Agricultural ammonia emissions and ammonium concentrations associated with aerosols and precipitation in the southeast United States

Temporal and spatial variations in ammonia (NH 3 ) emissions and ammonium (NH 4 + ) concentrations associated with aerosols and volume‐weighted NH 4 + concentration in precipitation are investigated over the period 1990–1998 in the southeast United States (Alabama, Florida, Georgia, Kentucky, North Carolina, South Carolina, Mississippi, and Tennessee). These variations were analyzed using an NH 3 emissions inventory developed for the southeast United States and ambient NH 4 + 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 NH 4 + concentration associated with aerosols increases with natural log‐transformed annual NH 3 emission density within the same county (R 2 = 0.86, p < 0.0001, N = 12). Natural log‐transformed annual volume‐weighted average NH 4 + concentration in precipitation shows only a very weak positive correlation with natural log‐transformed annual NH 3 emission densities within the corresponding county (R 2 = 0.12, p = 0.04, N = 29). Analysis of NH 4 + concentration associated with aerosols at CASTNet sites revealed that temperature, precipitation amount, and relative humidity are the most statistically significant (p < 0.05) parameters in predicting the weekly concentrations of NH 4 + during the period 1990–1998. Wind speed and wind direction were also statistically significant (p < 0.05) at several CASTNet sites, but the results were less consistent. Investigation into wet NH 4 + concentration in precipitation consistently yielded temperature as a statistically significant (p < 0.05) parameter at individual sites. Trends over the period 1990–1998 revealed a slight decrease in NH 4 + concentration at CASTNet site SPD, Claiborne County, Tennessee (2.14–1.88 μg m −3 ), while positive trends in NH 4 + 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.