regional atmospheric oxidizing capacity in specific situations. In particular, ice and snow are important for facilitating multiphase oxidative chemistry and as media upon which marine algae live. This paper reviews the nature of environmental ice substrates that participate in halogen chemistry, describes the reactions that occur on such substrates, presents the field evidence for ice-mediated halogen activation, summarizes our best understanding of ice-halogen activation mechanisms, and describes the current state of modeling these processes at different scales. Given the rapid pace of developments in the field, this paper largely addresses advances made in the past five years, with emphasis given to the polar boundary layer. The integrative nature of this field is highlighted in the presentation of work from the molecular to the regional scale, with a focus on understanding fundamental processes. This is essential for developing realistic parameterizations and descriptions of these processes for inclusion in larger scale models that are used to determine their regional and global impacts.

 

 

Estimating the climate significance of halogen-driven ozone loss in the tropical marine troposphere

A. Saiz-Lopez, J.-F. Lamarque, D. E. Kinnison, S. Tilmes, C. Ordóñez, J. J. Orlando, A. J. Conley, J. M. C. Plane, A. S. Mahajan, G. Sousa Santos, E. L. Atlas, D. R. Blake, S. P. Sander, S. Schauffler, A. M. Thompson, and G. Brasseur

Atmos. Chem. Phys., 12, 3939-3949, 2012

 

Abstract. We have integrated observations of tropospheric ozone, very short-lived (VSL) halocarbons and reactive iodine and bromine species from a wide variety of tropical data sources with the global CAM-Chem chemistry-climate model and offline radiative transfer calculations to compute the contribution of halogen chemistry to ozone loss and associated radiative impact in the tropical marine troposphere. The inclusion of tropospheric halogen chemistry in CAM-Chem leads to an annually averaged depletion of around 10% (~2.5 Dobson units) of the tropical tropospheric ozone column, with largest effects in the middle to upper troposphere. This depletion contributes approximately −0.10 W m−2 to the radiative flux at the tropical tropopause. This negative flux is of similar magnitude to the ~0.33 W m−2 contribution of tropospheric ozone to present-day radiative balance as recently estimated from satellite observations. We find that the implementation of oceanic halogen sources and chemistry in climate models is an important component of the natural background ozone budget and we suggest that it needs to be considered when estimating both preindustrial ozone baseline levels and long term changes in tropospheric ozone.

 

 

Bromine and iodine chemistry in a global chemistry-climate model: description and evaluation of very short-lived oceanic sources

C. Ordóñez, J.-F. Lamarque, S. Tilmes, D. E. Kinnison, E. L. Atlas, D. R. Blake, G. Sousa Santos, G. Brasseur, and A. Saiz-Lopez

Atmos. Chem. Phys., 12, 1423-1447, 2012

Abstract. The global chemistry-climate model CAM-Chem has been extended to incorporate an expanded bromine and iodine chemistry scheme that includes natural oceanic sources of very short-lived (VSL) halocarbons, gas-phase photochemistry and heterogeneous reactions on aerosols. Ocean emissions of five VSL bromocarbons (CHBr3, CH2Br2, CH2BrCl, CHBrCl2, CHBr2Cl) and three VSL iodocarbons (CH2ICl, CH2IBr, CH2I2) have been parameterised by a biogenic chlorophyll-a (chl-a) dependent source in the tropical oceans (20° N–20° S). Constant oceanic fluxes with 2.5 coast-to-ocean emission ratios are separately imposed on four different latitudinal bands in the extratropics (20°–50° and above 50° in both hemispheres). Top-down emission estimates of bromocarbons have been derived using available measurements in the troposphere and lower stratosphere, while iodocarbons have been constrained with observations in the marine boundary layer (MBL). Emissions of CH3I are based on a previous inventory and the longer lived CH3Br is set to a surface mixing ratio boundary condition. The global oceanic emissions estimated for the most abundant VSL bromocarbons – 533 Gg yr−1 for CHBr3 and 67.3 Gg yr−1 for CH2Br2 – are within the range of previous estimates. Overall the latitudinal and vertical distributions of modelled bromocarbons are in good agreement with observations. Nevertheless, we identify some issues such as the reduced number of aircraft observations to validate models in the Southern Hemisphere, the overestimation of CH2Br2 in the upper troposphere – lower stratosphere and the underestimation of CH3I in the same region. Despite the difficulties involved in the global modelling of the shortest lived iodocarbons (CH2ICl, CH2IBr, CH2I2), modelled results are in good agreement with published observations in the MBL. Finally, sensitivity simulations show that knowledge of the diurnal emission cycle for these species, in particular for CH2I2, is key to assess their global source strength.

 

 

Atmospheric Chemistry of Iodine

Alfonso Saiz-Lopez, John M. C. Plane, Alex R. Baker, Lucy J. Carpenter, Roland von Glasow, Juan C. Gómez Martín, Gordon McFiggans, and Russell W. Saunders

Chem. Rev., 2012, 112 (3), pp 1773–1804, doi: 10.1021/cr200029u

 

 

An active nitrogen cycle on Mars sufficient to support a subsurface biosphere

C.S. Boxe, K.P. Hand, K.H. Nealson, Y.L. Yung and A. Saiz-Lopez

International Journal of Astrobiology, 1(2): 109-115 (2012)

Abstract. Mars' total atmospheric nitrogen content is 0.2 mbar. One-dimensional (1D) photochemical simulations of Mars' atmosphere show that nitric acid (HNO3(g)), the most soluble nitrogen oxide, is the principal reservoir species for nitrogen in its lower atmosphere, which amounts to a steady-state value of 6×10−2 kg or 4 moles, conditions of severe nitrogen deficiency. Mars could, however, support ∼1015 kg of biomass (∼1 kg N m−2) from its current atmospheric nitrogen inventory. The terrestrial mass ratio of nitrogen in biomass to that in the atmosphere is ∼10−5; applying this ratio to Mars yields ∼1010 kg of total biomass – also, conditions of severe nitrogen deficiency. These amounts, however, are lower limits as the maximum surface-sink of atmospheric nitrogen is 2.8 mbar (9×1015 kg of N), which indicates, in contradistinction to the Klingler et al. (1989), that biological metabolism would not be inhibited in the subsurface of Mars. Within this context, we explore HNO3 deposition on Mars' surface (i.e. soil and ice-covered regions) on pure water metastable thin liquid films. We show for the first time that the negative change in Gibbs free energy increases with decreasing HNO3(g) (NO3(aq)) in metastable thin liquid films that may exist on Mars' surface. We also show that additional reaction pathways are exergonic and may proceed spontaneously, thus providing an ample source of energy for nitrogen fixation on Mars. Lastly, we explore the dissociation of HNO3(g) to form NO3(aq) in metastable thin liquid films on the Martian surface via condensed phase simulations. These simulations show that photochemically produced fixed nitrogen species are not only released from the Martian surface to the gas-phase, but more importantly, transported to lower depths from the Martian surface in transient thin liquid films. A putative biotic layer at 10 m depth would produce HNO3 and N2 sinks of −54 and −5×1012 molecules cm−2 s−1, respectively, which is an ample supply of available nitrogen that can be efficiently transported to the subsurface. The downward transport as well as the release to the atmosphere of photochemically produced fixed nitrogen species (e.g. NO2, NO and NO2) suggests the existence of a transient but active nitrogen cycle on Mars.

 

 

