Clean Air Status

and Trends Network

AAnnnnuuaallRReeppoorrtt

2011
Clean Air Status and Trends Network 

(CASTNET)

2011 Annual Report

Prepared by:

AMEC Environment & Infrastructure, Inc.

Prepared for:

U.S. Environmental Protection Agency

Office of Air and Radiation

Clean Air Markets Division

Washington, DC

EPA Contract No. EP-W-09-028

February 2013 
CASTNET 2011 Annual Report

i Table of Contents

Table of Contents

Executive Summary........................................................................................ii

Chapter 1 CASTNET Overview ........................................................................................ 1

Background....................................................................................................... 1 

Other Programs Contributing to Nitrogen Oxides Emission Reductions ............2

Cooperating Networks ...................................................................................... 5 

Locations of Monitoring Sites ............................................................................ 6 

CASTNET Reference Sites............................................................................... 7 

Measurements Recorded at CASTNET Sites.................................................... 8 

Modeling Dry Deposition................................................................................. 10 

2011 NAPAP Report to Congress ................................................................... 11 

Sulfur Dioxide and Nitrogen Oxides Emissions ............................................... 12 

Chapter 2 Atmospheric Concentrations....................................................................... 15

Sulfur Dioxide ................................................................................................. 15 

National Ambient Air Quality Standards for Particulate Matter.........................18

Particulate Sulfate...........................................................................................19

Total Nitrate .................................................................................................... 20 

The CASTNET Ammonia Study and AMoN Ammonia Intercomparison ..........23

Particulate Ammonium.................................................................................... 26 

Cloud Deposition Monitoring at Clingmans Dome, TN ....................................27

Chapter 3 Atmospheric Deposition............................................................................... 31

EPA Method for Substituting Historical Hourly Deposition Velocity Values 

 for Missing Hourly MLM Data ....................................................................... 32

Sulfur Deposition............................................................................................. 34 

The Effect of Differences in Annular Denuder System 

and CASTNET Filter Pack Concentration Data Relative to Estimates of 

Atmospheric Deposition..............................................................................38 

Nitrogen Deposition ........................................................................................ 40 

Estimates of Critical Loads throughout the United States................................44 

Chapter 4 Ozone Concentrations.................................................................................. 47

National Ambient Air Quality Standards for Ozone..........................................48 

Eight-Hour Ozone Concentrations .................................................................. 50 

Ozone and Filter Pack Concentrations at the Maine Howland 

Research Forest.........................................................................................55 

Ozone Flux Values for 2011............................................................................ 59 

W126 Values for 2011 ....................................................................................60

Air Quality in Western North Dakota................................................................62

Chapter 5 Data Quality................................................................................................... 64

Precision......................................................................................................... 65 

Accuracy .........................................................................................................73

Completeness................................................................................................. 75 

Results for 2011 Environment Canada Proficiency Testing............................. 76 

Summary of Data Quality Results ................................................................... 78 

References ....................................................................................................................... 79

Appendix A Locational and Operational Characteristics of CASTNET Sites

Appendix B Acronyms and Abbreviations
CASTNET 2011 Annual Report

ii Executive Summary

Executive Summary

The EPA Clean Air Status and Trends Network (CASTNET) measures atmospheric pollution 

concentrations in order to evaluate the effectiveness of national and regional air pollution control 

programs. This report presents 2011 maps of the geographic distribution of sulfur and nitrogen 

pollutant concentrations, ozone (O 

3) levels, and deposition fluxes and examines trends in air 

quality since 1990. CASTNET measured rural, regionally representative concentrations of sulfur 

and nitrogen species and O 

3 at 83 sites in the United States and Canada during 2011.

Key Results and Highlights through 2011 

• Mean annual sulfur dioxide (SO 

2) and particulate sulfate (SO2-4 ) concentrations declined 

significantly over the 22-year period 1990 through 2011 and showed a substantive decline 

since 2005. Three-year mean annual SO 

2 levels declined 72 percent while SO2-4

concentrations declined 52 percent at eastern reference sites from 1990 through 2011. SO 

2

concentrations measured at western reference sites were reduced by 54 percent, and SO2-

4

levels dropped by 21 percent over the 16-year period (1996 through 2011).

• Total (dry + wet) sulfur deposition estimated in the eastern United States declined by 

59 percent from 1990 through 2011. Total sulfur deposition aggregated over western 

reference sites fell by 35 percent over the 16 years 1996 through 2011. 

• Three-year mean annual concentrations of total nitrate (NO3), which is comprised of 

nitric acid (HNO 

3) plus particulate NO3 declined 37 percent at eastern sites over the 22-year 

period. Total NO3 levels measured at western reference sites dropped by 29 percent over 

the 16 years.

• Total nitrogen deposition [particulate NO3 (dry + wet) and HNO 

3] fell by 26 percent from 

1990 through 2011 at eastern reference sites. Total nitrogen deposition estimated for 

western reference sites was reduced by 22 percent over the 16 years. However, total 

nitrogen deposition increased significantly at eastern reference sites from 2010 to 2011 

because of increased nitrogen concentration in precipitation. The network recorded the 

highest annual average precipitation over the 22 years.

• The mean fourth highest daily maximum 8-hour average O 

3 concentration for 2011 was 

68 parts per billion (ppb), which was lower than the 2010 value of 69 ppb and higher than 

the 2009 concentration of 64 ppb, the lowest in the history of the network. Four eastern and 

three California sites recorded exceedances of the 8-hour standard of 0.075 parts per 

million during the most recent 3-year period (2009 through 2011). 

• Measurements taken during 2011 and historical data collected over the period 1990

through 2010 were analyzed relative to data quality indicators and their numerical 

measures. These analyses demonstrate that CASTNET data can be used with confidence 

and that CASTNET continues to produce information of the highest quality.

• In 2011, EPA-sponsored CASTNET sites became compliant with the requirements of 

40 CFR Part 58, and O 

3 data from EPA sites were delivered to the EPA Air Quality System. 
CASTNET 2011 Annual Report

iii Executive Summary

• EPA discontinued meteorological measurements at all but five EPA-sponsored CASTNET 

sites by December 31, 2010. NPS sites are continuing meteorological data collection. 

Consequently, missing deposition velocity (Vd) values resulting from missing 

meteorological data were replaced based on the results in Bowker et al. (2011), which 

substitutes hour-specific historical averages of Vd for missing Vd values at specific sites. 

The substitution procedure was shown to result in long-term, unbiased estimates of the 

annual mean Vd. 

• In 2010, in order to improve estimates of wet deposition, EPA used the Parameterelevation Regressions on Independent Slopes Model (PRISM) to develop a continuous grid 

of precipitation data. PRISM uses terrain elevation, slope, and aspect and climatic 

measurements to estimate precipitation on a 4-kilometer resolution grid.

• The Vd substitution procedure and the use of PRISM resulted in more complete, improved 

estimates of dry, wet, and total deposition for 2011. 

Prince Edward, VA (PED108)
CASTNET 2011 Annual Report

1 Chapter 1: CASTNET Overview

The Clean Air Status and Trends Network (CASTNET) began operation in 1991 to provide long-term 

monitoring of air quality in rural areas and to determine trends in regional atmospheric sulfur, nitrogen, and 

ozone concentrations and in deposition fluxes of sulfur and nitrogen pollutants. CASTNET was designed to 

provide data for use in assessing trends in air quality, atmospheric deposition, and ecological effects that 

result from air pollutant emission reductions by electric generating units. The Environmental Protection 

Agency and the National Park Service operate the network. CASTNET was established under the 1990 Clean 

Air Act Amendments and continued and expanded the National Dry Deposition Network, which began in 

1987. During 2011, the network operated 83 monitoring stations throughout the contiguous United States, 

Alaska, and Canada. Measurements collected in 2011 show the continuing decline in sulfur and nitrogen 

pollutant concentrations and ozone concentrations in the eastern United States relative to 2010. All pollutants 

have declined significantly over the last 22 years.

Chapter 1: CASTNET Overview

Background

The U.S. Congress established the Acid Rain Program (ARP) under Title IV of the 1990 Clean 

Air Act Amendments (CAAA). The ARP was promulgated to reduce emissions of sulfur dioxide 

(SO 

2) and nitrogen oxides (NOx) from electric generating units (EGUs). Congress further directed 

the U.S. Environmental Protection Agency (EPA) to institute CASTNET under Title IX of the 

CAAA in order to assess the effectiveness of the ARP and to assess and track real-world 

environmental results as the ARP was implemented, and emission reductions became effective. 

Congress mandated that CASTNET provide consistent, long-term measurements for determining 

relationships between changes in emissions and subsequent changes in air quality, atmospheric 

deposition, and ecological effects.

The ARP has produced significant reductions in SO 

2 and NOx emissions from EGUs since 1995. 

The SO 

2 program set a permanent cap on the total amount of SO 2 emissions allowed annually 

from EGUs and included the groundwork for trading and banking emission allowances. One 

allowance provides a regulated EGU limited authorization to emit one ton of SO 

2. The program 

was phased in with the final 2010 SO 

2 emissions cap set at 8.95 million tons, a level about onehalf of the total SO 

2 emissions from the power sector in 1980. Actual SO 2 emissions from EGUs 

in 2011 were about 4.5 million tons.
CASTNET 2011 Annual Report

2 Chapter 1: CASTNET Overview

NOx reductions under the ARP are achieved through a program that applies to a subset of coalfired EGUs by limiting the NOx emission rate based on a unit’s boiler type. Actual NOx emissions 

from ARP EGUs in 2011 were about 2.0 million tons. Although the ARP was responsible for a 

large portion of these annual NOx reductions, other emission control programs have also 

produced substantive declines in NOx emissions in the eastern United States. These programs 

include the Ozone Transport Commission (OTC) NOx Budget Program, the NOx State 

Implementation Plan (SIP) Call/NOx Budget Trading Program (NBP), and the Clean Air 

Interstate Rule (CAIR).

Other Programs Contributing to Nitrogen Oxides Emission Reductions

Ozone Transport Commission (OTC) – Established under the 1990 CAAA, this commission 

consists of states primarily located in the Northeast and Mid-Atlantic regions. The OTC developed the 

OTC NOx Budget Program, which operated from 1999 to 2002. As part of this program, 11 states 

and the District of Columbia entered into a memorandum of understanding to achieve regional 

emission reductions of NOx through the use of control technologies and an ozone (O 

3 ) season cap 

and trade program.

NOx State Implementation Plan (SIP) Call – Issued in 1998 to reduce the regional transport of 

ground-level O 

3, the NOx SIP Call required states to reduce O 3 season NOx emissions by meeting 

emission budgets.

NOx Budget Trading Program (NBP) – This market-based cap and trade program was developed 

under the NOx SIP Call and replaced the OTC NOx Budget Program in 2003. The NBP was created 

to reduce NOx emissions from power plants and other large combustion sources in 20 eastern states 

and the District of Columbia and operated from 2003 to 2008. 

Clear Air Interstate Rule (CAIR) – Promulgated in 2005, CAIR was designed to reduce emissions 

of SO 

2 and NOx in 27 eastern states and the District of Columbia and replaced the NBP in 2009. This 

rule created three separate trading programs: an annual NOx program, an O 

3 season NOx program, 

and an annual SO 

2 program. In December 2008, CAIR was remanded, but remained in place while 

EPA developed a replacement rule (EPA, 2012b).

Cross-State Air Pollution Rule (CSAPR) – On July 6, 2011, CSAPR was promulgated to require 

28 states in the eastern half of the United States to significantly improve air quality by reducing power

plant emissions that cross state lines and contribute to ground-level O 

3 and fine particle pollution in 

other states. The rule was scheduled to replace CAIR starting in 2012 and would have reduced annual 

SO 

2, annual NOx, and O 3 season NOx emissions, while providing sources with flexibility in how to 

comply with the program. On December 30, 2011, the court stayed CSAPR pending judicial review, 

and on August 21, 2012, the court decided to vacate the rule. On October 5, 2012, the U.S. 

Government filed a petition for rehearing en banc, asking the full court to reconsider that decision. The 

court denied the U.S. Government’s petition for an en banc rehearing on January 24, 2013. In the 

meantime, as the court stated in its opinion, CAIR remains in place, and states and affected sources 

are expected to comply with the rule. 

CASTNET was developed from the National Dry Deposition Network (NDDN), which began 

operation in 1987. The 50 NDDN sites were transferred to CASTNET in 1991. In 2011, the 
CASTNET 2011 Annual Report

3 Chapter 1: CASTNET Overview

network included 83 monitoring stations at 81 site locations throughout the contiguous United 

States, Alaska, and Canada. In September 2011, a new CASTNET site was installed in 

Howland, ME (HOW191) at the AmeriFlux Network site. This site represents the first 

collaboration between the two long-standing national networks. AmeriFlux’s purpose is to 

measure ecosystem level exchanges of carbon dioxide, water, energy, and momentum. 

AmeriFlux sites utilize a 30-meter (m) tower to enable measurements in the lower boundary 

layer (AmeriFlux, 2012). CASTNET is interested in similar processes for O 

3 and sulfur and 

nitrogen species. At the HOW191 site, hourly O 

3 concentrations are collected at eight elevations 

within and above the forest canopy on a scaffold tower. Filter pack sampling systems are 

operated at two elevations. 

CASTNET is sponsored by EPA and the National Park Service (NPS). NPS began its 

participation in CASTNET in 1994 under an agreement with EPA. With the participation of NPS, 

the network became a national, rather than a primarily eastern, network. NPS is responsible for 

protecting air quality and resources sensitive to air quality in national parks and wilderness 

areas. Twenty-five CASTNET sites were sponsored by NPS during 2011. Additional information 

on the NPS air monitoring program can be found on the Web site: 

http://www.nature.nps.gov/air/monitoring/.

CASTNET stations measure rural, regionally representative concentrations of sulfur and 

nitrogen pollutants and O 

3 in order to detect and quantify trends, define the geographic 

distribution of rural pollutants, and estimate dry deposition of pollutants. During 2011, O 

3

measurements at EPA-sponsored CASTNET sites became compliant with the requirements of 

Title 40 Code of Federal Regulations (CFR) Part 58, and O 

3 data from EPA sites were delivered 

to the EPA Air Quality System (AQS).

Dry deposition is calculated from CASTNET concentration measurements and modeled 

deposition velocities (Vd) using the Multi-Layer Model (MLM) or from historical Vd

measurements. The MLM requires several meteorological parameters and information on 

vegetation and land use specific to each site. CASTNET data are also used for atmospheric 

model evaluation and as input to ecological models.

EPA discontinued meteorological measurements at all but five EPA-sponsored CASTNET sites 

by December 31, 2010. NPS sites are continuing meteorological data collection. For this report, 

missing Vd values resulting from missing meteorological data were replaced using the method 

described in Bowker et al. (2011). Bowker’s method substitutes hour-specific historical averages 

of Vd for missing Vd values at specific sites. The substitution procedure was shown to result in 

long-term, unbiased estimates of the annual mean Vd. A variation of Bowker’s method was

applied to all sites with discontinued/missing meteorological data for the period October through 

December 2010 and for 2011 and also for sites missing historical Vd values. Sulfur and nitrogen 
CASTNET 2011 Annual Report

4 Chapter 1: CASTNET Overview

dry deposition fluxes based on updated values of Vd through 2011 are reported herein. The 

current MLM modeling approach that is used to estimate dry deposition fluxes will continue to 

be used for the five EPA-sponsored sites with continuing meteorological measurements and all 

NPS-sponsored sites.

In order to improve estimates of wet deposition, EPA used the Parameter-elevation Regressions 

on Independent Slopes Model (PRISM) to develop a continuous grid of precipitation data 

instead of using cubic inverse distance weighting (IDW). PRISM uses terrain elevation, slope,

and aspect and climatic measurements to estimate precipitation on a 4-kilometer (km) resolution 

grid. Pollutant concentrations in precipitation that were estimated for the PRISM grid were 

provided by National Atmospheric Deposition Program (NADP)/National Trends Network (NTN). 

These two grids were merged in order to estimate pollutant wet deposition rates. Wet deposition 

rates for CASTNET sites were estimated from four nearby gridded values for this report.

This report summarizes CASTNET monitoring activities and the resulting concentration and 

deposition data collected over the 22-year period from 1990 through 2011. Additional 

information, previous annual reports, and other CASTNET documents can be found on the EPA 

Web site, http://www.epa.gov/castnet/. The CASTNET database is also available to the public 

by accessing the “Download Data” link on EPA’s CASTNET Web page. The Web site provides a 

complete archive of concentration and deposition data for all EPA- and NPS-sponsored 

CASTNET sites. Fully validated data are available approximately 10 months following collection. 

NPS data collected during 2011 from the NPS-sponsored CASTNET sites and the NPS 

Gaseous Pollutant Monitoring Program are available in the NPS Annual Data Summary 2011 

(Ray, 2013) and the Data Quality Assurance Report 2011, Gaseous Pollutant Monitoring 

Program (NPS, 2012).

Pinedale, WY (PND165)
CASTNET 2011 Annual Report

5 Chapter 1: CASTNET Overview

Cooperating Networks

CASTNET monitors air quality and deposition in cooperation with other national networks.

• National Atmospheric Deposition Program (NADP) operates: 

o National Trends Network (NTN), which includes about 250 monitoring stations with wet 

deposition samplers to measure the concentrations and deposition rates of air pollutants 

removed from the atmosphere by precipitation. NADP/NTN operates wet deposition 

samplers at or near virtually every CASTNET site.

o Mercury Deposition Network (MDN), which operates samplers to measure mercury in 

precipitation. MDN samplers are operated at several CASTNET sites. 

o Ammonia Monitoring Network (AMoN), which operates passive ammonia (NH 

3) samplers 

at about 57 NTN and 40 CASTNET locations. AMoN, in operation for three years, provides 

information on 2-week average NH 

3 concentrations.

o Atmospheric Mercury Network (AMNet), which measures atmospheric concentrations of 

gaseous oxidized, particulate-bound, and elemental mercury at about 25 locations in the 

continental United States, Canada, Hawaii, and Taiwan in order to estimate dry and total 

mercury deposition.

See NADP’s Web site for more information on these complementary networks: 

http://nadp.isws.illinois.edu/.

• Canadian Air and Precipitation Monitoring Network (CAPMoN) operates 33 measurement 

sites throughout Canada and one in the United States. CASTNET and CAPMoN both operate 

filter pack samplers in Ontario, Canada. CAPMoN operates a wet deposition sampler at 

Pennsylvania State University. Visit CAPMoN’s Web site at http://www.ec.gc.ca/rsmn/default.asp?lang=En&n=752ce271-1.

• Interagency Monitoring of Protected Visual Environments (IMPROVE) measures aerosol 

pollutants near more than 20 CASTNET sites. IMPROVE measures particulate air pollutants that 

affect visibility. For more information on IMPROVE, see http://vista.cira.colostate.edu/IMPROVE/.

EPA relies on CASTNET and these other long-term national networks to generate the data 

needed to assess the effectiveness of promulgated emission control programs under several 

different mandates including the National Acid Precipitation Assessment Program (NAPAP), 

Title IX of the CAAA, the Canada – United States Air Quality Agreement, and the Government 

Performance and Results Act.

Although EPA and NPS are the primary sponsors of CASTNET, other organizations, Native 

American tribes, universities, and government agencies play a role in sponsoring individual 

CASTNET sites. In all, CASTNET has more than 50 partners. These co-sponsors provide inkind services that support the overall operation of a site, including site operation, land use, or 

both. All of the sites added to the network since 2001 have an associated co-sponsor. Three 

CASTNET monitoring sites are located on tribal lands: Cherokee Nation in eastern Oklahoma 

(CHE185), Alabama-Coushatta in eastern Texas (ALC188), and Santee Sioux in northern 
CASTNET 2011 Annual Report

6 Chapter 1: CASTNET Overview

Nebraska (SAN189). State agencies also operate special purpose air pollutant measurement 

devices at some CASTNET sites.

Locations of Monitoring Sites

The locations of CASTNET monitoring sites that were operational during 2011 are depicted in 

Figure 1-1. Eighty-three sites were operated at 81 distinct locations. To estimate precision 

across the network, collocated sites were operated at Mackville, KY (MCK131/231) and Rocky 

Mountain National Park, CO (ROM406/ROM206) during 2011. The ROM406/206 monitors 

ensure consistency between the two sponsoring agencies, EPA and NPS, since EPA operates 

ROM206 and NPS operates ROM406. ROM406 is considered the regulatory monitoring site for 

O 

3. Most CASTNET sites are located in rural or remote locations away from pollutant emission 

sources and heavily populated areas. Appendix A provides the location of each site by state and 

includes information on start date, latitude, longitude, elevation, and the parameters measured. 

For the purposes of this report, CASTNET sites are called “western” or “eastern” depending on 

whether they are west or east of 100 degrees west longitude. In general, sample flow rates are 

set to 1.5 liters per minute (lpm) in the east and at a higher rate of 3.0 lpm in the west due to the 

lower pollutant concentrations generally found in the western United States.

Figure 1-1 CASTNET Sites Operational During 2011
CASTNET 2011 Annual Report

7 Chapter 1: CASTNET Overview

CASTNET Reference Sites

Chapters 2, 3, and 4 show maps of pollutant concentrations and deposition fluxes across the 

United States. In addition, measurements from 34 CASTNET eastern reference sites 

(Figure 1-2) were analyzed for each pollutant in order to determine trends in concentrations and 

rates of dry, wet, and total deposition. These 34 sites have been reporting CASTNET 

measurements since at least 1990. The reference sites were selected using criteria such as site 

longevity and data completeness, which were used by EPA in its National Air Quality and 

Emissions Trends Report (2000). Trends in pollutant concentrations measured at CASTNET 

western reference sites (Figure 1-2) are also presented. The 17 western reference sites have 

been operating since at least 1996.

