Person:

Munger, J.

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.

Concentrations of HNO(g) and NH(g) determined in the field were compared to predictions from aerosol equilibrium models. The products of HNO(g) and NH(g) concentrations measured under cool and humid nonfoggy conditions agreed in magnitude with predictions from a comprehensive thermodynamic model for the atmospheric HSO‐HNO‐NH‐HO system. Observed concentrations of NH(g) in fogs were generally consistent with those predicted at equilibrium with fog water, but important discrepancies were noted in some cases. These discrepancies may be due to fluctuations in fog water composition over the course of sample collection or to the sampling of nonfoggy pockets of air present within the fog. Detectable concentrations of HNO(g) (up to 23 neq m) were often found in fogs with H 5 were below the detection limit of 4–8 neq m.

A systematic characterization of the atmospheric H2SO4-HNO3-NH3 system was conducted in the fog water, the aerosol, and the gas phase at a network of sites in the San Joaquin Valley of California. Spatial patterns of concentrations were established that reflect the distribution of SO2, NOx, and NH3 emissions within the valley. The concept of atmospheric alkalinity was introduced to interpret these concentrations in terms of the buffering capacity of the atmosphere with respect to inputs of strong acids. Regions of predominantly acidic and alkaline fog water were identified. Fog water was found to be alkaline in most of the valley, but small changes in emission budgets could lead to widespread acid fog. An extended stagnation episode was studied in detail: progressive accumulation of H2SO4-HNO3-NH3 species was documented over the course of the episode and interpreted in terms of production and removal mechanisms. Secondary production of strong acids H2SO4 and HNO3 under stagnant conditions resulted in a complete titration of available alkalinity at the sites farthest from NH3 sources. A steady SO2 conversion rate of 0.4–1.1% h−1 was estimated in the stagnant mixed layer under overcast conditions and was attributed to nonphotochemical heterogeneous processes. Removal of SO2 was enhanced in fog, compared to nonfoggy conditions. Conversion of NOx to HNO3 slowed down during the stagnation episode because of reduced photochemical activity; fog did not appear to enhance conversion of NOx. Decreases in total HNO3 concentrations were observed upon acidification of the atmosphere and were attributed to displacement of NO3− by H2SO4 in the aerosol, followed by rapid deposition of HNO3(g). The occurrence of fog was associated with general decreases of aerosol concentrations due to enhanced removal by deposition.