Internal acid buffering in San Joaquin Valley fog drops and its influence on aerosol processing

Although several chemical pathways exist for S(IV) oddation in fogs and clo uds, many are self-limiting: as sulfuric acid is produced and the drop pH d eclines, the rates of these pathways also decline. Some of the acid that is produced can be buffered by uptake of gaseous ammonia. Additional internal buffering can result from protonation of weak and strong bases present in solution. Acid titrations of high pH fog samples (median pH = 6.49) collect ed in California's San Joaquin Valley reveal the presence of considerable i nternal acid buffering. In samples collected at a rural location, the obser ved internal buffering could be nearly accounted for based on concentration s of ammonia and bicarbonate present in solution. In samples collected in t he cities of Fresno and Bakersfield, however, significant additional, unexp lained buffering was present over a pH range extending from approximately f our to seven. The additional buffering was found to be associated with diss olved compounds in the fogwater. It could not be accounted for by measured concentrations of low molecular weight (C-1-C-3) carboxylic acids, S(IV), p hosphate, or nitrophenols. The amount of unexplained buffering in individua l fog samples was found to correlate strongly with the sum of sample acetat e and formate concentrations, suggesting that unmeasured organic species ma y be important contributors. Simulation of a Bakersfield fog episode with a nd without the additional, unexplained buffering revealed a significant imp act on the fog chemistry. When the additional buffering was included, the s imulated fog pH remained 0.3-0.7 pH units higher and the amount of sulfate present after the fog evaporated was increased by 50%. Including the additi onal buffering in the model simulation did not affect fogwater nitrate conc entrations and was found to slightly decrease ammonium concentrations. The magnitude of the buffering effect on aqueous sulfate production is sensitiv e to the amount of ozone present to oxidize S(IV) in these high pH fogs. (C ) 1999 Elsevier Science Ltd. All rights reserved.