Characterization of non-methane hydrocarbons in the rural southeast United States

Measurements of non-methane hydrocarbons, as well as ozone, meteorological and trace gas data, made at four rural sites located within the southeastern United States as a part of the Southern Oxidants Study are compared. The C2–C10 hydrocarbons were obtained during the 1200–1300 local time period, once every six days from September 1992 through October 1993. The light molecular weight alkanes (ethane, propane, n-butane, iso-butane, ethene and acetylene) display a seasonal variation of a winter maximum and summer minimum. Isoprene was virtually non-existent during the winter at all sites, and averaged from 9.8 ppbC (Yorkville, Georgia) to 21.15 ppbC (Centreville, Alabama) during the summer. The C10 terpene concentration was largest during the summer period with 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 were 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 highly 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 season. A plot of ozone versus NOγ-NO highlights the NOx limited relationship of this region.