An advanced modeling study on the impacts and atmospheric implications of multiphase dimethyl sulfide chemistry

Significance

Climate models indicate the importance of dimethyl sulfide (DMS) oxidation in new aerosol particle formation and the activation of cloud condensation nuclei over oceans. These effects contribute to strong natural negative radiative forcing and substantially influence the Earth’s climate. However, the DMS oxidation pathway is not well-represented, because earlier model studies only parameterized gas-phase DMS oxidation and neglected multiphase chemistry. Here, we performed the most comprehensive current mechanistic studies on multiphase DMS oxidation. The studies imply that neglecting multiphase chemistry leads to significant overestimation of SO₂ production and subsequent new particle formation. These findings show that an advanced treatment of multiphase DMS chemistry is necessary to improve marine atmospheric chemistry and climate model predictions.

Abstract

Oceans dominate emissions of dimethyl sulfide (DMS), the major natural sulfur source. DMS is important for the formation of non-sea salt sulfate (nss-SO₄²⁻) aerosols and secondary particulate matter over oceans and thus, significantly influence global climate. The mechanism of DMS oxidation has accordingly been investigated in several different model studies in the past. However, these studies had restricted oxidation mechanisms that mostly underrepresented important aqueous-phase chemical processes. These neglected but highly effective processes strongly impact direct product yields of DMS oxidation, thereby affecting the climatic influence of aerosols. To address these shortfalls, an extensive multiphase DMS chemistry mechanism, the Chemical Aqueous Phase Radical Mechanism DMS Module 1.0, was developed and used in detailed model investigations of multiphase DMS chemistry in the marine boundary layer. The performed model studies confirmed the importance of aqueous-phase chemistry for the fate of DMS and its oxidation products. Aqueous-phase processes significantly reduce the yield of sulfur dioxide and increase that of methyl sulfonic acid (MSA), which is needed to close the gap between modeled and measured MSA concentrations. Finally, the simulations imply that multiphase DMS oxidation produces equal amounts of MSA and sulfate, a result that has significant implications for nss-SO₄²⁻ aerosol formation, cloud condensation nuclei concentration, and cloud albedo over oceans. Our findings show the deficiencies of parameterizations currently used in higher-scale models, which only treat gas-phase chemistry. Overall, this study shows that treatment of DMS chemistry in both gas and aqueous phases is essential to improve the accuracy of model predictions.

Gaseous sulfuric acid (H₂SO₄) and aqueous sulfate (HSO₄⁻, SO₄²⁻) contribute to the formation of new aerosol particles as well as secondary particulate matter and are, thus, important for human health and the Earth’s climate (1). Globally, anthropogenic sulfur emissions in the form of sulfur dioxide (SO₂) dominate atmospheric production of gaseous H₂SO₄ and particle-phase sulfate. However, the main natural source of sulfur is the oxidation of dimethyl sulfide (DMS) emitted by oceans (2), which is the most important precursor for non-sea salt sulfate (nss-SO₄²⁻) aerosols over the open ocean (3). Sulfate aerosols strongly influence the climate both by direct negative radiative forcing (4) and as a dominant source of cloud condensation nuclei (CCN) over the open ocean (5). Because oceans cover about 70% of Earth’s surface (6) and have generally low albedo, DMS oxidation plays a major role in influencing the natural radiative forcing of sulfate aerosols as well as cloud properties (3).

