Acid Deposition 

Q: What are the trends in outdoor air quality and their effects on human health and the

environment?

The above question pertains to all 'Outdoor Air' Indicators, however, the information on these pages

(overview, graphics, references and metadata) relates specifically to "Acid Deposition". Use the

right side drop list to view the other related indicators on this question.

Introduction

Every year, millions of tons of sulfur dioxide and nitrogen oxides are emitted to the atmosphere as a

result of the burning of fossil fuels and from other high temperature sources (the Sulfur Dioxide

Emissions indicator; the Nitrogen Oxides Emissions indicator). These gases react with water,

oxygen, and oxidants to form acidic compounds, which may be carried hundreds of miles by the

wind—even across state or national borders. Acid deposition occurs when these compounds fall to

the Earth in one of two forms: wet (dissolved in rain, snow, and fog) or dry (solid and gaseous

particles deposited on surfaces during periods of no precipitation). While wet deposition is the more

widely recognized form (more commonly referred to as “acid rain”), dry deposition can account for

20 to 80 percent of total acid deposition depending on location and climate (MACTEC Engineering

and Consulting, Inc., 2005). In the environment, acid deposition causes soils and water bodies to

acidify, which can make the water unsuitable for some fish and other wildlife. Some types of

ecosystems, those with less “buffering” capacity, are more sensitive to acid deposition than others. 

Scientists often use acid neutralizing capacity, a measure of the amount of anions, protons, and

non-proton cations in the water, as an indicator of which lakes and streams are most sensitive to

acidification (NAPAP, 1991). Most surface waters in the West do not exhibit many symptoms of

acidification, because relatively small amounts of acid deposition occur in acid-sensitive regions. In

the Northeast and along the Appalachian Mountains, however, relatively high levels of acid

deposition occur in acid-sensitive regions, or regions without enough geochemical buffering

capacity to prevent acidification of surface waters by acid deposition (the Lake and Stream Acidity

indicator). Therefore, reductions in acid deposition have the largest impact on acidification of lakes

and streams in those areas. 

Acid deposition damages some trees, particularly at high elevations, and speeds the decay of

buildings, statues, and sculptures that are part of our national heritage (U.S. EPA, 2003). The

nitrogen portion of acid deposition also contributes to eutrophication in coastal ecosystems, the

symptoms of which include potentially toxic algal blooms, fish kills, and loss of plant and animal

diversity. Acidification of lakes and streams can increase the amount of methylmercury available in

aquatic systems (Winfrey and Rudd, 1990). Finally, increased levels of sulfate in ground-level air, a

phenomenon related to dry deposition, can contribute to decreased visibility as well as a variety of

human health problems (U.S. EPA, 2003). 

Total acid deposition in this indicator is determined using wet deposition measurements and dry

deposition calculated from ambient air concentration measurements. Wet deposition is measured

through chemical analysis of rainwater collected at sites across the U.S. The primary source of wet

deposition information comes from the National Atmospheric Deposition Program/National Trends

Network. The chemical components of wet deposition include sulfate, nitrate, and ammonium. Dry

deposition is not measured directly. EPA’s Clean Air Status and Trends Network determines dry

deposition inferentially by measuring ambient air concentrations of acidic compounds and then
calculating deposition rates using a multi-layer model that depends on meteorological data collected

at the sites as well as local vegetative conditions ( http://www.epa.gov/castnet/). Chemicals

measured include components of particulate matter (sulfate [SO4

2-] and nitrate [NO3-]), gaseous

nitric acid (HNO3), sulfur dioxide (SO2), ammonia (NH3), and ammonium (NH4

+).

This indicator uses the 3-year average from 1989-1991 as a baseline, as this period immediately

predates controls on sulfur and nitrogen oxide emissions mandated by the 1990 Clean Air Act

Amendments. Baseline data are compared to the most recent 3-year average data available

(2005-2007). Use of 3-year average data helps ensure that trends reflect actual changes in acid

deposition, instead of shorter-term fluctuations in meteorological conditions. Additionally, this

indicator presents annual trend data for total deposition, which characterizes deposition over the

entire period of record, not just for the baseline and most recent 3-year average periods. 

What The Data Show

Wet Deposition Trends

Analyses of long-term monitoring data from the National Atmospheric Deposition Program show

that wet deposition of both sulfur and nitrogen compounds has decreased over the last 19 years

(Exhibits 2-29 and 2-30). 

