Dimethylsulfide Flux Research Articles

Influence of explicit Phaeocystis parameterizations on the global distribution of marine dimethyl sulfide

AbstractDimethyl sulfide (DMS) is a biogenic organosulfur compound which contributes strongly to marine aerosol mass and the determination of cloud condensation nuclei over the remote oceans. Since uncertainties in DMS flux to the atmosphere lead to large variations in climate forcing, the global DMS distribution has been the subject of increasingly complex dynamic simulations. DMS concentrations are directly controlled by marine ecosystems. Phaeocystis is a major DMS producer but is often omitted from global reduced sulfur mechanisms. Here we incorporate this phytoplankton group into the marine ecosystem‐biogeochemical module of the Community Earth System Model. To examine its role in the ocean sulfur cycle, an earlier DMS model has been enhanced to include new knowledge gained over the last few years. Results from the baseline run show that simulated Phaeocystis biomass generally agrees with observations, with high concentrations near the Antarctic continent and between 50° and 60° north. Given the new explicit Phaeocystis representation, the DMS distribution shows significant improvements, especially regarding the amplitude and location of high‐latitude peaks. The simulated global mean surface DMS value is 2.26 nM, comparable to an estimate of 2.34 nM from the latest climatology extrapolated based on observations. The total oceanic DMS source to the atmosphere is 20.4 Tg S/yr, on the low side of previous estimates. Comparisons with and without Phaeocystis show that the group dominates DMS distributions in temperate and cold waters, contributing 13% of the global flux. The proportion may increase as sea ice declines and should be considered in climate projections.

Variations in Concentrations and Fluxes of Dimethylsulfide (DMS) from the Indian Estuaries

In order to examine the variations in concentrations of dimethylsulfide (DMS) and its fluxes to the atmosphere, 25 major and medium estuaries from Indian subcontinent were sampled during wet and dry periods. River discharge brought substantial amount of nutrients and suspended particulate matter (SPM) to the Indian estuaries; however, the concentration of phytoplankton biomass was severely limited by latter due to shallowing of photic depth. Bacillariophyceae was the dominant phytoplankton group in the Indian estuaries followed by green algae, Cyanophyceae, and Dinophyceae. Relatively higher concentrations of DMS were observed in the estuaries located along the east (3.6 ± 5.7 nM) than the west coast of India (0.8 ± 0.3 nM) during wet period whereas no significant differences were observed during dry period. The concentrations of DMS were significantly lower during wet than dry period and it was consistent with the phytoplankton biomass. The slope of the relation between DMS and phytoplankton biomass displayed a significant spatial variation due to contribution of different groups of phytoplankton in the Indian estuaries. The concentrations of DMS in the Indian estuaries were higher than other estuaries in the world except some Chinese estuaries. The annual mean flux (1.95 ± 2.5 μmol m−2 day−1) from the Indian estuaries is lower than that of other estuaries in the world, except Pearl River estuary due to inhibition of phytoplankton growth by suspended load and low flushing rates.

Evaluation of diverse approaches for estimating sea-surface DMS concentration and air–sea exchange at global scale

Environmental context As climate models increasingly include detailed, process-based models of aerosol formation, they need to represent dimethylsulfide emissions from the ocean. Options for this include data-based climatologies and empirical or process-based models; there are diverse examples of each in the literature. This paper presents the first global-scale comparison of all available approaches and evaluation of their skill relative to observations and their possible roles in future climate models. Abstract Ocean emission and subsequent oxidation of dimethylsulfide (DMS) provides a source of sulfate in the atmosphere, potentially affecting the amount of solar radiation reaching the Earth’s surface through both direct and indirect radiative effects of sulfate aerosols. DMS concentration in the ocean is quite variable with season and location, which in turn leads to high spatial and temporal variability of ocean DMS emissions. This study tested currently available climatologies and empirical and prognostic models of DMS concentration in the surface ocean against each other and against observations. This analysis mainly reveals the limitations of estimating DMS with an empirical model based on variables such as chlorophyll and mixed-layer depth. The various empirical models show very different spatial patterns, and none correlate strongly with observations. There is considerable uncertainty in the spatial and temporal distribution of DMS concentration and flux, and in the global total DMS flux. Global total air–sea flux depends primarily on global mean surface ocean DMS concentration, and the spatial distribution of DMS concentration and the magnitude of the gas exchange coefficient are of secondary importance. Global total flux estimates range from 9 to 34TgSyear–1, with a best estimate of 18–24 Tg. Both empirical and prognostic models generally underestimate the total compared with the best available data-based estimates.

