Formation of particulate sulfur species (sulfate and methanesulfonate) during summer over the Eastern Mediterranean: A modelling approach

Add time:09/02/2019    Source:sciencedirect.com

To improve our understanding of the mechanisms of particulate sulfur formation (non sea-salt sulfate, nss-SO42−) and methanesulfonate (MSx used here to represent the sum of gaseous methanesulfonic acid, MSA, and particulate methanesulfonate, MS−) in the eastern Mediterranean and to evaluate the relative contribution of biogenic and anthropogenic sources to the S budget, a chemical box model coupled offline with an aerosol–cloud model has been used.Based on the measurements of gaseous dimethyl sulfide (DMS) and methanesulfonic acid (MSA) and the MSA sticking coefficient determined during the Mediterranean Intensive Oxidant Study (MINOS) experiment, the yield of gaseous MSA from the OH-initiated oxidation of DMS was calculated to be about 0.3%. Consequently, MSA production from gas-phase oxidation of DMS is too small to explain the observed levels of MS−. On the other hand, heterogeneous reactions of dimethyl sulfoxide (DMSO) and its gas-phase oxidation product methanesulfinic acid (MSIA) can account for most of the observed MS− levels. The modelling results indicate that about 80% of the production of MS− can be attributed to heterogeneous reactions.Observed submicron nss-SO42− levels can be fully explained by homogeneous (photochemical) gas-phase oxidation of sulfur dioxide (SO2) to sulfuric acid (H2SO4), which is subsequently scavenged by (mainly submicron) aerosol particles. The predominant oxidant during daytime is hydroxyl radical (OH) showing very high peak levels in the area during summer mostly under cloudless conditions. Therefore, during summer in the east Mediterranean, heterogeneous sulfate production appears to be negligible. This result is of particular interest for sulfur abatement strategy. On the other hand only about 10% of the supermicron nss-SO42− can be explained by condensation of gas-phase H2SO4, the rest must be formed via heterogeneous pathways.Marine biogenic sulfur emissions contribute up to 20% to the total oxidized sulfur production (SO2 and H2SO4) in good agreement with earlier estimates for the area.

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