Project at a Glance
Title: Characterization of reactants, reaction mechanisms and reaction products leading to extreme acid rain and acid aerosol conditions in Southern California.
Principal Investigator / Author(s): Hoffmann, Michael R.
Contractor: W. M. Keck Laboratories, California Institute of Technology
Contract Number: A0-141-32
Research Program Area: Atmospheric Processes
Topic Areas: Acid Deposition
Analyses of fog water collected by inertial impaction in the Los Angeles basin and the San Joaquin Valley indicated unusually high concentrations of major and minor ions. The dominant ions measured were N03-, S04 2-, NH4+ and H+. Nitrate exceeded sulfate on an equivalent basis by a factor of 2.5 in the central and coastal regions of the Los Angeles basin, but was approximately equal in the eastern Los Angeles basin and the San Joaquin Valley. Maximum observed values for NH4+, N03- and S042- were 10., 12., and 5. meq L -1, while the lowest pH observed was 2.2. Iron and lead concentrations over 0.1 mM and 0.01 mM, respectively, were observed. High concentrations of chemical components in fog appeared to correlate well with the occurrence of smog events. Concentrations in fog water were also affected by the physical processes of condensation and evaporation. Light, dissipating fogs routinely showed the highest concentrations.
The chemistry of urban fog has been modeled using a hybrid kinetic and equilibrium computer code. Extreme acidity found in Southern California fog may be due either to condensation and growth on acidic condensation nuclei or in situ S(IV) oxidation. Important oxidants of S(IV) were found to be O2 as catalyzed by Fe(III) and Mn(II), H202 and 03. Formation of hydroxymethanesulfonate ion (HMSA) via the nucleophilic addition of HSO3-, to CH2O (l) significantly increased the droplet capacity for S(IV) but did not slow down the net S(IV) oxidation rate leading to fog acidification. Gas phase nitric acid, ammonia and hydrogen peroxide were scavenged efficiently, although aqueous phase hydrogen peroxide was depleted rapidly by reduction with S(IV). Nitrate production in the aqueous phase was found to be dominated by HN03 gas phase scavenging. Major aqueous-phase species concentrations were controlled primarily by condensation, evaporation and pH.
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