Investigations of the effect of DMS oxidation on natural sulfate aerosol concentrations and cloud and aerosol properties require an accurate, reduced DMS oxidation scheme in chemical transport models (CTMs) and global climate models (GCMs). Current parameterizations use fixed yields of SO₂ and methyl sulfonic acid (MSA) to calculate new nss-SO₄²⁻ aerosol formation (7–13). In the gas phase, SO₂ can be oxidized to gaseous H₂SO₄, which may condense on existing particles or contribute to new particle formation in low-condensation sink environments. Other loss processes for SO₂ include dry deposition or aqueous-phase reactions. MSA mainly condenses on existing particles contributing to aerosol particle mass, and it is mainly removed by dry and wet deposition. The applied parameterizations usually consider only gas-phase reactions and should be treated with caution, because the mechanism of DMS oxidation to SO₂ and MSA is not completely understood (14). Various model studies have found significantly varying yields of DMS oxidation products (15). For example, the study by von Glasow and Crutzen (5) applied different mechanistic assumptions and calculated conversion efficiencies of DMS into SO₂ of between 0.14 and 0.95 in the marine boundary layer (MBL). Predictions of sulfate aerosol formation caused by DMS oxidation and their climate impact calculated by CTMs and GCMs are, therefore, highly uncertain. The formation of other stable DMS oxidation products, mainly MSA, lowers the SO₂ yield and constitutes the main source of uncertainty to aerosol formation, because MSA production predominantly leads to growth of existing particles and suppresses new particle formation (16). Recent studies suggest that MSA can significantly assist cluster formation between H₂SO₄ and amines and thereby, contribute to new particle formation, although it is a less potent clustering agent than H₂SO₄ (17, 18). Hence, a better estimate of MSA and SO₂ yields is necessary for improved nss-SO₄²⁻ aerosol predictions in CTMs and GCMs.

More detailed mechanistic studies of the DMS oxidation mechanism are needed to overcome these uncertainties. In particular, model studies that exclude multiphase chemistry and only treat gas-phase DMS chemistry do not reproduce observed MSA aerosol concentrations (16). In this context, it is important to note that altocumulus and altostratus clouds cover more than 20% and that stratus and stratocumulus clouds cover nearly 30% of the oceans. The residence time of an air parcel in these clouds is between 3 and 4 h (19). The review of Barnes et al. (14) concluded that multiphase DMS chemistry must also be taken into account to determine the oxidation of both DMS and its oxidation products. Early investigations of multiphase DMS chemistry were limited to 10 (20) and 7 (5) aqueous-phase reactions. However, kinetic studies have revealed many other significant reactions of DMS and its oxidation products in the aqueous phase (14). Zhu et al. (21) applied a trajectory ensemble model to show the importance of aqueous-phase DMS chemistry in cloud droplets. Their multiphase mechanism was restricted to nine DMS aqueous-phase reactions and neglected the contributions of other important chemical subsystems, especially halogen chemistry and aqueous-phase chemistry in deliquesced particles. Such small aqueous-phase mechanisms in box or local model studies exclude important DMS aqueous-phase reaction pathways. Despite the above-mentioned call by Barnes et al. (14) to incorporate more detailed multiphase DMS schemes in numerical models, no additional progress in DMS multiphase chemistry modeling has been made. As a result, our understanding of DMS oxidation to MSA and SO₂ and its contribution to aerosol production and mass is up to now limited. For example, measured MSA concentrations can only be reproduced using simplified model assumptions neglecting important reaction pathways (22). Furthermore, Berresheim et al. (23) indicated that modeled gaseous H₂SO₄ to measured concentrations does not agree with field measurements in coastal regions. These authors concluded that a better understanding of sulfur trioxide (SO₃) formation from DMS, an intermediate that rapidly forms H₂SO₄ with water vapor, could help to solve this gap (23). Overall, a better mechanistic understanding of DMS oxidation to SO₂, H₂SO₄, and other stable oxidation products, especially MSA, is needed to improve model predictions on DMS-related climate impacts.

