Atmospheric Deposition Program of the 

U.S. Geological Survey

INTRODUCTION

Nearly 200 million tons of criteria air pollutants (sulfur 

dioxide, nitrogen dioxide, carbon monoxide, lead, volatile 

organic compounds, and particulate matter) were emitted in the 

United States in 1997 (U.S. Environmental Protection Agency, 

1998). These and other pollutants can be returned to the earth 

through the processes of wet and dry atmospheric deposition. 

Wet deposition removes gases and particles in the atmosphere 

and deposits them to the Earthís surface by means of rain, sleet, 

snow, and fog. Dry deposition is the process through which 

particles and gases are deposited in the absence of precipitation. 

In 1980, the federally supported interagency National Acid 

Precipitation Assessment Program (NAPAP) was established to 

provide broadened support for research into the causes and 

effects of acid deposition. Under NAPAP, the U.S. Geological 

Survey (USGS) was designated as the lead Federal agency for 

the monitoring of wet atmospheric deposition in the United 

States. Currently, the USGS atmospheric-deposition program 

provides: 

ï Continuous measurement and assessment of the chemical 

constituents in precipitation throughout the United States 

through participation and leadership in the National Atmospheric Deposition Program (NADP).

ï Scientific research and assessment to evaluate the effects of 

atmospheric deposition on aquatic and terrestrial ecosystems.

This long-term, nationally consistent monitoring program, 

coupled with ecosystem research, provides critical information 

U.S. Geological Survey

U.S. Department of the Interior

Figure 1. An atmospheric-deposition-monitoring site at Little 

Bighorn Battlefield National Monument in Montana.

toward a national scorecard to evaluate the effectiveness of 

ongoing and future regulations intended to reduce atmospheric 

emissions and subsequent impacts to the Nationís land and water 

resources. Monitoring data from sites, such as the one shown in 

figure 1, and on-line program reports are available through the 

Internet at http://bqs.usgs.gov/acidrain.

PROVIDING FEDERAL LEADERSHIP IN THE 

NATIONAL ATMOSPHERIC DEPOSITION 

PROGRAM

The NADP consists of over 100 organizations, including 8 

Federal agencies, State governments, universities, tribal nations 

and private companies. These organizations share resources to 

provide the Nation with a long-term, high-quality data base of 

atmospheric deposition generated from networks of precipitationchemistry monitoring sites. The largest of these networks is the 

220-site National Trends Network (NTN) (fig. 2). Every NTN site 

uses identical equipment and operating protocols and operates 

continuously to monitor precipitation chemistry. Samples from 

NTN sites are analyzed for sulfate, nitrate, ammonium, 

phosphate, pH, chloride, calcium, magnesium, sodium, 

potassium, and specific conductance.

The USGS is the lead Federal agency in the NADP and 

supports 72 of the NADPís NTN stations. Since the inception of 

the network, the USGS has also provided an extensive external 

quality-assurance program for the entire NTN (Gordon, 1999). 

USGS Fact Sheet FS–112–00

December 2000

Alaska Puerto Rico Virgin Islands

National Atmospheric Deposition Program

National Trends Network

Figure 2. Location of sites in the National Atmospheric Deposition 

Program, National Trends Network.

1
0 10.4

10.4 15.6

15.6 20.8

20.8 26

26 31.2

31.2 36.4

36.4 41.6

41.6 46.8

46.8 52

52 53

Sulfate Concentrations, in

microequivalents per liter (1998)

Sulfate Trends (1981 1998)

Upward Trend

No Trend

Downward Trend

EXPLANATION

_ 

_ 

_ 

_ 

_ 

_ 

_ 

_ 

_ 

_ 

_ 

The NTN has one of the longest multisite records of 

precipitation chemistry in the world. 

