Tropospheric light alcohols, carbonyls, and acetonitrile: Concentrations in the southwestern United States and Henry's Law data

Abstract

Aliphatic alcohols (C 1 ‐ C 4), aldehydes (C 1 ‐ C 2) and ketones (C 3 ‐ C 4) have been determined at Tucson, Arizona, and at two rural sites about 40 km distant. Acetonitrile was also measured at the rural sites. The method involved condensation sampling, condensate preconcentration, and gas chromatography. Henry's law coefficients were required for all components and were determined. Mean concentrations in Tucson were higher than those in the rural areas by factors typically between 2 and 8; urban formaldehyde was only slightly elevated. Mean alcohol concentrations ranged from 7.9 ppb (C 1) to 0.12 ppb (C 4) within the city and from 2.6 ppb (C 1) to 0.06 ppb (C 4) at the rural sites. Acetone was found at 12 ppb in the city and 2.8 ppb at the rural sites. Concentrations of butanone were a factor of 5 lower. Acetaldehyde, at 23 ppb (city) and 6.9 ppb (rural), far exceeded formaldehyde concentrations (1.8 ppb in the city, and 1.5 ppb at the rural sites). Acetonitrile was found at the rural sites at a mean concentration of 60 ppt. A dimensionless Henry's law coefficient (mol L−1 of liquid/mol L−1 of vapor) was suprisingly similar for the alcohols at 0°C, ranging between 2×10 4 and 3.4×10 4 (900–1500 mol L−1 atm−1) the ketones were a factor of 10 lower. For acetaldehyde the coefficient was 1.7×10 3 (76 mol L−1 atm−1) and for acetonitrile 3.7×10 3 (165 mol L−1 atm−1). Concentrations of oxygenated organics in the condensates and in precipitation were compared; it was tentatively concluded that concentration differences of the carbonyls were consistent with these species being produced within the cloud, for example, by aqueous photochemistry.