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Two-dimensional monitoring of air pollution in Madrid usinga Multi-AXis Differential Optical Absorption Spectroscopytwo-dimensional (MAXDOAS-2D) instrument

David Garcia-Nieto, Nuria Benavent, Rafael Borge and Alfonso Saiz-Lopez

Atmos. Meas. Tech., 14, 2941–2955, 2021,


Trace gases play a key role in the chemistry of ur-ban atmospheres. Therefore, knowledge about their spatialdistribution is needed to fully characterize air quality in ur-ban areas. Using a new Multi-AXis Differential Optical Ab-sorption Spectroscopy two-dimensional (MAXDOAS-2D)instrument, along with an inversion algorithm (bePRO), wereport the first two-dimensional maps of nitrogen dioxide(NO2)and nitrous acid (HONO) concentrations in the city ofMadrid, Spain. Measurements were made during 2 months(6 May–5 July 2019), and peak mixing ratios of 12 and0.7 ppbv (parts per billion by volume) for NO2and HONO,respectively, were observed in the early morning in the south-ern part of the downtown area. We found good general agree-ment between the MAXDOAS-2D mesoscale observations– which provide a typical spatial range of a few kilometers– and the in situ measurements provided by Madrid’s airquality monitoring stations. In addition to vertical profiles,we studied the horizontal gradients of NO2in the surfacelayer by applying the different horizontal light path lengthsin the two spectral regions included in the NO2spectral anal-ysis: ultraviolet (UV, at 360 nm) and visible (VIS, 477 nm).We also investigate the sensitivity of the instrument to infervertically distributed information on aerosol extinction co-efficients and discuss possible future ways to improve theretrievals. The retrieval of two-dimensional distributions oftrace gas concentrations reported here provides valuable spa-tial information for the study of air quality in the city of Madrid.

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