Adsorbed water and thin liquid films on Mars,

C.S. Boxe, K. P. Hand, K. H. Nealson, Y. L. Yung, A. S. Yen, and A. Saiz-Lopez

International Journal of Astrobiology, doi:10.1017/S147355041200080, 2012

Abstract. At present, bulk liquid water on the surface and near-subsurface of Mars does not exist due to the scarcity of condensed- and gas-phase water, pressure and temperature constraints. Given that the nuclei of soil and ice, that is, the soil solid and ice lattice, respectively, are coated with adsorbed and/or thin liquid films of water well below 273 K and the availability of water limits biological activity, we quantify lower and upper limits for the thickness of such adsorbed/water films on the surface of the Martian regolith and for subsurface ice. These limits were calculated based on experimental and theoretical data for pure water ice and water ice containing impurities, where water ice containing impurities exhibit thin liquid film enhancements, ranging from 3 to 90. Close to the cold limit of water stability (i.e. 273 K), thin liquid film thicknesses at the surface of the Martian regolith is 0.06 nm (pure water ice) and ranges from 0.2 to 5 nm (water ice with impurities). An adsorbed water layer of 0.06 nm implies a dessicated surface as the thickness of one monolayer of water is 0.3 nm but represents 0.001–0.02% of the Martian atmospheric water vapour inventory. Taking into account the specific surface area (SSA) of surface-soil (i.e. top 1 mm of regolith and 0.06 nm adsorbed water layer), shows Martian surface-soil may contain interfacial water that represents 6–66% of the upper- and lower-limit atmospheric water vapour inventory and almost four times and 33%, the lower- and upper-limit Martian atmospheric water vapour inventory. Similarly, taking the SSA of Martian soil, the top 1 mm or regolith at 5 nm thin liquid water thickness, yields 1.10×1013 and 6.50×1013 litres of waters, respectively, 55–325 times larger than Mars’ atmospheric water vapour inventory. Film thicknesses of 0.2 and 5 nm represent 2.3×104–1.5×106 litres of water, which is 6.0×10−7–4.0×10−4%, respectively, of a 10 pr μm water vapour column, and 3.0×10−6–4.0×10−4% and 6.0×10−6–8.0×10−4%, respectively, of the Martian atmospheric water vapour inventory. Thin liquid film thicknesses on/in subsurface ice were investigated via two scenarios: (i) under the idealistic case where it is assumed that the diurnal thermal wave is equal to the temperature of ice tens of centimetres below the surface, allowing for such ice to experience temperatures close to 273 K and (ii) under the, likely, realistic scenario where the diurnal thermal wave allows for the maximum subsurface ice temperature of 235 K at 1 m depth between 30°N and 30°S. Scenario 1 yields thin liquid film thicknesses ranging from 11 to 90 nm; these amounts represent 4×106–3.0×107 litres of water. For pure water ice, Scenario 2 reveals that the thickness of thin liquid films contained on/within Martian subsurface is less than 1.2 nm, several molecular layers thick. Conversely, via the effect of impurities at 235 K allows for a thin liquid film thickness on/within subsurface ice of 0.5 nm, corresponding to 6.0×104 litres of water. The existence of thin films on Mars is supported by data from the Mars Exploration Rovers (MERs) Spirit and Opportunity's Alpha Proton X-ray Spectrometer instrumentation, which have detected increased levels of bromine beneath the immediate surface, suggestive of the mobilization of soluble salts by thin films of liquid water towards local cold traps. These findings show that biological activity on the Martian surface and subsurface is not limited by nanometre dimensions of available water.

 

 

The multi-seasonal NOy budget in coastal Antarctica and its link with surface snow and ice core nitrate: results from the CHABLIS campaign

A. E. Jones, E. W. Wolff, D. Ames, S. J.-B. Bauguitte, K. C. Clemitshaw, Z. Fleming, G. P. Mills, A. Saiz-Lopez, R. A. Salmon, W. T. Sturges, and D. R. Worton

Atmos. Chem. Phys., 11, 9271-9285, 2011

Abstract. Measurements of a suite of individual NOy components were carried out at Halley station in coastal Antarctica as part of the CHABLIS campaign (Chemistry of the Antarctic Boundary Layer and the Interface with Snow). Conincident measurements cover over half a year, from austral winter 2004 through to austral summer 2005. Results show clear dominance of organic NOy compounds (PAN and MeONO2) during the winter months, with low concentrations of inorganic NOy. During summer, concentrations of inorganic NOy compounds are considerably greater, while those of organic compounds, although lower than in winter, are nonetheless significant. The relative concentrations of the alkyl nitrates, as well as their seasonality, are consistent with an oceanic source. Multi-seasonal measurements of surface snow nitrate correlate strongly with inorganic NOy species (especially HNO3) rather than organic. One case study in August suggested that, on that occasion, particulate nitrate was the dominant source of nitrate to the snowpack, but this was not the consistent picture throughout the measurement period. An analysis of NOx production rates showed that emissions of NOx from the snowpack overwhelmingly dominate over gas-phase sources. This result suggests that, for certain periods in the past, the flux of NOx into the Antarctic boundary layer can be calculated from ice core nitrate data.

 

 

In situ detection of atomic and molecular iodine using Resonance and Off-Resonance Fluorescence by Lamp Excitation: ROFLEX

Gómez Martín, J. C., Blahins, J., Gross, U., Ingham, T., Goddard, A., Mahajan, A. S., Ubelis, A., and Saiz-Lopez, A.

Atmos. Meas. Tech., 4, 29-45, doi:10.5194/amt-4-29-2011, 2011

Abstract. We demonstrate a new instrument for in situ detection of atmospheric iodine atoms and molecules based on atomic and molecular resonance and off-resonance ultraviolet fluorescence excited by lamp emission. The instrument combines the robustness, light weight, low power consumption and efficient excitation of radio-frequency discharge light sources with the high sensitivity of the photon counting technique. Calibration of I2 fluorescence is achieved via quantitative detection of the molecule by Incoherent Broad Band Cavity-enhanced Absorption Spectroscopy. Atomic iodine fluorescence signal is calibrated by controlled broad band photolysis of known I2 concentrations in the visible spectral range at atmospheric pressure. The instrument has been optimised in laboratory experiments to reach detection limits of 1.2 pptv for I atoms and 13 pptv for I2, for S/N = 1 and 10 min of integration time. The ROFLEX system has been deployed in a field campaign in northern Spain, representing the first concurrent observation of ambient mixing ratios of iodine atoms and molecules in the 1–350 pptv range.

 

 

DOAS observations of formaldehyde and its impact on the HOx balance in the tropical Atlantic marine boundary layer

Mahajan, A. S., Whalley, L. K., Kozlova, E., Oetjen, H., Mendez, L., Furneaux, K. L., Goddard, A., Heard, D. E., Plane, J. M. C. and Saiz-Lopez, A.

J. Atmos. Chem., doi:10.1007/s10874-011-9200-7, 2011

Abstract. Measurements of formaldehyde (HCHO) were made at the Cape Verde Atmospheric Observatory between November 2006 and June 2007 using the Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) technique. Observations show that typical HCHO mixing ratios ranged between 350 and 550 pptv (with typical 2-σ uncertainties of ~110 pptv), with several events of high HCHO, the maximum being 1,885 ± 149 pptv. The observations indicate a lack of strong seasonal or diurnal variations, within the uncertainty of the measurements. A box model is employed to test whether the observations can be explained using known hydrocarbon photochemistry; the model replicates well the typical diurnal profile and monthly mean values. The model results indicate that on average 20% of HO2 production and 10% of OH destruction can be attributed to the mean HCHO levels, suggesting that even at these low average mixing ratios HCHO plays an important role in determining the HOx (HO2+OH) balance of the remote marine boundary layer.

 

 

Concurrent observations of atomic iodine, molecular iodine and ultrafine particles in a coastal environment

A. S. Mahajan, M. Sorribas, J. C. Gómez Martín, S. M. MacDonald, M. Gil, J. M. C. Plane, and A. Saiz-Lopez

Atmos. Chem. Phys., 11, 2545-2555, 2011

Abstract. Simultaneous measurements of atomic iodine (I), molecular iodine (I2) and ultrafine particles were made at O Grove, Galicia (42.50° N, 8.87° W), on the northwest coast of Spain. The observations show a strong tidal signature, and indicate that the most probable sources of reactive iodine species are the exposed macroalgae during low tide. For the first time, I2 and I were concurrently measured revealing a high average I2/I ratio of ~32, which is higher than previously inferred by modelling studies. A 1-dimensional photochemical model is employed to simulate the observations showing that the high I2/I ratio can be reproduced in the presence of fast vertical mixing close to the surface, or using an extra chemical loss for I atoms with an unknown species. There is a lack of strong correlation between the I2/I and ultrafine particles, indicating that although they both have macroalgal sources, these were not at the same location. The model simulations also suggest that the source of the observed ultrafine particles is likely not very close to the measurement site, in order for the particles to form and grow, but the source for I and I2 must be local. Finally, the effect of NOx levels on iodine oxides, and the conditions under which iodine particle bursts will be suppressed, are explored.

 

 

Reactive Halogens in the Marine Boundary Layer (RHaMBLe): the tropical North Atlantic experiments

J. D. Lee, G. McFiggans, J. D. Allan, A. R. Baker, S. M. Ball, A. K. Benton, L. J. Carpenter, R. Commane, B. D. Finley, M. Evans, E. Fuentes, K. Furneaux, A. Goddard, N. Good, J. F. Hamilton, D. E. Heard, H. Herrmann, A. Hollingsworth, J. R. Hopkins, T. Ingham, M. Irwin, C. E. Jones, R. L. Jones, W. C. Keene, M. J. Lawler, S. Lehmann, A. C. Lewis, M. S. Long, A. Mahajan, J. Methven, S. J. Moller, K. Müller, T. Müller, N. Niedermeier, S. O'Doherty, H. Oetjen, J. M. C. Plane, A. A. P. Pszenny, K. A. Read, A. Saiz-Lopez, E. S. Saltzman, R. Sander, R. von Glasow, L. Whalley, A. Wiedensohler, and D. Young

Atmos. Chem. Phys., 10, 1031-1055, 2010

 

Abstract. The NERC UK SOLAS-funded Reactive Halogens in the Marine Boundary Layer (RHaMBLe) programme comprised three field experiments. This manuscript presents an overview of the measurements made within the two simultaneous remote experiments conducted in the tropical North Atlantic in May and June 2007. Measurements were made from two mobile and one ground-based platforms. The heavily instrumented cruise D319 on the RRS Discovery from Lisbon, Portugal to São Vicente, Cape Verde and back to Falmouth, UK was used to characterise the spatial distribution of boundary layer components likely to play a role in reactive halogen chemistry. Measurements onboard the ARSF Dornier aircraft were used to allow the observations to be interpreted in the context of their vertical distribution and to confirm the interpretation of atmospheric structure in the vicinity of the Cape Verde islands. Long-term ground-based measurements at the Cape Verde Atmospheric Observatory (CVAO) on São Vicente were supplemented by long-term measurements of reactive halogen species and characterisation of additional trace gas and aerosol species during the intensive experimental period.

