Program Overview

CASTNET is a regional long-term 

environmental monitoring program 

administered and operated by 

EPA’s Clean Air Markets Division 

(CAMD). Developed from the 

existing National Dry Deposition 

Network (NDDN), CASTNET was 

established in 1991 under the Clean 

Air Act Amendments. The regional 

monitoring network was formed to 

assess trends in acidic deposition 

due to emission reduction 

regulations, such as the Acid Rain 

Program (ARP) and NOx Budget 

Trading Program (NBP). CASTNET 

has since become the nation’s 

primary monitoring network 

for measuring concentrations of 

air pollutants involved in acidic 

deposition affecting regional 

ecosystems and rural ambient 

ozone levels.

CASTNET is able to provide data 

needed to assess and report on 

geographic patterns and longterm temporal trends in ambient 

air pollution and dry atmospheric 

deposition. CASTNET can also 

be used to track changes in 

measurements associated with 

climate change (such as 

temperature and precipitation).

Network Description

Presently there are a total of 86 

operational CASTNET sites located 

in or near rural areas and sensitive 

ecosystems (see map on page 2) 

collecting data on ambient levels of 

pollutants where urban influences are 

minimal. As part of an interagency 

agreement, the National Park Service 

(NPS) sponsors 27 sites which are 

located in national parks and other 

Class-I areas designated as deserving 

special protection from air pollution.

A long-term network with consistency in monitoring methodologies, 

such as CASTNET, which has over 

40 sites with more than 15 years of 

continuous operations, is a valuable 

resource for regulatory accountability. CASTNET complements the 

National Atmospheric Deposition 

Program/National Trends Network 

(NADP/NTN), which is considered 

the nation’s primary source for wet 

deposition data. Together, these two 

monitoring programs provide data 

necessary to estimate temporal and 

spatial trends in total deposition 

(wet and dry) as well as ecosystem 

health (e.g., monitoring data on 

lakes, streams, and forests). Data 

from CASTNET and NADP are 

used to assess the effectiveness of 

emission reductions from EPA 

regulations such as reductions in 

SO2 and NOx from the NOx SIP 

Call and the ARP. CASTNET 

measurements also are used in the 

development and evaluation of 

numerical models created for 

regulatory assessment and an 

understanding of atmospheric 

processes. Over the next five years, 

CASTNET will also serve as an 

assessment tool for the Clean Air 

Interstate Rule (CAIR). 

Clean Air Status 

 and Trends Network(CASTNET)

Wet deposition is the portion 

of total atmospheric deposition dissolved in cloud droplets 

and deposited during rain and 

other forms of precipitation. 

Dry deposition is the portion 

of atmospheric deposition that 

settles as dust or deposits on 

dry surfaces during periods of 

no precipitation.

A monitoring network established to measure concentrations of key air pollutants involved in acid 

deposition, CASTNET has been an essential tool for evaluating EPA’s emission reduction programs.
Deposition Measurements

At each site, there is a temperaturecontrolled shelter which houses 

a computer, a data logger, and a 

continuous ozone monitor. Weekly 

samples of particulate matter and 

select gasses are collected using a 

3-stage filter pack with a controlled 

flow rate located atop a 10m tower. 

For quality assurance, site audits are 

performed once every six months 

and biennially by the CASTNET 

contractor and a third party auditor, 

respectively. 

Dry deposition fluxes (D) cannot be 

measured, but instead are calculated 

from the product of deposition velocities (Vd) and concentration of a pollutant (C). Dry deposition (D=CVd) is 

calculated from the weekly filter pack 

samples and meteorological measurements collected using the Multi-Layer 

Model (MLM).

Total deposition is the sum of estimated dry deposition fluxes from 

CASTNET sites and wet deposition 

measured at NADP sites. NADP sites 

can be collocated with CASTNET 

sites or the wet deposition is estimated 

by inverse distance weighting from 

wet deposition measurements taken at 

nearby NADP sites.

For more detailed information on sampling procedures and estimating deposition values, refer to the CASTNET 

web page at http://epa.gov/castnet.