Figure 1-2 CASTNET Western and Eastern Reference Sites

The data from the 34 eastern reference sites were aggregated and then presented using box 

plots for the 22-year period 1990 through 2011. Data from the 17 western reference sites were 

aggregated and then presented using box plots for the period 1996 through 2011.
CASTNET 2011 Annual Report

8 Chapter 1: CASTNET Overview

Measurements Recorded at CASTNET Sites

CASTNET was designed primarily to measure trends in seasonal and annual average 

concentrations and to model deposition fluxes over many years. Consequently, measurement of 

weekly (Tuesday to Tuesday) average concentrations was selected as the basic sampling 

strategy. Over the 7-day period, air is drawn at a controlled flow rate through an open-face 

3-stage filter pack (Figure 1-3) mounted atop a 10-m tower to collect air pollutants in the form of 

gases and particles. The first stage of the filter pack encloses a Teflon filter, the second a nylon 

filter, and the third holds two potassium carbonate (K 

2CO 3)-impregnated cellulose filters. The 

filter pack is changed out each Tuesday and shipped to 

the analytical chemistry laboratory for analysis.

Figure 1-3 Three-Stage Filter Pack

Note: Cellulose = Gaseous: sulfur dioxide

Nylon = Gaseous: nitric acid, sulfur dioxide

Teflon = Particulate: sulfate, nitrate, ammonium, sodium, calcium, magnesium, potassium, and chloride

CASTNET Ambient Measurements

 Sulfur species:

Sulfur dioxide

Particulate sulfate

 Nitrogen species:

Particulate nitrate

Nitric acid

Particulate ammonium

 Metal cations:

Particulate calcium

Particulate sodium

Particulate magnesium

Particulate potassium

 Particulate chloride

 Ozone

 Meteorological variables 

 Information on land use 

and vegetation

Quick Disconnect

Two

Cellulose

Filters

Nylon

Filter

Teflon

Filter

Shipping Cap

(removed during sampling)

Direction of Air Flow Teflon Spacers
CASTNET 2011 Annual Report

9 Chapter 1: CASTNET Overview

The filter packs are prepared, loaded, shipped, received, extracted, and analyzed at AMEC’s

Gainesville, FL laboratory. Following receipt from the field, exposed Teflon filters and blanks are 

extracted and then analyzed for concentrations of sulfate (SO2-

4 ), nitrate (NO3), and chloride (Cl-

) 

by micromembrane-suppressed ion chromatography (IC) and also for ammonium (NH+ 

4 ) by the 

automated indophenol method with the Bran+Luebbe AutoAnalyzer 3. Additionally, Teflon filter 

extracts are analyzed for calcium (Ca2+

 ), sodium (Na+ ), magnesium (Mg2+ ), and potassium (K+ ) 

by inductively coupled plasma-atomic emission spectrometry (ICP-AES) using a Perkin Elmer 

Optima 7300 Dual View spectrometer. The cellulose filter extracts are analyzed for SO 

2 as SO2-4

using IC. The nylon filter extracts are analyzed via IC for nitric acid (HNO 

3) as nitrogen and for 

SO 

2 as SO2-4 . The SO 2 concentrations from the cellulose and nylon filters are summed to obtain 

total SO 

2. 

CASTNET also measures hourly O 

3 concentrations, one of the major components of smog. O 3

measurements are recorded continually and archived as hourly averages. CASTNET O 

3

measurements meet 40 CFR Part 58 compliance regulations, and the hourly O 

3 concentrations 

are delivered to the EPA AQS. In addition to the air pollutant concentrations, five EPAsponsored and all NPS-sponsored CASTNET sites collect hourly meteorological 

measurements, which are used as input to the MLM, 

a numerical model used for estimating dry deposition 

to ecosystems in the atmospheric boundary layer. 

The five sites continuing all meteorological 

measurements are Beltsville, MD (BEL116); 

Bondville, IL (BVL130); Palo Duro Canyon State 

Park, TX (PAL190); CHE185, OK; and Beaufort, NC 

(BFT142). Temperature will continue to be measured 

at all sites in the network. While meteorological data 

collection is continuing at all NPS sites, NPS 

discontinued measurements of 2-m temperature and 

surface wetness at most sites. During 2011, delta 

temperature was measured at the five EPA sites plus 

the NPS sites at Acadia National Park, ME 

(ACA416); Great Smoky Mountains National Park, 

TN (GRS420); and ROM406, CO. Surface wetness 

was measured at the five EPA sites plus GRS420. 

The meteorological measurements are recorded 

continuously and archived as hourly averages.

CASTNET Meteorological Measurements

 Temperature:

Temperature (at 2 m and 9 m)

Delta temperature (difference 

 between 2 m and 9 m)

 Relative Humidity

 Solar Radiation

 Winds:

Speed

Direction

Sigma theta (standard deviation 

 of direction)

 Precipitation

 Surface Wetness
CASTNET 2011 Annual Report

10 Chapter 1: CASTNET Overview

Modeling Dry Deposition

Dry deposition processes are modeled as resistances to deposition. 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 (C) and modeled Vd. The equation to estimate flux 

is shown in Figure 1-4. Measured atmospheric concentrations are calculated based on the mass 

of each analyte in each filter extract and the volume of air sampled. The rate of deposition of a 

pollutant, also known as Vd, is influenced by meteorological conditions, vegetation, and 

atmospheric and plant chemistry. The Vd values for each site are calculated for each hour of 

each year using the MLM. The MLM was summarized by Meyers et al. (1998) and Finkelstein 

et al. (2000). The data used in the MLM to estimate dry deposition are derived from 

meteorological measurements and pollutant concentrations taken at the site together with an 

estimation of the vegetation leaf-out and leaf area index (LAI).

The schematic of the MLM in Figure 1-4 shows the relationships among the various resistances 

and illustrates the meteorological and other data that are required as model input. The 

deposition velocities and fluxes presented in Chapter 3 were calculated using the MLM or were 

estimated from historical data. The decision to discontinue meteorological measurements at all 

but five EPA CASTNET sites, plus the need to improve Vd data completeness, resulted in the 

development of a method by EPA (Bowker et al., 2011) to substitute historical average hourly Vd

values for missing MLM simulations. The substitution method was applied to the CASTNET 

database and used for the dry deposition estimates reported herein.

Figure 1-4 Multi-Layer Model

Flux VxC= d

a

bs cut b a, soil soil

d

r+

r+r

1

+

r+r

1

+

rr

1

1

=V1/

+

Resistances: 

ra = aerodynamic

ra, soil = aerodynamic near soil

rb = boundary layer in thin layer at surface

rcut = cuticular

rs = stomatal

rsoil = soil

Wetness,

Precip

Temp, Rel Hum,

Solar Rad, LAI

Wind Speed,

Sigma Theta
CASTNET 2011 Annual Report

11 Chapter 1: CASTNET Overview

2011 NAPAP Report to Congress

The National Acid Precipitation Assessment Program (NAPAP) was reauthorized in 1991 under 

Title IX of the 1990 CAAA and was tasked “to periodically assess and report to Congress on (1) the 

implementation of the ARP, (2) the most recent scientific information related to acid deposition and its 

effects, and (3) additional decreases in acid deposition necessary to prevent adverse ecological 

effects.” Title IX of the CAAA requested that NAPAP conduct acid rain research and monitoring and 

periodically assess the costs, benefits, and effectiveness of Title IV. The NAPAP member agencies 

are the EPA, the U.S. Department of Energy, the U.S. Department of Agriculture, the U.S. Department 

of Interior, the National Aeronautics and Space Administration, and the National Oceanic and 

Atmospheric Administration. The 2011 NAPAP report is the fourth published since 1990. It focuses on 

EGU emission reductions, summarizes changes in atmospheric deposition rates, and projects the 

ecological effects of additional decreases in emissions of SO 

2 and NOx. A copy of the report is 

available at http://ny.water.usgs.gov/projects/NAPAP/.

CASTNET data support the NAPAP report, which concludes that Title IV has successfully reduced 

emissions of SO 

2 and NOx from the power sector. CASTNET data depicted in the figures in 

Chapters 2, 3, and 4 of this report show significant reductions in ambient concentrations of sulfur and 

nitrogen pollutants, in total atmospheric deposition rates of sulfur and nitrogen species, and in fourth 

highest daily maximum O 

3 concentrations. These figures support the findings of the NAPAP report, 

which concludes that Title IV has successfully reduced power sector emissions of SO 

2 and NOx. EPA 

emissions data show SO 

2 emissions from EGUs (Figure 1a) were reduced to 4.5 million tons in 2011, 

64 percent lower than 1990 emissions and below the 2010 statutory cap of 8.95 million tons. NOx

emissions from EGUs (Figure 1b) were reduced to 2.0 million tons in 2011, 67 percent lower than 

1995 emissions and substantially exceeding the Title IV goal.

Figure 1a SO 

2 Emissions from CAIR SO 2 Annual Program and ARP Sources, 1980 through 2011 

(EPA, 2012b)

The success of the SO 

2

emission-reduction program is 

due to the combined use of an 

overall emission cap for SO 

2

with a trading system that 

encourages cost-effective 

emission reductions. The 

inherent flexibility for sources to 

choose how to control the SO 

2

emissions in the cap and trade 

approach for SO 

2 has been 

successful at reducing 

compliance costs to a fraction of 

the cost estimated in 1990. 

Several factors are responsible 

for the relatively low costs of 

SO 

2 reductions realized under 

Title IV, including the 

widespread availability of low sulfur coal and technical innovations that facilitated use of that coal; 

lower than anticipated scrubber costs; the opportunity to bank allowances; and development of an 

efficient, high-volume market for allowances. Annual SO2 Emissions (million tons)
CASTNET 2011 Annual Report

12 Chapter 1: CASTNET Overview

Figure 1b NOx Emissions from CAIR and ARP Sources, 1990 through 2011 (EPA, 2012b) 

The emission reductions achieved under the ARP have contributed to measurable improvements in air 

quality, decreases in acid deposition, the beginnings of recovery of acid sensitive lakes and streams in 

some areas, and improvements in visibility. The NAPAP report also estimates the benefits and costs 

of complying with Title IV. Despite the environmental benefits reported by NAPAP and the declines in 

pollutant concentrations and deposition rates reported in this CASTNET 2011 Annual Report, recovery 

from the effects of acidification is not likely for many sensitive forests and aquatic ecosystems without 

additional decreases in emissions and acid deposition.

Source: Burns et al. (2011); EPA (2012b) 

Sulfur Dioxide and Nitrogen Oxides Emissions

The ARP was designed to reduce the effects of acid deposition by requiring major reductions of 

SO 

2 and NOx emissions from the electric power industry. EPA issues annual progress reports 

(e.g., EPA, 2012a; 2012b) on the effects of the ARP and related programs. The 2011 EPA 

report (2012a) summarizes 2011 emission and compliance data, market analyses, and 

environmental and human health results.

Under the ARP, SO 

2 reductions are achieved using a cap and trade program that sets a 

permanent cap on the total amount of SO 

2 that may be emitted annually by all regulated EGUs 

in the contiguous United States. The program, which began in 1995, required incremental 

reductions in SO 

2 emissions yearly with the final 2010 SO 2 emissions cap set at 8.95 million 

tons, a level equal to about one-half of the emissions from the power sector in 1980. Use of a 

market-based cap and trade mechanism to reduce SO 

2 emissions allows flexibility for individual 

combustion units to select their own methods of compliance. Currently, one allowance provides 
CASTNET 2011 Annual Report

13 Chapter 1: CASTNET Overview

a regulated unit limited authorization to emit one ton of SO 

2. The CAAA allocated allowances to 

regulated units based on historic fuel consumption and specific emission rates prior to the start 

of the program. The total allowances allocated for each year equal the SO 

2 emission cap. The 

program encourages early reductions by allowing sources to bank unused allowances in one 

year and use them in a later year.

The ARP uses a more traditional approach to achieve NOx emission reductions. Rate-based 

NOx limits apply to most of the coal-fired electric utility boilers subject to the SO 

2 emission 

reductions, and no nationwide cap has been placed on NOx emissions. Other NOx emission 

control programs have resulted in significant reductions in NOx emissions during the O 

3 season. 

These programs include the OTC NOx Budget Program, the NOx SIP Call/NBP, and the CAIR. 

See the discussion on page 2. 

The ARP is comprised of two phases for the reduction of SO 

2 and NOx. Phase I applied 

primarily to the largest coal-fired EGUs from 1995 through 1999 for SO 

2 emissions and from 

1996 through 1999 for NOx emissions. Phase II for both pollutants began in 2000. In 2011, the 

Phase II ARP SO 

2 requirements affected 3,640 operating units. The Phase II ARP NOx

requirements applied to 930 of those operating units that exceed 25 megawatts and burned 

coal between 1990 and 1995 (EPA, 2012a). The CAIR NOx O 

3 season program affected 

3,345 units in 2011, and the NOx and SO 

2 annual programs affected 3,307 units. Under the 

ARP’s and CAIR’s emission reduction requirements, total SO 

2 emissions from affected 

sources were 4.5 million tons in 2011; total NOx emissions from ARP and CAIR sources were 

2.0 million tons.

Figure 1-5 presents state-by-state total annual SO 

2 emissions for CAIR and ARP electric utility 

plants for six years (1990, 1995, 2000, 2005, 2010, and 2011). The most significant reduction in 

SO 

2 emissions occurred in and near the Ohio River Valley where many coal-fired power plants 

are located. Many of the states east of the Mississippi River realized 50 percent reductions in

SO 

2 emissions over the 22 years; the larger emitting states (Illinois, Indiana, Kentucky, Ohio, 

Pennsylvania, and West Virginia) reduced SO 

2 emissions by more than 73 percent. The period 

2005 through 2011 experienced sharp reductions in SO 

2 emissions in several states as a result 

of CAIR requirements. For example, notable reductions were achieved in West Virginia

(80 percent), North Carolina and Massachusetts (72 percent), and Florida (78 percent).

Annual NOx emissions by state are depicted in Figure 1-6 for the same six years. States with the 

highest NOx emissions also produced the most significant declines. Every state east of the 

Mississippi River experienced a significant decline in annual NOx emissions from 2000 to 2011. 

For example, NOx emissions from Ohio EGUs declined by 73 percent from 2000 to 2011. 

Declines in NOx emissions from the larger emitting states ranged from 67 percent in 

Pennsylvania to 81 percent in Ohio over the entire 22 years.
CASTNET 2011 Annual Report

14 Chapter 1: CASTNET Overview

Figure 1-5 Annual SO 

2 Emissions (for CAIR and ARP Sources)

Figure 1-6 Annual NOx Emissions (for CAIR and ARP Sources)

Annual Emissions of Sulfur Dioxide

 

Scale: Largest bar equals 

2,212 thousand short 

tons (Ohio, 1990).

 

1990

1995

2000

2005

2010

2011

Annual Emissions of Nitrogen Oxides

 

Scale: Largest bar equals 

534 thousand short 

tons (Ohio, 1990).

 

1990

1995

2000

2005

2010

2011

Source: EPA Air Markets Program Data (2012c)

Source: EPA Air Markets Program Data (2012c)
CASTNET 2011 Annual Report

15 Chapter 2: Atmospheric Concentrations

Weekly average concentrations of SO 

2, SO2-4 , HNO 3, NO3, NH+ 4 , Cl-

, and 4 earth metals were measured at 

83 CASTNET monitoring stations using 3-stage filter packs. Trends in mean annual SO 

2, SO2-4 , total NO3, and 

NH+ 

4 concentrations aggregated over 34 eastern and 17 western reference sites are shown using box plots for 

each year. All four sulfur and nitrogen parameters measured at the eastern sites declined over the 22-year 

period from 1990 through 2011. Measured annual mean concentrations of SO 

2 and SO2-4 have decreased 

steadily with a major reduction since 2005. Concentrations of total nitrate began to drop in 2000 in response 

to NOx emission reduction programs and have continued to fall. SO 

2, SO2-4 , total NO3, and NH+ 4 concentrations 

measured at the 17 western reference sites declined over the last 16 years.

Chapter 2: Atmospheric Concentrations

Maps of 2011 annual mean concentrations of SO 

2, SO2-4 , total NO3 (HNO 3 + NO3), and NH+ 4 are 

presented in this chapter. Additional maps are provided in CASTNET quarterly reports (AMEC, 

2011a; 2011d; 2012a; 2012c). Trends in annual mean concentrations over the 22-year period 

(1990 through 2011) are presented using box plots derived from measurements from the 

34 CASTNET eastern reference sites (Figure 1-2). Data from the 17 CASTNET western 

reference sites (Figure 1-2) were used to illustrate trends for the period 1996 through 2011. This 

chapter also includes information on the National Ambient Air Quality Standards (NAAQS) for 

particulate matter (PM), NH 

3 measurements collected at five CASTNET sites, and final results 

from the Mountain Acid Deposition Program (MADPro). 

Sulfur Dioxide

SO 

2 is a reactive, gaseous pollutant emitted during the combustion of coal, oil, and other fossil 

fuels that contain sulfur. EGUs constitute the largest source of SO 

2 in the United States. 

Industrial facilities that fire fossil fuels and that extract metal from ore also emit SO 

2. It is a major 

air pollutant that affects human health and plant ecosystems. SO 

2 is a criteria pollutant, and, to 

be compliant under the SO 

2 NAAQS, which was promulgated on June 2, 2010, the 3-year 

average of the 99th percentile of the yearly distribution of daily maximum 1-hour SO 

2

concentrations must be at or below 75 parts per billion (ppb) [215 micrograms per cubic meter 

(μg/m3 ) of air]. SO 

2 gas reacts in the atmosphere to form various acidic compounds including 

sulfuric acid (H 

2SO 4), a significant contributor to acid rain, and ammonium sulfate [(NH 4)

 

2SO 4], a 

major component of fine particulate matter (PM2.5). Fine particles are also regulated by NAAQS 

since PM2.5 adversely affects health and contributes to degradation of atmospheric visibility. See 

page 18 for a summary of the PM NAAQS. CASTNET does not measure PM2.5, and CASTNET 
CASTNET 2011 Annual Report

16 Chapter 2: Atmospheric Concentrations

SO 

2 data are not used for attainment determinations. However, the results presented in this 

chapter depict long-term trends in the major components of PM2.5. It should be noted that the 

regulatory monitoring networks such as State and Local Air Monitoring Stations (SLAMS) and 

National Air Monitoring Stations do not provide long-term, consistent data on trends in SO 

2

and PM2.5. 

Annual mean SO 

2 concentrations measured in 2011 are shown in Figure 2-1. Two sites in the 

eastern United States measured mean concentrations greater than 5.0 μg/m3 of air. Quaker City, 

OH (QAK172) measured the highest concentration (6.5 μg/m3 ). The other site with a level above 

5.0 μg/m3 was Oxford, OH (OXF122). SO 

2 concentrations recorded in 2011 were lower than SO 2

levels measured in previous years. For comparison, the 3-state geographic average of mean SO 

2

concentrations measured in 2011 at 10 monitoring stations in Indiana, Ohio, and Pennsylvania 

was 3.8 μg/m3 versus a 3-state average of 4.4 μg/m3 in 2010. Concentrations measured at the 

western sites in 2011 were generally lower than those measured at the eastern sites. Western

concentrations were less than 1.0 μg/m3 with the exception of the site at PAL190, TX which 

measured a 2011 annual mean SO 

2 concentration equal to 1.0 μg/m3 

. 

Figure 2-1 Annual Mean SO 

2 Concentrations (μg/m3 ) for 2011

Figure 2-2 shows box plots of annual mean SO 

2 concentrations aggregated over the 34 eastern 

reference sites from 1990 through 2011 (right side) and the 17 western reference sites from 

1996 through 2011 (left side). Because concentrations measured at the western CASTNET 
CASTNET 2011 Annual Report

17 Chapter 2: Atmospheric Concentrations

0.0

0.5

1.0

1.5

1996199719981999200020012002200320042005200620072008200920102011

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

2

4

6

8

10

12

14

16

18

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

sites were much lower than concentrations measured at the eastern sites, the y-axes on the 

western and eastern plots have different scales.

The box plots for the eastern sites show a downward trend. A significant reduction in ambient 

SO 

2 was recorded in 1995 at the beginning of the ARP. SO 2 levels also declined sharply from 

2005 through 2011. Three-year mean concentrations for the eastern reference sites for 

1990–1992 and 2009–2011 were 8.9 μg/m3 and 2.5 μg/m3 , respectively. This change 

constitutes a 72 percent reduction in 3-year mean SO 

2 concentrations between the two periods. 

The 2011 mean level of 2.1 μg/m3 was the lowest mean value measured by the eastern 

reference sites in the history of the network and represents a significant decline over the six 

years from the 2005 concentration of 6.1 μg/m3 

. 

The box plots for the western reference sites indicate a decline in annual mean SO 

2

concentrations aggregated over the 17 sites. Three-year mean SO 

2 concentrations for 1996–1998 

and 2009–2011 were 0.6 μg/m3 and 0.3 μg/m3 , respectively. This change constitutes a 

54 percent reduction in 3-year mean SO 

2 concentrations over the 16 years. The aggregated mean 

SO 

2 concentrations from the western reference sites were typically more than 10 times lower than 

mean concentrations measured at the eastern reference sites. 

