NO EMISSIONS FROM SOILS IN THE SOUTHEASTERN UNITED-STATES

Biogenic soil emissions of nitric oxide (NO) are important in the form ation of tropospheric ozone (O-3) in the rural environment. This paper summarizes our recent estimates of soil NO emissions from a wide vari ety of sites in nine southeastern United States, A total of over 12,00 0 individual flux measurements was made from crop, pasture, and forest s representing major physiographic regions within the southeast. These measurements were used to determine flux algorithms relating NO emiss ions from land types. Strong relationships were found between NO flux and temperature that explained over 80% of the variation in emissions. Using these relationships and Geographic Information Systems land bas e information for the states, calculated seasonal emissions for NO are reported. Crop land regulated regional soil NO emissions, accounting for 83% of the emissions in summer, 61% in spring, and 55% in fall, Th e summertime contribution of soil NO to the overall NO inventory, incl uding anthropogenic sources, averaged 4.1% for the states, ranging fro m a high of 9.5% for Mississippi to a low of 2.2% for Florida. The imp ortance of high summertime temperatures in controlling soil NO was evi dent in that emissions on a prototypical summer day were only one fift h the emissions of a hot (36 degrees C, maximum daily high) summer day . Comparisons of the algorithms we developed to those of the current m odel used by the Environmental Protection Agency, BEIS 2.2, showed rea sonable agreement, that is, agreement within a factor of 2, given the inherent temporal and spatial variability of soil NO emissions.