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.

Dimethyl sulfide (CH3SCH3, DMS) emissions from the ocean are the most abundant source of reduced sulfur to the marine atmosphere (Andreae, 1990;Bates et al., 1987bBates et al., , 1992;;Carpenter et al., 2012). The role of DMS as a driver of cloud condensation nuclei (CCN) production, which ultimately impacts Earth's radiative budget in the marine atmosphere, has been studied extensively (Bates et al., 1987a;Carslaw et al., 2013;Charlson et al., 1987;Quinn and Bates, 2011). The oxidation of DMS in the atmosphere ultimately leads to the production of methane sulfonic acid (CH3SO3H, MSA) and sulfur dioxide (SO2) which can be further oxidized to sulfuric acid (H2SO4), contributing to particle formation and growth (Clarke et al., 1998;Hoffmann et al., 2016;Schobesberger et al., 2013;Sipila et al., 2010). Direct observations and mechanistic understanding of the intermediate products in the oxidation of DMS are limited, leading to large variability in estimates of SO2 yields (31 -98%), where SO2 is a precursor to sulfuric acid (H2SO4) and non-sea salt sulfate aerosol (nss-SO 4 2-) (Faloona, 2009;Hoffmann et al., 2016). Previous efforts to constrain the total budget of SO2 in the marine boundary layer (MBL) have required assigning a near 100% yield of SO2 from DMS oxidation (Faloona et al., 2009). A 100% yield of SO2 appears inconsistent with known production of MSA from DMS oxidation and with results from multiple laboratory studies (Faloona, 2009). The high yield of SO2 from DMS necessary for closure of the SO2 budget in that study prompted speculation on the existence of other unknown marine sulfur species which could contribute to SO2 production (Gray et al., 2011). Existence of other marine contributors to SO2 production would serve to reduce the implied SO2 yield from DMS and would potentially bring that yield closer to the 40-80% range typically determined in laboratory and modeling studies (Gray et al., 2011). Implementation of oceanic MeSH emissions and oxidation to SO2 in chemical transport models would result in an increase in sulfuric acid production with subsequent impacts on new particle formation and growth and CCN abundance.

Both DMS and the volatile reduced sulfur molecule methanethiol (CH3SH, MeSH) are produced in seawater from the same precursor metabolite, dimethylsulfoniopropionate (DMSP) (Kiene and Linn, 2000a). Bacterial cleavage of dissolved DMSP (DMSPd) primarily produces dissolved DMS (DMSd), and DMSP demethylation or demethiolation produces dissolved MeSH (MeSHd) (Yoch, 2002). MeSHd is the dominant product of DMSPd consumption with a total yield on the order of 75%, compared to an approximately 10% yield of DMSd (Kettle et al., 2001;Kiene andLinn, 2000b, 2000a). While yields of MeSHd are higher compared to DMSd, MeSHd is also more rapidly consumed by heterotrophic bacteria and phytoplankton, resulting in significantly faster turnover times (hours for MeSHd, days for DMSd) and lower steady state dissolved concentrations (Kiene, 1996). Both MeSHd and DMSd are persistently supersaturated in the dissolved phase resulting in ventilation to the atmosphere (Kettle et al., 2001;Lee and Brimblecombe, 2016). Surface ocean concentrations of DMSd have been measured extensively and are on the order of 1-7 nM with higher values in the summer (Lana et al., 2011). This extensive collection of measurements has permitted the development of global climatologies of DMSd and emission fluxes for implementation into global chemical transport models (Galí et al., 2018;Lana et al., 2011). In contrast, measurements of MeSHd are sparse. Underway measurements from a transect of the Atlantic in September and October of 1998 showed a mean MeSHd of 0.39 ± 0.34 nM and a maximum of 1.7 nM (Kettle et al., 2001). Mean MeSHd during that study was approximately 20% of DMSd. In the Baltic Sea, mean MeSHd was 0.16 ± 0.12 nM compared to 2.6 ± 1.6 nM for DMSd (Leck and Rodhe, 1991). MeSH emission fluxes were estimated to be 10% of DMS in that study. Significant variability in DMSd to MeSHd ratios were observed in those studies, emphasizing the need for more detailed study of the biogeochemical factors that control relative consumption and production of DMSd and MeSHd.

