Clean Air Status and 

Trends Network 

(CASTNET)

2003 

Program and

Technical Summary

May 2005
1 

CASTNET 2003 Program and Technical Summary 

Table of Contents 

About CASTNET....................................................................................................2 

CASTNET Operations ...........................................................................................3 

Cooperative Long-Term Environmental Monitoring at CASTNET Sites ................3 

CASTNET Monitoring Sites ...................................................................................4 

Atmospheric Species Measured by CASTNET .....................................................5 

Total Deposition.....................................................................................................6 

Ozone Monitoring ..................................................................................................7 

CASTNET Monitoring in Sensitive Ecosystems 

 Coastal Environments......................................................................................8 

 Great Smoky Mountains National Park and the Southern Appalachians.........9 

Documentation for the network including past annual reports and the CASTNET Quality 

Assurance Project Plan can be found at the CASTNET on-line library: 

www.epa.gov/castnet/library.html

The CASTNET database is available to the public via EPA’s 

CASTNET data web page:

www.epa.gov/castnet/data.html

Prepared by MACTEC Engineering and Consulting, Inc. for: 

U.S. Environmental Protection Agency 

Office of Air and Radiation 

Clean Air Markets Division 

Washington, DC 
2 

EPA’s Clean Air Status and Trends Network 

Air pollution has far-reaching effects. It negatively 

impacts human health, agricultural crops, natural 

ecosystems, national monuments, buildings, and 

other man-made products. Emissions of sulfur, 

nitrogen, and other substances react in the 

atmosphere with particles, water droplets, oxygen, 

and oxidants to form acidic compounds. These 

compounds are often transported long distances and 

then deposited to the environment by precipitation, 

clouds, and other meteorological events. Dry 

deposition, a component of total deposition, involves 

complex atmospheric processes such as settling, 

impaction, and absorption whereby, in the absence 

of precipitation, airborne particles and gases reach 

the earth’s surface. Dry deposition makes up 10 to 

70 percent of total deposition depending on location 

and climate. 

In 1990, Congress passed the Clean Air Act 

Amendments (CAAA), which mandated ambient 

monitoring to provide information and to track trends 

in pollutant concentrations and the dry deposition 

component of total atmospheric deposition. The 

information and trends data collected through 

ambient monitoring are used to determine the 

effectiveness of sulfur dioxide (SO 

2) and nitrogen 

oxides (NOx) emission reductions promulgated by 

the CAAA. To meet this monitoring need, the U.S. 

Environmental Protection Agency (EPA) established 

the Clean Air Status and Trends Network 

(CASTNET) in 1991, incorporating sites from the 

National Dry Deposition Network (NDDN), which 

began operation in 1987. Since 1991, many sites 

have been added, and there are now more than 85 

sites across the United States. The National Park 

Service (NPS) joined with EPA in 1994 as a principal 

sponsor and currently sponsors 30 CASTNET sites. 

CASTNET Principal Sponsors 

United States Environmental Protection Agency 

National Park Service

CASTNET Partners 

In addition to EPA and NPS, other partners support the operation of CASTNET sites. These partners include: 

Alabama-Coushatta Tribe Long Term Ecological Research Program (LTER) 

Canadian Air and Precipitation Monitoring 

Network (CAPMON) 

Interagency Monitoring of Protected 

Visual Environments (IMPROVE) 

Cherokee Nation St. Johns River Water Management District 

Kansas State University State University of New York 

National Atmospheric Deposition Program (NADP) U.S. Department of Agriculture Forest Service 

Pisgah National Forest, NC (PNF126)
3 

CASTNET Operations

The original network design was based on the 

assumption that dry deposition, or flux, could be 

estimated as the linear product of measured 

pollutant concentration and modeled deposition 

velocity. 

Weekly integrated averages of atmospheric 

concentrations are obtained through laboratory 

analysis of the 3-stage filter packs installed weekly 

at each CASTNET site. The influence of 

meteorological conditions, vegetation, and chemistry 

is simulated by the deposition velocity, which is 

calculated using the Multi-Layer Model. To support 

the estimation of deposition velocity, all CASTNET 

sites include equipment to measure a 

comprehensive suite of meteorological parameters. 

