41. Chemical Characteristics of Fine Organic Aerosols in the Great Smoky Mountains

Presenter: Lynn M. Hildemann
Coauthors: Liya Yu, Michelle Shulman, and Royal Kopperud

Water uptake by hydrophilic organic aerosols can be especially important in rural locations. An especially sizeable fraction of the aerosol organics in these more remote locales tends to be polar, due to the importance of biogenic emissions and the substantial photochemical alteration of any primary nonpolar anthropogenic emissions that have been transported long distances from urban areas.

As part of the Southeastern Aerosol and Visibility Study (SEAVS), 42 daytime and 10 nighttime filter samples of fine aerosols were collected during the period from July 15 to August 25, 1995 at the Great Smoky Mountain National Park, Tennessee (U.S.A.). Samples were extracted and derivatized to enable identification and quantification of the water-soluble organic compounds (WSOCs). The identified species were chemically classified into 7 groups: (1) monocarboxylic acids, (2) hydroxy-carboxylic acids, (3) dihydroxy-carboxylic acids, (4) dicarboxylic acids, (5) hydroxy-dicarboxylic acids, (6) polyols, and (7) other water-soluble organic compounds. Species concentrations ranged from $<$1 ng/m$^3$ to $>$200 ng/m$^3$.

Dicarboxylic acids were the most dominant identified compound class, and succinic acid was the most abundant dicarboxylic acid. The dominance of succinic acid over oxalic acid observed in this study contrasts with what has been reported for a number of other rural areas. This unique concentration distribution of dicarboxylic acids found in daytime SEAVS samples suggests to us that: (1) most WSOCs collected in the SEAVS samples were mainly generated from secondary photochemical reactions, especially during the first half of the sampling campaign, and (2) high relative humidity at the sampling site may have contributed to the high abundance of succinic acid. Concurrent trends in malic acid and malonic acid concentration support the hypothesis that succinic acid is being oxidized via hydroxyl radical addition. Analogous to the conversion of 3-hydroxy-propanoic acid to malonic acid, these measurements suggest that 4-hydroxy-butanoic acid could serve as a major precursor contributing to abundant succinic acid in the daytime samples. Nocturnal WSOCs exhibited different chemical compositions and lower concentrations than the daytime WSOCs. It appears that a nocturnal-to-diurnal ratio of succinic acid larger than 0.3 may indicate an atmospheric environment dominated by photochemical reactions, rather than by primary emissions.