Dimethyl sulfide (DMS: CH3SCH3) is the most abundant biological source of sulfate aerosol and has a significant influence on Earth's radiation budget and climate due to its contribution to atmospheric marine particles (Charlson et al., 1987; Fung et al., 2022). In the atmosphere, DMS reacts with hydroxyl radical (OH), nitrate radical (NO3), ozone (O3) and various halogen species (e.g., chlorine, Cl; bromine oxide, BrO), primarily forming sulfur dioxide (SO2) and methyl sulfonic acid (MSA: CH3SO3H) (Chen et al., 2018; Faloona, 2009; Hoffmann et al., 2016). These oxidation products are considered key influences on the formation and evolution of natural aerosols and clouds along with their associated climate impacts, especially in the marine boundary layer (MBL) (Carslaw et al., 2013; Sipilä et al., 2010; Schobesberger et al., 2013; Thomas et al., 2010; von Glasow and Crutzen, 2004). SO2 and MSA formed by DMS oxidation can be deposited on the Earth's surface or can further oxidize, affecting the size distribution of aerosol and cloud microphysics (Leaitch et al., 2013; Wollesen de Jonge et al., 2021). SO2 can either oxidize in the gas phase by reacting with the OH radical, forming H2SO4, which can participate in nucleation and the early growth of particles in the atmosphere, or it can be taken up by cloud droplets and undergo aqueous-phase oxidation by reacting with H2O2, O3 and O2 (catalyzed by transition metals: Mn, Fe), forming SO42- and generally only contributing to the growth of aerosol particles (Hoyle et al., 2016; Kulmala, 2003; Alexander et al., 2009). The hypohalous acids (HOBr, HOCl, HOI) also play a significant role in aqueous-phase sulfate production in the marine boundary layer (MBL) (Chen et al., 2016; Sherwen et al., 2016b). Recent studies have highlighted the importance of natural aerosols originating from DMS oxidation and their contribution to the uncertainty of aerosol radiative forcing in climate models (Carslaw et al., 2013; Fung et al., 2022; Rosati et al., 2022; Novak et al., 2021, 2022). Since DMS-derived aerosol is a major source of uncertainty when estimating the global natural aerosol burden and associated aerosol indirect radiative forcing, a more accurate representation of DMS oxidation and particle formation processes is an important step towards improved Earth system and climate modeling.

Although the chemistry of DMS oxidation has been previously studied in great detail, there are still known uncertainties in and omissions from the current mechanism in current air quality and chemical transport models (Barnes et al., 2006; Fung et al., 2022; Hoffmann et al., 2016, 2021). Furthermore, while increasingly complex and experimentally validated mechanisms are under ongoing development, DMS oxidation processes in many current chemical transport models continue to be represented through simplified gas-phase reactions with the tropospheric oxidants OH and NO3 that produce the two major oxidation products SO2 and MSA in a fixed ratio, as shown in Reactions (R1)–(R3) in Table  (Chen et al., 2018; Chin et al., 1996; Veres et al., 2020). This type of simplified mechanism neglects the formation and loss of important intermediates such as dimethyl sulfoxide (DMSO: CH3SOCH3), methanesulfinic acid (MSIA: CH3SO2H) and the recently discovered oxidation product hydroperoxymethyl thioformate (HPMTF: HOOCH2SCHO) (Berndt et al., 2019; Veres et al., 2020; Wu et al., 2015; Khan et al., 2021).

