CHARACTERIZATION OF NONMETHANE HYDROCARBONS IN THE RURAL SOUTHEAST UNITED-STATES

Measurements of non-methane hydrocarbons, as well as ozone, meteorolog ical and trace gas data, made at four rural sites located within the s outheastern United States as a part of the Southern Oxidants Study are compared. The C-2-C-10 hydrocarbons were obtained during the 1200-130 0 local time period, once every six days from September 1992 through O ctober 1993. The light molecular weight alkanes (ethane, propane, rr-b utane, iso-butane, ethene and acetylene) display a seasonal variation of a winter maximum and summer minimum. Isoprene was virtually non-exi stent during the winter at all sites, and averaged From 9.8 ppbC (York ville, Georgia) to 21.15 ppbC(Centreville, Alabama) during the summer. The Clo terpene concentration was largest during the summer period wi th averages ranging between 3.19 ppbC (Centreville, Alabama) and 6.38 ppbC (Oak Grove, Mississippi); winter time concentrations ranged from 1.25 to 1.9 ppbC for all sites. Propylene-equivalent concentrations we re calculated to account for differences in reaction rates between the hydroxyl radical and individual hydrocarbons, and to thereby estimate their relative contribution to ozone, especially in regard to the hig hly reactive biogenic compounds such as isoprene. The propy-equivalent concentrations from the biogenics represent at least 65% of the total non-methane hydrocarbon sum at these four sites during the summer sea son. A plot of ozone versus NOy-NO highlights the NOx limited relation ship of this region. (C) 1997 Elsevier Science Ltd.