Progress Report - Atmospheric Deposition

Atmospheric Deposition

Atmospheric deposition is the transfer of air pollutants (gases or particles) to the Earth’s surface. The effects of atmospheric deposition on the environment are determined by the amount and mixture of pollutants being deposited. Acid deposition, commonly known as “acid rain,” is a broad term referring to the mixture of wet and dry deposition from the atmosphere containing higher than normal amounts of oxidized sulfur and nitrogen-containing acidic pollutants. The precursors of acid deposition are primarily the result of emissions of sulfur dioxide (SO₂) and nitrogen oxides (NO_X) from fossil fuel combustion; however, natural sources, such as volcanoes and decaying vegetation, also contribute a small amount.

Atmospheric deposition of oxidized and reduced forms of nitrogen also cause changes in the environment other than acidification. For example, eutrophication of water bodies from the deposit of reduced nitrogen (ammonia/ammonium), in combination with other sources of nutrients (e.g., sources of nitrogen and phosphorus), can lead to harmful algal blooms and contaminated drinking water. Ammonia (NH₃) and ammonium (NH₄) are precursors of secondary PM2.5 and are often emitted from fossil fuel combustion emission controls (e.g., catalytic converters), agriculture, as well as natural sources (e.g., biomass burning).

Total nitrogen in the atmosphere is made up of inorganic and organic compounds that contribute to the global nitrogen cycle. The key difference between organic and inorganic nitrogen is that organic nitrogen is the nitrogen that occurs in organic compounds whereas the inorganic nitrogen is nitrogen that occurs in inorganic compounds. Organic compounds are chemical species containing carbon and hydrogen atoms as essential components, whereas inorganic compounds do not contain carbon and hydrogen as essential components. Inorganic nitrogen includes oxidized (nitrate, nitrite) and reduced (ammonia, ammonium) forms of nitrogen. Oxidized nitrogen compounds are formed by reactions containing oxygen atoms, while reduced nitrogen accepts a hydrogen atom during reactions. Routine monitoring networks capture the primary components of inorganic nitrogen. Organic nitrogen (e.g. peroxyacyl nitrates, alkyl nitrates) in the atmosphere remains poorly understood due to the lack of measurements, but is suspected to make up 6-20% of the total nitrogen budget in the U.S.¹

Wet Sulfate Deposition

• All areas of the eastern United States have shown significant improvement, with an overall 73 percent reduction in wet sulfate deposition from 2000–2002 to 2020–2022.
• Between 2000–2002 and 2020–2022, the Northeast and Mid-Atlantic experienced a 78 percent reduction in wet sulfate deposition.
• SO₂ emissions reductions and the consequent decrease in the formation of sulfates that are transported long distances have resulted in significantly 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 SO₂ emissions reductions in eastern Canada.²

Wet Inorganic Nitrogen Deposition

• Wet deposition of inorganic nitrogen decreased an average of 29 percent in the Mid-Atlantic and 37 percent in the Northeast but stayed neutral in the Mountain and North and South Central regions from 2000–2002 to 2020–2022. These neutral trends in the Rocky Mountain and North and South Central regions reflect increases in wet deposition of reduced nitrogen, combined with decreases in deposition of oxidized nitrogen between 2000 and 2022.
• 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. was 82 percent from 2000–2002 to 2020–2022, a value of similar magnitude to that of wet sulfate deposition.
• Decreases in oxidized nitrogen (NO_X) have generally been greater than that of reduced nitrogen (NH_X) deposition. Total oxidized nitrogen deposition decreased 59 percent in the East. In contrast, total deposition of reduced nitrogen increased by an average of 43 percent in the East from 2000–2002 to 2020–2022.

Background Information

Atmospheric Deposition

As SO₂ and NO_X 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 U.S. 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) programs. 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. Nitrogen is comprised of oxidized, reduced, and organic forms. Decreases in oxidized nitrogen also have resulted from emissions reductions from sources under the ARP and CSAPR programs. However, a portion of oxidized nitrogen, and reduced and organic forms of nitrogen, are emitted from unregulated sources and these emissions have remained neutral or increased over this period.

Monitoring Networks

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 2022, CASTNET operated 100 regional sites throughout the contiguous U.S., Alaska, and Canada. Sites are located in areas where stationary source 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. The NADP also operates the Ammonia Monitoring Network (NADP/AMoN), a nationwide network tracking air concentrations of gaseous ammonia (NH₃). Ammonia is a significant contributor to PM2.5 formation. In 2022, there were more than 100 AMoN sites across the U.S., Canada, and Puerto Rico. 

Together, these complementary networks provide long-term data needed to estimate spatial patterns and temporal trends in total deposition.³ 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 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 provided 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 (2002-2022).⁴ Improvements to the model have resulted in significant changes to the modeled deposition (e.g., reduced dry nitrogen deposition, non-measured oxidized nitrogen deposition).

More Information

• Acid Rain
• Clean Air Status and Trends Network (CASTNET)
• EPA Air QUAlity TimE Series (EQUATES) for the Community Multi-Scale Air Quality Modeling System (CMAQ)
• National Atmospheric Deposition Program (NADP)

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¹Jickells, T., Baker, A.R., Cape, J.N, Cornell, S.E., Nemitz (2013) The cycling of organic nitrogen through the atmosphere. Philos Trans R Soc Lond B Biol Sci 368(1621). DOI: 10.1098/rstb.2013.0115.
² Government of Canada, Environment Canada. (2023). Canada-United States Air Quality Agreement Progress Report 2020-2022. ISSN: 1910–5223: Cat. No.: En85-1E-PDF.
³ Schwede, DB and Lear, GG. (2014). A novel hybrid approach for estimating total deposition in the United States. Atmospheric Environment 92: 207-220.
⁴ 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.