These omissions can have major consequences for product yields of DMS oxidation, thereby affecting the aerosol burdens. For example, the OH-addition pathway of DMS forms DMSO and MSIA as the intermediates; these have been identified as a dominant source of MSA via their aqueous-phase oxidation, and a fraction of that MSA subsequently undergoes aqueous-phase oxidation to form sulfate aerosol (Chen et al., 2018; Ishino et al., 2021; Zhu et al., 2006; von Glasow and Crutzen, 2004). Previous studies have suggested that BrO is responsible for 8 %–30 % of the total DMS loss, highlighting the importance of this pathway as well (Breider et al., 2010; Boucher et al., 2003; Chen et al., 2018; Khan et al., 2016). More recent experimental and laboratory studies have confirmed the formation of methylthiomethylperoxy radicals (CH3CH2OO; abbreviated to MSP or MTMP) from the H-abstraction channel of OH oxidation, which can subsequently lead to a series of rapid intramolecular H-shift isomerization reactions, ultimately resulting in the formation of the stable intermediate HPMTF (Berndt et al., 2019; Veres et al., 2020; Vermeuel et al., 2020; Wu et al., 2015; Fung et al., 2022; Jernigan et al., 2022a). It has been reported that 30 %–46 % of the emitted DMS forms HPMTF according to different modeling studies, and this falls within the observational range from NASA Atmospheric Tomography (ATom) for ATom-3 and ATom-4 flight campaigns, where about 30 %–40 % of the DMS was oxidized to HPMTF along their flight tracks (Fung et al., 2022; Veres et al., 2020; Novak et al., 2021). Subsequent investigation of the isomerization rate and heterogeneous loss of HPMTF in cloud droplets and aerosol showed a high production rate of marine carbonyl sulfide (OCS) from the chemical loss of HPMTF, a potential precursor of stratospheric sulfate aerosol and significant inhibitor of the formation of cloud condensation nuclei (CCN) due to the resulting reduction in SO2 (Jernigan et al., 2022a). With the latest experimental findings on the heterogeneous HPMTF loss process and experimentally validated oxidation reactions for OCS formation directly from HPMTF, it is necessary to include these reactions as part of the DMS oxidation mechanism, as these will have an impact on the overall yield of SO2, thus affecting the formation probability of CCN (Jernigan et al., 2022a, b).

Considering these and other consequences of complex DMS oxidation processes, a heavily simplified oxidation scheme will necessarily neglect potentially important reaction intermediates along with their production and loss pathways, with implications for the concentration and distribution of the oxidation products, including particulate sulfate. Differing intermediate lifetimes further influence sulfur removal and transport depending on the relative dominance of pathways. Thus, the exclusion of key pathways and intermediate species can lead to errors in the representation of the spatial distributions of both gas- and particle-phase sulfur species as well as the global sulfur burden.

The DMS oxidation products sulfate and MSA play an important role in Earth's radiative budget through cloud droplet formation, and the extent of this role depends on how efficiently they can produce and grow new particles in the marine atmosphere (Thomas et al., 2010). In the gas phase, SO2 can oxidize forming H2SO4, which is a key product due to its contribution to nucleation and condensational growth, as shown in Fig. . SO2 oxidation through aqueous chemistry in cloud droplets does contribute to particle growth rates by providing larger aerosol particles during cloud evaporation, which act as more efficient CCN. (Kaufman and Tanré, 1994). On the other hand, MSA might participate in nucleation along with sulfuric acid in the presence of amines or ammonia (Johnson and Jen, 2023). Recent studies have highlighted the importance of aqueous-phase chemistry in the formation and loss of MSA (Boniface et al., 2000; Chen et al., 2015; Kaufman and Tanré, 1994; Kulmala et al., 2000).

Additionally, the recently identified intermediate HPMTF also has the potential for further gas-phase oxidation. Under cloud-free conditions, HPMTF can undergo gas-phase oxidation by OH, producing SO2 and eventually leading to the formation of non-sea-salt SO42-. This sulfate can contribute to aerosol formation and growth processes, with climate implications (Galí et al., 2019). Another work used direct airborne eddy covariance flux measurements to explain the chemical fate of HPMTF in the MBL and found that chemical loss due to aqueous-phase reactions in clouds is the major HPMTF removal process in cloudy conditions (Novak et al., 2021). In the same study, global model simulations showed a 35 % reduction in global annual average SO2 production from DMS and a 24 % reduction in the near-surface (0 to 3 km) global annual average SO2 concentrations over the ocean as a result of this process (Novak et al., 2021). Thus, a complete representation of cloud loss and aerosol uptake is needed to effectively evaluate the atmospheric impacts of marine DMS and their connections to cloud formation (Novak et al., 2021; Holmes et al., 2019).

To better understand the marine sulfur budget as well as the eventual formation, size distribution and seasonality of sulfate aerosol, we use the global chemical transport model GEOS-Chem, integrating previously developed mechanisms along with newly proposed pathways involving the formation and loss of the intermediates DMSO, MSIA and HPMTF. As part of this work, we further quantify the atmospheric impacts of individual reactions and mechanisms, evaluate uncertainties in the chemical mechanism, and identify the improvements necessary to better represent the impacts of DMS on atmospheric chemistry and climate more accurately. The resulting integrated scheme provides a more complete representation of marine sulfur and sulfate aerosol species in marine tropospheric environments compared to the simplified base GEOS-Chem mechanism, with improved comparisons to aircraft and surface observations. Since aerosols are a major contributor to uncertainty in climate forcing, improving oxidation and aerosol formation mechanisms by including and optimizing neglected reactions in models is a crucial step towards a more mechanistically robust representation of particle yields and sensitivities. We further perform multiple sensitivity tests to investigate how the uncertainty in the heterogeneous uptake of the newly identified HPMTF could influence DMS chemistry and tropospheric aerosol formation (Holmes et al., 2019; Novak et al., 2021). In a broader sense, our work provides a more detailed story of the heterogeneous loss, fate and ultimate impacts of DMS and its oxidation products, improving our understanding of a key ocean–atmosphere interaction in the context of global change.

