Carboxylic acids in clouds at a high-elevation forested site in central Virginia

During September 1990 we sampled coarse (>18-μm diameter) and fine (18- to 5.5-μm diameter) droplets and liquid-water content (LWC) in cloud from a tower on a forested ridge top in Shenandoah National Park, Virginia. Cloud-water pH and aqueous- and vapor-phase concentrations of carboxylic acids (HCOOH and CH3COOH) and formaldehyde (HCHO) were measured in parallel over 1- to 1.5-hour intervals. Both size fractions of cloud droplets contained similar concentrations of carboxylic species and H+ during most sampling; most cloud water was in coarse droplets. The pH of coarse (3.27–4.76) and fine (3.22–4.70) droplets coupled with total LWC of 0.04–0.56 g m−3 STP (standard m3 at 0°C and 1 atm) resulted in the partitioning of carboxylic acids primarily in the vapor phase. The observed phase partitioning for CH3COOH was within the uncertainty range of thermodynamic data. However, HCOOH exhibited significant phase disequilibria, which could not be explained by artifacts from variable LWC or from mixing droplets of different acidities. We hypothesize that the large volume of liquid water deposited on the forest canopy interacted with the near-surface cloud leading to apparent disequilibria based on time-integrated samples. HCOOH was selectively depleted relative to CH3COOH in cloud, particularly at higher pH, suggesting rapid removal of HCOOH by cloud-water deposition. We saw no evidence for significant production of HCOOH from the aqueous-phase oxidation of HCHO.Engineering and Applied Science

Carboxylic acids in the rural continental atmosphere over the eastern United States during the Shenandoah Cloud and Photochemistry Experiment

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.Engineering and Applied Science

Atmospheric selenium; geographical distribution and ocean to atmosphere flux in the Pacific

Approximately 700 weekly aerosol samples from seven island sites in the North Pacific have been analyzed for their selenium content. A much more limited set of selenium aerosol analysis is also presented for two South Pacific island sites. Selenium concentrations (0.25+ or - 0.16 ng m (super -3) ) in the high-latitude North Pacific atmosphere were a factor of 2 greater and much more variable than those found at mid-North Pacific basin sites. Aerosol concentrations at five sites in the mid- and southwestern North Pacific were uniformly low, with mean concentrations of 0.10-0.13 ng Se m (super -3) . Seasonal trends in the data were not evident at these locations. Near the equator, in the productive upwelling regime of the central North Pacific, mean particulate selenium concentrations were a factor of 3-4 greater than those observed in oligotrophic midocean regions. Effects of the 1982-1983 El Nino Southern Oscillation were also apparent in the aerosol record in this region. Consideration of aerosol selenium, nitrate, and non-sea-salt sulfate data from the Pacific suggests that source functions of aerosol selenium and excess sulfate may be similar, while removal mechanisms of selenium and nitrate may be analogous. The geographical distribution of aerosol selenium appears to be more closely related to primary productivity of the surrounding waters rather than to transport of continentally derived material into the central ocean basin. We estimate an ocean-to-atmosphere vapor phase selenium flux of 5-8 X 10 (super 9) g Se yr (super -1) . The anomalous enrichment of aerosol selenium which is observed in the marine atmosphere may be explained by the gas-to-particle conversion of this naturally produced vapor phase

Carboxylic acids in the rural continental atmosphere over the eastern United States during the Shenandoah Cloud and Photochemistry Experiment

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