Contribution Of H2s04 Hno3 Hono And Organic Acids

Although the term acid rain has been used extensively in the popular literature to describe the formation and deposition of acids at the earth's surface, the terminology acid deposition is more commonly encountered in the scientific literature. The reason for this is that deposition of acids can occur either as dry deposition or as wet deposition. The former refers to the direct transport of acidic gases or small particles to the surface, followed by adsorption, without first being dissolved in an aqueous phase such as rain, clouds, or fog. Wet deposition, on the other hand, refers to the transport of acids to, and deposition on, surfaces (including soil, trees, grass, buildings, etc.) after the acids have been dissolved in an aqueous medium. It should be noted that the surface itself can be either wet or dry; the terms wet and dry deposition refer to the mechanism of transport to the surface, not to the nature of the surface itself.

Acid deposition has been recognized for centuries. For example, as described in the fascinating historical perspectives provided by Brimblecombe (1978) and Cowling (1982), Robert Boyle referred in 1692 to "nitrous or salino-sulphureous spirits" in air in his book A General History of the Air. In 1872, Robert Smith published in the United Kingdom a book entitled Air and Rain: The Beginnings of a Chemical Climatology. In it, he described many of the factors affecting what he termed "acid rain," including coal combustion and the amount and frequency of precipitation as well as the effects on plants and materials. It is fascinating that he actually analyzed rain near Manchester, U.K., and noted a change in the chemistry from the town center to remoter surrounding areas:

"... carbonate of ammonia in the fields at a distance, ... sulphate of ammonia in the suburbs, and ... sulfuric acid or acid sulphate in the town."

Historically, the major acids believed to contribute to acid deposition in the troposphere have been sulfuric and nitric acids, formed by the oxidation in air of S02 and oxides of nitrogen, respectively. However, there is an increasing recognition that organic acids may contribute significantly to the total acid burden and indeed may represent the major acidic species even in polluted urban environments. In addition, since nitrous acid (HONO) is formed whenever N02 and water are present (see Chapter 7.B.3), its contribution to the total acidity, particularly to indoor air environments, has become of interest and concern.

The relative contributions of these acids to the total acid deposition depends, as one might expect, on the particular emission sources. For example, the ratio of H2S04 to HNO3 is typically 2:1 on the east coast of the United States, where there are significant sources of S02, but about f:2 on the west coast, where NOx emissions predominate. The contribution of HONO has not been well established, in part because it is difficult to measure in an accurate and specific manner (see Chapter ll.A.4a) so that the data base on its concentrations is somewhat limited. In addition, since it photolyzes rapidly, its concentrations and contributions to the total acidity during the day are not significant compared to sulfuric and nitric acids. It is only recently that the potential contribution of organic acids has also been recognized.

To treat the chemistry of oxides of nitrogen, which play such a central role in the chemistry of both the polluted and remote troposphere, in a consistent manner, we have discussed the formation and fates of

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