Therefore, this study investigates the role of multiphase DMS chemistry in the MBL of the open ocean. For this purpose, a comprehensive multiphase DMS chemistry mechanism, the CAPRAM DMS Module 1.0 (DM1.0), was developed and coupled to the multiphase chemistry mechanism Master Chemical Mechanism, version 3.2 (MCMv3.2)/Chemical Aqueous Phase Radical Mechanism 4.0α (CAPRAM4.0α) + CAPRAM Halogen Module 2.1 (HM2.1) (24–26). An open ocean scenario was simulated using the Spectral Aerosol Cloud Chemistry Interaction Model (SPACCIM) (27). To study the differences between multiphase DMS chemistry in clouds and deliquesced particles, a model scenario with nonpermanent clouds is applied. Compared with studies considering either cloud or noncloud conditions, the use of a nonpermanent cloud scenario allows more realistic simulations and the study of processed aerosols. The net effect of including aqueous-phase chemistry on the conversion of DMS into SO₂ and the formation of rather stable particulate products is evaluated in these two different chemical regimes (cloud-free and cloudy MBL). Finally, implications of this study on climate model predictions are discussed.

Results and Discussion

Model Simulations.

Studies with the box model SPACCIM (27) were performed to investigate multiphase DMS chemistry in the MBL. The chemical mechanism applied combines the MCMv3.2 (25, 26) for the description of the gas phase, the CAPRAM4.0α for the description of the aqueous phase, and marine chemistry described by the multiphase mechanism HM2.1 and the newly developed CAPRAM DM1.0. The HM2.1 was adapted from the CAPRAM Halogen Module 2.0 (24) for the use with the MCMv3.2. Multiphase DMS chemistry in the MBL under pristine ocean conditions was investigated based on the model scenario from the work by Bräuer et al. (24). This scenario was extended for multiphase DMS chemistry by implementing the DMS emission rate from the work by Lana et al. (28) as well as new initial concentrations and deposition rates for DMS oxidation products (Materials and Methods and SI Appendix). Other than the base run (termed as full), different sensitivity runs were performed to study the influence of various chemical subsystems on DMS product yields. All runs had identical meteorological conditions. An outline of the different sensitivity runs is given in Table 1.

Table 1.

Overview of the sensitivity runs

Model run	Specification
full	MCMv3.2, CAPRAM4.0α, HM2.1, and DM1.0
woIodine	MCMv3.2, CAPRAM4.0α, DM1.0, and HM2.1 without iodine chemistry
woHM2	MCMv3.2, CAPRAM4.0α, and DM1.0
O3	MCMv3.2, CAPRAM4.0α, HM2.1, and DM1.0; high initial concentrations of NO2, O3, and HNO3
woCloud	MCMv3.2, CAPRAM4.0α, HM2.1, and DM1.0; without cloud periods
woAqua	MCMv3.2, CAPRAM4.0α, HM2.1, and DM1.0; without aqueous-phase chemistry of DMS and its oxidation products

The DM1.0 was developed by an extensive literature study using the most recent kinetic and mechanistic data as well as the state of the art mechanism MCMv3.2. It contains 103 gas-phase reactions, five phase transfers, and 54 aqueous-phase reactions and is designed for varying atmospheric conditions ranging from a clean, remote marine atmosphere to polluted coastal conditions.

Multiphase DMS Oxidation.

A key issue in DMS chemistry is the contribution of different oxidants to its oxidation in both the gas and aqueous phases. The most important reaction pathways (those that contribute at least 5% to the total sink flux of multiphase DMS chemistry) including the mean reaction fluxes of each pathway over the whole simulation are presented for the full run in Fig. 1. It includes the main chemical processes in the gas and aqueous phases as well as phase transfer interactions. The percentage contributions outlined below generally correspond to the averages over the whole simulation time.

Fig. 1.

Depiction of mean multiphase source and sink fluxes (in 10² molecules cm⁻³ s⁻¹) of the full scenario run throughout the whole model. The temperature is 280 K during noncloud conditions and 276.5 K in clouds. Only oxidation fluxes exceeding 5% of the total flux are included. The blue area highlights aqueous-phase reactions, and stable compounds are shaded yellow. The width and color of arrows indicate the magnitude of the mass flux, and dashed arrows represent phase transfer processes. Numbers above and below the arrows give the mean fluxes, and chemical species at the beginning of the small curved arrows below and above the thicker arrows describe the main reactants.