Regional Changes 

in Deposition

The maps on the next page show corresponding trends in dry sulfur and 

nitrogen deposition for the Eastern 

United States, between 1989–1991 

and 2004–2006. With the implementation of the ARP in 1995, and the 

NBP in 1999, emission reductions 

have resulted in a dramatic decrease 

of total sulfur deposition in the 

Eastern United States (approximately 

40%). Total nitrogen deposition has 

also decreased in the East (approximately 20%). Dry sulfur deposition 

has decreased significantly (39%) in 

the Eastern region from 1989–1991 

and 2004–2006, while dry nitrogen 

deposition has decreased 17% during 

the same time period. The largest 

overall reductions for nitrogen and 

sulfur deposition have occurred in the 

Northeast region, downwind from the 

Ohio River Valley.

Changes in Deposition and Rural Ozone Concentrations

w

GAS153

EVE419

ESP127

EGB181 EGB281

DEV412

DCP114

CVL151

CTH110

COW137 CON186

CNT169

CND125

CKT136

CHE185

CHA467

CDZ171

CDR119 CAN407

CAD150

BWR139

BVL130

BFT142

BEL116

BBE401

ASH135

ARE128

ANA115

ALH157

ACA416

WNC4 ABT147 29

CAT175

YOS404

YEL408

WST109

WSP144

SEK430

ALC188

VPI120

VOY413

VIN140

UVL124

THR422

SUM156

STK138

SPD111

SND152

SHN418

SAL133

ROM206ROM406

QAK172

PSU106

PRK134

PNF126

PND165

PIN414

PET427

PED108

OXF122

NCS415

MOR409

MKG113

MEV405 MCK231 MCK131

MAC426

LYK123

LRL117

LAV410

KNZ184

PAL190

SAN189

JOT403

IRL141

HWF187

HOX148

HOW132

GTH161

GRS420

GRC474

GRB411

GLR468

DEN417

Current Sites

EPA

NPS

CASTNET Site Map (as of December 2007)

Network Summary

Initiated 1986*

Number of Sites (as of December 2007) 86**

Sampling Schedule 7-day averages (Tuesday to Tuesday)

Ambient Measurements

Gaseous

Sulfur Dioxide (SO2)

Nitric Acid (HNO3)

Ozone (O3) [hourly]

Particulate Sulfate (SO4

2-)

Nitrate (NO3

-

)

Ammonium (NH4

+)

Other relevant ions

Meteorological Parameters

(1-hour averages)

Temperature (2 and 9 meters)

Wind Speed and Direction

Sigma Theta

Solar Radiation

Relative Humidity

Precipitation

Surface Wetness

Land Surface Features Leaf Area Index and Vegetation

Land Use and Terrain

* The National Deposition Network (NDDN) was established in 1986 and field measurements began in 1987. 

With the passage of the Clean Air Act Amendments of 1990, NDDN was entirely subsumed by CASTNET in 1991.

** Two sites are collocated (MCK131/MCK231 and ROM406/ROM206).
Average Sulfate Concentration 1989–1991

Trends in Ambient Sulfate Concentrations 

From 1989–1991 to 2004–2006

Trends in Ambient Nitrate Concentrations 

From 1989–1991 to 2004–2006

Average Sulfate Concentration 2004–2006

Average Nitrate Concentration 1989–1991 Average Nitrate Concentration 2004–2006

The above maps are regional 

aggregations of sulfur and nitrogen 

deposition at Eastern sites averaged 

over the 2003–2006 period. The dry 

deposition component for sulfur varies 

between approximately 11%–60% 

at Eastern sites. The variation in dry 

sulfur deposition can be attributed 

to the proximity of large emission 

point sources in the Midwest region. 

Nitrogen deposition is typically more 

uniform across the East due to the large 

NOx contribution from motor vehicles 

(see map above).

Northeast

Mid-Atlantic

Midwest

Southeast

Dry Deposition

Wet Deposition

T=4.5

N=1

T=12.0

N=3

T=7.6

N=6

T=9.4

N=11

N: Number of sites (sites with valid deposition data 2003–2006)

T:Total Deposition (kg/ha/yr)

Total Dry Sulfur: SO2 + SO4

Percentage of Dry Sulfur to

Total Sulfur Deposition (2003–2006)

[Size of Pie Chart Represents Total

Sulfur Deposition (kg/ha/yr)]