EVALUATING THE EFFECTIVENESS OF THE 

CLEAN AIR ACT AMENDMENTS 

Through the end of 1994, the value of electric utility 

company investments in environmental protection measures 

amounted to more than $60 billion. Over the last two decades, 

utility companies have spent an additional $40 billion on 

operating and maintaining environmental control systems 

(Electric Power Research Institute, 1996). Estimates for the cost 

of implementing the sulfur dioxide emissions control program 

mandated by the 1990 Clean Air Act Amendments (CAAA) are 

expected to be about $600 million per year by 2015. These 

costs may be passed on to the public in the form of higher costs 

for energy, transportation, and other goods and services. The 

first phase of emission reductions under the 1990 CAAA, Title 

IV, began in January 1995, at 445 coal-burning electric utility 

plants, primarily located in the Midwest and Northeastern 

United States. Phase II began in 2000 and affects about five 

times the number of electric utility plants as Phase I. The USGS 

atmospheric-deposition program regularly provides data and 

assessments of trends in atmospheric deposition (for example; 

Lynch, 1996) to the scientific and policy community to 

determine if these regulatory actions are providing the desired 

outcome, namely an improvement in the quality of the Nationís 

precipitation chemistry with subsequent reduction in adverse 

environmental impacts to land and water resources. In the latest 

interagency report to Congress, the USGS reported that Phase I 

of the CAAAñ90, Title IV, has reduced acidic deposition by 10 

2

to 25 percent in the Eastern United States (National Acid 

Precipitation Assessment Program, 1998). A more recent analysis 

(Nilles and Conley, 2000) reinforces these findings (fig. 3).

ESTIMATING NITROGEN DEPOSITION AND ITS 

SIGNIFICANCE IN AFFECTING AQUATIC AND 

TERRESTRIAL ECOSYSTEMS

Nitrogen comprises 78 percent of the atmosphere by weight, 

primarily in the form of inert N2. A small fraction of atmospheric 

nitrogen exists in more chemically and biologically active forms, 

such as nitrogen oxide gases, nitric acid vapor, particulate and 

dissolved nitrate, gaseous ammonia, dissolved ammonium, 

ammonium particulates, and organic nitrogen compounds. These 

forms are active in the nitrogen cycle, which involves the 

atmosphere, photochemistry, plants, animals, microbes, soils, 

surface waters, and the oceans. Although reactive nitrogen 

compounds are produced naturally by several processes, a 

significant portion of the reactive nitrogen is released to the 

atmosphere through fossil-fuel combustion, fertilizer application, 

and intensive animal agriculture. Precipitation readily removes 

most reactive nitrogen compounds, such as ammonia and nitrogen 

oxides, from the atmosphere. These compounds are subsequently 

available as nutrients to aquatic and terrestrial ecosystems. 

Estimates of inorganic nitrogen as nitrate and ammonium nitrogen 

deposited in precipitation in the United States for 1999 are shown 

in figure 5.

Because nitrogen is commonly a growth-limiting nutrient in 

streams, lakes, and coastal waters, increased concentrations can 

lead to eutrophication, a process involving excess algae 

production, followed by depletion of oxygen in surface waters as 

algae die off and are decomposed. Data from the NTN are the 

primary national source of information from which to estimate 

nitrogen deposition to ecosystems in the United States. The USGS 

recently produced estimates of the atmospheric nitrogen inputs to 

the Mississippi River drainage area (Lawrence and others, 2000) 

Figure 3. 1981ñ98 SO4

2ñconcentrations trends at NADP/NTN 

sites (arrows) and 1998 estimate annual average SO4

2ñ concentration (adapted from National Atmospheric Deposition 

Program, 1999).