Figure 2-2 Trend in Annual Mean SO 

2 Concentrations (μg/m3 ) 

Western Reference Sites Eastern Reference Sites

Quaker City, OH (QAK172)
CASTNET 2011 Annual Report

18 Chapter 2: Atmospheric Concentrations

National Ambient Air Quality Standards for Particulate Matter

On December 14, 2012, EPA strengthened the NAAQS for PM2.5 while at the same time retaining the 

existing standard for coarse particulate matter (PM10). PM2.5 is comprised of (NH 

4)

 

2SO 4, ammonium 

nitrate (NH 

4NO 3), carbon aerosols, and other particles. PM2.5 is formed when SO 2 and NOx gaseous 

emissions from power plants, industrial facilities, and stationary diesel and gasoline engines react in 

the atmosphere. Other sources include fires and vehicles.

Annual standard – EPA lowered the level of annual PM2.5 concentrations from its previous level 

of 15 μg/m3 to a level of 12 μg/m3 . An area will meet the standard if the 3-year average of its annual 

average PM2.5 concentration is less than or equal to 12 μg/m3 

. 

24-hour standards – EPA retained the existing level of the 24-hour standard for PM2.5 at 35 μg/m3 

along with the current framework of the standard. An area would meet the 24-hour standard if the 

98th percentile of 24-hour PM2.5 concentrations averaged over three years is less than or equal 

to 35 μg/m3 

. 

EPA retained the existing 24-hour primary standard for PM10, which is 150 μg/m3 . An area meets the 

24-hour PM10 standard if it does not exceed the 150 μg/m3 level more than once per year on average 

over a 3-year period. 

24-hour secondary standards – EPA retained the levels of the existing secondary standards for 

PM2.5 and PM10. An area meets the annual PM2.5 secondary standard if the PM2.5 concentration is less 

than or equal to 15 μg/m3 . An area meets the 24-hour PM2.5 and PM10 secondary standard if the area 

does not exceed 35 μg/m3 and 150 μg/m3 

, respectively.

Implementation – EPA anticipates making final attainment/nonattainment designations by 

December 2014, with the designations becoming effective in early 2015. States will have until 2020 to 

meet the primary NAAQS. EPA intends to issue a final implementation rule in late 2014, about the 

same time as final designations. 
CASTNET 2011 Annual Report

19 Chapter 2: Atmospheric Concentrations

Particulate Sulfate

The transformation of SO 

2 in the atmosphere through both gas and aqueous (cloud) phase 

reactions produces particulate SO2-

4 . Sulfate exists in the atmosphere primarily as (NH 4)

 

2SO 4, 

a major component of PM2.5. Figure 2-3 depicts a map of annual mean particulate SO2-

4 concentrations measured during 2011. The map shows a fairly flat distribution of SO2-

4 concentrations 

greater than or equal to 3.0 μg/m3 centered on the major SO 

2 source region in the vicinity of the 

Ohio River Valley. CASTNET monitors in four adjacent states (Kentucky, Indiana, Ohio, and 

Pennsylvania) plus the monitor at ALC188, TX measured SO2-

4 concentrations greater than or 

equal to 3.0 μg/m3 . Most of the filter pack samplers in the eastern United States measured SO2-

4

levels greater than or equal to 2.0 μg/m3 . Big Bend National Park, TX (BBE401); PAL190, TX; 

and Sequoia National Park, CA (SEK430) are the only three western sites that measured SO2-

4

concentrations greater than or equal to 1.0 μg/m3 

. 

Figure 2-3 Annual Mean SO2-

4 Concentrations (μg/m3 ) for 2011

The right side of Figure 2-4 provides box plots of annual mean SO2-

4 concentrations from the 

34 eastern reference sites. The figure shows a substantial decline in SO2-

4 over the 22 years, 

and, in particular, concentrations declined significantly from 2005 through 2011. The difference 

between 3-year means from 1990–1992 to 2009–2011 represents a 52 percent reduction in 

SO2-

4 from 5.4 μg/m3 to 2.6 μg/m3 , respectively. The 2011 mean SO2-4 level of 2.5 μg/m3 was the 

lowest in the history of the network.
CASTNET 2011 Annual Report

20 Chapter 2: Atmospheric Concentrations

The box plots for the western reference sites are provided on the left side of Figure 2-4. The boxes 

show a 21 percent reduction in annual mean SO2-

4 concentrations aggregated over the 17 sites, 

with 1996–1998 and 2009–2011 concentrations of 0.8 μg/m3 and 0.6 μg/m3 , respectively.

Figure 2-4 Trend in Annual Mean SO2-

4 Concentrations (μg/m3 ) 

Western Reference Sites Eastern Reference Sites

Total Nitrate

Total NO3 is the sum of HNO 

3 and NO3, which are formed in the atmosphere from NOx emitted 

during the combustion of fossil fuels at high temperatures. Vehicular emissions (e.g., from 

automobiles and trucks) are the largest contributors of NOx emissions in the United States and 

produce more than 50 percent of nationwide NOx emissions. NOx is also emitted from EGUs 

and industrial processes. Lightning is a natural source of NOx in the atmosphere. HNO 

3 exists 

as a gas and contributes to acid deposition. NO3 is a component of PM2.5 in the form of NH 

4NO 3

and as larger particles such as sodium nitrate or calcium nitrate. Data on total NO3, rather than 

HNO 

3 and NO3 individually, are discussed because total NO3 is more representative of a 

response to changes in NOx emissions than either of its constituents alone. Also, 

measurements of the individual constituents are thought to include significant uncertainties. 

Figure 2-5 presents a map of annual mean total NO3 concentrations for 2011. The map shows 

annual mean concentrations greater than or equal to 3.0 μg/m3 were measured in Illinois and 

Indiana. SEK430 and Joshua Tree National Park (JOT403) in California measured annual mean 

total NO3 concentrations greater than or equal to 2.0 μg/m3 

. The BVL130, IL site measured the 

highest concentration (3.4 μg/m3 ) in the network.

0.0

0.5

1.0

1.5

2.0

1996199719981999200020012002200320042005200620072008200920102011

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

5

6

7

8

19901991199219931994199519961997199819992000

2001200220032004200520062007200820092010

2011

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET 2011 Annual Report

21 Chapter 2: Atmospheric Concentrations

Figure 2-6 provides box plots of total NO3 levels for eastern sites for the years 1990 through 

2011. The data shown on the right side of the figure were aggregated from the 34 eastern 

reference sites. The data show no trend in mean concentrations until 2000 when total NO3 levels 

began to decline in response to NOx emission control programs. Three-year mean levels 

declined from 3.0 μg/m3 to 2.0 μg/m3 from 1990–1992 to 2009–2011, producing a 37 percent 

reduction in total NO3. Total NO3 levels measured at the eastern reference sites declined from a 

mean value of 3.2 μg/m3 in 2000 to a mean value of 1.9 μg/m3 in 2011.

Figure 2-5 Annual Mean Total NO3 Concentrations (μg/m3 ) for 2011

Data aggregated from the 17 western sites are shown on the left side of Figure 2-6. The

3-year mean total nitrate concentration for 2009–2011 was 29 percent lower than the 

corresponding 1996–1998 level with 3-year mean concentrations of 0.7 μg/m3 and 

1.0 μg/m3 , respectively. 
CASTNET 2011 Annual Report

22 Chapter 2: Atmospheric Concentrations

Figure 2-6 Trend in Annual Mean Total NO3 Concentrations (μg/m3 ) 

Western Reference Sites Eastern Reference Sites

Bondville, IL (BVL130) 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1996199719981999200020012002200320042005200620072008200920102011

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

5

6

7

8

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET 2011 Annual Report

23 Chapter 2: Atmospheric Concentrations

The CASTNET Ammonia Study and AMoN Ammonia Intercomparison

AMEC conducted a reactive nitrogen (Nr) inter-comparison study at five CASTNET sites over the 

period April 2010 through September 2011 (Rogers et al., 2012). This study is known as the Ammonia 

CASTNET Chemical Speciation Network (CSN) Study (ACCS). The current CASTNET 3-stage filter 

pack captures particulate NH+ 

4 and NO3 on the first (Teflon) filter. There is a known bias of particulate 

NH+ 

4 and particulate NO3 due to the volatilization of NH 4NO 3 during the typical 1-week sampling period. 

In addition to the bias introduced by volatilization, there is also an error introduced in the partitioning 

between particulate NO3 and gaseous HNO 

3, which is collected on the second (nylon) filter.

The goals of the ACCS were to assess the weekly precision, accuracy, and bias of several different 

samplers, including passive NH 

3 samplers (Radiello), a traditional CASTNET filter pack with an 

additional fourth stage filter impregnated with phosphorous acid (H 

3PO 3) to collect atmospheric NH 3

and any volatilized NH+ 

4 (an NHx-filter), the Met One Super SASS mini-parallel plate denuder (MPPD) 

for NH 

3, and the Met One Super SASS ion canisters, with duplicate annular denuder systems (ADS) 

as the reference method.

Study sites were selected based upon location, predicted or known nearby NH 

3 emissions sources, 

site operator capability, and collocation with AMoN sites. AMoN sites are currently measuring NH 

3

concentrations at 2-week intervals. During ACCS, concentrations were calculated as an average of 

results obtained from triplicate Radiello samplers. Currently 5 percent of AMoN sites measure 

triplicate NH 

3 samples. The current AMoN protocol was followed for the passive samplers used in 

ACCS. The modified (4-stage) CASTNET filter pack was designed to run for two 1-week sampling 

periods at the same locations as the 3-stage filter pack. The Super SASS NH 

3 denuder plus ion 

canisters and ADS (EPA modified Compendium Method I.O. 4.2) were run for two 1-week periods 

every six weeks. The passive samplers were analyzed by the NADP Central Analytical Laboratory 

and reported in the AMoN database, which is accessible via the NADP Web page 

(http://nadp.sws.uiuc.edu/AMoN/sites/data/). The Super SASS MPPD and annular denuders, which 

were operated in duplicate, were shipped to and analyzed by the AMEC CASTNET laboratory. The 

Super SASS ion canisters were shipped to the RTI International laboratory for analysis as part of the 

CSN contract.

Sampling Sites

Table 2a lists the primary sites within CASTNET that were selected for this study along with the 

types of samplers originally expected to be deployed at each site. Each of the sites selected for 

ACCS deployed a standard CASTNET 3-stage filter pack. The ACCS site locations can be seen on 

Figure 1-1.

Table 2a CASTNET Sites Selected for Study

Site ID Sampler Configuration

CTH110, NY AMoN passive sampler

Single denuder NHx only ADS

PAL190, TX 4-Stage CASTNET filter pack

ARE128, PA AMoN passive sampler

Two denuder ADS

4-Stage CASTNET filter pack

Super SASS for NH 

3

Super SASS CSN ion module

CHE185, OK

ROM206, CO CTH110, NY
CASTNET 2011 Annual Report

24 Chapter 2: Atmospheric Concentrations

In addition to the primary sites, a test site was established at AMEC’s Gainesville, FL office to test the 

equipment and sampling configurations.

Results

A test program was performed at the Gainesville test site from April 2010 

to June 2010. Results of measurements at the test site showed problems 

with the 4-stage filter pack. NH 

3 on the H 3PO 3-impregnated filter was 

expected but not found. Use of a 4-stage filter pack in high humidity 

environments was found to be not viable. Reaction of NH 

3 with SO 2

collected by a hydrated K 

2CO 3-impregnated filter prior to encountering the 

H 

3PO 3-impregnated filter appeared to be responsible for the absence of 

NH 

3 on the H 3PO 3-impregnated filter. A 3-stage filter pack with the K 2CO 3

filter replaced with a H 

3PO 3 filter was successfully used to collect NH 3. 

Because of these results, 4-stage filter packs were not included during 

the ACCS field-sampling periods.

Following the test program, ACCS field sampling started at the end of 

August 2010 and continued through September 2011. Results from the field sampling component of 

the study are presented in Figures 2a through 2c. The results were used to evaluate the performance 

of the AMoN samplers and Super SASS MPPD.

In general, the AMoN passive sampler performed very well at all five sites compared with the ADS 

reference concentrations (Figures 2a–2c). Note that at ROM206/CO88, AMoN samples did not start 

until May 2011, and only four AMoN samples are currently included for comparison at that site. 

Super SASS MPPD samples consistently collected less NH 

3 than the reference ADS denuders 

(Figures 2b and 2c). This was especially evident at the high elevation site, ROM206 (Figure 2c). The 

CASTNET filter pack compared well with ADS-derived concentrations for all CASTNET analytes, but, 

as expected, CASTNET filter pack particulate NO3 concentrations were higher because of the 

collection of coarse-sized particles (Rogers et al., 2012). 

CHE185, OK

PAL190, TX

Super SASS 

AMoN

CASTNET 

Denuders

CASTNET Flow and Ozone
CASTNET 2011 Annual Report

25 Chapter 2: Atmospheric Concentrations

Figure 2a Comparison between AMoN and ADS NH 

3 Concentrations at PAL190/TX43

Note: TX43 is the AMoN site at Palo Duro Canyon State Park, TX.

Figure 2b Comparison between AMoN, ADS, and Super SASS MPPD NH 

3 Concentrations at

ARE128/PA00

Note: PA00 is the AMoN site at Arendtsville, PA.

Figure 2c Comparison between AMoN, ADS, and Super SASS MPPD NH 

3 Concentrations at

ROM206/CO88

Note: CO88 is the AMoN site at Rocky Mountain National Park, CO.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

8/31/10

10/12/10

11/23/10

1/4/11

2/15/11

3/29/115/10/116/21/11

8/2/11

9/13/11

Concentration (μg/m3)

PA00 AMoN

ADS Phosphorous Acid Denuder 1

ADS Phosphorous Acid Denuder 2

Super SASS MPPD 1

Super SASS MPPD 2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

8/31/10

10/12/10

11/23/10

1/4/11

2/15/113/29/115/10/116/21/11

8/2/11

9/13/11

Concentration (μg/m3)

CO88 AMoN

ADS Phosphorous Acid Denuder 1

ADS Phosphorous Acid Denuder 2

Super SASS MPPD 1

Super SASS MPPD 2

0

1

2

3

4

5

6

7

8/31/10

10/12/10

11/23/10

1/4/11

2/15/113/29/115/10/116/21/11

8/2/11

9/13/11

Concentration (μg/m3)

TX43 AMoN

ADS Phosphorous Acid Denuder 1

ADS Phosphorous Acid Denuder 2
CASTNET 2011 Annual Report

26 Chapter 2: Atmospheric Concentrations

Particulate Ammonium

Particulate NH+ 

4 is formed when gaseous NH 3 reacts with other gases and particles in the 

atmosphere including HNO 

3, particulate SO2-4 , and particulate NO3. The primary components of 

particulate NH+ 

4 are from (NH 4)

 

2SO 4 and NH 4NO 3. Figure 2-7 provides a map of 2011 annual 

mean NH+ 

4 concentrations. No monitors measured concentrations greater than 2.0 μg/m3 . The 

data show a relatively uniform geographic distribution across the central and eastern United 

States with more than half the sites reporting values less than or equal to 1.0 μg/m3 . Higher 

concentrations were measured in Illinois, Indiana, Ohio, Pennsylvania, Kentucky, and northern 

Alabama. The NH+ 

4 concentrations measured at western sites were less than 1.0 μg/m3 

. 

Figure 2-7 Annual Mean NH+ 

4 Concentrations (μg/m3 ) for 2011

Figure 2-8 shows box plots of NH+ 

4 concentrations. The trend diagram for the eastern sites 

shows a reduction in mean NH+ 

4 levels from 1990–1992 to 2009–2011. The 1990–1992 mean 

concentration was 1.8 μg/m3 , and the 2009–2011 value was 1.0 μg/m3 , a 43 percent decline. 

The box plots characterizing the western sites show about an 18 percent reduction.
CASTNET 2011 Annual Report

27 Chapter 2: Atmospheric Concentrations

Figure 2-8 Trend in Annual Mean NH+ 

4 Concentrations (μg/m3 ) 

Western Reference Sites Eastern Reference Sites

Cloud Deposition Monitoring at Clingmans Dome, TN 

The Mountain Acid Deposition Program (MADPro) began in 1993 to support CASTNET’s objective of 

gauging the impact of emission reductions on air pollution, atmospheric deposition, and ecological 

effects. Cloud water was measured at Clingmans Dome, TN (CLD303) in the Great Smoky Mountains 

National Park from 1993 through 2011, when the program was terminated. Monitoring stations were 

operated at other mountain locations in the early part of the program. Cloud water samples from 

MADPro were analyzed for their pollutant constituents and were used to estimate cloud deposition of 

these pollutants. During the last few years, MADPro was sponsored by the Tennessee Valley Authority 

(TVA) and EPA with infrastructure support from NPS.

For 2011, cloud water and meteorological parameters were measured at CLD303 (Figure 2d) atop a 

50-m tower near the mountain summit. Ambient pollutant concentrations were sampled at the 

CASTNET site GRS420, TN in order to estimate dry deposition. Wet deposition was measured at 

Elkmont, TN (TN11), which is operated by NPS for NADP/NTN. The cloud water collector is depicted

in Figure 2e. Atmospheric pollutant deposition experienced at mountain settings such as CLD303 

results from the combined contributions of cloud, dry, and wet deposition.

Cloud deposition was modeled using the CLOUD model (Lovett, 1984). Cloud deposition depends on 

pollutant concentrations in cloud water, liquid water content (LWC), wind speed, and other parameters 

that were assumed fixed in the model simulations. LWC was measured using a particle volume 

monitor (Gerber PVM-100). 

Figure 2f shows seasonal (June through September) cloud NO3 concentrations, CLOUD-modeled NO3

deposition fluxes, and NOx emissions from TVA power plants over the period 2000 through 2011. The 

figure shows an overall reduction in all three parameters. For example, the 3-year average NO3

deposition declined from 42.3 kilograms per hectare (kg/ha) for 2000–2002 to 12.6 kg/ha for 

2009-2011. NOx emissions declined from 270 thousand tons per year for 2000–2002 to about 

64 thousand tons per year in 2009–2011. 

Seasonal cloud SO2-

4 concentrations, CLOUD-modeled SO2-4 deposition fluxes, and SO 2 emissions from 

TVA power plants over the period 2000 through 2011 are shown in Figure 2g. All three parameters 

were reduced significantly. The three-year average SO2-

4 deposition declined from 70.4 kg/ha for 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1996199719981999200020012002200320042005200620072008200920102011

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET 2011 Annual Report

28 Chapter 2: Atmospheric Concentrations

2000–2002 to 19.6 kg/ha for 2009–2011. SO 

2 emissions declined from 630 thousand tons per year for 

2000–2002 to about 210 thousand tons per year in 2009–2011.

Figures 2h and 2i show time series of cloud, wet (precipitation), and dry deposition fluxes of nitrogen

and sulfur, respectively, for the period 2000 through 2011. The graphs show reductions in nitrogen 

and sulfur deposition fluxes associated with the three deposition mechanisms. Cloud deposition 

was the most significant pathway for deposition for CLD303, although the percentage contribution of 

cloud deposition decreased from about 90 percent at the beginning of the period to about 70 percent 

in 2011 (AMEC, 2012d). 

Figure 2d Clingmans Dome and Nearby Sampling Sites

Figure 2e Cloud Water Collector

GRS420

TN11

CLD303
CASTNET 2011 Annual Report

29 Chapter 2: Atmospheric Concentrations

Figure 2f Seasonal Cloud Water NO3 Concentrations and Deposition and TVA Annual

NOx Emissions

Figure 2g Seasonal Cloud Water SO2-

4 Concentrations and Deposition and TVA Annual

SO 

2 Emissions

Figure 2h Seasonal Nitrogen Deposition

0.0

5.0

10.0

15.0

20.0

25.0

30.0

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Nitrogen Deposition (kg/ha)

Dry Nitrogen

Wet Nitrogen

Cloud Nitrogen

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

0

50

100

150

200

250

300

350

200020012002200320042005200620072008200920102011

Nitrate Deposition (kg/ha)

NOx Emissions (1000 tons per year),

Nitrate Ion Concentration (μeq/L)

Nitrate Ion 

Concentration

Oxides of Nitrogen 

Emissions

Nitrate Ion 

Deposition

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0

100

200

300

400

500

600

700

800

2000200120022003

2004

2005200620072008200920102011

Sulfate Deposition (kg/ha)

SO2 Emissions (1000 tons per year), 

Sulfate Ion Concentration (μeq/L)

Sulfate Ion 

Concentration

Sulfur Dioxide 

Emissions

Sulfate Ion 

Deposition
CASTNET 2011 Annual Report

30 Chapter 2: Atmospheric Concentrations

Figure 2i Seasonal Sulfur Deposition

View from cloud water collection tower at CLD303

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Sulfur Deposition (kg/ha)

Dry Sulfur

Wet Sulfur

Cloud Sulfur
CASTNET 2011 Annual Report

31 Chapter 3: Atmospheric Deposition

CASTNET was designed to provide estimates of the dry deposition of sulfur and nitrogen pollutants across 

the United States. CASTNET primarily uses a hybrid approach to estimate dry deposition by combining 

measured pollutant concentrations and modeled Vd. The MLM is used to calculate hourly Vd for each 

monitoring site based on meteorological measurements and information on the vegetative cover within 1 km 

of each site. Recently, CASTNET adopted a Vd data substitution method developed by Bowker et al. (2011) to 

fill in missing values of Vd with long-term averages. This method results in nearly 100 percent data 

completeness for Vd data. Dry deposition is calculated as the product of the measured pollutant concentration 

and modeled/substituted Vd. In order to improve estimates of wet deposition, the Parameter-elevation 

Regressions on Independent Slopes Model (PRISM) was selected in 2010 to develop grids of precipitation 

based on data collected at NADP/NTN sites and on other related climate and terrain data. Total deposition is 

the sum of estimated dry and wet deposition. Three-year mean dry sulfur fluxes aggregated over the eastern

reference sites declined by 68 percent over the period from 1990–1992 to 2009–2011. Three-year mean dry 

sulfur deposition estimated for 17 western sites declined by 41 percent over the period 1996–1998 to 

2009–2011. Dry nitrogen deposition estimates declined 45 percent for the eastern reference sites and 

36 percent at the western reference sites.

