Dimethyl oceanic emission

Legrand, M., Feniet-Saigne, C., Saltzman, E. S. et al. (1991). Ice core record of oceanic emissions of dimethyl sulphide during the last climate cycle. Nature 350,144-146. 

Methanesulfonic acid, although it comprises a relatively small fraction of total non sea-salt aerosol sulfur, has been shown (2) to be a ubiquitous component of marine aerosols. Its occurrence and distribution have been suggested as of use as an in situ tracer (3.4) for oceanic emissions and subsequent reaction and deposition pathways of organosulfur compounds and dimethyl sulfide in particular. 

Another interesting example of the biological influence on atmospheric chemistry is provided by sulfur. Under natural conditions, sulfur compounds in the atmosphere are provided by the oceanic emission of dimethyl disulfide (DMS). This biogenic emission results from the breakdown of sulfoniopropionate (DMSP), which is thought to be used by marine phytoplankton to control their osmotic pre.ssure. The oxidation of DMS leads to the formation of sulfur dioxide, which is further converted to sulfate particles. As indicated above, these particles, by scattering back to space some of the incoming solar radiation, tend to cool the earth s surface. Their presence also affects the optical properties of the clouds, which introduces an indirect climatic effect. 

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

Gas-phase chemistry in remote areas is, in most cases, analogous to that in more polluted regions. The major difference is in lower NOx emissions and hence concentrations. In addition, in continental regions, there are substantial emissions of biogenic organics, many of which are highly reactive toward OH, 03, N03, and Cl atoms and in oceanic regions, dimethyl sulfide (DMS), which reacts with OH, N03, and Cl atoms. 

Tropospheric chemistry models have to take into account a significant number of chemical reactions required to simulate correctly tropospheric chemistry. In the global background marine troposphere, it seems reasonable to consider a simplified chemistry scheme based on O3/ NOx/ CH, and CO photochemical reactions. However, natural emissions of organic compounds from oceans (mainly alkenes and dimethyl sulphide-DMS) might significantly affect the marine boundary layer chemistry and in particular OH concentrations. Over continental areas both under clean and polluted conditions,... 

Over the past few years we have been studying the waters around the United Kingdom, including the North Sea, Irish Sea and N.E. Atlantic, in order to characterise dimethyl sulphide (DMS) emissions and assess the significance of this natural contribution to acidity of rainfall and the sulphur cycle. Biogenic DMS concentrations in seawater vary considerably both temporally and spatially and coastal and shelf water systems often contain higher concentrations of volatile sulphur than the open oceans (1.2). 

Bopp L, Boucher O, Aumont O, Belviso S, Dufresne JL, Pham M, Monfray P (2004) Will marine dimethylsulfide emissions amplify or alleviate global warming A model study. Can J Fish Aquat Sci 61 826-835 Bouillon RC, Miller WL (2004) Determination of apparent quantum yield spectra of DMS photo-degradation in an in situ iron-induced Northeast Pacific Ocean bloom. Geophys. Res. Lett. 31 Article no. L06310 Bouillon RC, Miller WL (2005) Photodegradation of dimethyl sulfide (DMS) in natural waters Laboratory assessment of the nitrate-photolysis-induced DMS oxidation. Environ Sci Technol 39 9471-9477... 

Dimethyl sulphide is the most dominant of the reduced sulphur gas found in surface layers of the ocean (Lovelock et al. 1972). The emission of dimethyl sulphide from seawater is expected to balance the excess sulphur deposition over the remote oceans (Charlson et al. 1992). Charlson et al. (1987) proposed a hypothesis, known as the CLAW (after the authors Charlson, Lovelock, Andreae and Warren) hypothesis connecting biogenic DMS emissions to changes in albedo, in which increased production of DMS due to global warming is expected to lead to more sulphate aerosols and subsequently to more cloud condensation nuclei (CCN) that in turn enhances back radiation. 

In the last 150 years the anthropogenic emission of sulfur has increased dramatically, primarily due to combustion processes [1]. In the 1950s anthropogenic emission surpassed natural emission and the atmospheric sulfur cycle is one of the most perturbed biogeochemical cycles [1,2]. The oceans are the largest natural source of atmospheric sulfur emissions, where sulfur is emitted in a reduced form, predominantly as dimethyl sulfide (DMS) and to a much lesser extent carbonyl sulfide (OCS) and carbon disulfide (CS2) [3]. Ocean emitted DMS and CS2 are initially oxidised to OCS, which diffuses through the troposphere into the stratosphere where further oxidation to sulfur dioxide (SO2), sulfur trioxide (SO3) and finally sulfuric acid (H2SO4) occurs [1-4]. 

Dimethyl sulphide (CH3SCH3, DMS) is the dominant natural sulphur eompound emitted from the world s oeeans (Berresheim et ai, 1995, Urbanski and Wine, 1999), accounting for about one quarter of global sulphur gas emissions. Oceanic DMS, through its oxidation products, is proposed to play a key role in climate regulation, especially in the remote marine atmosphere (Charlson et al, 1987). 

Dimethyl sulfide. 20-150 ppt Marine air Emission from oceans, 50 Reaction with OH 2d... 

Dimethyl sulfide is emitted mainly from the ocean where it is released from phytoplankton. Estimates of emission rates range from 30 to 68 Tg yr . Soils and vegetation contribute comparatively little to the global emission rate. The rate of DMS emissions evidently exceeds that of all other reduced sulfur compounds. Although this makes DMS the most important reduced sulfur compound globally, its impact is essentially confined to the marine atmosphere. The removal of DMS occurs primarily by reaction with OH radicals. 

Fig. 6.4 Schematic illustration of the key pathways in the atmospheric cycle of S involving (7) the natural emissions of reduced S compounds such as H2S frran terrestrial biota and dimethyl sulfide (CH3SCH3) from oceanic biota (2) anthropogenic emissions of S compounds, principally SO2 (3) the oxidation of reduced S compounds by OH and other photochemical oxidants leading to the production of intermediate oxidation state S compotmds such as SO2 and methane sulfonic acid (MSA) (4) the oxidation of these mtermediate oxidation state compounds within the gas phase by OH-producing H2SO4 vapor (5) the conversion of intermediate oxidation state compounds within liquid could droplets, which upon evaporation yield sulfate-containing particles (6) the conversion of H2SO4 to sulfate-containing particles and (7) the ultimate removal of S fiom the atmosphere by wet and dry deposition (Chameides and Perdue 1997)...

Particle precursor gases are emitted into the atmosphere either directly by natural and anthropogenic sources or by oxidation processes in the atmosphere. The most prominent precursor gas is probably sulfur dioxide (SO2). It is the precursor for particulate sulfates, such as sulfuric acid (H2SO4) or ammonium sulfate [(NH4)2S04]. Sulfur dioxide is directly emitted by natural sources (e.g., volcano emptions). Anthropogenic sources in industrial regions are mostly associated with combustion processes (e.g., coal combustion). Additional SO2 is derived from oxidation processes of dimethyl sulfide (DMS) over the oceans. Estimations of the global sulfur emissions from these sources are listed in Table 3. 

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