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DMS oxidation and sulfur aerosol formation in the marine troposphere: a focus on reactive halogen and multiphase chemistry

Abstract. The oxidation of dimethyl sulfide (DMS) in the troposphere and subsequent
chemical conversion into sulfur dioxide (SO2) and methane sulfonic
acid (MSA) are key processes for the formation and growth of
sulfur-containing aerosol and cloud condensation nuclei (CCN), but are highly
simplified in large-scale models of the atmosphere. In this study, we
implement a series of gas-phase and multiphase sulfur oxidation mechanisms
into the Goddard Earth Observing System-Chemistry (GEOS-Chem) global chemical transport model – including two important
intermediates, dimethyl sulfoxide (DMSO) and methane sulphinic acid (MSIA) – to
investigate the sulfur cycle in the global marine troposphere. We found that
DMS is mainly oxidized in the gas phase by OH (66 %), NO3
(16 %) and BrO (12 %) globally. DMS + BrO is important for the
model's ability to reproduce the observed seasonality of surface DMS mixing
ratio in the Southern Hemisphere. MSA is mainly produced from multiphase
oxidation of MSIA by OH(aq) (66 %) and O3(aq) (30 %)
in cloud droplets and aerosols. Aqueous-phase reaction with OH accounts for
only 12 % of MSA removal globally, and a higher MSA removal rate is needed
to reproduce observations of the MSA ∕ nssSO42- ratio. The
modeled conversion yield of DMS into SO2 and MSA is 75 % and
15 %, respectively, compared to 91 % and 9 % in the standard model run
that includes only gas-phase oxidation of DMS by OH and NO3. The
remaining 10 % of DMS is lost via deposition of intermediates DMSO and
MSIA. The largest uncertainties for modeling sulfur chemistry in the marine
boundary layer (MBL) are unknown concentrations of reactive halogens (BrO and
Cl) and OH(aq) concentrations in cloud droplets and aerosols. To
reduce uncertainties in MBL sulfur chemistry, we should prioritize
observations of reactive halogens and OH(aq).
