Pollutant Deposition in Radiation Fog

A study of atmospheric pollutant behavior was conducted in the southern San Joaquin Valley of California during periods of stagnation, both with and without dense fog. Measurements were made of gas-phase and aerosol pollutant concentrations, fogwater composition, and deposition of solutes to surrogate surfaces. Deposition rates for major species were 5 to 20 times greater during fogs compared to nonfoggy periods. Sulfate-ion deposition velocities measured during fog were 0.5 to 2 cm s^(-1). Rates measured for nitrate ion were generally 50% below those for sulfate, except for acidic fog (pH<5) conditions, because nitrate was less effectively scavenged by neutral or alkaline fogs. In radiation fogs, scavenging of ambient aerosol was observed to increase as liquid water content rose. The lifetimes for atmospheric sulfate and ammonium were short (6-12 h) during dense fog compared to the ventilation rate (>3 d) for valley air.