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




Chang, Hui, author

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Persistent radiation fogs often form in California' s San Joaquin Valley (SJV) during periods of winter air stagnation. 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). The purpose of the study was to gain more information about the role of fogs in aerosol processing in the San Joaquin Valley. In addition to the CRPAQS fog study, 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. Small drops contain higher concentrations of ammonium, nitrate, sulfate, and organic carbon, but lower concentrations of nitrite. The pH values measured in large and small fog drops were similar in this study, in contrast to previous measurements in the SJV. SJV fogs typically have high pH values, above 6, due to large atmospheric concentrations of water soluble ammonia. The high drop pH promotes rapid formation of aqueous aldehyde-S(IV) complexes and rapid oxidation of dissolved sulfur dioxide by ozone. 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. Fogs play an important role as cleansers of the polluted winter SJV atmosphere. Scavenging of atmospheric fine particles and soluble trace gases by fog drops, followed by drop deposition to the ground, removes large amounts of nitrate, sulfate, ammonium, and organic carbon from the atmosphere. Deposition fluxes of fog water were observed to vary with fog liquid water content and drop effective diameter. Deposition velocities for nitrate, ammonium, sulfate, and organic carbon were observed to be of the order of 1 cm/s, much larger than expected in the absence of fog. 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. This pattern results from enrichment of these species in smaller fog drops that settle more slowly and the dominance of sedimentation as a drop deposition mechanism in environments with low wind speeds and low surface roughness. The deposition velocity of nitrite was typically higher than for fog water, due to its enrichment in larger, faster settling droplets. 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. Typical reductions in boundary layer concentrations of organic carbon, nitrate, sulfate, and ammonium due to fog deposition were estimated to be of the order of 1 μg/m3·hr. Numerous reports have appeared in the recent literature indicating the likelihood of fog/cloud processing of carbonaceous aerosol particles. Fog composition measurements also indicate the important role fogs play as processors of carbonaceous species. 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. Together these compounds make up the majority of low molecular weight (< 500 Daltons) carbon in the fog water. Significant amounts of higher molecular weight organic matter were also identified in the fogs using ultrafiltration. Identification of the nature and properties of this higher molecular weight material should be a focus in future fog studies. In order to provide some additional insight into the types of organic molecules present in the fog water, we followed an approach previously outlined by Decesari et al. (2000) using anion exchange chromatography on a DEAE cellulose column. Chromatograms obtained using this approach have been claimed to indicate the composition of the sample as divided into neutral/basic, mono- and di-carboxylic acid, and polycarboxylic acid fractions. Results from this approach have been widely used in recent studies to derive new models for the organic composition of aerosols and fogs. Chromatograms obtained for the California fog samples are similar in appearance to those reported elsewhere in the literature. However, tests of this method using individual compound standards reveal a tendency for the method to misclassify important families of organic compounds (e.g., phenolic compounds and dicarbonyls) as carboxylic acids. Caution is certainly warranted, therefore, in interpretation of sample chromatograms obtained with this method.


July 2004.
Also issued as author's dissertation (Ph.D.) -- Colorado State University, 2004.

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Fog -- California -- San Joaquin Valley
Pollution -- California -- San Joaquin Valley


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