CLEAN AIR STATUS AND TRENDS NETWORK 1

Clean Air Status and Trends 

Network (CASTNET)

CASTNET is a long-term, rural 

monitoring network used to 

assess the environmental results 

due to emission reduction 

programs and pollutant impacts 

to sensitive ecosystems and 

vegetation. 

PROGRAM OVERVIEW

CASTNET is a long-term environmental monitoring network with 90 sites located throughout the US and Canada. CASTNET is managed and operated by 

EPA’s Clean Air Markets Division (CAMD) in cooperation with the National 

Parks Service (NPS) and other federal, state and local partners. The network 

was established under the 1991 Clean Air Act Amendments to assess the 

trends in acidic deposition due to emission reduction programs, such as the 

Acid Rain Program (ARP), NOx Budget Trading Program (NBP), and the Clean 

Air Interstate Rule (CAIR).

CASTNET measures ambient concentrations of sulfur and nitrogen species as 

well as rural ozone concentrations. Results from CASTNET are used to report 

on geographic patterns and temporal trends in acidic pollutants, deposition 

and regional ozone concentrations. CASTNET is the only network in the US 

that provides a consistent, long-term data record of acidic dry deposition 

fluxes.

CASTNET compliments the National Atmospheric Deposition Program’s 

(NADP) National Trends Network (NTN). The NTN is considered the nation’s 

primary source of wet deposition data and provides weekly wet deposition 

fluxes at more than 200 sites across the contiguous US, Canada, AK, and the 

US Virgin Islands. Nearly all CASTNET sites are collocated or near an NTN site. 

Together, these two monitoring programs provide data necessary to estimate 

long-term temporal and spatial trends in total deposition (wet and dry) as 

well as ecosystem health.

RECENT CHANGES TO CASTNET

In 2010, all ozone monitors at EPA-sponsored CASTNET sites were upgraded 

to meet the requirements in 40 Code of Federal Regulations (CFR) Part 58 

and can now be used to determine compliance with the National Ambient Air 

Quality Standard (NAAQS) for ozone. Hourly ozone measurements also are 

submitted to the AIRNow website (www.airnow.gov) and to EPA’s Air Quality 

System (AQS) database. CASTNET ozone data will be included in the 2011–

2013 ozone attainment designations.

Between 2007 and 2011, EPA and NPS deployed more than 30 NADP Ammonia Monitoring Network (AMoN) sites at CASTNET sites. AMoN sites measure 

bi-weekly concentrations of ambient ammonia. Results from the Community 

Multi-Scale Air Quality (CMAQ) model estimated CASTNET was missing 10–

40% of the total nitrogen budget before the addition of AMoN.
2 CLEAN AIR STATUS AND TRENDS NETWORK

Meteorological measurements were discontinued at all but 

5 EPA-sponsored CASTNET sites in 2010 due to budget constraints. EPA used hourly meteorological data from CASTNET 

sites as input to the Multi-Layer Model (MLM) to estimate dry 

deposition velocities (Vd). EPA began using historical hourly 

Vd to replace missing values for hours when one or more meteorological data values was invalid or missing. For sites with 

discontinued meteorological parameters, EPA now uses a historical average Vd.

In 2012, the EPA and NADP began using the Parameter-elevation Regression Independent Slopes Model (PRISM) to supplement NTN precipitation measurements to create interpolated 

wet deposition fluxes. Previously, the NADP only used precipitation data from NTN measurements to create the annual 

precipitation surfaces; however, sparse monitoring locations 

in the Western US with variable terrain caused known biases 

in the results. PRISM uses point observations, elevation, and 

climatic factors to estimate precipitation across the US. All historical wet deposition and total deposition maps have been using these new methods.

NETWORK OPERATIONS

As of January 2013, there are 90 CASTNET sites located in or 

near rural areas or sensitive ecosystems in 39 states and Ontario, Canada (see Figure 1). The NPS operates 26 sites, the 

Bureau of Land Management (BLM) operates 4 sites in Wyoming and NY DEC/NYSERDA operates 2 sites in New York. EPA 

operates a collocated site at Mackville, KY and NPS operates a 

collocated site at Yosemite NP, CA. EPA and NPS each operate 

independent sites at Rocky Mountain NP, CO for network-wide 

precision. 