Once emitted to the atmosphere, DMS is oxidized by hydroxyl (OH), nitrate (NO3), chlorine (Cl), and bromine oxide (BrO) radicals to produce lower volatility oxidized products which can contribute to aerosol particle formation and growth (Bates et al., 1987a;Charlson et al., 1987;Quinn and Bates, 2011). Gas phase mixing ratios of DMS in the MBL are typically on the order of 50 -600 ppt, with higher concentrations generally associated with regions of high phytoplankton abundance, and with diel maxima at night when oxidative loss is at a minimum (Kettle et al., 2001;Kettle and Andreae, 2000;Kim et al., 2017;Lana et al., 2011;Lawson et al., 2020;Sciare et al., 2000). Oxidation by OH, thought to be the largest loss pathway, proceeds either through OH addition or hydrogen abstraction. OH addition produces MSA, methane sulfinic acid (MSIA), dimethyl sulfoxide (DMSO), and SO2, while H-abstraction is traditionally thought to primarily produce SO2 (Barnes et al., 2006;Conley et al., 2009;Hoffmann et al., 2016). The branching fraction of the OH oxidation channels is highly temperature dependent with H-abstraction favored at higher temperatures (~70% at 298 K). The DMS H-abstraction product rapidly produces the methylthiomethyl peroxy radical (MTMP, CH3SCH2OO) following recombination with atmospheric oxygen. Until recently it was thought that MTMP primarily participates in further bimolecular reactions with the hydroperoxyl radical (HO2), nitric oxide (NO), or other peroxy radicals (RO2) which efficiently produce the methyl thiyl radical (CH3S•) and ultimately SO2 (Barnes et al., 2006;Hoffmann et al., 2016). Theoretical and laboratory studies have shown that MTMP can also undergo a series of intramolecular hydrogen shift rearrangements and additions of O2 to form the stable product hydroperoxymethyl thioformate (HPMTF; HOOCH2SCHO) at a rate that is competitive with bimolecular reactions (Berndt et al., 2019;Wu et al., 2015). HPMTF has been shown to be globally ubiquitous in the marine boundary layer from airborne observations (Veres et al., 2020) and at a coastal ocean ground site (Vermeuel et al., 2020). Global chemical transport modelling shows that 46% of emitted DMS goes on to form HPMTF (Novak et al., 2021). The atmospheric fate of HPMTF is an active topic of research but ambient observations show that dry deposition to the ocean surface is a significant loss term (lifetime ~30 hours, (Vermeuel et al., 2020)) and that HPMTF is efficiently lost to clouds (Novak et al., 2021;Veres et al., 2020;Vermeuel et al., 2020), resulting in a 35% decrease in global SO2 production from DMS oxidation (Novak et al., 2021) . These previously unconsidered loss processes for DMS derived sulfur may necessitate reevaluation of marine SO2 budgets (Bandy et al., 2011;Faloona et al., 2009).

Comparatively little is known about the atmospheric abundance of MeSH. To date, only two ambient observations of MeSH atmospheric mixing ratios have been presented. Measurements in the remote south-west Pacific showed MeSH mixing ratios from <10ppt to 65 ppt which were 3-36% of coincident DMS mixing ratios (Lawson et al., 2020). DMS and MeSH emission fluxes were inferred in that study from the rate of accumulation at night when oxidative loss was assumed to be zero, which showed that MeSH emission fluxes were 14-24% of the DMS flux. MeSH mixing ratios in the coastal and inshore waters west of the Antarctic peninsula were up to 3.6 ppt, which was 3% of coincident DMS (Berresheim, 1987). To our knowledge there have been no previous direct eddy covariance measurements of MeSH emission flux from the ocean.

While the bimolecular rate constants of MeSH with the primary atmospheric oxidants (OH, BrO, NO3, Cl) are known, there has been limited study on the reactive intermediates or yields of stable products from MeSH oxidation (Butkovskaya and Setser, 1999;Tyndall and Ravishankara, 1991). However, oxidation of MeSH by OH (R1) has been shown to produce the methyl thiyl radical (CH3S•) at a yield of 1.1 ± 0.2 (Tyndall and Ravishankara, 1989), providing a mechanistic link to known reactions in the DMS H-abstraction pathway (R1) (Barnes et al., 2006). CH3S• production from DMS H-abstraction is the major pathway for SO2 production from DMS, and the reactions of CH3S• are therefore well studied (Barnes et al., 2006).