Chapter 3: Atmospheric Deposition

Gaseous and particulate sulfur and nitrogen pollutants are deposited to the environment through 

dry and wet atmospheric processes. A principal objective of CASTNET is to estimate the rate, 

or flux, of dry deposition from the atmosphere to sensitive ecosystems. Flux values are estimated 

as the product of measured concentration data and modeled dry Vd. Recently, as summarized in 

the sidebar on the next page, CASTNET adopted a Vd data substitution method developed by 

EPA (Bowker et al., 2011) to fill in missing values using long-term averages of Vd. This method 

results in nearly 100 percent data completeness for Vd data. 

Wet deposition measurements were obtained from NADP/NTN. Wet deposition values used to 

estimate total deposition were based on a grid of PRISM-modeled precipitation amounts and 

gridded values of pollutant concentrations in precipitation. The concentrations were multiplied by 

precipitation rates to estimate wet fluxes. Wet flux data were interpolated to CASTNET sites and 

combined with CASTNET dry deposition data to estimate total deposition. 
CASTNET 2011 Annual Report

32 Chapter 3: Atmospheric Deposition

Speedwell, TN (SPD111)

EPA Method for Substituting Historical Hourly Deposition Velocity Values 

for Missing Hourly MLM Data

MLM calculations of hourly Vd values require hourly meteorological data plus information on vegetative 

cover as model input. This sidebar provides a synopsis of the analyses in the paper by Bowker et al. 

(2011) that are the bases for the data substitution method discussed herein. Historically, modeled Vd

values have been about 70 percent complete for a given year. The discontinuance of meteorological 

measurements at all but five EPA-sponsored CASTNET sites coupled with the need to improve Vd

data completeness resulted in the development of a method by EPA (Bowker et al., 2011) to substitute 

historical average hourly Vd values for missing MLM simulations. The approach was developed to 

capture the site-specific diurnal and seasonal patterns in modeled Vd values and has resulted in 

long-term unbiased estimates of the annual mean Vd and total dry deposition.

The metric selected to describe the dry flux was the annual average Vd value for SO 

2. The annual 

average Vd value depends on the year, the number of replaced values in the year, and distribution 

of missing values. The number of missing hours in a year was categorized by bins representing 

10 percent intervals from 0 percent to 100 percent missing hours. The percent error (%E) and percent 

difference (%D) between the original data and substituted long-term hourly values was calculated for 

more than 5,000 pairs of annual Vd values. The formulas for the two measures of bias are given in 

Equations 3a and 3b.

Equation 3a Equation 3b 

The distribution and magnitude of the percent errors and percent differences are shown in Tables 3a 

and 3b. Both measures of bias were found to be site dependent. The %E increased generally with the 

percentage of missing data for each of the 11 sites selected for evaluation.

The %D showed both positive and negative average percent differences across the range of percent 

missing data. For a particular year at any given site, the annual mean Vd resulting from substitution 

was either higher or lower than the true mean and, consequently, showed little bias. That is, the 

substitution procedure did not have a large impact on the analysis of deposition trend. In addition, the 

results indicated that for the majority of the sites and missing data categories, the average percent 

difference was relatively small (less than ± 5 percent).

1 %

1 X

XY

k

D j

k

j





 



 − = ∑=

E 1 %

1

x

X

XY

k

D j

k

j





 



 − = ∑=

x 100 x 100
CASTNET 2011 Annual Report

33 Chapter 3: Atmospheric Deposition

Speedwell, TN (SPD111)

Table 3a Percent Error of Replaced Values for Each Selected Site and Percent Missing Hours

Percent Missing Data Categories

SITE_ID 0-10% 11%-20% 21%-30% 31%-40% 41%-50% 51%-60% 60%-70% 71%-80% 81%-90% 90%-100%

ABT147, CT 0.50 1.52 1.97 3.32 4.62 5.66 5.31 3.15 0.00 7.21

ACA416, ME 0.58 1.58 3.23 4.41 5.75 8.85 7.02 9.85 0.00 11.84

CAD150, AR 0.46 0.93 1.99 2.12 2.82 3.42 4.83 3.47 1.64 5.10

DEN417, AK 0.93 2.48 4.47 6.23 6.35 8.73 7.67 11.54 0.00 14.56

DEV412, CA 0.06 0.12 0.18 0.19 0.22 0.21 0.31 0.40 0.00 0.54

GAS153, GA 0.61 1.89 3.03 4.29 4.55 4.85 6.53 6.21 1.09 7.37

GTH161, CO 0.38 0.95 1.72 1.65 2.02 1.43 2.81 2.52 1.70 4.49

NCS415, WA 0.42 1.06 1.12 2.19 2.25 1.86 3.58 2.82 3.39 3.71

PNF126, NC 0.38 1.28 1.76 1.87 2.25 3.24 3.22 3.62 0.00 3.33

PRK134, WI 0.93 3.30 5.49 7.43 8.64 10.92 10.73 12.72 0.00 17.80

WSP144, NJ 0.56 1.71 3.00 2.34 4.40 7.26 4.94 6.08 9.40 8.90

Table 3b Percent Difference of Replaced Values for Each Selected Site and Percent Missing Hours 

Percent Missing Data Categories

SITE_ID 0-10% 11%-20% 21%-30% 31%-40% 41%-50% 51%-60% 60%-70% 71%-80% 81%-90% 90%-100%

ABT147, CT 0.10 0.50 0.63 0.48 1.89 1.94 1.24 2.89 0.00 1.95

ACA416, ME -0.21 -0.88 -1.53 -1.84 -1.86 -6.09 2.66 -6.19 0.00 -4.76

CAD150, AR -0.13 -0.22 -0.66 -1.23 -1.89 -0.70 -2.84 -2.52 -1.64 -2.93

DEN417, AK -0.01 -0.03 0.11 -0.08 -0.47 0.33 1.37 -1.23 0.00 1.86

DEV412, CA -0.03 -0.07 -0.09 -0.17 -0.19 0.09 -0.31 -0.40 0.00 -0.46

GAS153, GA 0.15 0.83 0.15 1.33 1.98 -1.11 1.53 1.85 1.09 1.38

GTH161, CO 0.07 -0.04 0.80 0.28 0.08 -0.79 1.16 0.03 -1.70 1.16

NCS415, WA 0.05 0.00 0.30 -0.07 0.02 -1.03 0.08 -0.13 3.39 0.83

PNF126, NC -0.01 -0.35 0.04 0.57 -0.28 -1.37 -0.78 2.69 0.00 -0.86

PRK134, WI -0.38 -1.77 -1.54 -3.80 -3.42 -5.37 -6.41 -1.92 0.00 -8.66

WSP144, NJ 0.04 0.17 0.55 -0.40 -0.95 -0.26 -2.20 1.60 9.40 -1.45

Consequently, for this annual report, the Bowker et al. 

(2011) substitution method was applied to the 1987 

through 2011 database of hourly Vd values for all sites in 

order to improve Vd data completeness and to substitute 

for missing values. Consider the following example. A 

SO 

2 Vd value for hour 10:00 (Eastern Standard Time) is 

missing on December 24, 2011, which is in the 51st

week of the year, at Speedwell, TN (SPD111). SPD111 

began operation on June 12, 1989. To obtain a Vd value 

for that hour, the substituted value was calculated as the 

mean of 7 hourly values, if valid, for hour 10:00 for week 

51 for each year 1989 through 2010 plus 6 values, if valid, 

for hour 10:00 during week 51 for 2011, i.e., the mean of a 

possible 160 hourly Vd values. The substitution scheme resulted in nearly 100 percent completeness 

for all sites for the reporting period 1990 through 2011. The updated values of Vd were used to 

estimate dry deposition fluxes of individual sulfur and nitrogen species, which were then summed to 

obtain sulfur deposition as “S” (SO 

2 + SO2-4 ) and nitrogen deposition as “N” (HNO 3 + NO3 + NH+ 4 ) and 

their trends for 1990 through 2011.
CASTNET 2011 Annual Report

34 Chapter 3: Atmospheric Deposition

Sulfur Deposition

The MLM was run separately for SO 

2 and SO2-4 . The EPA substitution method was used to 

complete any missing flux values, resulting in 100 percent data completeness. The model 

calculations of SO 

2 and SO2-4 were summed to obtain the 2011 estimates of dry sulfur deposition 

(as S). Figure 3-1 shows a map of the estimated dry deposition fluxes. The magnitude of a 

deposition rate is illustrated by the size of the circle. The map shows three CASTNET sites in 

Indiana and Ohio with estimated fluxes greater than 3.0 kilograms per hectare per year 

(kg/ha/yr). The 3-state geographic average dry S deposition rate for Indiana, Ohio, and 

Pennsylvania was 2.5 kg/ha/yr compared to an average flux of 2.9 kg/ha/yr in 2010. The highest 

2011 deposition rate was estimated for QAK172, OH with a flux of 5.2 kg/ha/yr compared to a 

flux of 5.5 kg/ha/yr in 2010. The highest estimated dry sulfur deposition rates coincided with the 

major SO 

2 source region (Figures 1-5 and 2-1) and declined with distance from the source 

region. The dry deposition rates estimated for the western CASTNET sites were all less than 

1.0 kg/ha/yr with all but the two west Texas sites with fluxes less than 0.5 kg/ha/yr.

Figure 3-1 Dry Sulfur (SO 

2 + SO2-4 ) Deposition (kg/ha/yr) for 2011

2.3

0.7 1.4

1.3

1.9

0.8

3.2 1.0

1.4

1.0

0.7

1.0

1.6 0.9

1.5

2.1

0.2

0.4

0.1

0.3

2.0

0.1

0.8

0.3

1.6

1.1

0.3

0.6

0.5

0.3

0.8

1.8

0.8

0.2

0.3

1.5

0.6

2.6

0.8

0.7

0.4

1.4

0.1

0.1

0.1

0.3

0.6

0.2

0.6

0.3

1.2

0.6

0.1

2.2 0.2

0.2

0.1

0.9

0.5

2.9

0.2 5.2

1.5

1.1

1.1

0.5

1.4

3.2

0.2

2.3

1.7

0.2

0.2

0.1

1.1

0.2

0.4

0.6

0.7

Site not pictured:

DEN417, AK 0.1

Deposition

6.0

3.0

0.6
CASTNET 2011 Annual Report

35 Chapter 3: Atmospheric Deposition

Wet deposition values used to estimate total deposition were based on a grid of PRISMmodeled precipitation amounts and gridded values of pollutant concentrations in precipitation. 

The concentrations were multiplied by precipitation rates to estimate wet fluxes. Wet flux data 

were interpolated to CASTNET sites.

A map of estimates of total sulfur deposition is given in Figure 3-2. The map was constructed by 

adding dry and wet deposition. The diameters of the circles in the figure illustrate the magnitude 

of total sulfur deposition and also the relative contributions from wet and dry deposition. The 

dark shading (blue) signifies the percent wet deposition, and the light shading (tan) shows the 

percent dry deposition. The values estimated for 2011 were higher than 2010 values due to the 

atypically high contribution from wet deposition in 2011. Ten CASTNET sites in the states of 

Indiana, Ohio, Pennsylvania, and Kentucky and in Ontario had total (dry + wet) sulfur deposition 

fluxes greater than 7.0 kg/ha/yr in 2011. The highest total sulfur deposition rate was estimated 

for QAK172, OH with a value of 13.0 kg/ha/yr. The total sulfur deposition value for this site was 

9.8 kg/ha/yr in 2010 and 9.7 kg/ha/yr in 2009. Total sulfur deposition at western sites was less 

than or equal to 2.0 kg/ha/yr, which was the estimate for PAL190, TX. 

Figure 3-2 Total (Dry + Wet) Sulfur Deposition (kg/ha/yr) for 2011

3.7

8.0

5.0 9.3

5.6

3.4

4.5

6.8

6.9

5.4 4.1

4.2

2.7

6.3

5.3

7.9

2.2

0.8

5.1 5.0

5.1

0.7

4.5

1.0

4.8

5.8

3.4

1.7

3.6

5.5

4.0

8.5

7.4

5.1

3.9

3.5

0.8

0.8

0.6

0.7

2.2

4.0

3.2

4.8

0.4 0.4

6.8

3.0

0.7

5.6

7.1 1.2

1.0

8.0

1.1

0.5

0.6

3.5

2.9

7.9

1.3 13.0

5.9

5.0

4.7

2.8

1.3

4.4

9.3

1.1

5.7

5.9

2.7

0.6

1.0

4.4

1.1

0.8

2.2

2.0

Site not pictured:

DEN417, AK 0.2

Total Deposition

10

5

1

Dry Deposition

Wet Deposition
CASTNET 2011 Annual Report

36 Chapter 3: Atmospheric Deposition

0

2

4

6

8

10

12

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0.0

0.2

0.4

0.6

0.8

199619971998199920002001

20022003200420052006

20072008200920102011

Deposition (kg/ha/yr)

Figure 3-3 presents box plots that show the trend in dry sulfur deposition (as S) for the eastern 

(right side) and western (left side) reference sites. Figure 3-4 shows the trend in annual total 

(dry + wet) sulfur deposition (as S). The box plots were based on data obtained from the 

34 CASTNET eastern reference sites for the period 1990 through 2011 and from the 17 western 

reference sites for 1996 through 2011. The y-axes on the two figures have different scales. 

Aggregated dry sulfur deposition declined considerably at the eastern and western reference 

sites over their respective periods. Overall, dry sulfur deposition declined at the eastern 

reference sites from a 1990–1992 mean of 5.1 kg/ha/yr to a 2009–2011 mean of 1.6 kg/ha/yr, 

a 68 percent reduction. Dry sulfur deposition declined at the western reference sites from a 

1996–1998 mean of 0.3 kg/ha/yr to a 2009–2011 mean of 0.2 kg/ha/yr, a 41 percent reduction. 

Figure 3-3 Trend in Dry Sulfur Deposition (kg/ha/yr)

Western Reference Sites Eastern Reference Sites

Figure 3-4 shows the trend in annual total (dry + wet) sulfur deposition (as S) at the reference 

sites. Aggregated sulfur deposition declined considerably at the eastern and western reference 

sites over their respective periods. Overall, total sulfur deposition declined at the eastern 

reference sites from a 1990–1992 mean of 12.9 kg/ha/yr to a 2009–2011 mean of 5.3 kg/ha/yr, 

a 59 percent reduction. Total sulfur deposition declined at the western reference sites from a 

1996–1998 mean of 1.3 kg/ha/yr to a 2009–2011 mean of 0.8 kg/ha/yr, a 35 percent reduction. 

The total sulfur flux for the eastern reference sites was higher in 2011 than in 2010. Wet 

deposition in 2011 was a major component of sulfur flux with a contribution of 75 percent.

Figure 3-5 presents estimates of trends in dry, wet, and total deposition of sulfur (as S) for the 

western and eastern reference sites. The influence of precipitation on total sulfur deposition is 

illustrated by comparing the solid (top) line to the dotted blue line in Figure 3-5. The solid line 

shows total deposition, which depends on sulfur concentrations in precipitation and precipitation 

amounts. The dotted line shows concentrations in precipitation, which are independent of 
CASTNET 2011 Annual Report

37 Chapter 3: Atmospheric Deposition

0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.5

1.0

1.5

2.0

1996199719981999200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

Concentration (mg/l)

Wet Deposition

Dry Deposition

Total Deposition

Concentration in Precipitation

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

2

4

6

8

10

12

14

16

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

Concentration (mg/l)

Wet Deposition

Dry Deposition

Total Deposition

Concentration in Precipitation

0

5

10

15

20

25

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1996199719981999200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

precipitation amount. The figure shows wet, dry, and total sulfur depositions have all declined 

significantly. However, total sulfur deposition estimated for the eastern sites in 2011 increased 

relative to 2010, and the sulfur concentration in precipitation for the western sites increased 

in 2011.

Figure 3-4 Trend in Total Sulfur Deposition (kg/ha/yr)

Western Reference Sites Eastern Reference Sites

Figure 3-5 Trend in Sulfur Deposition (kg/ha/yr) with Concentrations in Precipitation 

[milligrams per liter (mg/l)]

Western Reference Sites Eastern Reference Sites
CASTNET 2011 Annual Report

38 Chapter 3: Atmospheric Deposition

The Effect of Differences in Annular Denuder System and CASTNET Filter Pack 

Concentration Data Relative to Estimates of Atmospheric Deposition

The primary purpose of the Ammonia CASTNET CSN Study (ACCS) was to conduct a Nr intercomparison study at five CASTNET sites for one year (Rogers et al., 2012). See the sidebar on 

page 23 in Chapter 2 for a synopsis of ACCS.

While the primary purpose of ACCS was to evaluate NH 

3 samplers and their bias, one of the benefits 

of ACCS was an increased understanding of how well the standard CASTNET filter pack performed 

relative to ADS (EPA Modified Compendium Method I.O. 4.2) in terms of estimating atmospheric 

deposition of sulfur and nitrogen species.

Results from ACCS (Rogers et al., 2012) show that, in general, the current CASTNET filter packs 

compared favorably with the ADS when measuring SO 

2, total NO3, SO2-4 , and NH+ 4 . Both systems 

consistently measured these standard CASTNET analytes. Total NO3 measurements were slightly 

higher for the CASTNET filter pack compared with the ADS because the CASTNET filter pack has an 

open face while the ADS has a 2.5 micrometer size cut cyclone separator. The cyclone feature allows 

coarse-sized NO3 particles (e.g., sodium and calcium nitrate) to be removed prior to sampling by the 

ADS. The filter pack samples are without size restriction, which results in the collection of both coarse 

and fine particles. 

Comparisons of sulfur species were consistently better than for nitrogen species. Figure 3a shows the 

comparison for SO 

2 between the CASTNET filter pack and two ADS at the Arendtsville, PA (ARE128) 

CASTNET site. The results between the two systems were very consistent. Similar results were also 

found when comparing SO2-

4 concentrations. Sulfur species are not subject to the volatilization and 

partitioning found with nitrogen species. In addition, particulate sulfur species are dominated by fine

particles, which are less affected by the size restriction imposed by the cyclone on the ADS. As a 

consequence, estimates of deposition of sulfur species based on either the CASTNET filter pack or 

the ADS data are similar.

Figure 3a Comparison between CASTNET Filter Pack and ADS SO 

2 Concentrations at the 

ARE128, PA Site

Note: FP = filter pack

ADSSC = sodium carbonate-impregnated ADS

0

2

4

6

8

10

12

8/31/10

9/7/10

10/12/1010/19/10

11/23/1011/30/10

1/4/11

1/11/11

2/15/112/22/113/29/11

4/5/11

5/10/115/17/116/21/116/28/11

8/2/118/9/11

9/13/119/20/11

Concentration (μg/m3)

ARE128 FP

ARE128 ADSSC1

ARE128 ADSSC2
CASTNET 2011 Annual Report

39 Chapter 3: Atmospheric Deposition

Results for nitrogen species are more uncertain. The use of a cyclone by the ADS to remove coarse 

particulate nitrogen affects the concentration levels. However, the bigger issue is that the absence of 

ammonia has historically been a major missing component of total deposition. The AMoN data resolve 

that problem; the results summarized in the ACCS sidebar on page 23 in Chapter 2 provide 

confidence in ammonia measurements. Figure 3b shows the comparison between CASTNET filter 

pack and ADS NH+ 

4 concentrations. Figure 3b also includes results from the Super SASS system used 

in the CSN. The figure shows that, relative to the ADS, the CASTNET filter pack measures higher 

concentrations of NH+ 

4 . 

Figure 3b Comparison between CASTNET Filter Pack, ADS, and Super SASS Ion Module NH+ 

4

Concentrations at the ARE128, PA Site

Note: FP = filter pack

Figure 3c shows similar data for total NO3. The results are not as consistent relative to the CASTNET 

filter pack versus the ADS. In some cases the values are higher; in others the values are lower. This is 

largely the result of the volatilization and partitioning of the nitrogen compounds on the filters of the 

CASTNET filter pack.

Figure 3c Comparison between CASTNET Filter Pack and ADS Total NO3 Concentrations at the 

ARE128, PA Site

Note: FP = filter pack

0

1

2

3

8/31/10

9/7/10

10/12/1010/19/10

11/23/1011/30/10

1/4/11

1/11/11

2/15/112/22/113/29/11

4/5/11

5/10/115/17/116/21/116/28/11

8/2/118/9/11

9/13/119/20/11

Concentration (μg/m3)

ARE128 FP

ARE128 ADS FP1

ARE128 ADS FP2

ARE128 CSN

0

1

2

3

4

5

6

8/31/10

9/7/10

10/12/1010/19/10

11/23/1011/30/10

1/4/11

1/11/11

2/15/112/22/113/29/11

4/5/11

5/10/115/17/116/21/116/28/11

8/2/118/9/11

9/13/119/20/11

Concentration (μg/m3)

ARE128 FP

ARE128 ADS 1

ARE128 ADS 2
CASTNET 2011 Annual Report

40 Chapter 3: Atmospheric Deposition

Nitrogen Deposition

Figure 3-6 illustrates estimates of dry fluxes of nitrogen (as N) for 2011. These fluxes are the 

sum of fluxes of HNO 

3 + NO3 + NH+ 4 , based on the individual MLM simulations or substituted 

values for the three species. A majority of CASTNET sites in the eastern United States had 

estimated dry nitrogen deposition rates greater than 1.0 kg/ha/yr. Five eastern sites had 

deposition rates greater than or equal to 2.0 kg/ha/yr. QAK172, OH had an estimated dry 

nitrogen flux of 2.1 kg/ha/yr, the highest value at eastern sites. The highest value for 

western sites was calculated for the CASTNET site at JOT403, CA with a dry nitrogen flux of 

1.9 kg/ha/yr. 

