Clean Air Status and Trends Network

(CASTNET)

2012 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

April 2014
CASTNET 2012 Annual Report

i Table of Contents

Table of Contents

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

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

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

Cooperating Networks ......................................................................................4

Locations of Monitoring Sites ............................................................................5

CASTNET Reference Sites...............................................................................6

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

Estimating Dry and Wet Deposition...................................................................9

Sulfur Dioxide and Nitrogen Oxides Emissions .................................................9

Chapter 2 Atmospheric Concentrations.......................................................................12

Sulfur Dioxide .................................................................................................12

Particulate Sulfate...........................................................................................14

Ambient Ammonia Measurements at Beaufort, NC.........................................16

Total Nitrate ....................................................................................................19

Particulate Ammonium....................................................................................20

Border 2020: U.S.-Mexico Environmental Program.........................................22

Chapter 3 Atmospheric Deposition...............................................................................26

Hybrid Approach to Estimating Total Deposition .............................................27

Sulfur Deposition.............................................................................................32

Total Reactive Oxides of Nitrogen Measurements at Beaufort, NC.................36

Nitrogen Deposition ........................................................................................38

An Analysis of Critical Loads of Streams near Seven CASTNET Sites in 

Eastern United States.................................................................................42

Chapter 4 Ozone Concentrations..................................................................................45

National Ambient Air Quality Standards for Ozone..........................................46

Eight-Hour Ozone Concentrations ..................................................................47

In-Canopy Measurements of Ozone and Other Gases and Particles at 

Howland Research Forest, ME...................................................................52

W126 Values for 2012 ....................................................................................55

Ozone Monitoring System Performance Audits...............................................57

Chapter 5 Data Quality...................................................................................................62

Summary of Data Quality Results ...................................................................63

Precision.........................................................................................................63

Filter Pack Bias Comparison...........................................................................67

Accuracy.........................................................................................................76

Completeness.................................................................................................78

Results for 2012 Environment Canada Proficiency Testing.............................80

References .......................................................................................................................82

Appendix A Locational and Operational Characteristics of CASTNET Sites

Appendix B Acronyms and Abbreviations
CASTNET 2012 Annual Report

ii Executive Summary

Executive Summary

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

concentrations across the continental United States, Canada, and Alaska in order to evaluate the 

effectiveness of national and regional air pollution control programs. This report presents 2012

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 at 91 monitoring 

stations at 88 sites and measured O 

3 at 83 sites in the United States and Canada during 2012.

Key Results and Highlights through 2012

• Mean annual sulfur dioxide (SO 

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

significantly over the 23-year period 1990 through 2012 and showed a substantive reduction

since 2005. Three-year mean annual SO 

2 levels declined 76 percent while SO2-4

concentrations declined 56 percent at eastern reference sites from 1990 through 2012. SO 

2

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

4

levels dropped by 22 percent over the 17-year period (1996 through 2012).

• The three-year mean annual total (dry + wet) sulfur deposition estimated in the eastern 

United States declined by 63 percent from 1990–1992 through 2010–2012. Total sulfur 

deposition aggregated over western reference sites fell by 37 percent from 1996–1998 

through 2010–2012.

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

acid (HNO 

3) plus particulate NO3, declined 39 percent at eastern sites over the 23-year 

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

17 years.

• Three-year mean total nitrogen deposition [particulate NO3 (dry + wet) and HNO 

3] fell by 

24 percent from 1990 through 2012 at eastern reference sites. Total nitrogen deposition 

estimated for western reference sites was reduced by 20 percent over the 17 years. 

However, total nitrogen deposition increased significantly at eastern reference sites from 

2010 to 2011 because of increased nitrogen concentrations in precipitation and abovenormal precipitation. In 2012 precipitation declined while the nitrogen concentration in 

precipitation increased slightly over the 2011 level. The net result was a decline in the total 

nitrogen deposition rate at eastern reference sites.

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

3 concentration for 2012 at eastern 

reference sites was 70 parts per billion (ppb) or, equivalently, 0.070 parts per million (ppm), 

which was higher than the 2011 value of 69 ppb and the 2009 concentration of 64 ppb, the 

lowest in the history of the network. Six eastern and three California sites plus the site at 

Rocky Mountain National Park, CO (ROM406) recorded exceedances of the 8-hour standard 

of 0.075 ppm during the most recent three-year period (2010 through 2012).
CASTNET 2012 Annual Report

iii Executive Summary

• Measurements from 2012 and historical data collected over the period 1990 through 2011 

were analyzed relative to data quality indicators and their numerical measures, e.g., for 

precision, accuracy, completeness, bias, and comparability. The results of these analyses, 

which are discussed in Chapter 5, demonstrate that CASTNET data can be used with 

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

• In 2012, EPA operated 83 CASTNET sites that were compliant with the requirements 

of 40 CFR Parts 50, 53, and 58. The precision of O 

3 data was evaluated as per 

Part 58 requirements.

• As with the 2011 CASTNET Annual Report, dry deposition estimates were based on the 

method developed by Bowker et al. (2011). This approach combines measured 

concentrations with deposition velocities modeled using the Multi-Layer Model (MLM) along 

with historical averages to replace missing values.

• For the wet deposition component of total deposition, EPA used the Parameter-elevation 

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. PRISM-based 

precipitation data were combined with measured values of concentrations in precipitation from 

the National Atmospheric Deposition Program’s National Trends Network to obtain estimates 

of wet deposition.

• The MLM/Bowker estimates of dry deposition were combined with PRISM-adjusted values of 

wet deposition. The use of the MLM/Bowker approach and NTN/PRISM resulted in improved 

completeness of dry, wet, and total deposition estimates for 2012.
CASTNET 2012 Annual Report

1 Chapter 1: CASTNET Overview

The Clean Air Status and Trends Network (CASTNET) provides long-term monitoring of air quality in rural 

areas to determine trends in regional atmospheric sulfur, nitrogen, and ozone concentrations and deposition 

fluxes of sulfur and nitrogen pollutants. CASTNET is managed and operated by the Environmental Protection 

Agency’s Clean Air Markets Division in cooperation with the National Park Service and other federal, state, 

and local partners. The Bureau of Land Management began operating four CASTNET sites in Wyoming in 

2012. CASTNET was established under the 1990 Clean Air Act Amendments and continued and expanded 

the National Dry Deposition Network, which began in 1987. During 2012, the network operated 91 monitoring 

stations at 88 site locations throughout the contiguous United States, Alaska, and Canada. Measurements 

collected in 2012 show the continuing decline in sulfur and nitrogen pollutant concentrations. Ozone 

concentrations in the eastern United States increased relative to 2011. All pollutants have declined 

significantly over the last 23 years.

Chapter 1: C ASTN ET Over view

Background

The U.S. Congress directed the U.S. Environmental Protection Agency (EPA) to institute 

CASTNET under Title IX of the 1990 Clean Air Act Amendments (CAAA) in order to assess the 

effectiveness of the Acid Rain Program (ARP) as emission reductions became effective in 1995 

and subsequent years. 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. CASTNET is sponsored 

by EPA and the National Park Service (NPS), which began its participation in 1994 and 

operated 26 sites in 2012. The Bureau of Land Management (BLM) began participation in 

CASTNET in late 2012 by operating four sites in Wyoming.

The U.S. Congress established the ARP under Title IV of the 1990 CAAA. The ARP was 

promulgated to reduce emissions of sulfur dioxide (SO 

2) and nitrogen oxides (NOx) from electric 

generating units (EGUs). 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 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 equals 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 

program was phased in with the final 2010 SO 

2 emissions cap set at 8.95 million tons, a level 

about one-half of the total SO 

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

from EGUs in 2012 were about 3.3 million tons.
CASTNET 2012 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 2012 were about 1.7 million tons. Although the ARP was responsible for a 

large portion of the 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), the Clean Air Interstate Rule (CAIR), and 

regional and state NOx emission control programs.

CAIR was promulgated in 2005 and was designed to reduce emissions of SO 

2 and NOx in 

27 eastern states and the District of Columbia. This rule created three separate trading 

programs: an annual NOx program, an ozone (O 

3) season NOx program, and an annual SO 2

program. The CAIR O 

3 seasonal and annual NOx programs began in 2009 while the CAIR 

annual SO 

2 program began in 2010. CAIR addresses regional interstate transport of O 3 and fine 

particles. CAIR benefits human health and results in improved visibility/visual quality in national 

parks and improved quality of freshwater ecosystems in the eastern United States.

In 2012, the network included 91 monitoring stations operated at 88 site locations throughout 

the contiguous United States, Alaska, and Canada. In mid-April 2012, NPS installed a 

collocated O 

3 analyzer at Yosemite National Park (YOS204, CA). The Howland, ME (HOW132) 

site was closed at the end of October 2012. As of November 1, 2012, the 23.5-meter (m) inlet at 

the Howland, ME (HOW191) AmeriFlux site was designated as the primary filter pack 

measurement for the Howland area. During November, new filter pack measurement systems 

were established in Vermont, New York, and Wyoming. The specific start dates and other 

locational and operational information for these new sites can be found in Appendix A to this 

report. The four Wyoming sites are sponsored by BLM and are operated to support the 

Wyoming Air Resources Monitoring System (WARMS). The two new New York sites are 

sponsored by the New York State Department of Environmental Conservation (NYSDEC) and 

New York State Energy Research and Development Authority (NYSERDA). The site in Vermont 

is sponsored by EPA.

The three new eastern sites are small footprint, filter pack-only sites. These sites do not have 

traditional CASTNET shelters and use pole-mounted “boxes” instead. See the photographs on 

page 3 for an example of the equipment box used in place of a shelter at a small footprint site.

New Filter Pack Site Locations and Names

Eastern Western

Nicks Lake, NY (NIC001) Basin, WY (BAS601)

Underhill, VT (UND002) New Castle, WY (NEC602)

Whiteface Mountain, NY (WFM105) Buffalo, WY (BUF603)

Sheridan, WY (SHE604)
CASTNET 2012 Annual Report

3 Chapter 1: CASTNET Overview

Additionally, trace-level gas monitoring for SO 

2, nitrogen oxide/total reactive oxides of nitrogen 

(NO/NOy), and carbon monoxide (CO) continued during 2012. During the last half of 2012, 

trace-level gas monitoring was added to the BVL130, IL and HWF187, NY sites. The three 

CASTNET sites with trace-level gas monitoring are now using Teledyne Advanced Pollution 

Instrumentation. The trace-level gas analyzers and data validation procedures meet all 

regulatory requirements in Title 40 of the Code of Federal Regulations (CFR). Data are 

submitted to EPA’s Air Quality System (AQS). The sites and the parameters measured at each 

site include:

Trace-level Gas Monitoring Sites

Site Location and Name Measurements

Bondville, IL (BVL130) CO, SO 

2 and NO/NOy

Huntington Wildlife Forest, NY (HWF187) NO/NOy

Beltsville, MD (BEL116) SO 

2 and NO/NOy

This report summarizes CASTNET monitoring activities and the resulting concentration and 

deposition data collected over the 23-year period from 1990 through 2012. Additional 

information, previous annual reports, other CASTNET documents, and the CASTNET database 

can be found on the EPA website, http://www.epa.gov/castnet/. The website provides a 

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

CASTNET sites.

Interior of box

Underhill, VT (UND002)
CASTNET 2012 Annual Report

4 Chapter 1: CASTNET Overview

Cooperating Networks

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

► National Atmospheric Deposition Program (NADP) operates: 

■ 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.

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

precipitation. MDN samplers are operated at several CASTNET sites. 

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

3) samplers 

at about 40 CASTNET locations. AMoN, in operation for four years, provides information on 

2-week average NH 

3 concentrations.

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

gaseous oxidized, particulate-bound, and elemental mercury at about 18 locations in the 

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

mercury deposition.

See NADP’s website 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 website at http://www.ec.gc.ca/rsmn/default.asp?lang=En&n=752ce271-1.

► EPA’s National Core Monitoring (NCore) operates trace-level measurement systems for SO 

2, 

NO/NOy, and CO. NCore also collects O 

3, particulate matter (PM), PM species, and 

meteorological measurements at approximately 80 sites.

► BLM’s Wyoming Air Resources Monitoring System (WARMS) operates eight sites in 

Wyoming. Four of the WARMS sites are CASTNET-protocol sites.

► 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 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 partners that play a role 

in sponsoring individual sites include BLM, Native American tribes, universities, and government 

agencies. In all, CASTNET has more than 50 partners.
CASTNET 2012 Annual Report

5 Chapter 1: CASTNET Overview

Locations of Monitoring Sites

The locations of CASTNET monitoring sites that were operational during 2012 are depicted in 

Figure 1-1. Ninety-one monitoring sites were operated at 88 distinct locations. To estimate 

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

and Rocky Mountain National Park, CO (ROM406/206) during 2012. The ROM406/206 monitors 

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

ROM206, and NPS operates ROM406. Of the two Rocky Mountain monitoring sites, ROM406 is 

considered the regulatory monitoring site for O 

3. In 2012, NPS installed a collocated O 3 analyzer 

at YOS204, CA. 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, filter pack 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.

Parsons, WV (PAR107)
CASTNET 2012 Annual Report

6 Chapter 1: CASTNET Overview

Figure 1-1 CASTNET Sites Operational During 2012

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.
CASTNET 2012 Annual Report

7 Chapter 1: CASTNET Overview

Figure 1-2 CASTNET Western and Eastern Reference Sites

To determine trends in concentrations and rates of dry, wet, and total deposition, the data from 

the 34 eastern reference sites were aggregated and then presented using box plots (e.g., 

Figure 2-2) for the 23-year period 1990 through 2012. Data from the 17 western reference sites 

were aggregated and then presented using box plots for the period 1996 through 2012.

Centennial, WY (CNT169)
CASTNET 2012 Annual Report

8 Chapter 1: CASTNET Overview

Measurements Recorded at CASTNET Sites

Each CASTNET site measures weekly concentrations of acidic pollutants, base cations, and 

chloride (Cl-

) using a 3-stage filter pack with a controlled flow rate (AMEC, 2012d). The filter 

pack is changed out each Tuesday and shipped to the analytical chemistry laboratory for 

analysis. Most CASTNET sites also include a temperature controlled shelter and continuous O 

3

monitoring system. The O 

3 inlet and filter pack are located atop a 10-m tower.

Network Summary

Number of Sites 91 at 88 locations

Weekly Ambient Measurements

Gaseous Sulfur dioxide (SO 

2)

Nitric acid (HNO 

3)

Particulate Sulfate (SO2-

4 )

Nitrate (NO3)

Ammonium (NH+ 

4 )

Base cations: magnesium (Mg2+

 ), 

calcium (Ca2+

 ), potassium (K

+ ), 

sodium (Na+ ), 

Chloride (Cl-)

Hourly Measurements Ozone (O 

3)

Shelter temperature

9-m temperature

Trace-level SO 

2*

Trace-level carbon monoxide (CO)*

nitrogen oxide/total reactive

oxides of nitrogen (NO/NOy)*

Other Meteorological 2-m temperature

Parameters** Wind speed and direction

(1-hour averages) Sigma theta

Solar radiation

Relative humidity

Precipitation

Surface wetness

* Measurements conducted at select locations.

** Meteorological measurements were discontinued in 2010 at most EPA- sponsored CASTNET 

sites. In 2012, meteorological parameters were measured at 6 EPA-, 25 NPS- and 4 BLMsponsored CASTNET sites.
CASTNET 2012 Annual Report

9 Chapter 1: CASTNET Overview

Estimating Dry and Wet Deposition

Dry deposition for 2012 was estimated using the method developed by Bowker et al. (2011), 

which was summarized in the 2011 CASTNET Annual Report (AMEC, 2013a). This approach 

combines measured concentrations with deposition velocities modeled using the Multi-Layer 

Model (MLM). When deposition velocities are unavailable due to data completeness or validity 

issues, historical deposition velocities are used as a substitute. The new hybrid approach (EPA, 

2013d), which incorporates air quality monitoring data with Community Multiscale Air Quality 

Modeling System (CMAQ) output, will be used for future spatial analyses of total deposition. The 

hybrid approach is summarized in the first callout in Chapter 3.

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. 

PRISM uses terrain elevation, slope, and aspect and climatic measurements to estimate 

precipitation on a 4-kilometer (km) resolution grid. Pollutant concentrations in precipitation were 

provided by NADP/NTN. The precipitation amount and pollutant concentration 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. Dry and wet deposition 

fluxes were added to obtain the estimates of total deposition that are discussed in Chapter 3.

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, 2013b; 2013c; 2012b) on the effects of the ARP and related programs such as the 

CAIR and the NBP.

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.

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

2 and NOx. Phase I required 

substantive SO 

2 emission reductions from the largest 435 coal-fired EGUs from 1995 through 

1999 and from 1996 through 1999 for NOx emissions. Phase II for both pollutants began in 

2000. In 2012, the Phase II ARP SO 

2 requirements affected 3,652 operating units. The Phase II 

ARP NOx requirements applied to 900 of those operating units that exceeded 25 megawatts and 

burned coal between 1990 and 1995 (EPA, 2013b). The CAIR NOx O 

3 season program affected 

3,273 units in 2012, and the NOx and SO 

2 annual programs affected 3,336 units. Under the 

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

2 emissions from affected sources 
CASTNET 2012 Annual Report

10 Chapter 1: CASTNET Overview

were 3.3 million tons in 2012; total annual NOx emissions from ARP and CAIR sources were 

1.7 million tons.