Other studies have shown that the reaction of MeSH with OH occurs primarily at the S-H group (0.87 ± 0.03) forming CH3S•, with a minor channel of H-abstraction from the methyl group (0.13 ± 0.03) (Butkovskaya and Setser, 2021). Recent ab initio/RKKM calculations determined the CH3S• yield from MeSH + OH at 298 K and 760 torr to be 0.98 (Mai et al., 2020).

Given the slight inconsistency between the directly measured CH3S• yield of 1.1 ± 0.2 and the MeSH + OH branching fractions, we take these experiments to provide an upper and lower bound on the CH3S• yield of 0.87 and 1.1, and assume a CH3S• yield of 1 for R1 throughout this analysis.

CH3S• efficiently produces SO2 through a series of competing reactions outlined in R2-R8 (Barnes et al., 2006;Hoffmann et al., 2016;Lucas, 2002;Mardyukov and Schreiner, 2018). As shown in R2, CH3S• experiences reversible addition of O2 producing a methyl thiyl peroxy radical (CH3SOO) which can then undergo a series of unimolecular reactions (R3-R4) to efficiently produce SO2.

Bimolecular reactions of CH3S can also occur to produce CH3SO3 which can react further to produce SO3 and MSA (Barnes et al., 2006).

The atmospheric yield of SO2 from the oxidation of MeSH by OH under low NOx was recently reported as 0.98 based on modeling results constrained by a laboratory oxidation study, which is in good agreement with the efficient production of SO2 from MeSH proposed in our reaction scheme (Chen et al., 2021). We exploit this link between MeSH oxidation by OH and known DMS oxidation chemistry to develop a MeSH oxidation mechanism for implementation into a 0-D chemical box model as described further in the subsequent text. The full set of reactions with rate equations are provided in Table S1.

Here we present eddy covariance (EC) flux measurements of DMS and MeSH emissions at a coastal ocean site using a high sensitivity Vocus PTR-TOF for detection (Krechmer et al., 2018). Results from this study show that emission fluxes of DMS (FDMS) and MeSH (FMeSH) were well correlated (R 2 =0.65) and that FMeSH is a significant contributor to marine sulfur emissions (mean FMeSH = 0.21 ppt m s -1 , compared to mean FDMS = 1.13 ppt m s -1 ). The average ratio of individual DMS to MeSH flux (FDMS:FMeSH) measurements for the full campaign was 5.5 ± 3.0. We assess the impact of the observed large MeSH emission flux on production of SO2 in the marine atmosphere through a coupled ocean flux -atmospheric chemistry 0-D box model with a newly compiled MeSH oxidation mechanism. Modeling results suggest that MeSH contributes approximately 30% of instantaneous afternoon SO2 production (PSO2). Together these results show that MeSH emissions are an important contributor to sulfur budgets in the marine atmosphere and further field studies and laboratory oxidation mechanism investigations are warranted.

Measurements of DMS and MeSH gas phase mixing ratios and EC flux were made continuously from the end of the Ellen Browning Scripps Memorial Pier (hereon SIO Pier) at the Scripps Institution of Oceanography in La Jolla, CA, USA during September 2019. The SIO Pier is 330 m long and extends over 100 m beyond the wave breaking zone. The SIO Pier site has been used regularly for EC studies of ocean-atmosphere trace gas exchange (Kim et al., 2014;Novak et al., 2020;Porter et al., 2018;Vermeuel et al., 2020). The SIO Pier experiences a characteristic sea-breeze circulation pattern during summer where winds are from the ocean at moderate windspeeds (0-6 m s -1 ) during daytime and are from land at night, which limits nighttime flux determinations. DMS and MeSH were detected with a latest generation Vocus PTR-TOF (Vocus) instrument (TOFWERK, Aerodyne), with an HTOF mass analyzer (resolution ca 5000 m/Δm for DMS and MeSH) (Krechmer et al., 2018). The Vocus was housed in a temperature-controlled trailer at the end of the pier and sampled through a 19 m long PFA inlet (0.625 cm i.d.) enclosed in an opaque housing to prevent photochemistry in the sampling line. The inlet was pumped at 22 slpm which maintained turbulent flow and short residence times in the sampling line (Reynolds number 4280, calculated volumetric evacuation time 1.7 s). The full inlet line was held at 40°C which was always above ambient temperatures to prevent condensation of water vapor on inlet surfaces. A bypass line through the Vocus front end subsampled from the main inlet at 5 slpm through a PFA tee located at the instrument interface. The Vocus subsampled from this bypass at a flow rate of 100 sccm immediately in front of the Vocus capillary inlet into the instrument drift tube. This sampling configuration was used to reduce residence times in the sampling lines as much as possible (total estimated inlet and instrument residence time ~1.9 s). In addition to the main inlet line, all surfaces in contact with the ambient sample flow, including unions and valves, were composed of PFA or PEEK in order to minimize known surface artifacts for MeSH sampling except for one stainless-steel union at the Vocus subsampling point (Perraud et al., 2016). The ambient inlet sampling point was collocated with a sonic anemometer recording three-dimensional winds at 10 Hz (Gil HS-50). The sonic anemometer and Vocus inlet were mounted on a 6.1 m long boom extended beyond the end of the pier to minimize flow distortions from the pier. The inlet was mounted on the boom at a height of 13 m above the mean lower low tide level. Ocean depth below the pier sampling point was ca. 6 m.