Figure 4. An emission plume from facilities at the Idaho National 

Engineering and Environmental Laboratory.
#

2.6

5.2

5.4

3.6

5.6 3.2

1.5

1.1

1.0

1.7

0.6

0.3

1.4

2.3

0.7

2.8

1.51.5

1.5 2.4

2.1

3.3

2.7

2.8

2.0

1.2

2.3

3.0

2.7

2.4 4.1

3.0

4.5

2.1

2.7

3.8

3.2

7.3

5.8

0.8 0.3

6.2

6.1

5.4

5.6 6.5

6.1

5.3

6.3

5.4 5.1

3.6

4.7 4.3

3.9

2.7 3.9

6.2

5.1

2.4

3.3

2.9

5.3

4.7 4.0

3.9

5.6

4.4 4.6 5.0

4.3

5.7

4.1

6.3

4.9

4.1

5.7

3.0

2.9

4.4

1.1

0.9

0.8

1.0

3.6

4.7

5.0 6.1 4.0

1.3

3.2

3.5

4.3 5.4

4.2

4.4

1.1

1.5

2.3

2.4

1.0

0.3

5.4

6.6

3.6

3.4 4.8

3.8

4.7

4.1

6.3 6.6

5.1

5.8

5.3

2.7

0.8

0.4

1.0 0.3

0.72.5

6.1

5.5 4.9

5.7

4.7

3.2 3.7

4.8

3.7

4.2

3.3

3.6

2.4

0.8 3.0

2.0

3.6

2.0

3.7

3.1

3.1 1.2

0.9

0.9

1.1 4.5

3.2 3.8

4.6

4.3

4.3

1.3 1.9

1.1 1.1

4.0

4.7

4.2

5.7

4.0

5.5

4.8 5.8

5.0

4.3 4.4

1.0

National Atmospheric Deposition Program/National Trends Network

http://nadp.sws.uiuc.edu

 Nitrogen

(kilogram per hectare)

Alaska

Puerto Rico Virgin Islands

Equal to or less than 1.0

1.0 2.0

2.0 3.0

3.0 4.0

4.0 5.0

5.0 6.0

6.0 7.0

Greater than 7.0

Estimated inorganic nitrogen deposition from nitrate

and ammonium, 1999

_

_

_

_

_

_ 0.2

0.6

0.2

as part of a detailed assessment by the National Science and 

Technology Councilís Committee on the Environment and 

Natural Resources into the causes of the low oxygen (hypoxic) 

zone, which has adversely affected the productivity of 

ecosystems and fisheries in the Gulf of Mexico. The NTN data 

are critical to evaluating the significance of atmospheric inputs 

of nutrients relative to other sources of nitrogen loading to the 

gulf and to the many estuarine ecosystems in the United States.

Continued elevated levels of nitrogen deposition have 

resulted in terrestrial ecosystems, such as some forested 

watersheds at high elevations in the Northeastern and Rocky 

Mountain regions of the United States, becoming nitrogen 

saturated. Nitrogen saturation means that the inputs of nitrogen 

into the soil exceed the uptake ability by plants and soil 

microorganisms. Under conditions of nitrogen saturation, 

excess nitrogen leaches into soil water and subsequently into 

ground water and surface water. Streams that drain these 

watersheds have shown increased levels of nitrogen in runoff 

(National Acid Precipitation Assessment Program, 1998; 

Williams and others, 1996). This leaching of excess nitrogen 

from the soils degrades water quality and removes other 

essential nutrients, such as calcium, needed for plant growth.

DETERMINING THE ROLE OF ATMOSPHERIC 

DEPOSITION IN SOIL CALCIUM DEPLETION IN 

THE EASTERN UNITED STATES

Calcium is an important nutrient for trees and aquatic 

ecosystems. Accumulating information and data indicates that 

atmospheric deposition has reduced the availability of acidneutralizing calcium in forest soils in the Eastern United States, 

with potentially significant adverse consequences for forest 

productivity and for the ability of surface waters to recover from 

acidification (Lawrence and others, 1999; Clow and Mast, 1999). 

Calcium in forest soils is increased by atmospheric deposition, 

weathering of rocks, and decomposition of calcium-containing 

organic matter, such as leaves and branches. The loss of calcium 

in soils is mainly by root uptake and by leaching.

Research in the 1990s, including several studies by the 

USGS, has shown that (1) calcium in forest soils has decreased at 

several locations studied in the Northeastern (Lawrence and 

others, 1997) and Southeastern United States (Lawrence and 

Huntington, 1999), and (2) acid rain and forest growth (uptake of 

calcium from the soil by roots) are factors that contribute to 

calcium depletion (Lawrence and Huntington, 1999). These 

findings provide regional-scale insight into the processes that 

contribute to the apparent lack of recovery in the acid-neutralizing 

capacity of lakes and streams in sensitive ecosystems of the 

Eastern United States. The lack of recovery at several sites 

3

Figure 5. Estimated nitrogen deposition in precipitation from nitrates and ammonium in the United States for 1999

(National Atmospheric Deposition Program , 2000) 
studied, despite reductions in acid rain and reductions of sulfate 

levels in surface-water, appears to be related to depletion of 

acid-neutralizing calcium and other base cations in forest soils 

(Lawrence and others, 1999; Clow and Mast, 1999). The 

occurrence and magnitude of calcium depletion also depends on 

soil depth, soil chemistry, forest-growth history, and harvesting 

patterns at specific locations. These studies help to address the 

policy questions now being debated by the scientific and policy 

communities as to whether the emission reductions mandated in 

the Clean Air Act Amendments are sufficient to initiate 

ecosystem recovery. The data provided by the deposition 

monitoring program are useful for quantifying the status and 

trends in acidic deposition and measuring the ongoing decline in 

atmospheric calcium deposition, while assessments supported by 

the program provide synthesis of existing USGS studies on 

calcium depletion.

