Fog and Cloud Water Deposition

Abstract

This paper is a summary of our findings from a four-year study of the chemical composition of fog and cloud water in California. Fog water was sampled at a number of sites with a rotating arm collector, which was developed in our laboratory and collects representative samples. Field investigations in the Los Angeles basin, the San Gabriel Mountains, and the San Joaquin Valley revealed very high ionic concentrations in polluted fogs, often coupled with very high acidities. Fogs and stratus clouds in the Los Angeles basin typically had pH values ranging from 2 to 4. Acidities were not as high in the San Joaquin Valley, mostly because of scavenging by the fogs of ammonia from agricultural sources. We showed that fogwater deposits efficiently on surfaces during fog events; this deposition was observed to be an important pollutant sink during stagnation episodes in the San Joaquin Valley, but at the same time it could be an important source of acid input to surfaces in some areas. Insight into the oxidation of S(IV) to S(VI), which is the major aqueous-phase source of acidity, was gained from field data, laboratory studies, and model development. Kinetic experiments showed that H_2O_2 was an important oxidant at low pH, and we predicted that metal-catalyzed autoxidation could also be an important source of sulfate. However, we found that the extreme acidities observed in fogs (below pH 3) require condensation on preexistent acidic nuclei and scavenging of gaseous nitric acid. Stabilization of S(IV) in the fog was observed, and this was attributed to the formation of S(IV)-aldehyde adduct.

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