The global chemical transport model GEOS-Chem v12.9.3 is used to simulate the chemistry of DMS and its oxidation products. Impacts on simulated aerosol size and number and mass concentrations are considered by coupling the TwO-Moment Aerosol Sectional (TOMAS) aerosol microphysics module with GEOS-Chem v12.9.3 (GC-TOMAS) (https://github.com/geoschem/geos-chem/tree/12.9.3, last access: 10 March 2024) (Adams and Seinfeld, 2002; Kodros and Pierce, 2017). The default GEOS-Chem chemical mechanism contains detailed HOx–NOx–VOC–O3–halogen tropospheric chemistry along with recently updated halogen chemistry and in-cloud processing (Bey et al., 2001; Holmes et al., 2019; Chen et al., 2017; Parrella et al., 2012; Schmidt et al., 2016; Wang et al., 2019). The DMS emission flux from the ocean is controlled by a gas transfer velocity which is dependent on sea surface temperature and wind speed (Johnson, 2010) and the climatology of concentrations in seawater (Lana et al., 2011; Nightingale et al., 2000). The aqueous-phase concentration of O3 in aerosols or cloud droplets is calculated assuming gas–liquid equilibrium, and the aqueous-phase concentration of OH is calculated based on [OH(aq)] = δ[OH(g)], where δ = 1 × 10-19 Mcm3molec.-1 (Jacob et al., 2005; Chen et al., 2018).

In this study, TOMAS tracks the aerosol number and the mass of each aerosol species in 15 logarithmically sized bins, with sizes in this analysis ranging from 3 nm to 10 µm (Lee and Adams, 2012; Lee et al., 2013). All binned aerosol species undergo interactive microphysics, allowing the calculation of aerosol number budgets (Westervelt et al., 2013). The version of GC-TOMAS used here includes 47 vertical levels, a horizontal resolution of 4° × 5° and the GEOS-FP data product for meteorological inputs. Simulations are performed for 2018 with 11 months of discarded model spin-up. Nucleation is simulated via a ternary nucleation scheme involving water, sulfuric acid and ammonia, with nucleation rates scaled by 10-5 (Napari et al., 2002; Westervelt et al., 2013). In low-ammonia regions (less than 1 pptv), a binary nucleation scheme involving water and sulfuric acid is used instead (Vehkamäki et al., 2002). Previously, GC-TOMAS was used for aerosol simulations to investigate topics such as the aerosol cloud-albedo effect and cloud condensation nuclei formation (Kodros et al., 2016; Kodros and Pierce, 2017; Pierce and Adams, 2006; Westervelt et al., 2013). The aerosol species available for GC-TOMAS simulations are sulfate, aerosol water, black carbon, organic carbon, mineral dust and sea salt (Alexander et al., 2005; Bey et al., 2001; Duncan Fairlie et al., 2007; Pye et al., 2009). The wet and dry deposition schemes for aerosols and gas species are based on previous studies (Amos et al., 2012; Emerson et al., 2020; Liu et al., 2001; Wesely, 1989; Wang et al., 1998).

We refer to simulations performed using only these three DMS oxidation reactions (Table ) as “BASE”; these involve only the direct formation of SO2 and MSA in the gas phase (Chin et al., 1996). We further implement and evaluate a custom chemical mechanism for DMS oxidation referred to as “MOD” (Tables –) and representing an integration of three individual DMS oxidation mechanism updates explored previously using GEOS-Chem and CAM6-Chem. This mechanism also includes HPMTF loss to clouds and aerosols via heterogeneous chemistry, dry and wet deposition of HPMTF, along with a further improvement based on recent literature updates to chemical kinetics (Chen et al., 2018; Fung et al., 2022; Veres et al., 2020; Novak et al., 2021; Cala et al., 2023). In GC-TOMAS, we use a specific subroutine that takes the amount of sulfate produced via in-cloud oxidation and condenses it into an existing aerosol size distribution. So, the mass of sulfate produced by oxidation is apportioned to the various size bins according to the number of particles in that size bin. The microphysics of TOMAS accounts for H2SO4 formation based on the gas-phase oxidation of SO2 included in the kinetic preprocessor (KPP) equation list valid for simulation BASE. Since there are additional sources of sulfate in the integrated DMS oxidation mechanism in both the gas and aqueous phases, we made the necessary changes in the KPP code to allow H2SO4 formation by gas-phase oxidation of SO2 to be explicitly tracked. On the other hand, code changes to account for sulfate formed by heterogeneous oxidation of MSA and HPMTF (in clouds and aerosols) were performed in the GEOS-Chem microphysics module, which also handles the in-cloud oxidation of SO2 in GC version 12.9.3 (Park et al., 2004; Trivitayanurak et al., 2008).

