Dimethylsulfide Flux Research Articles

Flux estimation of oceanic dimethyl sulfide around North America

Simultaneous measurements of atmospheric and surface water dimethyl sulfide (DMS) concentrations were taken aboard the icebreaker USCGC Polar Sea from July to October 1994, as part of a joint Canada/United States circumnavigation of North America, an expedition with a unique Arctic Ocean transect. Atmospheric DMS concentrations around North America varied between 0.25 and 50 nmol m−3 (mean = 5.1 nmol m−3, σ = 8.5 nmol m−3, n = 89) with highest values occurring near (south of) the Arctic ice edge. Surface water DMS concentrations ranged between 0.1 and 12.6 nmol L−1 (mean = 2.2 nmol L−1, σ = 2.7 nmol L−1, n = 46) with highest values in the western Arctic Ocean and off the U.S. east coast, near the Sargasso sea. In the Arctic Ocean, maximum concentrations in air and water were found along the ice edge in the Chukchi Sea region. Atmospheric DMS decay rates of 68% per day and 38% per day were deduced from observations between 70° and 76°N (continental shelf and slope of Chukchi Sea) on the west side and between 80° and 90°N (central Arctic Ocean) for the east side of the Arctic Ocean, respectively. Ocean to atmosphere flux estimates of DMS were determined using the Liss‐Merlivat empirical dependence of exchange coefficient on wind speed, DMS air concentrations, Henry's law constants, and DMS water concentrations. DMS fluxes varied between 0.0017 and 30 μmol m−2 d−1, respectively, with higher fluxes in regions with open water. The Arctic Ocean contributed 0.063 Tg S (DMS) (0.4% of DMS from the world oceans) during the summer of 1994. A simple one‐dimensional (1‐D) photochemical box model, applied to six case studies, showed that the atmospheric lifetime of DMS in the high Arctic was 2.5 to 8 days, whereas at 16°–33°N it was 1 to 2 days. Modeled DMS decay rates for these regions, using the 1‐D model, accounted for only 33% of the measured decay rate. This result also suggests that halogen chemistry, reactions with Br/BrO, may be an important sink for DMS in the Arctic atmosphere.

One‐dimensional modeling of sulfur species during the First Aerosol Characterization Experiment (ACE 1) Lagrangian B

A one‐dimensional Lagrangian model is used to simulate vertical profiles and temporal evolution of dimethylsulfide (DMS), sulfur dioxide (SO2), aerosol methane sulfonate, and non‐sea‐salt sulfate (nss sulfate) that were measured during the three flights of the second First Aerosol Characterization Experiment (ACE 1) Lagrangian (Lagrangian B) experiment. Entrainment rate, mixing heights, and cloud occurrence are calculated prognostically in this type of model. The model is forced by geostrophic winds and large scale subsidence from European Centre for Medium‐Range Weather Forecasts (ECMWF) analysis and sea surface temperature measured on board Research Vessel Discoverer. Gas phase oxidation and heterogeneous oxidation of SO2 to nss sulfate in clouds and sea‐salt particles are considered. The evolution of dynamical variables in the column is found to be well reproduced by the model. The model captures 82% of the variance of observed DMS assuming OH is the only oxidant and a DMS flux term calculated from Liss and Merlivat [1986] parameterization and seawater DMS concentrations measured aboard R/V Discoverer. However, uncertainties in DMS oxidation rates and regional seawater concentrations are too great to identify a best fit wind speed‐transfer velocity relationship. SO2 mixing ratios are correctly represented in the model (least squares correlation coefficient r2 = 75%) using a DMS to SO2 conversion efficiency of about 70%. Oxidation of SO2 in sea‐salt particle appears to be a dominant process and controls SO2 lifetime during the Lagrangian B at least in the well mixed lower layer. Removing heterogeneous loss of SO2 in sea salt significantly deteriorates the simulation (r2 = 50%). Under cloudy conditions, heterogeneous loss in cloud droplets and in sea‐salt particles are competitive (relative rates are 35% and 41%, respectively, during flight 26). Model‐generated aerosol methane sulfonate mixing ratios agree with the observations (r2 = 62.5%) when high branching ratio for an addition oxidation pathway is used. The model estimates nss sulfate mixing ratios with little bias (median simulated‐to‐observed concentration ratio 1.03 and slope of the regression line 0.7) but captures only one third of the observed variance of nss sulfate. Part of the discrepancy could be due to the assumption of a decrease of nss sulfate mixing ratios with altitude in the model, whereas observations revealed high concentrations at 4500 m during the last two flights suggesting that horizontal transport could be more important than vertical mixing in this region. Nss sulfate is found to be produced photochemically under non cloudy, low wind speed conditions encountered during the first flight. During the last two flights, nss sulfate is produced mainly by oxidation in cloud droplets (48% during flight 25 and 69% during flight 26) and sea‐salt particles (50% during flight 25 and 22% during flight 26).

