Hydrocarbon measurements in the southeastern United States: The Rural Oxidants in the Southern Environment (ROSE) Program 1990

Abstract

An automated gas Chromatographic system was employed at a rural site in western central Alabama to measure atmospheric hydrocarbons and oxygenated hydrocarbons (oxy‐hydrocarbons) on an hourly basis from June 8 to July 19, 1990. The location, which was a designated site for the Southern Oxidant Study (SOS), was instrumented for a wide variety of measurements allowing the hydrocarbon and oxy‐hydrocarbon measurements to be interpreted both in terms of meteorological data and as part of a large suite of gas phase measurements. Although the site is situated in a Loblolly pine plantation, isoprene was observed to be the dominant hydrocarbon during the daytime with afternoon maxima of about 7 parts per billion by volume (ppbv). Decrease of isoprene after sunset was too rapid to be accounted for solely on the basis of gas phase chemistry. During the nighttime, α‐pinene and β‐pinene were the dominant hydrocarbons of natural origin. The ratio of α‐pinene to β‐pinene showed a well‐defined diurnal pattern, decreasing by more than 30% during the night; a decrease that could be understood on the basis of local gas phase chemistry. Oxy‐hydrocarbons, dominated by methanol and acetone, were the most abundant compounds observed. On a carbon atom basis, the oxy‐hydrocarbons contributed about 46% of the measured atmospheric burden during the daytime and about 40% at night. The similarity of the observed diurnal methanol variation to that of isoprene and subsequent measurements [McDonald and Fall, 1993] indicate that much of the observed methanol was of local biogenic origin. Correlation of acetone with methanol suggests that it, also, has a significant biogenic source. In spite of the site's rural location, anthropogenic hydrocarbons constituted, on a carbon atom basis, about 21% of the hydrocarbon burden measured during the daytime and about 55% at night. Significant diurnal variations of the anthropogenic hydrocarbons, with increases at night, appeared to be driven by the frequent formation of a shallow nocturnal boundary layer.