Figure 3-6 Dry Nitrogen (HNO 

3 + NO3 + NH+ 4 ) Deposition (kg/ha/yr) for 2011

A map of total nitrogen deposition (as N) for 2011 is given in Figure 3-7. The map was 

constructed by summing the estimates of dry (light shading) and wet (dark shading) deposition. 

The figure shows that a majority of the eastern sites had estimated deposition rates greater than 

5.0 kg/ha/yr. The site at ARE128, PA recorded the highest total nitrogen flux (10.6 kg/ha/yr). 

Most of the western sites had estimated total nitrogen deposition rates below 3.0 kg/ha/yr. The 

CASTNET site at Wind Cave National Park, SD (WNC429) had the highest total nitrogen flux for 

the western sites with an estimated flux of 3.7 kg/ha/yr. Five other western sites in California, 

0.5

1.4

0.5

0.9

2.0 1.4

1.4

2.0 1.5

1.4

0.9

0.9

1.2

1.4 1.8

1.6

1.0

2.0

0.6

1.0

1.0

0.7

1.0

1.0

2.0

0.2

1.2

0.4

1.1

1.5

0.3

1.1

0.8

0.7

1.7

1.6

1.2

0.6

0.5

1.1

1.0

0.8

0.5

1.4

0.4

0.5

0.3

0.4

0.8

0.2

0.4

1.9

0.8

1.5

0.3

1.7 0.8

0.7

0.3

1.1

0.8

1.5

0.6 2.1

1.6

1.0

1.4

0.7

1.7

0.4

1.4

1.6

0.2

0.2

0.5

1.2

0.9

1.7

1.1

1.6

Site not pictured:

DEN417, AK 0.1

Deposition

3.0

1.5

0.3
CASTNET 2011 Annual Report

41 Chapter 3: Atmospheric Deposition

North Dakota, Wyoming, Colorado, and Texas had estimated total nitrogen fluxes greater than 

or equal to 3.0 kg/ha/yr. 

Dry nitrogen deposition contributed less than 50 percent of total deposition at the eastern and at 

the majority of western CASTNET sites. On the other hand, dry nitrogen deposition contributed 

more than half of total nitrogen deposition at two of the four monitors in California and at the two 

western monitors in Texas. The two California sites are in a region with elevated concentrations 

of nitrogen species and limited rainfall. Dry nitrogen deposition contributed about 80 percent of 

the total nitrogen deposition at JOT403, CA. Dry nitrogen deposition also contributed a minimum 

of 50 percent of total deposition in western Texas, which is also a region with limited rainfall. 

The site at BBE401, TX received a contribution of about 75 percent of dry nitrogen.

Figure 3-7 Total (Dry + Wet) Nitrogen Deposition (kg/ha/yr) for 2011

Figure 3-8 presents box plots that were constructed using data from the 34 eastern reference 

sites over the period 1990 through 2011 (right side) and 17 western reference sites for 1996 

through 2011 (left side). The box plots show the trends in dry nitrogen deposition (as N). The 

box plots for the eastern sites in Figure 3-8 show a reduction in dry nitrogen deposition 

beginning in 1999. Three-year mean fluxes declined from 2.2 kg/ha/yr for 1990–1992 to 

1.2 kg/ha/yr for 2009–2011, a 45 percent reduction over the 22 years. The box plots for the 

4.9

8.0

4.6 9.7

6.4

4.2

6.1

8.4

7.9

7.1 5.6

6.1

2.9

8.3

8.2

10.6

3.0

1.5

6.2 7.1

6.6

1.8

7.3

2.7

7.6

6.6

5.1

3.3

4.5

7.6

5.7

8.8

9.3

6.5

4.1

4.4

1.7

1.8

1.5

1.6

2.9

6.8

4.8

3.8

2.2

6.8

7.1

1.5

5.8

7.8 2.7

1.0

7.5

2.7

1.0

1.3

4.2

6.6

7.3

3.3 8.6

8.6

5.8

8.6

2.9

3.0

6.5

9.6

3.0

4.9

7.5

3.7

1.5

3.5

4.9

3.7

2.9

6.8

3.3

Site not pictured:

DEN417, AK 0.2

Total Deposition

10

5

1

Dry Deposition

Wet Deposition
CASTNET 2011 Annual Report

42 Chapter 3: Atmospheric Deposition

0.0

0.5

1.0

1.5

2.0

1996199719981999200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

5

6

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

western sites show a 36 percent decline, a change from a mean of 0.9 kg/ha/yr for 1996–1998 

to 0.6 kg/ha/yr for 2009–2011.

Figure 3-9 depicts the trends in annual total (dry + wet) nitrogen deposition for the eastern 

(right side) and western (left side) reference sites. The figure shows that total nitrogen flux in the 

eastern United States has decreased since 1996 with a substantial decline from 2003 through 

2010. Total nitrogen flux increased significantly in 2011. Total nitrogen flux for the western sites 

shows a relatively flat distribution. Estimates of trends in wet, dry, and total deposition of 

atmospheric nitrogen (as N) are presented in Figure 3-10 for both eastern and western 

reference sites. The trend line (dotted) for precipitation-weighted mean nitrogen concentrations 

in precipitation measured at eastern sites shows an overall slow downward trend since 1998 

with increases in 2010 and 2011. This trend line is reasonably representative of the effect of 

changes in NOx emissions because it does not depend on precipitation amounts. Total N 

deposition declined 26 percent in the East over the 22 years and 22 percent for the aggregated 

western sites over the 16-year period. However, total N deposition at eastern sites increased by 

33 percent from 2010 to 2011. This significant increase can be explained by an increase in N 

concentration in precipitation and the highest average annual rainfall rate (138 centimeters) at 

eastern sites over the 22-year period.

Figure 3-8 Trend in Dry Nitrogen Deposition (kg/ha/yr)

Western Reference Sites Eastern Reference Sites

Abington, CT (ABT147)
CASTNET 2011 Annual Report

43 Chapter 3: Atmospheric Deposition

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1996199719981999200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

Concentration (mg/l)

Wet Deposition

Dry Deposition

Total Deposition

Concentration in Precipitation

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

2

4

6

8

10

12

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

Concentration (mg/l)

Wet Deposition

Dry Deposition

Total Deposition

Concentration in Precipitation

0

1

2

3

4

5

6

19961997

19981999

200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

2

4

6

8

10

12

14

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

Figure 3-9 Trend in Total Nitrogen Deposition (kg/ha/yr)

Western Reference Sites Eastern Reference Sites

Figure 3-10 Trend in Nitrogen Deposition (kg/ha/yr) with Concentrations in 

Precipitation (mg/l)

Western Reference Sites Eastern Reference Sites

Arendtsville, PA (ARE128)
CASTNET 2011 Annual Report

44 Chapter 3: Atmospheric Deposition

Estimates of Critical Loads throughout the United States

The critical load approach is an assessment tool that can help determine the degree to which acidic 

deposition may be affecting the ecological health of surface waters. The 2011 NAPAP Report to 

Congress (Burns et al., 2011) used critical loads to gauge the effectiveness of emission reductions 

associated with the ARP and other programs by assessing a selected number of sensitive lakes and 

streams in the eastern United States affected by acidification.

A critical load is a quantitative estimate of exposure to one or more pollutants below which significant 

harmful effects on specific sensitive elements of the environment do not occur according to present 

knowledge (Nilsson and Grennfelt, 1988). A critical load provides a benchmark against which to 

assess the extent to which a water body is potentially at risk due to current acidic deposition levels. If 

pollutant exposure is less than the critical load, adverse ecological effects (e.g., reduced reproductive 

success, stunted growth, or loss of biological diversity) are not anticipated, and recovery is expected 

over time if an ecosystem has been damaged by past exposure. A critical load exceedance is the 

measure of pollutant exposure above the critical load. An exceedance indicates the ecosystem 

continues to be exposed to damaging levels of S and N deposition. 

This critical load assessment examined selected lakes and streams within 25 km of 30 CASTNET 

monitoring locations in the United States (Figures 3d and 3e). Critical loads were obtained for 

600 lakes and streams from the National Critical Load Database, developed in conjunction with the 

NADP Critical Loads of Atmospheric Deposition (CLAD) science committee. Critical loads were 

estimated using the Steady-State Water Chemistry mass-balance model (Henriksen et al., 2002). 

The critical load assumed in this study represents the amount of acidic deposition of S and N that a 

stream/lake’s watershed could receive and still maintain an acid neutralizing capacity (ANC) of 

50 microequivalents per liter (μeq/L) or higher, which supports a healthy aquatic ecosystem. 

Critical loads are expressed in terms of ionic charge balance as milliequivalents per square meter 

per year (meq/m2 /yr).

Total S and N deposition were determined for each lake and stream. Wet deposition for SO2-

4 , NO3, 

and NH+ 

4 was based on PRISM-corrected NADP estimates. Dry deposition of SO 2, SO2-4 , NO3, HNO 3, 

and NH+ 

4 was based on CASTNET modeled values for each CASTNET monitoring location. At 

CASTNET sites with AMoN monitors, dry NH 

3 deposition values were calculated using AMoN’s air 

concentration and a Vd from a 2002 Community Multiscale Air Quality Modeling System (CMAQ) 

model run (EPA, 2011). For CASTNET sites without AMoN monitors, dry NH 

3 was determined using a 

dry NH 

3 to wet NH+ 

4 concentration ratio (NH 

3 /NH+ 

4 ) calculated for AMoN monitoring locations and 

applied to CASTNET locations.

Critical load estimates for surface water acidification for selected lakes and streams within 25 km of 

selected CASTNET locations are shown in Figures 3d and 3e. Gray circles indicate CASTNET 

monitoring locations, and red and green symbols represent lakes or streams that exceed or do not 

exceed, respectively, their critical load for an ANC of 50 μeq/L. Graphs represent the 15 percentile 

critical load and total S and N deposition for years 2010 and 2011 at CASTNET locations. On the 

graphs, dark and light red bars represent the maximum critical load (CLmaxS) of S and the amount 

(CLminN) of N that is removed by the ecosystem, e.g., through immobilization and/or denitrification. 

Dark to light blue represents total wet and dry inorganic and organic N and S deposition, respectively.

Figures 3d and 3e show a comparison between the 15 percentile critical load for lakes and streams 

within 25 km of a CASTNET location and total average N and S deposition.
CASTNET 2011 Annual Report

45 Chapter 3: Atmospheric Deposition

The 15 percentile critical load represents the lakes and streams most vulnerable to acidic deposition 

near the CASTNET monitoring locations. Many of these vulnerable lakes and streams are in pristine 

natural areas that are located within national parks, wilderness areas, and other protected areas. 

For the 30 selected CASTNET locations, 16 of them received total S and N deposition greater than 

the 15 percentile critical load. The greatest density (7 of 16 lakes and streams that exceed the

15 percentile critical load) is located along the Appalachian Mountain chain from Pennsylvania 

to North Carolina, despite significant reductions of upwind emissions of NOx and SO 

2 since 1990. 

This analysis also shows that lakes and streams near CASTNET monitoring sites in the central 

Rocky Mountains and Sequoia, Lassen Volcanic, and Mount Rainier National Parks are at risk 

of acidification.

Figure 3d Critical Load Estimates for Selected Lakes and Streams within 25 km of Eastern 

CASTNET Sites (2010–2011) 

Source: J. Lynch
CASTNET 2011 Annual Report

46 Chapter 3: Atmospheric Deposition

Figure 3e Critical Load Estimates for Selected Lakes and Streams within 25 km of Western 

CASTNET Sites (2010–2011)

Source: J. Lynch
CASTNET 2011 Annual Report 

47 Chapter 4: Ozone Concentrations 

CASTNET is the principal network for monitoring rural, ground-level O 

3 concentrations in the United States. 

The network provides critical information on geographic patterns in rural O 

3 levels as well as O 3 data for 

evaluation of compliance with NAAQS. CASTNET was not originally designed to operate as a regulatory 

network. However, during 2010 and 2011, CASTNET O 

3 monitoring systems were upgraded in order to 

comply with regulatory monitoring requirements, and data collected during the 2011 O 

3 season were 

submitted to EPA’s AQS. In this report, O 

3 measurements collected during 2009–2011 were evaluated with 

respect to the current NAAQS (EPA, 2008a) of 0.075 parts per million (ppm). For the 2009–2011 period, three 

California and four eastern sites recorded exceedances of the 0.075 ppm standard. 

Chapter 4: Ozone Concentrations 

Most CASTNET sites operate an O 

3 analyzer that measures hourly concentrations, which are 

archived in the CASTNET database and delivered to AQS. At the end of 2011, 80 ozone 

analyzers were operated throughout the network. Data from these sites, with the exception of 

ROM206, CO and HOW191, ME, which are designated as non-regulatory, will be used to 

calculate design values as three years of Part 58-compliant data become available. CASTNET’s 

geographic coverage of the United States provides data that are essential in terms of presenting 

information on geographic patterns in regional O 

3 and for evaluating rural O 3 concentrations in 

the context of the O 

3 NAAQS. The analyses presented in this chapter provide maps and 

examine trends in the annual fourth highest daily maximum 8-hour average (DM8A) O 

3

concentrations measured at CASTNET sites. Maps of 2011 dry O 

3 fluxes and W126 values are 

also presented. Additional maps of O 

3 concentrations from the NPS Air Atlas can be viewed at 

http://nature.nps.gov/air/maps/airatlas/index.cfm. 

Ground-level O 

3 is an air pollutant of major concern that can cause adverse effects to the 

human respiratory system, damage to vegetation and ecosystems, and reduce crop yield and 

quality. O 

3 is formed in the lower atmosphere when volatile organic compounds (VOCs) and 

NOx react in the presence of sunlight. O 

3 is the main component of smog. Emissions from motor 

vehicle exhaust, industrial facilities, chemical solvents, surface coatings, gasoline vapors, and 

power plants are the major anthropogenic sources of NOx and VOCs. Also, biogenic VOC 

emissions from trees and other vegetation contribute to O 

3 formation in rural areas. 

Meteorological conditions play a significant role in O 

3 formation. O 3 concentrations are generally 

higher in the warmer months because sunlight and hot weather accelerate its formation, but 
CASTNET 2011 Annual Report

48 Chapter 4: Ozone Concentrations

episodes of high O 

3 concentrations can vary by geographical region. Ozone concentrations 

typically increase during daylight and peak in the late afternoon after temperature and sunlight 

intensity peak. 

The DM8A O 

3 standard can be used to assess the status and trends in rural O 3 levels in order to 

gauge the success of EPA emission reduction programs such as the NOx SIP Call/NBP, and 

CAIR. On May 1, 2009, the CAIR NOx O 

3 season trading program began, replacing the NBP in 

states covered by the new program and requiring further NOx emission reductions from the 

power sector. Implementation of the NOx emission reductions that were mandated by the ARP 

and other NOx emission control programs have contributed to reductions in O 

3 concentrations, 

especially in rural areas with elevated DM8A levels, because regional transport of pollutants 

contributes to O 

3 formation. VOC emission reduction strategies also have been successful in 

reducing higher, short-term O 

3 concentrations in and downwind of urban areas. 

The O 

3 NAAQS are summarized in the following sidebar. The primary standard is designed to 

protect public health, including the health of at-risk populations. The secondary standard is 

designed to protect public welfare and the environment, including sensitive vegetation and 

ecosystems. Both the primary and secondary O 

3 NAAQS are set at a level of 0.075 ppm 

averaged over 8 hours. EPA is in the midst of its regularly scheduled review of the O 

3 NAAQS.

O 

3 concentrations can also be analyzed in terms of W126, a cumulative metric that sums 

weighted hourly O 

3 concentrations during the O 3 season. For this report, W126 was calculated 

for the months of May through September. The highest of the rolling 3-month sums during the 

five months is the W126 value. See page 60 for an additional discussion of W126.

National Ambient Air Quality Standards for Ozone

Ozone1

Primary Standard Secondary Standard

Level Averaging Time Level Averaging Time

0.075 ppm 8-hour1 0.075 ppm 8-hour1

Note: 1 To attain this standard, the 3-year average of the fourth-highest DM8A O 

3 concentrations measured at each monitor 

within a specified area must not exceed 0.075 ppm or 75 ppb in practice (effective May 27, 2008; EPA, 2008a).

Assessing Compliance with NAAQS

EPA and other federal, tribal, and state agencies collect O 

3 measurements on an hourly basis through 

national and local monitoring networks. AMEC followed EPA procedures (1998) to use CASTNET data 

to estimate O 

3 design values and 2011 fourth-highest DM8A O 3 concentrations. It should be noted that 

CASTNET O 

3 data will not be used to gauge compliance with NAAQS until Part 58 compliant data are 

available for three years.
CASTNET 2011 Annual Report

49 Chapter 4: Ozone Concentrations

• Hourly O 

3 measurements were used to compute 8-hour average concentrations.

Running 8-hour averages were computed from the hourly O 

3 concentration data for each hour 

of the year, and the result was stored in the first, or start, hour of each 8-hour period. In the 

event that only 6 (or 7) hourly averages were available, the 8-hour average was computed using 

6 (or 7) as the divisor.

When more than two hours of monitoring data were missing, the 8-hour average concentration 

was computed by substituting one-half the minimum detectable limit for the missing hourly 

concentrations and using 8 as the divisor. In this case, if the computed 8-hour average 

concentration was greater than 0.075 ppm, it was retained; otherwise, the 8-hour concentration 

for this 8-hour period was considered to be missing.

In all cases, the computed 8-hour average O 

3 concentration was reported in ppm to three 

decimal places. 

• The daily maximum 8-hour average was recorded for each day.

An O 

3 monitoring day was counted as a valid O 3 monitoring day if 8-hour averages were 

available for at least 18 of the 24 possible 8-hour average periods that start within the day, or if 

the DM8A O 

3 concentration for the day was greater than 0.075 ppm.

• For each year, the fourth highest DM8A concentration was calculated for the designated O 

3

monitoring season for each CASTNET site. The designated O 

3 monitoring seasons are listed by 

state in 40 CFR 58, Appendix D (Table D-3). The listed O 

3 seasons were used to compute the 

data completeness requirements.

• The annual fourth highest daily DM8A O 

3 concentrations were averaged over 3-year periods to 

compute design values.

For the purpose of judging compliance with the standard, a valid year had to have valid DM8A O 

3

concentrations for at least 75 percent of the required monitoring days in the O 

3 season unless 

the fourth highest DM8A was above 0.075 ppm, in which case completeness was not 

considered, and the year was automatically considered valid. A design value requires that all 

three years be valid and that the combined data completeness for the 3-year period has to 

average at least 90 percent to demonstrate attainment.

If the 3-year average of DM8A O 

3 concentrations exceeded 0.075 ppm (or 75 ppb), the site was 

considered nonattainment. EPA (2012b) made final designations using air quality monitoring data from 

2008, 2009, and 2010, generally the most recent three years of certified data available, and in some 

cases using data from 2009, 2010, and 2011. EPA considered data through 2011 if a state certified 

them complete and submitted the data for consideration by February 29, 2012. Based on 2008 

through 2010 or 2009 through 2011 data, EPA designated 46 areas nonattainment.
CASTNET 2011 Annual Report

50 Chapter 4: Ozone Concentrations

Eight-Hour Ozone Concentrations

Figure 4-1 presents 3-year averages of the fourth highest DM8A O 

3 concentrations for 2009–2011. 

If the 3-year average was not available because of incomplete data (see the discussion in the 

sidebar beginning on page 48), the results were not included on the maps in this chapter. Sites 

with no O 

3 concentrations are shown as dots with no value. During this period, 3-year averages of 

the three highest DM8A O 

3 concentrations were greater than 75 ppb at four eastern sites and three

sites in California. The eastern sites include Washington Crossing, NJ (WSP144); BEL116, MD; 

Blackwater National Wildlife Refuge (BWR139); and GRS420, TN. The three California sites 

include Yosemite National Park (YOS404), SEK430, and JOT403. Three-year average 

concentrations were generally lower in 2009–2011 than in 2008-2010.

The 2009–2011 3-year averages of fourth highest DM8A O 

3 concentrations shown on 

Figure 4-1 provide examples of the design value for achieving the DM8A O 

3 NAAQS if 

CASTNET 2009 and 2010 O 

3 data were Part 58 compliant. A design value is a statistic that 

describes the air quality status of a given area relative to the level of the NAAQS. Design values 

change as each new 3-year database of monitored O 

3 concentrations becomes available. 

Design values are used to classify nonattainment areas, assess progress towards meeting the 

NAAQS, and develop control strategies to achieve the NAAQS. For example, the value of 

96 ppb at SEK430 that was measured during 2009–2011 would have to be reduced to 75 ppb to 

achieve the level of the current standard. Similarly, the concentration of 78 ppb at BEL116 

would have to be reduced to 75 ppb to meet the NAAQS.

Beltsville, MD (BEL116)
CASTNET 2011 Annual Report

51 Chapter 4: Ozone Concentrations

Figure 4-1 Three-year Average of Fourth Highest DM8A Ozone Concentrations (ppb) for 

2009–2011

Figure 4-2 Three-year Average of Fourth Highest DM8A Ozone Concentrations (ppb) for 

2000–2002
CASTNET 2011 Annual Report

52 Chapter 4: Ozone Concentrations

Three-year averages of the fourth highest DM8A O 

3 concentrations for 2000–2002 are 

presented in Figure 4-2 for comparison with the 2009–2011 design values shown in Figure 4-1. 

