The chemical composition of fogwater at some urban and rural sites

Abstract

The chemical composition of ambient fogwater was studied at several urban and rural sites in California and at a site in Albany, NY, using a rotating arm collector (accompanying paper by Jacob et al.). The ionic concentrations observed were generally much higher than those commonly found in precipitation. In the Los Angeles basin and the San Joaquin Valley of California extremely high acidities were found. Fog sampled in rural areas frequently had high ionic concentrations, indicating long-range pollutant transport. Low stratus clouds in the Los Angeles basin were found to be chemically similar to ground-level fogs. At some California sites the aerosol and gas-phase chemistry were monitored prior to fog formation and after fog evaporation. From these measurements it appears that fog droplets are efficient scavengers of ambient aerosol particles and gases. At Albany data was available on the liquid water content, droplet microphysics and boundary layer meteorological parameters; it is found that condensation and evaporation of water vapor on preexisting nuclei are the dominant processes controlling the chemical composition of fog droplets. Absorption of SO_2 followed by aqueous-phase oxidation to sulfate, with the concomitant production f acidity, is also expected to contribute to observed concentrations. Aqueous-phase oxidation of S(IV) by oxygen could in some cases be important in view of the high concentrations of transition metal catalysts found in the fogwater.