Figure 1-3 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 2012). The states with the highest 

emitting sources in 1990 have typically seen the greatest reductions in SO 

2 emissions under the 

ARP, and this trend has continued under CAIR. From 1990 to 2012, annual SO 

2 emissions 

dropped in 43 states and D.C. by a total of approximately 12 million tons. The most significant 

reductions in SO 

2 emissions achieved between 2005 and 2012 were realized in Massachusetts 

(91 percent), North Carolina (89 percent), Florida (86 percent), and West Virginia (83 percent).

Annual NOx emissions by state are depicted in Figure 1-4 for the same six years. Between 

1990 and 2012, all states participating in the CAIR NOx annual program decreased their NOx

emissions. The greatest NOx emissions reductions between 2000 and 2012 were realized in 

Ohio (78 percent reduction).

Oxford, OH (OXF122)
CASTNET 2012 Annual Report

11 Chapter 1: CASTNET Overview

Figure 1-3 Annual SO2 Emissions (for CAIR and ARP Sources)

Figure 1-4 Annual NOx Emissions (for CAIR and ARP Sources)
CASTNET 2012 Annual Report

12 Chapter 2: Atmospheric Concentrations

Three-stage filter packs were used to sample weekly average concentrations of SO 

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

Cl-

, and 4 base cations at 91 CASTNET monitoring stations in 88 locations. HNO 

3 and particulate NO3 were 

summed to estimate total NO3. The sulfur and nitrogen pollutants measured at the 34 eastern reference sites 

declined over the 23-year period from 1990 through 2012. Measured annual mean concentrations of SO 

2 and 

SO2-

4 have decreased steadily with major annual reductions 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 17 years.

Chapter 2: Atmospheric C oncentrations

This chapter presents maps of 2012 annual mean concentrations of SO 

2, SO2-4 , total NO3

(HNO 

3 + NO3), and NH+ 4 . Additional maps are provided in CASTNET quarterly reports 

(AMEC, 2012b; 2012e; 2013b; 2013e). Trends in annual mean concentrations over the 23-year 

period (1990 through 2012) were derived from measurements from the 34 CASTNET eastern 

reference sites (Figure 1-2) and for the period 1996 through 2012 from data collected at 

17 CASTNET western reference sites (Figure 1-2). This chapter also includes information on 

NH 

3 measurements collected at Beaufort, NC (BFT142) and summarizes the United StatesMexico Border 2020 Program.

Sulfur Dioxide

SO 

2 is a major air pollutant that affects human health and plant ecosystems. It is emitted during 

the combustion of coal, oil, and other fossil fuels that contain sulfur. EGUs constitute the largest 

source (69 percent) of SO 

2 in the United States (EPA, 2013a). Industrial facilities that fire fossil 

fuels and extract metal from ore compose another source (20 percent) of SO 

2 emissions 

(EPA, 2013a). Some transportation sources that combust high sulfur fuels, such as ships, 

locomotives, and off-road equipment, also discharge SO 

2.

Since SO 

2 is a criteria pollutant, concentrations are required to not exceed the National Ambient 

Air Quality Standard (NAAQS), which was promulgated on June 2, 2010. For compliance, 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]. Gas phase SO 

2 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.
CASTNET 2012 Annual Report

13 Chapter 2: Atmospheric Concentrations

Annual mean SO 

2 concentrations measured in 2012 are shown in Figure 2-1. No sites in the 

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

City, OH (QAK172) and Horton Station, VA (VPI120) measured the highest concentration of 

SO 

2 at 3.8 μg/m3 . SO 2 concentrations recorded in 2012 were lower than SO 2 levels measured in 

previous years. One simple comparison is to calculate the average of annual mean SO 

2

concentrations measured in 2012 versus 2011 at 10 monitoring stations in Indiana, Ohio, and 

Pennsylvania. The 2012 average concentration for these 10 sites was 2.6 μg/m3 

versus an

average concentration of 3.8 μg/m3 in 2011. Concentrations measured at the western sites in 

2012 were generally lower than those measured at the eastern sites with no SO 

2 concentrations 

greater than 1.0 μg/m3 

.

Figure 2-1 Annual Mean SO2 Concentrations (μg/m3

) for 2012

Figure 2-2 shows box plots of annual mean SO 

2 concentrations aggregated over the 34 eastern 

reference sites from 1990 through 2012 (right side) and the 17 western reference sites from 

1996 through 2012 (left side). Because concentrations measured at the western CASTNET 

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 2012. Three-year mean concentrations for the eastern reference sites for 
CASTNET 2012 Annual Report

14 Chapter 2: Atmospheric Concentrations

1990–1992 and 2010–2012 were 8.9 μg/m3 and 2.1 μg/m3 , respectively. This change 

constitutes a 76 percent reduction in 3-year mean SO 

2 concentrations between the two periods. 

The 2012 mean level of 1.5 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 seven 

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 2010–2012 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 17 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 SO2 Concentrations (μg/m3

)

Western Reference Sites Eastern Reference Sites

Particulate Sulfate

Sulfate exists in the atmosphere primarily as (NH 

4)

 

2SO 4, a major component of PM2.5. Particulate 

SO2-

4 is produced by the transformation of SO 2 in the atmosphere through both gas and aqueous 

(cloud) phase reactions. A map of annual mean particulate SO2-

4 concentrations measured 

during 2012 is shown in Figure 2-3. The map shows the highest SO2-

4 concentrations were 

measured in or near the Ohio River Valley. A majority of the CASTNET filter pack samplers in 

the eastern United States measured SO2-

4 levels greater than or equal to 2.0 μg/m3 

. SO2-

4

concentrations measured at the eastern CASTNET sites were higher than those measured at 

western sites. In the West, Big Bend National Park, TX (BBE401); Palo Duro Canyon State 

Park, TX (PAL190); Joshua Tree National Park, CA (JOT403); and Sequoia National Park, CA 

(SEK430) measured SO2-

4 concentrations greater than or equal to 1.0 μg/m3 

.

0.0

0.5

1.0

1.5

19961997199819992000200120022003

2004

20052006200720082009201020112012

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Mean

25th Percentile

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0

2

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18

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CASTNET 2012 Annual Report

15 Chapter 2: Atmospheric Concentrations

Figure 2-3 Annual Mean SO2-

4 Concentrations (μg/m3

) for 2012

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 23 years, 

and, in particular, concentrations declined significantly from 2005 through 2012. The difference 

between 3-year means from 1990–1992 to 2010–2012 represents a 56 percent reduction in 

SO2-

4 from 5.4 μg/m3 to 2.4 μg/m3 , respectively. The 2012 mean SO2-4 level of 2.1 μg/m3 was the 

lowest in the history of the network. 

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

The boxes show a 22 percent reduction in annual mean SO2-

4 concentrations aggregated 

over the 17 sites with 1996–1998 and 2010–2012 concentrations of 0.8 μg/m3 and

0.6 μg/m3 

, respectively.
CASTNET 2012 Annual Report

16 Chapter 2: Atmospheric Concentrations

Figure 2-4 Trend in Annual Mean SO2-

4 Concentrations (μg/m3

)

Western Reference Sites Eastern Reference Sites

Ambient Ammonia Measurements at Beaufort, NC

CASTNET collected measurements of ambient ammonia (NH 

3) at Beaufort, NC (BFT142) in order 

to supplement filter pack and AMoN measurements. The goals of the Beaufort study were to 

continue and expand analyses started during the Ammonia CASTNET CSN Study (ACCS). The 

Beaufort location expanded ACCS to include a coastal site located in an area of interest for NH 

3

monitoring because of the proximity to agricultural sources (animal and crop production) in eastern 

North Carolina.

Reference ammonia concentrations were measured using a modified annular denuder system (ADS) 

that included a phosphorus acid-coated annular denuder for collecting gaseous NH 

3. The Beaufort 

sampling included:

■ Seven daily ADS (one for each day) that ran only for the first week of the 2-week 

sampling period,

■ One field blank, and

■ Two weekly ADS (one for each week) that ran for one week in conjunction with the daily ADS 

and then for a second consecutive week for comparison with AMoN.

The ADS was designed for automated and unattended operation and required only a weekly site 

operator visit. The weekly ADS samples were run using a flow rate of 3.0 lpm compared with a flow 

rate of 16.7 lpm normally used for daily ADS sampling. The denuders were analyzed by automated 

colorimetry using distilled water as the extraction fluid. Chemical analysis of the denuders was 

performed by the CASTNET laboratory.

ADS sampling began in May 2011 and was completed in November 2012. Similar to ACCS, the 

Beaufort sampling included exposing a weekly ADS for two consecutive 1-week periods during the 

designated measurement weeks. The daily ADS ran for a 24 hour period at 16.7 lpm each day of the 

first week of each 2-week measurement period to provide better temporal resolution. The ADS used a 

cyclonic separator with a 2.5 micrometer (μm) size cut to remove larger particles. The ADS (daily, 

weekly, and field blanks) were installed and operated at 10 m. Figure 2a provides photographs of the 

BFT142 site configuration.

0.0

0.5

1.0

1.5

2.0

199619971998199920002001200220032004200520062007

2008

2009201020112012

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

5

6

7

8

19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET 2012 Annual Report

17 Chapter 2: Atmospheric Concentrations

Figure 2b shows the weekly and daily ADS NH 

3 concentrations from the Beaufort study along with 

AMoN NH 

3 concentrations. The figure also shows ADS concentrations that were averaged from daily 

to weekly values. The daily ADS samples (black diamonds) showed greater variability than the weekly 

integrated samples. Daily ADS concentrations averaged over a week (red x) matched the weekly ADS 

levels (green squares) very well, indicating that weekly ADS run at reduced flow rates is a viable

sampling method.

The 2-week average ADS NH 

3 concentrations also showed good agreement with the integrated 

concentration measurements made at the AMoN site (NC06) as shown in Figure 2b. A linear 

regression based on ADS values aggregated to two weeks versus the 2-week NC06 values gives a 

regression equation of

y = 0.7792x + 0.0963,

with an R2 value of 0.8178. This comparison demonstrates that AMoN provides viable measurements 

of atmospheric NH 

3 concentrations.

Figure 2a Annular Denuder System and Related Sampling Systems at the BFT142 Site

Note: 1 NADP/NTN precipitation sampling systems

2 AMoN sampler

3 Meteorological tower

4 NOy converter tower

5 Filter pack and ADS tower

ADS tower configuration Solenoids used to control ADS flow

Beaufort, NC (BFT142)

3

1

4

5

2
CASTNET 2012 Annual Report

18 Chapter 2: Atmospheric Concentrations

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

4/24/115/24/116/23/117/23/118/22/119/21/11

10/21/11

11/20/11

12/20/11

1/19/122/18/123/19/124/18/125/18/126/17/127/17/128/16/129/15/12

10/15/12

11/14/12

Concentration (μg/m3)

AMoN

Weekly

Daily

Daily Averaged to Weekly

Figure 2b Time Series of Measured NH 

3 Concentrations (μg/m3 )

Yosemite National Park, CA (YOS404)
CASTNET 2012 Annual Report

19 Chapter 2: Atmospheric Concentrations

Total Nitrate

The sum of HNO 

3 and particulate NO3 is defined herein as total NO3. HNO 3 and NO3 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 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 ammonium nitrate (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 its constituents are in equilibrium with each other 

and total NO3 is considered to be 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. 

A map of annual mean total NO3 concentrations for 2012 is provided in Figure 2-5. The map 

shows annual mean concentrations greater than 3.0 μg/m3 were sampled in Illinois and Indiana. 

In the West, the highest concentrations (2.7 μg/m3 and 2.3 μg/m3 ) were measured in California 

at SEK430 and JOT403, respectively. The Salamonie Reservoir, IN (SAL133) site measured the 

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

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

) for 2012
CASTNET 2012 Annual Report

20 Chapter 2: Atmospheric Concentrations

Figure 2-6 provides box plots of total NO3 levels for the eastern and western reference sites

through 2012. 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 1990–1992 to 2010–2012, from 3.0 μg/m3 to 1.8 μg/m3 , producing a 

39 percent reduction in total NO3. Total NO3 levels measured at the eastern reference sites 

declined from a mean value of 3.1 μg/m3 in 2000 to a mean value of 1.8 μg/m3 in 2012.

Data aggregated from the 17 western sites are shown on the left side of Figure 2-6. The 3-year 

mean total NO3 concentration for 2010–2012 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. 

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

)

Western Reference Sites Eastern Reference Sites

Particulate Ammonium

Particulate NH+ 

4 is formed when gaseous NH 3 reacts with acidic gases, including HNO 3 and 

H2SO4, in the atmosphere. The primary components of particulate NH+ 

4 are (NH 4)

 

2SO 4 and 

NH 

4NO 3. A map of 2012 annual mean NH+ 4 concentrations is given in Figure 2-7. 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 two-thirds 

of the sites reporting values less than or equal to 1.0 μg/m3 . Higher concentrations were 

measured at eastern CASTNET sites in Illinois, Indiana, Michigan, Ohio, Pennsylvania, and 

Kentucky. The NH+ 

4 concentrations measured at western CASTNET sites were less than 

1.0 μg/m3 

.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

19961997199819992000200120022003200420052006200720082009201020112012

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

5

6

7

8

19901991199219931994199519961997

19981999

2000200120022003200420052006200720082009

20102011

2012

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET 2012 Annual Report

21 Chapter 2: Atmospheric Concentrations

Figure 2-7 Annual Mean NH+

4 Concentrations (μg/m3

) for 2012

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 2010–2012. The 1990–1992 mean 

concentration was 1.8 μg/m3 , and the 2010–2012 value was 0.9 μg/m3 , a 46 percent decline. 

The box plots characterizing the western sites show about an 18 percent reduction from 

1996–1998 to 2010–2012.

Figure 2-8 Trend in Annual Mean NH+

4 Concentrations (μg/m3

)

Western Reference Sites Eastern Reference Sites

0.0

0.1

0.2

0.3

0.4

0.5

0.6

19961997199819992000200120022003200420052006200720082009201020112012

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

199019911992

19931994

1995199619971998

19992000

20012002200320042005

2006

20072008200920102011

2012

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET 2012 Annual Report

22 Chapter 2: Atmospheric Concentrations

Border 2020: U.S.-Mexico Environmental Program

In 2012, the United States and Mexico established Border 2020: U.S.-Mexico Environmental Program 

to address the environmental and public health problems in the U.S.-Mexico border region. Border 

2020 is a continuation of the Border 2012 program. Border 2020 established six strategies and five 

long-term goals (EPA, 2012c). The goals are:

1. Reduce air pollution,

2. Improve access to clean and safe water,

3. Promote materials and waste management and clean sites,

4. Enhance joint preparedness for environmental response, and

5. Assure environmental compliance and stewardship.

This discussion focuses on the first goal and describes air quality data collected in the border region 

and their trends.

Figure 2c provides a map of the border region and shows the locations of four nearby CASTNET sites. 

Chiricahua National Park (CHA467) is located in southeastern Arizona and has been operating since 

April 25, 1989. Big Bend National Park, TX (BBE401) and Alabama-Coushatta, TX (ALC188) have 

been operating since July 18, 1995 and April 2, 2004, respectively. Palo Duro Canyon State Park, TX 

(PAL190) has been operating since April 24, 2007. CHA467 and BBE401 also operate IMPROVE 

visibility-related monitoring sites. CHA467 and BBE401 are located within the designated 

border region.

Figure 2c U.S.-Mexico Border Region

Source: EPA

A field data collection and data analysis study (Pitchford et al., 2004) was performed from July through 

October 1999 to understand atmospheric haze in Big Bend National Park (BBNP). The study was 

titled, “Big Bend Regional Aerosol and Visibility Observational (BRAVO) Study,” and it was sponsored

by NPS, EPA, and the Texas Commission on Environmental Quality (TCEQ). BRAVO scientists 
CASTNET 2012 Annual Report

23 Chapter 2: Atmospheric Concentrations

concluded that SO2-

4 particles were the single largest contributor to haze in BBNP. The haze in the park 

peaked in the spring months when both SO2-

4 and carbonaceous particles contributed to the haze and 

in the early fall months (August–October) when SO2-

4 particles dominated. Key BRAVO findings can be 

found in the 2004 report (Pitchford et al., 2004).

The findings include: 

■ Eastern United States and eastern Texas sources of SO 

2 were the largest contributors to peak 

SO2-

4 episodes during BRAVO.

■ On average, Mexican sources contributed about one-fifth of the total SO2-

4 . In particular, the 

Carbon I and II power plants, which are located at Nava, Coahuila, Mexico, contributed to SO2-

4

levels during easterly winds toward BBNP.

■ The least hazy days at BBNP were produced during westerly winds.