This paper presents a summary of the measurements made within the RHaMBLe remote experiments and discusses them in their meteorological and chemical context as determined from these three platforms and from additional meteorological analyses. Air always arrived at the CVAO from the North East with a range of air mass origins (European, Atlantic and North American continental). Trace gases were present at stable and fairly low concentrations with the exception of a slight increase in some anthropogenic components in air of North American origin, though NOx mixing ratios during this period remained below 20 pptv (note the non-IUPAC adoption in this manuscript of pptv and ppbv, equivalent to pmol mol−1 and nmol mol−1 to reflect common practice). Consistency with these air mass classifications is observed in the time series of soluble gas and aerosol composition measurements, with additional identification of periods of slightly elevated dust concentrations consistent with the trajectories passing over the African continent. The CVAO is shown to be broadly representative of the wider North Atlantic marine boundary layer; measurements of NO, O3 and black carbon from the ship are consistent with a clean Northern Hemisphere marine background. Aerosol composition measurements do not indicate elevated organic material associated with clean marine air. Closer to the African coast, black carbon and NO levels start to increase, indicating greater anthropogenic influence. Lower ozone in this region is possibly associated with the increased levels of measured halocarbons, associated with the nutrient rich waters of the Mauritanian upwelling. Bromide and chloride deficits in coarse mode aerosol at both the CVAO and on D319 and the continuous abundance of inorganic gaseous halogen species at CVAO indicate significant reactive cycling of halogens.

Aircraft measurements of O3 and CO show that surface measurements are representative of the entire boundary layer in the vicinity both in diurnal variability and absolute levels. Above the inversion layer similar diurnal behaviour in O3 and CO is observed at lower mixing ratios in the air that had originated from south of Cape Verde, possibly from within the ITCZ. ECMWF calculations on two days indicate very different boundary layer depths and aircraft flights over the ship replicate this, giving confidence in the calculated boundary layer depth.

 

 

Measurement and modelling of tropospheric reactive halogen species over the tropical Atlantic Ocean

A. S. Mahajan, J. M. C. Plane, H. Oetjen, L. Mendes, R. W. Saunders, A. Saiz-Lopez, C. E. Jones, L. J. Carpenter, and G. B. McFiggans

Atmos. Chem. Phys., 10, 4611-4624, 2010

Abstract. Although tropospheric reactive halogen chemistry is well studied in coastal and polar environments, the presence of halogens over the open ocean environment has not been widely reported. The impacts of halogens on the tropical open ocean marine boundary layer (MBL), in particular, are not well characterised. This paper describes observations of iodine monoxide (IO) and bromine oxide (BrO) over eight months in the tropical open ocean MBL, on the north-eastern side of São Vicente (Cape Verde Islands, 16.85° N, 24.87° W). The highest BrO mixing ratio observed was 5.6±1 pmol mol−1, while the maximum observed IO mixing ratio was 3.1±0.4 pmol mol−1. The average values seen between 09:00–17:00 GMT were ~2.8 pmol mol−1 for BrO and ~1.5 pmol mol−1 for IO; these averages showed little variability over the entire campaign from November 2006 to June 2007. A 1-dimensional chemistry and transport model is used to study the evolution of iodine species and quantify the combined impact of iodine and bromine chemistry on the oxidising capacity of the MBL. It appears that the measured fluxes of iodocarbons are insufficient to account for the observed levels of IO, and that an additional I atom source is required, possibly caused by the deposition of O3 onto the ocean surface in the presence of solar radiation. Modelling results also show that the O3 depletion observed at Cape Verde cannot be explained in the absence of halogen chemistry, which contributes ~45% of the observed O3 depletion at the height of measurements (10 m) during summer. The model also predicts that halogens decrease the hydroperoxy radical (HO2) concentration by ~14% and increase the hydroxyl radical (OH) concentration by ~13% near the ocean surface. The oxidation of dimethyl sulphide (DMS) by BrO takes place at a comparable rate to oxidation by OH in this environment. Finally, the potential of iodine chemistry to form new particles is explored and conditions under which particle formation could be important in the remote MBL are discussed.

 

 

Coupling of HOx, NOx and halogen chemistry in the antarctic boundary layer

W. J. Bloss, M. Camredon, J. D. Lee, D. E. Heard, J. M. C. Plane, A. Saiz-Lopez, S. J.-B. Bauguitte, R. A. Salmon, and A. E. Jones

Atmos. Chem. Phys., 10, 10187-10209, 2010

Abstract. A modelling study of radical chemistry in the coastal Antarctic boundary layer, based upon observations performed in the course of the CHABLIS (Chemistry of the Antarctic Boundary Layer and the Interface with Snow) campaign at Halley Research Station in coastal Antarctica during the austral summer 2004/2005, is described: a detailed zero-dimensional photochemical box model was used, employing inorganic and organic reaction schemes drawn from the Master Chemical Mechanism, with additional halogen (iodine and bromine) reactions added. The model was constrained to observations of long-lived chemical species, measured photolysis frequencies and meteorological parameters, and the simulated levels of HOx, NOx and XO compared with those observed. The model was able to replicate the mean levels and diurnal variation in the halogen oxides IO and BrO, and to reproduce NOx levels and speciation very well. The NOx source term implemented compared well with that directly measured in the course of the CHABLIS experiments. The model systematically overestimated OH and HO2 levels, likely a consequence of the combined effects of (a) estimated physical parameters and (b) uncertainties within the halogen, particularly iodine, chemical scheme. The principal sources of HOx radicals were the photolysis and bromine-initiated oxidation of HCHO, together with O(1D) + H2O. The main sinks for HOx were peroxy radical self- and cross-reactions, with the sum of all halogen-mediated HOx loss processes accounting for 40% of the total sink. Reactions with the halogen monoxides dominated CH3O2-HO2-OH interconversion, with associated local chemical ozone destruction in place of the ozone production which is associated with radical cycling driven by the analogous NO reactions. The analysis highlights the need for observations of physical parameters such as aerosol surface area and boundary layer structure to constrain such calculations, and the dependence of simulated radical levels and ozone loss rates upon a number of uncertain kinetic and photochemical parameters for iodine species.

 

 

Reactive iodine species in semi-polluted environments

Mahajan, A. S., Oetjen, H., Saiz-Lopez, A., Lee, J. D., McFiggans, B. G., and Plane, J. M. C.

Geosphys. Res. Letters., 36, L16803, doi:10.1029-2009GL038018, 2009

Abstract. Iodine chemistry in the marine boundary layer has attracted increasing attention over the last few years because iodine oxides cause ozone destruction, change the atmospheric oxidising capacity, and can form ultrafine particles. However, the chemistry of iodine in polluted environments is not well understood: its effects are assumed to be inhibited by reactions involving NOx (NO2 & NO). This paper describes Differential Optical Absorption Spectroscopy (DOAS) observations of iodine species (I2, OIO and IO) and the nitrate radical (NO3) at a semi-polluted coastal site in Roscoff, France. Significant concentrations of IO and I2 were measured during daytime, indicating efficient recycling of iodine from iodine nitrate (IONO2). I2, IO, OIO and NO3 were observed at night. These observations are interpreted using a one dimensional model to demonstrate that iodine plays an important role even in polluted environments due to recycling mechanisms not considered previously.

 

 

Iodine in the air: Origin, Transformation and Exchange to Mammals

Saunders, R. W., and A. Saiz-Lopez

In: Comprehensive Handbook of Iodine. V. R. Preedy, G. Burrow, and R. Watson (eds), Oxford: Academic Press, 2009, pp. 76-82

 

Influence of thin liquid films on polar ice chemistry: Implications for Earth and planetary science

Boxe, C. S., and Saiz-Lopez, A.