The Vocus was operated at a drift tube pressure of 1.5 mbar and an axial electric field gradient of 41.5 V cm -1 resulting in a high overall effective field strength (E/N) of 143 Td. Mass spectra were recorded at 10 Hz for the full mass range of 19-500 m/Q. The second RF-only focusing quadrupole in the instrument was operated at an amplitude of 275 V at the start of the campaign before being reduced to 235 and ultimately 215 V later in the deployment. The reduced amplitude on the quadrupole increased transmission of low mass (<50 m/Q) ions as described by Krechmer et al., (2018). MeSH (m/Q 49) transmission was increased by 10% at the reduced amplitude of 215 V, compared to 275 V, which was accounted for in the data processing.

Transmission efficiency of DMS was independent of the quadrupole amplitude as its nominal mass (m/Q 63) is larger than the mass discrimination window of the quadrupole. High resolution peak fitting and integration of the mass spectra was performed in the Tofware 3.2.0 software developed by the instrument manufacturer (TOFWERK).

Additional ancillary measurements made continuously from the pier included O3 mixing ratios, temperature, relative humidity, Observing System (Wright, 2016).

Instrument calibration factors for DMS were determined during ambient sampling by a two-point standard addition of a DMS gas standard (Praxair, 5.08 ppm ± 5%) to the full sampling inlet every 2.5 to 4 hours. Campaign mean sensitivity to DMS was 3.9 cps ppt -1 . Sensitivity to DMS was independent of RH. MeSH was not directly calibrated for during field sampling. Post campaign calibrations for MeSH were performed in the laboratory using a MeSH compressed gas standard (Airgas, 6.11 ± 5% ppm) yielding a calibration factor of 1.3 cps ppt -1 for dry conditions. MeSH sensitivity was slightly humidity dependent resulting in the sensitivity at 80% RH being 11% lower than at 0% RH. Calibrations for MeSH were performed with the same 19 m sampling line and flow conditions used for ambient sampling. We expect that this reduced sensitivity for MeSH at high RH is at least partially due to conversion on inlet surfaces which is a known complication in MeSH sampling (Perraud et al., 2016). Ambient MeSH calibration factors were determined by scaling the measured in-field DMS calibration factors by the RH dependent DMS:MeSH sensitivity ratio determined from the laboratory calibrations. We note that the observed sensitivity ratio of DMS:MeSH is different from what would be predicted based on measured proton transfer rate coefficients which are approximately equal for DMS and MeSH (Williams et al., 1998), emphasizing the need for direct calibration of MeSH.

Instrument backgrounds were determined by overflowing the full inlet line with dry UHP N2 at the tip of the ambient sampling point. Ambient sampling periods were subdivided into 30-minute blocks and were matched to the nearest temporal calibration and background determination point. The campaign mean, median, and interquartile range for DMS were 72.6, 49.2, and 28.0-89.8 ppt respectively. MeSH mean, median, and interquartile ranges mixing ratios were 19.1, 13.4, and 7.6-24.5 ppt. Limits of detection (LOD) at a signal-to-noise (S/N) ratio of three were 2.6 ppt for DMS and 3.6 ppt for MeSH for a 10 s averaging time following the calculation method of Bertram et al., (2011).

The flux (F) of trace gas across the interface is described by Eq. 1, as a function of both the gas-phase (Cg) and liquid phase (Cl) concentrations and the dimensionless gas over liquid Henry's law constant (H), where Kt, the total transfer velocity for the gas, encompasses all of the chemical and physical processes that govern air-sea gas exchange (Liss and Slater, 1974).