The CASTNET sites CHA467 and BBE401 began operating before BRAVO began and provide a good 

record of trends in sulfur and nitrogen pollutants and O 

3 in the border region. Figure 2d depicts 

3-month (August–October) average SO2-

4 concentrations for the two border region CASTNET sites plus 

ALC188 and PAL190. ALC188 reported the highest August–October SO2-

4 concentrations. Figure 2e

shows annual average concentrations of total NO3 for the sites; Figure 2f provides fourth highest daily 

maximum 8-hour average (DM8A) O 

3 concentrations over the histories of the sites.

Figure 2d shows overall reductions in 3-month average SO2-

4 concentrations over the lifetime of the 

monitoring at the sites. Data from the two sites (BBE401 and CHA467) in the border region are 

presented using solid lines. Data from the other sites are presented using dashed lines. PAL190 data 

were not analyzed for trends because of its limited data record. The percent declines were 26 percent 

for CHA467, 16 percent for BBE401, and 35 percent for ALC188. The downward trends in 3-month 

average SO2-

4 concentrations resulted from SO 2 emission reductions in Texas and nearby states 

(Figure 1-3).

The data from two of the monitoring sites (Figure 2e) show decreases in total NO3: 12 percent for 

BBE401 and 11 percent for ALC188. An 18 percent increase was measured at CHA467.

Trends in fourth highest DM8A O 

3 concentrations for the sites are shown in Figure 2f. The 75 ppb 

NAAQS is identified in the figure by a black line. Fourth highest DM8A O 

3 concentrations averaged 

less than 75 ppb with the exception of 78 ppb at ALC188 in 2008 and 80 ppb at BBE401 in 2011, both 

during periods with extensive wildfires.

Relatively high annual mean total NO3 and fourth highest DM8A O 

3 concentrations were observed in 

2011 with moderate declines in 2012. The high 2011 annual total NO3 and DM8A O 

3 concentrations 

measured at the sites were likely influenced by frequent wildfires over Texas and Arizona during the 

summer and fall fire season (AMEC, 2013a). 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 O 

3 season. The fires continued into the 2012 fire season. 

These wildfires affected air quality in the southwestern states and the border region; some plumes 

were transported all the way to the Baltimore-Washington metropolitan area (AMEC, 2013a).
CASTNET 2012 Annual Report

24 Chapter 2: Atmospheric Concentrations

0

1

2

3

4

5

6

19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012

Concentration (μg/m3)

ALC188, TX

BBE401, TX

CHA467, AZ

PAL190, TX

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2.0

2.5

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Concentration (μg/m3)

ALC188, TX

BBE401, TX

CHA467, AZ

PAL190, TX

55

60

65

70

75

80

85

199019911992199319941995199619971998199920002001

2002

2003200420052006200720082009201020112012

Concentration (ppb)

ALC188, TX

BBE401, TX

CHA467, AZ

PAL190, TX

Figure 2d Three-month (August–October) Average SO2-

4 Concentrations

Figure 2e Annual Average Concentrations of Total NO3 (HNO 

3 + NO3)

Note: Total NO3 concentrations measured at all four sites were influenced by wildfires in 2011.

Figure 2f Fourth Highest DM8A O 

3 Concentrations

Note: Fourth highest DM8A O 

3 concentrations measured at BBE401, PAL190, and CHA467 in 2011 were influenced 

by wildfires.
CASTNET 2012 Annual Report

25 Chapter 2: Atmospheric Concentrations

CASTNET measurements have shown that SO2-

4 concentrations have declined in the border region. 

Total NO3 levels declined at BBE401 and ALC188 and increased at CHA467. Fourth highest DM8A O 

3

concentrations remained fairly steady over the monitoring periods. Both NO3 and O 

3 levels were 

elevated during periods with frequent wildfires.

Alabama-Coushatta, TX (ALC188)

Big Bend National Park, TX (BBE401) Chiricahua National Park, AZ (CHA467)
CASTNET 2012 Annual Report

26 Chapter 3: Atmospheric Deposition

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

the United States. CASTNET currently estimates dry deposition by combining measured pollutant 

concentrations and modeled deposition velocities (Vd). The MLM/Bowker method was used to produce values 

of hourly Vd, which were combined with values of air pollutant concentrations to produce dry deposition fluxes. 

PRISM was used 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 73 percent over the period 

from 1990–1992 to 2010–2012. Three-year mean dry sulfur deposition estimated for the 17 western reference 

sites declined by 40 percent over the period 1996–1998 to 2010–2012. Dry nitrogen deposition estimates 

declined 46 percent for the eastern reference sites and 36 percent at the western reference sites. In 

summary, sulfur and nitrogen deposition are at historic lows.

Chapter 3: Atmospheric D epositi on

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. Dry flux values were 

estimated as the product of Vd estimates produced by the MLM and measured concentration 

data. In 2012, CASTNET adopted a Vd data substitution method developed by Bowker et al. 

(2011) to fill in long-term averages of Vd for missing values to improve data completeness. This 

method resulted 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. The grid of wet deposition data was combined with the 

MLM/Bowker point estimates of dry deposition data to estimate total deposition.

The MLM/Bowker and NTN/PRISM approaches were used to produce the deposition data to 

generate the maps and trends analyses for this annual report. For future spatial analyses of 

total deposition, the hybrid model, which is also called the Total Deposition Science Committee 

(TDEP) model (EPA, 2013d), will be used. The hybrid model is summarized in the first callout in 

this chapter.
CASTNET 2012 Annual Report

27 Chapter 3: Atmospheric Deposition

Hybrid Approach to Estimating Total Deposition

A hybrid approach has been developed for estimating total deposition of nitrogen and sulfur and 

producing data products of deposition for use by the scientific community. The development of the 

hybrid approach and map production efforts are part of the specific charges for the Total Deposition 

Science Committee (TDEP), which was officially established in October 2011 as part of the NADP. 

The committee was formed by a group of interested scientists in order to discuss scientific and other 

issues related to estimating total deposition across North America. The mission of TDEP is to improve 

estimates of atmospheric deposition by advancing the science of measuring and modeling 

atmospheric wet, dry, and total deposition of pollutant species, such as sulfur, nitrogen, and mercury, 

by providing a forum for exchange of information on current and emerging issues within a broad multiorganization context, including atmospheric and ecosystem scientists, resource managers, and policy 

makers. TDEP coordinates its efforts with the Critical Loads of Atmospheric Deposition Science 

Committee (CLAD) and other groups to advocate use of the most scientifically defensible deposition 

estimates for critical loads and other environmental assessments. 

The TDEP committee’s first effort involved a needs assessment that categorized work and research 

currently being conducted, described projects and project status, and listed partner organizations. 

Compiling these types of information served the primary purpose of prioritizing early actions for TDEP 

including the generation of total sulfur and nitrogen deposition estimates as a grid-based product.

The hybrid modeling approach summarized here has been submitted for publication in a scientific 

journal. For more information on the approach, see the Total Deposition Project (EPA, 2013d). In the 

meantime, the MLM/Bowker method for dry deposition estimates was used in this report and will 

continue to be used until the hybrid approach becomes operational in the CASTNET program.

The hybrid approach uses a combination of actual air pollutant measurements and multi-year average 

Community Multiscale Air Quality Modeling System (CMAQ; Byun and Schere, 2006) modeled results. 

Developers of the hybrid approach methodology (EPA, 2013d) determined this to be the most practical 

method for producing relatively near-term total deposition estimates while avoiding the limitations (e.g.,

three- to five-year time lag and use of historical data) of constructing the emission inventories and 

meteorological databases required for atmospheric chemistry and transport models, such as CMAQ. 

Although actual field measurements are more accurate than modeled results for specific locations, it is 

beneficial to fill in spatial and chemical gaps in the measurements with modeled results in order to 

improve coverage for the continental United States. CMAQ is currently the most accessible modeling 

system for use by CASTNET and TDEP and was, therefore, selected for the hybrid approach. 

CMAQ output for 2002 through 2009 was used during the development of the methodology and 

initial releases.

The monitoring data used to support the dry deposition estimates included CASTNET air pollutant 

concentrations of HNO 

3, SO 2, NO3, NH+ 4 , SO2-4 ; AMoN NH 3 data; and the Southeastern Aerosol 

Research and Characterization Network (SEARCH) rural concentrations of HNO 

3, SO 2, and NH 3. 

SEARCH measurements are collected in Alabama, Florida, Georgia, and Mississippi. The CMAQ 

Centers for Disease Control (CDC) Public Health Air Surveillance Evaluation (PHASE; CDC, 2013) 

runs were used for generating the modeling data for CMAQ v4.7 for 2002 through 2006. These runs 

utilized Pennsylvania State University/National Center for Atmospheric Research MM5 mesoscale 

model meteorological data and 2002 National Emissions Inventory (NEI) emissions with year-specific 

major point sources, mobile sources, fires, biogenic and agricultural activities. CMAQ v4.7.1 was used 

for 2007 through 2009 with Weather Research and Forecasting (WRF) Model meteorological data, 

National Land Cover Database (NLCD) 2001 land use, and the NEI 2005 emissions for 2007, and NEI 

2008 emissions for years 2008 and 2009. Year-specific emissions from major point sources, 
CASTNET 2012 Annual Report

28 Chapter 3: Atmospheric Deposition

Cranberry, NC (PNF126)

mobile sources, fires, and biogenic and agricultural sources were utilized in the modeling. For wet 

deposition estimates, NADP/NTN concentrations in precipitation, including SO2-

4 , NO3, and NH+ 4 , were 

combined with precipitation measurements from NADP sites and the PRISM precipitation product 

using the method previously developed and described in the 2010 CASTNET Annual Report (AMEC, 

2012a). Modeled air pollutant concentrations were adjusted for bias where sufficient monitoring data 

were available.

The specific hybrid approach involved calculation of hourly mean, minimum, maximum, and standard 

deviation of Vd, concentration, and flux values of nitrogen and sulfur species from the CMAQ runs for 

eight years, 2002 through 2009. Hourly data were then aggregated to the standard Tuesday to 

Tuesday CASTNET week. Weekly ambient air concentrations were estimated using data from the 

networks listed previously using inverse-distance weighting applied to 12 km grids. Estimations of 

the weekly dry deposition fluxes were calculated by multiplying the air concentration grids by the 

CMAQ Vd values to create deposition product grids for measured species. The grid resolution for the 

CMAQ Vd values was 36 km for 2002–2006 and 12 km for 2007–2009. The 2002–2006 data were 

regridded to a 12-km resolution. For the years 2002–2008, the year-specific weekly average 

concentration was multiplied by the weekly average Vd for the specific year. For 2000–2001 and 

2010–2012, the concentrations were multiplied by weekly average Vd determined from the years 

2002 and 2009 data, respectively.

The final steps consisted of summing the weekly dry deposition values into annual aggregations for all 

measured species and the addition of the CMAQ-modeled deposition of unmeasured species [nitrous 

acid (HNO 

2), dinitrogen pentoxide, NO, nitrogen dioxide (NO 2), peroxyacytyl nitrate (PAN), 

peroxypropionyl nitrate (PANx), and organic 

nitrates (NTR)] to obtain annual dry 

deposition fluxes. The sum of dry deposition 

and wet deposition estimates resulted in 

total deposition fluxes across the United 

States. For more information view 

ftp//ftp.epa.gov/castnet/tdep.

Figures 3a through 3d show example maps 

for 2012 of total sulfur (S) deposition as 

kilograms per hectare per year (kg/ha/yr);

percentage of total S deposition due to dry S 

deposition; total nitrogen (N) deposition as 

kg/ha/yr; and percentage of total N 

deposition due to dry N deposition. 

CASTNET, AMoN, and rural SEARCH sites 

are shown on the four maps. The shading in 

the figures indicates the magnitude of the 

fluxes and the percentages of dry 

deposition. Maps of 2012 dry and total 

deposition of sulfur and nitrogen based on 

the MLM/Bowker and the NADP/PRISM 

approaches are presented later in this 

chapter in Figures 3-1 through 3-10.
CASTNET 2012 Annual Report

29 Chapter 3: Atmospheric Deposition

The TDEP map in Figure 3a shows the highest total S deposition fluxes as kilograms per hectare 

(kg/ha) were observed along the Ohio River Valley from southern Illinois to New Jersey and from 

Tennessee south to Louisiana and west to Texas. High S fluxes were also observed in North Dakota. 

Figure 3b shows the percentages of total S deposition attributed to dry deposition.

As shown in Table 3a, the five highest total S fluxes in 2012 were modeled using TDEP and 

MLM/NTN/PRISM for the indicated CASTNET sites.

Table 3a Highest Total Sulfur Fluxes for 2012

TDEP MLM/NTN/PRISM

Site Flux (kg/ha) Percent Dry Deposition Flux (kg/ha) Percent Dry Deposition

VPI120, VA 17.0 84 4.6 42

QAK172, OH 14.1 70 8.1 40

LRL117, PA 12.8 70 3.6 26

KEF112, PA 12.5 70 4.8 20

PSU106, PA 11.6 73 5.6 37

Figure 3c (TDEP) shows several areas in the eastern United States with total N fluxes greater than or 

equal to 10.0 kg/ha/yr. Regions with noticeably high N fluxes were modeled in Iowa, eastern North 

Carolina, the San Joaquin Valley in CA, and a few other areas. Figure 3d shows the percentages of 

total N deposition attributed to dry deposition.

Table 3b shows the CASTNET sites with the five highest total 2012 N fluxes.

Table 3b Highest Total Nitrogen Fluxes for 2012

TDEP MLM/NTN/PRISM

Site Flux (kg/ha) Percent Dry Deposition Flux (kg/ha) Percent Dry Deposition

SND152, AL 18.2 77 5.4 24

STK138, IL 17.2 72 5.5 27

ARE128, PA 17.2 57 9.3 21

CDZ171, KY 14.8 73 5.2 31

ALH157, IL 13.9 65 6.0 20

Comparing Figure 3a with Figure 3-2 and Figure 3c with Figure 3-7 provides insights into the hybrid 

TDEP modeling system versus the MLM/Bowker/NTN/PRISM system. The big difference between the 

two approaches is that the TDEP maps show significantly higher percentages of dry deposition for 

both S and N than the MLM-based maps. Dry S deposition for 2012 contributes more than half the 

total S deposition for about half of the United States. TDEP total S fluxes (Figure 3a) are significantly 

higher than those shown in Figure 3-2. Dry N deposition constitutes more than half total N deposition 

for 90 percent of United States. TDEP total N fluxes (Figure 3c) are significantly higher than those 

shown in Figure 3-7.

Some possible explanations of the differences between TDEP/CMAQ and MLM/NTN/PRISM 

results include:

■ Different algorithms are used in CMAQ to simulate dry deposition than in MLM;

■ Different meteorological conditions are used to drive the two sets of model calculations;

■ Differences in land use in the grid and at the site affect the model results;

■ Higher N deposition is in TDEP because of the addition of deposition of NH 

3 and NOy;
CASTNET 2012 Annual Report

30 Chapter 3: Atmospheric Deposition

■ More extensive monitoring data, e.g., AMoN and SEARCH data, are used in the TDEP 

hybrid simulations;

■ CMAQ modeling includes estimates of N deposition where monitoring results were 

unavailable, e.g., North Dakota, San Joaquin Valley in California, Iowa, and eastern

North Carolina.

