Transport and oxidation of SO2 in a stagnant foggy valley

The fate of SO_2 emitted in the San Joaquin Valley of California under stagnant foggy conditions was determined by the release of an inert tracer and the concurrent monitoring of SO_2 and SO_4^(2−) concentrations. At night, SO_2 was found to be trapped in a dense fog layer below a strong and persistent inversion based a few hundred meters above the valley floor. This lack of ventilation led to the accumulation of SO_2 and SO_4^(2−) over a major SO_2 source region in the valley. The rate of oxidation of SO_2 to SO_4^(2−) in fog was estimated at 3 ± 2%h^(−1). Production of acidity from the oxidation of SO_2 fully titrated the NH_3(g) present before the fog, and led to a progressive drop of the fogwater pH over the course of the night. In the afternoon, the valley was found to be efficiently ventilated by a buoyant upslope flow through the inversion. The tracer data indicated that about 40 % of the air transported upslope in the afternoon was returned to the valley in the night-time drainage flow. The fates of SO_2 and SO_4^(2−) in the valley during extended high-inversion episodes appear to depend considerably on the presence of fog or stratus, and on the extent of daytime insolation.