Biogeochemistry of dimethylsulfoniopropionate, dimethylsulfide and acrylic acid in the Yellow Sea and the Bohai Sea during autumn

Environmental context Dimethylsulfide (DMS) is a climatically important biogenic trace gas that is emitted from oceans. This research focuses on the spatiotemporal distributions of DMS and its related compounds, i.e. dimethylsulfoniopropionate (DMSP) and acrylic acid (AA), and the influencing factors in the Yellow Sea and the Bohai Sea during autumn. In addition, the sea-to-air flux of DMS, kinetic responses of DMSP consumption as well as DMS and AA production are also investigated. This study is helpful in understanding the marine sulfur cycle in marginal seas in China. Abstract The biogeochemistry of dimethylsulfoniopropionate (DMSP), dimethylsulfide (DMS) and acrylic acid (AA) in the Yellow Sea (YS) and the Bohai Sea (BS) was investigated in November 2013. The concentrations (and ranges) of total DMSP (DMSPt), dissolved DMSP (DMSPd), DMS and AA in surface waters were 30.71 (1.07–122.50), 6.60 (0.85–35.67), 1.48 (0.53–5.32) and 42.2 (13.8–352.8) nmol L–1 respectively. The concentrations of DMSPd and AA were positively correlated with chlorophyll-a levels, which suggests that phytoplankton biomass has an important function in controlling DMSPd and AA distributions. Furthermore, DMS and AA concentrations revealed significant positive relationships with DMSPd concentrations. The average ratios of AA/(DMSP+AA) and DMS/AA were 53.98 and 7.62% respectively. The vertical profiles of DMSP, DMS and AA were characterised by high concentrations that mostly occur near the surface. Even under highly variable hydrographic conditions, a positive relationship was observed between DMSPt and chlorophyll-a concentrations. The rates of DMSPd consumption, as well as DMS and AA production, significantly varied with marine environments. The sea-to-air fluxes of DMS from the YS and the BS to the atmosphere were estimated to be in the range of 3.01 to 6.91μmol m–2day–1.

Dimethylsulfide model calibration in the Barents Sea using a genetic algorithm and neural network

Environmental context Future changes in marine biogenic aerosol emissions in Arctic seas are likely to affect the radiative budget of the region. Here we employ a calibrated biogeochemical model to simulate change in sulfate aerosol emissions in the Barents Sea, and find strong increases occur by the late 21st century. If replicated across the Arctic Ocean, such increases in sulfate aerosol loading to the Arctic atmosphere may decrease the rate of warming at polar latitudes. Abstract Global warming of climate is connected to ecosystem change, especially in the polar oceans. Biogenic emissions of dimethylsulfide (DMS) are the main biogenic source of sulfate aerosols to the marine atmosphere and may change in the Arctic, where warming is currently very rapid. Here, we simulate DMS distribution and sea-to-air flux in the Barents Sea (30–40°E and 70–80°N) for the period 2003–05. A genetic algorithm is used to calibrate the key parameters in the DMS model. We use MODIS satellite chlorophyll-a data and regional DMS field data to calibrate the model. Owing to limited DMS observations in the Arctic Ocean, multiple data sources were used and compared. A back-propagation neural network is used for predicting regional DMS based on previous history time series. Parameter sensitivity analysis is done based on DMS flux output. Global climate model forcings for 1×CO2 to 3×CO2 conditions are used to force the biogeochemical model under future climate warming (c. 2080). The simulation results show that under tripled CO2, DMS flux would increase 168 to 279% from autumn through winter and would increase 112% during ice melting season. DMS flux would increase much more in ice-melt-affected water. The increased DMS flux under 3×CO2 indicates that regional warming could slow owing to the emission of DMS in the Arctic, if the increase in emissions of anthropogenic greenhouse gases is controlled.

