Oceanic emissions of dimethyl sulfide and methanethiol and their contribution to sulfur dioxide production in the marine atmosphere
Authors: Novak Gordon A. , Kilgour Delaney B. , Jernigan Christopher M. , Vermeuel Michael P. , Bertram Timothy H. 
Published: May 2022

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 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 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 in 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 h−1, which results in a 43 % increase in total SO2 production compared to a case
where only DMS emissions are considered and 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.