Atmospheric DMS Research Articles

Evidence for Abiotic Dimethyl Sulfide in Cometary Matter

Technological progress related to astronomical observatories such as the recently launched James Webb Space Telescope (JWST) allows searching for signs of life beyond our solar system, namely, in the form of unambiguous biosignature gases in exoplanetary atmospheres. The tentative assignment of a 1σ–2.4σ spectral feature observed with JWST in the atmosphere of exoplanet K2-18b to the biosignature gas dimethyl sulfide (DMS; sum formula C2H6S) raised hopes that, although controversial, a second genesis had been found. Terrestrial atmospheric DMS is exclusively stemming from marine biological activity, and no natural abiotic source has been identified—neither on Earth nor in space. Therefore, DMS is considered a robust biosignature. Since comets possess a pristine inventory of complex organic molecules of abiotic origin, we have searched high-resolution mass spectra collected at comet 67P/Churyumov–Gerasimenko, target of the European Space Agency’s Rosetta mission, for the signatures of DMS. Previous work reported the presence of a C2H6S signal when the comet was near its equinox, but distinction of DMS from its structural isomer ethanethiol remained elusive. Here we reassess these and evaluate additional data. Based on differences in the electron ionization-induced fragmentation pattern of the two isomers, we show that DMS is significantly better compatible with the observations. Deviations between expected and observed signal intensities for DMS are <1σ, while for ethanethiol they are 2σ–4σ. The local abundance of DMS relative to methanol deduced from these data is (0.13 ± 0.04)%. Our results provide the first evidence for the existence of an abiotic synthetic pathway to DMS in pristine cometary matter and hence motivate more detailed studies of the sulfur chemistry in such matter and its analogs. Future studies need to investigate whether or not the present inference of cometary DMS could provide an abiotic source of DMS in a planetary atmosphere.

Improving Estimates of Dynamic Global Marine DMS and Implications for Aerosol Radiative Effect

AbstractDimethyl sulfide (DMS) is the predominant natural sulfur source and plays a pivotal role in regulating global climate. However, the current method for estimating seawater DMS concentrations has limitations, and the existing DMS‐induced radiative effect heavily relies on bottom‐up DMS climatologies. This study aims to improve the method for estimating seawater DMS concentrations as well as to evaluate its induced aerosol direct radiative effect (DRE) and indirect radiative effect (IRE) using a state‐of‐the‐art aerosol microphysics scheme integrated with a chemical transport model. The predicted seawater DMS concentrations based on data‐driven methods were verified with multi‐year in situ measurements, revealing a marked reduction in mean bias by over 80%. Results show that our estimates generally indicate lower seawater DMS concentrations (1.48–1.88 μmol/m3) compared to previous seawater DMS climatologies, with differences ranging from −37% to 11%, and that interannual variability in DMS concentrations is varies significantly, particularly in polar regions. The DRE and cloud‐albedo IRE induced by DMS were −0.06 and −0.19 W/m2, respectively, representing a cooling effect on radiative effect that was weaker by 31.4% and 27.0% of those derived from the commonly used bottom‐up DMS climatology. The comprehensive evaluation of the model's performance of atmospheric DMS prediction based on global‐scale observations shows a significant improvement after using our estimates. Thus, we conclude that the global DMS fluxes provided in the past are overestimated, including its resulting DMS radiative effect, which highlights the need for refining the estimation of global aerosol radiative effect to enhance the accuracy of assessing aerosol‐induced climate impacts.