Figure 3a TDEP Total Sulfur Deposition for 2012

Figure 3b TDEP Percent Dry Sulfur Deposition for 2012
CASTNET 2012 Annual Report

31 Chapter 3: Atmospheric Deposition

Figure 3c TDEP Total Nitrogen Deposition for 2012

Figure 3d TDEP Percent Dry Nitrogen Deposition for 2012
CASTNET 2012 Annual Report

32 Chapter 3: Atmospheric Deposition

2.3

1.1 0.5

1.7

0.1

0.8

0.5

0.9

1.0 0.6

1.2

1.7

0.6

0.3

1.3

1.0

1.4

0.3

0.6

1.4 0.8

0.5

0.2

0.6

0.3

0.7

1.2

0.5

0.2

0.3

0.9

0.5

1.8

0.7

0.5

0.4

0.9

0.1

0.1

0.1

0.1

0.3

0.5

0.5

0.3

0.9

0.6

0.1

0.9

1.6 0.2

0.1

0.9

0.4

0.2

0.1

0.6

0.4

2.1

0.2 3.2

1.2

0.7

1.0

0.4

0.2

1.2

2.2

0.2

1.8

1.1

0.1

0.2

0.2

0.9

0.2

0.4

0.5

0.6

Site not pictured:

DEN417, AK 0.1

Deposition

4.0

2.0

0.4

Sulfur Deposition

The MLM/Bowker approach was utilized to produce separate products for SO 

2 and SO2-4 . The 

calculations of SO 

2 and SO2-4 dry deposition were summed to obtain the 2012 estimates of dry 

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

of deposition is illustrated by the size of the circle. The map shows only one CASTNET site in

Ohio (QAK172) with an estimated flux greater than 3.0 kg/ha/yr. This site had the highest 

estimated deposition rate for 2012 with a flux of 3.2 kg/ha/yr, which is a marked decrease from 

5.2 kg/ha/yr in 2011 and 5.5 kg/ha/yr in 2010. The highest estimated dry sulfur deposition rates 

occurred along the Ohio River Valley and coincided with the major SO 

2 source region 

(Figures 1-3 and 2-1). The deposition rates declined with distance from the Ohio River 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 (SO2 + SO2-

4 ) Deposition (kg/ha/yr) for 2012

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 

blue shading signifies the percent of wet deposition, and the tan shading shows the percent of 

dry deposition. The values estimated for 2012 were lower than 2011 and 2010 values 

(Figure 3-3) continuing the overall trend of declining sulfur deposition and represent the lowest 
CASTNET 2012 Annual Report

33 Chapter 3: Atmospheric Deposition

2.0

3.6

5.6

5.3

3.3

2.1

3.3

3.5 4.0 3.6

4.1

4.6

2.4

2.3

4.4

3.7

4.8

1.9

1.4

4.4 2.8

2.9

0.4

3.2

0.9

2.8

4.2

2.8

1.4

3.0

3.6

2.8

4.8

6.7

3.4

3.3

3.2

0.6

0.7

0.5

0.7

1.8

3.2

3.2

0.5

4.7

2.2

0.7

4.8

4.3 0.9

1.1

4.7

0.9

0.5

0.5

2.8

1.7

5.1

1.1 7.4

3.6

3.4

3.1

2.4

0.9

3.4

6.2

1.4

4.5

3.8

2.6

0.9

0.7

4.8

0.8

0.9

1.6

2.1

Site not pictured:

DEN417, AK 0.1

Total Deposition

10

5

1

Dry Deposition

Wet Deposition

historic values. Six CASTNET sites in the states of Indiana, Ohio, and Pennsylvania and the 

Canadian province of Ontario had total (dry + wet) sulfur deposition fluxes greater than 

5.0 kg/ha/yr in 2012. The highest total sulfur deposition rate was estimated for QAK172, OH 

with a value of 7.4 kg/ha/yr. The total sulfur deposition values for QAK172 were 13.0 kg/ha/yr in 

2011 and 9.8 kg/ha/yr in 2010. Total sulfur deposition at western sites was less than or equal to 

2.1 kg/ha/yr, which was the estimate for PAL190, TX in 2012. PAL190 began operating in the 

spring of 2007 and started reporting annual deposition values in 2008. Except for 2010, PAL190 

has had the highest annual total sulfur deposition values among the western sites.

Figure 3-2 Total (Dry + Wet) Sulfur Deposition (kg/ha/yr) for 2012

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. The box plots were based on data obtained 

from the 34 CASTNET eastern reference sites for the period 1990 through 2012 and from the 

17 western reference sites for 1996 through 2012. 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 2010–2012 mean of

1.4 kg/ha/yr, a 73 percent reduction. Dry sulfur deposition declined at the western reference 

sites from a 1996–1998 mean of 0.3 kg/ha/yr to a 2010–2012 mean of 0.2 kg/ha/yr, a 

40 percent reduction.
CASTNET 2012 Annual Report

34 Chapter 3: Atmospheric Deposition

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 CASTNET 

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 2010–2012 mean of 

4.6 kg/ha/yr, a 64 percent reduction. Total sulfur deposition declined at the western reference 

sites from a 1996–1998 mean of 1.3 kg/ha/yr to a 2010–2012 mean of 0.8 kg/ha/yr, a 

36 percent reduction. The 2012 total sulfur deposition values for eastern and western sites were 

the lowest in the history of the network. Wet deposition in 2012 was a major component of sulfur

flux with a contribution of 74 percent, which is approximately the same as the contribution from 

wet deposition in 2011 (75 percent). The average contribution from wet deposition from 1990 

through 2010 was 62 percent. The 74 percent and 75 percent contributions from wet flux for the 

past two years were the highest in 

the history of the project.

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, precipitation Quaker City, OH (QAK172)

0.0

0.2

0.4

0.6

0.8

19961997199819992000200120022003200420052006200720082009201020112012

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

2

4

6

8

10

12

19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET 2012 Annual Report

35 Chapter 3: Atmospheric Deposition

amounts and dry deposition. The dotted line shows concentrations in precipitation, which are 

independent of precipitation amount. The figure shows that wet, dry, and total sulfur deposition

have declined substantially for the eastern sites. The aggregated total sulfur deposition 

(3.7 kg/ha/yr) for 2012 estimated for the eastern sites decreased after the upswing in 2011 

(5.5 kg/ha/yr) to below 2010 levels (4.8 kg/ha/yr). Both the total sulfur deposition and sulfur 

concentrations in precipitation at the western sites show slight declines from 2011 values while 

the dry deposition value has been steady.

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

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1996199719981999200020012002200320042005200620072008

2009

201020112012

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

5

10

15

20

25

1990199119921993199419951996199719981999200020012002200320042005

20062007

20082009201020112012

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.5

1.0

1.5

2.0

19961997199819992000200120022003200420052006200720082009201020112012

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

19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012

Deposition (kg/ha/yr)

Concentration (mg/l)

Wet Deposition

Dry Deposition

Total Deposition

Concentration in Precipitation
CASTNET 2012 Annual Report

36 Chapter 3: Atmospheric Deposition

Total Reactive Oxides of Nitrogen Measurements at Beaufort, NC

In addition to the NH 

3 sampling discussed in Chapter 2, another measurement study was conducted at 

the CASTNET BFT142, NC site (Figure 1-1). The study focused on continuous measurement of NOy, 

which is defined as NOx plus NOz (HNO 

3, HNO 2, PAN, NTR, and nitrite). NOy measurements began in 

February 2012 and were completed in April 2013. Trace-level NOy measurements were obtained using 

a Thermo Scientific Model 42i-Y NOy analyzer, which is a chemiluminescent trace-level gas analyzer 

that measures all reactive oxides of nitrogen in the ambient air. A second converter that permitted the 

measurement of NOx in addition to NOy and NO was used for the BFT142 NOy study. The first 

converter was located at 10 m to measure NOy, and the second was situated in the shelter at the 

instrument to measure NOx. A program was written for the data logger to switch between the 

converters. Thus, NO 

2 could be calculated using differences (NOx - NO). For every minute of data 

collection, 20 seconds were dedicated to the NO measurement, 20 seconds to a pre-reactor 

measurement, and 10 seconds each to NOy and NOx measurements. Performance of the data logger 

program was verified at the AMEC field equipment calibration laboratory before deployment to 

BFT142. The sampling configuration produced hourly concentrations of NOy, NOx, NO, and calculated 

NO 

2 and NOz.

Figures 3e through 3g illustrate results from the NOy monitoring study. The data cover the entire 

measurement period from February 2012 through May 2013. Figure 3e shows six plots of composite 

NOy, NOx, NO 

2, NOz, NO, and O 3 concentrations. The curves were constructed by averaging all hours 

for each hour of the day for each parameter for the entire measurement period. The results show 

typical diurnal distributions with NO values at 0.0 for much of the night due to oxidation by O 

3 and 

conversion to NO 

2. NOz levels showed little variation during the day. NOy, NOx, and NO 2 levels 

increased after sunrise with fresh NO emissions produced by vehicular traffic and other sources. O 

3

concentrations peaked around 1500 local time. The growth of the convective boundary layer, 

photochemical transformations, and dry deposition decreased nitrogen concentrations during the day 

as O 

3 levels increased.

Figure 3f provides a time series of weekly average concentrations of NOy, NOx, calculated NO 

2, 

calculated NOz, and NO concentrations collected over the entire measurement period. The results 

show reasonable relationships between the measurements.

Time series of weekly average concentrations of continuous NOy and weekly concentrations of total 

nitrate (HNO 

3 + particulate NO3) measured by the CASTNET filter pack deployed at the BFT142 site 

are shown in Figure 3g.To evaluate the components of NOy measurements NOy concentrations were 

compared with total NO3 concentrations. Since NOy species include HNO 

3 and particulate NO3, the 

ratio of total NO3 to NOy should always be less than or equal to 1.0. Other than for two weeks, the NOy

concentrations were consistently higher than the total NO3 levels, as expected.

Beaufort, NC (BFT142)
CASTNET 2012 Annual Report

37 Chapter 3: Atmospheric Deposition

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2/10/12

3/2/12

3/23/124/13/12

5/4/12

5/25/126/15/12

7/6/12

7/27/128/17/12

9/7/12

9/28/12

10/19/12

11/9/12

11/30/12

12/21/12

1/11/13

2/1/13

2/22/133/15/13

4/5/13

4/26/13

Nitrogen Concentration (ppb)

NOY

NOX

NO2_calc

NOZ_calc

NO

NOy

NOx

NO2 _calc

NOz _calc

NO

Figure 3e Composite NOy, NOx, NO 

2, NOz, NO, and O 3 Concentrations (ppb)

Figure 3f Weekly NOy, NOx, NO 

2, NOz, and NO Concentrations (ppb)

Figure 3g Weekly Total Filter Pack NO3 and Weekly Average Continuous NOy Concentrations (ppb)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2/7/12

2/21/12

3/6/12

3/20/12

4/3/12

4/17/12

5/1/12

5/15/125/29/126/12/126/28/127/10/127/24/12

8/7/12

8/21/12

9/4/12

9/18/1210/2/12

10/16/1210/30/1211/13/1211/27/1212/11/1212/26/12

1/10/131/21/13

2/5/13

2/19/13

3/5/13

3/19/13

4/2/13

4/16/134/30/13

Concentration (ppb)

Total NO3

NOY

10

15

20

25

30

35

40

45

0.0

0.5

1.0

1.5

2.0

2.5

0 4 8 12 16 20 24

Ozone Concentration (ppb)

Nitrogen Concentration (ppb)

Hour

NOY

NOX

NO2_calc

NOZ_calc

NO

Ozone

NOy

NOx

NO2 _calc

NOz _calc

NO

Ozone

Total NO3

NOy
CASTNET 2012 Annual Report

38 Chapter 3: Atmospheric Deposition

0.7

1.3

0.9

1.3

0.2

1.2

0.4 1.8

1.3 1.5

1.1

1.5

1.1

0.4

1.1

1.3

1.8

0.3

1.1

1.1 1.4

0.7

0.7

1.3

0.7

1.4

1.4

1.0

0.6

0.4

1.0

0.9

1.8

1.0

0.7

0.4

1.2

0.1

0.5

0.5

0.2

0.4

0.8

0.4

1.8

0.7

1.5

0.4

0.8

1.6 0.9

0.1

0.8

1.0

0.7

0.3

1.0

0.7

1.3

0.7 1.8

1.6

0.9

1.4

0.5

0.6

1.2

1.5

0.5

1.2

1.5

0.2

0.2

0.6

1.1

0.9

1.9

1.4

1.5

Site not pictured:

DEN417, AK 0.1

Deposition

2.0

1.0

0.2

Nitrogen Deposition

Figure 3-6 illustrates estimates of dry fluxes of nitrogen (as N) for 2012. The year 2012 was the 

first year with dry nitrogen deposition values less than 2.0 kg/ha/yr at all sites in the network. 

The dry nitrogen deposition depicted in Figure 3-6 represents the sum of dry deposition

estimates of HNO 

3 + NO3 + NH+ 4 . A majority of CASTNET sites in the eastern United States had 

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

deposition rates equal to 1.8 kg/ha/yr. The highest value for all sites was calculated for the 

western CASTNET site SEK430, CA with a dry nitrogen flux of 1.9 kg/ha/yr.

Figure 3-6 Dry Nitrogen (HNO3 + NO3 + NH+

4) Deposition (kg/ha/yr) for 2012

A map of total nitrogen deposition (as N) for 2012 is given in Figure 3-7. The map was 

constructed by summing the estimates of dry (yellow shading) and wet (blue 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 

(9.2 kg/ha/yr). Most of the western sites had estimated total nitrogen deposition rates below 

3.0 kg/ha/yr. The CASTNET site at PAL190, TX had the highest total nitrogen flux for the 

western sites with an estimated flux of 5.6 kg/ha/yr. Three other western sites in California, 

South Dakota, and Wyoming had estimated total nitrogen fluxes greater than or equal to 

3.0 kg/ha/yr. 
CASTNET 2012 Annual Report

39 Chapter 3: Atmospheric Deposition

3.4

3.4

5.6

4.2

5.9

6.6

5.0

4.7

4.5 5.4

5.7

5.6

4.1

2.7

5.9

6.4

9.2

2.7

2.4

6.1 4.5

3.5

1.5

5.8

2.5

4.6

5.9

5.3

3.3

5.8

4.1

5.8

8.5

5.4

3.1

4.4

1.6

2.2

1.8

5.9

1.9

2.9

6.6

2.9

2.4

5.2

4.9

2.6

5.5

6.1 2.3

1.7

5.5

2.2

1.1

1.3

4.0

4.8

5.9

2.7 6.3

6.4

5.4

6.2

2.5

2.5

5.9

6.8

4.2

4.9

6.3

4.2

2.4

2.4

5.9

3.2

3.0

4.7

5.6

Site not pictured:

DEN417, AK 0.1

Total Deposition

10

5

1

Dry Deposition

Wet Deposition

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

the majority of western CASTNET sites. Dry nitrogen deposition did contribute more than half of 

total nitrogen deposition at three of the five monitors in California. These three California sites 

are in a region with limited rainfall. Dry nitrogen deposition contributed about 75 percent of the 

total nitrogen deposition at JOT403 and about 65 percent at Pinnacles National Monument 

(PIN414) and SEK430.

Figure 3-7 Total (Dry + Wet) Nitrogen Deposition (kg/ha/yr) for 2012

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

sites over the period 1990 through 2012 (right side) and 17 western reference sites for 1996 

through 2012 (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 2000. Three-year mean fluxes declined from 2.2 kg/ha/yr for 1990–1992 to 

1.2 kg/ha/yr for 2010–2012, a 46 percent reduction over the 23 years. The box plots for the 

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 2010–2012.
CASTNET 2012 Annual Report

40 Chapter 3: Atmospheric Deposition

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

Western Reference Sites Eastern Reference Sites

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. The trend in total nitrogen flux for the eastern sites has fluctuated since 2009. Total 

nitrogen deposition for the eastern sites was reduced from a 1990–1992 mean of 7.5 kg/ha/yr to 

a 2010–2012 mean of 2.1 kg/ha/yr, a 72 percent reduction. Total nitrogen deposition for the 

western sites shows a relatively flat trend from a 1996–1998 mean of 2.5 kg/ha/yr to a 

2010–2012 mean of 2.1 kg/ha/yr, which constitutes a 19 percent decline.

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 from 2000 through 2009. However, precipitationweighted mean nitrogen concentrations in precipitation have increased since 2009. This trend is 

reasonably representative of the effect of changes in NOx emissions because it does not 

depend on precipitation amounts. Although total N deposition (solid top line) at eastern sites 

increased by 34 percent from 2010 to 2011, this increase can be explained by an increase in N 

concentration in precipitation coinciding with the highest average annual rainfall rate 

(138 centimeters) at eastern sites over the 23-year period. A 19 percent decrease in total 

nitrogen deposition was observed from 2011 to 2012.

0.0

0.5

1.0

1.5

2.0

19961997199819992000200120022003200420052006200720082009201020112012

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

5

6

19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET 2012 Annual Report

41 Chapter 3: Atmospheric Deposition

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

Western Reference Sites Eastern Reference Sites

0

1

2

3

4

5

6

19961997199819992000200120022003200420052006200720082009201020112012

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

2

4

6

8

10

12

14

19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

Everglades National Park, FL (EVE419)
CASTNET 2012 Annual Report

42 Chapter 3: Atmospheric Deposition

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

Precipitation (mg/l)

Western Reference Sites Eastern Reference Sites

An Analysis of Critical Loads of Streams near

Seven CASTNET Sites in Eastern United States

Air pollutant emission reduction programs have been successful in reducing the extent of lakes and 

streams impacted by acidic deposition in acid sensitive regions of the United States (Burns et al., 

2011). In recent years, emissions of SO 

2 and NOx from power generating plants have continued to 

decline sharply, likely providing greater protection of forest and aquatic resources. 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 can be used as an assessment tool that provides a benchmark against which to assess 

whether a water body is 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 exceeding its critical load. A critical load exceedance is the measure 

of pollutant exposure above the critical load and is higher than, or “exceeds,” the critical load, which 

exposes water bodies to damaging levels of S and N deposition.

In this case study, wet and dry S and N deposition at selected streams were estimated and compared 

with available surface water critical loads estimates. Two periods, 2000–2002 and 2010–2012, were 

compared in order to investigate how wet and dry S and N deposition and their proportional 

contributions to critical loads have changed. Wet and dry S and N deposition were obtained from the 

new TDEP maps (EPA, 2013d) for each year. Critical loads for each stream in this case study 

represent the amount of acidic deposition of total S and N that a stream’s watershed could receive and 

still allow the surface water chemistry to maintain an acid neutralizing capacity (ANC) of 

50 microequivalents per liter (μeq/L) or higher. Fifty μeg/L is the threshold above which healthy aquatic 

ecosystem are supported (Henriksen et al., 2002). Critical loads were obtained from the National

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

19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012

Deposition (kg/ha/yr)

Concentration (mg/l)

Wet Deposition

Dry Deposition

Total Deposition

Concentration in Precipitation
CASTNET 2012 Annual Report

43 Chapter 3: Atmospheric Deposition

Critical Load Database (NCLD) (http://nadp.isws.illinois.edu/committees/clad/db/) and are expressed 

in terms of ionic charge balance as milliequivalents per square meter per year (meq/m2

/yr).

