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Title: Oceanic emissions of dimethyl sulfide and methanethiol and their contribution to sulfur dioxide production in the marine atmosphere
Abstract. Oceanic emissions of dimethyl sulfide (CH3SCH3,DMS) have long been recognized to impact aerosol particle composition andsize, the concentration of cloud condensation nuclei (CCN), and Earth'sradiation balance. The impact of oceanic emissions of methanethiol(CH3SH, MeSH), which is produced by the same oceanic precursor as DMS,on the volatile sulfur budget of the marine atmosphere is largelyunconstrained. Here we present direct flux measurements of MeSH oceanicemissions using the eddy covariance (EC) method with a high-resolutionproton-transfer-reaction time-of-flight mass spectrometer (PTR-ToFMS)detector and compare them to simultaneous flux measurements of DMS emissionsfrom a coastal ocean site. Campaign mean mixing ratios of DMS and MeSH were72 ppt (28–90 ppt interquartile range) and 19.1 ppt (7.6–24.5 pptinterquartile range), respectively. Campaign mean emission fluxes of DMS (FDMS) and MeSH (FMeSH) were 1.13 ppt m s−1 (0.53–1.61 ppt m s−1 interquartile range) and 0.21 ppt m s−1 (0.10–0.31 ppt m s−1 interquartile range), respectively. Linear least squares regression of observed MeSH and DMS flux indicates the emissions are highly correlatedwith each other (R2=0.65) over the course of the campaign,consistent with a shared oceanic source. The campaign mean DMS to MeSH fluxratio (FDMS:FMeSH) was 5.5 ± 3.0, calculated from the ratio of 304 individual coincident measurements of FDMS and FMeSH. Measured FDMS:FMeSH was weakly correlated (R2=0.15) withocean chlorophyll concentrations, with FDMS:FMeSH reaching a maximumof 10.8 ± 4.4 during a phytoplankton bloom more » period. No other volatilesulfur compounds were observed by PTR-ToFMS to have a resolvable emissionflux above their flux limit of detection or to have a gas-phase mixing ratio consistently above their limit of detection during the study period,suggesting DMS and MeSH are the dominant volatile organic sulfur compoundsemitted from the ocean at this site. The impact of this MeSH emission source on atmospheric budgets of sulfurdioxide (SO2) was evaluated by implementing observed emissions in a coupled ocean–atmosphere chemical box model using a newly compiled MeSHoxidation mechanism. Model results suggest that MeSH emissions lead toafternoon instantaneous SO2 production of 2.5 ppt h−1, which results in a 43 % increase in total SO2 production compared to a casewhere only DMS emissions are considered and accounts for 30% of theinstantaneous SO2 production in the marine boundary layer at the meanmeasured FDMS and FMeSH. This contribution of MeSH to SO2production is driven by a higher effective yield of SO2 from MeSHoxidation and the shorter oxidation lifetime of MeSH compared to DMS. Thislarge additional source of marine SO2 has not been previouslyconsidered in global models of marine sulfur cycling. The field measurementsand modeling results presented here demonstrate that MeSH is an importantcontributor to volatile sulfur budgets in the marine atmosphere and must be measured along with DMS in order to constrain marine sulfur budgets. Thislarge additional source of marine–reduced sulfur from MeSH will contribute to particle formation and growth and CCN abundance in the marine atmosphere, with subsequent impacts on climate. « less
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Atmospheric Chemistry and Physics
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6309 to 6325
Sponsoring Org:
National Science Foundation
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