Sulfate formation, from atmospheric sulfur

Similar data for sulfate have been reported in many studies. Figure 9.36, for example, shows overall average sulfate distributions measured in marine areas as well as at continental sites (Milford and Davidson, 1987). The marine data show two modes, a coarse mode associated with sea salt and a fine mode associated with gas-to-particle conversion. Sulfate in seawater, formed, for example, by the oxidation of sulfur-containing organics such as dimethyl sulfide, can be carried into the atmosphere during the formation of sea salt particles by processes described earlier and hence are found in larger particles. The continental data show only the fine particle mode, as expected for formation from the atmospheric oxidation of the S02 precursors. 

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). 

Figure 1. Transformations in the ocean and overlying atmosphere which lead to the production of sulfate from a marine biogenic source (dark arrows). DMS is produced in the ocean after the uptake of seawater sulfate by phytoplankton and the production and breakdown of DMSP. Sulfate formation occurs after DMS is transferred across the sea-air interface and undergoes atmospheric oxidation. The S S values for the individual sulfur pools are indicated in the boxes and measured or estimated discriminations (D) are indicated above the arrows. Clearly, data for the remote atmosphere are limited.

Large quantities of sulfur dioxide enter the atmosphere each year from anthropogenic sources, mainly the combustion of fossil fuels and the smelting of metals. S02 indisputably ranks as a prominent pollutant, and it is understandable that research of the past 30 yr dealing with atmospheric sulfur has concentrated on such problems as the dispersal of S02 from power stations and urban centers, its conversion to sulfuric acid, the formation of sulfate aerosols, and the deposition of sulfate and S02 at the ground surface. 

In simple terms, the global sulfur cycle has two components. One is biochemical involving the conversion of sulfate to sulfide and the formation of DMS the other is atmospheric photochemical oxidation of DMS to sulfur oxyacids. DMS is formed mainly in the oceans by microorganisms and to a lesser extent in plants. About 38M0 Tg year-1 of DMS are released to the atmosphere from the oceans. The major precursor for DMS formation is the sulfonium salt, dimethylsulfoniopropionate, (CH3)2 S+ CH2 CH2 COOH, DMSP. DMSP lyase enzymes catalyze an elimination of acrylic acid from DMSP (Equation 12) with the release of DMS ... 

Chemical radicals—such as hydroxyl, peroxyhydroxyl, and various alkyl and aryl species—have either been observed in laboratory studies or have been postulated as photochemical reaction intermediates. Atmospheric photochemical reactions also result in the formation of finely divided suspended particles (secondary aerosols), which create atmospheric haze. Their chemical content is enriched with sulfates (from sulfur dioxide), nitrates (from nitrogen dioxide, nitric oxide, and peroxyacylnitrates), ammonium (from ammonia), chloride (from sea salt), water, and oxygenated, sulfiirated, and nitrated organic compounds (from chemical combination of ozone and oxygen with hydrocarbon, sulfur oxide, and nitrogen oxide fragments). ... 

Many deleterious effects have been associated with photochemically polluted air ozone is deflnitely associated with respiratory problems, plant damage, and material damage PAN has deflnitely been associated with plant damage, and some other members of this class of chemical compounds have been associated with eye irritation the hydroxyl radical is considered to be an important factor in the conversion of gas-phase intermediates to end products, such as sulfur dioxide to particulate sulfate the particulate complex is responsible for haze formation and has also been associated with eye irritation and respiratory effects. The aldehydes have been associated with eye irritation. Ozone and PAN themselves do not cause eye irritation. For purposes of control, much more research is needed, in order to relate the laboratory data about the concentrations of these various materials that have significant effects to their formation in the atmosphere from emission and their atmospheric distribution. The lack of convenient measurement methods has hindered progress in gaining this understanding. 

Further studies are needed to give better dose-response information and to provide a frequency distribution of the population response to oxidants alone and in combination with other pollutants at various concentrations. Such studies should include the effects of mixed pollutants over ranges corresponding to the ambient atmosphere. With combinations of ozone and sulfur dioxide, the mixture should be carefully characterized to be sure of the effects of trace pollutants on sulfate aerosol formation. The design of such studies should consider the need to use the information for cost-benefit analysis and for extrapolation from animals to humans and from small groups of humans to populations. Recent research has indicated the possibility of human a ptation to chronic exposure to oxidants. Further study is desirable. 

Calvert and McQuigg suggest that yet unknown radicals, such as 0CH20 or those derived from it, formed in the 03-olefin-air mixtures may oxidize S02 in the homogeneous reaction. It is known that OH and H02 radicals combine rapidly with S02. The addition products may eventually be transformed into sulfuric acid, peroxysulfuric acid, sulfates, and nitrates in a polluted atmosphere probably in a liquid phase of aerosol particles, although the detailed steps are still unknown. Finlayson and Pitts (357) believe that the oxidation of aromatic compounds by such species as OH, H02, 03, and 0(3P) may also be significant for the formation of organic aerosol. 

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