Factors controlling atmospheric DMS and its oxidation products (MSA and nssSO42−) in the aerosol at Terra Nova Bay, Antarctica

Abstract. This paper presents the results of simultaneous high time-resolution measurements of biogenic aerosol (methane sulfonic acid (MSA), non-sea salt sulfate nssSO42-) with its gaseous precursor dimethylsulfide (DMS), performed at the Italian coastal base Mario Zucchelli Station (MZS) in Terra Nova Bay (MZS) during two summer campaigns (2018–2019 and 2019–2020). Data on atmospheric DMS concentration are scarce, especially in Antarctica. The DMS maximum at MZS occurs in December, one month earlier than at other Antarctic stations. The maximum of DMS concentration is connected with the phytoplanktonic senescent phase following the bloom of Phaeocystis antarctica that occurs in the polynya when sea ice opens up. The second plankton bloom occurs in January and, despite the high dimethylsufoniopropionate (DMSP) concentration in seawater, atmospheric DMS remains low, probably due to its fast biological turnover in seawater in this period. The intensity and timing of the DMS evolution during the two years suggest that only the portion of the polynya close to the sampling site produces a discernible effect on the measured DMS. The closeness to the DMS source area and the occurrence of air masses containing DMS and freshly formed oxidation products allow us to study the kinetic of biogenic aerosol formation and the reliable derivation of the branch ratio between MSA and nssSO42- from DMS oxidation that is estimated to be 0.84±0.06. Conversely, for aged air masses with low DMS content, an enrichment of nssSO42- with respect to MSA, is observed. We estimate that the mean contribution of freshly formed biogenic aerosol to PM10 is 17 % with a maximum of 56 %. The high contribution of biogenic aerosol to the total PM10 mass in summer in this area highlights the dominant role of the polynya on biogenic aerosol formation. Finally, due to the regional and year-to-year variability of DMS and related biogenic aerosol formation, we stress the need for long-term measurements of seawater and atmospheric DMS and biogenic aerosol along the Antarctic coast and in the Southern Ocean.

Rate of non-linearity in DMS aerosol-cloud-climate interactions

Abstract. The degree of non-linearity in DMS-cloud-climate interactions is assessed using the ECHAM5-HAMMOZ model by taking into account end-to-end aerosol chemistry-cloud microphysics link. The evaluation is made over the Southern oceans in austral summer, a region of minimal anthropogenic influence. In this study, we compare the DMS-derived changes in the aerosol and cloud microphysical properties between a baseline simulation with the ocean DMS emissions from a prescribed climatology, and a scenario where the DMS emissions are doubled. Our results show that doubling the DMS emissions in the current climate results in a non-linear response in atmospheric DMS burden and subsequently, in SO2 and H2SO4 burdens due to inadequate OH oxidation. The aerosol optical depth increases by only ~20 % in the 30° S–75° S belt in the SH summer months. This increases the vertically integrated cloud droplet number concentrations (CDNC) by 25 %. Since the vertically integrated liquid water vapor is constant in our model simulations, an increase in CDNC leads to a reduction in cloud droplet radius of 3.4 % over the Southern oceans in summer. The equivalent increase in cloud liquid water path is 10.7 %. The above changes in cloud microphysical properties result in a change in global annual mean radiative forcing at the TOA of −1.4 W m−2. The results suggest that the DMS-cloud microphysics link is highly non-linear. This has implications for future studies investigating the DMS-cloud climate feedbacks in a warming world and for studies evaluating geoengineering options to counteract warming by modulating low level marine clouds.

Closing the dimethyl sulfide budget in the tropical marine boundary layer during the Pacific Atmospheric Sulfur Experiment