In the gas phase, oxidation of DMS occurs by either an H-abstraction or an addition pathway. The abstraction pathway leads to a peroxyl radical in the first generation, whereas the addition pathway mainly results in DMSO formation. The addition pathway dominates over the abstraction pathway, mainly because of the reaction with BrO, which accounts for 46% of DMS oxidation averaged over the whole model run. The remaining gas-phase oxidation is mostly triggered by reaction with the chlorine atom (Cl; 3% by addition and 15% by H-abstraction) and the hydroxyl radical (OH; 9% by addition and 11% by H-abstraction). These results show the importance of DMS oxidation by halogens, which are mainly ignored in current parameterizations of CTMs and GCMs. Neglecting this chemistry will lead to inaccurate predictions of the formation of new aerosol particles, the activation of CCN, and natural radiative forcing (5, 29).

Clouds significantly lower the gas-phase concentrations of OH, Cl, and BrO (24, 30) and modify the prevailing chemistry, such that aqueous-phase reactions take over and dominate the oxidation of DMS and its oxidation products. During cloud periods, 78% of DMS (in all phases) is oxidized in the aqueous phase by ozone (O₃) to DMSO (SI Appendix, Multiphase Chemistry of DMS and Its Oxidation Products, and Figs. S4 and S5). This reaction contributes 7% to multiphase DMS oxidation over the whole model run. Although consistent with the previous simulations by von Glasow and Crutzen (5), this finding is unexpected, because less than 0.01% of DMS partitions to the aqueous phase (Fig. 2). The contribution is explained by the very fast kinetics of the O₃ reaction, so that a large flux is established because of the reactive uptake of DMS. During in-cloud periods, the overall rate of DMS degradation decreases, with the result that the gas-phase DMS concentration increases slightly.

Fig. 2.

Partitioning of DMS, DMSO, DMSO₂, MSIA, and MSA into the aqueous phase of deliquesced particles and cloud droplets throughout the second model day. The y axis is logarithmic. Gray shaded bars denote the night periods, and blue bars denote the cloud periods.

In conclusion, this study has shown that halogens contribute significantly to DMS oxidation and that, during cloud periods, the aqueous-phase oxidation of DMS by O₃ is dominant, whereas the overall degradation decreases. An overview of the fractional contributions of different oxidants in the DM1.0 to DMS oxidation in the gas and aqueous phases is given in SI Appendix, Table S13 along with fractional contributions to the oxidation of DMSO, methyl sulfinic acid (MSIA), and MSA. For the sake of completeness, the modeled evolution of the concentration–time profiles of DMS, DMSO, MSIA, and MSA along with comparisons with field measurements and mass flux analyses are discussed in detail in SI Appendix, Multiphase Chemistry of DMS and Its Oxidation Products.

Processing of the Main DMS Oxidation Products.

The additional conversion of DMS oxidation products must also be considered to determine the effect of the overall DMS oxidation on natural radiative forcing and thus, Earth’s climate. In this section, we first consider the additional processing of H-abstraction products and then, examine the reactions of DMS products of the addition pathway (Fig. 1).

