Agricultural Ammonia Emissions and Ammonium Concentrations Associated with Precipitation in the Southeast United States
Viney P. Aneja
Department of Marine, Earth, and Atmospheric Sciences,
North Carolina State University
Raleigh, North Carolina, USA
The conversion of ammonia gas (NH3) into ammonium ion (NH4+) is a fundamental process that is of great environmental significance. Excessive amounts of NH4+ can lead to acidification of soils and other pollution problems. An agricultural ammonia emissions inventory for the Southeast United States (Alabama, Florida, Georgia, Kentucky, North Carolina, South Carolina, Mississippi, and Tennessee) was developed using data from the United States Department of Agriculture 2002 Census and published ammonia emission factors for agricultural and anthropogenic sources. Moreover, temporal and spatial variations in ammonia (NH3) emissions and ammonium (NH4+) concentrations associated with aerosols and volume-weighted NH4+ concentration in precipitation are investigated over the period 1990–1998 in the southeast United States. These variations were analyzed using an NH3 emissions inventory developed for the southeast United States and ambient NH4+ data from the various Clean Air Status and Trends Network (CASTNet) and the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). Results show that natural log-transformed annual NH4+ concentration associated with aerosols increases with natural log-transformed annual NH3 emission density within the same county (R2 = 0.86, p < 0.0001, N = 12). Natural log-transformed annual volume-weighted average NH4+ concentration in precipitation shows only a very weak positive correlation with natural log-transformed annual NH3 emission densities within the corresponding county (R2 = 0.12, p = 0.04, N = 29). Investigation into wet NH4+ concentration in precipitation consistently yielded temperature as a statistically significant (p < 0.05) parameter at individual sites. Positive trends in NH4+ concentration in precipitation were evident at NADP sites NC35, Sampson County, North Carolina (0.2–0.48 mg L-1) and KY35, Rowan County, Kentucky (0.2–0.35 mg L-1) over the period 1990–1998.