All field equipment is subject to semiannual 

inspections and multipoint calibrations using 

standards traceable to the National Institute of 

Standards and Technology (NIST). Results of field 

calibrations are used to assess sensor accuracy and 

validate field data. 

CASTNET operations include a comprehensive 

quality assurance (QA) program that was designed 

to ensure that all reported data are of known and 

documented quality. QA is emphasized in order for 

CASTNET data to meet CASTNET objectives and 

be reproducible and comparable with other 

monitoring networks and laboratories. 

Cooperative Long-Term Environmental Monitoring at CASTNET Sites 

Many CASTNET site locations serve multiple monitoring efforts, some of which belong to long-term networks 

designed to monitor environmental factors not covered by the CASTNET design. The following table summarizes 

10 such networks whose activities include a wide range of measurements. The number of sites listed for each 

network represents the number of sites located within 10 kilometers of a CASTNET site. 

Network Year 

Began 

Sites Collocated 

with CASTNET 

Sponsoring Agency 

Types of Measurements 

Atmospheric Integrated Research 

Monitoring Network (AIRMoN) 

1992 3 National Atmospheric Deposition Program (NADP) 

Wet deposition – event-based sampling 

http://nadp.sws.uiuc.edu

Canadian Air and Precipitation 

Monitoring (CAPMoN) 

1978 2 Environment Canada 

Dry and wet components of total deposition 

http://www.msc.ec.gc.ca/capmon

Climate Reference Network (CRN) 2004 5 active 

 17 planned 

National Oceanic and Atmospheric Administration (NOAA) 

Homogenous measurements of temperature/ precipitation 

http://www.ncdc.noaa.gov/oa/climate/uscrn

Interagency Monitoring of Protected 

Visual Environments (IMPROVE) 

1985 35 Multi-agency collaboration 

Visibility in Class I areas 

http://vista.cira.colostate.edu/improve

Long-Term Ecological Research 

Network (LTER) 

1980 4 National Science Foundation 

Ecological processes 

http://www.lternet.edu

Mercury Deposition Network (MDN) 1995 12 National Atmospheric Deposition Program (NADP) 

Wet deposition/ methyl mercury 

http://nadp.sws.uiuc.edu

National Dioxin Air Monitoring Network 

(NDAMN) 

1997 6 Environmental Protection Agency (EPA) 

Dioxins and PCBs 

National Trends Network (NTN) 1978 58 National Atmospheric Deposition Program (NADP) 

Wet deposition component of total deposition 

http://nadp.sws.uiuc.edu

Surface Radiation Budget Network 

(SURFRAD) 

1993 2 National Oceanic and Atmospheric Administration (NOAA) 

U.S. radiation budget 

http://www.srrb.noaa.gov/surfrad

UV-B Monitoring and Research 

Program 

1992 6 U.S. Department of Agriculture 

Ultraviolet-B radiation 

http://nadp.nrel.colostate.edu
4 

 

