Acid generation in the troposphere by gas-phase chemistry

Abstract

Some of the important aspects of the problem of acid rain are presented. They are: identifying the important chemical reactions that form acids in the troposphere, the related reactions that generate and remove the reactive precursors required for these reactions, and the theoretical estimation of the rates at which acid generation is expected to occur in these reactions. Computer simulations of summertime conditions are used to evaluate the reactions. Major emphasis is on the role of gas-phase chemistry in acid development in the troposphere. The reaction sequence involves 130 chemical reactions that show the inorganic and organic chemistry of the polluted troposphere. The important gas-phase tropospheric chemistry of SO/sub 2/ and NO-NO/sub 2/ leading to acid generation are presented. Results indicate that the average rate of gas-phase oxidation of SO/sub 2/ to H/sub 2/SO/sub 4/ is in the range of 13 - 24% per 24-h period (relative humidity 50%). The theoretical conversion rates of NO/sub x/ oxidation to HONO/sub 2/ were found to be much higher than those for SO/sub 2/. The theoretical influence of the gas-phase tropospheric chemistry on the liquid-phase processes leading to acid rain is discussed. When clouds are abundant in the troposphere, the very fast in-cloudmore » conversion of SO/sub 2/ to H/sub 2/SO/sub 4/ through O/sub 3/ and H/sub 2/O/sub 2/ dominate the mechanisms of acid generation. The model calculations suggest that from 5% to 20% of the total acids in the troposphere may be composed of organic acids.« less