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

Abstract

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).

Heterogeneous sulfate aerosol formation mechanisms during wintertime Chinese haze events: air quality model assessment using observations of sulfate oxygen isotopes in Beijing

“…S(IV) oxidation by H 2 O 2 (6 % in both HPP and CP) and NO 2 (6 % in HPP and 8 % in CP) represent a minor heterogeneous sulfate production pathway. Previous studies suggested that oxidation of SO 2 by NO 2 in aerosol water dominates heterogeneous sulfate formation in Beijing during winter (Wang et al, 2016;Cheng et al, 2016) at neutral aerosol pH. However, subsequent studies showed that these high aerosol pH values were unlikely during wintertime Beijing haze events and they calculated aerosol pH values in the range of 4.2-4.7 (Guo et al, 2017;Liu et al, 2017).…”

“…We assume that Ca 2+ and Mg 2+ cations constitute 3.0 % and 0.6 % of the dust by mass, respectively, based on observations near East Asian dust source regions (Fairlie et al, 2010). In addition, we considered the impacts of acidity and ionic strength on TMI-catalyzed reaction rates following Cheng et al (2016) (Table 2), since the ionic strength of aerosol liquid water can reach 20 M during polluted periods (He et al, 2018;Herrmann et al, 2015). We performed seven sensitivity studies based on Run_Het but with prescribed values of aerosol pH to examine the dependence of model results on aerosol pH alone (Table 1).…”

“…Sulfate is one of the most important components of PM 2.5 , representing 10 %-30 % of PM 2.5 mass in eastern China (Huang et al, 2014;Zhang et al, 2013;Wang et al, 2014;Shi et al, 2017). Recent observations show that the sulfate mass fraction of PM 2.5 increases during haze pollution periods, indicating that sulfate is a key driver for severe haze events (Wang et al, 2014(Wang et al, , 2016Cheng et al, 2016;. Previous simulations have shown that most models fail to predict severe haze pollution in Beijing, at least in part because of sulfate underestimation (Jiang et al, 2013;Park et al, 2014;Pozzer et al, 2012).…”

“…However, recent studies have shown that traditional gas-and aqueous-phase chemistry in cloud droplets cannot explain rapid sulfate production observed during haze, suggesting missing sulfate formation mechanisms on aerosols in the models (Zheng et al, 2015;D. Chen et al, 2016;Zhang et al, 2015;Wang et al, 2014;Cheng et al, 2016;Huang et al, 2014). These missing sulfate formation mechanisms on aerosols include heterogeneous oxidation of SO 2 by NO 2 (Cheng et al, 2016;Wang et al, 2016Wang et al, , 2018Gao et al, 2016;Zhang et al, 2015) and O 2 catalyzed by TMIs (G. Li et al, 2011) and via a free radical chain mechanism (Huie and Neta, 1984;Hung and Hoffman, 2015).…”

“…Chen et al, 2016;Zhang et al, 2015;Wang et al, 2014;Cheng et al, 2016;Huang et al, 2014). These missing sulfate formation mechanisms on aerosols include heterogeneous oxidation of SO 2 by NO 2 (Cheng et al, 2016;Wang et al, 2016Wang et al, , 2018Gao et al, 2016;Zhang et al, 2015) and O 2 catalyzed by TMIs (G. Li et al, 2011) and via a free radical chain mechanism (Huie and Neta, 1984;Hung and Hoffman, 2015). The importance of these heterogeneous reactions remains highly uncertain, due in part to uncertainties regarding the aerosol liquid water content, pH, and ionic strength, all of which impact heterogeneous reaction rates (Herrmann et al, 2015;Cheng et al, 2016).…”