Measurements

Each CASTNET site measures weekly concentrations of acidic 

pollutants, base cations and Cl- using a 3-stage filter pack with 

a controlled flow rate. Most CASTNET sites also include a temperature controlled shelter and continuous ozone monitoring system. The ozone inlet and filter pack are located atop a 

10-m tower. Many sites are collocated with other monitoring 

networks such as NTN, AMoN, EPA’s National Core Monitoring 

Network (NCore), and the Interagency Monitoring of Protected Visual Environments (IMPROVE).

Wet deposition is the fraction 

of atmospheric deposition deposited to the earth’s surface 

by precipitation, predominately as rain, snow or cloud 

droplets.

Dry deposition is the fraction 

of atmospheric deposition 

that is deposited to the earth’s 

surface by settling, impaction 

or adsorption.

Wet and dry deposition are 

combined to estimate the total 

deposition of pollutants (gases and particles) to the earth’s 

surface.

Figure 1: CASTNET Site Map

EPA-sponsored CASTNET sites

NPS-sponsored CASTNET sites

BLM-sponsored CASTNET sites

PAL190

ALC188

GAS153

ESP127

EGB181

DCP114

CVL151

CTH110

COW137

CNT169

CND125

CKT136

CHE185

CHA467

CDZ171

CDR119

CAD150

BWR139

BFT142

BBE401

ASH135

ARE128

ANA115

ALH157

ABT147

YOS404

YEL408

WST109

WSP144

SEK430

VOY413

VIN140

UVL124

THR422

SUM156

STK138

SPD111

SND152

SHN418

SAL133

VPI120

ROM206ROM406

QAK172

PSU106

PRK134

PNF126

PND165

PIN414

PED108

PAR107

OXF122

MAC426

MKG113

MEV405 MCK231

MCK131

LRL117

LAV410

KNZ184

KEF112

WNC429

PET427

MOR409

EVE419

CAT175

CAN407

BEL116

SAN189

ACA416

IRL141

HOX148

GTH161

GRS420

GRC474

GRB411

GLR468

YOS204

HWF187

BVL130

SHE604

BUF603

UND002

NIC001

NEC602

HOW191

WFM105

JOT403

BAS601

January 29, 2013
CLEAN AIR STATUS AND TRENDS NETWORK 3

Data is polled remotely from each EPA-sponsored CASTNET 

site and updated daily on the CASTNET website. Data is quality 

assured using automated screening techniques, semi-annual 

calibration results, site operator comments and manual data 

review. Near real-time ozone data is loaded to EPA’s AIRNow 

site. The data in AIRNow have not undergone rigorous QA 

checks. Final hourly ozone concentrations are reported to AQS 

monthly.

Direct measurement of dry deposition flux is expensive and 

time consuming so dry deposition fluxes (D) are calculated as 

the measured pollutant concentration (C) multiplied by the 

modeled Vd. However, alternative measurement methods are 

currently being developed and, once available, will be used to 

compare estimated dry deposition fluxes from CASTNET to direct deposition measurement results.

ACCOUNTABILITY AND RESULTS

Emission reduction programs such as the ARP, NBP and CAIR 

have led to significant reductions in sulfur and nitrogen pollutants as well as ambient ozone concentrations. The purpose 

of CASTNET is to quantify the environmental results of these 

emission reduction programs. Results of these programs are 

summarized in CAMD’s annual reports (www.epa.gov/airmarkets/progress/progress-reports.html).

Acidic deposition

CASTNET reports on the long-term, regional trends of acid deposition. Figures 2 and 3 show the dry, wet and total deposition trends in sulfur and nitrogen deposition in the Eastern US 

between 1989 and 2011. The 3-year averages of total sulfur 

deposition decreased by 62% (13.9 kg/ha to 5.3 kg/ha) between 1989–1991 and 2009–2011 in the Eastern US. Total nitrogen deposition decreased by 31% (8.0 kg/ha to 5.5 kg/ha) 

over the same time period.

Ozone

CASTNET is the primary network for measuring rural ozone 

concentrations and used to assess changes in background 

ozone concentrations, a known indicator of climate change. 