State Site ID Station 

AL SND152 Sand Mountain 

AK DEN417 Denali National Park 

POF425 Poker Flats Research Range 

AZ CHA467 Chiricahua National Monument 

GRC474 Grand Canyon National Park 

PET427 Petrified Forest National Park 

AR CAD150 Caddo Valley 

CA CON186 Converse Station 

DEV412 Death Valley National Monument 

JOT403 Joshua Tree National Monument 

LAV410 Lassen Volcanic National Park 

PIN414 Pinnacles National Monument 

SEK402 Sequoia National Park 

YOS404 Yosemite National Park 

CO GTH161 Gothic 

MEV405 Mesa Verde National Park 

ROM206 Rocky Mountain National Park 

ROM406 Rocky Mountain National Park 

CT ABT147 Abington 

FL EVE419 Everglades National Park 

IRL141 Indian River Lagoon 

SUM156 Sumatra 

GA GAS153 Georgia Station 

HI HVT424 Hawaii Volcanoes National Park 

IL ALH157 Alhambra 

 BVL130 Bondville 

 STK138 Stockton 

IN SAL133 Salamonie Reservoir 

VIN140 Vincennes 

KS KNZ184 Konza Prairie 

KY CDZ171 Cadiz 

CKT136 Crockett 

MAC426 Mammoth Cave National Park 

MCK131 Mackville 

ME ACA416 Acadia National Park 

 ASH135 Ashland 

 HOW132 Howland 

MD BEL116 Beltsville 

BWR139 Blackwater National Wildlife Refuge 

MI ANA115 Ann Arbor 

 HOX148 Hoxeyville 

 UVL124 Unionville 

MN VOY413 Voyageurs National Park 

MS CVL151 Coffeeville 

MT GLR468 Glacier National Park 

NV GRB411 Great Basin National Park 

NH WST109 Woodstock 

NJ WSP144 Washington’s Crossing 

NY CAT175 Claryville 

CTH110 Connecticut Hill 

HWF187 Huntington Wildlife Forest 

NC BFT142 Beaufort 

 CND125 Candor 

 COW137 Coweeta 

PNF126 Pisgah National Forest 

ND THR422 Theodore Roosevelt National Park 

OH DCP114 Deer Creek State Park 

 LYK123 Lykens 

 OXF122 Oxford 

 QAK172 Quaker City 

OK CHE185 Cherokee Nation 

ON EGB181 Egbert, Ontario, Canada 

PA ARE128 Arendtsville 

KEF112 Kane Experimental Forest 

LRL117 Laurel Hill State Park 

MKG113 M.K. Goddard State Park 

PSU106 Pennsylvania State University 

SD WNC429 Wind Cave National Park 

TN ESP127 Edgar Evins State Park 

GRS420 Great Smoky Mountains National Park 

SPD111 Speedwell 

TX BBE401 Big Bend National Park 

UT CAN407 Canyonlands National Park 

VT LYE145 Lye Brook 

VI VII423 Virgin Islands National Park 

VA PED108 Prince Edward 

 SHN418 Shenandoah National Park 

VPI120 Virginia Poly Tech 

WA MOR409 Mount Rainier National Park 

NCS415 North Cascades National Park 

OLY421 Olympic National Park 

WV CDR119 Cedar Creek State Park 

 PAR107 Parsons 

WI PRK134 Perkinstown 

WY CNT169 Centennial 

 PND165 Pinedale 

 YEL408 Yellowstone National Park 

CASTNET Monitoring Sites 

As of December 2003, 87 CASTNET monitoring sites 

were operational (Figure 1). Most CASTNET sites are 

located in rural, regionally representative locations away 

from pollution emission sources and heavily populated 

areas. The map shows reasonable geographic coverage 

in most regions of the United States. Less coverage is 

provided in the central United States from South Dakota 

to Texas. The addition of new sites in the central United 

States will enhance the ability of the network to 

determine patterns and trends in air pollutant 

concentrations and deposition rates and will help assess 

the effectiveness of emission control regulations. 

CASTNET’s primary goal is to monitor trends in 

regional-scale air quality and dry deposition. For the 

time period 1990 through 2003, 34 CASTNET sites 

were selected as reference sites (shown in red on 

Figure 1) based on the completeness of their 14-year 

data record and on criteria similar to those used by the 

U.S. Environmental Protection Agency in its National Air 

Quality and Emission Trends Report (2000). As the 

network ages and the period of interest changes, 

additional sites will meet these requirements and will be 

added as reference sites. 

Figure 1. CASTNET Sites as of December 2003 

Reference

Sites

Not Displayed: 

Alaska – 2 sites 

Hawaii – 1 site 

U.S. Virgin Islands – 1 site 
5 

Atmospheric Species Measured by CASTNET

The table below details the atmospheric species routinely 

measured at CASTNET sites. Following laboratory analysis 

of a filter pack, atmospheric concentrations for each of the 

analytes, except ozone, are calculated by dividing the mass 

on each filter by the total flow volume through the filter pack 

during the sampling period. Ozone concentrations are 

measured hourly by a separate instrument. 

The maps to the left (Figure 2) display annual mean SO 

2

concentrations in μg/m3

 for 1991 through 1993 (top) and 

2001 through 2003 (bottom). The dramatic reduction of SO 

2

between the two periods indicates the success of the 

Phase I and II SO 

2 emission reductions mandated by the 

Clean Air Act Amendments of 1990. Only CASTNET sites 

with valid measurements during both 3-year periods are 

included on the maps. For CASTNET sites along the Ohio 

River Valley, SO 

2 concentrations decreased by an average 

of approximately 60 percent. 