Abstract. Fourteen research flights were conducted with the National Center for Atmospheric Research (NCAR) C-130 near Christmas Island (2° N, 157° W) during the summer of 2007 as part of the Pacific Atmospheric Sulfur Experiment (PASE). In order to tightly constrain the scalar budget of DMS, vertical eddy fluxes were measured at various levels in the marine boundary layer (MBL) from ~30 m to the top of the mixed layer (~500 m) providing improved accuracy of the flux divergence calculation in the DMS budget. The observed mean mole fraction of DMS in the MBL exhibited the well-known diurnal cycle, ranging from 50–95 pptv in the daytime to 90–110 pptv at night. Contributions from horizontal advection are included using a multivariate regression of all DMS flight data within the MBL to estimate the mean gradients and trends. With this technique we can use the residual term in the DMS budget as an estimate of overall photochemical oxidation. Error analysis of the various terms in the DMS budget indicate that chemical losses acting on time scales of up to 110 h can be inferred with this technique. On average, photochemistry accounted for ~7.4 ppt hr −1 loss rate for the seven daytime flights, with an estimated error of 0.6 ppt hr−1. The loss rate due to expected OH oxidation is sufficient to explain the net DMS destruction without invoking the action of additional oxidants (e.g., reactive halogens.) The observed ocean flux of DMS averaged 3.1 (±1.5) μmol m−2 d−1, and generally decreased throughout the sunlit hours. Over the entire mission, the horizontal advection contribution to the overall budget was merely -0.1 ppt hr−1, indicating a mean atmospheric DMS gradient nearly perpendicular to the east-southeasterly trade winds and the chlorophyll gradient in the equatorial upwelling ocean. Nonetheless, horizontal advection was a significant term in the budget of any given flight, ranging from −1.2 to 2.5 ppt hr−1 , indicating a patchy and variable surface seawater DMS distribution, and thus needs to be accounted for in budget studies.

Analysis of a potential “solar radiation dose–dimethylsulfide–cloud condensation nuclei” link from globally mapped seasonal correlations

The CLAW postulate states that an increase in solar irradiance or in the heat flux to the ocean can trigger a biogeochemical response to counteract the associated increase in temperature and available sunlight. This natural (negative) feedback mechanism would be based on a multistep response: first, an increase in seawater dimethylsulfide concentrations (DMSw) and therefore its fluxes to the atmosphere (DMSflux); second, an increase in the atmospheric cloud condensation nuclei (CCN) burden as a consequence of DMS oxidation to form biogenic CCN (CCNbio); and third, an increase in cloud albedo due to higher CCN numbers. Monthly global climatological fields of the solar radiation dose in the upper mixed layer (SRD), surface oceanic DMSw, model outputs of hydroxyl radical concentrations (OH), and satellite‐derived CCN numbers (CCNs) are analyzed in order to evaluate the proposed “solar radiation dose‐DMS‐CCN” link from a global point of view. OH is included as the main atmospheric oxidant of the estimated DMSflux to produce CCNbio. Global maps of seasonal correlations between the variables show that the solar radiation dose is highly (positively) correlated with seawater dimethylsulfide over most of the global ocean and that atmospheric DMS oxidation is highly (positively) correlated with CCNs over large regions. These couplings are stronger at high latitudes, whereas the regions with negative or no correlation are located at low latitudes around the equator. However, CCNbio estimates for 15 regions of the global ocean show that DMS oxidation can be an important contributor to the CCNs burden only over pollution‐free regions, while it would have a minor contribution over regions with high loads of continental aerosols. Globally, the mean annual contribution of CCNbio to total CCNs is estimated to be ≈30%. Our results support that an oceanic biogenic mechanism that modulates cloud formation and albedo can indeed occur, although its impact seems rather weak over regions under a strong influence of continental aerosols. Nevertheless, our approach does not fully rule out that the observed correlations are due to an independent seasonal variation of the studied variables; seasonal couplings are necessary but not sufficient conditions to prove the CLAW hypothesis.

Effects of aqueous organosulfur chemistry on particulate methanesulfonate to non–sea salt sulfate ratios in the marine atmosphere

The oxidation of dimethyl sulfide (DMS, CH3SCH3) in the atmosphere could influence climate by affecting cloud condensation nuclei concentrations and cloud properties. This work focuses on elucidating the importance of DMS‐cloud interactions, especially the poorly understood aqueous phase chemical transformations of DMS oxidation products. For this purpose, we incorporate an oxidation mechanism of atmospheric DMS and its products within the modeling framework of a trajectory ensemble model (TEM). Both marine cumulus and stratocumulus clouds are considered. It is found that the aqueous phase reactions of sulfur compounds contribute &gt;97% of methanesulfonate (MS, CH3(O)S(O)O−) and &gt;80% of non–sea salt sulfate (NSS) production in particles and that about 30% of total MS and NSS production is from the aqueous phase oxidation of the organosulfur compounds. The aqueous phase methanesulfinate (MSI, CH3S(O)O−) + Cl2− reaction is found to be more important than MSI + OH as an MS source. The MS + OH reaction could consume almost 20% of MS and produce about 8% of total NSS within 3 days under typical marine atmospheric conditions.