16%

39%

42%

23%

14

Percentage of Dry to 

Total Sulfur Deposition

Percentage of Dry to 

Total Nitrogen Deposition

Percent Change in 90th Percentile

1-Hour Ozone Concentration

Decrease Between 10% and 15%

Decrease Between 5% and 10%

Decrease Less Than 5%

Increase Less Than 5%

Increase Between 5% and 10%

At each site there is a continuous 

ozone analyzer which measures and 

records hourly average concentrations. CASTNET is the primary 

network for measuring rural ozone 

concentrations and a vital assessment 

tool for EPA’s NBP. As shown in the 

map on the right, there has been a 

decrease in the 90th percentile ozone 

concentration measured at each site 

located in the NBP region (shaded 

states) between 2000–2002 (prior to 

NBP implementation) and 2004–2006 

(post NBP implementation).

Northeast

Mid-Atlantic

Midwest

Southeast

T=3.6

N=1

T=7.2

N=3

T=6.3

N=5

T=7.8

N=11

Dry Deposition

Wet Deposition

N: Number of sites (sites with valid deposition data 2003–2006)

T:Total Deposition (kg/ha/yr)

Total Dry Nitrogen: HNO3 + NO3 + NH4

13

Percentage of Dry Nitrogen to

[Size of Pie Chart Represents Total

Total Nitrogen Deposition (2003–2006)

Nitrogen Deposition (kg/ha/yr)]

21%

26%

28%

26%
1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 

14

13

12

11

10

9

8

7

6

5

4

3

2

1

0

Year

Dry Deposition (kg/ha)

Trend in Sulfur Dry Deposition in Eastern U.S.

1989 – 2006

Regional Trends in Sulfur and Nitrogen Deposition

Trend in Sulfur Dry Deposition in Eastern U.S. 1989–2006

CASTNET has been operational for almost 20 years, giving EPA and other 

data users the ability to study trends in dry deposition for an individual site, 

region, or the entire nation. Due to EPA emission reduction programs, such 

as the ARP, regional dry sulfur deposition has decreased significantly in the 

Eastern United States. The largest decrease in regional dry sulfur deposition 

was in the Northeast (72%). Dry nitrogen deposition also has decreased 

in the Eastern region. The largest decrease (47%) in regional dry nitrogen 

deposition was, again, in the Northeast. The trend plots of dry nitrogen and 

sulfur deposition represent the 5th, 25th, 50th, 75th, and 95th percentiles calculated for the Eastern reference sites from 1989–2006.

1989 1991 1993 1995 1997 1999 2001 2003 2005 2007 

5

4

3

2

1

0

Year

Dry Deposition (kg/ha)

Trend in Nitrogen Dry Deposition in Eastern U.S.

1989 – 2006

Trend in Nitrogen Dry Deposition in Eastern U.S. 1989–2006

Monitoring Networks

For more information about networks 

that CASTNET collaborates with 

to provide regional and nationwide 

assessments of ambient air quality, 

please refer to the following websites:

National Atmospheric Deposition 

Program-National Trends Network 

(NADP/NTN) (wet deposition)

http://nadp.sws.uiuc.edu

National Atmospheric Deposition 

Program-Mercury Deposition Network 

(NADP/MDN) (mercury deposition) 

http://nadp.sws.uiuc.edu/mdn/

Interagency Monitoring of 

Protected Visual Environments 

(IMPROVE) (visibility)

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

State and Local Ambient Monitoring 

Stations and National Air Monitoring 

Stations (SLAMS and NAMS)

(urban ozone)

http://www.epa.gov/oar/oaqps/qa/

monprog.html

Canadian Air and Precipitation 

Monitoring Network (CAPMON) 

(Canadian air quality monitoring)

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

index_e.cfm

Online Resources

CASTNET has 10 sites which report 

real-time hourly ozone data to the 

AIRNow website so users can access 

air quality forecasts or real-time air 

quality conditions by visiting http://

airnow.gov. AIRNow is a web-based 

database created by EPA, NOAA, NPS, 

and tribal, state, and local agencies to 

improve the accessibility of air quality 

data to the public. 

Note: The ozone data submitted to 

AIRNow have not gone through the 

extensive quality assurance that is 

used for regulatory purposes; therefore 

the data should be used as a public 

reference, not for publication.

For more information about 

CASTNET—including CASTNET 

data and maps, annual reports, and 

quality assurance information—visit 

http://www.epa.gov/castnet/.