Particulate sulfate concentrations also decreased between 

the two time periods while atmospheric concentrations of 

nitrogen species and ozone showed no significant change. 

For maps of concentrations of other analytes, please see the 

CASTNET 2003 Annual Report. 

Species Chemical 

Symbol State Description 

Sulfur Compounds 

 Sulfur dioxide SO 

2 gas Emitted through the combustion of coal and oil that contain sulfur; reacts to form various 

acidic compounds including sulfuric acid (H2SO4), a significant contributor to acid rain. 

 Sulfate SO2-

4 particle Typically exists in the atmosphere as ammonium sulfate (NH4)2SO4, major component of fine 

particle matter (PM2.5). 

Nitrogen Compounds 

 Nitric acid HNO 

3 gas Created by emissions of nitrogen oxides (NOx) produced during combustion; a significant 

contributor to acid rain. 

 Nitrate NO3 particle Created by emissions of nitrogen oxides (NOx) produced during combustion; typically exists 

as ammonium nitrate (NH4)NO3, a major component of PM2.5. 

 Ammonium NH+ 

4 particle Formed when ammonia (NH3) reacts with sulfate and nitrate particles in the atmosphere. 

Other Ions and Cations 

 Potassium K+ particle 

 Sodium Na+ particle 

 Magnesium Mg2+ particle 

 Calcium Ca2+ particle 

 Chloride Cl- particle 

These particles occur naturally in the atmosphere through weathering and soil erosion and 

from urban and industrial sources. Cation particles counteract the acidity of the sulfur and 

nitrogen anion particles through their ability to neutralize the anions. 

Surface Level Ozone 

 Ozone O 

3 gas Formed through complex chemical reactions among precursor emissions of volatile organic 

compounds (VOC) and nitrogen oxides (NOx) in the presence of sunlight. 

Figure 2. Mean Annual SO2 

Concentrations (μg/m3

) 

(1991-1993)

(2001-2003)

18

14

10

6

2

(1991-1993)

(2001-2003)

18

14

10

6

2
6 

Total Deposition 

Total deposition at CASTNET sites is estimated by 

combining dry deposition fluxes modeled using the 

Multi-Layer Model (see page 3) with wet deposition 

estimated from National Atmospheric Deposition 

Program/National Trends Network (NADP/NTN) 

sites. Dry deposition accounts for about 10 to 

70 percent, depending on location and climate, of 

total deposition for both sulfur and nitrogen. Median 

values are approximately 40 percent for sulfur 

deposition and 30 percent for nitrogen deposition. 

Figure 3 illustrates the magnitude of total sulfur 

deposition during 2003, shown by the diameter of 

the circles and the relative contributions from dry 

and wet deposition. Relatively high rates of sulfur 

deposition were observed in the states along and 

downwind of the Ohio River Valley. Lower rates of 

sulfur deposition were observed along the 

peripheries of the network, i.e., in the northernmost 

states, the Great Plains, Florida, and throughout the 

western states. 

Figure 4 depicts the magnitude of total nitrogen 

deposition rates during 2003. In the states along the 

Ohio River Valley, contributions to total nitrogen 

deposition from dry and wet deposition were more 

evenly distributed than the respective contributions 

to total sulfur deposition for the same area. Lower 

nitrogen fluxes were observed in the northernmost 

states, Florida, and throughout the West. 

Figure 5 shows that total sulfur deposition at the 

reference sites (Figure 1) has declined significantly 

over the last 14 years. The sharp decline in 1995 

closely parallels the decline in sulfur dioxide 

emissions that resulted from the electric generating 

utilities’ compliance with Phase I of the Acid Rain 

Program. The trend line for total nitrogen deposition 

shows no significant change over the time period. 

However, there has been an increase in rates over 

the last two years. Increases in sulfur and nitrogen 

are associated, in part, with increased precipitation 

in the Eastern United States particularly in 2003. 