The period 2000–2002 was selected because the NOx SIP Call/NBP began in 2003 for the 

eastern United States. The changes in O 

3 concentrations from 2000–2002 to 2009–2011 

illustrate the effectiveness of the emission reduction programs. Based on the 75 ppb NAAQS 

level, almost all eastern sites recorded 3-year averages greater than 75 ppb during 2000–2002. 

Five monitors in California and the site at ROM406, CO also recorded concentrations greater 

than 75 ppb. The regions with relatively low DM8A O 

3 concentrations in the East were located in 

northern New England and in Florida.

O 

3 concentrations for 2011 are shown in Figure 4-3. These data comply with Part 58 

requirements. All valid data are included, and exceptional events are not excluded. Three 

California, three other western, and nine eastern CASTNET sites measured fourth highest 

DM8A O 

3 concentrations above 75 ppb. The geographic extent of measured fourth highest 

DM8A O 

3 concentrations above 75 ppb was greater in 2011 than in 2010 for both the western 

and eastern sites. The highest western concentration of 98 ppb was measured at SEK430, CA, 

and the highest eastern level of 84 ppb was measured at BEL116, MD. 

In 2011, CASTNET sites at BBE401, TX; PAL190, TX; and CHE185, OK all measured a fourth 

highest DM8A O 

3 concentration greater than the NAAQS for the first time in the last five years. 

Also, two other southwestern sites measured relatively high concentrations: Chiricahua National 

Monument, AZ (CHA467) – 75 ppb and Grand Canyon National Park, AZ (GRC474) – 74 ppb. 

The following concentration data illustrate fourth highest DM8A O 

3 concentrations over the last 

five years for the five sites.

2011 2010 2009 2008 2007

BBE401, TX 80 64 65 65 68

PAL190, TX 76 61 63 65 65

CHE185, OK 78 71 64 68 73

CHA467, AZ 75 71 65 68 67

GRC474, AZ 74 69 66 71 69

The high DM8A levels measured at these five sites were likely influenced by frequent wildfires in

Texas and Arizona during the 2011 summer and fall fire season. The National Oceanic and 

Atmospheric Administration (2012) and the National Climatic Data Center (2012) reported 

extensive fires in Arizona, New Mexico, Texas, and Oklahoma throughout the 2011 summer O 

3

season. These wildfires affected air quality locally, as indicated by the preceding O 

3 data, as 

well as throughout the eastern United States all the way to the Baltimore-Washington 

metropolitan area.
CASTNET 2011 Annual Report

53 Chapter 4: Ozone Concentrations

Figure 4-3 Fourth Highest DM8A Ozone Concentrations (ppb) for 2011

Figure 4-4 provides box plots depicting trends in fourth highest DM8A O 

3 concentrations from 

the 34 CASTNET eastern reference sites (right side) and 17 western reference sites (left side). 

The eastern O 

3 data show an overall reduction since 2003. The mean aggregated value 

(68 ppb) for the eastern reference sites for 2011 was the second lowest in the history of the 

network and shows a small decline from the 2010 level of 69 ppb. The concentrations measured 

in 2009 through 2011 represent a substantive improvement over the mean value of 90 ppb 

measured in 1998. The box plots constructed from the aggregated western O 

3 sites show no 

trend. The 2009–2011 average of the fourth highest DM8A O 

3 concentrations for the western 

reference sites was 68 ppb.

Washington Crossing, NJ (WSP144)
CASTNET 2011 Annual Report

54 Chapter 4: Ozone Concentrations

Figure 4-4 Trend in Fourth Highest DM8A Ozone Concentrations (ppb)

Western Reference Sites Eastern Reference Sites

50

60

70

80

90

100

110

1996199719981999200020012002200320042005200620072008200920102011

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

50

60

70

80

90

100

110

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

Georgia Station, GA (GAS153)
CASTNET 2011 Annual Report

55 Chapter 4: Ozone Concentrations

Ozone and Filter Pack Concentrations at the Maine Howland Research Forest

CASTNET began collecting O 

3 and filter pack measurements at the Howland Research Forest 

AmeriFlux site (designated HOW191) in Maine in September 2011. This study represents the first 

collaboration between the two long-standing national networks. AmeriFlux’s goal is to measure 

ecosystem level exchanges of carbon dioxide, water, energy, and momentum. AmeriFlux sites use a 

30-m tower to enable flux measurements in the lower boundary layer. CASTNET is interested in 

similar atmospheric processes for O 

3 and sulfur and nitrogen species. 

Hourly average O 

3 concentrations were collected at eight elevations on a 30-m scaffold tower during 

fourth quarter 2011 at the HOW191 site. The tower is located in a 20-m natural canopy of hemlockspruce-fir (66 percent), aspen-birch (23 percent), and hemlock-hardwood (11 percent) mixtures. The 

site is situated in a mature, lowland evergreen forest with trees between 40 and 160 years old. O 

3

concentrations are measured at 2, 6, 10, 14, 17, 20.5, 23.5, and 28 m. The inlet for the sampler at 

28 m is above the canopy, and the inlet at 23.5 m is near the top of the canopy. All other sampler 

inlets are located within the canopy. Filter pack sampling systems are operated at 2 and 23.5 m.

Design for the O 

3 system centered on the use of a single O 3 analyzer and site transfer standard in 

conjunction with a solenoid system to allow for all eight levels to be measured using the same 

analyzer (Figure 4a). Even though the measurement heights vary from 2 m to more than 28 m above 

the ground, the residence times for all levels are less than 20 seconds, and there are no indications of 

line loss. Although not a regulatory O 

3 site, the analyzer is challenged in a similar manner by 

performing daily checks using a calibration gas created by the site transfer standard and delivered 

through-the-probe at the 23.5-m inlet.

Figure 4a O 

3 Sampling System

Data from fourth quarter 2011 are presented in 

this report. Figure 4b depicts three plots of 

aggregated O 

3 concentrations versus height. The 

graphs were constructed from measurements at 

the eight sampling heights. The blue diamonds 

show concentrations aggregated over all hours of 

the day for the entire quarter. These data show a 

decrease in concentrations with decreasing 

elevation throughout the canopy and fit a 

conceptual model of the distribution of reactive 

pollutants in a treed canopy. The red boxes show 

concentrations aggregated for the hour 0600. As 

expected, the early morning concentrations were 

lower than midday values, likely because of 

nighttime scavenging. Finally, the green triangles 

show concentrations by elevation for the hour 

1400. The afternoon values were higher than the 

other two sets and were more uniformly 

distributed with height. The magnitude and 

distribution of the 1400 values were influenced 

by daytime O 

3 production and atmospheric 

mixing of O 

3 from aloft.
CASTNET 2011 Annual Report

56 Chapter 4: Ozone Concentrations

Figure 4b Aggregated O 

3 Concentrations for Fourth Quarter 2011 for Specific Hours at HOW191, ME

HOW191 site shelter

HOW191 site tower

0

5

10

15

20

25

30

10 15 20 25 30 35

Height (m)

Concentration (ppb)

All Hours

0600

1400
CASTNET 2011 Annual Report

57 Chapter 4: Ozone Concentrations

Figure 4c SO 

2 Concentrations for Fourth Quarter 2011 for HOW191, ME Above and Below Canopy 

and HOW132, ME Standard 10-m Filter Packs

HOW191 O 

3 sampling solenoid system

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

10/18/1110/25/11

11/1/1111/8/11

11/15/1111/22/1111/29/11

12/6/11

12/13/11

12/20/11

Concentration (μg/m3)

HOW191 - Below Canopy

HOW191 - Above Canopy

HOW132
CASTNET 2011 Annual Report

58 Chapter 4: Ozone Concentrations

A standard CASTNET sampling tower is located in a clearing approximately 5 km east of HOW191 at 

the HOW132 site. O 

3 concentrations aggregated over the entire quarter and measured at the standard 

10-m level at HOW132, ME conform to the 28-m values measured at HOW191.

All Hours (ppb) 0600 (ppb) 1400 (ppb)

HOW132, ME (10 m) 33.0 28.3 36.9

HOW191, ME (28 m) 34.0 30.8 37.0

SO 

2 concentrations from the two sampling heights at HOW191 and from the 10-m height at HOW132 

are shown in Figure 4c using a time series of bars to illustrate how concentrations compared during 

the fourth quarter. The chart shows the above-canopy concentration values from HOW191 and the 

10-m values from HOW132 are comparable. The SO 

2 concentrations measured at 2 m were 

noticeably lower than the above-canopy values, indicating depletion by deposition near the ground. 

Similar results were found for measured total NO3 concentrations.

These data collected from HOW191 provide insight into the deposition and scavenging mechanisms 

that exist within a forest canopy. Hourly O 

3 concentrations show evidence of episodic within-canopy 

nighttime loss events. Deposition is also evident in the above- and below-canopy filter pack 

concentrations. The below-canopy SO 

2 concentrations are approximately 60 percent of the abovecanopy concentration values, and the below-canopy total NO3 concentrations, which include gaseous 

HNO 

3, are approximately 64 percent of the above-canopy concentrations. As would be expected, 

concentrations of particles, such as SO2-

4 and NH+ 4 , show that deposition rates of particles within the 

canopy are slower than for gaseous species.

HOW191 tower looking down inlets
CASTNET 2011 Annual Report

59 Chapter 4: Ozone Concentrations

Ozone Flux Values for 2011

Recent research (Klingberg et al., 2011) has concluded that the effects of O 

3 on vegetation 

depend more on the flux of O 

3 to the plant than on ambient O 3 concentrations. In particular, 

damage to vegetation is related to the uptake of O 

3 through the stomata. The MLM modeling 

approach was used to simulate dry O 

3 deposition. Figure 1-4 illustrates the MLM used for O 3 dry 

deposition modeling. The EPA data substitution method was used to fill in long-term averages of 

O 

3 Vd for missing values. A map of modeled 2011 annual O 3 fluxes is presented in Figure 4-5. 

These estimates represent the dry deposition of O 

3 to the environment. Dry deposition fluxes 

greater than 50.0 kg/ha/yr were simulated for 12 CASTNET sites in 10 states, including North 

Carolina, Tennessee, Kentucky, Virginia, Ohio, Kansas, Oklahoma, Texas, Arizona, and 

California. The highest flux (68.3 kg/ha/yr) was modeled for GRS420, TN and the lowest 

(7.7 kg/ha/yr) for the site at Indian River Lagoon, FL (IRL141). 

Figure 4-5 Modeled Annual O 

3 Flux (kg/ha/yr) for 2011

44.0

40.6

43.1 34.6

33.8

54.1

44.7

37.9

50.8

33.1 68.3

35.1

62.2

33.8

28.2

48.2

32.3

48.0

26.2 31.1

30.0

32.0

31.2

58.0

47.5

48.8

45.0

34.4

43.0

39.9

40.5

28.4

45.3

24.5

39.2

33.9

35.8

31.3

31.3

17.6

7.7

41.3

37.8

60.5

32.4

41.4

54.3 39.2

24.0

24.5

47.5

53.2

29.7

36.2

60.5

29.1

30.5

41.0 54.4

37.5

47.1

54.4

39.3

38.0

39.0

37.5

29.3

39.5

32.9

51.3

42.1

19.3

27.7

34.2

Site not pictured:

DEN417, AK 22.9

Deposition

60

30

6
CASTNET 2011 Annual Report

60 Chapter 4: Ozone Concentrations

W126 Values for 2011

The W126 O 

3 exposure index is a biologically-based, cumulative peak-weighted exposure index 

(Lefohn and Runeckles, 1987; EPA, 1996; 2006) for relating reduced vegetation yields to O 

3

exposure. Many scientists (e.g., Musselman et al., 2006) concluded that both cumulative effects 

and peak concentrations are important in relating atmospheric O 

3 to vegetation damage. 

Consequently, W126 was developed as a seasonal measure from the sum of weighted hourly 

values measured during the approximately 12-hour daylight period during the O 

3 season with 

the higher hourly concentrations being weighted more heavily than lower concentrations 

(EPA, 2010b). In this report, W126 is used to characterize high O 

3 concentrations over a 

seasonal, 3-month exposure period. The highest rolling 3-month W126 is presented.

The W126 index is represented as the sum of all hourly O 

3 concentrations, where each 

measured concentration is weighted by a function, wi

, to assign greater emphasis to the higher 

measured concentrations. This weighting function provides a weighting value that is unique for 

each hourly O 

3 concentration. The weighting function, as described by Lefohn et al. (1988), is:

Hourly Weighting = Equation 4-1 

Where:

wi = weighting value for hourly concentration ci

, and

 ci = hourly concentration in ppb for period i.

Each hourly weighting value is multiplied by its corresponding hourly concentration. This product 

is summed over all valid hours to calculate the W126 exposure. Thus, the W126 exposure is:

Equation 4-2 

Where:

n = number of hours with valid O 

3 concentrations.

The exposure unit is ppm-hours. For CASTNET, W126 exposures are calculated using O 

3 values 

measured during the 12 hours (n = 12) from 8:00 a.m. to 8:00 p.m. in the months of May through 

September of each year.

CASTNET hourly O 

3 concentrations were used to calculate W126 values for 2011. First, the 

daily value for each site was calculated. The daily values were then used to calculate the 

monthly values for the months of May through September. The highest of the rolling 3-month 

sums for a consecutive 3-month period was the W126 value for each site. 

wi 1 + 4403 exp (-0.126ci

)

1 =

=∑=

n

i 1

W126 wi ci
CASTNET 2011 Annual Report

61 Chapter 4: Ozone Concentrations

W126 levels for 2011 are presented in Figure 4-6. The higher W126 values were measured in 

California, at western sites in high terrain, and at eastern sites with relatively high DM8A O 

3

concentrations. The highest W126 value (61.5 ppm-hour) was measured at SEK430, CA. O 

3

concentrations measured at high elevation sites (AMEC, 2003) typically persist during the night 

because dry deposition and scavenging of O 

3 by nitric oxide are low at night, resulting in the 

persistence of daytime values. Nighttime dry deposition is low because shallow boundary layers 

typically do not form at elevated sites, and scavenging is low because little fresh nitric oxide is 

available to react with the existing O 

3. The persistence of moderate O 3 concentrations at night 

produces steady exposure and high W126 levels. The 2011 W126 values were higher than 

values in 2010, reflecting higher ambient concentrations in 2011. For example, GRS420 

measured a W126 value of 22.3 ppm-hour in 2011 versus a value of 13.7 ppm-hour in 2010. 

W126 values observed at southwestern CASTNET sites were higher in 2011, consistent with 

higher ambient O 

3 concentrations associated with frequent wildfires in Texas and Arizona during 

the summer and fall fire season. 

Figure 4-6 W126 Values (ppm-hour) for 2011
CASTNET 2011 Annual Report

62 Chapter 4: Ozone Concentrations

Air Quality in Western North Dakota

North Dakota has surpassed Alaska and California as the nation’s second highest oil producing state. 

The state is drawing oil and gas from the Bakken shale formation in the northwest corner of North 

Dakota and eastern Montana. About 7,000 wells currently operate in an area centered on Williston, 

ND. Oil production increased from 28 million barrels in 2008 to 86 million barrels in 2010. Many more 

wells are under construction, and oil production is growing rapidly. The Bakken field is producing 

domestic energy for the United States, creating jobs, and generating royalty revenues for the state, 

tribes, and taxpayers.

The Bureau of Land Management, which is the lead agency for permitting and enforcement, and the 

general public have expressed concern that Class I areas in the region may experience a deterioration 

in visibility and air quality in addition to an increase in O 

3 concentrations as the result of the 

development of the Bakken field. The North Dakota Department of Health, Division of Air Quality, 

operates and maintains a network of ambient air quality monitors in North Dakota. Eight

department-operated sites are shown in Figure 4d. The Theodore Roosevelt National Park (TRNP) – 

South Unit (Painted Canyon) site is also a CASTNET site (THR422) sponsored by NPS. The North

Dakota Division of Air Quality operates and maintains SO 

2, O 3, and continuous PM2.5 analyzers at the 

THR422 site in addition to the CASTNET filter pack sampling system.

Fourth highest DM8A O 

3 concentrations from the eight North Dakota monitors are shown in Table 4a 

for 2011. Three-year averages of the fourth highest DM8A values are also shown in the table. The 

measurements show O 

3 concentrations are below the NAAQS. Figure 4e illustrates changes in sulfur 

and nitrogen pollutants and O 

3 at the THR422 site over the period 1999 through 2011. The figure 

shows annual mean concentrations of SO 

2 and total NO3 and fourth highest DM8A O 3 levels for the 13 

years. The plots suggest no significant change in O 

3 or total NO3 concentrations and a 21 percent 

reduction in 3-year mean SO 

2 levels from 1999–2001 to 2009–2011.

Figure 4d Air Quality Monitoring Sites
CASTNET 2011 Annual Report

63 Chapter 4: Ozone Concentrations

Table 4a 2011 O 

3 Concentrations in North Dakota

Pollutant: O 

3 (ppb)* M A X I M A

Valid

Days

1 – Hour 8 – Hour 3yr 1HR 8HR

Location Year 1st 2nd 1st 2nd 3rd 4th Avg #>120 #>75

Beulah North 2011 151 66 64 62 62 59 59 58 0 0

Bismarck Residential 2011 146 63 63 62 60 58 56 57 0 0

Dunn Center 2011 152 60 60 58 57 54 54 55 0 0

Fargo NW 2011 128 66 64 61 60 58 57 59 0 0

Hannover 2011 139 78 65 62 62 60 58 58 0 0

Lostwood NWR 2011 149 68 65 65 62 61 60 60 0 0

TRNP - NU 2011 142 65 64 62 62 60 58 59 0 0

THR422 2011 151 65 63 59 59 58 57 56 0 0

* The air quality standards for O 

3 are:

STATE – 75 ppb three year average of the annual 4th highest daily maximum 8-hour concentrations.

FEDERAL – 75 ppb three year average of the annual 4th highest daily maximum 8-hour concentrations.

NW = northwest, NWR = national wildlife refuge, NU = north unit

Source: North Dakota Department of Health, Division of Air Quality (2012)

Figure 4e Trend in Pollutant Concentrations at THR422, ND

Theodore Roosevelt National Park, ND (THR422)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

35

40

45

50

55

60

65

70

75

80

85

199920002001200220032004200520062007200820092010

2011

SO2 / Total NO3

- Concentration (μg/m3)

Fourth Highest DM8A

Ozone Concentration (ppb)

Ozone

Sulfur Dioxide

Total Particulate Nitrate
CASTNET 2011 Annual Report

64 Chapter 5: Data Quality

CASTNET measurements and supporting activities are assessed routinely in order to provide high-quality 

data to meet project objectives. The CASTNET quality assurance program is based on data quality objectives 

that are evaluated using data quality indicators (DQI) such as precision, accuracy, and completeness. 

Measurements taken during 2011 and historical data collected over the period 1990–2010 were analyzed 

relative to DQI and their associated measures. These analyses demonstrate that CASTNET data can be used 

with confidence and that CASTNET continues to produce information of the highest quality.

Chapter 5: Data Quality

The CASTNET Quality Assurance (QA) program was designed to ensure that all reported data 

are of known and documented quality in order to meet CASTNET objectives and to be 

reproducible and comparable with data from other monitoring networks and laboratories. The 

2011 QA program elements are documented in the CASTNET Quality Assurance Project Plan 

(QAPP), Revision 7.0 (AMEC, 2011c). The QAPP is comprehensive and includes standards and 

policies for all components of project operation from site selection through final data reporting. 

The QAPP contains a project overview and sections on field measurements, chemical analysis 

of field samples, data management, and QA (e.g., assessments and response actions). The 

appendices provide standard operating procedures (SOPs), plans, and other procedures for all 

aspects of CASTNET operations.

DQI are quantitative statistics and qualitative descriptors used to interpret the degree of 

acceptability and utility of the data collected. The DQI for CASTNET are precision, accuracy, 

completeness, bias, representativeness, and comparability. Precision, accuracy, and 

completeness for CASTNET 2011 data were analyzed and compared with historical data 

collected during the period 1990 through 2010. The information in this report is supplemented 

by analyses that are discussed in quarterly QA reports (AMEC, 2011b; 2011e; 2011f; 2012b). 

These QA reports are produced four times per year with the fourth quarter report including an 

annual summary.

To estimate precision across the network, collocated sites were operated at Mackville, KY 

(MCK131/231) and at Rocky Mountain National Park, CO (ROM406/ROM206) during 2011. The 

ROM406/206 monitors are operated independently and also ensure consistency between the 

two sponsoring agencies, NPS and EPA, respectively. ROM406 is considered the regulatory 

monitoring site. Beginning with 2012 measurements, precision of O 

3 measurements will be 
CASTNET 2011 Annual Report

65 Chapter 5: Data Quality

estimated based on requirements in 40 CFR Part 58 and the EPA QA Handbook for Air 

Pollution Measurement Systems, Volume II, Appendix D (2008b). In other words, precision of O 

3

measurements will be calculated based on single instrument data, not collocated data. Precision 

of filter measurements will continue to be estimated from collocated data.

Meteorological measurements (excluding temperature) were discontinued at all but five 

EPA-sponsored sites by the end of 2010. Meteorological data collection is continuing at all NPS 

sites. However, NPS discontinued measurements of 2-m temperature and surface wetness at 

all but three sites. Precision was calculated for 2011 O 

3, flow, and temperature data, only, 

because the other continuous measurements were terminated at EPA-sponsored collocated 

sites. Accuracy was estimated for all parameters.

Precision

Exposed Filter Concentrations

Historical (1990 through 2010) mean absolute relative percent difference (MARPD) data for all 

11 collocated site pairs operated over the history of the network and the 2011 data for the 

current collocated sites at MCK131/231 and ROM406/206 are provided in Figure 5-1. Precision 

estimates for trace cations and Cl- are shown later in Figure 5-2. Table 5-1 lists the precision 

and accuracy criteria for laboratory filter pack measurements.