Isolation and characterization of Methanomethylovorans hollandica gen. nov., sp. nov., isolated from freshwater sediment, a methylotrophic methanogen able to grow on dimethyl sulfide and methanethiol.

A newly isolated methanogen, strain DMS1(T), is the first obligately anaerobic archaeon which was directly enriched and isolated from a freshwater sediment in defined minimal medium containing dimethyl sulfide (DMS) as the sole carbon and energy source. The use of a chemostat with a continuous DMS-containing gas stream as a method of enrichment, followed by cultivation in deep agar tubes, resulted in a pure culture. Since the only substrates utilized by strain DMS1(T) are methanol, methylamines, methanethiol (MT), and DMS, this organism is considered an obligately methylotrophic methanogen like most other DMS-degrading methanogens. Strain DMS1(T) differs from all other DMS-degrading methanogens, since it was isolated from a freshwater pond and requires NaCl concentrations (0 to 0.04 M) typical of the NaCl concentrations required by freshwater microorganisms for growth. DMS was degraded effectively only in a chemostat culture in the presence of low hydrogen sulfide and MT concentrations. Addition of MT or sulfide to the chemostat significantly decreased degradation of DMS. Transient accumulation of DMS in MT-amended cultures indicated that transfer of the first methyl group during DMS degradation is a reversible process. On the basis of its low level of homology with the most closely related methanogen, Methanococcoides burtonii (94.5%), its position on the phylogenetic tree, its morphology (which is different from that of members of the genera Methanolobus, Methanococcoides, and Methanohalophilus), and its salt tolerance and optimum (which are characteristic of freshwater bacteria), we propose that strain DMS1(T) is a representative of a novel genus. This isolate was named Methanomethylovorans hollandica. Analysis of DMS-amended sediment slurries with a fluorescence microscope revealed the presence of methanogens which were morphologically identical to M. hollandica, as described in this study. Considering its physiological properties, M. hollandica DMS1(T) is probably responsible for degradation of MT and DMS in freshwater sediments in situ. Due to the reversibility of the DMS conversion, methanogens like strain DMS1(T) can also be involved in the formation of DMS through methylation of MT. This phenomenon, which previously has been shown to occur in sediment slurries of freshwater origin, might affect the steady-state concentrations and, consequently, the total flux of DMS and MT in these systems.

Use of a mixed‐layer model to estimate dimethylsulfide flux and application to other trace gas fluxes