Polar Science, 3, 73-81, 2009

Abstract. The polar snowpack (and sea-ice) plays a major role in affecting overlying boundary layer chemistry and has only recently come to light. Furthermore, the understanding of this system and its importance is steadily growing. Investigations done thus far, nonetheless, examined the subsets of the polar environment as an uncoupled system. Analogous to some materials, the surface of snow/ice exhibits thin liquid layers (e.g., the quasi-liquid layer (QLL) and brine layer (BL)). This paper gives an overview of thin liquid films and illustrations of their function in Earth science. The impact of such films in polar science (i.e., polar snowpack photochemistry) is discussed within the context of how field data has been elucidated through laboratory data and modeling techniques. Specifically, what laboratory and modeling investigations have revealed about the effect of thin liquid layers on constraining field observations and, more importantly, the physicochemical mechanisms that govern the behavior of trace gases within the snowpack (and sea-ice) and how they are released from the polar snowpack. Current and future impacts of these findings are discussed, along with putative implications of the effect of thin liquid films in planetary science.

 

 

High bromine oxide concentrations in the semi-polluted boundary layer

Mahajan, A. S., Oetjen, H., Lee, J. D., Saiz-Lopez, A., McFiggans, B. G., and Plane, J. M. C.

Atmos. Environ., 43, 3811-3818, 2009

Abstract. Bromine chemistry in the marine boundary layer is recognized to play an important role through catalytic ozone destruction, changes to the atmospheric oxidising capacity (by changing the OH/HO2 and NO/NO2 ratio) and oxidation of compounds such as dimethyl sulphide (DMS). However, the chemistry of bromine in polluted environments is not well understood and its effects are thought to be inhibited by reactions involving NOx (NO2 & NO). This paper describes long-path Differential Optical Absorption Spectroscopy (DOAS) observations of bromine oxide (BrO) at a semi-polluted coastal site in Roscoff, France. Significant concentrations of BrO (up to 7.5 ± 1.0 pptv) were measured during daytime, indicating the presence of unknown sources or efficient recycling of reactive bromine from bromine nitrate (BrONO2), which should be the major reservoir for bromine in a high NOx environment. These measurements indicate that bromine chemistry can play an important role in polluted environments.

 

 

Year- round observations of NO, NO2, O3, SO2 and toluene measured with a DOAS system in Central Spain (Puertollano)

Saiz-Lopez, A., Notario, A., Albaladejo, J., Poblete, J., Adame, J. A., and Bolivar, J. P.

Water Air and Soil Pollution, DOI 10.1007/s11270-008-9912-8, 2008

Abstract. We report observations of primary and secondary atmospheric pollutants such as nitrogen oxides, sulfur dioxide, toluene, and ozone during the period February 2002 to August 2003 in Puertollano, an industrial area located in central–southern Spain. The measurements were performed using a commercial differential optical absorption spectroscopy instrument. From the hourly data, we have analyzed the mean seasonal levels and the daily evolution and we have examined the occurrence of elevated pollution episodes. The daily cycles of NO, NO2, SO2, and toluene were characterized by an early-morning maximum whereas O3 peaks were monitored around noon. Seasonally, the highest hourly mean concentrations of NO, NO2, SO2, and toluene, 14.2, 27.0, 34.4, and 12.1 μg m−3 respectively, were found in the winter while O3 summer levels reached 119.1 μg m−3. The dataset presented here shows episodic occurrences of elevated concentrations that exceeded the maximum levels established in the European Directives. For instance, hourly values for SO2 were repeatedly measured above 350 μg m−3. During the period of measurements, the O3 thresholds (i.e., hourly value of 240 μg m−3) defined to protect the human health have also been exceeded numerous times. Finally, we investigate daily and seasonal patterns in pollution levels within the context of local meteorology and photochemistry, vehicular traffic, and industrial emissions.

 

 

Multiphase modelling of nitrate photochemistry in the quasi- liquid layer (QLL): implications for NOx release from the Arctic and coastal Antarctic snowpack

Boxe, C. S. and A. Saiz-Lopez

Atmospheric Chemistry and Physics. 8, 4855-4864, 2008

Abstract. We utilize a multiphase model, CON-AIR (Condensed Phase to Air Transfer Model), to show that the photochemistry of nitrate (NO3) in and on ice and snow surfaces, specifically the quasi-liquid layer (QLL), can account for NOx volume fluxes, concentrations, and [NO]/[NO2] (γ=[NO]/[NO2]) measured just above the Arctic and coastal Antarctic snowpack. Maximum gas phase NOx volume fluxes, concentrations and γ simulated for spring and summer range from 5.0×104 to 6.4×105 molecules cm−3 s−1, 5.7×108 to 4.8×109 molecules cm−3, and ~0.8 to 2.2, respectively, which are comparable to gas phase NOx volume fluxes, concentrations and γ measured in the field. The model incorporates the appropriate actinic solar spectrum, thereby properly weighting the different rates of photolysis of NO3 and NO2. This is important since the immediate precursor for NO, for example, NO2, absorbs at wavelengths longer than nitrate itself. Finally, one-dimensional model simulations indicate that both gas phase boundary layer NO and NO2 exhibit a negative concentration gradient as a function of height although [NO]/[NO2] are approximately constant. This gradient is primarily attributed to gas phase reactions of NOx with halogens oxides (i.e. as BrO and IO), HOx, and hydrocarbons, such as CH3O2.

 

 

On the vertical distribution of boundary layer halogens over coastal Antarctica: implications for O3, HOx, NOx and the Hg lifetime

Saiz-Lopez, A., J. M. C. Plane, A. S. Mahajan, P. S. Anderson, S. J.-B. Bauguitte, A. E. Jones, H. K. Roscoe, R. A. Salmon, W. J. Bloss, J. D. Lee and D. E. Heard

Atmospheric Chemistry and Physics, 8, 887-900, 2008

Abstract. A one-dimensional chemical transport model has been developed to investigate the vertical gradients of bromine and iodine compounds in the Antarctic coastal boundary layer (BL). The model has been applied to interpret recent year-round observations of iodine and bromine monoxides (IO and BrO) at Halley Station, Antarctica. The model requires an equivalent I atom flux of ~1010 molecule cm−2 s−1 from the snowpack in order to account for the measured IO levels, which are up to 20 ppt during spring. Using the current knowledge of gas-phase iodine chemistry, the model predicts significant gradients in the vertical distribution of iodine species. However, recent ground-based and satellite observations of IO imply that the radical is well-mixed in the Antarctic boundary layer, indicating a longer than expected atmospheric lifetime for the radical. This can be modelled by including photolysis of the higher iodine oxides (I2O2, I2O3, I2O4 and I2O5), and rapid recycling of HOI and INO3 through sea-salt aerosol. The model also predicts significant concentrations (up to 25 ppt) of I2O5 in the lowest 10 m of the boundary layer. Heterogeneous chemistry involving sea-salt aerosol is also necessary to account for the vertical profile of BrO. Iodine chemistry causes a large increase (typically more than 3-fold) in the rate of O3 depletion in the BL, compared with bromine chemistry alone. Rapid entrainment of O3 from the free troposphere appears to be required to account for the observation that on occasion there is little O3 depletion at the surface in the presence of high concentrations of IO and BrO. The halogens also cause significant changes to the vertical profiles of OH and HO2 and the NO2/NO ratio. The average Hg0 lifetime against oxidation is also predicted to be about 10 h during springtime. An important result from the model is that very large fluxes of iodine precursors into the boundary layer are required to account for the observed levels of IO. The mechanisms which cause these emissions are unknown. Overall, our results show that halogens profoundly influence the oxidizing capacity of the Antarctic troposphere.

 

 

DMS and MSA measurements in the Antarctic boundary layer: impact of BrO on MSA production

Read, K. A., A. C. Lewis, S. Bauguitte, A. M. Rankin, R. A. Salmon, E. W. Wolff, A. Saiz- Lopez, W. J. Bloss, D. E. Heard, J. D. Lee, and J. M. C. Plane

Atmospheric Chemistry and Physics, 8, 2985-2997, 2008

Abstract. In situ measurements of dimethyl sulphide (DMS) and methane sulphonic acid (MSA) were made at Halley Station, Antarctica (75°35' S, 26°19' W) during February 2004–February 2005 as part of the CHABLIS (Chemistry of the Antarctic Boundary Layer and the Interface with Snow) project. DMS was present in the atmosphere at Halley all year (average 38.1±43 pptV) with a maximum monthly average value of 113.6±52 pptV in February 2004 coinciding temporally with a minimum in sea extent. Whilst seasonal variability and interannual variability can be attributed to a number of factors, short term variability appeared strongly dependent on air mass origin and trajectory pressure height. The MSA and derived non-sea salt sulphate (nss-SO42−) measurements showed no correlation with those of DMS (regression R2=0.039, and R2=0.001 respectively) in-line with the complexity of DMS fluxes, alternative oxidation routes, transport of air masses and variable spatial coverage of both sea-ice and phytoplankton. MSA was generally low throughout the year, with an annual average of 42 ng m−3 (9.8±13.2 pptV), however MSA: nss-SO42− ratios were high implying a dominance of the addition oxidation route for DMS. Including BrO measurements into MSA production calculations demonstrated the significance of BrO on DMS oxidation within this region of the atmosphere in austral summer. Assuming an 80% yield of DMSO from the reaction of DMS+BrO, an atmospheric concentration of BrO equal to 3 pptV increased the calculated MSA production from DMS by a factor of 9 above that obtained when considering only reaction with the hydroxyl radical. These findings have significant atmospheric implications, but may also impact on the interpretation of ice cores which previously relied on the understanding of MSA and nss-SO42− chemistry to provide information on environmental conditions such as sea ice extent and the origins of sulphur within the ice.

