Airborne measurements of isoprene, CO, and anthropogenic hydrocarbons and their implications

Measurements of the mixing ratios of light hydrocarbons (greater than or equal to C-6) were made aboard the National Oceanic and Atmospheric Administration Lockheed Orion WP-3 aircraft during June and July of 1994 and 1995 during a series of flights over the region around Nashville, Tennessee. The measurements were carried out as part of the Nashville/ Middle Tennessee Study of the Southern Oxidant Study (SOS) whose purpose was to describe the sources, variation, and distribution of ozone and its precursors in the southeastern United States during the summer season. The spatial (altitude and latitude) distribution of isoprene and anthropogenic nonmethane hydrocarbons (NMHCs) is described. The isoprene distribution measured within the planetary boundary layer over the region is compared to the recent inventories. The correlations between the various anthropogenic hydrocarbons are used to demonstrate the combined influence of OH photochemistry and dilution on their concentrations. Finally, the distributions of those NMHCs that were measured are compared to those of CO and methane and discussed in terms their implications for odd hydrogen photochemistry and our understanding of regional ozone production. In the boundary layer, OH reactions are dominated by isoprene in the southern part of the region explored and by CO and methane in the northern part. The other measured NMHCs are seen to play only a minor role in ozone production in the nonurban atmosphere.