HYDROCARBON MEASUREMENTS IN THE SOUTHEASTERN UNITED-STATES - THE RURAL OXIDANTS IN THE SOUTHERN ENVIRONMENT (ROSE) PROGRAM 1990

An automated gas chromatographic system was employed at a rural site i n western central Alabama to measure atmospheric hydrocarbons and oxyg enated hydrocarbons (oxy-hydrocarbons) on an hourly basis from June 8 to July 19, 1990. The location, which was a designated site for the So uthern Oxidant Study (SOS), was instrumented for a wide variety of mea surements 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 i n 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 t oo rapid to be accounted for solely on the basis of gas phase chemistr y. During the nighttime, alpha-pinene and beta-pinene were the dominan t hydrocarbons of natural origin. The ratio of alpha-pinene to beta-pi nene showed a well-defined diurnal pattern, decreasing by more than 30 % during the night; a decrease that could be understood on the basis o f local gas phase chemistry. Oxy-hydrocarbons, dominated by methanol a nd acetone, were the most abundant compounds observed. On a carbon ato m basis, the oxy-hydrocarbons contributed about 46% of the measured at mospheric burden during the daytime and about 40% at night. The simila rity of the observed diurnal methanol variation to that of isoprene an d 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 bioge nic source. In spite of the site's rural location, anthropogenic hydro carbons constituted, on a carbon atom basis, about 21% of the hydrocar bon burden measured during the daytime and about 55% at night. Signifi cant diurnal variations of the anthropogenic hydrocarbons, with increa ses at night, appeared to be driven by the frequent formation of a sha llow nocturnal boundary layer.