The main product of the H-abstraction pathway is SO₂, whereas only small amounts of MSA and SO₃ are formed. Furthermore, accumulation of reservoir gases, especially S-methyl thioformate (CH₃SCHO), may reduce the conversion of DMS to SO₂ because of CH₃SCHO deposition to the ocean. In principle, the understanding is that SO₂ forms gaseous H₂SO₄ from OH oxidation. However, our results indicate that the thermal decay of CH₃SO₃^• into SO₃ represents the main contributor to gaseous H₂SO₄ over the open ocean. Gaseous CH₃SO₃^• is predominantly formed from gaseous CH₃SO₂^•, which is mainly generated by the isomerization of gaseous CH₃S(OO^•) (Fig. 1). In the atmosphere, SO₃ reacts rapidly with water vapor to form gaseous H₂SO₄. In total, 94% of the gaseous H₂SO₄ is formed because of thermal decay of CH₃SO₃^•. Consequently, the isomerization of gaseous CH₃S(OO^•) is very important in determining gaseous concentrations of H₂SO₄ (additional details are in SI Appendix). However, only 4% of DMS is converted into gaseous SO₃, whereas 24% of DMS is converted into SO₂ (Table 2). The strongly reduced importance of SO₂ to gaseous H₂SO₄ formation results from the presence of noon clouds, which enhance uptake processes and significantly reduce both the OH and SO₂ concentrations, thereby suppressing gaseous production of H₂SO₄. Daytime clouds, therefore, greatly reduce the significance of SO₂ as an intermediate species in the formation of gaseous H₂SO₄. Still, even in the woCloud run, in which no clouds are implemented, 80% of H₂SO₄ is formed via the thermal decomposition of CH₃SO₃^•. This result suggests that, other than the gas-phase oxidation of SO₂, the thermal decomposition of CH₃SO₃^• is a main contributor to new particle formation in the MBL in the presence of clouds. Hence, the thermal decomposition of CH₃SO₃^• may explain the gap between modeled and measured gaseous H₂SO₄ concentrations in coastal regions as it has been proposed before (23).

Table 2.

Summary of effective conversion yields of DMS into (i) SO₂ and (ii) MSA, and (iii) conversion of MSA to S(VI) in the gas and aqueous phases for all sensitivity runs

Sensitivity run	DMS→SO2 (%)	DMS→MSA (%)	MSA→S(VI) (%)
full	23.6	41.0	2.2
O3	56.5	14.9	34.4
woIodine	20.5	46.9	2.5
woHM2	30.7	29.2	3.9
woCloud	32.7	30.3	0.8
woAqua	60.3	0.9	0.1

DMSO, formed via the oxidation of DMS by OH and BrO, is a key intermediate of the addition pathway. The subsequent oxidation of DMSO strongly alters new aerosol formation in the MBL (5), because it reduces the yield of SO₂ from DMS oxidation by forming additional stable oxidation products, like dimethyl sulfone (DMSO₂) and MSA. DMSO is mostly oxidized by OH to MSIA in both the gas and aqueous phases. Oxidation by halogens is of minor importance. Fig. 2 presents the partitioning of DMSO, DMSO₂, MSIA, and MSA in deliquesced particles and cloud droplets. About 1% and 2% of DMSO and DMSO₂, respectively, partition into deliquesced particles. Because of the less effective partitioning of DMSO into aerosols (Fig. 2) compared with the more oxidized products MSIA and MSA, the aqueous-phase oxidation mainly occurs in cloud droplets. It is also noteworthy that, although the production of DMSO is suppressed in cloud droplets, its oxidation is increased (SI Appendix, Fig. S7 and Table S13). This effect is explained by the partitioning of DMSO. With the onset of cloud formation, DMSO and DMSO₂ partition almost completely into cloud droplets because of their high Henry’s Law coefficients (14).

The simulation results imply that DMSO₂ formed through the OH-addition pathway or DMSO oxidation is only a minor product of DMS oxidation in a marine environment. Therefore, DMSO₂ oxidation is not further considered in this study.