Figure 5-1 Historical and 2011 Precision Data for Atmospheric Concentrations

0

5

10

15

20

25

30

Sulfur Dioxide Sulfate Nitric Acid Nitrate Ammonium

MARPD

Overall - 1990 through 2010

Mackville, KY - 2011

Rocky Mtn, CO - 2011
CASTNET 2011 Annual Report

66 Chapter 5: Data Quality

Table 5-1 Data Quality Indicator Criteria for CASTNET Laboratory Measurements

Precision1 Accuracy2

Nominal

Reporting Limits

Analyte Method (MARPD) (%) mg/l μg/Filter

Ammonium (NH+ 

4 ) AC 20 90 – 110 0.020* 0.5

Sodium (Na+ ) ICP-AES 20 95 – 105 0.005 0.125

Potassium (K+ ) ICP-AES 20 95 – 105 0.006 0.15

Magnesium (Mg2+) ICP-AES 20 95 – 105 0.003 0.075

Calcium (Ca2+) ICP-AES 20 95 – 105 0.006 0.15

Chloride (Cl-) IC 20 95 – 105 0.020 0.5

Nitrate (NO3) IC 20 95 – 105 0.008* 0.2

Sulfate (SO2-

4 ) IC 20 95 – 105 0.040 1.0

Note: 1 This column lists precision goals for both network precision calculated from collocated filter samples and 

 laboratory precision based on replicate samples. 

2 This column lists laboratory accuracy goals based on reference standards and continuing calibration 

 verification spikes. The criterion is 90-110 percent for ICP-AES reference standards.

AC = automated colorimetry

μg/Filter = micrograms per filter

* = as nitrogen

Values are rounded according to American Society for Testing and Materials (Standard Practice for Using Significant Digits 

in Test Data to Determine Conformance with Specifications E 29).

For more information on analytical methods and associated precision and accuracy criteria, see the CASTNET QAPP, 

Revision 7.0 (AMEC, 2011c)

The historical results vary from just over 4 percent for particulate SO2-

4 to more than 12 percent 

for particulate NO3. The historical MARPD results and the MARPD values for 2011 met the 

20 percent criterion for the CASTNET filter pack measurements shown in Table 5-1. The 

precision of historical and 2011 NO3 measurements was consistently worse than for the other 

analytes, largely because NO3 concentrations include sampling artifacts as discussed in 

Chapter 2.

The 2011 precision results shown in Figure 5-1 indicate that the MARPD data for MCK131/231

were lower than (i.e., more precise) than the historical results for all parameters, except SO2-

4 . 

The 2011 results for ROM406/206 showed better precision than historical precision for SO 

2 and 

NH+ 

4 . All the 2011 filter pack concentration data met the 20 percent criterion. The MARPD results 

were higher for ROM406/206 than for MCK131/231 because of lower concentrations measured 

at this site. Overall, the filter pack precision estimates for MCK131/231 were about the same in 

2011 as in 2010. The precision values for ROM406/206 showed improvement in 2011.

Historical and 2011 precision statistics for four metal cations and Cl- are summarized in 

Figure 5-2. The historical and 2011 MARPD statistics for both MCK131/231 and ROM406/206 

met the DQI criterion of 20 percent. The MARPD values for K+ were higher than for the other 

ions at ROM406/206. In 2009 and 2010, the MARPD values for K+ exceeded the DQI criterion. 

The precision of the K+ concentration data was improved by ensuring SOPs for filter pack 
CASTNET 2011 Annual Report

67 Chapter 5: Data Quality

assembly and disassembly, extraction and analysis, and impregnation of the potassium 

carbonate filters were followed carefully; by refining existing procedures for isolating the room 

where filters are handled to avoid filter contamination; and by improving air handling in the filter 

room. The 2011 precision results for the four metal cations and Cl- were about the same as 

2010 at MCK131/231 and showed improvement at ROM406/206.

Table 5-2 summarizes 2011 precision results by quarter for the two sets of collocated sites. See 

the 2011 Quarterly Data Reports (AMEC, 2011a; 2011d; 2012a; 2012c) and QA Quarterly 

Reports (AMEC, 2011b; 2011e; 2011f; 2012b) for discussions of quarterly precision data.

Figure 5-2 Historical and 2011 Precision Data for Cation and Cl- Concentrations

Table 5-2 Collocated Precision Results (MARPD) for 2011 Filter Pack Data by Quarter

Sampling

Period

SO2-

4 NO3 NH+ 4 Ca2+ Mg2+ Na+ K+ Cl- HNO 3 SO 2

Total 

NO3

MCK131/231, KY

Quarter 1 2.21 4.46 2.15 7.76 6.69 5.02 5.25 8.46 5.63 2.47 3.98

Quarter 2 6.02 10.67 6.28 9.03 7.32 7.17 6.77 6.08 4.76 4.76 6.78

Quarter 3 3.91 10.68 3.92 6.80 5.51 4.81 7.72 1.78 5.57 4.28 5.70

Quarter 4 6.29 10.22 7.34 9.85 10.24 7.88 8.89 8.35 5.99 3.86 6.55

2011 4.61 9.01 4.92 8.36 7.44 6.22 7.16 6.17 5.49 3.84 5.75

ROM406/ROM206, CO

Quarter 1 4.74 11.69 4.97 11.29 9.74 10.98 19.26 2.46 7.66 5.56 6.38

Quarter 2 3.37 12.95 3.93 5.99 7.55 4.16 12.64 7.08 10.66 6.22 8.66

Quarter 3 3.30 11.21 3.47 3.57 4.83 4.01 10.71 3.35 12.64 4.46 11.11

Quarter 4 6.75 19.90 6.41 11.31 10.47 10.37 13.51 17.53 11.63 9.03 9.18

2011 4.54 13.94 4.70 8.04 8.15 7.38 14.03 7.61 10.65 6.32 8.83

0

5

10

15

20

25

30

1 2 3 4 5

Overall - 2000 through 2010

Mackville, KY - 2011

Rocky Mtn, CO - 2011

MARPD

Calcium Potassium Magnesium Sodium Chloride
CASTNET 2011 Annual Report

68 Chapter 5: Data Quality

The 2011 analytical precision results for eight analytes and the three filter types are presented 

in Figure 5-3. The results were based on analysis of 5 percent of the samples that were 

randomly selected for replication in each batch. The results of in-run replicate analyses were 

compared to the original concentration results. The laboratory precision data met the 

measurement criterion listed in Table 5-1.

Figure 5-3 Precision Results for Laboratory Replicate Samples (2011)

Ozone Concentrations

CASTNET was originally designed to report on regional, rural O 

3 levels. The QA procedures for 

the EPA-sponsored O 

3 analyzers at CASTNET sites have differed from the EPA QA 

requirements for SLAMS monitoring (EPA, 2010a). Consequently, EPA directed AMEC to 

update EPA-sponsored CASTNET O 

3 monitoring to meet 40 CFR Part 58 monitoring 

requirements. This effort was completed in 2011.

In September 2011, AMEC began submitting O 

3 data collected at EPA-sponsored CASTNET 

sites to the EPA AQS database. The beginning of the data record in AQS was determined by 

the date of conversion to 40 CFR Part 58 O 

3 monitoring protocols for a given site, the earliest of 

which occurred in late 2010. Approximately one third of EPA-sponsored CASTNET sites have 

an AQS data record that starts January 1, 2011. The second group of sites has a start date of 

April 1, 2011, and the final third of the sites has a start date of June 1, 2011. Data are now 

uploaded to the AQS database monthly. One month of data from all EPA-sponsored CASTNET 

sites is submitted no later than 90 days after the end of a given month (e.g., August 2011 data 

for all sites were uploaded in November 2011). NPS-sponsored CASTNET sites and CHE185, 

OK, which is operated in partnership with the Cherokee Nation, have been configured as 

0

1

2

3

4

5

Nylon

Sulfate

Cellulose

Sulfate

Teflon

Sulfate

Nylon

Nitrate

Teflon

Nitrate

Teflon

Ammonium

Teflon

Sodium

Teflon

Potassium

Teflon

Magnesium

Teflon

Calcium

Teflon

Chloride

contribute to sulfur dioxide

MARPD

for In-Run Replicates

nitric acid
CASTNET 2011 Annual Report

69 Chapter 5: Data Quality

40 CFR Part 58 protocol sites since the beginning of site operation, and their entire data records 

are in AQS. Submittal schedules of O 

3 data to AQS are similar for these sites.

Beginning in 2012, O 

3 data quality will be evaluated in terms of AQS-protocol O 3 quality control

(QC) criteria (e.g. precision will be calculated as per 40 CFR Part 58 instead of using MARPD of 

collocated sites). In any event, despite historical differences in QA procedures, operation of the 

collocated EPA and NPS O 

3 analyzers at ROM406/206 provides an opportunity to evaluate the 

precision of the independent systems. Table 5-3 provides the DQI criteria for the CASTNET 

continuous measurements including O 

3. The precision criterion for the collocated O 3 data is 

10 percent.

Table 5-3 Data Quality Indicator Criteria for CASTNET Field Measurements

Measurement Criteria*

Parameter Method MARPD/MAD Precision Accuracy

Filter Pack Flow Mass Flow Controller MARPD ± 10% ± 5%

Ozone UV Absorbance MARPD ± 10% (of reading) ± 10%

Ambient Temperature Platinum RTD MAD ± 1.0°C ± 0.5°C

Wind Speed Anemometer MAD ± 0.5 m/s The greater of ± 0.5 m/s for 

winds < 5 m/s or ± 5% for 

winds ≥ 5 m/s

Wind Direction Wind Vane MAD ± 5° ± 5°

Sigma Theta Wind Vane MAD Undefined Undefined

Delta Temperature Platinum RTD MAD ± 0.5°C ± 0.5°C

Relative Humidity Thin Film Capacitor MARPD ± 10% (of full scale) ± 10 percent

Solar Radiation Pyranometer MARPD ± 10% (of reading taken 

at local noon)

± 10%

Precipitation Tipping Bucket Rain Gauge MARPD ± 10% (of reading) ± 0.05 inch†

Surface Wetness Conductivity Bridge MAD Undefined Undefined

Note: MARPD is the precision measure for percentage criteria such as flow.

Mean absolute difference (MAD) is the precision measure for difference criteria such as temperature.

°C = degrees Celsius

m/s = meters per second

RTD = resistance-temperature device

* Precision criteria apply to collocated instruments, and accuracy criteria apply to calibration of instruments. Precision was 

calculated for only ozone, flow, and temperature because the other continuous measurements were terminated at 

MCK131, KY; MCK231, KY; and ROM206, CO. Accuracy was estimated for all parameters. † For target value of 0.50 inch

MARPD statistics were calculated from hourly O 

3 measurements obtained from the collocated 

sites MCK131/231 and ROM406/206 during 2011. In addition, quarterly historical precision 

statistics were compiled for all collocated sites. The historical data were based on the operation 

of 11 collocated site pairs over the history of the network. Quarterly precision results are 

summarized in Figure 5-4. Table 5-4 provides precision results for O 

3 concentrations by quarter 

for 2011. The data show the historical and MCK131/231 2011 results met the 10 percent 

criterion. However, the 2011 MARPD values for first and second quarter 2011 at ROM406/206 

did not meet the criterion. The high MARPD values were caused by an erroneous calibration, 

which dropped readings by about 2 ppb, as well as ongoing differences in systems and 
CASTNET 2011 Annual Report

70 Chapter 5: Data Quality

procedures at the two independently operated sites. Also, the completeness of the O 

3

concentrations combined from the two monitoring stations at ROM406/206 was only 63 percent 

in the first quarter. The relatively low completeness was caused by ice in the sample line at 

ROM206. This resulted in a corresponding inability to perform zero/precision/span checks over 

an extended period in early 2011.

Scatter plots of the hourly O 

3 concentrations measured at the MCK131/231 and ROM406/206 

collocated site pairs during 2011 are presented in Figure 5-5. The results from MCK131/231 

were excellent with an R2 of 0.9982 and small y-intercept of 0.48. The ROM406/206 regression 

line shows a consistent bias between ROM406 and ROM206 O 

3 measurements with ROM406 

measuring higher concentrations with a slope of 0.9224 and a y-intercept of 1.04. Efforts are 

underway to eliminate the bias by reducing differences in hardware configuration, systems, and 

procedures between the two networks. In any event, ROM206 is considered a special study site 

and ROM206 O 

3 data are not used to gauge compliance.

Figure 5-4 Historical and 2011 Precision Data by Quarter for Ozone Concentrations

0

5

10

15

20

25

30

First

Quarter

Second

Quarter

Third

Quarter

Fourth

Quarter

Overall - 1990 through 2010

Mackville, KY - 2011

Rocky Mtn, CO - 2011

MARPD
CASTNET 2011 Annual Report

71 Chapter 5: Data Quality

Table 5-4 Collocated Precision Results (MARPD) for 2011 Ozone Concentrations 

by Quarter 

Sampling Period MCK131/231, KY ROM406/206, CO

Quarter 1 1.29 12.03

Quarter 2 0.91 11.24

Quarter 3 1.07 9.17

Quarter 4 0.83 6.07

2011 1.03 9.63

Figure 5-5 2011 Hourly Ozone Concentrations at Collocated Sites

Continuous Measurements

Precision criteria for the continuous measurements are listed in Table 5-3. Because continuous 

measurements, excluding O 

3, flow, and temperature, were discontinued at all but five EPA sites 

at the end of 2010, including MCK131, MCK231, and ROM206, precision estimates were made 

only for flow and temperature data. Figure 5-6 provides precision results for flow and 

temperature for historical data (1990 through 2010) at all collocated site pairs and 2011 

precision statistics at the two current collocated site pairs. Precision was calculated in terms of 

the MARPD or mean absolute difference (MAD) of hourly measurements. All historical and 2011 

precision results met the DQI criteria. Table 5-5 gives precision data by quarter for 2011 for flow 

and temperature for the two collocated sites. The results show overall good instrument precision 

with all annual and quarterly precision results meeting the DQI criteria. 
CASTNET 2011 Annual Report

72 Chapter 5: Data Quality

Figure 5-6 Historical and 2011 Precision Data for Flow and Temperature Measurements

Note: The left y-axis indicates MARPD for flow (Table 5-3), and the right y-axis indicates MAD for temperature (Table 5-3).

Table 5-5 Collocated Precision Results for 2011 Continuous Measurements by Quarter 

Sampling Period

MARPD MAD Flow Rate Temperature

MCK131/231, KY

Quarter 1 0.08 0.09

Quarter 2 0.12 0.13

Quarter 3 0.08 0.10

Quarter 4 3.88 0.16

2011 1.04 0.12

ROM406/206, CO

Quarter 1 0.68 0.14

Quarter 2 6.64 0.14

Quarter 3 4.59 0.17

Quarter 4 8.07 0.23

2011 4.99 0.17

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0

2

4

6

8

10

12

14

16

Flow Rate Temperature

MAD

MARPD

MARPD: Overall - 1990 through 2010

MARPD: Mackville, KY - 2011

MARPD: Rocky Mtn, CO - 2011

MAD: Overall - 1990 through 2010

MAD: Mackville, KY - 2011

MAD: Rocky Mtn, CO - 2011
CASTNET 2011 Annual Report

73 Chapter 5: Data Quality

Accuracy

Laboratory Filter Concentrations

Accuracy of laboratory measurements is assessed through the analysis of reference and 

continuing calibration verification (CCV) samples. Reference samples and CCV are procured 

from independent suppliers and are traceable to the National Institute for Standards and 

Technology (NIST). Reference samples are analyzed at the beginning and end of each 

analytical batch to verify the accuracy and stability of the calibration curve. The target value of 

the CCV solution is set to the midrange of the calibration curve. The CCV are analyzed every 

tenth sample to verify that instrument calibration has not drifted beyond established limits. 

Table 5-6 presents the percent recoveries and standard deviations for reference samples and 

CCV relative to target concentrations for 2011. The table shows that the DQI goals (see Table 

5-1) were met in 2011.

Table 5-6 Filter Pack Quality Control Summary for 2011 Filter Type Parameter

Reference

Sample1 Recovery

(%R)

Continuing Calibration 

Verification Samples (%R)

In-Run Replicate2

(RPD)

MeanStandard

Deviation

Count3MeanStandard

Deviation

Count3MeanStandard

Deviation

Count3

Teflon SO2-

4 98.97 1.49 130 99.72 1.41 661 0.37 0.56 310

NO3 100.85 0.91 130 100.51 1.12 661 0.99 1.18 326

NH+ 

4 102.96 2.34 132 101.32 2.18 677 0.78 0.91 347

Ca2+ 99.18 2.34 133 100.45 0.94 678 0.80 0.81 312

Mg2+ 103.41 1.29 133 100.03 0.88 678 1.39 1.61 312

Na+ 99.96 1.66 133 100.01 0.96 678 1.11 1.48 312

K+ 100.80 2.23 133 100.03 0.78 678 1.79 2.21 312

Cl- 103.48 0.83 130 100.83 1.59 661 1.61 1.78 296

Nylon SO2-

4 98.83 1.27 129 99.97 1.59 651 1.55 2.20 298

NO3 100.54 1.16 129 100.72 1.17 651 1.07 1.19 312

Cellulose SO2-

4 98.28 1.49 170 99.42 1.58 671 1.54 2.19 337

Note: % R = percent recovery

RPD = relative percent difference 1 Results of reference sample analyses provide accuracy estimates

2 Results of replicate analyses provide precision estimates

3 Number of quality control samples
CASTNET 2011 Annual Report

74 Chapter 5: Data Quality

Continuous Measurements

Table 5-7 presents field accuracy results for 2011 based on instrument challenges performed 

using independent reference standards during site calibration visits. CASTNET sites were 

calibrated every six months with NIST-traceable standards. The calibration results were 

evaluated using the accuracy criteria listed in Table 5-3. Each parameter was within its criterion 

with at least 90 percent frequency with the exception of wind direction north at 87.5 percent and 

solar radiation at 87.5 percent. However, these results did not adversely affect data collection 

because data are not considered invalid unless criteria are exceeded by more than two times 

the criterion. Using the two times standard, the two parameters passed with 100.0 percent

frequency. The 2011 results for flow rate accuracy were lower than for 2010. 

Table 5-7 Accuracy Results for 2011 Field Measurements 

Parameter Percent Within Criterion Parameter Percent Within Criterion

Flow Rate 90.9 percent Temperature (ambient) 97.1 percent

Ozone Slope 97.0 percent Delta Temperature (0°C) 100.0 percent

Ozone Intercept 100.0 percent Delta Temperature (ambient) 100.0 percent

Wind Speed < 5 m/s 100.0 percent Relative Humidity 100.0 percent

Wind Speed ≥ 5 m/s 100.0 percent Precipitation 100.0 percent

*Wind Direction North 87.5 percent *Solar Radiation 87.5 percent

Wind Direction South 100.0 percent Wetness (w/in 0.5 volts) 100.0 percent

Temperature (0°C) 100.0 percent

Note: °C = degrees Celsius

 m/s = meters per second

 * = Per CASTNET project protocols, data are flagged as “suspect” but still considered valid if the calibration criterion is 

not exceeded by more than its magnitude (i.e., if within 2x the criterion).

Cedar Creek State Park, WV (CDR119)
CASTNET 2011 Annual Report

75 Chapter 5: Data Quality

Completeness

Completeness is defined as the percentage of valid data points obtained from a measurement 

system relative to total possible data points. The CASTNET measurement criterion for 

completeness requires a minimum completeness of 90 percent for every measurement for 

each quarter. For 2011 all the parameters listed in Figure 5-7 were measured at the five 

EPA-sponsored sites with continuing meteorological measurements. All parameters, except delta 

temperature and wetness, were measured at NPS-sponsored sites. Ozone concentrations were 

measured at 80 of 83 sites. Atmospheric concentrations, filter pack flow, and ambient 

temperature were measured at all sites. Delta temperature was measured at the five EPA sites 

plus ACA416, ME; GRS420, TN; and ROM406, CO. Surface wetness was measured at the five 

EPA sites plus GRS420, TN. Figure 5-7 presents historical (black) and 2011 completeness data 

for all sites for measured filter concentrations and continuous measurements. The figure shows 

that the 2011 direct measurements met the 90 percent completeness goal, and four 

meteorological measurements and filter pack flow exceeded 95 percent. Atmospheric 

concentrations also exceeded 95 percent.

Figure 5-7 Historical and 2011 Percent Completeness of Measurements 

(black bars are 1990–2010)

65 70 75 80 85 90 95 100

Surface Wetness

Filter Pack Flow

Ozone

Delta Temperature

Ambient Temperature

Precipitation

Solar Radiation

Relative Humidity

Sigma Theta

Wind Direction

Scalar Wind Speed

Vector Wind Speed

Atmospheric Concentrations

Percent Completeness

Continuous

Parameters

DQI Measurement Criterion
CASTNET 2011 Annual Report

76 Chapter 5: Data Quality

Results for 2011 Environment Canada Proficiency Testing

During 2011, the CASTNET laboratory participated in the Environment Canada (ECAN) Proficiency 

Testing Program for Inorganic Environmental Substances (http://www.ec.gc.ca/inrenwri/default.asp?lang=En&n=7A20877C-1). Specifically, the laboratory is one of 32 laboratories 

that participated in the 2011 Rain and Soft Waters round robin studies, which consist of natural 

waters supplied by the National Laboratory for Environmental Testing. The CASTNET laboratory 

receives 10 samples of mixed rain and Canadian Shield waters for chemical analysis from ECAN 

every 6 months. The laboratory reported the eight CASTNET parameters for samples in two studies 

(study codes 0098 and 0099) during 2011. 

