Search

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.

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.

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.

An instrument is presented which collects large samples of ambient fogwater by impaction of droplets on a screen. The collection efficiency of the instrument is determined as a function of droplet size, and it is shown that fog droplets in the range 3–100-µm diameter are efficiently collected. No significant evaporation or condensation occurs at any stage of the collection process. Field testing indicates that samples collected are representative of the ambient fogwater. The instrument may easily be automated, and is suitable for use in routine air quality monitoring programs.

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. The in-cloud scavenging of aerosols and soluble gases coupled with the small size of fog droplets results in higher chemical concentrations in fog water than in rainwater. In the urban regions of southern California and the southern San Joaquin Valley, fog water chemistry is dominated by sulfate, nitrate, and ammonium ions, which are measured at millimolar levels. The formation of fog is shown to accelerate deposition rates for water-scavenged atmospheric constituents. 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.

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.

The chemistry of fogs, clouds, dew and rain at selected locations in California bas been observed over the last several years. Fog and cloud water in southern California often has extremely low pH values (i.e., 1.7 ≤ pH ≤ 4) and extremely high concentrations of sulfate, nitrate, ammonium ion and trace metals. A representative set of values reported by Waldman et al and Munger et al are given in Table I along with values reported for other sites around the world. Of special interest are the high values observed for SO_4^(2-), NO_3-, S(IV), CH_2O, Fe, Mn, Pb and Cu in fog water. These values and their time-dependent changes as shown in Figures 1 and 2 indicate that fogs provide a very reactive environment for the accumulation of HNO_3 and H_25O_4. Concomitant incorporation of NH_3 gas and calcareous dust into the droplet phase neutralizes some of the acidity.

An instrument is presented which collects large samples of ambient fogwater by impaction of droplets on a screen. The collection efficiency of the instrument is determined as a function of droplet size, and it is shown that fog droplets in the range 3–100-µm diameter are efficiently collected. No significant evaporation or condensation occurs at any stage of the collection process. Field testing indicates that samples collected are representative of the ambient fogwater. The instrument may easily be automated, and is suitable for use in routine air quality monitoring programs.