Total Deposition

10

5

1

Dry Deposition

Wet Deposition

Total Deposition

10

5

1

Dry Deposition

Wet Deposition

Figure 3. Total Sulfur Deposition for 2003 (kg/ha/yr) 

 Figure 5. Timeline of Total Deposition (kg/ha/yr)

Total Deposition

16

8

1.6

Dry Deposition

Wet Deposition

Total Deposition

16

8

1.6

Dry Deposition

Wet Deposition

Figure 4. Total Nitrogen Deposition for 2003 (kg/ha/yr)

4

6

8

10

12

14

16

19901991199219931994199519961997199819992000200120022003

Deposition

Total Sulfur Deposition

Total Nitrogen Deposition
7 

Figure 6. Fourth Highest Daily Maximum 8-Hour

Ozone Concentrations (ppb) for 2003

Ozone Monitoring 

Although not a regulatory network, CASTNET ozone 

(O 

3) monitoring provides information on geographic 

patterns in regional ozone and on how 

concentrations in rural areas compare to the national 

standard for the pollutant. The 8-hour ozone 

standard is 0.080 ppm and is assessed by 

calculating an 8-hour “design value,” which is 

determined by taking the fourth highest daily value 

measured at a site for each year of a 3-year period 

and finding the average. In practice, sites with 

design values greater than or equal to 85 ppb are 

considered to be in violation of the standard. 

CASTNET ozone data are not explicitly used in the 

determination of attainment, and, therefore, design 

values are not calculated for CASTNET sites. 

However, fourth highest daily maximum 8-hour 

concentrations are calculated according to EPA 

protocol for specific years for the purpose of 

examining non-urban ozone levels. 

Figure 6 shows the fourth highest daily maximum 

8-hour concentrations for CASTNET sites in the 

eastern United States for 2003. To demonstrate 

where nonattainment regions exist and relate the 

depicted CASTNET concentrations to these areas, 

EPA attainment designations for counties in the 

eastern United States are also shown on the map. 

The maps are based on designations released 

in 2004. 

CASTNET sites with fourth highest daily maximum 

8-hour concentrations for 2003 that are greater than 

or equal to 85 ppb are shown in blue. Sites with 

concentrations lower than 85 ppb are shown in 

black. Of the 42 sites considered, five eastern sites 

observed a fourth highest daily maximum 8-hour 

value greater than or equal to 85 ppb. For 2003, the 

mean of the fourth highest daily maximum 8-hour 

concentrations for the 42 sites is 77 ppb. 

Figure 7 depicts trends in fourth highest daily 

maximum 8-hour O3 concentrations aggregated over 

all eastern reference sites (Figure 1) for 1990 -

 2003. The diagram shows that the 2003 value was 

the lowest in CASTNET history. No overall trend is 

evident in the time series. The lower value in 2003 

was produced in part by cool, cloudy, wet weather 

during the ozone season. 

Figure 7. Timeline of Fourth Highest Daily Maximum 

8-hour Ozone Concentrations (ppb)

90

95

64

7581

80

89

86

87

80

80

84

83

58

83

75

74

79

73

78

82

76

76

63

83

72

81

72

7675

81

70

81

79

77 81

77

74

84

82

79

68

Unclassified Areas

Nonattainment Areas (partial county)

Nonattainment Areas (full county)

Eastern Sites

(2003)

70

80

90

100

19901991199219931994199519961997199819992000200120022003

Concentration
8 

CASTNET Monitoring in Sensitive Ecosystems: 

Coastal Environments 

CASTNET includes five sites along the eastern 

coast of the United States that provide valuable 

information about the coastal and estuarine 

environment (Figure 8). Two of these sites are in 

national parks: Acadia National Park, ME (ACA416) 

and Everglades National Park, FL (EVE419). The 

other three sites are located in estuaries that are 

part of EPA’s National Estuary Program: Maryland 

Coastal Bays, MD (BWR139); Albemarle-Pamlico 

Sounds, NC (BFT142); and Indian River Lagoon, FL 

(IRL141). The National Estuary Program was 

established by Congress in 1987 to improve the 

quality of important estuaries. 

