The processing of aerosol particles and soluble trace gases by chemically heterogeneous radiation fogs

During winter 2000/2001 an extensive fog study was conducted in the San Joaquin Valley (SJV) in California, within the framework of the California Regional PM10/PM2.5 Air Quality Study (CRPAQS). Samples from an additional radiation fog field campaign in January 2004 in Fresno, California were also analyzed as part of this project.; The dominant contributors to SJV fog composition include ammonium, nitrate, sulfate, nitrite, acetate, formate, formaldehyde, glyoxal, and methyl glyoxal. Significant differences were observed between the composition of small and large SJV fog drops.; Model simulations indicate that formation of the S(IV)-formaldehyde adduct hydroxymethanesulfonate is the dominant fate of dissolved sulfur dioxide in both large and small fog drops. S(IV) oxidation is limited due to finite rates of reactant mass transport into the drops and competition for available sulfite and bisulfite by formaldehyde.; Deposition velocities for nitrate, sulfate, ammonium, and organic carbon in fog were observed to be slightly lower than the deposition velocity of fog water itself. The deposition velocity of nitrite was typically higher than for fog water. This pattern results from enrichment of these species in different size fog drops. Overall, the observed deposition fluxes of these major inorganic aerosol species are significant and can offset or exceed aqueous phase production of new aerosol mass.; This study reveals the presence of numerous organic species in SJV fog, including formaldehyde, low molecular weight carboxylic acids such as acetate and formate, dicarboxylic acids, carbonyls and dicarbonyls including glyoxal and methyl glyoxal. We followed an approach previously outlined by Decesari et al. (2000) using anion exchange chromatography on a DEAF cellulose column. Chromatograms obtained for the California fog samples are similar in appearance to those reported. However, tests of this method using individual compound standards reveal a tendency for the method to misclassify important families of organic compounds. Caution is certainly warranted, therefore, in interpretation of sample chromatograms obtained with this method.