We discuss two techniques based on mixed‐layer scaling for estimating trace gas surface emission fluxes from aircraft using instruments that do not have sufficient frequency response for direct eddy correlation measurements. The first is the mixed‐layer gradient technique, which requires accurate measurements of mean concentrations at several heights in the clear convective planetary boundary layer (CBL) to resolve gradients from even strong surface sources of short‐lived trace gases. The flux‐gradient relationship is obtained from large‐eddy numerical simulations of the CBL. We show that this technique is limited to trace gases with lifetimes of about a day or less. An example is dimethylsulfide (DMS), which is emitted from the ocean and has a lifetime of about a day. Surface DMS flux was estimated from data collected from the NASA P‐3B research aircraft during the Pacific Exploratory Mission‐Tropics (PEM‐Tropics) flight 7 (August 24, 1996), when the aircraft flew a sequence of constant altitude circles about 50 km in diameter at different heights in and above the boundary layer, following the boundary layer air trajectory. The flight took place between 0530 and 1330 local solar time, providing a good opportunity to observe diurnal changes within a Lagrangian framework under nearly clear‐sky conditions. The resulting DMS flux of 2.5±0.8 pptv m s−1 (6.1±1.9)×1013 molecules m−2 s−1, or 8.8±2.8 μmols m−2 d−1 is on the high end of previous measurements in this milieu. The second technique is the mixed‐layer variance technique, which uses measurements of the variance at several heights in the CBL to estimate the surface flux. A major problem with the variance technique is accounting for the contribution of mesoscale variability to the measured variance. Several sources of mesoscale variability were identified: clusters of small cumulus clouds rising through the top of the boundary layer, mesoscale variations in the horizontal wind leading to inaccurate tracking of the air mass and, to a lesser extent, the presence of horizontal roll vortices in some areas of the boundary layer. We show that the variance technique should be applicable to estimating surface fluxes of short‐lived trace gases in cumulus‐free boundary layers that are horizontally uniform, if sample‐collection times of about 10 s or less are used. We also show that it may be possible to utilize mesoscale variance measurements to estimate surface fluxes and lifetimes of species which have lifetimes of perhaps a week or more.

Dimethyl sulfide emissions to the atmosphere from lakes of the Canadian boreal region

Dimethyl sulfide (DMS) concentrations were measured over 2 years in 10 lakes situated in northwestern Ontario. Concentrations varied from 0.1 nmol L−1 to 100 nmol L−1 (geometric mean of 1.2 nmol L−1) during the ice‐free season (April to November) of 1995 and 1996. Shallow (1–5 m depth and 1–5 ha area) and medium depth (5–10 m depth and 5–15 ha area) lakes exhibited higher surface water DMS concentrations than deeper and larger lakes (32 m depth and 20–56 ha area). During the fall in the medium size lakes (5–12 m depth and <15 ha area), DMS concentrations increased markedly by a factor of as much as 100 because of circulation of deeper, high‐DMS water from the hypolimnion (lake turnover). The estimated seasonal mean fluxes of DMS from shallow and medium depth lakes to the atmosphere ranged between 0.058 and 15 μmol S m−2 d−1 respectively. The shallow and medium depth lakes tend to release higher DMS than the large and deeper lakes. Extrapolation of flux estimates indicates that the lakes of the Canadian Boreal Shield emit 1.5 Gg yr−1 biogenic sulfur (DMS) to the atmosphere, and this is approximately 83% of the total annual biogenic sulfur (DMS) emissions from the Canadian Boreal Shield (lakes plus terrestrial). Compared to the annual anthropogenic emissions by five smelters in the boreal region, these emissions are small (0.08%).

A STUDY OF PROCESSES THAT GOVERN THE MAINTENANCE OF AEROSOLS IN THE MARINE BOUNDARY LAYER

We systematically evaluate the relative influence of sources and sinks of particles in the remote marine boundary layer (MBL) to elucidate the principal factors that govern MBL aerosol behavior. Processes considered include: (1) surface flux of dimethyl sulfide (DMS); (2) gas-phase oxidation of DMS to SO 2; (3) gas-phase oxidation of SO 2 to H 2SO 4; (4) mass transfer of SO 2 and H 2SO 4 to pre-existing particles; (5) homogenous nucleation of H 2SO 4/H 2O; (6) entrainment of air from the free troposphere; (7) deposition to the sea surface; (8) cycling of air through clouds and rain scavenging; (9) oxidation of SO 2 in sea salt aerosols and cloud droplets; and (10) sea-salt particle production at sea surface. The average aerosol number concentration is found to be quite sensitive to the rate of entrainment of aerosol-containing air from the free troposphere. The path that leads to the greatest accumulation of non-sea-salt (nss) sulfate involves SO 2 (rather than H 2SO 4) absorption into existing particles. Because of scavenging of SO 2 and H 2SO 4 by sea-salt aerosol, a considerable fraction of nss-sulfate is internally mixed with sea-salt aerosol. Under the conditions assumed in this study, MBL aerosol number concentration is dominated by free tropospheric aerosol under virtually all conditions, 89% in the base case, and even 69% at a 17 m s -1 wind speed. Aerosol mass, on the other hand, is dominated by sea-salt particles, 62% in the base case and 98% at a wind speed of 17 m s -1. Evaporation of cloud droplets provides 4.6% of the particle number in the base case, but 28% of the particle mass. At the high nucleation rate case considered here, there is notably little change in the overall contributions to aerosol number and mass from the base case; only about 5% of the total particle number is provided by nucleation events. Variation in precipitation frequency also has only a minor effect on the overall contributions. One concludes that the MBL aerosol is remarkably robust in the face of ever-changing conditions. Free tropospheric aerosol entrainment tends to sustain particle number concentrations, and sea-salt emissions maintain most of the aerosol mass. Cloud processing, while not a major contributor to aerosol number, does provide, except under high wind conditions, the order of 20% of the aerosol mass. Although nucleation occurs only infrequently and does not contribute appreciably to long-term average aerosol number or mass, nucleation is, nonetheless, the mechanism that replenishes aerosol number in brief, intense episodes when aerosol surface area levels are substantially reduced by precipitation.