MSIA is the major DMSO oxidation product, and about 10–40% of MSIA partitions into marine aerosols. MSIA has a diurnal profile, showing stronger partitioning at night (gray shaded bars in Fig. 2). The high solubility and formation in the aqueous phase increase the importance of aqueous-phase chemistry for the MSIA fate. MSIA is almost completely degraded during daytime in the aqueous phase. Oxidation is decreased during night, explaining the diurnal partitioning profile in Fig. 2. The main oxidants are dissolved O₃, OH, and Cl₂⁻. The strongest sink flux is the reaction of MSIA and dissociated methyl sulfinic acid (MSI⁻) with O₃ (42%) to form MSA. The DM1.0 is the first mechanism that has implemented these reactions. Cl₂⁻ and OH react mainly with MSI⁻. These two oxidations have to be differentiated between oxidation in deliquesced particles and cloud droplets. The oxidation by Cl₂⁻ is dominant in deliquesced particles, whereas oxidation by OH dominates in cloud droplets (SI Appendix, Table S13). These results are clearly different from the assumption in the work by Zhu et al. (21), in which the cloud droplet Cl₂⁻ concentration was kept constant. The total contributions of Cl₂⁻ and OH to aqueous-phase MSIA oxidation are 10% and 19%, respectively. As explained in SI Appendix, Model and Mechanism Description, both reactions occur via an electron transfer reaction leading to high amounts of CH₃SO₂(O₂^•), which reacts further with MSI⁻ to form dissolved MSA and CH₃SO₃^• (31). CH₃SO₃^• mainly decomposes to CH₃ and SO₃, but small reaction fluxes with MSI⁻ occur, also forming MSA. As a result, the oxidation of MSI⁻ by Cl₂⁻ and OH leads to a net destruction of at least two MSIA molecules. Overall, the reactions of MSI⁻ with CH₃SO₂(O₂^•) and CH₃SO₃^• account for 29% of MSIA depletion. Because of these reactions, MSI⁻ oxidation by Cl₂⁻ and OH produces dissolved MSA and SO₃ in nearly equal amounts and oxidizes more MSIA/MSI⁻ than the O₃ reactions.

In total, 58% of the modeled MSA is produced by the aqueous-phase reactions of MSIA/MSI⁻ with O₃, and 40% is caused by the aqueous-phase reactions of MSI⁻ with its oxidation products. In contrast, the gas-phase formation only accounts for 2% of MSA. However, the gas-phase MSA as well as gaseous H₂SO₄ production depend strongly on the rate constant of the thermal decay of CH₃SO₃^•, which is very uncertain (details are in SI Appendix, Model and Mechanism Description). Hence, additional laboratory investigations of this thermal decay are warranted. MSA remains almost entirely in the aqueous phase throughout the whole model run (Fig. 2). Our results, therefore, indicate that only aqueous-phase oxidation of MSA is important. Because of its high stability, only 2% of MSA is oxidized further by Cl₂⁻ and dissolved OH. Hence, MSA sinks do not compete with its production, and MSA accumulates in the aerosol phase. Because the tropospheric lifetime of marine aerosols is between 1 and 7 d (32), wet and dry deposition are likely the dominant tropospheric sinks of MSA.

Overall, modeling of DMS oxidation shows that the H-abstraction pathway in the gas phase leads predominantly to SO₂ and that the thermal decay of gaseous CH₃SO₃^• is the main contributor to gaseous H₂SO₄ formation, whereas the addition pathway leads to MSA involving multiphase processes. Furthermore, DMS oxidation leads to MSA and sulfate production in nearly equal amounts. The large fraction of MSA formed through aqueous-phase processes is different from parameterizations of the DMS addition pathway currently applied in GCMs mostly implemented after the work by Chin et al. (8).

SO₂ and MSA Yields and Atmospheric Implications.

The contribution of DMS oxidation to natural radiative forcing is strongly influenced by the yields of SO₂ and MSA. Fig. 1 shows the importance of both gas- and aqueous-phase reactions in determining product yields. Multiphase chemistry causes a shift in the product distribution of DMS chemistry from SO₂ toward MSA. This result highlights the deficiencies of current parameterizations in climate models. Apart from multiphase chemistry in general, the results presented in Fig. 1 also highlight specific chemical subsystems (e.g., halogen chemistry or chemistry in cloud droplets) that strongly influence DMS conversion yields. Different sensitivity runs (Table 1) were performed to investigate the influence of these chemical subsystems on DMS conversion. Table 2 shows the results of the sensitivity runs for (i) the total yield of SO₂, (ii) the yield of total MSA from DMS, and (iii) the conversion of MSA into sulfur VI [S(VI)]. High background concentrations of O₃ and NO₂ enhance the production of SO₂ and decrease the conversion into MSA. Under these conditions, oxidation of MSA increases strongly. A similar, albeit weaker, effect appears when halogen chemistry is limited. The decreased conversion to MSA in the woHM2 run arises from the orders of magnitude lower gas-phase concentrations of BrO and Cl owing to the restricted halogen chemistry schemes in the MCMv3.2 and the CAPRAM4.0α. The higher MSA oxidation is caused by the slightly higher OH concentration. The higher OH concentration arises from reduced ozone depletion by halogens, which enhance the photolysis of O₃ and therefore, OH production. Thus, reactive Cl and bromine species strongly affect DMS oxidation and conversion efficiency into SO₂ and MSA. However, knowledge of tropospheric halogen chemistry is still quite restricted and needs additional investigation as Simpson et al. (33) have outlined in detail.