Comparison of modeled versus measured MSA:nss SO4= ratios: A global analysis

The MSA:nss SO4= ratio, which is a measure of the relative marine biogenic contribution to the total atmospheric sulphur burden, has long been measured in various parts of the globe. Transect studies and observations from a network of stations have provided some idea of the spatial and temporal behavior of the ratio in various regions, but gaps in knowledge still exist in other parts of the globe. Here we present results of a global 3‐D chemical transport modeling study which complement these measurements and provide a globe‐wide picture of the spatial variation and distribution of this ratio. Comparison of modeled versus measured data on the MSA:nss SO4= ratio resulting from all sulphur sources considered shows fair model performance (i.e., a general overestimation of 23%; degrees of freedom = 90) in all areas of the globe where actual measurements of the ratio have been made. On the other hand, the model‐observation comparisons for the MSA:nss SO4= ratio derived solely from the oceanic DMS source are not as satisfactory (an overall overestimation of a factor of 3; degrees of freedom = 50). The MSA:nss SO4= ratio that is derived from the oceanic DMS source alone provides information on the relative yields of MSA and SO4= from atmospheric DMS oxidation. Our model results are consistent with measurements, showing that the ratio is highest around the polar regions and lowest within the tropics. This spatial trend is attributed to the fact that MSA production occurs best under low temperatures (maximum ambient temperature of 27°C). Despite MSA being preferably produced under low temperatures, observations at high latitudes have consistently shown summer maxima and winter minima in the MSA:nss SO4= ratio. This has raised many questions on the robustness of the theory of the MSA production mechanism. Diminished marine biological activity and low seawater DMS conditions in winter have widely been cited as the cause of this observed trend. In this study, we further propose that since photochemical hydroxyl radical (OH) production during the dark winter months at polar latitudes is non‐existent, reduced wintertime oxidation of DMS by OH to form MSA results in summer maxima and winter minima in MSA concentrations at these latitudes. Temperature and marine biological activity are, therefore, not the only major determining factors for MSA production at high latitudes on a seasonal scale. Light conditions are also important. Throughout the year, the highest ratios occur in the Southern Hemisphere, where the atmospheric DMS burden is highest. This is in agreement with both short‐ and long‐term measurements in literature.

The contribution of ocean‐leaving DMS to the global atmospheric burdens of DMS, MSA, SO2, and NSS SO4=

The contribution of ocean‐derived DMS to the atmospheric burdens of a variety of sulphur compounds (DMS, MSA, SO2, and nss SO4=) is quantified from season to season. Such quantification, especially for nss SO4= (the climate‐relevant product of DMS oxidation), is essential for the quantification of the radiative forcing of climate that may be attributable to marine phytoplankton under possible future climate conditions. Three‐dimensional chemical transport modeling up to the stratosphere is used as a tool in realizing this aim. Global data sets on oceanic and terrestrial sulphur sources are used as input. We find that the contribution of ocean‐leaving DMS to the global annually averaged column burdens of the modeled compounds is considerable: 11.9 μmol m−2 (98% of total global burden) for DMS; 0.95 μmol m−2 (94% of total global burden) for MSA; 2.8 μmol m−2 (32% of total global burden) for SO2 and 2.5 μmol m−2 (18% of total global burden) for nss SO4=. The mean annual contribution of DMS to the climate‐relevant nss SO4= column burden is greatest in the relatively pristine Southern Hemisphere, where it is estimated at 43%. This contribution is only 9% in the Northern Hemisphere, where anthropogenic sulphur sources are overwhelming. The marine algal‐derived input of the other modeled sulphur compounds (DMS, MSA, and SO2) is also greatest in the Southern Hemisphere where a lower oxidative capacity of the atmosphere, a larger sea‐to‐air transfer of DMS and a larger emission surface area lead to an elevation of the atmospheric DMS burden.