Impact of river discharge on distribution of dimethyl sulfide (DMS) and its fluxes in the coastal Bay of Bengal

A systematic study was conducted to examine the influence of river discharge on variations in DMS and DMSPt during peak discharge period along the coastal Bay of Bengal. The southern coastal Bay of Bengal (SCB) received a large quantity of suspended particulate matter (SPM) and nitrogen nutrients from the monsoonal estuaries and vice versa to the northern coastal Bay of Bengal (NCB) by glacial river (Ganges). The higher concentration of DMSPt was observed in the NCB compared with SCB associated with low nitrate, and low salinity suggesting that their stress on phytoplankton increased DMSPt production in the former region. The concentrations of DMS and DMSPt in the surface waters displayed a linear relation with biovolume of Bacillariophyceae and Dinophyceae suggesting significant contributions by these two groups. In contrast, the weak spatial variability in DMS was attributed to removal of DMS to the atmosphere or bacterial consumption. The mean sea-to-air flux of DMS amounted to 5.5±4μmolSm−2d−1 suggesting that the coastal Bay of Bengal is a significant source to atmospheric DMS during study period. This study revealed that river discharge brought a significant modification in the chemical characteristics of the coastal Bay of Bengal leading to variable stress (salinity and nutrients) on phytoplankton resulting in a significant flux of DMS to the atmosphere.

Biological production and spatial variation of dimethylated sulfur compounds and their relation with plankton in the North Yellow Sea

The concentrations of dimethylated sulfur compounds and chlorophyll a, as well as biological production and consumption rates of dimethylsulfide (DMS), were measured in the surface water of the North Yellow Sea (NYS, 37–40°N to 121–124°E) in winter 2007. Surface DMS, dissolved and particulate dimethylsulfoniopropionate (DMSPd and DMSPp) concentrations in the study area increased significantly from offshore to inshore sites, with the average values of 2.00, 4.52 and 7.21nM, respectively. The biological production and consumption rates of DMS were estimated, with the average values of 5.41 and 3.84nMd−1, respectively. The spatial variation of chlorophyll a was consistent with that of DMS and DMSP, as well as with that of DMS biological production, suggesting that phytoplankton biomass might play an important role in controlling the distribution of DMS and DMSP in the study area. According to the collective data of dimethylated sulfur compounds and DMS biological conversion in China Seas, the dimethylated sulfur compounds concentrations in the NYS during winter were a factor of 2 and 1.3 higher than those in the East China Sea and South China Sea, respectively. Less DMS (DMSP) in NYS was released in winter than that in spring and summer, which could be attributed to the shift in phytoplankton community composition dominated by diatoms to non-diatoms from winter to summer. Quantitative comparison analysis pointed to DMSPp rather than DMSPd as an important precursor of DMS in the surface water. The estimated sea-to-air fluxes of DMS using Liss and Merlivat (LM86), Wanninkhof (W92) and Nightingale (N2000) formulae were 2.72, 5.12 and 4.28μmolm−2d−1, respectively. In the surface water, the biological turnover time of DMS varied from 0.21 to 1.73d with an average of 0.83d, which was about 5.43-fold faster than the mean DMS sea–air turnover time (3.12d), implying that microbial consumption was a main sink of DMS in the surface water.

