Transport and oxidation of SO2 in a stagnant foggy valley

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.