A systematic characterization of the atmospheric H2SO4‐HNO3‐NH3system 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 NH3emissions 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‐NH3species was documented over the course of the episode and interpreted in terms of production and removal mechanisms. Secondary production of strong acids H2SO4and HNO3under stagnant conditions resulted in a complete titration of available alkalinity at the sites farthest from NH3sources. A steady SO2conversion rate of 0.4–1.1% h−1was estimated in the stagnant mixed layer under overcast conditions and was attributed to nonphotochemical heterogeneous processes. Removal of SO2was enhanced in fog, compared to nonfoggy conditions. Conversion of NOxto HNO3slowed down during the stagnation episode because of reduced photochemical activity; fog did not appear to enhance conversion of NOx. Decreases in total HNO3concentrations were observed upon acidification of the atmosphere and were attributed to displacement of NO3−by H2SO4in 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