Measurements and Analysis of Photochemical Oxidants and Trace Gases in the Rural Troposphere of the Southeast United States

Ambient concentrations of photochemical oxidants (O 3 , PAN, HNO 3 , H 2 O 2 ) and various trace species including reactive nitrogen compounds as well as total NO y were measured during June and early July 1992 at a rural site, SONIA, in the central Piedmont region of North Carolina, as a part of the Southern Oxidants Study. The measurements were made in an effort to provide a comprehensive understanding of tropospheric photochemistry in the rural Southeastern United States. NO y , NO 2 , and NO showed diurnal variations with maxima in the morning between 0600 and 0900 EST. The maximum NO y , NO, and NO 2 concentrations reached were 14.5, 5.4, and 7.8 ppbv, respectively. The mean NO y concentration was found to be 2.63 ± 1.72 ppbv ( n = 819) with an average daily maximum of 3.6 ppbv. The mean concentrations of NO and NO 2 for the entire period of measurement were found to be 0.18 ± 0.37 ppbv ( n = 794) and 1.31 ± 0.99 ppbv/( n = 769). H 2 O 2 , HNO 3 , and PAN showed diurnal variation with maxima in the afternoon and minima at night. Mean concentrations were found to be 0.52 ± 0.36 ppbv ( n = 312), 0.67 ± 0.33 ppbv ( n = 250), and 0.41 ± 0.24 ppbv ( n = 578). The NO x /NO y ratio was used as an indicator of the chemical age of airmasses and the ratio showed strong positive correlations with the photochemical oxidants HNO 3 ( r = 0.76), PAN ( r = 0.68), and O 3 ( r = 0.79) measured at the site. The relationship between the accumulation rate of O 3 and the deviation from the photostationary state was examined based on the measured PSS constant obtained from the values of [O 3 ], [NO], and [NO 2 ] measured at the site.