Comparison of Ammonium in USA Wet Deposition to Ammonia Emission Estimates

The Acid Deposition Control Title (IV) of the 1990 Clean Air Act Amendments calls for reductions in annual sulfur dioxide and nitrogen oxide emissions in order to “reduce the adverse effects of acid deposition,” without addressing emissions of ammonia. Ammonia is present in precipitation and aerosols as the ammonium cation, NH4. Ammonium is rich in nitrogen and together with nitrate comprises virtually all of the inorganic nitrogen deposited by precipitation. A substantial database exists for ammonium in precipitation while the air quality record for ammonium is limited. This paper presents spatial and temporal patterns in the precipitation ammonium record and compares these patterns to those for ammonia emission estimates. Data from the National Trends Network (NTN) of the National Atmospheric Deposition Program (NADP) are used to examine 5-year average ammonium concentrations in precipitation for two time periods: 1985-89 and 1995-99. The spatial distributions of ammonium concentrations in precipitation in the contiguous United States are shown as maps. These maps were based on objectively-analyzed data from 150 NTN stations that met pre-selected data completeness criteria in both periods. To examine the ammonium changes between these two periods, a difference map was developed in which the objectively-analyzed 1985-89 concentrations were subtracted from the objectively-analyzed 1995-99 concentrations. Ammonium concentrations generally increased over this 10-year interval (mean difference: .054 [s.d. .052] mg/L or +25.6% [s.d. 23.5%], median difference: +.045 mg/L or +22.5%, range: -.051 mg/L to +.317 mg/L or -24.9% to +121%). For each of the time periods the area of highest ammonium concentration extends from northwestern Texas to North Dakota and eastward to southern Michigan. Emissions of airborne ammonia in the two 5-year periods were estimated in an effort to identify a potential explanation for the ammonium increases in precipitation. Sources considered were livestock manure (beef cattle, dairy cows, pigs, laying hens, broiler chickens, and turkeys), synthetic fertilizer, and automobile exhaust. Emissions from all of these source categories increased, except for dairy cattle (decreased) and beef cattle (about the same). Ammonia emissions from these categories increased by 7.8%, considerably less than the increase in precipitation ammonium concentrations. Possible reasons for this apparent inconsistency are discussed. Mass balance considerations suggest that the estimated ammonia emissions are biased substantially low.