Figure 8. Costal CASTNET Sites 

The site at IRL141, FL is sponsored by Florida’s 

St. Johns River Water Management District to help 

quantify the contribution of atmospheric deposition 

to the total nutrient load to the estuary. Using the dry 

deposition estimates from the CASTNET site with 

wet deposition data from an adjacent National 

Atmospheric Deposition Program / National Trends 

Network (NADP/NTN) style collector, preliminary 

analyses suggest that approximately 13 percent of 

total external nitrogen loading into the Sebastian 

Inlet region of the Indian River Lagoon results from 

atmospheric deposition processes. Estimates of the 

dry deposition of nitrogen are being used in similar 

estuarine research efforts as interest increases 

regarding the role of atmospheric deposition in the 

nutrient enrichment and eutrophication of sensitive 

water bodies. 

Figure 9 shows 3-year annual averages (2001-2003) 

of the wet and dry components of total nitrogen 

deposition for the five coastal CASTNET sites 

(Figure 7). Also included for comparison is the total 

nitrogen deposition profile for the same time period 

for the 34 CASTNET reference sites (Figure 1). 

Nitrogen deposition levels are highest at the coastal 

sites in North Carolina and Maryland with total 

deposition for BWR139, MD greater than the mean 

of the reference sites. Sites in Maine and Florida 

have lower deposition rates. The Florida sites show 

dry deposition of nitrogen as contributing the lowest 

percentage of the total deposition with an average of 

17 percent. The splits for the other sites are similar 

to the split for the reference sites. 

Maryland Coastal Bays

(BWR139)

Everglades

(EVE419)

Acadia

(ACA416)

Indian River Lagoon

(IRL141)

Albemarle-Pamlico Sounds

(BFT142)

Figure 9. Components of Annual Mean Total 

Nitrogen Deposition (kg/ha/yr) 2001-2003

0

1

2

3

4

5

6

7

8

9

10

Acadia

National

Park

(ACA416, ME)

Maryland

Coastal

Bays

(BWR139, MD)

AlbemarlePamlico

Sounds

(BFT142, NC)

Indian

River

Lagoon

(IRL141, FL)

Everglades

National

Park

(EVE419, FL)

Mean of

CASTNET

Reference

Sites

Deposition

NH4 Wet Deposition (as N)

NO3 Wet Deposition (as N)

NH4 Dry Deposition (as N)

NO3 Dry Deposition (as N)

HNO3 Dry Deposition (as N)

NH4

+

 Wet Deposition (as N)

NO3

-

 Wet Deposition (as N)

NH4

+

 Dry Deposition (as N)

NO3

-

 Dry Deposition (as N)

HNO3

+

Dry Deposition (as N)
9 

CASTNET Monitoring in Sensitive Ecosystems: 

Great Smoky Mountains National Park and the Southern Appalachians 

High elevation ecosystems are often sensitive to 

deposition because they lack the ability to neutralize 

and/or assimilate atmospheric inputs. Several 

CASTNET sites span the Southern Appalachian 

Mountains (Figure 10) including, from north to south, 

Shenandoah National Park, VA (SHN418); Virginia 

Polytechnic Institute, VA (VPI120); Pisgah National 

Forest, NC (PNF126); Great Smoky Mountains 

National Park, TN (GRS420); and Coweeta 

Hydrologic Laboratory, NC (COW137). 

Figure 10. Southern Appalachian CASTNET Sites 

These sites experience relatively high rates of total 

atmospheric deposition. Figures 11 and 12 show 

annual mean contributions to total sulfur and 

nitrogen deposition rates, respectively, for the 3-year 

period 2001 – 2003. Annual total sulfur deposition 

was more than 9 kg/ha/yr at all of the Appalachian 

sites except COW137, NC, which experienced a rate 

of 6 kg/ha/yr. Annual total nitrogen deposition 

ranged from 5 kg/ha/yr at COW137, NC to more 

than 8 kg/ha/yr at GRS420, TN. 

