CHARACTERIZATION OF NITROGEN-OXIDE FLUXES FROM SOIL OF A FALLOW FIELDIN THE CENTRAL PIEDMONT OF NORTH-CAROLINA

NO(x) emissions from soils may contribute to the formation of O3 in ru ral areas, especially when there is substantial emissions of natural h ydrocarbons from surrounding vegetation, such as in the southeastern U nited States. Soil NO(x) flux measurements were made from 6 June 1992 to 7 July 1992 in the central Piedmont region of North Carolina (South east Oxidants and Nitrogen intensive Analysis site, SONIA) in an effor t to determine the role of natural emissions of NO(x) on rural atmosph eric photochemistry. The overall average NO and NO2 emission rates, us ing a dynamic chamber technique, were found to be 1.79 +/- 1.37 ng-N m -2s-1 (range: 0.13 to 6.67 ng-N m-2s-1) and -1.07+/-0.87 ng-N m-2s-1 ( range: -6.71 to 3.16 ng-N m-2s-1), respectively. Over 85% of the NO2 f lux measurements were negative indicating net deposition to the soil s urface. No negative NO flux rates were observed. NO flux was correlate d with soil temperature. There was a positive correlation between NO c oncentration near the soil surface (approximately 50 cm) and NO flux ( r = 0.35). The NO compensation point (1.12 ppbv) was estimated from th e relationship between NO emission rate and ambient NO concentrations measured at 10 m. Both positive and negative vertical gradients of NO concentration between 10 m and soil surface were detected. The positiv e vertical gradients are indicative of NO transport to the site from p olluted air masses. A significant negative correlation between NO flux and ambient O3 concentration (r=0.66), however, supports the hypothes is that soil emissions of NO contribute to local production Of O3 in r ural areas.