CARBOXYLIC-ACIDS IN THE RURAL CONTINENTAL ATMOSPHERE OVER THE EASTERNUNITED-STATES DURING THE SHENANDOAH CLOUD AND PHOTOCHEMISTRY EXPERIMENT

The Shenandoah Cloud and Photochemistry Experiment (SCAPE) was conduct ed during September 1990 in the rural continental atmosphere at a moun tain top site (1014 m) in Shenandoah National Park, Virginia, We repor t here the extensive set of trace gas measurements performed during cl ear sky periods of SCAPE, with particular focus on the carboxylic acid s, formic, acetic, and pyruvic. Median mixing ratios were 5.4 and 2.1 parts per billion by volume (ppbv) for formic and acetic acid, respect ively, and they did nor exhibit the diurnal variation characteristic o f low-elevation sites. Mixing ratios of formic acid often approached o r 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. Seco ndary production from suspected pathways appears to be relatively unim portant. The observed lack of correlation between formic and acetic ac id with peroxide species argues against a significant source from perm utation reactions of peroxy radicals. In addition, model calculations using the SCAPE data indicate minimal production of carboxylics from o lefin/O-3 oxidation reactions. The tight correlation (r(2) = 0.88) bet ween mixing ratios of formic and acetic acid is strongly suggestive of a commonality in their sources. The seasonal cycle of carboxylic acid s in the atmosphere and precipitation over the eastern United States i s evidence that combustion emissions are not a principal source of the se 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 r atio ranging 4 - 266 parts per trillion by volume (pptv) (median = 63) and most likely supported by biogenic emissions and possibly photoche mical sources.