A mass‐balance/photochemical assessment of DMS sea‐to‐air flux as inferred from NASA GTE PEM‐West A and B observations

This study reports dimethyl sulfide (DMS) sea‐to‐air fluxes derived from a mass‐balance/photochemical‐modeling approach. The region investigated was the western North Pacific covering the latitude range of 0°–30°N. Two NASA airborne databases were used in this study: PEM‐West A in September‐October 1991 and PEM‐West B in February‐March 1994. A total of 35 boundary layer (BL) sampling runs were recorded between the two programs. However, after filtering these data for pollution impacts and DMS lifetime considerations, this total was reduced to 13. Input for each analysis consisted of atmospheric DMS measurements, the equivalent mixing depth (EMD) for DMS, and model estimated values for OH and NO3. The evaluation of the EMD took into account both DMS within the BL as well as that transported into the overlying atmospheric buffer layer (BuL). DMS fluxes ranged from 0.6 to 3.0 μmol m−2 d−1 for PEM‐West A (10 sample runs) and 1.4 to 1.9 μmol m−2 d−1 for PEM‐West B (3 sample runs). Sensitivity analyses showed that the photochemically evaluated DMS flux was most influenced by the DMS vertical profile and the diel profile for OH. A propagation of error analysis revealed that the uncertainty associated with individual flux determinations ranged from a factor of 1.3 to 1.5. Also assessed were potential systematic errors. The first of these relates to our noninclusion of large‐scale mean vertical motion as it might appear in the form of atmospheric subsidence or as a convergence. Our estimates here would place this error in the range of 0 to 30%. By far the largest systematic error is that associated with stochastic events (e.g., those involving major changes in cloud coverage). In the latter case, sensitivity tests suggested that the error could be as high as a factor of 2. With improvements in such areas as BL sampling time, direct observations of OH, improved DMS vertical profiling, direct assessment of vertical velocity in the field, and preflight (24 hours) detailed meteorological data, it appears that the uncertainty in this approach could be reduced to ±25%.

Global nitrogen and sulfur inventories for oceangoing ships

We present geographically resolved global inventories of nitrogen and sulfur emissions from international maritime transport for use in global atmospheric models. Current inventories of globally resolved sources of natural and anthropogenic emissions do not include the significant contribution of SO2 or NOx from oceangoing ships [Benkovitz et al., 1996]. We estimate the global inventory of ship emissions, using current emission test data for ships [Carlton et al., 1995] and a fuel‐based approach similar to that used for automobile inventories [Singer and Harley, 1996]. This study estimates the 1993 global annual NOx and SO2 emissions from ships to be 3.08 teragrams (Tg, or 1012 g) as N and 4.24 Tg S, respectively. Nitrogen emissions from ships are shown to account for more than 14% of all nitrogen emissions from fossil fuel combustion, and sulfur emissions exceed 5% of sulfur emitted by all fuel combustion sources including coal. Ship sulfur emissions correspond to about 20% of biogenic dimethylsulfide (DMS) emissions. In regions of the Northern Hemisphere, annual sulfur emissions from ships can be of the same order of magnitude as estimates of the annual flux of DMS [Chin et al., 1996]. Monthly inventories of ship sulfur and nitrogen emissions presented in this paper are geographically characterized on a 2° × 2° resolution. Temporal and spatial characteristics of the inventory are presented. Uncertainty in inventory estimates is assessed: the fifth and ninety‐fifth percentile values for global nitrogen emissions are 2.66 Tg N and 4.00 Tg N, respectively; the fifth and ninety‐fifth percentile values for sulfur emissions are 3.29 Tg S and 5.61 Tg S, respectively. We suggest that these inventories, available via the Ship Emissions Assessment (SEA) web site, be used in models along with the Global Emissions Inventory Activity (GEIA) inventories for land‐based anthropogenic emissions and modeled with ocean‐biogenic inventories for DMS.