 

 

Chemistry of the Antarctic boundary layer and interface with snow: An overview of the CHABLIS campaign

A. E. Jones, E. W. Wolff, R. A. Salmon, S. J.-B. Bauguitte, H. K. Roscoe, P. S. Anderson, D. Ames, K. C. Clemitshaw, Z. L. Fleming, W. J. Bloss, D. E. Heard, J. D. Lee, K. A. Read, P. Hamer, D. E. Shallcross, A. V. Jackson, S. L. Walker, A. C. Lewis, G. P. Mills, J. M. C. Plane, A. Saiz-Lopez, W. T. Sturges, and D. R. Worton

Atmos. Chem. Phys., 8, 3789-3803, 2008

Abstract. CHABLIS (Chemistry of the Antarctic Boundary Layer and the Interface with Snow) was a collaborative UK research project aimed at probing the detailed chemistry of the Antarctic boundary layer and the exchange of trace gases at the snow surface. The centre-piece to CHABLIS was the measurement campaign, conducted at the British Antarctic Survey station, Halley, in coastal Antarctica, from January 2004 through to February 2005. The campaign measurements covered an extremely wide range of species allowing investigations to be carried out within the broad context of boundary layer chemistry. Here we present an overview of the CHABLIS campaign. We provide details of the measurement location and introduce the Clean Air Sector Laboratory (CASLab) where the majority of the instruments were housed. We describe the meteorological conditions experienced during the campaign and present supporting chemical data, both of which provide a context within which to view the campaign results. Finally we provide a brief summary of highlights from the measurement campaign. Unexpectedly high halogen concentrations profoundly affect the chemistry of many species at Halley throughout the sunlit months, with a secondary role played by emissions from the snowpack. This overarching role for halogens in coastal Antarctic boundary layer chemistry was completely unanticipated, and the results have led to a step-change in our thinking and understanding.

 

 

A mechanism for biologically-induced iodine emisiones from sea-ice

Saiz-Lopez, A. and C. S. Boxe

Atmospheric Chemistry and Physics Discussions. 8, 2953-2976, 2008

Abstract. Only recently, ground- and satellite-based measurements have reported high concentrations of IO in coastal Antarctica. The sources of such a large iodine burden in the Antarctic atmosphere remain unknown. We propose a novel mechanism for iodine release from sea-ice surfaces. The release is triggered by the biological production of iodide (I-) and hypoiodous acid (HOI) from marine algae, contained within and underneath sea-ice, and their diffusion through sea-ice brine channels to accumulate in the quasi-liquid layer on the surface of sea-ice. A multiphase chemical model of polar atmospheric chemistry has been developed to investigate the biology-ice-atmosphere coupling in the polar environment. Model simulations were conducted to interpret recent observations of elevated IO in the coastal Antarctic springtime. The results show that the levels of inorganic iodine (i.e. I2, IBr, ICl) released from sea-ice through this mechanism account for the observed IO concentrations in the Antarctic springtime environment. The model results also indicate that iodine may trigger the catalytic release of bromine from sea-ice through phase equilibration of IBr. Considering the extent of sea-ice around the Antarctic continent, we suggest that the resulting high levels of iodine may have widespread impact on catalytic ozone destruction and aerosol formation in the Antarctic lower troposphere.


 

Extensive halogen-mediated ozone destruction over the sub-tropical Atlantic Ocean

Read, K. A., A. A. Mahajan, L. C. Carpenter, M. J. Evans, B. V. Faria, D. E. Heard, J. R. Hopkins, J. D. Lee, S. Moller, A. C. Lewis, L. Mendes, J. B. McQuaid, H. Oetjen, A. Saiz- Lopez, M. J. Pilling and J. M. C. Plane

Nature, 453, 1232-1235, 2008

Abstract. Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation; the prediction of future trends in tropospheric ozone will require a full understanding of both its precursor emissions and its destruction processes. A large proportion of tropospheric ozone loss occurs in the tropical marine boundary layer and is thought to be driven primarily by high ozone photolysis rates in the presence of high concentrations of water vapour. A further reduction in the tropospheric ozone burden through bromine and iodine emitted from open-ocean marine sources has been postulated by numerical models, but thus far has not been verified by observations. Here we report eight months of spectroscopic measurements at the Cape Verde Observatory indicative of the ubiquitous daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. A year-round data set of co-located in situ surface trace gas measurements made in conjunction with low-level aircraft observations shows that the mean daily observed ozone loss is approx50 per cent greater than that simulated by a global chemistry model using a classical photochemistry scheme that excludes halogen chemistry. We perform box model calculations that indicate that the observed halogen concentrations induce the extra ozone loss required for the models to match observations. Our results show that halogen chemistry has a significant and extensive influence on photochemical ozone loss in the tropical Atlantic Ocean boundary layer. The omission of halogen sources and their chemistry in atmospheric models may lead to significant errors in calculations of global ozone budgets, tropospheric oxidizing capacity and methane oxidation rates, both historically and in the future.

 

 

Variability of the mesospheric nightglow during the 2002 Leonid storms

Plane, J. M. C., A. Saiz-Lopez, B. J. Allan, S. H. Ashworth and P. Jenniskens

Advances in Space Research, 39, 562-566, 2007

Abstract. There was no significant increase in the intensities of three prominent components of the terrestrial nightglow during the 2002 Leonid storm peaks. The atomic oxygen line at 557.7 nm, the sodium D lines at 589.0 and 589.6 nm, and the OH(6,2) band at 826–862 nm were monitored using an airborne spectrometer over the North Atlantic (40–50°N). The results indicate that the meteor storm produced a negligible change in both atomic sodium and oxygen compared to the background concentrations. The spectrometer resolved the sodium doublet, and showed that the ratio of the D2 and D1 lines is not 2.0, as had been thought hitherto, but is highly variable on distances of a few tens of kilometers. The mean value is about 1.8, with values ranging from 1.3 to 2.4.

 

 

Seasonal variation of NOx loss processes coupled to the HNO3 formation in a daytime urban atmosphere: a model study

Saiz-Lopez, A., Notario, A., Albaladejo, J. and McFiggans, G.

Water Air and Soil Pollution, 182, 197- 206, 2007

Abstract. We studied the seasonal cycle of the coupling between atmospheric denoxification processes and in-situ daytime formation of gas phase HNO3 using a photochemical air pollution model. The model is constrained with urban atmospheric boundary layer observations of O3, NO2 and NO made in Ciudad Real, central Spain. The highest daytime HNO3 mixing ratio of 0.3 ppbv was predicted to occur in summer, following a modelled OH concentration peak of ∼1.4 × 106 molecules cm−3 and subsequent reaction with NO2. During winter, calculated values of HNO3 are lower due to less incoming radiation and higher wet removal of atmospheric HNO3. The predicted mixing ratios are in good agreement with observations of atmospheric HNO3 at similar urban environments in central Spain. Additionally, a marked seasonal cycle is predicted with minimum HNO3 concentrations occurring in winter, indicative that traffic emissions and photochemistry dominate the in-situ formation of gas phase HNO3 at this location. This process has implications in the removal of NOx from the urban atmosphere.

 

 

Night-time radical chemistry during the NAMBLEX campaign

Sommariva, R., S. M. Ball, M. Bitter, W. J. Bloss, Z. L. Fleming, D. E. Heard, R. L. Jones, J. D. Lee, P. S. Monks, M. J. Pilling, J. M. C. Plane and A. Saiz-Lopez

Atmospheric Chemistry and Physics, 7, 587-598, 2007

Abstract. Night-time chemistry in the Marine Boundary Layer has been modelled using a number of observationally constrained zero-dimensional box-models. The models were based upon the Master Chemical Mechanism (MCM) and the measurements were taken during the North Atlantic Marine Boundary Layer Experiment (NAMBLEX) campaign at Mace Head, Ireland in July–September 2002.

The model could reproduce, within the combined uncertainties, the measured concentration of HO2 (within 30–40%) during the night 31 August–1 September and of HO2+RO2 (within 15–30%) during several nights of the campaign. The model always overestimated the NO3 measurements made by Differential Optical Absorption Spectroscopy (DOAS) by up to an order of magnitude or more, but agreed with the NO3 Cavity Ring-Down Spectroscopy (CRDS) measurements to within 30–50%. The most likely explanation of the discrepancy between the two instruments and the model is the reaction of the nitrate radical with inhomogeneously distributed NO, which was measured at concentrations of up to 10 ppt, even though this is not enough to fully explain the difference between the DOAS measurements and the model.

