Ammonium Wet Deposition in Eastern North America From 1980 to 1999

Since the late 1970s, air and precipitation monitoring programs have been in place in Canada and the U.S.A. Since then, 12 Canadian precipitation-monitoring networks and 11 US networks have been operational in eastern North America. The data from the networks have been combined to produce wet deposition and concentration maps.

Annual and long-term average concentration and deposition patterns of NH₄⁺ were produced for eastern North America from 1980 to 1999. High values of NH₄⁺ concentration (>0.4 mg/L) and deposition (>3.0 kg/ha/yr) occurred in an area covering southern Manitoba, Ontario, Quebec, and most of the U.S. Great Plains and Midwest. The major land use in this area is agricultural, suggesting a strong influence of fertilizer and livestock on precipitation composition. This appears to be confirmed by the local concentration and deposition maxima (>0.5 mg/L and 4.0 kg/ha/yr, respectively) located in the intensive agricultural areas of southwestern Ontario and South Dakota, Nebraska and Iowa. The extension of the 0.2 mg/L and 2 kg/ha/yr isopleths into northern Ontario and Quebec is somewhat surprising given the boreal forest land use that far north. This strongly suggests an influence of long-range transport from the agricultural areas to the south.

The patterns of NH₄⁺ concentration and deposition and the NH₃ emission patterns illustrate the relationship between precipitation chemistry and emissions, and provide evidence of long-range transport into northern Canada. NH₄⁺ wet deposition increased slightly between early 1980s and late 1990s. More than 40% of the total nitrogen wet deposition in eastern North America is from NH₄⁺ and the rest is from NO₃^-.