NO emissions from soils in the southeastern United States

Biogenic soil emissions of nitric oxide (NO) are important in the formation of tropospheric ozone (O3) in the rural environment. This paper summarizes our recent estimates of soil NO emissions from a wide variety of sites in nine southeastern United States. A total of over 12,000 individual flux measurements was made from crop, pasture, and forests representing major physiographic regions within the southeast. These measurements were used to determine flux algorithms relating NO emissions 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 base 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. The summertime contribution of soil NO to the overall NO inventory, including anthropogenic sources, averaged 4.1% for the states, ranging from a high of 9.5% for Mississippi to a low of 2.2% for Florida. The importance of high summertime temperatures in controlling soil NO was evident in that emissions on a prototypical summer day were only one fifth the emissions of a hot (36°C, maximum daily high) summer day. Comparisons of the algorithms we developed to those of the current model used by the Environmental Protection Agency, BEIS 2.2, showed reasonable agreement, that is, agreement within a factor of 2, given the inherent temporal and spatial variability of soil NO emissions.