A rate of production and destruction analysis showed that radicals were generated during the night mainly by the reaction of ozone with light alkenes. The cycling between HO2/RO2 and OH was maintained during the night by the low concentrations of NO and the overall radical concentration was limited by slow loss of peroxy radicals to form peroxides. A strong peak in [NO2] during the night 31 August–1 September allowed an insight into the radical fluxes and the connections between the HOx and the NO3 cycles.

 

 

First observations of iodine oxide from space

Saiz-Lopez, A., K. Chance, X. Liu, T. P. Kurosu and S. P. Sander

Geophysical Research Letters, 34, L12812, doi:10.1029/2007GL030111

Abstract. We present retrievals of IO total columns from the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) satellite instrument. We analyze data for October 2005 in the polar regions to demonstrate for the first time the capability to measure IO column abundances from space. During the period of analysis (i.e. Southern Hemisphere springtime), enhanced IO vertical columns over 3 × 1013 molecules cm−2 are observed around coastal Antarctica; by contrast during that time in the Artic region IO is consistently below the calculated instrumental detection limit for individual radiance spectra (2–4 × 1012 molecules cm−2 for slant columns). The levels reported here are in reasonably good agreement with previous ground-based measurements at coastal Antarctica. These results also demonstrate that IO is widespread over sea-ice covered areas in the Southern Ocean. The occurrence of elevated IO and its hitherto unrecognized spatial distribution suggest an efficient iodine activation mechanism at a synoptic scale over coastal Antarctica.

 

 

Boundary layer halogens in coastal Antarctica

Saiz-Lopez, A., A. S. Mahajan, R. A, Salmon, S. Bauguitte, A. E. Jones, H. K. Roscoe, and J. M. C. Plane

Science, 317, 348-351, 2007

Abstract. Halogens influence the oxidizing capacity of Earth's troposphere, and iodine oxides form ultrafine aerosols, which may have an impact on climate. We report year-round measurements of boundary layer iodine oxide and bromine oxide at the near-coastal site of Halley Station, Antarctica. Surprisingly, both species are present throughout the sunlit period and exhibit similar seasonal cycles and concentrations. The springtime peak of iodine oxide (20 parts per trillion) is the highest concentration recorded anywhere in the atmosphere. These levels of halogens cause substantial ozone depletion, as well as the rapid oxidation of dimethyl sulfide and mercury in the Antarctic boundary layer.

 

 

UV-visible Differential Optical Absorption Spectroscopy (DOAS)

J.M.C. Plane and A. Saiz-Lopez

In: D.E. Heard (ed). Analytical Techniques for Atmospheric Measurement. Blackwell Publishing: Oxford, 2006

 

The North Atlantic Marine Boundary Layer Experiment (NAMBLEX): Overview of the campaign held at Mace Head, Ireland, in summer 2002

D. E. Heard, K. A. Read, J. Methven, S. Al-Haider, W. J. Bloss, G. P. Johnson, M. J. Pilling, P. W. Seakins, S. C. Smith, R. Sommariva, J. C. Stanton, T. J. Still, T. Ingham, B. Brooks, G. De Leeuw, A. V. Jackson, J. B. McQuaid, R. Morgan, M. H. Smith, L. J. Carpenter, N. Carslaw, J. Hamilton, J. R. Hopkins, J. D. Lee, A. C. Lewis, R. M. Purvis, D. J. Wevill, N. Brough, T. Green, G. Mills, S. A. Penkett, J. M. C. Plane, A. Saiz-Lopez, D. Worton, P. S. Monks, Z. Fleming, A. R. Rickard, M. R. Alfarra, J. D. Allan, K. Bower, H. Coe, M. Cubison, M. Flynn, G. McFiggans, M. Gallagher, E. G. Norton, C. D. O'Dowd, J. Shillito, D. Topping, G. Vaughan, P. Williams, M. Bitter, S. M. Ball, R. L. Jones, I. M. Povey, S. O'Doherty, P. G. Simmonds, A. Allen, R. P. Kinnersley, D. C. S. Beddows, M. Dall'Osto, R. M. Harrison, R. J. Donovan, M. R. Heal, S. G. Jennings, C. Noone, and G. Spain

Atmos. Chem. Phys., 6, 2241-2272, 2006

Abstract. The North Atlantic Marine Boundary Layer Experiment (NAMBLEX), involving over 50 scientists from 12 institutions, took place at Mace Head, Ireland (53.32° N, 9.90° W), between 23 July and 4 September 2002. A wide range of state-of-the-art instrumentation enabled detailed measurements of the boundary layer structure and atmospheric composition in the gas and aerosol phase to be made, providing one of the most comprehensive in situ studies of the marine boundary layer to date. This overview paper describes the aims of the NAMBLEX project in the context of previous field campaigns in the Marine Boundary Layer (MBL), the overall layout of the site, a summary of the instrumentation deployed, the temporal coverage of the measurement data, and the numerical models used to interpret the field data. Measurements of some trace species were made for the first time during the campaign, which was characterised by predominantly clean air of marine origin, but more polluted air with higher levels of NOx originating from continental regions was also experienced. This paper provides a summary of the meteorological measurements and Planetary Boundary Layer (PBL) structure measurements, presents time series of some of the longer-lived trace species (O3, CO, H2, DMS, CH4, NMHC, NOx, NOy, PAN) and summarises measurements of other species that are described in more detail in other papers within this special issue, namely oxygenated VOCs, HCHO, peroxides, organo-halogenated species, a range of shorter lived halogen species (I2, OIO, IO, BrO), NO3 radicals, photolysis frequencies, the free radicals OH, HO2 and (HO2+Σ RO2), as well as a summary of the aerosol measurements. NAMBLEX was supported by measurements made in the vicinity of Mace Head using the NERC Dornier-228 aircraft. Using ECMWF wind-fields, calculations were made of the air-mass trajectories arriving at Mace Head during NAMBLEX, and were analysed together with both meteorological and trace-gas measurements. In this paper a chemical climatology for the duration of the campaign is presented to interpret the distribution of air-mass origins and emission sources, and to provide a convenient framework of air-mass classification that is used by other papers in this issue for the interpretation of observed variability in levels of trace gases and aerosols.

 

 

The mass balance of mercury in the springtime polar environment

Brooks, S. B., A. Saiz-Lopez, H. Skov, S. E. Lindberg, J. M. C. Plane and M. E. Goodsite

Geophysical Research Letters, 33, L13812, doi:10.1029/2005GL025525, 2006

Abstract. The load of mercury in the Arctic environment is to a large extent controlled by atmospheric mercury depletion events. At Barrow, Alaska, these depletion events have been linked with the near-surface air formation of reactive gaseous mercury (Hg(II)) (RGM), to a much lesser extent in a particulate-bound form, and the accumulation of total mercury in the snow pack (>100 ng/L in late Spring). This transport of Hg from atmospheric conversion, to deposition, to bio-available forms is likely to be the predominant pathway for mercury into Arctic biota. For the first time we combine flux rate measurements, atmospheric chemistry measurements, and air mass trajectories to give a comprehensive two-week window into the springtime dynamics and mass balance of Arctic mercury. We have conducted polar-sunrise to snowmelt mercury monitoring at Barrow from 1998 to 2004, and the time period March 25th–April 7th (Julian days 84–97), 2003 appears typical for this time of year. A clear link was observed between air of marine origin, the build-up of BrO together with removal of gaseous elemental mercury (GEM)), and the formation of RGM. This provides the most direct evidence so far for Br and Hg chemistry as the direct source of RGM. The fluxes of RGM and GEM were determined and the net flux calculated.

 

 

Seasonal evolution of levels of gaseous pollutants in an urban area (Ciudad Real) in central-southern Spain: a DOAS study

Saiz-Lopez, A., A. Notario, E. Martinez and J. Albaladejo

Water Air and Soil Pollution, 171, 153-167, 2006

 

Abstract. Long term continuous monitoring measurements of urban atmospheric concentrations of O3, NO2, NO, and SO2 were performed for the first time in Ciudad Real, a city in central-southern Spain. The measurements were carried out using the differential optical absorption spectroscopy (DOAS) technique, with a commercial system (OPSIS, Lund-Sweden), covering the summer and winter seasons (from 21st July 2000 to 23rd March 2001). Mean levels of O3, NO2 and SO2 monitored during this period were: 27 μg m−3, 50 μg m−3 and 7 μg m−3 respectively. The highest hourly averaged value of O3 (160 μg m−3) was measured during the summer period, while NO2 was enhanced in wintertime (highest values 90 μg m−3). In the coldest period, when central heating installations were operating, SO2 showed maximum levels of 20 μg m−3. The daily, weekly and seasonal analysis of the data shows that photochemical air pollution dominates in this urban atmosphere and is strongly influenced by levels of motor traffic and domestic heating system emissions. These measurements were compared with other studies in Spain and Europe. Also, the long path averaged DOAS measurements were compared with in situ observations made in Ciudad Real, from 23rd August 2000 to 25th September 2000, using a mobile air pollution control station. All gas concentrations reported in this paper are below the WHO guidelines and the different thresholds introduced by the European Environmental Legislation.

