A Regional Analysis of Non-Methane Hydrocarbons And Meteorology of The Rural Southeast United States

Measurements of non-methane hydrocarbons, as well as ozone, meteorological and trace gas data, were made at four rural sites located within the southeastern United States as a part of the Southern Oxidants Study. Fifty-six C2-C10 hydrocarbons were collected from 1200-1300 local time, once every six days from September 1992 through October 1993. The measurements were made in an effort to enhance the understanding of the behavior and trends of ozone and other photochemical oxidants in this region. The light molecular weight alkanes (ethane, propane, n-butane, iso-butane), ethene and acetylene display a seasonal variation with a winter maximum and summer minimum. Isoprene was virtually non-existent during the winter at all sites, and averaged from 9.8 ppbC (Yorkville, GA) to 21.15 ppbC (Centreville, AL) during the summer. The terpene concentration was greatest in the summer with averages ranging between 3.19 ppbC (Centreville, AL) to 6.38 ppbC (Oak Grove, MS), but was also emitted during the winter months, with a range of 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 regards to the highly reactive biogenic compounds such as isoprene. It was calculated that biogenics represent at least 65% of the total non-methane hydrocarbon sum at these four sites during the summer season when considering propylene-equivalent concentrations. An ozone episode which occurred from July 20 to July 24 1993 was used as an example to show ozone profiles at each of the sites, and to show the effect of synoptic meteorology on high ozone by examining NOAA daily weather maps and climatic data.