The fate of SO2 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 SO2 and SO42− concentrations. At night, SO2 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 SO2 and SO42− over a major SO2 source region in the valley. The rate of oxidation of SO2 to SO42− in fog was estimated at 3 ± 2%h−1. Production of acidity from the oxidation of SO2 fully titrated the NH3(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 SO2 and SO42− in the valley during extended highinversion episodes appear to depend considerably on the presence of fog or stratus, and on the extent of daytime insolation.Engineering and Applied Science