Spatial distribution of dimethylsulfide and dimethylsulfoniopropionate in the Yellow Sea and Bohai Sea during summer

The distributions of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) in surface water of the Yellow Sea and the Bohai Sea were studied during June 2011. The mean concentrations and ranges of DMS, dissolved DMSP (DMSPd), and particulate DMSP (DMSPp) in surface waters were 6.85 (1.60-12.36), 7.25 (2.28-19.05) and 61.87 (6.28-224.01) nmol/L, respectively. There were strong correlations between DMSPp and chlorophyll a in the Bohai Sea and the North Yellow Sea, respectively, and concentrations of DMS and DMSP were high, with a relatively high proportion of dinoflagellates, in the region of the South Yellow Sea Cold Water Mass. Results show that phytoplankton biomass and species composition were important factors that controlled the distribution of DMS and DMSP. Complex environmental factors, including nutrients, transparency, and terrestrial runoff, might also influence the variability in DMS and DMSP. Biological production and consumption rates of DMS in the Bohai Sea were higher than those in the Yellow Sea. DMS production rates were closely correlated with DMSPd concentrations. DMS and DMSP exhibited obvious diel variations, with high concentrations occurring in the late afternoon (16:00-19:00) and low concentrations occurring during the night, implying that the intensity of solar radiation had a significant influence on these variations. Size distributions of chlorophyll a and DMSPp were also investigated and large nanoplankton (5-20 mu m), mainly diatoms, contributed significantly to chlorophyll a and DMSPp at most stations. The average sea-to-air flux of DMS in the study area was estimated to be 11.07 mu mol/(m(2)center dot d) during the summer.

Distribution of biogenic sulfur in the Bohai Sea and northern Yellow Sea and its contribution to atmospheric sulfate aerosol in the late fall

Spatial distributions of biogenic sulfur compounds including dimethylsulfide (DMS), dissolved and particulate dimethylsulfoniopropionate (DMSPd and DMSPp) were investigated in the Bohai Sea (BS) and northern Yellow Sea (NYS) in November 2011. DMS and DMSPp concentrations were significantly correlated with chlorophyll a (Chl-a) in the surface water and larger nanoplankton (5–20μm) was the most important producer of DMSPp in the study area. Silicate had statistically significant influence on the growth of phytoplankton and DMSP concentration. The vertical profiles of DMS and DMSP were characterized by a maximum at the upper layer and the bottom concentrations were sometimes relatively higher compared with the overlying layer. The sea-to-air fluxes of DMS in the study area were estimated to be from 0.05 to 27.4μmolm−2d−1, with a mean of 4.21μmolm−2d−1. The contribution of biogenic non-sea-salt SO42− (nss-SO42−) to total nss-SO42− in the atmospheric aerosol over the study area varied from 0.46% to 5.49% with an average of 1.40%. Therefore, the contribution of biogenic sulfur to nss-SO42− was not significant and anthropogenic source was dominant for atmospheric sulfate.

Seasonal variation and biogeochemical cycling of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in theYellowSea andBohaiSea

AbstractThe concentrations of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP) and chlorophylla(Chl‐a) as well as the size distribution of particulate DMSP (DMSPp) were determined in the Yellow Sea (YS) and Bohai Sea (BS) during two cruises from 1 to 20 November 2012 and from 21 June to 11 July 2013. Besides, the biological production and consumption rates of DMS and the degradation rates of dissolved DMSP (DMSPd) were also measured. The concentrations of DMS and DMSP showed a significant seasonal variation with higher values in summer, which corresponded well with the seasonal change of Chl‐ain the study area. Both DMS and DMSPp concentrations were significantly correlated with Chl‐alevels, implying that phytoplankton biomass might play an important role in controlling the distributions of DMS and DMSP in the study area. The size‐fractionation research showed that nanophytoplankton (5–20 μm) contributed to most of Chl‐aand DMSPp in autumn and summer. The average biological production and consumption rates of DMS during summer were 13.35 and 9.80 nmol L−1d−1, respectively, which were much higher than those during autumn. The degradation rates of DMSPd during summer ranged from 7.10 to 21.70 nmol L−1d−1, with an average of 14.71 nmol L−1d−1, which was nearly equal to the average biological production rate (13.35 nmol L−1d−1) of DMS. The average sea‐to‐air flux of DMS in the YS and BS were 12.06 and 20.81 μmol m−2d−1in autumn and in summer, respectively, which were much higher than the values reported in the ocean.