Carbon dioxide flux techniques performed during GasEx-98

A comprehensive study of air–sea interactions focused on improving the quantification of CO2 fluxes and gas transfer velocities was performed within a large open ocean CO2 sink region in the North Atlantic. This study, GasEx-98, included shipboard measurements of direct covariance CO2 fluxes, atmospheric CO2 profiles, atmospheric DMS profiles, water column mass balances of CO2, and measurements of deliberate SF6–3He tracers, along with air–sea momentum, heat, and water vapor fluxes. The large air–sea differences in partial pressure of CO2 caused by a springtime algal bloom provided high signals for accurate CO2 flux measurements. Measurements were performed over a wind speed range of 1–16 m s−1 during the three-week process study. This first comparison between the novel air-side and more conventional water column measurements of air–sea gas transfer show a general agreement between independent air–sea gas flux techniques. These new advances in open ocean air–sea gas flux measurements demonstrate the progress in the ability to quantify air–sea CO2 fluxes on short time scales. This capability will help improve the understanding of processes controlling the air–sea fluxes, which in turn will improve our ability to make regional and global CO2 flux estimates.

Physico‐chemical modeling of the First Aerosol Characterization Experiment (ACE 1) Lagrangian B: 2. DMS emission, transport and oxidation at the mesoscale

A three‐dimensional mesoscale meteorological model was used to study the interplay between the dynamical (turbulent mixing and advection) and physico‐chemical (sea‐air flux and photochemical sink by OH) processes that control dimethylsulfide DMS concentrations and their distribution in the marine boundary layer (MBL) during the First Aerosol Characterization Experiment ACE 1. Atmospheric DMS concentrations were constrained using observed seawater DMS concentrations and box model derived OH concentrations. Lateral boundary values of dynamical parameters were derived from the 6‐hourly meteorological analysis of the European Centre for Medium‐Range Weather Forecasts. Calculated DMS concentrations, wind speed and direction, and cloud cover were compared with measurements made aboard the R/V Discoverer and on the three NCAR/C130 aircraft flights during the LagB experiment. Model‐generated atmospheric DMS concentrations agreed with the DMS observations from the NCAR/C130 aircraft flights during the LagB experiment (R2 = 0.69) assuming OH is the only oxidant and DMS flux parameterization based on Liss and Merlivat [1986]. Comparison with Eulerian measurements made aboard the R/V Discoverer showed that the model simulated the range of observed values but not the hour‐to‐hour variation observed in the atmospheric DMS concentrations. Part of the discrepancy was attributed to uncertainties in DMS sea‐to‐air transfer velocity, small scale features of seawater DMS that are beyond the model resolution, and uncertainties in the venting of the boundary layer by shallow clouds. A quantitative budget at the ship location revealed a strong impact of advection processes in determining DMS levels and temporal evolution. The three‐dimensional mesoscale meteorological model was also used to estimate the effect of the low spatial resolution used in global models on seawater DMS concentrations and atmospheric OH concentrations.

Physico‐chemical modeling of the First Aerosol Characterization Experiment (ACE 1) Lagrangian B: 1. A moving column approach