The results reported by the 32 laboratories are evaluated for systematic bias and precision. 

Systematic bias is assessed using Youden (1969) non-parametric analysis, while precision is 

calculated using algorithm A from the International Organization for Standardization (ISO) standard 

13528 (ISO, 2005). 

Individual sample results are flagged based on the robust standard deviation (R-σ) obtained from the 

ISO 13528 computation (ISO, 2005). Samples within 2 standard deviations of the assigned (median 

laboratory) value are not flagged; samples between 2 and 3 standard deviations are given a warning 

flag; samples greater than 3 standard deviations from the assigned value are flagged as above the 

action limit (remedial action is required). Laboratory results are considered systematically biased when 

individual parameters are ranked by the Youden analysis to be consistently and significantly higher or 

lower than the assigned value without regard to flagged results. 

The CASTNET laboratory’s 2011 ECAN results for the eight CASTNET parameters are presented in 

Table 5a. All scored parameters received the highest possible rating of “ideal” indicating an unbiased 

score in study 0098. Two of 10 individual results for NH 

3 were flagged “warning high” for study 0099, 

indicating that the flagged results were between 2 and 3 R-σ from their assigned values. Both of these 

results were at concentrations below 3 times the method reporting limit. Investigation by the laboratory 

revealed the root cause for these results to be a slightly degraded low-level standard. Corrective 

action was implemented to require an acceptance criterion of ±5 percent of the expected value for 

each point in a calibration curve.

Table 5a AMEC Results for Studies 0098 and 0099

Test

Parameter

Analytical

Method

Reference

Method

Laboratory Performance Rating

Study 0098

Summer 2011

Study 0099

Winter 2011

NH 

3 AC EPA Method 350.1 Insufficient Data* Biased High

Ca2+

 ICP-AES EPA Method 6010 Ideal Ideal

Cl- IC EPA Method 300.0 Ideal Ideal

Mg2+ ICP-AES EPA Method 6010 Ideal Ideal

NO3 + NO2 IC EPA Method 300.0 Ideal Ideal

K

+ ICP-AES EPA Method 6010 Ideal Ideal

Na+ ICP-AES EPA Method 6010 Ideal Ideal

SO2-

4 IC EPA Method 300.0 Ideal Ideal

Note: * Fewer than five samples above method reporting limit.

Source: Environment Canada (2012)
CASTNET 2011 Annual Report

77 Chapter 5: Data Quality

The overall laboratory performance rating was “Very Good” for study 0098 and “Good” for study 0099.

The overall laboratory rating indicates a percent score as described in Table 5b. 

Table 5b Laboratory Performance Rating

Laboratory Performance Rating

Rating % Score*

Very Good 0 - 5

Good > 5 - 12.5

Fair > 12.5 - 30

Poor > 30

Note: * Sum of Parameters Biased and Results Flagged

Source: Environment Canada (2012)

The report for study 0099 included a summary of the laboratory’s performance over the last five years. 

The laboratory achieved the highest possible rating for 9 of the 10 studies conducted during that 

period. The results are listed below in Table 5c.

Table 5c Five-Year Historical Laboratory Performance

Study No. / Date Score*

No. 0090, Summer 2007 1.0

No. 0091, Winter 2007 3.0

No. 0092, Summer 2008 0.0

No. 0093, Winter 2008 0.0

No. 0094, Summer 2009 5.0

No. 0095, Winter 2009 2.0

No. 0096, Summer 2010 5.0

No. 0097, Winter 2010 0.0

No. 0098, Summer 2011 0.0

No. 0099, Winter 2011 6.0

5-Year Median 1.5

Note: * Sum of Parameters Biased and Results Flagged

Source: Environment Canada (2012)

Mackville, KY (MCK131/231)
CASTNET 2011 Annual Report

78 Chapter 5: Data Quality

Summary of Data Quality Results

DQI results demonstrate that CASTNET field and laboratory processes were adequately 

monitored through QA/QC procedures and were generally free of systemic bias during 2011 

with the exception of O 

3 measurements at ROM406/206, CO. The O 3 concentrations measured 

at ROM406 were consistently higher than ROM206. However, ROM206 is considered a special 

study site, and ROM206 O 

3 data are not used to gauge compliance with NAAQS. 

Historical and 2011 precision data for filter pack parameters are considered acceptable. 

The 2011 precision data for O 

3 concentrations met the 10 percent DQI criterion for six of the 

eight quarters. The 2011 O 

3 MARPD values for the first and second quarter 2011 at 

ROM406/206 did not meet the criterion. The precision of quarterly and annual flow and 

temperature measurements for 2011 was good with all quarterly and annual results meeting the 

DQI criteria.

Accuracy data met the established criteria for field and laboratory parameters. Completeness 

criteria were met for all parameters in 2011. Completeness results for 2011 are considered 

excellent. ECAN proficiency test studies (0098 and 0099) showed that the CASTNET laboratory 

performed well for the interlaboratory chemical analyses.

Rocky Mountain National Park, CO (ROM406/206)
CASTNET 2011 Annual Report 

79 References

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CASTNET 2011 Annual Report

Appendix A

Appendix A

Locational and Operational Characteristics of

CASTNET Sites
CASTNET 2011 Annual Report

A-1 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

Alabama

SND152 Sand Mountain 12/27/88 34.2894 85.9704 352 • • AL99 Agri. Rolling Y EPA

Alaska

DEN417 Denali National Park 10/06/98 63.7258 148.9633 661 • • AK03 Forested Complex N NPS

Arizona

CHA467 Chiricahua National Monument 04/25/89 32.0092 109.3892 1570 • • AZ98 Range Complex N NPS

GRC474 Grand Canyon National Park 05/16/89 36.0597 112.1822 2073 • • AZ03 Forested Complex M NPS

PET427 Petrified Forest National Park 09/24/02 34.8225 109.8919 1723 • • AZ97 Desert Flat Y NPS

Arkansas

CAD150 Caddo Valley 10/04/88 34.1792 93.0989 71 • • AR03 Forested Rolling N EPA

California

JOT403 Joshua Tree National Park 02/16/95 34.0714 116.3906 1244 • • CA67 Desert Complex M NPS

LAV410 Lassen Volcanic National Park 07/25/95 40.5403 121.5764 1756 • • CA96 Forested Complex M NPS

PIN414 Pinnacles National Monument 05/16/95 36.4850 121.1556 335 • • CA66 Forested Complex M NPS

SEK430 Sequoia National Park 04/07/05 36.4894 118.8269 457 • • CA75 Forested Mountaintop N NPS

YOS404 Yosemite National Park 09/25/95 37.7133 119.7061 1605 • • CA99 Forested Complex N NPS

Colorado

GTH161 Gothic 05/16/89 38.9573 106.9854 2926 • • CO10 Range Complex N EPA

MEV405 Mesa Verde National Park 01/10/95 37.1983 108.4903 2165 • • CO99 Forested Complex M NPS

ROM206 Rocky Mountain National Park 07/03/01 40.2778 105.5453 2743 • • CO98 Forested Complex M EPA

ROM406 Rocky Mountain National Park 12/20/94 40.2778 105.5453 2743 • • CO98 Forested Complex M NPS

Connecticut

ABT147 Abington 12/28/93 41.8402 72.0111 209 • • CT15 Urban-Agri. Rolling M EPA
CASTNET 2011 Annual Report

A-2 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

Florida

EVE419 Everglades National Park 10/06/98 25.3911 80.6806 2 • •4 FL11 Swamp Flat Y NPS

IRL141 Indian River Lagoon 07/09/01 30.1065 80.4554 2 • • FL99 Beach Flat Y EPA

SUM156 Sumatra 12/28/88 30.1065 84.9938 14 • • FL23 Forested Flat Y EPA

Georgia

GAS153 Georgia Station 06/28/88 33.1812 84.4100 270 • • GA41 Agri. Rolling M EPA

Illinois

ALH157 Alhambra 06/28/88 38.8690 89.6229 164 • • IL46 Agri. Flat Y EPA

BVL130 Bondville 02/09/88 40.0520 88.3725 212 • • IL11 Agri. Flat Y EPA

STK138 Stockton 12/28/93 42.2872 89.9998 274 • • IL18 Agri. Rolling M EPA

Indiana

SAL133 Salamonie Reservoir 06/28/88 40.8164 85.6608 250 • • IN20 Agri. Flat Y EPA

VIN140 Vincennes 08/04/87 38.7406 87.4844 134 • • IN22 Agri. Rolling M EPA

Kansas

KNZ184 Konza Prairie 03/26/02 39.1021 96.6096 348 • • KS31 Prairie Flat Y EPA

Kentucky

CDZ171 Cadiz 10/01/93 36.7841 87.8500 189 • • KY99 Agri. Rolling M EPA

CKT136 Crockett 08/24/93 37.9211 83.0658 455 • • KY35 Agri. Rolling Y EPA

MAC426 Mammoth Cave National Park 07/24/02 37.1319 86.1478 243 • • KY10 Agri./Forested Rolling M NPS

MCK131 Mackville 07/31/90 37.7044 85.0483 353 • • KY03 Agri. Rolling M EPA

Maine

ACA416 Acadia National Park 12/01/98 44.3769 68.2608 158 • • ME98 Forested Complex M NPS

ASH135 Ashland 12/20/88 46.6039 68.4142 235 • • ME00 Agri. Flat Y EPA

HOW132 Howland 11/24/92 45.2158 68.7085 69 • • ME09 Forested Rolling Y EPA

HOW191 Howland AmeriFlux 09/16/2011 45.2041 68.7402 60 • • ME09 Forested Rolling N EPA
CASTNET 2011 Annual Report

A-3 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

Maryland

BEL116 Beltsville 11/01/88 39.0283 76.8175 46 • • MD99 Urban-Agri. Flat N EPA

BWR139 Blackwater National Wildlife Refuge 07/04/95 38.4448 76.1115 4 • • MD15 Forest-Marsh Coastal M EPA

Michigan

ANA115 Ann Arbor 06/28/88 42.4164 83.9019 267 • • MI52 Forested Flat M EPA

HOX148 Hoxeyville 10/31/00 44.1809 85.7390 298 • • MI53 Forested Flat Y EPA

UVL124 Unionville 06/28/88 43.6139 83.3597 201 • • MI51 Agri. Flat Y EPA

Minnesota

VOY413 Voyageurs National Park 06/13/96 48.4128 92.8292 429 • • MN32 Forested Rolling M NPS

Mississippi

CVL151 Coffeeville 12/27/88 34.0028 89.7989 134 • • MS30 Forested Rolling M EPA

Montana

GLR468 Glacier National Park 12/27/88 48.5103 113.9956 976 • • MT05 Forested Complex N NPS

Nebraska

SAN189 Santee Sioux 07/05/06 42.8292 97.8541 429 • • SD99 Agri. Rolling N EPA

Nevada

GRB411 Great Basin National Park 05/16/95 39.0053 114.2158 2060 • • NV05 Forested Complex M NPS

New Hampshire

WST109 Woodstock 12/27/88 43.9446 71.7008 258 • • NH02 Forested Complex N EPA

New Jersey

WSP144 Washington Crossing 12/27/88 40.3133 74.8726 61 • • NJ99 Urban-Agri. Rolling M EPA

New York

CAT175 Claryville 05/10/94 41.9423 74.5519 765 • •4, 5 NY68 Forested Complex N EPA

CTH110 Connecticut Hill 09/28/87 42.4010 76.6535 515 • • NY67 Forested Rolling N EPA

HWF187 Huntington Wildlife Forest 05/28/02 43.9732 74.2232 502 • • NY20 Forested Complex N EPA
CASTNET 2011 Annual Report

A-4 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

North Carolina

BFT142 Beaufort 12/28/93 34.8843 76.6213 2 • • NC06 Agri. Flat Y EPA

CND125 Candor 09/25/90 35.2643 79.8382 198 • • NC36 Forested Rolling M EPA

COW137 Coweeta 11/04/87 35.0605 83.4302 686 • • NC25 Forested Complex N EPA

PNF126 Cranberry 12/27/88 36.1040 82.0448 1250 • • NC45 Forested Mountaintop M EPA

North Dakota

THR422 Theodore Roosevelt National Park 10/06/98 46.8947 103.3778 850 • • ND00 Range Rolling Y NPS

Ohio

DCP114 Deer Creek State Park 09/28/88 39.6358 83.2600 267 • • OH54 Agri. Rolling Y EPA

OXF122 Oxford 08/18/87 39.5314 84.7231 284 • • OH09 Agri. Rolling N EPA

QAK172 Quaker City 09/28/93 39.9431 81.3378 372 • • OH49 Agri. Rolling M EPA

Oklahoma

CHE185 Cherokee Nation 04/02/02 35.7507 94.6700 299 • • AR27 Agri. Rolling Y EPA

Ontario

EGB181 Egbert, Ontario 12/27/94 44.2317 79.7840 251 • •4 NY10 Agri. Rolling Y EPA

Pennsylvania 

ARE128 Arendtsville 06/28/88 39.9231 77.3078 269 • • PA00 Agri. Rolling M EPA

KEF112 Kane Experimental Forest 01/03/89 41.5981 78.7683 622 • • PA29 Forested Rolling Y EPA

LRL117 Laurel Hill State Park 12/15/87 39.9883 79.2522 615 • • MD08 Forested Complex N EPA

MKG113 M.K. Goddard State Park 01/12/88 41.4250 80.1447 384 • • NY10 Forested Rolling N EPA

PSU106 Penn. State University 01/06/87 40.7209 77.9316 376 • • PA42 Agri. Rolling M EPA

South Dakota

WNC429 Wind Cave National Park 11/18/03 43.5578 103.4839 1292 • • SD04 Prairie Rolling M NPS
CASTNET 2011 Annual Report

A-5 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

Tennessee

ESP127 Edgar Evins State Park 03/22/88 36.0389 85.7330 302 • • KY10 Forested Rolling N EPA

GRS420 Great Smoky Mountains National Park 10/06/98 35.6331 83.9422 793 • • TN11 Forested Complex N NPS

SPD111 Speedwell 06/12/89 36.4698 83.8265 361 • • TN04 Agri. Rolling Y EPA

Texas

ALC188 Alabama-Coushatta 04/02/04 30.4210 94.4045 101 • • TX10 Forested Rolling Y EPA

BBE401 Big Bend National Park 07/18/95 29.3022 103.1772 1052 • • TX04 Forested Complex M NPS

PAL190 Palo Duro Canyon State Park 04/24/07 34.8803 101.6649 1050 • • TX43 Prairie Complex M EPA

Utah

CAN407 Canyonlands National Park 01/24/95 38.4586 109.8211 1809 • • UT09 Desert Complex M NPS

Virginia

PED108 Prince Edward 11/03/87 37.1653 78.3070 150 • • VA24 Forested Rolling M EPA

SHN418 Shenandoah National Park 06/28/88 38.5231 78.4347 1073 • • VA28 Forested Mountaintop M NPS

VPI120 Horton Station 06/02/87 37.3300 80.5573 920 • • VA13 Forested Mountaintop N EPA

Washington

MOR409 Mount Rainier National Park 08/29/95 46.7583 122.1244 415 • • WA99 Forested Complex N NPS

West Virginia

CDR119 Cedar Creek State Park 11/10/87 38.8794 80.8478 234 • • WV05 Forested Complex N EPA

PAR107 Parsons 01/19/88 39.0906 79.6614 510 • • WV18 Forested Complex N EPA

Wisconsin

PRK134 Perkinstown 09/27/88 45.2066 90.5972 472 • • WI35 Agri. Rolling M EPA
CASTNET 2011 Annual Report

A-6 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

Wyoming

CNT169 Centennial 08/19/91 41.3722 106.2422 3178 • • WY95 Range Complex M EPA

PND165 Pinedale 12/27/88 42.9214 109.7900 2388 • • WY06 Range Rolling M EPA

YEL408 Yellowstone National Park 06/26/96 44.5597 110.4006 2400 • • WY08 Forested Rolling N NPS

1. The dry deposition filters are analyzed for the following constituents:

Teflon = SO2-

4 , NO3, NH +4, Cl -, K +, Na +, Mg2+ , Ca2+ 

Nylon = SO2-

4 , NO3 (reported as HNO 3) 

Cellulose = SO2-

4 (reported as SO 2) 

2. Temperature is measured at all sites. Other meteorological measurements have been 

discontinued at all 100 and 200 series sites with the exception of PAL190,TX; CHE185, 

OK; BVL130, IL, BEL116, MD; and BFT142, NC. Delta temperature was discontinued at 

all 400 series sites with the exception of ACA416, ME; GRS420, TN; and ROM406, CO. 

Surface wetness was discontinued at all 400 series sites with the exception of GRS420. 

Meteorological sensors include: temperature, delta temperature, relative humidity, solar 

radiation, vector wind speed, scalar wind speed, wind direction, sigma theta, surface 

wetness, and precipitation via tipping bucket rain gauge.

3. N = No; Y = Yes; M = Marginal.

4. O 

3 not measured.

5. Solar-powered site.

• Indicates current monitoring.

100 and 200 series = EPA – Operated Sites

400 series = NPS – Operated Sites
CASTNET 2011 Annual Report

Appendix B

Appendix B

Acronyms and Abbreviations
CASTNET 2011 Annual Report

B-1 Appendix B

List of Acronyms and Abbreviations

ACCS Ammonia CASTNET Chemical Speciation Network Study

ADS annular denuder system

AMEC AMEC Environment & Infrastructure, Inc.

AMNet Atmospheric Mercury Network

AMoN Ammonia Monitoring Network 

ANC acid neutralizing capacity

AQS EPA’s Air Quality System

ARP Acid Rain Program

C concentration

Ca2+

 particulate calcium ion

CAAA Clean Air Act Amendments

CAIR Clean Air Interstate Rule

CAPMoN Canadian Air and Precipitation Monitoring Network

CASTNET Clean Air Status and Trends Network

CFR Code of Federal Regulations

CCV continuing calibration verification samples

Cl- particulate chloride ion

CLAD Critical Loads of Atmospheric Deposition science committee

CLD303 Clingmans Dome, TN cloud sampling site

CLmaxS maximum critical load of sulfur

CMAQ Community Multiscale Air Quality Modeling System

CLminN nitrogen removed by an ecosystem

CSAPR Cross-State Air Pollution Rule

CSN Chemical Speciation Network

%D percent difference

DM8A daily maximum 8-hour average

DQI data quality indicator

%E percent error

ECAN Environment Canada 

EGU electric generating unit

EPA U.S. Environmental Protection Agency

H 

2SO 4 sulfuric acid

HNO 

3 nitric acid

H 

3PO 3 phosphorous acid

IC ion chromatography

ICP-AES inductively coupled plasma-atomic emission spectrometry

IDW inverse distance weighting

IMPROVE Interagency Monitoring of Protected Visual Environments
CASTNET 2011 Annual Report

B-2 Appendix B

List of Acronyms and Abbreviations (continued)

ISO International Organization for Standardization

K+ particulate potassium ion

K 

2CO 3 potassium carbonate

kg/ha kilograms per hectare

kg/ha/yr kilograms per hectare per year

km kilometer

LAI leaf area index

lpm liters per minute

LWC liquid water content

m meter

MAD mean absolute difference

MADPro Mountain Acid Deposition Program

MARPD mean absolute relative percent difference

MDN Mercury Deposition Network

meq/m2 /yr milliequivalents per square meter per year

Mg2+ particulate magnesium ion

mg/l milligrams per liter

MLM Multi-Layer Model

MPPD mini-parallel plate denuder

N nitrogen (HNO 

3 + NO3 + NH +4 ) 

Na+ particulate sodium ion

NAAQS National Ambient Air Quality Standards

NADP National Atmospheric Deposition Program

NAPAP National Acid Precipitation Assessment Program

NBP NOx Budget Trading Program

NDDN National Dry Deposition Network

NH 

3 ammonia

NH +

4 particulate ammonium

NH 

4NO 3 ammonium nitrate

(NH 

4)

 

2SO 4 ammonium sulfate

NHx NH 

3 + volatilized NH +4

NIST National Institute of Standards and Technology

NO -

3 particulate nitrate 

NOx nitrogen oxides [nitric oxide (NO) + nitrogen dioxide (NO 

2)]

NPS National Park Service

Nr reactive nitrogen

NTN National Trends Network
CASTNET 2011 Annual Report

B-3 Appendix B

List of Acronyms and Abbreviations (continued)

O 

3 ozone

OTC Ozone Transport Commission

PM particulate matter

PM10 coarse particulate matter

PM2.5 fine particulate matter

PRISM Parameter-elevation Regressions on Independent Slopes Model

ppb parts per billion

ppm parts per million

QA quality assurance

QAPP Quality Assurance Project Plan

QC quality control

R-σ robust standard deviation

RTI RTI International, Inc.

S sulfur (SO 

2 + SO2-4 ) 

SIP State Implementation Plan

SLAMS State and Local Air Monitoring Stations

SO 

2 sulfur dioxide

SO2-

4 particulate sulfate

SOP standard operating procedure

TN11 Elmont, TN wet deposition monitoring site

total NO3 gaseous nitric acid (HNO 

3) + particulate nitrate (NO3) 

TRNP Theodore Roosevelt National Park

TVA Tennessee Valley Authority

μeq/L microequivalents per liter

μg/m3 micrograms per cubic meter

Vd deposition velocity

VOCs volatile organic compounds
For More Information:

U.S. Environmental Protection Agency

Office of Air and Radiation

Clean Air M kt ar e s Di i i v s on

Washington, DC

On the Web:

CASTNET CASTNET H P ome age: Cl Ai M kt EPA H P

www.epa.gov/castnet

Clean Air Markets

Division Home Page:

www.epa.gov/airmarkets

EPA Home Page:

www.epa.gov