Exploiting satellite earth observation to quantify current global oceanic DMS flux and its future climate sensitivity

We used coincident Envisat RA2 and AATSR temperature and wind speed data from 2008/2009 to calculate the global net sea-air flux of dimethyl sulfide (DMS), which we estimate to be 19.6 Tg S a−1. Our monthly flux calculations are compared to open ocean eddy correlation measurements of DMS flux from 10 recent cruises, with a root mean square difference of 3.1 μmol m−2 day−1. In a sensitivity analysis, we varied temperature, salinity, surface wind speed, and aqueous DMS concentration, using fixed global changes as well as CMIP5 model output. The range of DMS flux in future climate scenarios is discussed. The CMIP5 model predicts a reduction in surface wind speed and we estimate that this will decrease the global annual sea-air flux of DMS by 22% over 25 years. Concurrent changes in temperature, salinity, and DMS concentration increase the global flux by much smaller amounts. The net effect of all CMIP5 modelled 25 year predictions was a 19% reduction in global DMS flux. 25 year DMS concentration changes had significant regional effects, some positive (Southern Ocean, North Atlantic, Northwest Pacific) and some negative (isolated regions along the Equator and in the Indian Ocean). Using satellite-detected coverage of coccolithophore blooms, our estimate of their contribution to North Atlantic DMS emissions suggests that the coccolithophores contribute only a small percentage of the North Atlantic annual flux estimate, but may be more important in the summertime and in the northeast Atlantic.

Characterizing reduced sulfur compounds emissions from a swine concentrated animal feeding operation

Reduced sulfur compounds (RSCs) emissions from concentrated animal feeding operations (CAFOs) have become a potential environmental and human health concern, as a result of changes in livestock production methods. RSC emissions were determined from a swine CAFO in North Carolina. RSC measurements were made over a period of ≈1 week from both the barn and lagoon during each of the four seasonal periods from June 2007 to April 2008. During sampling, meteorological and other environmental parameters were measured continuously. Seasonal hydrogen sulfide (H2S) barn concentrations ranged from 72 to 631 ppb. Seasonal dimethyl sulfide (DMS; CH3SCH3) and dimethyl disulfide (DMDS; CH3S2CH3) concentrations were 2–3 orders of magnitude lower, ranging from 0.18 to 0.89 ppb and 0.47 to 1.02 ppb, respectively. The overall average barn emission rate was 3.3 g day−1 AU−1 (AU (animal unit) = 500 kg of live animal weight) for H2S, which was approximately two orders of magnitude higher than the DMS and DMDS overall average emissions rates, determined as 0.017 g day−1 AU−1 and 0.036 g day−1 AU−1, respectively. The overall average lagoon flux was 1.33 μg m−2 min−1 for H2S, which was approximately an order of magnitude higher than the overall average DMS (0.12 μg m−2 min−1) and DMDS (0.09 μg m−2 min−1) lagoon fluxes. The overall average lagoon emission for H2S (0.038 g day−1 AU−1) was also approximately an order of magnitude higher than the overall average DMS (0.0034 g day−1 AU−1) and DMDS (0.0028 g day−1 AU−1) emissions. H2S, DMS and DMDS have offensive odors and low odor thresholds. Over all four sampling seasons, 77% of 15 min averaged H2S barn concentrations were an order of magnitude above the average odor threshold. During these sampling periods, however, DMS and DMDS concentrations did not exceed their odor thresholds. The overall average barn and lagoon emissions from this study were used to help estimate barn, lagoon and total (barn + lagoon) RSC emissions from swine CAFOs in North Carolina. Total (barn + lagoon) H2S emissions from swine CAFOs in North Carolina were estimated to be 1.22*106 kg yr−1. The barns had significantly higher H2S emissions than the lagoons, contributing ≈98% of total North Carolina H2S swine CAFO emissions. Total (barn + lagoon) emissions for DMS and DMDS were 1–2 orders of magnitude lower, with barns contributing ≈86% and ≈93% of total emissions, respectively. H2S swine CAFO emissions were estimated to contribute ≈18% of North Carolina H2S emissions.