During Lagrangian experiment B (LB in the following) of the First Aerosol Characterization Experiment (ACE 1), a clean maritime air mass was followed over a period of 28 hours. During that time span, the vertical distribution of aerosols and their gas phase precursors were characterized by a total of nine aircraft soundings which were performed during three research flights that followed the trajectory of a set of marked tetroons. The objective of this paper is to study the time evolution of gas phase photochemistry in this Lagrangian framework. A box model approach to the wind shear driven and vertically stratified boundary layer is questionable, since its basic assumption of instantaneous turbulent mixing of the entire air column is not satisfied here. To overcome this obstacle, a one‐dimensional Lagrangian boundary layer meteorological model with coupled gas phase photochemistry is used. To our knowledge, this is the first time that such a model is applied to a Lagrangian experiment and that enough measurements are available to fully constrain the simulations. A major part of this paper is devoted to the question of to what degree our model is able to reproduce the time evolution and the vertical distribution of the observed species. Comparison with observations of O3, OH, H2O2, CH3OOH, DMS, and CH3I, made on the nine Lagrangian aircraft soundings shows that this is in general the case, although the dynamical simulation started to deviate from the observations on the last Lagrangian flight. In agreement with experimental findings reported by Q. Wang et al. (unpublished manuscript, 1998b), generation of turbulence in the model appears to be most sensitive to the imposed sea surface temperature. Concerning the different modeled and observed chemical species, a number of conclusions are drawn: (1) Ozone, having a relatively long photochemical lifetime in the clean marine boundary layer, is found to be controlled by vertical transport processes, in particular synoptic‐scale subsidence or ascent. (2) Starting with initally constant vertical profiles, the model is able to “create” qualitatively the vertical structure of the observed peroxides. (3) OH concentrations are in agreement with observations, both on cloudy and noncloudy days. On the first flight, a layer of dry ozone rich air topped the boundary layer. The model predicts a minimum in OH and peroxides at that altitude consistent with observations. (4) Atmospheric DMS concentrations are modeled correctly only when using the Liss and Merlivat [1986] flux parameterization, the Wanninkhof [1992] flux parameterization giving values twice those observed. To arrive at this conclusion, OH is assumed to be the major DMS oxidant, but no assumptions about mixing heights or entrainment rates are necessary in this type of model. DMS seawater concentrations are constrained by observations.

Dimethyl sulfide, methane sulfonic acid and physicochemical aerosol properties in Atlantic air from the United Kingdom to Halley Bay

The concentrations of dimethyl sulfide in air were obtained during a cruise between the United Kingdom and the Antarctic in the period October 1992 to January 1993 using a method of sampling and analysis optimized to avoid interferences from oxidants. In equatorial regions (30°N to 30°S) the atmospheric DMS concentration ranged from 3 to 46 ng (S) m−3, with an average of 18 ng (S) m−3. In the polar waters and regions south of the Falkland Islands, concentrations from 3 to 714 ng (S) m−3 were observed, with a mean concentration of 73 ng (S) m−3. The concentrations of a range of DMS oxidation products were also obtained. No clear relationships between reactant and product concentrations were seen. Information on particle number concentration, Fuchs surface area and the thermal volatility characteristics of the ambient aerosol was obtained, but again no clear relationships with sulfur concentrations were observed. Accumulation mode particle concentrations averaged 25 cm−3 in the clean marine and polar air masses south of 58°S while background condensation nuclei (CN) concentrations were of the order of 400–600 cm−3. Simplistic calculations suggest that a particle source strength of about 20–60 particles cm−3 d−1 is required to sustain this background CN concentration. It is not clear whether boundary layer nucleation of new CN or entrainment from the free troposphere provided the source of CN. Periods of elevated CN concentrations (&gt;4000 cm−3) were regularly observed in the boundary layer over the Weddell Sea and were attributed to “bursts” of new particle formation. However, shortly after these nucleation events the CN concentration rapidly decayed to the background level through coagulation losses, suggesting little impact on the background CN or cloud condensation nuclei (CCN) concentration.

Dimethyl sulfide and its oxidation products in the atmosphere of the Atlantic and Southern Oceans

Dimethyl sulfide, methane sulfonate, non-sea-salt sulfate and sulfur dioxide concentrations in air were obtained during a cruise between the U.K. and the Antarctic during the period October 1992–January 1993. In equatorial regions (30°N to 30°S) the atmospheric DMS concentration ranged from 3 to 46 ng (S)m −3 with an average of 18 ng(S)m −3. In the polar waters and regions south of the Falkland Islands concentrations from 3 to 714ng(S)m −3 were observed with a mean concentration of 73ng(S)m −3. Methane sulfonate concentrations were also enhanced in the vicinity of the Antarctic Peninsula and in the Weddell Sea. A simple model of DMS oxidation was used to estimate the ocean to atmosphere flux rate, and this was found to be within the range of previous estimates, with a mean value of 1011 ng(S) m −2 h −1.