In addition, critical loads were summarized for streams in the NCLD located near seven CASTNET 

sites: Parsons, WV (PAR107); Kane Experimental Forest, PA (KEF112); Beltsville, MD (BEL116); 

Shenandoah National Park, VA (SHN418); Horton Station, VA (VPI120); Cranberry, NC (PNF126); 

and Coweeta, NC (COW137). Critical Loads were summarized for the selected streams near each of 

the seven CASTNET monitoring locations as the 15th percentile critical load value (Figure 3h). The 

15th percentile is the critical load value below which 15 percent of the observations were summarized

for a given CASTNET site. These are the streams with the lowest critical load values and are most 

vulnerable to acidic deposition. The selection of the 15th percentile critical load value is just one 

choice among many options that could be selected. It is also important to note while the NCLD is 

extensive in its coverage of water bodies, the water bodies analyzed herein are not intended to 

represent all water bodies that are within 25 km of CASTNET locations.

This analysis examined over 250 streams within 25 km of the seven CASTNET monitoring locations in 

the eastern United States (Figure 3h). Gray circles indicate CASTNET monitoring sites while 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 sulfur and 

nitrogen deposition for years from 2010–2012 at CASTNET locations. On the graphs, light and dark 

red bars represent the maximum critical load for S (CLmaxS) and the amount of N that is removed by 

the ecosystem (CLminN), respectively. Dark to light blue represents inorganic and organic wet sulfur, 

wet nitrogen, dry sulfur, and dry nitrogen deposition, respectively.

Figure 3h shows comparisons of the 15th percentile critical load value with total average N and S 

deposition from 2000–2002 and 2010–2012. For all the CASTNET locations, total acid deposition 

decreased on average by 56 meq/m2

/yr or 32 percent between 2000–2002 and 2010–2012, indicating 

an important reduction in the acidic loading to these aquatic ecosystems. KEF112, PA had the largest 

decline of 129 meq/m2

/yr or 45 percent during this period. The contributions of wet and dry deposition 

varied among the seven CASTNET locations. On average, total dry deposition (43.18 meq/m2

/yr) 

decreased twice as much as wet deposition (21.87 meq/m2

/yr), and total dry deposition was on 

average about 32 percent higher than total wet deposition, making dry depositional loading the largest 

contributor to the critical load exceedances.

The contribution of N versus S deposition also varied at the seven CASTNET locations for both time 

periods. Three CASTNET locations had on average more total N deposition, compared with four sites 

that had higher total S deposition relative to N. For 2010–2012, N total deposition was on average only 

slightly higher (9.54 meq/m2

/yr) than total S deposition among the CASTNET locations. However, total 

S deposition on average declined three times more than total N deposition over the period of 

comparison, making declining S the largest contributor to the declining acid loading.

For all but the BEL116, MD site, the 15th percentile critical load is less than current total N and S 

deposition, indicating that acidification critical loads are being exceeded and that some vulnerable 

streams surrounding CASTNET monitoring locations are still being affected by acid deposition today 

despite significant reductions in deposition in recent years.
CASTNET 2012 Annual Report

44 Chapter 3: Atmospheric Deposition

Figure 3h Critical Load Estimates (2000–2002 and 2010–2012) for Selected Streams with 25 km of 

Seven Eastern CASTNET Sites

State Boundary

Selected CASTNET Sites

Does Not Exceed Critical Load

Exceeds Critical Load

PAR107, WV KEF112, PA BEL116, MD

SHN418, VA

VPI120, VA

PNF126, NC

COW137, NC

CASTNET Site
CASTNET 2012 Annual Report

45 Chapter 4: Ozone Concentrations

CASTNET is the primary air quality network for monitoring rural, ground-level O 

3 concentrations in the United 

States and for providing critical information on geographic patterns in rural O 

3 levels. CASTNET was not 

originally designed to operate as a regulatory network; however, NPS-sponsored CASTNET sites have 

complied with regulatory monitoring requirements from site inception, and during 2010 and 2011, EPAsponsored CASTNET O 

3 monitoring systems were upgraded in order to comply with regulatory monitoring 

requirements. Data collected from the NPS-sponsored sites and data collected from EPA-sponsored sites for 

2011 and 2012 were submitted to EPA’s AQS. In this report, O 

3 measurements collected during 2010–2012 

were evaluated with respect to the current NAAQS (EPA, 2008a) of 0.075 parts per million (ppm). For the 

2010–2012 period, three California, one other western, and eight eastern sites recorded concentrations above 

the 0.075 ppm standard. The geographic extent of O 

3 levels above 0.075 ppm was greater in 2012 than in the 

previous four years.

Chapter 4: Ozone C oncentr ations

Most CASTNET sites operate an O 

3 analyzer that measures hourly average concentrations, 

which are archived in the CASTNET database and delivered to AQS. At the end of 2012, 83 O 

3

analyzers were operated throughout the network. Data from these sites, with the exception of 

YOS204, CA; ROM206, CO; and HOW191, ME, which are designated as non-regulatory, can be 

used to calculate design values as three years of Title 40 CFR-compliant data become available. 

CASTNET’s geographic coverage of the United States provides data that are essential for 

evaluating rural O 

3 concentrations in the context of the O 3 NAAQS and in terms of presenting 

information on trends and geographic patterns in regional O 

3. 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. A map of 2012 W126 values is 

also presented and discussed on pp. 55-56. Vertical gradient profile O 

3 data within a forest 

canopy collected at the HOW191, ME AmeriFlux site are analyzed and summarized. 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 a criteria pollutant and a component of smog that can affect the human 

respiratory system, cause damage to vegetation and ecosystems, and reduce crop yield and 

quality. O 

3 is formed in the lower atmosphere when NOx and volatile organic compounds (VOC) 

react in the presence of sunlight. Emissions from motor vehicles, industrial facilities, chemical 

solvents, surface coatings, gasoline, and power plants are the major anthropogenic sources of 

NOx and VOCs. Also, biogenic VOC emissions from trees and other vegetation may 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 episodes of high O 

3 concentrations vary by geographical region and 
CASTNET 2012 Annual Report

46 Chapter 4: Ozone Concentrations

can occur in any season. O 

3 concentrations typically increase during daylight and peak in the 

late afternoon after temperature and sunlight intensity peak. Different diurnal patterns of O 

3 can 

be observed across sites depending on site elevation, complexity of terrain, and latitude. See 

the discussion of O 

3 levels in the Upper Green River Basin, WY in the CASTNET 2010 Annual 

Report (AMEC, 2012a). Also, view the figures of diurnal distributions of O 

3 and NOy levels at 

four sites in the recent CASTNET Quarterly Data Reports for the second and third quarters of 

2013 (AMEC, 2013d; 2014).

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 ARP, 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 CAIR and 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. 

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). O 

3

concentrations are commonly presented in units of ppb.

EPA and other federal, tribal, state, and local agencies perform O 

3 measurements on an hourly basis 

through national and local monitoring programs. AMEC followed EPA procedures (1998) to estimate 

O 

3 design values and 2012 fourth highest DM8A O 3 concentrations at CASTNET sites. O 3 data from 

EPA-sponsored CASTNET sites will not be used to gauge compliance with NAAQS until 40 CFRcompliant data are available for three years. O 

3 data from NPS-sponsored sites can be used to gauge 

compliance with NAAQS since these sites have met regulatory quality assurance criteria since 

site inception. The primary O 

3 NAAQS standard is designed to protect public health. The secondary 

standard is designed to protect public welfare and the environment. Both O 

3 NAAQS are set at a level 

of 0.075 ppm averaged over eight hours.

W126 Index

O 

3 concentrations can also be analyzed in terms of W126, a measure of exposure that can indicate 

damage to vegetation. W126 is a cumulative metric that sums weighted hourly O 

3 concentrations 

during the O 

3 season. For this report (see pp. 55-56), W126 was calculated for CASTNET sites for the 

months of May through September. The highest of the rolling 3-month sums during the five months is 

the W126 value for 2012.
CASTNET 2012 Annual Report

47 Chapter 4: Ozone Concentrations

Eight-Hour Ozone Concentrations

Figure 4-1 presents 3-year averages of the fourth highest DM8A O 

3 concentrations for 

2010–2012. O 

3 concentrations were not included on the maps in this chapter if the 3-year 

average was not available because of incomplete data; these sites are shown as dots with no 

value. During this period, 3-year averages of the fourth highest DM8A O 

3 concentrations were 

greater than 75 ppb at eight eastern sites, three sites in California, and the site at ROM406, CO. 

The eastern sites include Washington Crossing, NJ (WSP144); Blackwater National Wildlife 

Refuge, MD (BWR139); BEL116, MD; Great Smoky Mountains National Park, TN (GRS420); 

Oxford, OH (OXF122); Vincennes, IN (VIN140); Alhambra, IL (ALH157); and Cherokee Nation, 

OK (CHE185). The three California sites include Yosemite National Park (YOS404), SEK430, 

and JOT403. Three-year average concentrations were generally higher in 2010–2012 than in 

2009–2011.

The 3-year averages of fourth highest DM8A O 

3 concentrations for 2010–2012, which are 

shown on Figure 4-1, provide examples of the design values for CASTNET sites used to 

determine compliance with the O 

3 NAAQS if all CASTNET O 3 data for 2010 through 2012 were 

regulatory 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, if it is assumed that the 3-year averages of 

2010–2012 fourth highest DM8A O 

3 concentrations are design values, then any value higher 

than 75 ppb will have to be reduced to this level to achieve the current O 

3 NAAQS.

Great Smoky Mountains National Park, TN (GRS420)
CASTNET 2012 Annual Report

48 Chapter 4: Ozone Concentrations

Figure 4-1 Three-year Average of Fourth Highest DM8A O3 Concentrations (ppb) for

2010–2012

Figure 4-2 Three-year Average of Fourth Highest DM8A O3 Concentrations (ppb) for

2000–2002
CASTNET 2012 Annual Report

49 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 2010–2012 values shown in Figure 4-1. The 

period 2000–2002 was selected because it shows O 

3 concentrations prior to reductions in NOx

emissions under the NOx SIP Call/NBP, which began in 2003 for the eastern United States. The 

changes in O 

3 concentrations from 2000–2002 to 2010–2012 illustrate the effectiveness of the 

emission reduction programs. Approximately 70 percent of the 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. During 2010–2012, relatively 

low DM8A O 

3 concentrations in the East were monitored in northern New England and in the 

Florida panhandle, and only 12 monitors nationwide measured concentrations greater than 

75 ppb.

O 

3 concentrations for 2012 are shown in Figure 4-3. These data comply with regulatory

requirements. All valid data are included; exceptional events have not been analyzed and, 

therefore, are not excluded on the map. “Exceptional events are unusual or naturally occurring 

events that can affect air quality but are not reasonably controllable using techniques that state, 

tribal, or local air agencies may implement in order to attain and maintain the NAAQS” (EPA, 

2013e). Twenty eastern, three California, and two other western CASTNET sites measured 

fourth highest DM8A O 

3 concentrations greater than 75 ppb. The geographic extent of 

measured fourth highest DM8A O 

3 concentrations above 75 ppb was greater in 2012 than in the 

previous four years for the eastern sites and greater than the previous three years for the 

western sites. The highest eastern concentration of 85 ppb was measured at OXF122, OH, and 

the highest western level of 95 ppb was observed at two sites in California, JOT403 and 

SEK430.

The CASTNET 2011 Annual Report (AMEC, 2013a) reported that CASTNET sites at 

BBE401, TX; PAL190, TX; and CHE185, OK each measured a fourth highest DM8A O 

3

concentration greater than 75 ppb during 2011, which was likely influenced by frequent wildfires 

in Texas and Arizona during the summer and fall fire season. Moderately high fourth highest 

DM8A O 

3 concentrations persisted at these sites during 2012, again because of an active 

wildfire season.

Quaker City, OH (QAK172)
CASTNET 2012 Annual Report

50 Chapter 4: Ozone Concentrations

Figure 4-3 Fourth Highest DM8A O3 Concentrations (ppb) for 2012

Figure 4-4 provides box plots depicting trends in 1-year mean values and distributions of 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. On the other hand, the mean aggregated value of 72 ppb for the eastern reference sites 

for 2012 shows an increase from the minimum value of 64 ppb that was recorded in 2009. The 

recent increase over the last three years in mean fourth highest DM8A O 

3 concentrations 

occurred despite decreases in NOx emissions (Figure 1-4). The higher fourth highest DM8A O 

3

concentrations measured from 2010 through 2012 may be explained by warmer weather 

throughout the United States. Figure 4-5 presents box plots of hourly temperatures measured at 

the eastern reference sites. The boxes for 2010 through 2012 show higher mean and 75th

percentile temperatures relative to 2008 and 2009, years with relatively low fourth highest DM8A 

O 

3 concentrations. Ninetieth percentile values in 2011 and 2012 were also higher than in 2008 

and 2009. In short, the NOx emission decreases were offset by warmer weather, which likely 

contributed to higher mean fourth highest DM8A O 

3 concentrations over the last three years.

The box plots constructed from the aggregated western O 

3 sites show an increase since the

2009 minimum. The 2010–2012 average of the fourth highest DM8A O 

3 concentrations for the 

western reference sites was 71 ppb.
CASTNET 2012 Annual Report

51 Chapter 4: Ozone Concentrations

Figure 4-4 Trend in Fourth Highest DM8A O3 Concentrations (ppb)

Western Reference Sites Eastern Reference Sites

Figure 4-5 Trend in Hourly Temperature Values (degrees Celsius)

Western Reference Sites Eastern Reference Sites

Sand Mountain, AL (SND152)

0

5

10

15

20

25

19961997199819992000200120022003200420052006200720082009201020112012

Degrees Celsius

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

5

10

15

20

25

19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012

Degrees Celsius

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

50

60

70

80

90

100

110

19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012

Concentration (ppb)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

50

60

70

80

90

100

110

1996199719981999

200020012002200320042005200620072008200920102011

2012

Concentration (ppb)
CASTNET 2012 Annual Report

52 Chapter 4: Ozone Concentrations

0

5

10

15

20

25

30

15 20 25 30 35 40

Height (m)

Concentration (ppb)

All Hours

0600

1400

In-Canopy Measurements of Ozone and Other Gases and Particles at

Howland Research Forest, ME

Ozone Concentrations

CASTNET began collecting O 

3 and filter pack measurements at the Howland Research Forest 

AmeriFlux site (designated HOW191) in Maine in September 2011. The study ended in April 2013. 

This study represented the first collaboration between the two long-standing national networks. The 

primary goal of AmeriFlux is to measure ecosystem level exchanges of carbon dioxide, water, energy, 

and momentum.

The design of the O 

3 system (AMEC, 2013a; Rogers et al., 2012) 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. O 

3 concentrations were 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. Levels were sampled in 

succession for three minutes each. All eight inlets were sampled within a 24-minute period at least 

twice per hour. Two sampling periods were required each hour to calculate an hourly value.

Figure 4a 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 period of record. These data show a 

decrease in concentrations with decreasing elevation throughout the 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, dry deposition and other depletion 

mechanisms (Talbot et al., 2005). 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.

Figure 4a Aggregated O 

3 Concentrations for Entire Database (September 2011 through 

April 2013) for Specific Hours at HOW191, ME
CASTNET 2012 Annual Report

53 Chapter 4: Ozone Concentrations

HOW191 28 m

HOW191 23.5 m

HOW191 2 m

Figure 4b Time Series of O 

3 Concentrations at the Highest Inlet above the Canopy (28 m), the Inlet 

Just above the Forest Canopy (23.5 m), and the Lowest Inlet below the Canopy (2 m)

The HOW191 O 

3 concentration data were analyzed in order to understand nighttime depletion/loss 

mechanisms that produce lower O 

3 concentrations at 2 m relative to values measured near the canopy 

top. Figure 4b shows a time series of O 

3 concentrations (the 1-week period chosen at random) at the 

highest inlet (28 m), the inlet just above the forest canopy (23.5 m), and the lowest inlet (2 m). The plot 

shows that there is little difference between measurements taken above the canopy, but losses were 

observed between the above canopy and below canopy measurements. These O 

3 losses in the lower 

canopy occurred during the nighttime but did not occur every night. The analyses can be summarized 

as follows:

■ Non-loss events were defined as relative percent difference (RPD) from 5 to -10 percent O 

3

concentration values measured at 28 m. They occurred more frequently than loss events 

(RPD from -50 to -100 percent) with ~25 to 55 percent relative humidity (RH), and loss events 

occurred more frequently than non-loss events with RH from ~60 to 100 percent (Figure 4c). 