Spatial variation of biogenic sulfur in the south Yellow Sea and the East China Sea during summer and its contribution to atmospheric sulfate aerosol

Spatial distributions of biogenic sulfur compounds including dimethylsulfide (DMS), dissolved and particulate dimethylsulfoniopropionate (DMSPd and DMSPp) were investigated in the South Yellow Sea (SYS) and the East China Sea (ECS) in July 2011. The concentrations of DMS and DMSPp were significantly correlated with the levels of chlorophyll a in the surface water. Simultaneously, relatively high ratio values of DMSP/chlorophyll a and DMS/chlorophyll a occurred in the areas where the phytoplankton community was dominated by dinoflagellates. The DMSPp and chlorophyll a size-fractionation showed that larger nanoplankton (5–20μm) was the most important producer of DMSPp in the study area. The vertical profiles of DMS and DMSP were characterized by a maximum at the upper layer and the bottom concentrations were also relatively higher compared with the overlying layer of the bottom. In addition, a positive linear correlation was observed between dissolved dimethylsulfoxide (DMSOd) and DMS concentrations in the surface waters. The sea-to-air fluxes of DMS in the study area were estimated to be from 0.03 to 102.35μmolm−2d−1 with a mean of 16.73μmolm−2d−1 and the contribution of biogenic non-sea-salt SO42− (nss-SO42−) to the measured total nss-SO42− in the atmospheric aerosol over the study area varied from 1.42% to 30.98%, with an average of 8.2%.

Measurement of Air-Sea Exchange of Dimethyl Sulfide and Acetone by PTR-MS Coupled with Gradient Flux Technique

We developed a new method for in situ measurement of air-sea fluxes of multiple volatile organic compounds (VOCs) by combining proton transfer reaction-mass spectrometry (PTR-MS) and gradient flux (GF) technique. The PTR-MS/GF system was first deployed to determine the air-sea flux of VOCs in the open ocean of the western Pacific, in addition to carbon dioxide and water vapor. Each profiling at seven heights from the ocean surface up to 14 m took 7 min. In total, 34 vertical profiles of VOCs in the marine atmosphere just above the ocean surface were obtained. The vertical gradient observed was significant for dimethyl sulfide (DMS) and acetone with the best-fit curves on quasi-logarithmic relationship. The mean fluxes of DMS and acetone were 5.5 ± 1.5 and 2.7 ± 1.3 μmol/m(2)/day, respectively. These fluxes are in general in accordance with those reported by previous expeditions.

Lagrangian evolution of DMS during the Southern Ocean gas exchange experiment: The effects of vertical mixing and biological community shift

Concentrations of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) are highly variable in time and space. What is driving the variability in DMS(P), and can those variability be explained by physical processes and changes in the biological community? During the Southern Ocean Gas Exchange Experiment (SO GasEx) in the austral fall of 2008, two 3He/SF6 labeled patches were created in the surface water. SF6 and DMS were surveyed continuously in a Lagrangian framework, while direct measurements of air‐sea exchange further constrained the gas budgets. Turbulent diffusivity at the base of the mixed layer was estimated from SF6 profiles and used to calculate the vertical fluxes of DMS and nutrients. Increasing mixed layer nutrient concentrations due to mixing were associated with a shift in the phytoplankton community structure, which in turned likely affected the sulfur dynamics on timescales of days. DMS concentration as well as air‐sea DMS flux appeared to be decoupled from the DMSP concentration, possibly due to grazing and bacterial DMS production. Contrary to expectations, in an environment with high winds and modest productivity, physical processes (air‐sea exchange, photochemistry, vertical mixing) only accounted for a small fraction of DMS loss from the surface water. Among the DMS sinks, inferred biological consumption most likely dominated during SO GasEx.