Atmospheric sulfur cycling in the tropical Pacific marine boundary layer (12°S, 135°W): A comparison of field data and model results: 1. Dimethylsulfide

Shipboard measurements of atmospheric and seawater DMS were made at 12°S, 135°W for 6 days during March 1992. The mean seawater DMS concentration during this period was 4.1 ± 0.45 nM (1σ, n = 260) and the mean atmospheric DMS mole fraction was 453 ± 93 pmol mol−1 (1σ, n = 843). Consistent atmospheric diel cycles were observed, with a nighttime maximum and daytime minimum and an amplitude of approximately 85 pmol mol−1. Photochemical box model calculations were made to test the sensitivity of atmospheric DMS concentrations to the following parameters: 1) sea‐to‐air flux, 2) boundary layer height, 3) oxidation rate, and 4) vertical entrainment velocities. The observed relationship between the mean oceanic and atmospheric DMS levels require the use of an air‐sea exchange coefficient which is at the upper limit end of the range of commonly used parameterizations. The amplitude of the diel cycle in atmospheric DMS is significantly larger than that predicted by a photochemical model. This suggests that the sea‐to‐air DMS flux is higher than was previously thought, and the rate of daytime oxidation of DMS is substantially underestimated by current photochemical models of DMS oxidation.

Seasonal and short-term variability in dimethyl sulfide, sulfur dioxide and biogenic sulfur and sea salt aerosol particles in the arctic marine boundary layer during summer and autumn

The International Arctic Ocean Expedition (IAOE), lasting from August to mid-October 1991, provided a unique opportunity to characterize and quantify relationships within the natural sulfur cycle in the marine boundary layer under conditions of limited anthropogenic influence. Concentrations of dimethyl sulfide, sulfur dioxide, and submicrometer aerosol concentrations of methane sulfonate, non-sea-salt sulfate, ammonium and elemental carbon ranged from 17 to 0.05 nmol m3 (380–1 ppt(v)), 1.7–0.04 nmol m3 (38–0.9 ppt(v)), 1.4–0.002 nmol m-3 (31–0.05 ppt(v), 6.9–0.03 nmol m-3 (155–0.7 ppt(v)), 3.9–0.03 nmol m-3 (90–0.7 ppt(v)) and 0.51–0.009 nmol m-3 (11–0.2 ppt(v)), respectively. Observations showed a seasonal variation of concentrations with highest values for all the marine biogenic sulfur gas-aerosol parametres along the ice edge zone in August and lowest values over the pack ice in late September On average concentrations fell with a decline rate of about 20–40% per week. A similar seasonal change was also reflected in particulate ammonium. This therefore indicates links between the different sulfur compounds as well as between the biogenic sulfur and nitrogen cycles. Concentrations over the pack ice region were generally lower than over the open waters at the ice edge with an estimated net loss rate of roughly 35% per day of transport over the pack ice. Contrary to earlier marine sulfur studies performed outside the Arctic region, a constant methane sulfonate to non-sea-salt sulfate molar ratio was found in the submicrometer size fraction for samples with a minimal influence from fog and anthropogenic sources. This ratio had a value of 0.22 in spite of large seasonal changes in temperature and concentrations of methane sulfonate and non-sea-salt sulfate. Thus we conclude that the sum of the proceses controlling the measured particle properties do not exhibit a net temperature dependence. The one to one molar ratio of ammonium to non-sea-salt sulfate indicated a partly neutralised ammoniated sulfate aerosol. This was further verified by single particle analysis. Measurements of non-sea-salt sulfate and ammonium revealed a bimodal size distribution with about 70% of their mass found in the submicrometer size fraction. Methane sulfonate was mainly associated with submicrometer particles, with less than 8% of the mass observed in the largest particles. We have also shown that the interchange of air between the surface mixed layer and clouds, caused by atmospheric wave motions, dominated the short time variations in atmospheric DMS and submicrometer aerosol concentrations. This interchange will have a strong influence on the chemical and physical processes that control the properties of the aerosol, and deserves more attention in future work.

