The atmospheric sulfur cycle under a changing climate (SULCLIM) PID2019-111677RB-100
Naturally emitted from the oceans, dimethyl sulfide (DMS, CH3SCH3) is the most abundant biological source of sulfur to the marine atmosphere. DMS is produced from a variety of marine phytoplankton and is emitted to the atmosphere where it undergoes radical-initiated oxidation by hydroxyl (OH), halogen radicals (e.g. chlorine, Cl, and bromine oxide, BrO), and the nitrate radical (NO3) to form sulfur dioxide (SO2) and methane sulfonic acid (MSA, CH3SO3H), according to traditional description of DMS oxidation chemistry. Gas phase SO2 can be further oxidized to sulfuric acid (H2SO4), a key precursor to new particles formed via homogeneous nucleation. These newly formed particles may grow by further condensation and coagulation to sizes large enough to serve as cloud condensation nuclei (CCN), thus affecting cloud optical properties and climate.
Studies of DMS oxidation have focused so far on the fate of SO2 and MSA and their impact on the concentration of CNN. Many of the proposed intermediates in the complex DMS oxidation scheme have not been directly observed, thus creating uncertainty in the DMS product branching ratios and oxidation timescales.
Climate change can affect the emissions fluxes of DMS due to concurrent sea ice changes and ocean ecosystems compositions shifts caused by changes in temperature, ocean mixing, nutrient and light regimes. Modelling studies show that these changes can increase the emissions of DMS (up to 150%) over regions of the Southern ocean. Therefore, these changes can lead to potential climate feedbacks between DMS emissions and atmospheric processing, CCN formation and radiative forcing. To date, climate models do not include a complete representation of sulfur chemistry in the atmosphere, mainly due to the lack of mechanistic information on the chemical reactions involved in DMS processing in the atmosphere, particularly its auto-oxidation chemistry. This omission leads to a fundamental gap in our understanding of the global sulfur chemical cycle and its climate impact. Here, through a combination of theoretical, experimental and climate modelling methods, we propose to evaluate for the first time the global impact of evolving DMS emissions and new sulfur chemistry on atmospheric aerosol loading and radiative balance under climate change during the 21st century.
Chjange in the radiative effect of sulfate aerosol due to the implementation of the new sulfur scheme in CESM, in present time (a) and in the future according to RCP 6.0 (b) and RCP 8.5 (c) scenarios.