Dimethylsulfide and its oxidation products at two sites in Brittany (France)

Dimethylsulfide (DMS), sulfur dioxide (SO 2), dimethylesulfoxide (DMSO), sulfate (SO 2- 4) and methanesulfonate (MSA) were measured at a coastal site, Pointe de Penmarc’h, over a 25 day period during May and June 1993. Measurements were also performed to estimate the DMS flux from the algal beds around the Penmarc’h lighthouse and from the surrounding sea water. Similar atmospheric measurements were conducted at a second site approximately 2 km inland from Penmarc’h. A comparison between the 2 sites indicates similar patterns of DMS concentration at each site, with values at the inland site being generally 10–15% lower. The concentrations of the aerosol phase products such as methanesulfonic acid and sulfate are examined in the light of air mass origins. The fraction of nss SO 2- 4 produced from biogenic sources is estimated to be less than 25%. The yield of MSA and DMSO from DMS oxidation are assessed from their relative concentrations and estimates of their atmospheric lifetime.

Biogeochemistry of dimethylsulfide in the South China Sea

The distribution of dimethylsulfide (DMS) was studied in surface seawater and vertical profiles at nineteen stations in the Nansha Islands sea area of the South China Sea. The concentrations of DMS in surface-layer (0-1 m) seawater vary from 64 to 140 ng S/L with high values found in the productive regions, in agreement with the horizontal distribution of chrorophyll a. The vertical profiles of DMS show a single peak shape with maximum concentrations occurring at depths between 30-75 m. The DMS concentrations are correlated with chlorophyll a levels both in the upper 20 m of seawater as well as in vertical profiles. A clear diel variation in DMS concentration is observed at the 50-m water layer at a fixed station with the highest DMS concentration found in the late afternoon. The DMS concentrations are associated with environmental factors such as seawater temperature, dissolved O 2 and nutrient contents. Although DMS is correlated to chlorophyll a, the phytoplankton species is a major factor responsible for the obviously higher DMS concentration than expected from the phytoplankton biomass in this sea area. The sea-to-air flux of DMS from this sea area is calculated to be 7.6 μmol m -2 d - 1 .

Modelling the production and cycling of dimethylsulphide during the vernal bloom in the Barents Sea

Recent field work suggests an important ro^le for the Arctic Ocean in the global budget of dimethylsulphide (DMS), a climatically active volatile sulphur compound. Here, we have used an existing DMS production model and local field data to examine the temporal dynamics of the DMS cycle during the spring bloom in the Arctic shelf of the Barents Sea. The timing and duration of the spring phytoplankton bloom has been shown to be a key determinant of the flux of DMS to the atmosphere. Particular oceanic conditions due to the retreating ice-edge (e.g., a shallow mixed layer) can have an important effect on the timing of the phytoplankton bloom and thus the effux of DMS in this region. Model simulations support the view that algal taxonomy is not the most important factor determining DMS production in these waters. The mean vernal DMS flux is predicted to be 0.063 mg S m-2 d-1 which is in general agreement with previous summer season averages in the Arctic.