■ Loss events did not occur at high wind speeds and were only observed when wind speeds 

were less than 3.5 meters per second. 

■ Loss events were more frequent during hours with lower sigma theta values.

■ Loss events were more frequent in stable conditions when temperature difference (ΔT) from 

2 m to 23.5 m was positive. The ratio of loss to non-loss events increased with increasing ΔT 

until no non-loss events occurred when ΔT was greater than 1.5 oC.
CASTNET 2012 Annual Report

54 Chapter 4: Ozone Concentrations

Figure 4c Percent Frequency of Non-loss Events (RPD = 5 to -10 percent; orange) and Loss Events 

(RPD = -50 to -100 percent; red) Binned to Percent Relative Humidity

Note: RPD = relative percent difference

Filter Pack and Ozone Concentrations

CASTNET filter pack sampling systems were operated at 2 m and 23.5 m. Weekly ambient 

concentrations measured with filter packs support the O 

3 concentration observations. The belowcanopy concentrations were approximately 62 percent and 64 percent of above-canopy 

concentrations for SO 

2 and HNO 3, respectively. Above and below-canopy concentrations of particulate 

matter components (i.e., SO2-

4 , NO3, and NH+ 4 ) show much less difference, which is indicative of slower 

deposition rates compared with those of the gaseous species. 

Hourly O 

3 and weekly filter pack concentration data were collected at the nearby HOW132, ME

CASTNET monitoring site, where measurements occurred in a clearing at a height of 10 m. For the 

first 10 months of measurements, November 2011 through August 2012, concentrations of SO 

2

(4 percent higher) and SO2-

4 (5 percent lower), HNO 3 (3 percent lower), and NO3 (3 percent lower) at 

HOW132 were very similar to the above canopy concentrations measured at HOW191. NH+ 

4

concentrations showed the greatest difference and were 13 percent lower at HOW132. Hourly O 

3

concentrations at HOW132 were about 9 percent lower than the HOW191 above-canopy (28 m) 

measurements (R2

= 0.93).

Howland, ME (HOW191) AmeriFlux site
CASTNET 2012 Annual Report

55 Chapter 4: Ozone Concentrations

W126 Values for 2012

The W126 O 

3 exposure index is a cumulative peak-weighted exposure index (Lefohn and 

Runeckles, 1987; EPA, 1996; 2006) for relating reduced vegetation yields to O 

3 exposure. 

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, 2010). 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 value for each site is 

presented in Figure 4-6. 

W126 exposures (ppm-hours) were calculated using O 

3 values measured during the 12 hours 

from 8:00 a.m. to 8:00 p.m. in the months of May through September 2012. 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. For reference, a W126 index 

value of 21 ppm-hours is associated with a level of crop protection of approximately no more 

than 10 percent yield loss in 50 percent of crop cases studied in the National Crop Loss 

Assessment Network (NCLAN) experiments (2013). A W126 index value of 13 ppm-hours is 

associated with a level of crop protection of approximately no more than 10 percent yield 

loss in 75 percent of NCLAN cases. In addition, vegetative sensitivity to O 

3 varies between and 

within species.

W126 levels for 2012 are presented for CASTNET sites in Figure 4-6. The higher W126 values 

were measured in California, at western sites in high terrain, and at a few eastern sites with 

relatively high DM8A O 

3 concentrations. The highest W126 value (57.8 ppm-hour) was 

measured at SEK430, CA. O 

3 concentrations measured at high elevation sites (AMEC, 2003; 

Talbot et al., 2005) typically persist during the night because dry deposition and scavenging of 

O 

3 by nitric oxide are low at night. 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 2012 W126 values were higher 

than values in 2011, reflecting higher ambient concentrations in 2012. For example, OXF122, 

OH measured a W126 value of 24.5 ppm-hour in 2012 versus a value of 14.6 ppm-hour in 2011.
CASTNET 2012 Annual Report

56 Chapter 4: Ozone Concentrations

Figure 4-6 W126 Values (ppm-hour) for 2012

Sequoia National Park, CA (SEK430) south view
CASTNET 2012 Annual Report

57 Chapter 4: Ozone Concentrations

Ozone Monitoring System Performance Audits

CASTNET Ozone Monitoring

CASTNET measures ground-level O 

3 concentrations at 83 sites nationwide. Measurements are made 

via ultraviolet absorbance using Thermo Scientific 49i O 

3 analyzers. CASTNET O 3 monitoring systems 

comply with the regulatory monitoring requirements described in 40 CFR Parts 50, 53, and 58 and 

Appendix A to Part 58 (EPA, 2011; 2012a). CASTNET is the primary source of rural O 

3 monitoring 

data for the United States. The data collected are used to calculate design values for all sites where 

data completeness requirements are met and are also used to assess the effectiveness of CAIR and 

other EPA NOx emission reduction programs. Additionally, CASTNET data are used to support the 

development of primary and secondary air quality standards.

CASTNET O 

3 data are collected as hourly averages and submitted in near real-time to the EPA 

AIRNow system. Hourly concentration data for all sites are also submitted to EPA’s AQS each month. 

Regulatory Auditing Requirements

Federal air monitoring regulations require application of a specified program of internal, independent, 

and external (i.e., regulatory agency) audits of technical systems and instrument performance. These 

required audits are summarized in Table 4a. Audit criteria are listed in Table 4b.

Table 4a Summary of 40 CFR Regulatory Audit Requirements

Required Audits Performed By Administrator Audit Frequency Results Submitted

Recertification of site 

transfer standard*

Program contractor** Program contractors Once every 6 

months

EPA/NPS/BLM

program contractors

Certification to Level 1 

standard for Level 2 

standards

NIST and EPA 

regional laboratories

Program contractors Annually EPA/NPS/BLM

program contractors

Annual performance 

evaluation

Independent auditor EPA Annually 

(all sites)

AQS

EPA

National Performance 

Audit Program (NPAP)

EPA (Regional office 

coordination)

EPA 20% of sites each 

year

AQS

Technical systems 

facilities audit

Third party auditor EPA/NPS Each facility every 3 

years

EPA/NPS 

AQS

program contractors

Note: *CASTNET site transfer standards generally carry Level 3 authority in the EPA traceability hierarchy (please refer to 

Figure 4d on page 60).

**AMEC (EPA contractor), Air Resource Specialists (NPS and BLM contractor). Managed subcontractors may perform 

on-site work.

NIST = National Institute of Standards and Technology
CASTNET 2012 Annual Report

58 Chapter 4: Ozone Concentrations

Table 4b Operational Criteria for O 

3 Audits

Requirement / 

Type of Audit Frequency Acceptance Criteria Information /Action

Annual performance 

evaluation of single 

analyzer

Every site once 

per year within

period of monitor 

operation,

25% of sites 

quarterly

Percent difference of audit 

levels 3-10 ≤ ± 15%

Audit levels 1 and 2 ± 1.5 

ppb difference or ± 15%

1,240 CFR Part 58, App A, Section 3.2.2

3Recommendation: Three audit 

concentrations not including zero. AMTIC 

guidance 2/17/2011 

http://www.epa.gov/ttn/amtic/cpreldoc.html

Federal audits 

(NPAP)

Once per year at 

selected sites, 

20% of sites 

audited per year

Audit levels 1 and 2 

± 1.5 ppb difference 

All other levels percent 

difference ± 10%

140 CFR Part 58, App A, Section 2.4 

2NPAP adequacy requirements on AMTIC,

3NPAP QAPP/SOP

Ozone Transfer Standard – Level 2 

Certification/

recertification to 

SRP (Level 1)

Once per year Single point difference 

≤ ± 3%

140 CFR Part 50, App D, Section 5.4 

2,3Transfer Standard Guidance, EPA-454/B10-001

Level 2 and greater 

transfer standard 

precision

Once per year Standard deviation less 

than 0.005 ppm or 3%, 

whichever is greater

140 CFR Part 50, App D, Section 3.1 

2Recommendation: part of re-verification 

340 CFR Part 50, App D, Section 3.1

If recertified via a 

transfer standard

Once per year Regression slopes = 1.00 

± 0.03 and two intercepts 

are 0 ± 3 ppb

1,2,3Transfer Standard Guidance, EPA545/B-10-001

Ozone Transfer Standard – Level 3 and Level 4

Certification After qualification 

and upon receipt/ 

adjustment/repair

RSD of six slopes ≤ 3.7% 

Standard deviation of 

6 intercepts ≤ 1.5 ppb

1,2,3Transfer Standard Guidance, EPA545/B-10-001 1

Technical systems 

audit of monitoring 

organization 

program and 

facilities

Once per 3 years Compliance with 

established regulations

1,2 40 CFR Part 58, App A, Section 2.5, 

3QA Handbook, Volume 2, Section 15.3

Note: AMTIC = Ambient Monitoring Technology Information Center

App = appendix

NPAP = National Performance Audit Program

QAPP = quality assurance project plan

SOP = standard operating procedures

SRP = standard reference photometer

RSD = relative standard deviation 1 Audit requirement citation 2 Frequency requirement citation 3 Acceptance criteria citation
CASTNET 2012 Annual Report

59 Chapter 4: Ozone Concentrations

Semiannual Recertification for Site Transfer Standards

Each site utilizes a second in-station photometer as a transfer standard to verify site analyzer 

performance each day. To perform the semiannual recertifications of these transfer standards, the 

concentration of a calibration gas is introduced to the system, and the concentration detected by the 

site transfer standard is compared with the concentration detected by a traveling transfer standard with 

Level 2 authority using the statistical criteria listed in Table 4b. During the semiannual recertification at 

EPA-sponsored sites, the internal O 

3 generation system of the on-site transfer standard is used to 

generate specific concentrations of O 

3 as the calibration gas. At NPS-sponsored sites, the site 

analyzer generates the calibration gas. 

Semiannual Verification for Level 2 Transfer Standards

The bench standards that are used in the CASTNET instrumentation laboratories and the traveling 

transfer standards that are used for the recertification of site transfer standards are defined as Level 2 

transfer standards in the EPA traceability hierarchy depicted in Figure 4d. Level 2 standards are 

verified annually by a National Institute of Standards and Technology (NIST) standard reference 

photometer (SRP) or EPA Regional SRP. Additionally, traveling transfer standards are audited 

approximately twice per calendar quarter against the Level 2 laboratory bench standards. Failing 

standards are removed from service until they are repaired and recertified. 

Annual Performance Evaluation

Using a technical process similar to that used for recertification of site transfer standards, an 

independent auditor brings a Level 2 traveling transfer standard to each site on an annual basis to 

verify site analyzer performance. The auditor compares the results reported by the site analyzer with 

the Level 2 traveling transfer standard as described in Table 4b.

National Performance Audit Program

The National Performance Audit Program (NPAP) audits follow a technical process similar to that 

utilized by auditors performing the annual performance evaluation but are managed directly by the 

EPA Office of Air Quality Planning and Standards. NPAP audit acceptance criteria are tighter than 

criteria used for the annual performance evaluation while audits may be performed less frequently 

(Table 4b), depending on the site. However, all sites will be audited at least once every five years. 

State Audit Programs

Monitoring programs in states where CASTNET monitors are located may require additional audits. 

State, tribal, or local monitoring organizations are welcome to perform audits at CASTNET sites as 

needed for their individual programs with advance notice provided to EPA, NPS, or BLM.

Coweeta, NC (COW137)
CASTNET 2012 Annual Report

60 Chapter 4: Ozone Concentrations

Figure 4d Ozone Standard Verification and Traceability* Hierarchy

Audit Summary for 2012

Table 4c summarizes audits performed for 2012. Since this was the first complete year of 

regulatory protocol network operation for EPA-sponsored sites, some portions of the audit program 

were initiated in the third calendar quarter. The audit procedures used by NPS and the audit results 

through 2011 are provided on the web site: http://www.nature.nps.gov/air/monitoring/network.cfm.

Speedwell, TN (SPD111)

AMEC traveling transfer 

standards

(Level 2)

A A B B

D

CASTNET station analyzer

CASTNET station transfer standard

(Level 3)

Stationary transfer standard at the 

AMEC Gainesville, FL facility

(Level 2)

National Performance Audit 

Program (NPAP) 40CFR58 

Appendix A Section 2.4

traveling transfer standard

(Level 2)

Third party auditor traveling 

transfer standard 

(Level 2)

EPA SRP

(Level 1)

NIST SRP

(Level 1)

NIST SRP

(Level 1)

EPA SRP

(Level 1)

E

F

G

C

Legend

A = Annual reverification

B = Quarterly reverification

C = Audited ~ once/6 weeks

D = reverification once/6 months

E = Zero, span and single point QC check daily

F = Audited annually

G = Audited once/5 years

Level 2 O3

Transfer Standard

Level 3 O3

Transfer Standard

Level 4 O3

Transfer Standard

NIST

SRP2 EPA SRP1 or SRP7

Regional EPA SRPs

Level 1

Standard Ref. Photometers

Site or Field Ozone Analyzer

1

2

3

Steps from SRP

4

*Traceability
CASTNET 2012 Annual Report

61 Chapter 4: Ozone Concentrations

Table 4c 2012 Audit Summary

Sites* or Equipment Audited Audit/Certification Performed Results

All sites Recertification of site transfer standard All sites passed

All Level 2 standards utilized 

during the year

Certification to Level 1 standard for Level 2 

standards

All standards passed

All sites Annual performance evaluation All sites passed

BEL116, MD

BWR139, MD

HOW132, ME

HWF187, NY

KEF112, PA

LAV410, CA

SEK430, CA

National Performance Audit Program All sites passed

BFT142, NC

CND125, NC

AMEC field instrumentation 

laboratory, FL 

Technical systems facilities audit Minor findings related to 

documentation.**

BEL116, MD

BWR139, MD

State of Maryland technical systems and 

performance audit

All sites passed

GTH161, CO

ROM206, CO

ROM406, CO

State of Colorado technical systems and 

performance audit

All sites passed

Note: *Please refer to site locations table in Appendix A for detailed information.

**The complete audit report and AMEC response are posted to the EPA CASTNET website - 

http://epa.gov/castnet/javaweb/ozone.html

Rocky Mountain National Park, CO (ROM206)
CASTNET 2012 Annual Report

62 Chapter 5: Data Quality

Data quality indicators (DQI), such as precision, accuracy, and completeness, are used to assess CASTNET 

measurements and supporting activities. Routine assessment and analysis help guarantee the production of 

high-quality data and information to meet project objectives. Measurements taken during 2012 and historical 

data collected over the period 1990–2011 were analyzed relative to DQI and their associated metrics. These 

analyses demonstrate that CASTNET data can be used with confidence and that CASTNET continues to 

produce information of the highest quality.

Chapter 5: D ata 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. The QA program 

was developed to deliver data that are reproducible and comparable with data from other 

monitoring networks and laboratories. The 2012 QA program elements are documented in the 

CASTNET Quality Assurance Project Plan (QAPP), Revision 8.0 (AMEC, 2012d). The QAPP 

includes standards and policies for all components of project operation from site selection 

through final data reporting. The QAPP is comprised of a project overview and sections on field 

measurements, chemical analysis of field samples, data management, and 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 2012 data were analyzed and compared with historical data 

collected during the period 1990 through 2011. Comparability is supported by the use of 

conventional and standard units for reporting results and participation in laboratory 

intercomparison studies. The information in this report is supplemented by analyses that are 

discussed in quarterly QA reports (AMEC, 2012c; 2012f; 2012g; 2013c). 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 (ROM406/206) during 2012. The 

MCK131/231 sites are operated by EPA. The ROM406/206 monitors are operated 

independently by NPS and EPA, respectively, and ensure consistency between NPS and EPA

monitoring activities. At the MCK131/231 site pair, both O 

3 monitors are regulatory. At the 

ROM406/206 site pair, ROM406 is the regulatory O 

3 monitoring site. NPS began operating a 

collocated O 

3 monitoring site at YOS204, CA in June 2012. At the YOS404/204 site pair, 

YOS404 is the regulatory O 

3 monitoring site. Beginning with 2012 measurements, precision of 
CASTNET 2012 Annual Report

63 Chapter 5: Data Quality

O 

3 measurements was estimated based on requirements in 40 CFR and the EPA QA Handbook 

for Air Pollution Measurement Systems, Volume II, Appendix D (2008b). In other words, 

precision of O 

3 measurements was assessed based on comparisons with a transfer standard.

In addition to precision of O 

3 measurements, precision of other measurement parameters based 

on collocated data is also presented in this report. Meteorological measurements were 

performed at 6 EPA, 25 NPS, and 4 BLM sites in 2012. Precision was calculated for 2012 filter 

pack flow and temperature data, which are two measurements that are continuing at all 

CASTNET sites. Accuracy was estimated for all parameters.

Summary of Data Quality Results

DQI results demonstrate that CASTNET field and laboratory processes were sufficiently 

monitored through QA/quality control (QC) procedures and were free of systemic bias 

during 2012.