Air–sea dimethylsulfide (DMS) gas transfer in the North Atlantic: evidence for limited interfacial gas exchange at high wind speed

Abstract. Shipboard measurements of eddy covariance dimethylsulfide (DMS) air–sea fluxes and seawater concentration were carried out in the North Atlantic bloom region in June/July 2011. Gas transfer coefficients (k660) show a linear dependence on mean horizontal wind speed at wind speeds up to 11 m s−1. At higher wind speeds the relationship between k660 and wind speed weakens. At high winds, measured DMS fluxes were lower than predicted based on the linear relationship between wind speed and interfacial stress extrapolated from low to intermediate wind speeds. In contrast, the transfer coefficient for sensible heat did not exhibit this effect. The apparent suppression of air–sea gas flux at higher wind speeds appears to be related to sea state, as determined from shipboard wave measurements. These observations are consistent with the idea that long waves suppress near-surface water-side turbulence, and decrease interfacial gas transfer. This effect may be more easily observed for DMS than for less soluble gases, such as CO2, because the air–sea exchange of DMS is controlled by interfacial rather than bubble-mediated gas transfer under high wind speed conditions.

Sulfur isotope homogeneity of oceanic DMSP and DMS

Oceanic emissions of volatile dimethyl sulfide (DMS) represent the largest natural source of biogenic sulfur to the global atmosphere, where it mediates aerosol dynamics. To constrain the contribution of oceanic DMS to aerosols we established the sulfur isotope ratios ((34)S/(32)S ratio, δ(34)S) of DMS and its precursor, dimethylsulfoniopropionate (DMSP), in a range of marine environments. In view of the low oceanic concentrations of DMS/P, we applied a unique method for the analysis of δ(34)S at the picomole level in individual compounds. Surface water DMSP collected from six different ocean provinces revealed a remarkable consistency in δ(34)S values ranging between +18.9 and +20.3‰. Sulfur isotope composition of DMS analyzed in freshly collected seawater was similar to δ(34)S of DMSP, showing that the in situ fractionation between these species is small (<+1‰). Based on volatilization experiments, emission of DMS to the atmosphere results in a relatively small fractionation (-0.5 ± 0.2‰) compared with the seawater DMS pool. Because δ(34)S values of oceanic DMS closely reflect that of DMSP, we conclude that the homogenous δ(34)S of DMSP at the ocean surface represents the δ(34)S of DMS emitted to the atmosphere, within +1‰. The δ(34)S of oceanic DMS flux to the atmosphere is thus relatively constant and distinct from anthropogenic sources of atmospheric sulfate, thereby enabling estimation of the DMS contribution to aerosols.

Global simulations of the impact on contemporary climate of a perturbation to the sea-to-air flux of dimethylsulfide

The sea-to-air flux of the biogenic sulfur (S) compound dimethylsulfide (DMS) is thought to constitute an important radiative impact on climate, especially in remote marine areas. Previous biogeochemical modelling analyses simulate medium to large changes in the sea-to-air flux of DMS in polar regions under warming scenarios. Here we assess the global radiative impact of such a prescribed change in DMS flux on contemporary climate using a low-resolution atmospheric general circulation model. This impact operates through the atmospheric oxidation of DMS to radiatively-active sulfate aerosols, which are known to both reflect incoming short-wave radiation and to affect the microphysical properties of clouds, for example, through an increase in cloud albedo. We use an atmospheric GCM with incorporated sulfur cycle, coupled to a mixed-layer ('q-flux') ocean, to estimate the climatic response to a prescribed meridionally-variable change in zonal DMS flux, as simulated in a previous modelling analysis. We compare baseline sulfur emissions (contemporary anthropogenic S and contemporary DMS sea-to-air flux), with contemporary anthropogenic S and a perturbed DMS flux. Our results indicate that the global mean DMS vertically integrated concentration increases by about 41 per cent. The relative increase in DMS annual emission is around 17 per cent in 70-80°N, although the most significant increase is in 50-70°S, up to 70 per cent. However, concentrations of atmospheric SO2 and SO4 2- increase by only about eight per cent. The oxidation of DMS by OH increases by about 20 per cent. Oxidation of SO2 to SO 4 2 by H2O2 increases seven per cent. The oxidation of SO2 by O3 increases around six per cent. Overall sulfur emissions increase globally by around 4.6 per cent. Global mean aerosol optical depth (AOD) increases by 3.5 per cent. Global mean surface temperature decreases by 0.6 K. There is a notable difference between the impacts in the southern and northern hemispheres. In general, most processes and chemical species related to the sulfur cycle show a larger increase in the southern hemisphere, except SO2 and the oxidation of DMS by NO 3. The global mean direct radiative forcing due to the DMS change is -0.05 Wm-2 with total forcing (direct + indirect effects) of -0.48 Wm-2. This perturbation on DMS flux leads to a mean surface temperature decrease in the southern hemisphere of around 0.8 K, compared with a decrease of 0.4 K in the northern hemisphere.