ESTIMATING DEPOSITION RATES TO SENSITIVE 

HIGH-ELEVATION ECOSYSTEMS IN THE 

WESTERN UNITED STATES

Federal land managers and State regulatory agencies need 

estimates of atmospheric deposition to prepare resource impact 

statements and to grant permits for new emission sources that 

may affect sensitive ecosystems. In addition to deposition 

estimates from data provided by the NADP sites in the West, the 

USGS has supported production of enhanced deposition estimates 

for sensitive high-elevation Western ecosystems by using a total 

deposition survey. The ecosystems are characterized by dilute 

surface waters, exposed bedrock or very shallow unbuffered soils, 

and runoff patterns that are dominated by melting of large 

snowpacks (National Acid Precipitation Assessment Program, 

1990). These areas are not optimally represented by the NTN sites 

because of logistical problems associated with accessing and 

operating automated samplers at harsh, snow-dominated, highelevation sites. The annual total-deposition survey roughly 

follows the Continental Divide northward from New Mexico to 

the Canadian Border and measures total wintertime deposition by 

Figure 6. USGS and U.S. Department of Agriculture Forest Service 

scientists examining the effects of acid rain on soil calcium depletion 

and sugar maple health near Frost Valley, New York. Photograph 

courtesy of Ken Dudzik, Forest Service, Northeastern Forest 

Experiment Station, Durham, New Hampshire.

SUPPORTING THE USE OF NADP DATA BY PROVIDING UNIVERSAL CUSTOMER ACCESS

A fundamental program objective is to provide scientific investigators worldwide with a long-term, high-quality data base of 

atmospheric deposition for research support in the areas of air quality, water quality, agricultural effects, forest productivity, materials 

effects, ecosystem studies, watershed studies, and human health. All NADP data are made available free of charge through the 

Internet at http://nadp.sws.uiuc.edu. In 1999, investigators downloaded 16,000 datasets in support of research and education. Major 

data applications include gaging the effectiveness of ongoing emission-control measures, determining nitrogen inputs from 

atmospheric sources to sensitive ecosystems, and providing regional estimates of mercury deposition in the United States and Canada 

(Conley and Nilles, 2000). Some recent applications for the data by external customers include:

National Park Service: Determining the effect of rainfall pH on efforts to reintroduce the leopard frog to Mesa Verde National 

Park, Colorado.

National Center for Atmospheric Research: Enhancing the accuracy of global climate-change models by accounting for the 

influence of nitrogen deposition on global carbon-fixation rates.

U.S. Environmental Protection Agency: Determining the relative contribution of atmospheric deposition of nitrogen to the 

overall nitrogen budget of the Chesapeake Bay watershed.

City of New York: Estimating atmospheric chemical loads to public-supply reservoirs.

National Acid Precipitation Assessment Program: Assessing the effectiveness of industrial and powerplant emission reductions 

under the Clean Air Act Amendments, Title IV, on reducing acidic atmospheric deposition.

U.S. Department of Defense: Providing precipitation chemistry data for a study examining maintenance practices to reduce 

galvanic corrosion on F-18 fighter jets.

State of North Carolina: Documenting changes in ammonium deposition in North Carolina relative to growth of confined 

animal agriculture facilities and estimating the potential impacts of ammonia emissions on air and water quality.

Ford Motor Company: Examining possible effects of precipitation chemistry on automotive paints.

Teachers and students: About 40 percent of all applications for the programís monitoring data are for use in science education, 

ranging from elementary school to college graduate-level studies.

