The H_2SO_4-HNO_3-NH_3 System at High Humidities and in Fogs: 1. Spatial and Temporal Patterns in the San Joaquin Valley of California

Abstract

A systematic characterization of the atmospheric H_2SO_4-HNO_3-NH_3 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 SO_2, NO_x, and NH_3 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 H_2SO_4-HNO_3-NH_3 species was documented over the course of the episode and interpreted in terms of production and removal mechanisms. Secondary production of strong acids H_2SO_4 and HNO_3 under stagnant conditions resulted in a complete titration of available alkalinity at the sites farthest from NH_3 sources. A steady SO_2 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 SO_2 was enhanced in fog, compared to nonfoggy conditions. Conversion of NO_x to HNO_3 slowed down during the stagnation episode because of reduced photochemical activity; fog did not appear to enhance conversion of NO_x. Decreases in total HNO_3 concentrations were observed upon acidification of the atmosphere and were attributed to displacement of NO_3− by H_2SO_4 in the aerosol, followed by rapid deposition of HNO_3(g). The occurrence of fog was associated with general decreases of aerosol concentrations due to enhanced removal by deposition.

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