Sulfate oceanic emission

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. 

On the early Earth, ions were mobilized from volcanic rocks by chemical weathering. Rivers and hydrothermal emissions transported these chemicals into the ocean, making seawater salty. These salts are now recycled within the crustal-ocean-atmosphere fectory via incorporation into sediments followed by deep burial, metamorphosis into sedimentary rock, uplift, and weathering. The last process remobilizes the salts, enabling their redelivery to the ocean via river runoff and aeolian transport. In the case of sodium and chlorine, evaporites are the single most important sedimentary sink. This sedimentary rock is also a significant sink for magnesium, sulfate, potassium, and calcium. 

Biogenic Sulfur Emissions from the Ocean. The ocean is a source of many reduced sulfur compounds to the atmosphere. These include dimethylsulfide (DMS) (2.4.51. carbon disulfide (CS2) (28). hydrogen sulfide (H2S) (291. carbonyl sulfide (OCS) (30.311. and methyl mercaptan (CH3SH) ( ). The oxidation of DMS leads to sulfate formation. CS2 and OCS are relatively unreactive in the troposphere and are transported to the stratosphere where they undergo photochemical oxidation (22). Marine H2S and CH3SH probably contribute to sulfate formation over the remote oceans, yet the sea-air transfer of these compounds is only a few percent that of DMS (2). 

Summary of Biogenic Sulfur Emissions from the Ocean. To summarize the source fractionation patterns for biogenic sulfur emissions from the oceam a simple isotopic model has been constructed (Figure 1). From this, the S S value of sulfate from a marine biogenic source of atmospheric sulfur can be estimated. 

Miller, J. M. Tellus. in press) have examined the transport of North American sulfur emissions across the north Atlantic Ocean to Europe. In a review of available precipitation-sulfate data from the north Atlantic and adjacent coastal regions, they report a concentration field consistent with known source distributions and meteorological factors. The excess sulfate concentration of marine background... 

NH3 and to a lesser extent mono-, di-, and trimethylamines are the only significant gaseous bases in the atmosphere, and there has been considerable interest in whether the oceans are a source or sink of these gases. Early attempt to assess the air-sea flux from concentration measurements are probably suspect because of the ease with which sample contamination can occur during laboratory processing and analysis. It should be noted here that due to its high solubihty (low value of Henry s law constant), the air-water transfer of NH3 (and the methylamines for the same reason) is under gas phase control (see Section 6.03.2.1.1). The first reliable measurements were probably from the North and South Pacific and indicated that the flux of NH3 from sea to air is of a size similar to that for emission of DMS (Quinn et al., 1990, 1988). Indeed, the authors showed that this similarity was mirrored in the molar ratio of (non-sea-salt) sulfate to ammonium (1.3 0.7) in atmospheric aerosol particles collected on the cruise, indicating that for clean marine air remote from terrestrial sources, the emission of DMS and NH3 from the sea appears to control the composition of the aerosol. 

The low amount of liquid water associated with particles (volume fraction 10-10, compared to clouds, for which the volume fraction is on the order of 10-7) precludes significant aqueous-phase conversion of S02 in such droplets. These particles can contribute to sulfate formation only for very high relative humidities (90% or higher) and in areas close to emissions of NH3 or alkaline dust. Seasalt particles can also serve as the sites of limited sulfate production (Sievering et al. 1992), as they are buffered by the alkalinity of seawater. The rate of such a reaction as a result of the high pH of fresh seasalt particles is quite rapid, 60 pM min-1, corresponding to 8% h 1 for the remote oceans (S02 = 0.05 ppb). Despite this initial high rate of the reaction, the extent of such production may be quite limited. For a seasalt concentration of 100 nmol m 3, the alkalinity of seasalt... 

Primary particles, such as road dust, salt (sea-) spray from the oceans and cement dust do not change form after emission, whereas a substantial fraction of mass of the secondary particles, such as photochemically produced sulfates and photochemical smog, is formed by in situ chemical reactions involving gases. 

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)...

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