MEASUREMENTS AND ANALYSIS OF REACTIVE NITROGEN SPECIES IN THE RURAL TROPOSPHERE OF SOUTHEAST UNITED-STATES - SOUTHERN OXIDANT STUDY SITE SONIA

Ambient concentrations of reactive nitrogen compounds as well as total NO, were measured during June and early July 1992 at a rural site, si te SONIA, in the central Piedmont region of North Carolina as a part o f the Southern Oxidants Study. The measurements of reactive nitrogen s pecies were made in an effort to provide a comprehensive understanding of nitrogen chemistry and to investigate the total nitrogen budget at the site. NOy NO2 and NO showed diurnal variations with maxima in the morning between 0600 and 0900 EST. The maximum NOy concentration reac hed was similar to 14.5 ppbv, and the maximum concentrations of NO and NO2 were 5.4 and 7.8 ppbv, respectively. The mean NOy mixing ratio wa s found to be 2.88 +/- 1.58 ppbv (n = 743) with an average daily maxim um of 3.6 ppbv. The mean mixing ratios of NO and NO2 were found to be 0.15 +/- 0.29 ppbv (n = 785) and 1.31 +/- 0.99 ppbv (n = 769). Average daily maxima of NO and NO2 were 0.4 and 2.0 ppbv, respectively. HNO3 and PAN showed diurnal variation with maxima in the afternoon and mini mum in the night, and mean mixing ratios were found to be 0.67 +/- 0.3 3 ppbv (n = 250) and 0.40 +/- 0.24 ppbv (n = 578). The fractions of in dividual reactive nitrogen species to total NOy were investigated and contrasted to the results from a remote marine site and rural continen tal sites. As in two other rural continental sites in the U.S., NOx wa s found to be the most abundant constituent (similar to 45%) of NOy; w hile HNO3 was the most abundant compound in NOy measured at a remote m arine site. The discrepancy between the NOy partitioning at site SONIA and the marine site is attributed to the influence of local and regio nal anthropogenic sources of NOx and the continental origin of the maj ority of air masses encountered at the site. The NOx/NOy ratio and NOz (= NOy - NOx) were used as an indicator of the chemical age of airmas ses. The NOx/NOy ratio showed strong positive correlations with the ph otochemical oxidants HNO3 (r = 0.76), PAN (r = 0.68) and O-3 (r = 0.79 ) measured at the site. Positive correlations were found between surfa ce wind direction and both the magnitude of NO, and the NOx/NOy ratio. These correlations suggest that synoptic meteorological conditions an d transport of NO, are important in the distribution of NOy and its re lationship with photochemical oxidants at the site. The ozone producti on efficiency was illustrated by correlation of O-3 and NOz and compar ed with other published measurements made in the Southeast U.S., and p ublished results from a 3D Eulerian model simulation.