Summertime Seawater Concentrations of Dimethylsulfide in the Western Indian Ocean: Reconciliation of Fluxes and Spatial Variability with Long-Term Atmospheric Observations

Dimethylsulfide (DMS) measurements in the seawater of the subtropical and the temperate western Indian Ocean were conducted for the first time from 3 December to 20 December 1997. In total, 443 surface seawater DMS determinations were performed between 24°–49° S and 50° E–77° E with a frequency of 1 sample every 10 km. An important spatial variability was observed in seawater DMS concentrations with values ranging from 0.9 to 35.8 nM. DMS maxima coincided in most cases with thermal fronts and were in reasonable agreement with mean pigment figures obtained from satellite observations. The deduced DMS fluxes are consistent with long-term observations of atmospheric DMS and rainwater concentrations of nss- SO4= and MSA measured at Amsterdam island (37° S, 77° E); then account for the differences observed in atmospheric DMS concentrations between Amsterdam island and Cape Grim, Indian Ocean monitoring stations.

Spring–summer Emission of Dimethyl Sulphide from the North-western Mediterranean Sea

Sea-to-air emission fluxes of dimethyl sulphide (DMS) from the NW Mediterranean Sea were estimated from aqueous concentrations, seawater temperatures and wind speeds measured during three cruises in spring and summer of 1993 and 1994. Fluxes averaged 11·2 and 2·5μmolm−2day−1for shelf and open-sea stations, respectively. Annual extrapolation of these spring–summer fluxes resulted in a DMS emission of 8·8×103t(S) year−1for the whole NW Mediterranean basin. This is the first estimate of the outflux of biogenic sulphur in the region based on experimental data. It is concluded that the NW Mediterranean is a source of biogenic sulphur to the atmosphere, but DMS fluxes are moderate overall and closer to fluxes from temperate open oceanic regions than to those from other marginal seas. The marine emission represents only 0·3% of the surrounding continental anthropogenic sulphur emissions.

The distribution of dimethylsulfide, DMS, and dimethylsulfoniopropionate, DSMP, in waters off the Western Coast of Ireland

During June and early July of 1996, we measured concentrations of dimethylsulfide, DMS, and dimethylsulfoniopropionate, DMSP, in surface waters on the continental shelf and slope off the western coast of Ireland. We examined the distribution of DMS, DMSP, and the calculated flux of DMS to the atmosphere during four time series. A band of water with high DMS concentrations (>5 nM) was found around the 200 m isobath; DMS levels decreased across horizontal concentration gradients on either side of this band. The Irish Shelf Front seems to act as the inshore boundary of the enhanced DMS water, and the movement of the front may be related to the varying position and width of this area of high DMS. The distribution of DMSP and the atmospheric emission of DMS followed a similar pattern of that found in the DMS, low values near the coast and offshore, and the highest values found in between, around the 200 m isobath.

The impact of simulated climate change on the air-sea flux of dimethylsulphide in the subantarctic Southern Ocean

The impact of simulated climate change on the air-sea flux of dimethylsulphide in the subantarctic Southern Ocean

A one‐dimensional sectional model to simulate multicomponent aerosol dynamics in the marine boundary layer: 2. Model application

The dynamics of aerosols in the marine boundary layer (MBL) are simulated with the marine boundary layer aerosol model (MARBLES), a one‐dimensional, multicomponent sectional aerosol model [Fitzgerald et al., this issue; Gelbard et al., this issue]. First, to illustrate how the various aerosol processes influence the particle size distribution, the model was run with one or two processes operating on the same initial size distribution. Because of current interest in the effects of cloud processing of aerosols and exchange of aerosols with the free troposphere (FT) on marine aerosol size distributions, these two processes are examined in considerable detail. The simulations show that the effect of cloud processing (characteristic double‐peaked size distribution) in the upper part of the MBL is manifested at the surface on a timescale that is much faster than changes due to exchange with the FT, assuming a typical exchange velocity of 0.6 cm s−1. The model predicts that the FT can be a significant source of particles for the MBL in the size range of the cloud‐processing minimum, between the unactivated interstitial particles and the cloud condensation nuclei (CCN) which have grown as a result of conversion of dissolved SO2 to sulfate in cloud droplets. The model was also used to simulate the evolution of the aerosol size distribution in an air mass advecting from the east coast of the United States out over the ocean for up to 10 days. The modification of a continental aerosol size distribution to one that is remote marine in character occurs on a timescale of 6–8 days. Nucleation was not observed in the base case 10‐day advection simulation which assumed rather typical meteorological conditions. However, significant nucleation was predicted under a more favorable (albeit, atypical) combination of conditions which included significant precipitation scavenging (5 mm h−1 of rain for 12 hours), colder temperatures by 10°C (283 K at the surface decreasing to 278 K at 1000 m) and a high DMS flux (40 μmol m−2 d−1). In a test of model self initialization, long‐term (8–10 days) predictions of marine aerosol size distributions were found to be essentially independent of initial conditions.

