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

Abstract

<p><strong>Abstract.</strong> The oxidation of dimethyl sulfide (DMS) in the troposphere and subsequent chemical conversion into sulfur dioxide (<span class="inline-formula">SO₂</span>) 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<span class="thinspace"></span>%), <span class="inline-formula">NO₃</span> (16<span class="thinspace"></span>%) and BrO (12<span class="thinspace"></span>%) globally. DMS<span class="thinspace"></span><span class="inline-formula">+</span><span class="thinspace"></span>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 <span class="inline-formula">OH_(aq)</span> (66<span class="thinspace"></span>%) and <span class="inline-formula">O_3(aq)</span> (30<span class="thinspace"></span>%) in cloud droplets and aerosols. Aqueous-phase reaction with OH accounts for only 12<span class="thinspace"></span>% of MSA removal globally, and a higher MSA removal rate is needed to reproduce observations of the MSA<span class="thinspace"></span><span class="inline-formula">∕</span><span class="thinspace"></span>nss<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="70c2dca1cdebf0791ac6d03f5c421763"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-18-13617-2018-ie00001.svg" width="29pt" height="17pt" src="acp-18-13617-2018-ie00001.png"/></svg:svg></span></span> ratio. The modeled conversion yield of DMS into <span class="inline-formula">SO₂</span> and MSA is 75<span class="thinspace"></span>% and 15<span class="thinspace"></span>%, respectively, compared to 91<span class="thinspace"></span>% and 9<span class="thinspace"></span>% in the standard model run that includes only gas-phase oxidation of DMS by OH and <span class="inline-formula">NO₃</span>. The remaining 10<span class="thinspace"></span>% 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 <span class="inline-formula">OH_(aq)</span> concentrations in cloud droplets and aerosols. To reduce uncertainties in MBL sulfur chemistry, we should prioritize observations of reactive halogens and <span class="inline-formula">OH_(aq)</span>.</p>