Acid Deposition Figures
Last updated: 01/2023
Wet Sulfate Deposition
- All areas of the eastern U.S. have shown significant improvement, with an overall 71 percent reduction in wet sulfate deposition from 2000–2002 to 2019–2021.
- Between 2000–2002 and 2019–2021, the Northeast and Mid-Atlantic experienced a 77 percent reduction in wet sulfate deposition.
- SO₂ emissions and the consequent decrease in the formation of sulfates that are transported long distances have resulted in reduced sulfate deposition in the Northeast. The sulfate reductions documented in the region, particularly across New England and portions of New York, were also affected by lowered SO₂ emissions in eastern Canada.1
Wet Inorganic Nitrogen Deposition
- Wet deposition of inorganic nitrogen decreased an average of 19 percent in the Mid-Atlantic and 32 percent in the Northeast but increased in the Mountain and Central regions from 2000–2002 to 2019–2021. Increases in wet deposition of inorganic nitrogen in the Rocky Mountain and Central regions are attributed to 36 and 34 percent increases in wet deposition of reduced nitrogen (NH4+), respectively, between 2000 and 2021.
- Reductions in nitrogen deposition recorded since the early 1990s have been less pronounced than those for sulfur. Emissions from other source categories (e.g., mobile sources, agriculture, biomass burning, and manufacturing) contribute to air concentrations and deposition of nitrogen.
Regional Trends in Total Deposition
- The reduction in total sulfur deposition (wet plus dry) in the eastern U.S. has been of similar magnitude to that of wet deposition with an overall average reduction of 82 percent from 2000–2002 to 2019–2021.
- Decreases in oxidized nitrogen (NOX) have generally been greater than that of reduced nitrogen (NHX) deposition. Total oxidized nitrogen deposition decreased 59 percent in the east. In contrast, total deposition of reduced nitrogen increased by an average of 46 percent in the east from 2000–2002 to 2019–2021.
As SO₂ and NOₓ gases react in the atmosphere with water, oxygen, and other pollutants, they form acidic compounds that are deposited to the earth’s surface in the form of wet and dry deposition.
Long-term monitoring network data show significant improvements in the primary indicators of acid deposition. For example, wet sulfate deposition (sulfate that falls to the earth through rain, snow, and other forms of precipitation) has decreased in much of the eastern United States due to SO₂ emission reductions achieved through implementation of the Acid Rain Program (ARP), the Clean Air Interstate Rule (CAIR) and the Cross-State Air Pollution Rule (CSAPR). Some of the most dramatic reductions have occurred in the mid-Appalachian region, including Maryland, New York, West Virginia, Virginia, and most of Pennsylvania. Along with wet sulfate deposition, precipitation acidity, expressed as hydrogen ion (H+ or pH) concentration, has also decreased by similar percentages.
Reductions in nitrogen deposition compared to the early 1990s have been less pronounced than those for sulfur. As noted earlier, emissions from source categories other than ARP and CSAPR regulated sources contribute to changes in air concentrations and deposition of oxidized, reduced, and organic forms of nitrogen.
The Clean Air Status and Trends Network (CASTNET) provides long-term monitoring of regional air quality to determine trends in atmospheric concentrations and deposition of nitrogen, sulfur, and ozone in order to evaluate the effectiveness of national and regional air pollution control programs. In 2021, CASTNET operated 100 regional sites throughout the contiguous U.S., Alaska, and Canada. Sites are located in areas where urban influences are minimal.
The National Atmospheric Deposition Program/National Trends Network (NADP/NTN) is a nationwide, long-term network tracking the chemistry of precipitation. The NADP/NTN provides concentration and wet deposition data on hydrogen ion (acidity as pH), sulfate, nitrate, ammonium, chloride, and base cations. The NADP/NTN has grown to more than 250 sites spanning the U.S., Canada, Puerto Rico, and the Virgin Islands.
Together, these complementary networks provide long-term data needed to estimate spatial patterns and temporal trends in total deposition.2 Maps and regional trends provided in this chapter were produced using the measurement-model fusion method developed by NADP’s Total Deposition Science Committee. Briefly, CASTNET and NADP/NTN data are combined with modeled deposition results from EPA’s Community Multiscale Air Quality Model (CMAQ) to produce gridded estimates of total deposition. The deposition values provuded in this report have been updated using CMAQv5.3.2, incorporating the state of the science input data for emissions, meteorology, and air quality over the timeseries (2002-2019).3 Improvements to the model have resulted in significant changes to the modeled deposition (e.g., reduced dry nitrogen deposition, non-measured oxidized nitrogen deposition).
- Acid Rain
- Clean Air Status and Trends Network (CASTNET)
- EPA’s Air QUAlity TimE Series (EQUATES) for the Community Multi-Scale Air Quality Modeling System (CMAQ)
- National Atmospheric Deposition Program (NADP) Exit
- Government of Canada, Environment Canada. (2018). Canada-United States Air Quality Agreement Progress Report 2016. ISSN: 1910–5223: Cat. No.: En85-1E-PDF.
- Schwede, DB and Lear, GG. (2014). A novel hybrid approach for estimating total deposition in the United States. Atmosphere Environment 92: 207-220.
- Appel, K.W., Bash, J.O., Fahey, K.M., Foley, K.M., Gilliam, R.C., Hogrefe, C., Hutzell, W.T., Kang, D., Mathur, R., Murphy, B.N., Napelenok, S.L., Nolte, C.G., Pleim, J.E., Pouliot, G.A., Pye, H.O.T., Ran, L., Roselle, S.J., Sarwar, G., Schwede, D.B., Sidi, F.I., Spero, T.L., and Wong, D.C. The Community Multiscale Air Quality (CMAQ) model versions 5.3 and 5.3.1: system updates and evaluation, Geosci. Model Dev., 14, 2867-2897, https://doi.org/10.5194/gmd-14-2867-2021, 2021.