Both DMS and MeSH are persistently supersaturated in the liquid phase leading to an emission flux to the atmosphere (where a positive flux value indicates emission). Fluxes of DMS and MeSH were measured in the turbulent planetary boundary layer with the well-established eddy covariance (EC) technique, where F is calculated as the time average of the instantaneous covariances from the mean of vertical wind (w) and the scalar magnitude (x, here either DMS or MeSH) as shown in Eq. 2.

Overbars are means and primes are the instantaneous variance from the mean. N is the total number data points during the flux averaging period. Ambient data was subdivided into approximately 30-minute flux averaging periods for the EC flux calculation.

Several standard EC data processing steps, data filters, and quality control checks were applied during flux analysis including:

1) filtering by wind direction for periods of onshore winds (true wind direction 200-360°), 2) coordinate rotation of threedimensional wind components by the planar fit method to remove unintentional tilts in the sonic mounting and account for local flow distortions (Wilczak et al., 2001), 3) application of a friction velocity (𝑈 * ) threshold of 0.05 cm s -1 to reject periods of low shear driven turbulence, 4) despiking of DMS and MeSH data using a mean absolute deviation filter before the EC flux calculation (Mauder et al., 2013), 5) linear detrending of the scalar timeseries, and 6) flux stationarity filtering with flux periods rejected if they were non-stationary at a 30% threshold (Foken and Wichura, 1996). Additional discussion of EC flux processing and quality control steps including lag time determination, spectral analysis, frequency attenuation corrections, and flux LOD determination are provided in SI. A total of 304 out of 696 flux measurment periods passed all quality control filters.

Campaign mean flux of DMS was 1.13 ppt m s -1 with a mean of individual flux period limits of detection at the 3σ confidence interval of 0.35 ppt m s -1 . MeSH mean flux was 0.21 ppt m s -1 with a mean flux 3σ LOD of 0.11 ppt m s -1 .

A coupled ocean-atmosphere 0-D chemical box model was developed using the Master Chemical Mechanism (MCM) v3.3.1 (http://mcm.york.ac.uk, Jenkin et al., 1997;Saunders et al., 2003) in the Framework for 0-D Atmospheric Modeling (F0AM, Wolfe et al., 2016) with added chlorine chemistry (Riedel et al., 2014). The model was used to assess the impact of observed MeSH emissions on production of secondary marine SO2. Model ability to reproduce observed diel profiles of DMS and MeSH mixing ratios was also tested. Measured emission fluxes of MeSH and DMS are coupled into the model to set the source term for those molecules. Meteorological inputs were acquired from the NOAA National Data Buoy Center (Stations LJPC1 and LJAC1) as well as from an onsite temperature and relative humidity data logger (OM-62, Omega Engineering). Chemical constraints included coincident measurements of O3 and nitryl chloride (ClNO2) measured at the same site in August 2018 (Vermeuel et al., 2020), an assumed OH profile that followed the solar cycle with a peak at 4.0 x 10 6 molecules cm -3 , and constant concentrations of other major trace gases as listed in Table S2. A constant first-order dilution loss term was used with a one-day lifetime to approximate mixing out of the boundary layer. A static boundary layer height (BLH) of 500 m was assumed (Faloona et al., 2005;Stull, 1988;Wei et al., 2018). Clear sky conditions were also assumed (i.e. no heterogeneous loss from reactive uptake on cloud droplets (Vermeuel et al., 2020)). An updated oxidation mechanism for DMS and MeSH was implemented, expanding upon the default DMS oxidation scheme in the MCM v3.3.1 to include oxidation of MeSH to form CH3S• (R1) and to include HPMTF chemistry, detailed in Table S1. The model was allowed to spin up for two days to allow reactive intermediates to reach equilibrium, with all reported values taken from day three of the model run.

Two distinct model cases were developed which differ in how they treat the diel profile of FDMS and FMeSH. The first (termed the Pier Model Case) used the observed diel profile of FDMS and FMeSH at Scripps Pier to test model ability to reproduce observed diel profile of DMS and MeSH gas phase mixing ratios. Nighttime flux measurements from 23:00 to 09:00 were limited during this study due to persistent off-shore wind conditions. Instead, we apply a constant nighttime emission flux taken as the average of the 9:00-10:00 and 21:00-22:00 flux observations for DMS and MeSH. A three-hour moving mean was also applied to the observed diel flux profiles to reduce the influence of experimental variability on the model. This Pier Model Case is used in the analysis of diel profiles presented in Section 3.4.

