Characterization of nitrogen oxide fluxes from soil of a fallow field in the central piedmont of North Carolina

Abstract

Abstract NO x emissions from soils may contribute to the formation of 0 3 in rural areas, especially when there is substantial emissions of natural hydrocarbons from surrounding vegetation, such as in the southeastern United States. Soil NO, flux measurements were made from 6 June 1992 to 7 July 1992 in the central Piedmont region of North Carolina (Southeast Oxidants and Nitrogen Intensive Analysis site, SONIA) in an effort to determine the role of natural emissions of NO, on rural atmospheric photochemistry. The overall average NO and N0 2 emission rates, using a dynamic chamber technique, were found to be 1.79 ± 1.37 ng-N m −2 s −1 (range: 0.13 to 6.67 ng-N m − s −1 ') and -1.07 ± 0.87 ng-N m −2 s −1 (range: -6.71 to 3.16ng-Nm −2 s −1 respectively. Over 85% of the N0 2 flux measurements were negative indicating net deposition to the soil surface. No negative NO flux rates were observed. NO flux was correlated with soil temperature. There was a positive correlation between NO concentration near the soil surface (∼50 cm) and NO flux ( r =0.35). The NO compensation point (1.12 ppbv) was estimated from the 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 positive vertical gradients are indicative of NO transport to the site from polluted air masses. A significant negative correlation between NO flux and ambient 03 concentration ( r =0.66), however, supports the hypothesis that soil emissions of NO contribute to local production of 03 in rural areas.