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, 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. NOy NO2 and NO showed diurnal variations with maxima in the morning between 0600 and 0900 EST. The maximum NOy concentration reached 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 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 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 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 NOy 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., NOx was found to be the most abundant constituent (similar to 45%) of NOy; while HNO3 was the most abundant compound in NOy measured at a remote marine site. The discrepancy between the NOy partitioning at site SONIA and the marine site is attributed to the influence of local and regional anthropogenic sources of NOx and the continental origin of the majority 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 airmasses. The NOx/NOy ratio showed strong positive correlations with the photochemical oxidants HNO3 (r = 0.76), PAN (r = 0.68) and O-3 (r = 0.79) measured at the site. Positive correlations were found between surface wind direction and both the magnitude of NO, and the NOx/NOy ratio. These correlations suggest that synoptic meteorological conditions and transport of NO, are important in the distribution of NOy and its relationship with photochemical oxidants at the site. The ozone production efficiency was illustrated by correlation of O-3 and NOz and compared with other published measurements made in the Southeast U.S., and published results from a 3D Eulerian model simulation.