03 Nov 2021

03 Nov 2021

Review status: this preprint is currently under review for the journal ACP.

Oceanic emissions of dimethyl sulfide and methanethiol and their contribution to sulfur dioxide production in the marine atmosphere

Gordon A. Novak1,a, Delaney B. Kilgour1, Christopher M. Jernigan1, Michael P. Vermeuel1,b, and Timothy H. Bertram1 Gordon A. Novak et al.
  • 1Department of Chemistry, University of Wisconsin – Madison, Madison, WI
  • anow at: Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309 and NOAA Chemical Sciences Laboratory (CSL), Boulder, CO 80305
  • bnow at: Department of Soil, Water, and Climate, University of Minnesota – Twin Cities, St. Paul, MN, USA, 55108

Abstract. Oceanic emissions of dimethyl sulfide (CH3SCH3, DMS) have long been recognized to impact aerosol particle composition and size, the concentration of cloud condensation nuclei (CCN), and Earth’s radiation 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 largely unconstrained. Here we present direct flux measurements of MeSH oceanic emissions using the eddy covariance (EC) method with a high-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToFMS) detector and compare them to simultaneous flux measurements of DMS emissions from a coastal ocean site. Campaign mean mixing ratios of DMS and MeSH were 72 ppt (28–90 ppt interquartile range) and 19.1 ppt (7.6–24.5 ppt interquartile range) respectively. Campaign mean (and interquartile range) emission fluxes of DMS (FDMS) and MeSH (FMeSH) were 1.13 (0.53–1.61) and 0.21 (0.10–0.31) ppt m s-1 respectively. Linear least squares regression of observed MeSH and DMS flux indicates the emissions are highly correlated with each other (R2 = 0.65) over the course of the campaign, consistent with a shared oceanic source. The campaign mean DMS to MeSH flux ratio (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) with ocean chlorophyll concentrations, with FDMS:FMeSH reaching a maximum of 10.8 ± 4.4 during a phytoplankton bloom period. No other volatile sulfur compounds were observed by PTR-ToFMS to have a resolvable emission flux 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 compounds emitted from the ocean at this site.

The impact of this MeSH emission source on atmospheric budgets of sulfur dioxide (SO2) was evaluated by implementing observed emissions into a coupled ocean-atmosphere chemical box model using a newly compiled MeSH oxidation mechanism. Model results suggest that MeSH emissions lead to afternoon instantaneous SO2 production of 2.5 ppt hr-1, which accounts for 30 % of the instantaneous SO2 production in the marine boundary layer at the mean measured FDMS and FMeSH. This contribution of MeSH to SO2 production is driven by a higher effective yield of SO2 from MeSH oxidation and the shorter oxidation lifetime of MeSH compared to DMS. This large additional source of marine SO2 has not been previously considered in global models of marine sulfur cycling. The field measurements and modeling results presented here demonstrate that MeSH is an important contributor to volatile sulfur budgets in the marine atmosphere, and must be measured along with DMS in order to constrain marine sulfur budgets. This large 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.

Gordon A. Novak et al.

Status: open (until 29 Dec 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2021-891', Anonymous Referee #1, 25 Nov 2021 reply
    • RC2: 'Correction for Referee comment 1', Anonymous Referee #1, 25 Nov 2021 reply

Gordon A. Novak et al.

Gordon A. Novak et al.


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Short summary
We describe field measurements of the concentration and vertical flux of dimethyl sulfide (DMS) and methanethiol (MeSH) from a coastal ocean site. DMS is known to impact aerosol formation and growth in the marine atmosphere, with subsequent impacts on cloud formation and climate. Measurements of MeSH, which is produced by the same oceanic source as DMS are rare. We show that MeSH emissions are large and must be measured alongside DMS to understand marine sulfur chemistry and aerosol formation.