 

 

Peroxy radical chemistry and the control of ozone photochemistry at Mace Head, Ireland during the summer of 2002

Fleming, Z. L, P. S. Monks, A. R. Rickard, D. E. Heard, W. J. Bloss, P. I. Seakins, T. J. Still, R. Sommariva, M. J. Pilling, R. Morgan, T. J. Green, N. Brough, S. A. Penkett, A. C. Lewis, J. D. Lee, A. Saiz-Lopez and J. M. C. Plane

Atmos. Chem. Phys., 6, 2193-2214, 2006

Abstract. Peroxy radical (HO2+ΣRO2) measurements, using the PEroxy Radical Chemical Amplification (PERCA) technique at the North Atlantic Marine Boundary Layer EXperiment (NAMBLEX) at Mace Head in summer 2002, are presented and put into the context of marine, boundary-layer chemistry. A suite of other chemical parameters (NO, NO2, NO3, CO, CH4, O3, VOCs, peroxides), photolysis frequencies and meteorological measurements, are used to present a detailed analysis of the role of peroxy radicals in tropospheric oxidation cycles and ozone formation. Under the range of conditions encountered the peroxy radical daily maxima varied from 10 to 40 pptv. The diurnal cycles showed an asymmetric shape typically shifted to the afternoon. Using a box model based on the master chemical mechanism the average model measurement agreement was 2.5 across the campaign. The addition of halogen oxides to the model increases the level of model/measurement agreement, apparently by respeciation of HOx. A good correlation exists between j(HCHO).[HCHO] and the peroxy radicals indicative of the importance of HCHO in the remote atmosphere as a HOx source, particularly in the afternoon. The peroxy radicals showed a strong dependence on [NO2] with a break point at 0.1 ppbv, where the radicals increased concomitantly with the reactive VOC loading, this is a lower value than seen at representative urban campaigns. The HO2/(HO2+ΣRO2) ratios are dependent on [NOx] ranging between 0.2 and 0.6, with the ratio increasing linearly with NOx. Significant night-time levels of peroxy radicals were measured up to 25 pptv. The contribution of ozone-alkenes and NO3-alkene chemistry to night-time peroxy radical production was shown to be on average 59 and 41%. The campaign mean net ozone production rate was 0.11±0.3 ppbv h-1. The ozone production rate was strongly dependent on [NO] having linear sensitivity (dln(P(O3))/dln(NO)=1.0). The results imply that the N(O3) (the in-situ net photochemical rate of ozone production/destruction) will be strongly sensitive in the marine boundary layer to small changes in [NO] which has ramifications for changing NOx loadings in the European continental boundary layer.

 

 

Modelling the molecular iodine emissions at coastal marine environments: the link to new particle formation

Saiz-Lopez, A., J. M.C. Plane, G. McFiggans, P. I. Williams, S. M. Ball, M. Bitter, R. L. Jones, C. Hongwei and T. Hoffmann

Atmospheric Chemistry and Physics, 6, 883-895, 2006

Abstract. A model of iodine chemistry in the marine boundary layer (MBL) has been used to investigate the impact of daytime coastal emissions of molecular iodine (I2). The model contains a full treatment of gas-phase iodine chemistry, combined with a description of the nucleation and growth, by condensation and coagulation, of iodine oxide nano-particles. In-situ measurements of coastal emissions of I2 made by the broadband cavity ring-down spectroscopy (BBCRDS) and inductively coupled plasma-mass spectrometry (ICP/MS) techniques are presented and compared to long path differential optical absorption spectroscopy (DOAS) observations of I2 at Mace Head, Ireland. Simultaneous measurements of enhanced I2 emissions and particle bursts show that I2 is almost certainly the main precursor of new particles at this coastal location. The ratio of IO to I2 predicted by the model indicates that the iodine species observed by the DOAS are concentrated over a short distance (about 8% of the 4.2 km light path) consistent with the intertidal zone, bringing them into good agreement with the I2 measurements made by the two in-situ techniques. The model is then used to investigate the effect of iodine emission on ozone depletion, and the production of new particles and their evolution to form stable cloud condensation nuclei (CCN).

 

 

Measurements and modelling of I2, IO, OIO, BrO and NO3 in the mid-latitude marine boundary layer

Saiz-Lopez, A., J. A. Shillito, H. Coe and J. M. C. Plane

Atmospheric Chemistry and Physics, 6, 1513-1528, 2006

Abstract. Time series observations of molecular iodine (I2), iodine oxides (IO, OIO), bromine oxide (BrO), and the nitrate radical (NO3) in the mid-latitude coastal marine boundary layer (MBL) are reported. Measurements were made using a new long-path DOAS instrument during a summertime campaign at Mace Head on the B3Π(0+u)-X1Σ+g electronic transition between 535 and 575 nm. The I2 mixing ratio was found to vary from below the detection limit (~5 ppt) up to a nighttime maximum of 93 ppt. Along with I2, observations of IO, OIO and NO3 were also made during the night. Surprisingly, IO and OIO were detected at mixing ratios up to 2.5 and 10.8 ppt, respectively. A model is employed to show that the reaction between I2 and NO3 is the likely nighttime source of these radicals. The BrO mixing ratio varied from below the detection limit at night (~1 ppt) to a maximum of 6 ppt in the first hours after sunrise. A bromine chemistry model is used to simulate the diurnal behaviour of the BrO radical, demonstrating the importance of halogen recycling through sea-salt aerosol. In the same campaign a zenith sky DOAS was employed to determine the column density variation of NO3 as a function of solar zenith angle (SZA) during sunrise, from which vertical profiles of NO3 through the troposphere were obtained. On several occasions a positive gradient of NO3 was observed over the first 2 km, possibly due to dimethyl sulphide (DMS) removing NO3 at the ocean surface.

 

 

OH and HO2 chemistry during NAMBLEX: roles of oxygenates, halogen oxides and heterogeneous uptake

Sommariva, R., W. J. Bloss, N. Brough, N. Carslaw, M. Flynn, A.-L. Haggerstone, D. E. Heard, J. R. Hopkins, J. D. Lee, A. C. Lewis, G. McFiggans, P. S. Monks, S. A. Penkett, M. J. Pilling, J. M. C. Plane, K. A Read, A. R. Rickard, A. Saiz-Lopez and P. I. Williams

Atmos. Chem. Phys., 6, 1135-1153, 2006

Abstract. Several zero-dimensional box-models with different levels of chemical complexity, based on the Master Chemical Mechanism (MCM), have been used to study the chemistry of OH and HO2 in a coastal environment in the Northern Hemisphere. The models were constrained to and compared with measurements made during the NAMBLEX campaign (Mace Head, Ireland) in summer 2002.

The base models, which were constrained to measured CO, CH4 and NMHCs, were able to reproduce [OH] within 25%, but overestimated [HO2] by about a factor of 2. Agreement was improved when the models were constrained to oxygenated compounds (acetaldehyde, methanol and acetone), highlighting their importance for the radical budget. When the models were constrained to measured halogen monoxides (IO, BrO) and used a more detailed, measurements-based, treatment to describe the heterogeneous uptake, modelled [OH] increased by up to 15% and [HO2] decreased by up to 30%. The actual impact of halogen monoxides on the modelled concentrations of HOx was dependant on the uptake coefficients used for HOI, HOBr and HO2. Better agreement, within the combined uncertainties of the measurements and of the model, was achieved when using high uptake coefficients for HO2 and HOI (γHO2=1, γHOI=0.6).

A rate of production and destruction analysis of the models allowed a detailed study of OH and HO2 chemistry under the conditions encountered during NAMBLEX, showing the importance of oxygenates and of XO (where X=I, Br) as co-reactants for OH and HO2 and of HOX photolysis as a source for OH.

 

 

Impact of halogen monoxide chemistry upon boundary layer OH and HO2 concentrations at a coastal site

W.J. Bloss, J.D. Lee, G.P. Johnson, R. Sommariva, D.E. Heard, A. Saiz-Lopez, J.M.C. Plane, A. Rickard, Z. Fleming,G. McFiggans, H. Coe, M. Flynn and P. Williams

Geophysical Research Letters, 32, L06814, doi: 10.1029/2004GL022084, 2005

Abstract. The impact of iodine oxide chemistry upon OH and HO2 concentrations in the coastal marine boundary layer has been evaluated using data from the NAMBLEX (North Atlantic Marine Boundary Layer Experiment) campaign, conducted at Mace Head, Ireland during the summer of 2002. Observationally constrained calculations show that under low NOx conditions experienced during NAMBLEX (NO ≤ 50 pptv), the reaction IO + HO2 → HOI + O2 accounted for up to 40% of the total HO2 radical sink, and the subsequent photolysis of HOI to form OH + I comprised up to 15% of the total midday OH production rate. The XO + HO2 (X = Br, I) reactions may in part account for model overestimates of measured HO2 concentrations in previous studies at Mace Head, and should be considered in model studies of HOx chemistry at similar coastal locations.

