Monitoring Chemical Climate Change in America - The Case for Ammonia

The National Atmospheric Deposition Program - National Trends Network (NTN) has measured the acids, nutrients, and base cations in U.S. precipitation for more than two decades, long enough to identify a “chemical climate” and observe its changes. Much as climatologists describe the physical characteristics of our average weather (i.e., climate) and use long-term temperature records to evaluate climate change, scientists use NTN data to calculate mean chemical concentrations and deposition fluxes to evaluate chemical climate changes. Precipitation chemistry is an indicator of chemical climate, since precipitation scavenges airborne gases and particles, which are affected by emissions, chemical transformations, and weather. NTN data indicate that significant changes have occurred in precipitation chemistry, particularly for the ammonia/ammonium system and important related chemical species.

A Seasonal Kendall Trend Test was applied to precipitation-weighted concentration data from 139 NTN sites operational from 1985 to 2002 (see Nilles and Conley, Water, Air, Soil Pollution 130:409-414, 2001). Ammonium increased over this 18-year period at 124 sites (89%) and the increases were statistically significant (p<0.05) at 58 sites (42%). Central and northern Midwestern states generally had the largest ammonium increases. Significant decreases occurred at just two sites. The median annual ammonium change over all 139 sites was 0.17 microequivalents/liter/year; the median ammonium concentration increased ~30% from 1985 to 2002. Citing U.S. Environmental Protection Agency data (www.epa.gov/ttn/chief/trends/inex.html), ammonia emissions increased, as well, from 3.9 teragrams (Tg) in 1990 to 4.5 Tg in 2001. Ammonia emissions and ammonium concentrations have increased, signaling important changes in our chemical climate over the last 15 - 20 years.

Generally higher ammonium concentrations in precipitation have been accompanied by significant sulfate decreases (e.g., Lynch, et al., Environmental Science & Technology 34:940-949) and locally significant nitrate increases and decreases. The relative amounts of sulfate, nitrate, and ammonium in precipitation have changed markedly. This signals a change in the mix of gases and particles being scavenged by precipitation. This change is important to our understanding of the changing chemical climate of America.

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