To examine the sensitivities of size-resolved aerosol formation and growth to DMS chemistry modifications, model simulations are conducted as summarized in Table . Output from simulations MOD and MOD_noHetLossHPMTF was then compared against simulation BASE to understand the contribution of these additional chemical reactions to the spatial pattern of the surface concentrations of major oxidation products of DMS.

As shown in Table , the modified DMS chemistry simulations examined here include the gas- and aqueous-phase oxidation of DMS and the intermediate products of its oxidation by OH, NO3, O3 and halogenated species, as previously explored in an older version of GEOS-Chem (Chen et al., 2018). The aqueous-phase reactions in cloud droplets and aerosols were parameterized assuming first-order loss of the gas-phase sulfur species (Chen et al., 2018). Further building upon the previous mechanism, the scheme used here also includes the formation and loss of HPMTF, previously tested in the global climate model CAM6-Chem, as shown in Table  (Veres et al., 2020). Table  presents the third piece of the mechanism: a gas-phase MSA-producing branch of the H-abstraction pathway in the DMS chemistry that bridges the other two sets of reactions (Fung et al., 2022). To avoid the addition of SO3 oxidation chemistry, we have replaced SO3 with H2SO4 following previous work on the decomposition reaction of CH3SO3 (Table ). A similarly integrated mechanism (Tables –) was previously explored using the CAM6-Chem model with a focus on radiation budget impacts, which is improved in this work through updates to rate constants and the inclusion of additional relevant reactions (Fung et al., 2022; Novak et al., 2021; Wollesen de Jonge et al., 2021; Cala et al., 2023). The newly added reactions and their respective rate constants are largely based on the Master Chemical Mechanism (MCM) v3.3.1 and the literature cited in the reference lists in Tables –. We use a rate constant of 1.40 × 10-11 cm3molec.-1s-1 for HPMTF + OH, which was previously determined based on the concentrations of other known sulfur species (DMS, DMSO, SO2 and methyl thioformate (MTF); CH3SCHO, a structurally similar proxy for HPMTF) and evaluated by the box model (Jernigan et al., 2022a). An exploration of the reaction rate uncertainty for the HPMTF+OH reaction (Table ), including both high- and low-end limits of 5.5 × 10-11 cm3molec.-1s-1 and 1.4 × 10-12 cm3molec.-1s-1, respectively, resulted in only minor impacts on the fate of HPMTF and on sulfate formation in our simulations (Novak et al., 2021; Wu et al., 2015).

Model sensitivity simulations were also performed with (MOD simulations) and without (“MOD_noHetLossHPMTF” simulations) HPMTF heterogeneous uptake by clouds and aerosols to account for how much of the DMS-derived HPMTF eventually forms SO2 in the presence of these additional loss processes (Table ). Previous work has shown that aerosol surface chemistry causes additional decreases in HPMTF mixing ratios, primarily over land, and that the loss of HPMTF in clouds is larger (36 %) than the losses from aerosols (15 %) when using an uptake coefficient of γ = 0.01 for both processes (Novak et al., 2021). In this work, based on recent laboratory measurements, we use a smaller uptake coefficient (γ = 0.0016) for HPMTF loss to aerosols and clouds (Table ) (Jernigan et al., 2022b). Based on previous work, we assume that HPMTF directly produces sulfate in cloud and aerosol, though there is some uncertainty regarding the fate of HPMTF during heterogeneous loss (Zhang and Millero, 1993; Novak et al., 2021; Jernigan et al., 2022a). For the aqueous-phase reactions listed in Table , including the oxidation of the intermediates DMSO and MSIA in cloud droplets and aerosols, a first-order loss of the gas-phase sulfur species was assumed, following previously used parameterizations and physical parameter values (Chen et al., 2018). Alongside the gas-phase and aqueous-phase reactions relevant to the added DMS oxidation mechanism contributing to the formation of SO2 and sulfate, the default version of GC-TOMAS used here also includes in-cloud oxidation of SO2 by H2O2, O3 and O2 (catalyzed by transition metals: Mn, Fe) as well as the loss of dissolved SO2 by HOBr and HOCl, all of which are passed to TOMAS to account for sulfate production (Chen et al., 2017; Wang et al., 2021).