Historical and 2012 precision data for filter pack parameters are considered acceptable. An 

analysis of filter pack bias revealed no convincing pattern of bias at either ROM406/206 or 

MCK131/231. O 

3 precision was estimated based on requirements in 40 CFR and the EPA QA 

Handbook for Air Pollution Measurement Systems, Volume II, Appendix D (2008b). O 

3 precision 

was acceptable at all but two sites. The precision of quarterly and annual flow and temperature 

measurements for 2012 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 2012. Completeness results for 2012 are considered 

excellent. Results from Environment Canada (ECAN) proficiency test studies (0100 and 0101) 

showed that the CASTNET laboratory performed well for the interlaboratory chemical analyses

(ECAN, 2013).

Precision

Exposed Filter Concentrations

Historical (1990 through 2011) mean absolute relative percent difference (MARPD) data for filter 

pack concentrations for all 11 collocated site pairs operated over the history of the network and 

the 2012 data for the current collocated sites at MCK131/231 and ROM406/206 are provided in 

Figure 5-1. Precision estimates for base cations and Clare shown later in Figure 5-2. Table 5-1

lists the precision and accuracy criteria for laboratory filter pack measurements.
CASTNET 2012 Annual Report

64 Chapter 5: Data Quality

Figure 5-1 Historical and 2012 Precision Data for Atmospheric Concentrations

Table 5-1 Data Quality Indicator Criteria for CASTNET Laboratory Measurements

Analyte Method

Precision1

(MARPD)

Accuracy2

(%)

Nominal

Reporting Limits

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

ICP-AES = inductively coupled plasma-atomic emission spectrometry

IC = ion chromatography

mg/l = milligrams per liter

μg/Filter = micrograms per filter

* = as nitrogen

Values are rounded according to American Society for Testing and Materials (2008).

For more information on analytical methods and associated precision and accuracy criteria, see the CASTNET QAPP, 

Revision 8.0 (AMEC, 2012d)

0

5

10

15

20

25

30

Sulfur Dioxide Sulfate Nitric Acid Nitrate Ammonium

MARPD

Overall - 1990 through 2011

Mackville, KY - 2012

Rocky Mtn, CO - 2012
CASTNET 2012 Annual Report

65 Chapter 5: Data Quality

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 2012 were within

the 20 percent criterion (dashed line in Figure 5-1) for the CASTNET filter pack measurements 

shown in Table 5-1. The precision of historical and 2012 NO3 measurements was worse than for 

the other analytes, largely because NO3 concentrations include sampling artifacts.

The precision results for all parameters shown in Figure 5-1 indicate that the MARPD data for 

MCK131/231 were lower in 2012 (i.e., more precise) than the historical results. The 2012 results 

for ROM406/206 showed better precision than historical precision for NO3 and NH+ 

4 and were 

about the same as historical precision for the other three parameters. 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 2012 

as in 2011. The precision values for ROM406/206 showed some improvement in 2012.

Historical and 2012 precision statistics for four base cations and Clare summarized in 

Figure 5-2. The historical and 2012 MARPD statistics for both MCK131/231 and ROM406/206 

met the DQI criterion of 20 percent (dashed line). The 2012 precision results for the four base

cations and Clwere about the same as 2011 at both collocated sites.

Table 5-2 summarizes 2012 precision results by quarter for the two sets of collocated sites. See 

the 2012 Quarterly Data Reports (AMEC, 2012b; 2012e; 2013b; 2013e) and Quarterly QA 

Reports (AMEC, 2012c; 2012f; 2012g; 2013c) for discussions of quarterly precision data.

Figure 5-2 Historical and 2012 Precision Data for Cation and Cl

-

Concentrations

0

5

10

15

20

25

30

1 2 3 4 5

Overall - 2000 through 2011

Mackville, KY - 2012

Rocky Mtn, CO - 2012

MARPD

Ca2+ K+ Mg2+ Na+ Cl- Calcium Potassium Magnesium Sodium Chloride
CASTNET 2012 Annual Report

66 Chapter 5: Data Quality

Table 5-2 Collocated Precision Results (MARPD) for 2012 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

Quarter 1 3.25 5.80 2.88 9.68 8.52 5.26 4.86 3.82 3.63 3.35 4.70

Quarter 2 3.44 8.02 3.67 7.60 7.76 4.41 5.36 3.14 3.86 3.76 0.99

Quarter 3 3.05 7.77 2.59 6.11 5.98 4.15 4.84 5.08 6.81 4.67 1.98

Quarter 4 4.36 5.61 3.81 9.13 9.43 5.58 5.20 3.34 4.56 4.26 5.09

2012 3.53 6.80 3.24 8.13 7.92 4.85 5.07 3.85 4.72 4.01 3.19

ROM406/206

Quarter 1 5.95 6.73 4.73 6.05 8.12 8.03 5.88 10.16 5.38 7.85 9.67

Quarter 2 3.84 5.73 4.25 4.43 6.05 3.87 6.94 8.23 4.06 6.15 7.73

Quarter 3 4.42 10.19 5.25 4.70 5.21 5.26 8.03 7.65 6.68 7.13 6.25

Quarter 4 3.56 13.87 7.53 5.56 7.54 7.45 14.52 9.36 12.17 7.66 3.66

2012 4.45 9.13 5.44 5.19 6.73 6.15 8.84 8.85 7.07 7.20 6.83

Towers at Mackville, KY (MCK131/231) and Rocky Mountain 

National Park, CO (ROM406/206)
CASTNET 2012 Annual Report

67 Chapter 5: Data Quality

Filter Pack Bias Comparison

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

and to model depositions over many years. Consequently, measurement of weekly average 

concentrations was selected as the basic sampling strategy. Over the course of seven days, air is 

drawn at a controlled flow rate through an open-face, 3-stage filter pack (Figure 5a) mounted atop a 

10-m tower to collect air pollutants in the form of gases and particles. CASTNET’s principal 

measurements are SO 

2, SO2-4 , HNO 3, NO3, and NH+ 4 . In addition to measurements of sulfur and 

nitrogen pollutants, each CASTNET site also includes measurements of base cations and Cl-

. The first 

stage of the filter pack encloses a Teflon filter to collect particles; the second, a nylon filter to collect 

gaseous HNO 

3 and SO 2; and the third holds two potassium carbonate-impregnated cellulose filters to 

collect gaseous SO 

2. The filter pack is changed out each Tuesday and shipped to the analytical 

chemistry laboratory for analysis.

Figure 5a Three-Stage Filter Pack

Two CASTNET sites include collocated sampling systems for determining network precision. Both of 

the monitors at the collocated site pair in Kentucky, MCK131 and MCK231, are sponsored by EPA and 

have the same site operator. Each of the monitors at the collocated site pair in Colorado, ROM406 and 

ROM206, has a different sponsor and site operator. ROM406 is sponsored by NPS, and ROM206 is 

sponsored by EPA. Historical concentration data from these collocated sites were analyzed for 

potential bias. A result exceeding the collocated filter pack sample precision criterion listed in 

Table 5-1 may indicate that equipment, procedures, or site configuration have not been sufficient to 

prevent the introduction of bias. If such a result had been obtained, further investigation including, but 

not limited to, equipment, field and laboratory procedures, and site configuration would have been

initiated to determine the cause and appropriate corrective action. All results from the comparison 

presented here were well within the listed criterion. Airflow
CASTNET 2012 Annual Report

68 Chapter 5: Data Quality

Two methods of data analysis were chosen for this investigation: 1) standard correlation analysis 

and 2) a bias calculation from 40 CFR Part 58 Appendix A, Section 4.1.3 (EPA, 2012a), based on the 

upper bound on the mean absolute value of the percent differences (%D) of each collocated pair as 

described in Equation 5a.

Equation 5a

|AB| = AB + t0.95, n − 1 ∗

AS

√n

where n is the number of single point checks being aggregated; t0.95, n – 1 is the 95th quantile of a 

t-distribution with n – 1 degrees of freedom; the quantity AB is the mean of the absolute values of the 

di and is calculated using Equation 5b.

Equation 5b

AB = 1

n ∗ �|di|

n

i=1

where di = (measured concentration – audit concentration)/audit concentration, and the audit 

concentrations were measured at MCK131 or ROM406 (i.e., the primary of the collected pair), and the 

quantity AS is the standard deviation of the absolute value of the di and is calculated using Equation 

5c.

Equation 5c

AS = �n�|di|2

n

i=1

− ��|di|

n

i=1

�

2

n(n − 1)

Since the bias statistic calculated in Equation 5a uses absolute values, it does not have a negative or 

positive bias associated with it. A sign is designated by rank ordering the percent differences of the 

collocated pair. The 75th and 25th percentiles are calculated, and the absolute bias upper bound is

flagged as positive if both percentiles are positive and flagged as negative if both percentiles are 

negative. The absolute bias upper bound is not flagged if the percentiles are different signs.

The bias of particulate SO2-

4 and NO3 was investigated, along with gaseous HNO 3, due to their 

importance to the project. However, because SO2-

4 concentrations differ by an order of magnitude at 

the two monitoring locations, Ca2+

 was selected to provide a stable on-filter particulate that showed 

similar concentrations at both locations. Data analyses were performed for data collected from 

July 2001 through December 2012 using only valid reported values above their established 

reporting limits.
CASTNET 2012 Annual Report

69 Chapter 5: Data Quality

ROM406/206 Bias (%)

n ∑|d| "AB" (eq 5b)

562 3602.581 6.410

n-1 ∑|d|

2 "AS" (eq 5c)

561 111664.336 12.565

Bias (%) (eq 5a) Both Signs Positive

7.28 TRUE

Signed Bias (%) Both Signs Negative

+7.28 FALSE

MCK131/231 Bias (%)

n ∑|d| "AB" (eq 5b)

554 1717.814 3.101

n-1 ∑|d|

2 "AS" (eq 5c)

553 15084.434 4.201

Bias (%) (eq 5a) Both Signs Positive

3.39 FALSE

Signed Bias (%) Both Signs Negative

-3.39 TRUE

y = 1.012x + 0.029

R2 = 0.991

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20

MCK Sulfate

Linear (MCK Sulfate)

Summary

Particulate SO2-

4 correlation (Figure 5b) at MCK131/231 showed an R2 of 0.991 and slope of 1.012 

compared with R2 and slope of 0.930 and 0.923, respectively, for ROM406/206. The intercepts were 

similar at 0.029 and 0.021, respectively. Both sites showed signed biases (Figure 5c) with 

MCK131/231 at -3.39 percent and ROM406/206 at +7.28 percent.

Figure 5b Correlation Analysis for SO2-

4

Figure 5c Bias Analysis for SO2-

4

-30

-20

-10

0

10

20

30

Percent Differences

MCK131/231

%D

-30

-20

-10

0

10

20

30

Percent Differences

ROM406/206

%D

y = 0.923x + 0.021

R2 = 0.930

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.0 0.5 1.0 1.5 2.0

ROM Sulfate

Linear (ROM Sulfate)
CASTNET 2012 Annual Report

70 Chapter 5: Data Quality

ROM406/206 Bias (%)

n ∑|d| "AB" (eq 5b)

545 6333.635 11.621

n-1 ∑|d|

2 "AS" (eq 5c)

544 156103.240 12.315

Bias (%) (eq 5a) Both Signs Positive

12.49 FALSE

Signed Bias (%) Both Signs Negative

±12.49 FALSE

MCK131/231 Bias (%)

n ∑|d| "AB" (eq 5b)

551 5395.138 9.792

n-1 ∑|d|

2 "AS" (eq 5c)

550 231335.089 18.016

Bias (%) (eq 5a) Both Signs Positive

11.06 FALSE

Signed Bias (%) Both Signs Negative

±11.06 FALSE

y = 1.030x + 0.014

R2 = 0.983

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

MCK Nitrate

Linear (MCK Nitrate)

Particulate NO3 correlation (Figure 5d) at MCK131/231 showed an R2 of 0.983 and slope of 1.030

while R2 for ROM406/206 was 0.947 and the slope 0.915. Intercepts were again similar at 0.014 and 

0.010, respectively. There was no signed bias (Figure 5e) at either site with MCK131/231 showing 

11.06 percent and ROM406/206 showing 12.49 percent.

Figure 5d Correlation Analysis for NO3

Figure 5e Bias Analysis for NO3

-30

-20

-10

0

10

20

30

Percent Differences

MCK131/231

%D

-30

-20

-10

0

10

20

30

Percent Differences

ROM406/206

%D

y = 0.915x + 0.010

R2 = 0.947

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.5 1.0 1.5

ROM Nitrate

Linear (ROM Nitrate)
CASTNET 2012 Annual Report

71 Chapter 5: Data Quality

MCK131/231 Bias (%)

n ∑|d| "AB" (eq 5b)

578 4697.215 8.127

n-1 ∑|d|

2 "AS" (eq 5c)

577 118542.762 11.802

Bias (%) (eq 5a) Both Signs Positive

8.94 FALSE

Signed Bias (%) Both Signs Negative

-8.94 TRUE

The R2, slope, and intercept values for particulate Ca2+

 (Figure 5f) were 0.957, 1.058, and 0.001 for 

MCK131/231 and 0.969, 0.938, and 0.002 for ROM406/206. There was no signed bias (Figure 5g) for 

ROM406/206 at 15.46 percent, but MCK131/231 showed a signed bias at -8.94 percent.

Figure 5f Correlation Analysis for Ca2+

 

Figure 5g Bias Analysis for Ca2+

 

-30

-20

-10

0

10

20

30

Percent Differences

MCK131/231

%D

-30

-20

-10

0

10

20

30

Percent Differences

ROM406/206

%D

ROM406/206 Bias (%)

n ∑|d| "AB" (eq 5b)

562 7206.192 12.822

n-1 ∑|d|

2 "AS" (eq 5c)

561 899254.330 37.924

Bias (%) (eq 5a) Both Signs Positive

15.46 FALSE

Signed Bias (%) Both Signs Negative

±15.46 FALSE

y = 0.938x + 0.002

R2 = 0.969

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

ROM Calcium

Linear (ROM Calcium)

y = 1.058x + 0.001

R2 = 0.957

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

MCK Calcium

Linear (MCK Calcium)
CASTNET 2012 Annual Report

72 Chapter 5: Data Quality

MCK131/231 Bias (%)

n ∑|d| "AB" (eq 5b)

554 2497.598 4.508

n-1 ∑|d|

2 "AS" (eq 5c)

553 22455.832 4.500

Bias (%) (eq 5a) Both Signs Positive

4.82 FALSE

Signed Bias (%) Both Signs Negative

±4.82 FALSE

Finally, results for gaseous HNO 

3 (Figure 5h) for R2

, slope, and intercept at MCK131/231 were 0.980, 

0.994, and 0.030 while ROM406/206 showed 0.950, 0.969, and 0.001. Neither site showed a signed 

bias (Figure 5i) with unsigned values of 4.82 percent and 8.22 percent, respectively. Overall, these 

analyses reveal no convincing pattern showing filter pack sampling bias at either site.

Figure 5h Correlation Analysis for HNO 

3

Figure 5i Bias Analysis for HNO 

3

-30

-20

-10

0

10

20

30

Percent Differences

MCK131/231

%D

-30

-20

-10

0

10

20

30

Percent Differences

ROM406/206

%D

ROM406/206 Bias (%)

n ∑|d| "AB" (eq 5b)

561 4188.226 7.466

n-1 ∑|d|

2 "AS" (eq 5c)

560 96344.428 10.780

Bias (%) (eq 5a) Both Signs Positive

8.22 FALSE

Signed Bias (%) Both Signs Negative

±8.22 FALSE

y = 0.969x + 0.001

R2 = 0.950

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.0 0.5 1.0 1.5 2.0

ROM Nitric Acid

Linear (ROM Nitric Acid)

y = 0.994x + 0.030

R2 = 0.980

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

MCK Nitric Acid

Linear (MCK Nitric Acid)
CASTNET 2012 Annual Report

73 Chapter 5: Data Quality

The 2012 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 criteria listed in Table 5-1.

Figure 5-3 Precision Results for Laboratory Replicate Samples (2012)

Ozone Concentrations

O 

3 data quality was evaluated in terms of AQS protocol O 3 QC criteria (i.e., precision per 

40 CFR Part 58). AQS protocol O 

3 precision was estimated based on the acceptance criteria set 

forth in 40 CFR Part 58 Appendix A (EPA, 2012a). The O 

3 instrument was challenged daily with 

a known O 

3 concentration of 90 ppb. The acceptance criterion is an upper 90 percent 

confidence limit (CL) for the coefficient variation (CV) of 7 percent, i.e., 90 percent CL CV 

≤ 7 percent. The 2012 results for EPA-sponsored sites are presented in Table 5-3, which

provides the 90 percent CL and the number of values used to calculate it. O 

3 precision

data collected at only two EPA-sponsored sites, ALC188, TX and SUM156, FL, did not meet 

the criterion. Ambient data were not affected in either case. Both problems were isolated to the 

pneumatic system used to introduce known O 

3 concentrations.

