A field investigation of physical and chemical mechanisms affecting pollutant concentrations in fog droplets

High ionic loadings were found in fogwater collected at Bakersfield. California during an extended stagnation episode. The major ions were NH₄⁺, NO₃^-, and SO₄^2-, with concentrations usually in the millimolar range. Droplet growth played an important rôle in determining fogwater concentrations. The amount of solute decreased substantially over the course of each fog event; this was attributed, at least in part, to deposition of fog droplets on surfaces. The occurrence of dense fogs thus seemed to limit particle build-up during stagnation episodes. The sulfate fraction in the aerosol increased appreciably over several days of stagnation, but no statistical evidence for in situ S(IV) aqueous-phase oxidation was found. The high ammonia concentrations present were sufficient to neutralize a large fraction of the ambient acidity. As a result, fogwater pH values rarely attained the extremely low values found in other polluted environments.