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.

Dimethyl Sulfide Measurement by Fluorine-Induced Chemiluminescence

We have developed a high-speed sensor for dimethyl sulfide (DMS) based on its fast chemiluminescent reaction with molecular fluorine. Emission in the wavelength range 450-650 nm is monitored via photon counting. The instrument can continuously measure DMS with a response time of 0.1 s and is highly linear and sensitive. Limits of detection (S/N = 1) are 39, 12, and 4 pptv DMS for 0.1-, 1-, and 10-s integration times, respectively. Sensitivity and response time allow the direct measurement of DMS fluxes in the marine atmospheric boundary layer by the eddy correlation technique. Selectivity has previously been measured and is sufficient for monitoring DMS in the marine boundary layer without significant interferences.

The impact of simulated climate change on the air-sea flux of dimethylsulphide in the subantarctic Southern Ocean

Dimethylsulphide is an important sulphur-containing trace gas produced by enzymatic cleavage of its precursor compound, dimethylsulphoniopropionate (DMSP) which is released by marine phytoplankton in the upper ocean. After ventilation to the atmosphere, DMS is oxidised to form sulphate aerosols which in the unpolluted marine atmosphere are a major source of cloud condensation nuclei (CCN). Because the microphysical properties of clouds relevant to climate change are sensitive to CCN density, it has been postulated that marine sulphur emissions may play a role in climate regulation. Here we examine the DMS cycle in the subantarctic Southern Ocean west of the atmospheric baseline station at Cape Grim, NW Tasmania, where a long time series of atmospheric data has been collected. The Southern Ocean is relatively free of anthropogenic sulphur emissions and thus sulphate aerosols will be mainly due to the biogenic source of DMS. An atmospheric general circulation model has been used to provide meteorological forcings under current and doubled atmospheric CO2 conditions. We have used an existing DMS production model to investigate the sensitivity of the sea-to-air flux to the simulated changes in temperature and wind speed at the ocean surface. Under doubled CO2 conditions sea-surface temperature is simulated to increase by 4 °C throughout the year. Annual mean surface wind speed is simulated to decrease by about 3%. The changes in temperature and wind speed cause a net increase of DMS transfer velocity in the range 3-11%. The increase in temperature will also increase the growth rate of phytoplankton. The annual integrated DMS flux is simulated to increase between 2% and 8% under doubled CO2 conditions. A median change of + 5% in DMS flux corresponds to an increase in the range 2-4% in cloud condensation nuclei (CCN) concentration and a perturbation to radiative forcing of — 0.29 W m-2, confirming a minor role for DMS-derived aerosols in climate regulation.

Effect of UV- radiation on DMSP content and DMS formation of Phaeocystis antarctica

DMSP (dimethyl sulphonium propionate) contents produced by an Antarctic marine phytoplankton species, Phaeocystis antarctica (Prymnesiophyta), which were incubated under light conditions with radiations of different UV wavebands, were measured by gas chromatography after various exposure times. Full light (UV-B + UV-A + PAR) caused the strongest decrease in the production of DMSP in the alga. A marked depression of DMSP content was also observed with short UV-B and UV-A wavebands after 3 h. It was therefore hypothesised that DMSP production in Phaeocystis antarctica was inhibited by UV radiation. There was a negative correlation on change of DMSP contents under UV radiation. There was a negative correlation on change of DMSP contents under UV radiation with exposure times. The conversion rate of DMSP dissolved to DMS (dimethyl sulphide) was significantly increased with UV radiation. The possibility could not be excluded that a high concentration of free chemical radicals in seawater due to UV radiation resulted in an increase of DMSP cleavage in seawater. The oxidation of DMS in seawater due to UV-B radiation could result in a decrease of its flux to the atmosphere. The effect of UV radiation on DMSP production and oxidation of DMS may be an important factor in the variability of DMSP and the global flux of DMS from ocean to atmosphere.