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A 1 month intensive summertime field study conducted in rural western Maryland resulted in a comprehensive set of fine particle, gaseous and meteorological data. Sulfur in the assumed form of ammonium sulfate accounted for 67% of the average fine particle mass and had a very high correlation with fine particle mass (r=0.99). Other measured species, includiqg carbon and nitrate, made only minor contributions to the fine mass. Peak sulfate concentrations, averaged over 6-h intervals, exceeded 50 [mu]g m-3. Nitric acid concentrations showed strong daytime maxima and on a 24 h basis were about four times those of fine particle nitrate. Sulfur in the gas phase (SO2) constituted more than half of the total sulfur, indicating that the sampling site was being influenced by local sources. Fine particle selenium was well correlated with fine particle sulfur (r=0.70). The ratio of fine particle sulfur to selenium was 2800, characteristic of a rural site downwind of coal-burning areas. Mixed-layer back trajectories were used to identify possible source regions for the measured parameters. Wind frequency-normalized concentrations of parameters associated with coal-burning (S, Se, SO2 and mass) were highest for back trajectories arriving at Deep Creek Lake from the west-northwest. Use of Rahn and Lowenthal regional signatures showed an overwhelming dominance by the Lower Midwest region, and a surprisingly weak impact by the Upper Midwest region, at the site.

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 two-electron reduction of molecular oxygen to hydrogen peroxide can be catalyzed by metal oxide semiconductor particles in the presence of visible and near-UV light. Even though very high quantum yields for this process are observed, rather low steady-state concentrations of H_2O_2 are reached. Detailed mechanisms are presented to explain these experimental findings. Metal oxide particles are found in atmospheric and surface waters. The environmental significance of photocatalytic formation of H_2O_2 on these particles in natural systems is discussed.