Trends in Agricultural Ammonia Emissions and Ammonium Concentrations in Precipitation over the Southeast and Midwest United States

Abstract

Emissions from agricultural activities, both crop and animal, are known to contain gaseous ammonia (NH3) which through chemical reaction in rainwater changes into ammonium ion (NH4+). Using wet deposition data of ammonium from several National Atmospheric Deposition Program/National Trends Network (NADP⁄NTN) and Clean Air Status Trends Network (CASTNet) sites, as well as calculated ammonia emissions from North Carolina and the Southeast and Midwest regions of the United States, trends in ammonium concentrations in precipitation were analyzed for the period of 1983-2004. In addition, HYSPLIT back-trajectory model was used to determine that when ambient air in downwind sites arrived from the high ammonia emissions source region, ammonium concentrations in precipitation were enhanced. For the Southeast United States domain, analysis shows that NH4+ concentrations generally increased with increasing NH3 emissions from within the same region. Similar analysis has been performed over the Midwest United States and compared to the results from the Southeast United States. Emissions from the Midwest are attributed to larger animals, including hogs and cattle, whereas the Southeast has a higher percentage of emissions coming from smaller livestock, such as chickens. In addition, the Midwest United States region has a much more uniform spatial distribution of emissions.

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 Midwest United States and Southeast United States was developed using data from the United States Department of Agriculture 2002 Census. This inventory indicates total annual ammonia emissions to be nearly the same over the two regions, with 1417 X 106 kg NH3⁄km2⁄year over the Southeastern United States and 1691 X 106 kg NH3⁄km2⁄year over the Midwestern United States. The emission rates are similar to those of model simulations from the Carnegie Mellon University Ammonia Model. Comparing these rates to the ammonium ion concentration and wet deposition obtained from the National Atmospheric Deposition Program (NADP) monitoring network reveals discrepancies from the results projected by the emissions inventory. The NADP network shows deposition rates of ammonium over the Southeast U.S. region at nearly twice those of the Midwest U.S. region. These contrasts have been explored and reasons for the differences are discussed.

The beginning of 1997 coincides with the implementation of a moratorium on new hog farms in the state of North Carolina. Results from the analysis in North Carolina indicate a lessening in the rate of increases in NH4+ concentration in precipitation since the moratorium went into effect. Sampson County, NC, saw stable ammonium ion concentrations from 1983-1989, an average rise of 9.5% from 1989-1996, and an average increase of only 4% from 1997-2004. In analyzing the trends in NOx, the effect of implementation of the Clean Air Act Amendments of 1990 is clearly seen in the precipitation chemistry as well.