Independent marine and atmospheric model estimates of the sea‐air flux of dimethylsulfide in the Southern Ocean

Marine and atmospheric models have been combined with data collected at the Cape Grim atmospheric baseline station, to estimate the flux of dimethylsulfide to the atmosphere during the spring‐summer period in the Subantarctic Southern Ocean in the vicinity of the Cape Grim baseline atmospheric monitoring station. The marine model predicts that production of phytoplankton and of dissolved DMS will increase during spring to reach a maximum in summer consistent with the data on atmospheric DMS. Local wind and sea temperature data have been used to calculate the DMS transfer velocity which was used to compute the sea‐to‐air flux of DMS. Independent predictions of the DMS flux using an atmospheric model and Cape Grim data are in excellent agreement with the marine based prediction.

Relation between the concentrations of DMS in surface seawater and air in the temperate North Pacific region

The concentrations of DMS were simultaneously measured in both water and air at the sea surface on board a vessel during a trans-Pacific cruise around 40° N in August 1988. Those in the surface seawater varied widely with a mean of 162 ng S/1 and a standard deviation of 134 ng S/1 (n=37), but the variation was not a mere fluctuation and the high concentration (376 ng S/1) was found in the area between 145° W and 170° W. The atmospheric DMS concentration varied more widely with a mean value of 177 ng S/m3 and a standard deviation of 203 ng S/m3 (n=23). The diurnal variation of DMS was not significant in the air near the sea surface. However, the concentrations in the surface water was fairly well correlated with those in the surface air. The correlation coefficient (r2=0.86) was larger than that between the atmospheric concentration and outflux of DMS (r2=0.64). These findings mean that the turnover time of DMS in the atmosphere is not extremely short. Based on the linear relation between the atmospheric and seawater DMS, the turnover time of the atmospheric DMS has been calculated to be 0.9 days with an uncertainty of around 50%. The oxidation rate agrees fairly well with that expected from the OH radical concentration in the marine atmosphere.

Seasonal variation of methanesulfonic acid in precipitation at Amsterdam island in the southern Indian Ocean

A 29-month record of methanesulfonate (MSA) concentration in 103 rainwater samples has been performed at Amsterdam Island in the southern Indian Ocean. Rain water MSA concentrations range from 0.008 to 1.150 μeq l −1 with a mean value of 0.187 ± 0.054 μeq l −1. A strong seasonal variation in rain water MSA concentration was found with a minimum in winter and a maximum in summer, similar to that observed for atmospheric DMS concentrations measured during the same period. The annual average MSA wet deposition during the studied period was 0.51 μeq m −2 d −1 which represents roughly 20% of the annual average DMS flux.

Seasonal variations of atmospheric sulfur dioxide and dimethylsulfide concentrations at Amsterdam Island in the southern Indian Ocean

Daily measurements of atmospheric sulfur dioxide (SO2) concentrations were performed from March 1989 to January 1991 at Amsterdam Island (37°50′ S–77°30′ E), a remote site located in the southern Indian Ocean. Long-range transport of continental air masses was studied using Radon (222Rn) as continental tracer. Average monthly SO2 concentrations range from less than 0.2 to 3.9 nmol m-3 (annual average = 0.7 nmol m-3) and present a seasonal cycle with a minimum in winter and a maximum in summer, similar to that described for atmospheric DMS concentrations measured during the same period. Clear diel correlation between atmospheric DMS and SO2 concentrations is also observed during summer. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces the seasonal variations in the measured atmospheric SO2 concentrations within ±30%. Comparing between computed and measured SO2 concentrations allowed us to estimate a yield of SO2 from DMS oxidation of about 70%.