4
Continental Divide

Minimum, 1.4 microequivalents per liter

Maximum, 13.3 microequivalents per liter

EXPLANATION

Ammonium Ion Levels in Snowpack for

1993 97 in the Rocky Mountain Region _

Figure 7. Ammonium ion levels in snowpack for 1993ñ1997 in

the Rocky Mountain region.

analyses of snow cores collected at the time of maximum 

snowpack accumulation (early spring). Preliminary results from 

this sampling are provided in figure 7, which indicates 

substantial variation in ammonium deposition. Accurately 

determining nitrogen inputs from ammonium and nitrate 

deposition is of major interest because increasing nitrogen 

concentrations in stream water have been linked to nitrogen 

saturation of soils in sensitive watersheds (Williams and others, 

1996). 

USGS scientists are now comparing data from these 

surveys with data from NADP/NTN sites and data from a 

network of snowpack-accumulation sensors located throughout 

the Rocky Mountains (SNOTEL). Data synthesis and modeling 

of relationships between elevation and deposition will provide 

Federal and State land managers with a new, high-resolution 

picture of atmospheric deposition in the Rocky Mountain region. 

As Federal and State policies that are aimed at understanding 

and improving air-quality-related issues in protected public 

lands in the region evolve, enhanced estimates of deposition to 

these sensitive ecosystems are vital to relate atmospheric 

emissions to observed aquatic and terrestrial effects.

THE FUTURE

Some emerging issues provide direction for program 

priorities in data interpretation, research, and infrastructure needs 

for the next decade and beyond.

Emission trading. The acid rain reduction provisions of the 

Clean Air Act Amendments incorporate innovative ìfree marketî 

approaches to accomplishing environmental objectives (Burtraw 

and Mansur, 1999). These approaches include emission banking 

and trading, whereby utilities can accumulate, trade, and sell 

emissions allowances earned by exceeding emission-reduction 

targets. These provisions are controversial in the Northeastern 

United States where critics contend that these mechanisms may 

increase allowable emissions in the Midwest and result in 

increased deposition of acidic compounds to sensitive 

ecosystems ìdownwindî in the East. Although Federal 

legislation places no regional restrictions on emission allocation 

trading, States in the East have called for restrictions on trading 

of emission credits from Eastern utilities to utilities in 

Midwestern States ìupwindî of sensitive ecosystems. Since 

1995, emission reductions have significantly exceeded mandates, 

resulting in a buildup of emission allowances during the first 

phase of the program that ended in 1999. Citizens and 

environmental groups have raised concern that depletion of builtup allowances, beginning in 2000 and continuing through 2010, 

may increase emissions and subsequent atmospheric deposition 

in the first decade of this century. The monitoring network the 

USGS supports will provide the data to determine if, indeed, 

acidic deposition patterns have changed in 2000 and beyond.

Electric utility deregulation. The process of electric utility 

deregulation is accelerating at the State and Federal level. There 

is concern that deregulation of utilities may increase the use of 

coal and emissions from large coal-fired powerplants, resulting 

in significant adverse changes in the magnitude and areal extent 

of chemical substances deposited from the atmosphere. Data 

from the program will be used to ascertain if this is occurring.

Nitrogen deposition. Ammonium deposition is increasing 

at some of the NADP monitoring sites and may be coincident 

with growth in confined animal agriculture facilities in these 

regions. Although stricter controls of sulfur dioxide emissions 

have reduced sulfate deposition, emissions of nitrogen oxides 

and subsequent nitrate deposition have not decreased in the 

United States (Nilles and Conley, 2000). Increased nitrogen 

mobilization and distribution from ammonia compounds and 

nitrates can result in changes in nutrient runoff, precipitation 

acidity, and watershed nutrient status. Assessments to quantify 

nitrogen loading to sensitive ecosystems will continue to grow in 

importance for the program.

Infrastructure renewal. Initial sites in the NADP 

deposition-monitoring network were established in 1978, and a 

large portion of the network was in place by 1984. The current 

sampling equipment has been operating continuously in the field 

for an average of 17 years, and there has been no substantial 

5
upgrade or refurbishment of the sampling equipment since 

installation. A major refurbishment and/or replacement program 

is needed to prevent the aging equipment from adversely 

affecting the operational readiness of the program. Testing of 

off-the-shelf modern recording precipitation gages is well 

underway, and design of a new prototype deposition sampler 

began in 2000. The overall objective of this work is to develop 

and deploy, on a network-wide basis, a modern and more 

flexible deposition sampling system, one that would not only 

support all current NADP sampling protocols but also provide 

the capability to sample precipitation for chemicals of emerging 

interest (for example, organic compounds).