A second case (termed Open Ocean Model Case) was developed to provide a general assessment of the relative contribution of DMS and MeSH emissions to SO2 production as described in Section 3.5. This Open Ocean case uses fixed values for FDMS and FMeSH taken as the campaign mean fluxes, rather than a variable diel profile. This case avoids the ambiguity of uncertain nighttime emission flux in the observations and better represents conditions on the open ocean where there is little diel variability in wind speed and thus emission fluxes for DMS and MeSH are relatively constant (Archer and Jacobson, 2005).

Observed meteorology and ocean physical and biogeochemical parameters showed minimal variance over the sampling period.

interquartile ranges of air temperatures and relative humidity were 22°C (19.5 to 23.6°C) and 79.9% (72.3 to 88.3%) respectively. Chlorophyll concentrations suggest moderate biological productivity with an observed campaign mean of 1.86 μg L -1 (1.5 to 2.0 μg L -1 interquartile range). O3 mixing ratios showed a clear diel pattern peaking in mid-afternoon with a campaign mean of 32.6 ppbv (27.6 to 38.9 ppbv interquartile range). Wind speeds during onshore wind periods were from 0 to 6 m s -1 , typically peaking in late afternoon with a campaign mean of 2.8 m s -1 . Clear sky conditions were observed for most afternoons during the study period, with total solar irradiance peaking near noon. The period from day of year (DOY) 268-271 saw occasional overcast skies during the afternoon. Light rainfall during the early morning of the DOY 271 was the only precipitation during the campaign. Morning and late evening periods showed occasional presence of marine stratocumulus clouds which drove day-to-day variability in solar irradiance during those times. The campaign time series of wind speed, air temperature, sea surface temperature (SST), O3 mixing ratios, solar irradiance, and DMS and MeSH mixing ratios are presented in Fig. 1. Gaps in the DMS and MeSH timeseries are from instrument maintenance periods, power outages at the site, and periods where the instrument was operated in an alternative sampling mode.

The campaign mean, median, and interquartile range of DMS mixing ratios were 72.6, 49.2, and 28.0-89.8 ppt respectively.

MeSH mean, median, and interquartile range mixing ratios were 19.1, 13. (Carpenter et al., 2012;Huebert et al., 2004;Kim et al., 2017;Marandino et al., 2007). Measurement of FDMS and FMeSH during nighttime was limited due to persistent winds from the to the mean ratio from the full campaign of 5.5 ± 3.0. The relative production and consumption of DMS and MeSH in seawater is known to be a complex function of the speciation and abundance of phytoplankton and bacteria as well as available organic sulfur and other biogeochemical parameters (Kiene et al., 2000;Kiene and Linn, 2000b). No other measured meteorological parameters including wind speed, SST, and solar irradiance showed a significant correlation with FDMS:FMeSH. The underlying cause for the correlation between FDMS:FMeSH and chlorophyll in our dataset is not clear without additional constraints on the ocean biochemistry. However, this result highlights that biological activity can drive variations in dissolved ratios of DMS and MeSH resulting in variability in ambient FDMS:FMeSH emission ratios, and that further study is needed to elucidate this mechanism and its spatiotemporal variability. Measurements during an induced mesocosm phytoplankton bloom experiment using seawater collected as SIO pier immediately before this study showed that the ratio of gas phase DMS to MeSH varied by more than a factor of five over the course of a phytoplankton bloom and decay (Kilgour et al., 2021). DMS to MeSH ratios in that study were strongly linked to changes in bacterial sulfur demand and changes in the available pool of dissolved sulfur across the phytoplankton bloom and decay cycle. While induced mesocosm blooms are not fully representative of the ambient ocean, those results demonstrate the controlling role of ocean biology on FDMS and FMeSH and ultimately on marine SO2 production which must be better constrained through further ambient observations. In the campaign mean case MeSH emissions contribute 30% of the overall SO2 production (or a 43% increase in total SO2 production compared to the FDMS only case). The model yield of SO2 from MeSH oxidation was 0.99 which is comparable to an experimentally constrained model determination of the atmospheric yield of 0.98 (Chen et al., 2021). We include the full MeSH oxidation mechanism for completeness due to its overlap with known DMS chemistry and ease of implementation in the box model. But given our determined model yield of SO2 from MeSH of 0.99, and a recently determined yield of 0.98 constrained by laboratory oxidation studies (Chen et al., 2021), future modeling efforts may be justified in simplifying this mechanism by including only a direct MeSH + OH → SO2 reaction at a yield of 1. Prior efforts to constrain the total SO2 budget in the marine boundary layer required a unit yield of SO2 from DMS (Faloona et al., 2009) which stands in contrast to the 40-80% range typically determined in laboratory and modeling studies (Faloona, 2009;Gray et al., 2011). This discrepancy has prompted consideration that other potential SO2 precursors might be present which might reduce the yield of SO2 from DMS needed to close the SO2 budget (Bandy et al., 2011). Oceanic MeSH emissions as observed in our study are likely one such additional contributor to secondary marine SO2 which has not been previously considered. Eddy covariance flux measurements of ocean-atmosphere trace gas exchange have generally been limited to a small set of molecules (e.g. DMS, acetone, methanol, acetaldehyde) (Novak and Bertram, 2020). As demonstrated with the MeSH measurements presented here, marine EC flux observations of new molecules are critical to constrain marine sulfur and volatile organic compound (VOC) budgets. Global spatiotemporal variability of MeSH emission flux magnitude and the ratio of FDMS:FMeSH are both highly uncertain due to the sparsity of ambient observations, which will need to be better constrained through future studies.