Wet sulfate deposition decreased across much of the U.S. since 1989 (Exhibit 2-29). The greatest

reductions in wet sulfate deposition occurred in the Mid-Appalachian region (Maryland, New York,

West Virginia, Virginia, and most of Pennsylvania) and the Ohio River Valley. Less dramatic

reductions were observed across much of New England and portions of the Southern Appalachians.

Average regional decreases in wet deposition of sulfate between the periods 1989-1991 (panel A)

and 2005-2007 (panel B) were approximately 35 percent in the Northeast, 33 percent in the

Midwest, 29 percent in the Mid-Atlantic, and 28 percent in the Southeast. 

Wet nitrate deposition decreased approximately 31 percent across the Northeast and 29 percent in

the Mid-Atlantic between the periods 1989-1991 (Exhibit 2-30, panel A) and 2005-2007 (panel B).

However, there is a high degree of variability in the measurements used to calculate these

percentages, complicating efforts to reliably estimate trends for wet nitrate deposition. Wet

deposition of inorganic nitrogen has not changed substantially in the rest of the country over this

period. 

Total Deposition Trends

As with wet deposition, total deposition (the sum of wet and dry deposition) decreased between

1989-1991 and 2005-2007, and reductions were more substantial for sulfur compounds than for

nitrogen compounds (Exhibits 2-31 and 2-32). In the eastern U.S., where data are most abundant,

total sulfur deposition decreased by 44 percent between 1990 and 2007 (Exhibit 2-33), while total

nitrogen deposition decreased by 25 percent over the same time frame (Exhibit 2-34). Note that total

nitrogen deposition in this indicator does not include nitrogen components, such as ammonia, which

can be a significant portion of the dry deposition. 







Limitations

Geographic coverage is limited, particularly for dry deposition (and thus total deposition as

well), but the concentration of sites in the Midwest and Northeast is justified by the fact that

acid rain is much more of a problem in those regions than it is in the West, Great Plains, or

Southeast.

Measurement techniques for dry deposition have improved substantially, but characterization

of dry deposition still requires a combination of measurements and modeling, which has

inherent uncertainties. Further, dry deposition presented in this indicator does not include

contributions from deposition of gaseous ammonia.

Data Sources
Summary data in this indicator were provided by EPA’s Office of Atmospheric Programs, based on

deposition data from two sources. Wet deposition data are from the National Atmospheric

Deposition Program/National Trends Network (NADP, 2008) ( http://nadp.sws.uiuc.edu/), and dry

deposition data are from the Clean Air Status and Trends Network (U.S. EPA, 2008)

(http://www.epa.gov/castnet). This indicator aggregates data across 3-year periods to avoid

influences from short-term fluctuations in meteorological conditions, and wet deposition data were

interpolated among monitoring stations to generate the maps shown in Exhibits 2-29 and 2-30. 

References

MACTEC Engineering and Consulting, Inc. 2008. Data from the Clean Air Status and Trends

Network. Accessed 2008. http://www.epa.gov/castnet/data.html

MACTEC Engineering and Consulting, Inc. 2005. Clean Air Status and Trends Network

(CASTNET): 2004 annual report. Prepared for U.S. EPA, Office of Air and Radiation. 

http://www.epa.gov/castnet/docs/annual_report_2004.pdf

NADP (National Atmospheric Deposition Program). 2008. Data from the NADP/National Trends

Network. Accessed 2008. http://nadp.sws.uiuc.edu

NAPAP (National Acid Precipitation Assessment Program). 1991. 1990 integrated assessment

report. Washington, DC.

U.S. EPA (United States Environmental Protection Agency). 2008. Data from the Clean Air Status

and Trends Network. Accessed 2008. http://www.epa.gov/castnet/

U.S. EPA. 2003. Latest findings on national air quality: 2002 status and trends. EPA/454/K-03/001.

Research Triangle Park, NC. http://www.epa.gov/air/airtrends/aqtrnd02/2002_airtrends_final.pdf

Winfrey, M.R., and J.W.M. Rudd. 1990. Environmental factors affecting the formation of methyl

mercury in low pH lakes. Environ. Toxicol. Chem. 9(7):853-869.