REFERENCES CITED

Burtraw, D., and Mansur, E., 1999, Environmental effects of SO2 trading and banking: Environmental Science and Technology, v. 33, 

no. 20, p. 3489ñ3494.

Clow, D.W., and Mast, M.A., 1999, Long-term trends in stream water 

and precipitation chemistry at five headwater basins in the Northeastern United States: Water-Resources Research, v. 35, no. 2, 

p. 541ñ554.

Conley, B., and Nilles, M.A., 2000, Uses of National Atmospheric 

Deposition Program/National Trends Network Data, May 1998 to 

June 1999; U.S. Geological Survey Open-File Report 00ñ52, 

p. 1ñ6.

Electric Power Research Institute, 1996, Environmental risk analysis: 

Environmental update, v. 10, no. 1, p. 1ñ3.

Gordon, J.D., 1999, External quality-assurance results for the National 

Atmospheric Deposition Program/National Trends Network, 

1995ñ96: U.S. Geological Survey Water-Resources Investigations 

Report 99ñ4072, 62 p.

Lawrence, G.B., David, M.B., Bailey, S.W., and Shortle, W.C., 1997, 

Assessment of calcium status in soils of red spruce forests in the 

northeastern United States: Biogeochemistry, v. 38, p. 19ñ39.

Lawrence, G.B., David, M.B., Lovett, G.M., Murdoch, P.S., Burns, 

D.A., Stoddard, J.L., Baldigo, B.P., Porter, J.H., and Thompson, 

A.W., 1999, Soil calcium status and the response of stream chemistry to changing acidic deposition levels: Ecological Applications, v. 9, no. 3, p. 1059ñ1072.

Lawrence, G.B., Goolsby, D.A., Battaglin, W.A. and Stensland, G.J., 

2000, Atmospheric nitrogen in the Mississippi River Basin ó

Emissions, deposition and transport: The Science of the Total 

Environment v. 248 p. 87ñ99

Lawrence, G.B., and Huntington, T.G., 1999, Soil-calcium depletion 

linked to acid rain and forest growth in the Eastern United States: 

U.S. Geological Survey Water-Resources Investigations Report 

98ñ4267, 12 p.

Lynch, J.A., 1996, Trends in precipitation chemistry in the United 

States, 1983ñ94 ó An analysis of the effects of phase I of the 

Clean Air Act Amendments of 1990, Title IV: U.S. Geological 

Survey Open-File Report 96ñ0346, p. 12ñ13.

National Acid Precipitation Assessment Program, 1990, Acidic 

deposition ó State of science and technology report 12, episodic 

acidification of surface waters due to acidic deposition: 

Washington, D.C., p. 100.

National Acid Precipitation Assessment Program, 1998, NAPAP biennial 

report to Congress ó An integrated assessment: Silver Springs, Md., 

p. 35.

National Atmospheric Deposition Program, 2000, 1999 Wet deposition: 

Champaign, Illinois State Water Survey, 16 p.

National Atmospheric Deposition Program, 1999, 1998 Wet deposition: 

Champaign, Illinois State Water Survey, 16 p.

Nilles, M.A., and Conley, B.E., in press, Changes in the chemistry

of precipitation in the United States, 1981ñ1998: Water, Air and Soil 

Pollution.

Williams, M.W., Baron, J.S., Caine, N., Sommerfeld, R., and Sanford, R., 

1996, Nitrogen saturation in the Rocky Mountains: Environmental 

Science and Technology, v. 30, no. 2, p. 640ñ645.

U.S. Environmental Protection Agency, 1998, National air quality and 

emissions trend report, 1997: Research Triangle Park, North 

Carolina, EPA 454/Rñ98ñ016, p. 112ñ117.

óBy Mark A. Nilles

For more information contact:

Atmospheric Deposition Program Coordinator

U.S. Geological Survey

Box 25046, MS401

Denver, CO 80225

(303) 236ñ1878

This report and related information can be accessed on the 

World Wide Web at http://bqs.usgs.gov/acidrain

The use of brand, trade, or firm names in this report is for 

descriptive purposes only and does not constitute endorsement by 

the U.S. Government.

6