Heterogeneous chemistry of the DMS oxidation product HPMTF is not included in our base model case. HPMTF heterogeneous chemistry has been proposed to be a potentially large sink for HPMTF which would reduce SO2 production from DMS (Novak et al., 2021;Veres et al., 2020;Vermeuel et al., 2020). These details of HPMTF heterogeneous chemistry do not impact the yield of SO2 from MeSH described previously but do impact the calculated relative production of SO2 from MeSH compared to DMS. Inclusion of HPMTF heterogeneous chemistry (at γ=0.01 and aerosol surface area of 48 μm 2 cm -3 ) reduces model SO2 production from DMS to 2.7 ppt h -1 compared to 5.8 ppt h -1 in the model base case. In the HPMTF heterogeneous chemistry case MeSH oxidation accounts for 48% of marine SO2 production. Further details on HPMTF chemistry are given in SI S3.

We also note that the yield of SO2 production from MeSH does not have a temperature dependence, unlike DMS, which may result in MeSH being an especially important source of SO2 in colder high latitude regions. At lower temperatures, the DMS OH-addition reaction pathway becomes more favored, resulting in less efficient production of SO2 from DMS oxidation as production of highly soluble intermediates begins to dominate compared to at higher temperatures. Model calculations presented here used the measured diel temperature profile at the SIO pier during this study which had a mean of 293 K.

Modeled daytime PSO2 as a function of DMS and MeSH emission flux magnitude are shown in Fig. 8 with the interquartile range of FMeSH and FDMS measured SIO pier emission fluxes overlaid as a constraint. These results highlight the potential variability in PSO2 in varying regimes of FDMS:FMeSH. Model results were from the base Open Ocean Case described in Section 2.4.2.2. Given the potential biological control on FDMS:FMeSH, the temperature dependence of SO2 yield from DMS, and the impact of HPMTF heterogeneous chemistry on SO2 yield from DMS, we expect there may be significant temporal and regional variability in the relative contribution of DMS and MeSH to marine PSO2 across the global oceans. This additional SO2 production from MeSH will likely contribute to new particle formation and growth through enhanced production of sulfuric acid (H2SO4) with subsequent impacts on CCN abundance in the MBL. Given the newly determined significance of both MeSH emissions and HPMTF heterogeneous chemistry on marine SO2 production, a critical reevaluation of the global marine 435 SO2 budget is likely warranted.

In addition to DMS and MeSH, several other volatile organic sulfur compounds (VOSCs) have been reported to be significant in the marine atmosphere, either from direct oceanic emissions or gas phase oxidation of precursor species. Recent shipborne observations in the Arabian Sea reported high mixing ratios of dimethyl sulfone (DMSO2, 40-120 ppt) and methane sulfonamide (MSAM, 20-50 ppt) downwind of a biologically productive upwelling region (Edtbauer et al., 2020). Reported DMS in the same region was from 100-300 ppt. The authors in that study propose a direct oceanic emission source of MSAM as there is no known gas phase oxidation pathway to produce MSAM from DMS. For our observations at Scripps Pier, both MSAM and DMSO2 were found to have no observable emission flux above the flux limit of detection at either the standard 3σ LOD threshold or a more relaxed 1σ LOD threshold. Gas phase mixing ratios of MSAM, DMSO, and DMSO2 were also not found to be significant, with none of those species consistently observed above the 10 s averaging time LOD of 2.4, 7.0, and 9.2 ppt respectively. All determinations of MSAM, DMSO, and DMSO2 mixing ratio and LOD assume they have the same detection sensitivity as DMS. Our measurements of DMSO and DMSO2 are consistent with box model calculated mixing ratios which show hourly maxima of 4.9 ppt and <0.1 ppt for DMSO and DMSO2, respectively which are both below the instrument LOD. The box model conditions used were the Pier Model Case with the addition of [BrO]max of 1 ppt. Oxidation of DMS by BrO favors DMSO and DMSO2 production and the inclusion of [BrO]max of 1 ppt was done as a test of the reasonable upper limit case for DMSO and DMSO2 production for conditions during this study. Dimethyl disulfide (DMDS, CH3S2CH3) was also not consistently observed above the LOD in our measurements.

