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 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.
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Marine gas-phase sulfur emissions during an induced phytoplankton bloom
Abstract. The oxidation of dimethyl sulfide (DMS;CH3SCH3), emitted from the surface ocean, contributes to theformation of Aitken mode particles and their growth to cloud condensationnuclei (CCN) sizes in remote marine environments. It is not clear whetherother less commonly measured marine-derived, sulfur-containing gases sharesimilar dynamics to DMS and contribute to secondary marine aerosolformation. Here, we present measurements of gas-phase volatile organosulfurmolecules taken with a Vocus proton-transfer-reaction high-resolutiontime-of-flight mass spectrometer during a mesocosm phytoplankton bloomexperiment using coastal seawater. We show that DMS, methanethiol (MeSH;CH3SH), and benzothiazole (C7H5NS) account for on averageover 90 % of total gas-phase sulfur emissions, with non-DMS sulfur sourcesrepresenting 36.8 ± 7.7 % of sulfur emissions during the first 9 d of the experiment in the pre-bloom phase prior to major biologicalgrowth, before declining to 14.5 ± 6.0 % in the latter half of theexperiment when DMS dominates during the bloom and decay phases. The molarratio of DMS to MeSH during the pre-bloom phase (DMS : MeSH = 4.60 ± 0.93) was consistent with the range of previously calculated ambient DMS-to-MeSH sea-to-air flux ratios. As the experiment progressed, the DMS to MeSHemission ratio increased significantly, reaching 31.8 ± 18.7 duringthe bloom and decay. Measurements of dimethylsulfoniopropionate (DMSP),heterotrophic bacteria, and enzyme activity in the seawater suggest theDMS : MeSH ratio is a sensitive indicator of the bacterial sulfur demand andthe composition and magnitude of available sulfur sources in seawater. Theevolving DMS : MeSH ratio and the emission of a new aerosol precursor gas,benzothiazole, have important implications for secondary sulfate formationpathways in coastal marine environments.
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- Award ID(s):
- 1801971
- PAR ID:
- 10329719
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 22
- Issue:
- 2
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 1601 to 1613
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The organic sulfur compound dimethylsulfoniopropionate (DMSP) is synthesized by numerous species of marine phytoplankton, and its volatile degradation products are a major source of biogenic sulfur to the atmosphere. A massive bloom of the dinoflagellate
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/acp-22-1601-2022
