Project at a Glance

Title: Characterization of reactants, reaction mechanisms, and reaction products in atmospheric water droplets: fog, cloud, dew, and rain water chemistry

Principal Investigator / Author(s): Hoffmann, Michael R.

Contractor: W. M. Keck Laboratories, California Institute of Technology

Contract Number: a2-048-32

Research Program Area: Atmospheric Processes

Topic Areas: Acid Deposition, Chemistry & Reactivity


This report is a summary of our findings from a two-year study of the chemical composition of fogwater in California. Fogwater 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 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. By comparing our data to previous case studies of health-threatening pollution events, we find evidence that suspended "acid fog" can constitute a hazard to public health.

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 H202 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 adducts.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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