All simulations are conducted for the year 2018, which was chosen to match the model simulation with the dates of the NASA Atmospheric Tomography flight campaign (ATom-4) that offers observational data for HPMTF, DMS and SO2. Rate coefficients for all gas-phase sulfur reactions are obtained from the most recent JPL report and other references, while sulfur product yields for gas-phase reactions are obtained from various laboratory and modeling studies (Burkholder et al., 2020; Lucas and Prinn, 2002; Hoffmann et al., 2016; Gershenzon et al., 2001; Kowalczuk et al., 2003; Zhou et al., 2019; Jernigan et al., 2022a). The simulations included sea salt debromination except for some sensitivity tests described below (Zhu et al., 2019; Schmidt et al., 2016). In all our simulations that include MOD, DMS is advected and undergoes chemical loss and transport but does not undergo dry or wet deposition. However, dry and wet deposition of oxidation products such as DMSO, MSIA, MSA and HPMTF are included.

We note that previous work has explored the impact of MSA on aerosol growth, including modifications within TOMAS to represent this process (Hodshire et al., 2019). We do not include this process here. Future work to examine its importance in the context of the chemistry updates presented here is recommended.

In this study we have updated the default DMS oxidation scheme in the GEOS-Chem model by implementing an integrated oxidation mechanism. The new scheme includes gas-phase and aqueous-phase reactions involving DMSO, MSIA and HPMTF formation as well as newly identified HPMTF loss processes yielding considerable changes in the seasonal concentrations of major oxidation products and sulfur-derived aerosols. With this new chemistry scheme, the global annual mean surface DMS concentration decreases by 36 % relative to the BASE scheme in GEOS-Chem globally due to the presence of additional loss processes in the integrated mechanism, reducing the bias with respect to ATom-4 DMS measurement.

In this new scheme, OH, BrO, O3 and NOx species act as important sinks of DMS, contributing 66.2 %, 18.4 %, 3.1 % and 11.2 % of the global annual mean surface DMS loss and highlighting the relative importance of these loss processes in determining the surface DMS budget. We also find that at higher latitudes, gas-phase and multiphase oxidation of DMS by O3 and BrO become important in determining the budget of DMS. On the other hand, overall, OH is responsible for the largest loss of DMS via the addition and abstraction reactions relative to other sinks that contribute more from the addition reaction than the abstraction reaction. For the global distribution of simulated HPMTF, our updated scheme in GEOS-Chem provides a reduced high bias with respect to observations compared to previous studies. While emissions of BrO are uncertain in this version of GEOS-Chem, we find that the compound acts as a key sink of DMS, especially over the Southern Ocean. Overall, we find a large reduction in SO2 (40 %) and an increase in sulfate (17 %) due to the addition of heterogeneous HPMTF loss processes.

The lower SO2 obtained with the new DMS chemistry scheme contributes to a reduction of 16.8 % in the global annual mean surface-layer number concentration of particles with diameters less than 80 nm, contributed to by reductions in gas-phase precursors for new particle formation. There is a concurrent increase of 3.8 % in the global annual mean number of particles with diameters larger than 80 nm. This latter global mean particle number change varies in sign seasonally, with a 6.7 % increase for JJA and a 5.4 % decrease for DJF. This decrease is dominated by Southern Hemisphere summertime changes connected to suppressed new particle formation/growth and enhanced coagulation following additional sulfate production through aqueous chemistry. Cloud loss processes related to HPMTF make key contributions to these simulated changes through the enhancement of the aqueous-phase particle growth of those particles large enough to act as CCN.

Although the increased chemical mechanism complexity described in this work will necessarily increase the model's computational cost (MOD simulation runtimes are approximately 16 % longer), this study highlights the value of including a more realistic chemical oxidation mechanism for DMS and its stable intermediates to allow a better representation of DMS-derived aerosol in the marine atmosphere as well as its seasonal size distributions. A reduced form of this reaction mechanism including key chemical species and pathways should be able to capture the key processes with less computational impact and will be a priority in future work.