Document Type

Article

Date of Original Version

1995

Abstract

The Shenandoah Cloud and Photochemistry Experiment (SCAPE) was conducted during September 1990 in the rural continental atmosphere at a mountain top site (1014 m) in Shenandoah National Park, Virginia. We report here the extensive set of trace gas measurements performed during clear sky periods of SCAPE, with particular focus on the carboxylic acids, formic, acetic, and pyruvic. Median mixing ratios were 5.4 and 2.1 parts per billion by volume (ppbv) for formic and acetic acid, respectively, and they did not exhibit the diurnal variation characteristic of low‐elevation sites. Mixing ratios of formic acid often approached or exceeded 10 ppbv, which are the largest values yet reported for the nonurban troposphere. Over the rural eastern United States, formic and acetic acid appear to have significant nonphotochemical sources. Secondary production from suspected pathways appears to be relatively unimportant. The observed lack of correlation between formic and acetic acid with peroxide species argues against a significant source from permutation reactions of peroxy radicals. In addition, model calculations using the SCAPE data indicate minimal production of carboxylics from olefin/O3 oxidation reactions. The tight correlation (r2 = 0.88) between mixing ratios of formic and acetic acid is strongly suggestive of a commonality in their sources. The seasonal cycle of carboxylic acids in the atmosphere and precipitation over the eastern United States is evidence that combustion emissions are not a principal source of these species. It appears that direct biogenic emissions from vegetation and soils cannot be ruled out as important sources. In particular, the correlation between the seasonal variation of formic and acetic acid and the ambient temperature is consistent with a soil microbial source. Similar conclusions were reached for pyruvic acid, with its mixing ratio ranging 4–266 parts per trillion by volume (pptv) (median = 63) and most likely supported by biogenic emissions and possibly photochemical sources.

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