Measurements and analysis of reactive nitrogen species in the rural troposphere of Southeast United States: Southern oxidant study site SONIA

Abstract

Ambient concentrations of reactive nitrogen compounds as well as total NOY were measured during June and early July 1992 at a rural site, site SONIA, in the central Piedmont region of North Carolina as a part of the Southern Oxidants Study. The measurements of reactive nitrogen species were made in an effort to provide a comprehensive understanding of nitrogen chemistry and to investigate the total nitrogen budget at the site. NO y , NO 2 and NO showed diurnal variations with maxima in the morning between 0600 and 0900 EST. The maximum NO y concentration reached was ∼ 14.5 ppbv, and the maximum concentrations of NO and NO 2 were 5.4 and 7.8 ppbv, respectively. The mean NO y mixing ratio was found to be 2.88 ± 1.58 ppbv ( n = 743) with an average daily maximum of 3.6 ppbv. The mean mixing ratios of NO and N0 2 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 N0 2 were 0.4 and 2.0 ppbv, respectively. HNO 3 and PAN showed diurnal variation with maxima in the afternoon and minimum in the night, and mean mixing ratios were found to be 0.67 ± 0.33 ppbv ( n = 250) and 0.40 ± 0.24 ppbv ( n = 578). The fractions of individual reactive nitrogen species to total NO y , were investigated and contrasted to the results from a remote marine site and rural continental sites. As in two other rural continental sites in the U.S., NO x was found to be the most abundant constituent (∼45%) of NOY; while HNO 3 was the most abundant compound in NO y measured at a remote marine site. The discrepancy between the NO y , partitioning at site SONIA and the marine site is attributed to the influence of local and regional anthropogenic sources of NO x and the continental origin of the majority of air masses encountered at the site. The NO x NO y ratio and NO z ( = NO y − NO x ) were used as an indicator of the chemical age of airmasses. The NO x NO y ratio showed strong positive correlations with the photochemical oxidants HNO 3 ( r = 0.76), PAN ( r = 0.68) and 0 3 ( r = 0.79) measured at the site. Positive correlations were found between surface wind direction and both the magnitude of NO y and the NO x NO y ratio. These correlations suggest that synoptic meteorological conditions and transport of NO x are important in the distribution of NO y and its relationship with photochemical oxidants at the site. The ozone production efficiency was illustrated by correlation of O 3 and NO z and compared with other published measurements made in the Southeast U.S., and published results from a 3D Eulerian model simulation.