In a cloud-free MBL, the MSA yield decreases, and SO₂ and MSA are produced in equal amounts. This effect results from the higher production of Cl and BrO in a cloud-free MBL, because clouds strongly suppress halogen activation. Cl activation in particular is reduced during cloud periods (24). The H-abstraction pathway is more important under these circumstances. Overall, marine clouds shift the DMS oxidation toward a higher production of MSA. Neglecting aqueous-phase DMS chemistry (as in most GCMs) leads to an overestimation of SO₂ formation and an underestimation of MSA production. The majority of MSA is formed and resides in the aqueous phase. This is an important result, because the described aqueous-phase chemistry does not lead to the formation of new particles but rather, results in an increase of aerosol mass of already existing particles. This crucial finding implies that not considering multiphase DMS chemistry in CTMs and GCMs will strongly affect the modeled number concentration and mass of marine aerosols and thereby, modify radiative forcing and climate predictions. The supposed influences are schematically depicted in Fig. 3. Ignoring multiphase DMS chemistry could lead to an overestimation of the nss-SO₄²⁻ aerosol concentration and the number of CCNs. Accordingly, predicted cloud albedos would be higher under these conditions, and GCMs would strongly overestimate the negative natural radiative forcing by clouds.

Fig. 3.

Schematic depiction of the multiphase DMS conversion efficiency to SO₂ and MSA for (A) the sensitivity run without aqueous-phase DMS chemistry and (B) the model run including aqueous-phase DMS chemistry. The supposed changes to cloud albedo and related radiation effects are also shown. The blue circle in B represents aqueous-phase chemistry in deliquesced particles and cloud droplets by the simulation. The red plus and minus signs represent changes of the DMS conversion yields into SO₂ and MSA between the two runs. The numbers represent the modeled yields of SO₂ and MSA. The yellow arrows represent the supposed effects on incoming and reflected solar radiation. Tg, teragram.

This study shows that the role of DMS on Earth’s climate is still not well-understood, despite many global model studies. Our simulations suggest that enhanced DMS emission results in higher particulate mass but not necessarily to appreciably higher aerosol number concentrations. The model findings are also relevant to geoengineering concepts based on aerosol production via DMS generated by ocean fertilization. Previous calculations on the efficiency of this approach often assumed a total DMS to SO₂ yield of unity (34). The results of advanced multiphase modeling of DMS oxidation indicate that SO₂ production is likely to be greatly suppressed and that changes in cloud albedo will likely be weaker than expected.

Conclusions

Atmospheric multiphase chemistry simulations of a pristine ocean scenario show that the conversion of DMS to MSA is strongly underestimated when DMS aqueous-phase chemistry is omitted and correspondingly, that formation of SO₂ is overestimated. Halogen–DMS interactions are also essential for the conversion efficiency of DMS to SO₂ and MSA. The simulations show that, despite the low solubility of DMS, marine clouds can have a major influence on DMS oxidation product yields and formation and growth of new nss-SO₄²⁻ aerosols. Overall, halogen–DMS interactions and DMS aqueous-phase chemistry have a strong impact on: (i) the conversion of DMS into SO₂ and MSA, (ii) the aging of marine aerosols, (iii) the production of nss-SO₄²⁻ aerosols, and (iv) the radiative properties of marine clouds and aerosols. Hence, neglecting multiphase DMS chemistry in CTMs and GCMs greatly increases the uncertainties of model predictions. Implementation of a near-explicit multiphase DMS mechanism, such as the DM1.0, is computationally expensive. Therefore, future work will focus on the reduction of the DM1.0 for application in CTMs and parameterization development for application in GCMs.