 

 

Variability of the mesospheric nightglow sodium D2/D1 ratio

Slanger, T. G., P. C. Cosby, D. L. Huestis, A. Saiz-Lopez, B. J. Murray, D. A. O'Sullivan, J. M. C. Plane, C. Allende Prieto, F. J. Martin-Torres, and P. Jenniskens

Journal of Geophysical Research,110, D23302, doi:10.1029/2005JD006078. 2005

Abstract. Measurements of the intensity ratio of the 589.0/589.6 nm sodium doublet in the terrestrial nightglow over an 8-year period, involving >300 separate determinations, have established that it is variable, the value RD = I(D2)/I(D1) lying between 1.2 and 1.8. Sky spectra from the Keck I telescope with the High-Resolution Échelle Spectrometer (HIRES) échelle spectrograph and the Keck II telescope with the Échellette Spectrograph and Imager (ESI) échelle spectrograph were used in this analysis. The result contrasts with the accepted view, from earlier measurements at midlatitude, that the ratio is 2.0, as expected on statistical grounds. The lack of dependence of the ratio on viewing elevation angle, and hence Na slant column, allows self-absorption to be ruled out as a cause of the variability. The data suggest a semiannual oscillation in the ratio, maximum at the equinoxes and minimum at the solstices. Airborne measurements over the North Atlantic (40°–50°N) in 2002 show an even larger range in the nightglow ratio and no correlation with the upper mesospheric temperature determined from the OH 6–2 bands. A laboratory study confirms that the ratio does not depend on temperature; however, it is shown to be sensitive to the [O]/[O2] ratio. It is therefore postulated that the variable ratio arises from a competition between O reacting with NaO(A3Σ+), produced from the reaction of Na with O3, to yield D-line emission with a D2/D1 ratio greater than about 2.0, and quenching by O2 to produce NaO(X2Π), possibly with vibrational excitation, which then reacts with O to produce emission with a ratio of less than 1.3.

 

 

Recent applications of Differential Optical Absorption Spectroscopy: halogen chemistry in the lower troposphere

Saiz-Lopez, A. and J.M.C. Plane

J. Phys. IV France 121 (2004) 223-238

Abstract. This chapter provides a comprehensive review of the atmospheric chemistry of halogens in the lower troposphere, including a discussion of the important ways in which halogens affect this region of the atmosphere. It then describes the recent progress made in observing these species by the Differential Optical Absorption Spectroscopy (DOAS) technique. A brief description of the technique and its capabilities is also provided.

 

 

Direct evidence for coastal iodine particles from Laminaria macroalgae – linkage to emissions of molecular iodine

G. McFiggans, H. Coe, R. Burgess, J. Allan, M. Cubison, M. R. Alfarra, R. Saunders, A. Saiz-Lopez, J. M. C. Plane, D. Wevill, L. Carpenter, A. R. Rickard, and P. S. Monks

Atmos. Chem. Phys., 4, 701-713, 2004

Abstract. Renewal of ultrafine aerosols in the marine boundary layer may lead to repopulation of the marine distribution and ultimately determine the concentration of cloud condensation nuclei (CCN). Thus the formation of nanometre-scale particles can lead to enhanced scattering of incoming radiation and a net cooling of the atmosphere. The recent demonstration of the chamber formation of new particles from the photolytic production of condensable iodine-containing compounds from diiodomethane (CH2I2), (O'Dowd et al., 2002; Kolb, 2002; Jimenez et al., 2003a; Burkholder and Ravishankara, 2003), provides an additional mechanism to the gas-to-particle conversion of sulphuric acid formed in the photo-oxidation of dimethylsulphide for marine aerosol repopulation. CH2I2 is emitted from seaweeds (Carpenter et al., 1999, 2000) and has been suggested as an initiator of particle formation. We demonstrate here for the first time that ultrafine iodine-containing particles are produced by intertidal macroalgae exposed to ambient levels of ozone. The particle composition is very similar both to those formed in the chamber photo-oxidation of diiodomethane and in the oxidation of molecular iodine by ozone. The particles formed in all three systems are similarly aspherical. When small, those formed in the molecular iodine system swell only moderately when exposed to increased humidity environments, and swell progressively less with increasing size; this behaviour occurs whether they are formed in dry or humid environments, in contrast to those in the CH2I2 system. Direct coastal boundary layer observations of molecular iodine, ultrafine particle production and iodocarbons are reported. Using a newly measured molecular iodine photolysis rate, it is shown that, if atomic iodine is involved in the observed particle bursts, it is of the order of at least 1000 times more likely to result from molecular iodine photolysis than diiodomethane photolysis. A hypothesis for molecular iodine release from intertidal macroalgae is presented and the potential importance of macroalgal iodine particles in their contribution to CCN and global radiative forcing are discussed.

 

 

Absolute absorption cross-section and photolysis rate of I2

A. Saiz-Lopez, R. W. Saunders, D. M. Joseph, S. H. Ashworth, and J. M. C. Plane

Atmos. Chem. Phys., 4, 1443-1450, 2004

Abstract. Following recent observations of molecular iodine (I2) in the coastal marine boundary layer (MBL) (Saiz-Lopez and Plane, 2004), it has become important to determine the absolute absorption cross-section of I2 at reasonably high resolution, and also to evaluate the rate of photolysis of the molecule in the lower atmosphere. The absolute absorption cross-section (σ) of gaseous I2 at room temperature and pressure (295K, 760Torr) was therefore measured between 182 and 750nm using a Fourier Transform spectrometer at a resolution of 4cm-1 (0.1nm at λ=500nm). The maximum absorption cross-section in the visible region was observed at λ=533.0nm to be σ=(4.24±0.50)x10-18cm2molecule-1. The spectrum is available as supplementary material accompanying this paper. The photo-dissociation rate constant (J) of gaseous I2 was also measured directly in a solar simulator, yielding J(I2)=0.12±0.03s-1 for the lower troposphere. This is in excellent agreement with the value of 0.12±0.015s-1 calculated using the measured absorption cross-section, terrestrial solar flux for clear sky conditions and assuming a photo-dissociation yield of unity. A two-stream radiation transfer model was then used to determine the variation in photolysis rate with solar zenith angle (SZA), from which an analytic expression is derived for use in atmospheric models. Photolysis appears to be the dominant loss process for I2 during daytime, and hence an important source of iodine atoms in the lower atmosphere.

 

 

Novel iodine chemistry in the marine boundary layer

Alfonso Saiz-Lopez, John M.C. Plane

Geophysical Research Letters, 31, L04112, doi: 10.1029/2003GL019215, 2004

Abstract. The atmospheric chemistry of iodine is important for several reasons, including the influence of iodine oxides on the oxidising capacity of the troposphere, the formation of new particles, and the enrichment of iodine in marine aerosols and the transport of this essential dietary element to the continents. It is shown here that a substantial iodine source is I2, most likely emitted from macro-algae at low tide. This source accounts for the daytime production of new particles in the coastal marine boundary layer, and also explains the discovery of significant night-time levels of iodine oxides.

 

 

Bromine oxide in the mid-latitude marine boundary layer

Saiz-Lopez, A., J.M.C. Plane and J. A. Shillito

Geophysical Research Letters, 31, LO3111, doi: 10.1029/2003GL018956, 2004

Abstract. We report direct observations of bromine oxide (BrO) in the mid-latitude marine boundary layer (MBL), using long-path Differential Optical Absorption Spectroscopy (DOAS). The measurements were made at the Mace Head observatory on the west coast of Ireland. Over six days of observations, the BrO concentration varied from below the detection limit (≈0.8 parts per trillion (ppt)) at night, to a maximum daytime concentration of 6.5 ppt. At the average daytime concentration of 2.3 ppt, BrO causes significant O3 depletion in the MBL through catalytic cycles involving the iodine oxide and hydroperoxy radicals, and also oxidises dimethyl sulfide much more rapidly than the hydroxyl radical. A post-sunrise pulse of BrO was observed, consistent with the build up of photolabile precursors produced by heterogeneous reactions on sea-salt aerosol during the previous night. This indicates that significant bromine activation occurs over the open ocean.

 

Copyright © 2016. AC2 CSIC, Institute of Physical Chemistry Rocasolano. Serrano 119 - 28006 Madrid - Spain. Tfno +34 91 561 94 00