In addition to wet and dry deposition, higher 

elevation sites in the Appalachians may experience 

significant cloud water deposition. Clingmans 

Clingmans Dome, TN (CLD303) Dome, TN (CLD303), at an elevation of 2014 meters 

Figure 11. Components of Annual Mean Total 

Sulfur Deposition (kg/ha/yr) 2001-2003 

0

2

4

6

8

10

12

14

Shenandoah

National

Park

(SHN418, VA)

Virginia

Polytechnic

Institute

(VPI120, VA)

Pisgah

National

Forest

(PNF126, NC)

Great Smoky

Mountains

National Park

(GRS420, TN)

Coweeta

Hydrologic

Laboratory

(COW137, NC)

Deposition

SO4 Wet Deposition (as S)

SO4 Dry Deposition (as S)

SO2 Dry Deposition (as S)

SO4

2- Wet Deposition (as S)

SO4

2- Dry Deposition (as S)

SO2 Dry Deposition (as S)

Figure 12. Components of Annual Mean Total 

Nitrogen Deposition (kg/ha/yr) 2001-2003

0

2

4

6

8

10

12

Shenandoah

National

Park

(SHN418, VA)

Virginia

Polytechnic

Institute

(VPI120, VA)

Pisgah

National

Forest

(PNF126, NC)

Great Smoky

Mountains

National Park

(GRS420, TN)

Coweeta

Hydrologic

Laboratory

(COW137, NC)

Deposition

NH4 Wet Deposition (as N)

NO3 Wet Deposition (as N)

NH4 Dry Deposition (as N)

NO3 Dry Deposition (as N)

HNO3 Dry Deposition (as N)

NH4

+

 Wet Deposition (as N)

NO3

-

 Wet Deposition (as N)

NH4

+

 Dry Deposition (as N)

NO3

-

 Dry Deposition (as N)

HNO3

+

Dry Deposition (as N)

Virginia Tech

(VPI120)

Pisgah National Forest

(PNF126)

Shenandoah

(SHN418)

Coweeta Hydrologic Lab

(COW137)

Great Smoky Mountains

(GRS420)
10 

Great Smoky Mountains National Park, TN 

(GRS420) 

Sulfur

Nitrogen

0

2

4

6

8

10

12

14

16

18

Deposition

Cloud Deposition

Wet Deposition

Dry Deposition

in Great Smoky Mountains National Park, is the only 

CASTNET site at which cloud water deposition is 

measured. The site is sponsored by the National 

Park Service and the Tennessee Valley Authority in 

cooperation with EPA’s Region IV. Figure 13 shows 

the large amount of sulfur and nitrogen deposition 

that is contributed by clouds bringing total 

deposition estimates up to 17 kg/ha and 12 kg/ha, 

respectively, for June through September 2003. 

Because the CASTNET cloud monitor at CLD303, 

TN only operates during the summer season, the 

park also measures throughfall, which incorporates 

wet, dry, and cloud water deposition on a year-round 

basis. Throughfall measurements conducted at the 

CLD303, TN site during 2003 were approximately 

24 kg/ha/yr for sulfur and 17 kg/ha/yr for nitrogen. 

Atmospheric deposition at Great Smoky Mountains 

National Park is one of many stressors affecting the 

park’s ecosystems. Research shows that both 

chronic and episodic acidification adversely affect 

streams, soils, and sensitive plants including spruce 

fir forests. Due to high rates of atmospheric nitrogen 

deposition, high elevation ecosystems are 

experiencing advanced stages of nitrogen 

saturation. This condition limits the availability of 

nutrients (mainly calcium) to plants and causes the 

release of toxic aluminum that can harm vegetation 

and stream biota. 

Dry deposition monitoring by CASTNET began at 

GRS420, TN in 1998. Wet deposition monitoring by 

the National Atmospheric Deposition Program 

(NADP) began in 1980. As shown in Figure 14, 

GRS420, TN experienced relatively constant sulfur 

and nitrogen deposition from 1999 to 2003 with a 

slight increase over the past two years.

0

2

4

6

8

10

12

19992000200120022003

Deposition

0

50

100

150

200

250

300

Precipitation

Total Sulfur Deposition

Total Nitrogen Deposition

Precipitation

Figure 14. Timeline of Total Deposition (kg/ha/yr) 

with Precipitation (cm) at GRS420, TN

Figure 13. Total Deposition (kg/ha) at CLD303, TN 

for June – September 2003 
U.S. Environmental Protection Agency

Office of Air and Radiation

Clean Air Markets Division

Washington, D.C.