The 3-year average of the 4th highest daily maximum rolling 

8-hour average ozone concentration has decreased 19% beNetwork Summary

Number of Sites 90

Weekly Ambient Measurements

Gaseous Sulfur Dioxide (SO2)

Nitric Acid (HNO3)

Particulate Sulfate (SO4

2-)

Nitrate (NO3

_

)

Ammonium (NH4

+

)

Base Cations (Mg2+, Ca2+, K+)

Chloride ion (Cl_)

Hourly Measurements Ozone (O3)

Shelter Temperature

9-m temperature

Meteorological 

Parameters* 

(1-hour averages)

2-m Temperature

Wind Speed and Direction

Sigma Theta

Solar Radiation

Relative Humidity

Precipitation

Surface Wetness

*Meteorological measurements were discontinued at all but 

5 EPA-sponsored CASTNET sites in 2010.

Figure 2: Trend in Sulfur Deposition (kg/ha/yr) 

with Concentration in Precipitation (mg/l) in the 

Eastern US

16

10

12

14

8

6

4

2

0

1.2

1.0

0.8

0.6

0.4

0.2

0

Deposition (kg/ha/yr)

Concentration (mg/l)

19901995200020052010

Dry Deposition

Wet Deposition

Total Deposition

Concentration in Precipitation
4 CLEAN AIR STATUS AND TRENDS NETWORK

tween 2000–2002 (83 ppb) and 2009–2011 (67 ppb) in the 

Eastern US. The Western US realized a 5% reduction over the 

same time period (73 ppb reduced to 67 ppb). CASTNET ozone 

measurements will be used in non-attainment designations for 

the 2011–2013 monitoring season. Figure 4 shows the 3-year 

averages of the 4th highest daily maximum for 2009–2011 

from CASTNET sites meeting the completeness criteria.

RESEARCH AREAS

Direct Deposition Measurements

CAMD has deployed an 8-inlet ozone sampling system on a 

30 meter tower at the Howland AmeriFlux site to determine 

if this is a viable method for measuring the gradient above and 

below a forest canopy. Preliminary results show ozone loss 

at night within the canopy which is consistent with previous 

direct measurement techniques. In addition, filter packs were 

deployed within and below the forest canopy to determine if 

loses could be seen in a weekly integrated sample.

EPA’s Office of Research and Development (ORD) is evaluating a direct deposition analyzer which can be used to validate 

model results.

Total Deposition

CASTNET is continuing development of an improved method 

for estimating total deposition that uses measurement data 

where available and modeled results where measurements 

are lacking. These products will support critical loads research 

and help prioritize improvements in existing routine monitoring networks.

DATA AVAILABILITY

CASTNET Data: Aggregates of ambient 

concentrations and dry and total deposition fluxes, and site information can be 

downloaded from the CASTNET website 

under Download Data (www.epa.gov/

castnet/clearsession.do). The CASTNET annual report provides long-term 

trends, regional patterns of pollutants 

and network quality assurance results. 

The annual report can be found under 

Documents and Reports (www.epa.gov/

castnet/documents.do).

NADP Data: NTN wet deposition estimates and ambient NH3 concentrations 

can be downloaded from the NADP website (nadp.isws.illinois.edu/).

Figure 3: Trend in Nitrogen Deposition (kg/ha/yr) 

with Concentrations in Precipitation (mg/l) in the 

Eastern US

Figure 4: Three-year averages of the 4th highest daily maximum ozone 

concentrations (ppb) for 2009–2011

January 29, 2013

78

65

56

77

67

62

72

66

56

59

63

67

72

96

62

69 74

70

59

65

64

70

66

65

66

54

71

68 69

70 68

68

64

69

65

93

58

65

54

77

69

70

51

71

67

71

65

65

68

70

71

70

61

68

76 78

69

53

65

72

69

3-year average of the 4th 

highest daily maximum (ppb)

Less than 60 ppb

Between 60 and 70 ppb

Between 70 and 75 ppb

Between 75 and 85 ppb

Greater than 85 ppb

Wet Deposition

Total Deposition

Dry Deposition

Concentration in Precipitation

12

10

8

6

4

2

0

Deposition (kg/ha/yr)

19901995200020052010

1.2

1.0

0.8

0.6

0.4

0.2

0

Concentration (mg/l)