While a direct oceanic emission source of MSAM to the atmosphere has been proposed (Edtbauer et al., 2020), we did not measure a detectable MSAM emission flux in this study. As noted by Edtbauer et. al. (2020), given the Henry's law constant of MSAM (KH = 3.3 × 10 4 M atm -1 ), the waterside concentration of MSAM necessary to drive a net emission flux of MSAM to the atmosphere is on the order of 1700 nM. Given that maximum total dissolved organic sulfur (DOS) in ocean surface waters are on the order of 200 nM (Ksionzek et al., 2016), even if all surface ocean DOS was in the form of MSAM, we still would not expect an emission flux of MSAM to the atmosphere. This exercise suggests MSAM may instead be formed through an unknown reaction pathway in the atmosphere which was active in the Arabian Sea but not in our observations at SIO Pier in coastal southern California. Taken together our observations indicate that MSAM, DMSO, and DMSO2 emission fluxes and mixing ratios were small at the SIO Pier site consistent with model calculations. This discussion on the lack of observations of other VOSCs only extends to molecules that are detectable with the Vocus PTR instrument, such as DMSO, DMSO2, MSAM, and DMDS. For example, it does not include inorganic sulfur containing molecules such as carbonyl sulfide (OCS) and carbon disulfide (CS2) which also have a known oceanic source but are not readily detectable by PTR. In particular our measurements do not include the recently observed DMS oxidation product HPMTF which was found to be a globally ubiquitous sulfur reservoir from airborne observations in the marine atmosphere (Veres et al., 2020) and in prior surface observations at the SIO Pier (Vermeuel et al., 2020). HPMTF is not detectable with the Vocus PTR ion chemistry used in this study. HPMTF was previously observed in the summer of 2018 at the SIO Pier site to have a strong diel profile, peaking in the early afternoon with an average daytime mixing ratio of 12.1 pptv (Vermeuel et al., 2020). While observations of HPMTF were made a year prior to the DMS and MeSH observations reported here, it suggests that HPMTF may comprise up to 10-20% of the total daytime VOSC concentration at this site during summer.

We present the first direct eddy covariance flux measurement of MeSH emissions from the ocean which show that MeSH emissions account for a mean of 16% (FMeSH/(FMeSH + FDMS)) of emitted volatile organic sulfur measurable with the Vocus PTR-ToF during this study. DMS and MeSH emission fluxes were correlated with each other (R 2 = 0.65), consistent with their shared oceanic source from the degradation of DMSP. Measured FDMS:FMeSH was found to have a weak correlation (R 2 = 0.15) with chlorophyll concentrations, which highlights the need for further study of biogeochemical cycling in the ocean surface which may drive significant spatiotemporal variability in FMeSH. The atmospheric implications of ocean MeSH emissions were assessed by development of a MeSH oxidation mechanism and incorporation into a coupled ocean-atmosphere 0-dimensional chemical box model. Modeling results show that oxidation of MeSH by OH produces SO2 at a high yield (~100%) and is an important contributor to SO2 production in the marine atmosphere, driving an increase in afternoon PSO2 of 2.5 ppt hr -1 , corresponding to 30% of total afternoon SO2 production rates. In a model case including HPMTF heterogeneous uptake to aerosols, SO2 yield from DMS is reduced and MeSH becomes even more important, accounting for 48% of marine SO2 production. Taken together these results demonstrate that MeSH is an important contributor to volatile sulfur budgets in the marine atmosphere and that further studies are needed to constrain spatiotemporal trends of MeSH emission and oxidation relative to DMS.