Dimethylsulphide (DMS) emissions from the western Pacific Ocean: a potential marine source for stratospheric sulphur?

Abstract. Sea surface and atmospheric measurements of dimethylsulphide (DMS) were performed during the TransBrom cruise in the western Pacific Ocean between Japan and Australia in October 2009. Air–sea DMS fluxes were computed between 0 and 30 μmol m−2 d−1, which are in agreement with those computed by the current climatology, and peak emissions of marine DMS into the atmosphere were found during the occurrence of tropical storm systems. Atmospheric variability in DMS, however, did not follow that of the computed fluxes and was more related to atmospheric transport processes. The computed emissions were used as input fields for the Lagrangian dispersion model FLEXPART, which was set up with actual meteorological fields from ERA-Interim data and different chemical lifetimes of DMS. A comparison with aircraft in situ data from the adjacent HIPPO2 campaign revealed an overall good agreement between modelled versus observed DMS profiles over the tropical western Pacific Ocean. Based on observed DMS emissions and meteorological fields along the cruise track, the model projected that up to 30 g S per month in the form of DMS, emitted from an area of 6 × 104 m2, can be transported above 17 km. This surprisingly large DMS entrainment into the stratosphere is disproportionate to the regional extent of the area of emissions and mainly due to the high convective activity in this region as simulated by the transport model. Thus, if DMS can cross the tropical tropopause layer (TTL), we suggest that the considerably larger area of the tropical western Pacific Ocean can be a source of sulphur to the stratosphere, which has not been considered as yet.

Contribution of aboveground plants, the rhizosphere and root-free-soils to total COS and DMS fluxes at three key growth stages in rice paddies

Carbonyl sulfide (COS) and dimethyl sulfide (DMS) are trace sulfur gases that contribute substantially to sulfate aerosols or cloud condensation nuclei in the upper and lower atmosphere and therefore play great roles in the earth's radiative balance. COS and DMS fluxes in rice (Oryza sativa L.) have been measured previously in rice paddies, and this study was designed to further investigate the role of aboveground plants, the rhizosphere and the root-free-soils in the exchange of COS and DMS by comparing COS and DMS fluxes among planted plots (PP), non-planted plots (NPP), plots with aboveground plants just cut before test (plants-cut plots, PCP) at three key growth stages (tillering, jointing–booting and mature stage). The average COS fluxes in NPP, PCP, and PP were 10.5±3.8, 6.1±4.6, and −12.3±5.0pmolm−2s−1, suggesting that COS emission from root-free-soils (10.5pmolm−2s−1) was surpassed by uptake of the rhizosphere (−4.4pmolm−2s−1) and aboveground plants (−18.4pmolm−2s−1). DMS emission rates were 60.5±25.2, 20.2±7.3 and 30.8±10.8pmolm−2s−1 in PP, NPP and PCP, respectively. Calculated DMS emission from the aboveground plants, rhizosphere and the root-free-soils were 29.7, 10.6 and 20.2pmolm−2s−1, accounting respectively for 49.0%, 17.6% and 33.4% of total DMS emission. Directly measured COS and DMS fluxes from aboveground plants averaged −14.2±7.2 and 22.3±10.6pmolm−2s−1, approximating that of −18.4 and 29.7pmolm−2s−1 calculated by subtracting fluxes in PP and PCP, respectively. Preliminary diurnal flux observation at jointing–booting stage revealed significant linear correlation between COS uptake rates by above ground plants (MFCOS) and photosynthesis rates (Pn) with a MFCOS/Pn slope of 0.48pmolμmol−1. DMS emission from the aboveground plants was also found to be significantly increased with temperature.