Depositional Aspects of Pollutant Behavior in Fog

Droplet deposition during fog is shown to play an important role in the removal of anthropogenic pollutants from the atmosphere. Relevant theoretical principles are reviewed, and a survey of previous investigations is made. Results of extensive field monitoring programs are presented, and characterizations of fog chemistry and deposition in several environments are reported. The in-cloud scavenging of aerosols and soluble gases coupled with the small size of fog droplets are found to result in higher chemical concentrations in fogwater than in rainwater. In the urban regions of southern California and the southern San Joaquin Valley, fogwater chemistry is dominated by sulfate, nitrate, and ammonium ions, which are measured at millimolar levels. High fog- and cloudwater acidities (pH 2 to 4) are routinely found in the western Los Angeles basin where ammonia emissions are low. San Joaquin Valley fogwater samples are less acidic due to greater ammonia release from local sources. The formation of fog is shown to accelerate deposition rates for water-scavenged atmospheric constituents. Surrogate-surface measurements made in the San Joaquin Valley indicate that major species were removed at rates 5 to 20 times greater during fogs compared to nonfoggy periods. During stagnation episodes, pollutant removal by ventilation of valley air requires at least 5 days, while the enhancement of deposition by fog formation leads to pollutant lifetimes on the order of 6-12 h. Thus, in an environment characterized by flat, open landscape and low wind speed, droplet sedimentation can be the dominant removal mechanism of pollutants during prolonged stagnation episodes with fog.