Materials and Methods

Model Description.

Marine multiphase chemistry simulations were carried out with the box model SPACCIM [details are in the work by Wolke et al. (27)]. SPACCIM allows the complex chemical processing in clouds and deliquesced particles to be investigated by combining a detailed microphysical model with a fine-resolved particle/droplet spectrum and a complex multiphase chemistry model. The applied chemical mechanism combined the MCMv3.2, the CAPRAM4.0α, the HM2.1, and the DM1.0. In total, 10,212 species and 21,928 reactions were included. The simulations started on June 19 at 45° N and ran for 108 h with an assumed temperature of 280 K. In the model scenario, an air parcel is moved along a predefined trajectory starting at 850 hPa. The scenario includes eight cloud passages, which are achieved by lifting the air parcel from 850 to 800 hPa for about 2 h at noon and midnight, respectively. Because of the adiabatic cooling as the air parcel rises, the temperature falls to about 276.5 K. The in-cloud residence time of about 2 h was chosen based on the calculations by Pruppacher and Jaenicke (35). Noon and midnight clouds were chosen to study differences in nighttime and daytime clouds on multiphase chemistry.

Mechanism Development.

Based on the gas-phase reactions considered in the MCMv3.2, an extended multiphase DMS chemistry mechanism, the DM1.0, has been developed in this work. For this purpose, an extensive aqueous-phase chemistry mechanism was designed using the most recent kinetic and mechanistic data.

The gas-phase reaction scheme in the DM1.0 is largely based on the DMS gas-phase oxidation mechanism of the MCMv3.2. The MCMv3.2 contains one of the most detailed gas-phase DMS oxidation mechanisms and is able to reproduce experimental findings (36). However, the MCMv3.2 does not contain DMS reactions with halogen species and is also missing some reactions that are potentially important in the marine environment (e.g., reactions with the hydroperoxyl radical). These reactions can be critical for the multiphase chemistry in the MBL (5). The chemical scheme in the MCMv3.2 was extended by 23 reactions of reactive halogen compounds and 14 reactions of nonhalogen compounds. Overall, the gas-phase mechanism scheme of the DM1.0 contains 103 reactions, including nine photolysis processes.

The SPACCIM calculates phase transfer processes according to the approach by Schwartz (37) by considering the gas-phase diffusion coefficient D_g, the mass accommodation coefficient α, and the Henry’s Law coefficient H_A. These parameters were implemented in the DM1.0 for uptake of DMS, DMSO, DMSO₂, MSIA, and MSA.

The aqueous-phase mechanism of the DM1.0 contains many more intermediate steps and reaction pathways of DMS, DMSO, DMSO₂, MSIA, and MSA than any other DMS aqueous-phase mechanism. Rate constants for these reactions were chosen from recent measurements. No literature data were available for some types of reactions (such as oxygen addition at carbon-centered radicals), and they were estimated according to the treatment of this reaction kind in the CAPRAM (38). Overall, the aqueous-phase mechanism of the DM1.0 contains 49 reactions and five dissociations and represents the most detailed aqueous-phase DMS chemistry mechanism to date.

Briefly, the module described explicitly the tropospheric oxidation of DMS and its oxidation products by several oxidants (mainly OH, NO₃, Cl, ClO, Br, BrO, Cl₂⁻, and SO₄⁻) in the gas and aqueous phases. For additional information, the reader is referred to SI Appendix, Model and Mechanism Description.