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

contribute to sulfur dioxide nitric acid Nitric Acid
CASTNET 2012 Annual Report

74 Chapter 5: Data Quality

Table 5-3 Single Point Precision for 2012 O3 Measurements at EPA-Sponsored Sites

Site

Precision

Site

Precision

Site

Precision

Single 

Point QC 

Count

Single 

Point QC 

CL

Single 

Point QC 

Count

Single 

Point QC 

CL

Single 

Point QC 

Count

Single 

Point QC 

CL

ABT147, CT 358 0.06 CVL151, MS 357 0.17 PED108, VA 367 0.05

ALC188, TX 353 41.83a DCP114, OH 370 3.99 PND165, WY 304 0.06

ALH157, IL 341 0.04 ESP127, TN 372 0.37 PNF126, NC 366 0.03

ANA115, MI 365 0.54 GAS153, GA 363 0.46 PRK134, WI 338 0.27

ARE128, PA 356 1.88 GTH161, CO 309 0.57 PSU106, PA 362 0.04

ASH135, ME 366 0.11 HOW132, ME 294 0.09 QAK172, OH 335 0.73

BEL116, MD 324 1.06 HOX148, MI 350 0.12 ROM206, CO 364 0.23

BFT142, NC 313 0.60 HWF187, NY 368 0.18 SAL133, IN 367 0.07

BVL130, IL 376 1.16 IRL141, FL 369 0.10 SAN189, NE 366 1.17

BWR139, MD 368 4.64 KEF112, PA 368 0.05 SND152, AL 365 0.04

CAD150, AR 371 0.52 KNZ184, KS 368 0.27 SPD111, TN 368 0.14

CDR119, WV 355 0.06 LRL117, PA 318 0.89 STK138, IL 338 0.47

CDZ171, KY 364 0.67 MCK131, KY 368 0.08 SUM156, FL 343 21.14b

CKT136, KY 349 0.06 MCK231, KY 371 0.16 UVL124, MI 359 0.09

CND125, NC 364 1.10 MKG113, PA 356 0.93 VIN140, IN 371 4.14

CNT169, WY 343 2.28 OXF122, OH 351 0.11 VPI120, VA 330 0.37

COW137, NC 360 0.09 PAL190, TX 364 6.14 WSP144, NJ 353 0.09

CTH110, NY 343 0.05 PAR107, WV 293 0.08 WST109, NH 363 0.48

Note: a There were problems with the zero air system.

b Transfer standard tubing was improperly connected.

Basin, WY (BAS601)
CASTNET 2012 Annual Report

75 Chapter 5: Data Quality

Table 5-4 Data Quality Indicator Criteria for CASTNET Field Measurements

Measurement Criteria1

Parameter2 Method Precision Accuracy

Filter pack flow Mass flow controller ± 10% ± 5%

Ozone3 UV absorbance All points within ± 2% of full scale of best fit straight line

Linearity error < 5%

Wind speed Anemometer ± 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 ± 5° ± 5°

Sigma theta Wind vane Undefined Undefined

Ambient temperature Platinum RTD ± 1.0°C ± 0.5°C

Delta temperature Platinum RTD ± 0.5°C ± 0.5°C

Relative humidity Thin film capacitor ± 10% (of full scale) ± 10%

Precipitation Tipping bucket rain gauge ± 10% (of reading) ± 0.05 inch4

Solar radiation Pyranometer ± 10% (of reading taken at 

local noon)

± 10%

Surface wetness Conductivity bridge Undefined Undefined

Note: °C = degrees Celsius

m/s = meters per second

RTD = resistance-temperature device

UV = ultraviolet 1 Precision criteria apply to collocated instruments, and accuracy criteria apply to calibration of instruments. Collocated 

precision criteria do not apply to AQS-protocol ozone measurements. 2 As of the end of 2012, meteorological parameters were only measured at 6 EPA-sponsored sites, 25 NPS-sponsored sites, 

and 4 BLM-sponsored sites. 3 Ozone is measured at 83 CASTNET sites. 4 For target value of 0.50 inch

Continuous Measurements

Precision criteria for the continuous measurements are listed in Table 5-4. Because continuous 

measurements, excluding O 

3, flow, and temperature, were discontinued at most EPA-sponsored

sites at the end of 2010, including MCK131, MCK231, and ROM206, precision estimates were 

made only for flow and temperature data. Figure 5-4 provides precision results for flow and 

temperature for historical data (1990 through 2011) at all collocated site pairs and 2012 

precision statistics at the two current collocated site pairs. Precision was calculated in terms of 

the MARPD for flow and mean absolute difference (MAD) for temperature. All historical and 

2012 precision results met the DQI criteria. Table 5-5 gives precision data by quarter for 2012 

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 2012 Annual Report

76 Chapter 5: Data Quality

Figure 5-4 Historical and 2012 Precision Data for Flow and Temperature Measurements

Note: The left y-axis indicates MARPD for flow (Table 5-5), and the right y-axis indicates MAD for temperature (Table 5-5).

Table 5-5 Collocated Precision Results for 2012 Continuous Measurements by Quarter

Sampling Period

MARPD MAD

Flow Rate Temperature

MCK131/231, KY

Quarter 1 0.17 0.15

Quarter 2 0.14 0.13

Quarter 3 0.13 0.14

Quarter 4 0.13 0.17

2012 0.14 0.15

ROM406/206, CO

Quarter 1 1.09 0.19

Quarter 2 3.88 0.16

Quarter 3 3.85 0.22

Quarter 4 0.11 0.17

2012 2.23 0.18

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 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 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0

2

4

6

8

10

Flow Rate Temperature

MAD

MARPD

MARPD: Overall - 1990 through 2011

MARPD: Mackville, KY - 2012

MARPD: Rocky Mtn, CO - 2012

MAD: Overall - 1990 through 2011

MAD: Mackville, KY - 2012

MAD: Rocky Mtn, CO - 2012
CASTNET 2012 Annual Report

77 Chapter 5: Data Quality

established limits. Table 5-6 presents the percent recoveries and standard deviations for 

reference samples and CCV relative to target concentrations for 2012. The table shows that the 

DQI goals (see Table 5-1) were met in 2012.

Table 5-6 Filter Pack Quality Control Summary for 2012 Filter Type Parameter

Reference

Sample1 Recovery

(%R)

Continuing Calibration 

Verification Samples (%R)

In-Run Replicate2

(RPD)

Mean

Standard Deviation

Count3MeanStandard DeviationCount3MeanStandard DeviationCount3

Teflon SO2-

4 99.07 1.48 129 100.80 1.38 644 0.52 0.69 276

NO3 101.09 1.43 129 100.75 1.32 644 1.29 1.51 306

NH+ 

4 101.92 1.75 128 100.30 1.42 642 0.57 0.64 338

Ca2+ 99.13 1.97 129 100.57 0.93 647 0.88 0.88 300

Mg2+ 102.91 1.52 129 100.01 0.84 647 1.47 1.52 300

Na+ 100.32 1.29 129 100.01 0.81 647 1.11 1.21 300

K

+ 100.77 2.03 129 100.00 0.66 647 1.76 2.12 300

Cl- 102.09 1.56 129 101.90 1.30 644 1.29 1.41 270

Nylon SO2-

4 99.48 1.25 122 100.65 1.42 609 1.57 1.65 259

NO3 101.13 1.25 122 100.42 1.25 609 1.23 1.20 293

Cellulose SO2-

4 98.18 1.19 144 98.85 1.42 623 2.10 2.90 305

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

Continuous Measurements

CASTNET sites are calibrated every six months with NIST-traceable standards. Table 5-7 

presents field accuracy results for 2012 based on instrument challenges performed at EPAsponsored sites during site calibration visits. The calibration results were evaluated using the 

accuracy criteria listed in Table 5-4. Each parameter was within its criterion with at least 

90 percent frequency with the exception of wind speed less than 5 meters per second (m/s) at 

87.5 percent, wind speed greater than or equal to 5 m/s at 87.5 percent, wind direction south at 

88.9 percent, and solar radiation at 88.9 percent. However, these results did not adversely 

affect data collection because data are not considered invalid unless the calibration criterion is 

exceeded by more than two times the criterion. Using the two times standard, the four 

parameters passed with 100.0 percent frequency. The 2012 results for flow rate accuracy were 

better than for 2011.
CASTNET 2012 Annual Report

78 Chapter 5: Data Quality

Table 5-7 Accuracy Results for 2012 Field Measurements

Parameter* Percent Within 

Criterion Parameter Percent Within Criterion

Flow Rate 97.3 percent Delta Temperature (0°C) 100.0 percent Wind Speed < 5 m/s 87.5 percent Delta Temperature (ambient) 100.0 percent Wind Speed ≥ 5 m/s 87.5 percent Relative Humidity 100.0 percent

Wind Direction North 100.0 percent Precipitation 100.0 percent Wind Direction South 88.9 percent Solar Radiation 88.9 percent

Temperature (0°C) 99.1 percent Wetness (w/in 0.5 volts) 100.0 percent

Temperature (ambient) 99.1 percent

Note: °C = degrees Celsius

m/s= meters per second

* = Results are based on all EPA-sponsored sites for flow rate and temperature. Results for all other parameters, 

including delta temperature, are based on the six EPA-sponsored sites that measured meteorological parameters

during 2012.

 = Per CASTNET project protocols, data are flagged as “suspect” (S) but still considered valid if the calibration criterion is 

not exceeded by more than its magnitude (i.e., if within 2x the criterion). All calibration failures reported in 2012 for the 

indicated parameters were within 2x the criterion.

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 2012, all the parameters listed in Figure 5-5, except delta temperature and 

wetness, were measured at the NPS-sponsored sites, BLM-sponsored sites, and the six EPAsponsored sites with ongoing meteorological measurements. Ozone concentrations were 

measured at 83 of 91 monitoring sites. Atmospheric concentrations, filter pack flow, and 

ambient temperature were measured at all sites. Delta temperature was measured at five out of 

six EPA sites plus the NPS sites at Acadia, ME (ACA416); GRS420, TN; and ROM406, CO. 

Surface wetness was measured at the six EPA sites plus ACA416, ME. Figure 5-5 presents 

historical (black) and 2012 completeness data for all sites for measured filter concentrations and 

continuous measurements. The figure shows that the 2012 direct measurements met the 

90 percent completeness goal, and seven meteorological measurements plus filter pack flow 

exceeded 95 percent. Atmospheric concentrations also exceeded 95 percent.
CASTNET 2012 Annual Report

79 Chapter 5: Data Quality

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

Figure 5-5 Historical and 2012 Percent Completeness of Measurements (black bars are

1990–2011)

Palo Duro Canyon State Park, TX (PAL190)
CASTNET 2012 Annual Report

80 Chapter 5: Data Quality

Results for 2012 Environment Canada Proficiency Testing

During 2012, the AMEC CASTNET laboratory participated in the ECAN Proficiency Testing Program 

for Inorganic Environmental Substances (http://www.ec.gc.ca/inrenwri/default.asp?lang=En&n=7A20877C-1). Specifically, the CASTNET laboratory is one of 

35 laboratories that participated in the 2012 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 six months. The laboratory reported the eight CASTNET parameters for samples in two 

studies (study codes 0100 and 0101) during 2012.

The results reported by the 35 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; and 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 2012 ECAN results for the eight CASTNET parameters are presented in 

Table 5a. All eight parameters received the highest possible rating of “Ideal” indicating an unbiased 

score in study 0100. One of the 10 individual results for ammonia was flagged “warning high” for study 

0101, indicating that the flagged result was between 2 and 3 R-σ from its assigned value. Investigation 

by the laboratory revealed the root cause to be incorrect significant figures used in reporting laboratory 

results. The laboratory had been reporting results to two significant figures when three significant 

figures were required, resulting in inappropriate rounding of the final result. If three significant figures 

had been used, the data would have passed with no bias flags. Corrective action was implemented to 

correct this on the forms used to report ECAN study data.

Table 5a AMEC Results for Studies 0100 and 0101

Test Parameter Analytical 

Method Reference Method

Laboratory Performance Rating

Study 0100 

Summer 2012

Study 0101 Winter 

2012

Ammonia AC EPA Method 350.1 Ideal Biased High

Calcium ICP-AES EPA Method 6010 Ideal Ideal

Chloride IC EPA Method 300.0 Ideal Ideal

Magnesium ICP-AES EPA Method 6010 Ideal Ideal

Nitrate + Nitrite IC EPA Method 300.0 Ideal Ideal

Potassium ICP-AES EPA Method 6010 Ideal Ideal

Sodium ICP-AES EPA Method 6010 Ideal Ideal

Sulfate IC EPA Method 300.0 Ideal Ideal

Note: AC = automated colorimetry

ICP-AES = inductively coupled plasma-atomic emission spectrometry

IC = ion chromatography

Source: Environment Canada (2013)
CASTNET 2012 Annual Report

81 Chapter 5: Data Quality

The overall laboratory performance rating was “Very Good” for both studies. The overall laboratory 

rating indicates a percent score as described in Table 5b. The 5-year historical laboratory rating is 

listed by ECAN as “Very Good.”

Table 5b Laboratory Performance Rating

Laboratory Performance Rating

Rating % Score*

Very Good 0 - 5

Good > 5 - 12.5

Fair > 12.5 - 30

Poor > 30

*Sum of Parameters Biased & Results Flagged

Source: Environment Canada (2013)

Egbert, Ontario, Canada (EGB181)
CASTNET 2012 Annual Report

82 References

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Trends Report, 1998. Report No. EPA-454-R-00-003. OAQPS, Research Triangle 

Park, NC 27711.

U.S. Environmental Protection Agency (EPA). 1998. Guideline on Data Handling Conventions 

for the 8-Hour Ozone NAAQS. EPA-454/R-98-017. OAQPS, Research Triangle Park, 

NC 27711.

U.S. Environmental Protection Agency (EPA). 1996. Air Quality Criteria for Ozone and Related 

Photochemical Oxidants. EPA Report No. EPA/600/P-93/004bF. ORD, Research 

Triangle Park, NC. 

Youden, W.J. (Ku, H.H., ed). 1969. Precision Measurement and Calibration. NBS Special 

Publication 300-Volume 1. U.S. Government Printing Office, Washington, DC.
CASTNET 2012 Annual Report

Appendix A

Appendix A

Locational and Operational Characteristics of

CASTNET Sites
CASTNET 2012 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 Pack1Ozone and Meteorology2Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3Sponsor

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 2012 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 Pack1Ozone and Meteorology2Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3Sponsor

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 2012 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 Pack1Ozone and Meteorology2Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3Sponsor

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

NIC001 Nicks Lake 11/20/12 43.6806 74.98917 525 • •4 NY29 Forested Rolling N NYSDEC

WFM105 Whiteface Mountain 11/20/12 44.39 73.86 570 • •4 NY98 Forested Complex N NYSERDA
CASTNET 2012 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 Pack1Ozone and Meteorology2Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3Sponsor

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 2012 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 Pack1Ozone and Meteorology2Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3Sponsor

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

Vermont

UND002 Underhill 11/13/12 44.52839 72.8688 399 • •4 VT99 Forested Complex 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 2012 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 Pack1Ozone and Meteorology2Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3Sponsor

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

BAS601 Basin 11/06/12 44.28 108.0411 1242 • • MT00 Prairie Rolling M BLM

NEC602 Newcastle 11/07/12 43.87306 104.1919 1468 • • WY99 Prairie Rolling M BLM

BUF603 Buffalo 11/06/12 44.1442 106.1089 1324 • •4 WY99 Prairie Rolling M BLM

SHE604 Sheridan 11/06/12 44.93 106.85 1115 • •4 MT00 Prairie Rolling M BLM

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.

BLM = Bureau of Land Management

EPA = U.S. Environmental Protection Agency

NPS = National Park Service

NYSDEC = New York State Development of Environmental Conservation

NYSERDA = New York State Energy Research and Development Authority
CASTNET 2012 Annual Report

Appendix B

Appendix B

Acronyms and Abbreviations
CASTNET 2012 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

BART Best Available Retrofit Technology

BBNP Big Bend National Park, TX

BLM Bureau of Land Management

BRAVO Big Bend Regional Aerosol and Visibility Observational Study

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

CDC Centers for Disease Control

CL confidence limit

Cl- particulate chloride ion

CLAD Critical Loads of Atmospheric Deposition science committee

CLmaxS maximum critical load of sulfur

CMAQ Community Multiscale Air Quality Modeling System

CLminN nitrogen removed by an ecosystem

CO carbon monoxide

CSN Chemical Speciation Network

CV coefficient variation

%D percent difference

DM8A daily maximum 8-hour average

DQI data quality indicator

ECAN Environment Canada 

EGU electric generating unit

EPA U.S. Environmental Protection Agency

H 

2SO 4 sulfuric acid

HNO 

2 nitrous acid

HNO 

3 nitric acid

IMPROVE Interagency Monitoring of Protected Visual Environments
CASTNET 2012 Annual Report

B-2 Appendix B

List of Acronyms and Abbreviations (continued)

ISO International Organization for Standardization

K+ particulate potassium ion

kg/ha kilograms per hectare

kg/ha/yr kilograms per hectare per year

km kilometer

lpm liters per minute

m meter

m/s meters per second

MAD mean absolute difference

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

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

NCLAN National Crop Loss Assessment Network 

NCLD National Critical Load Database

NCore EPA’s National Core Monitoring

NEI National Emissions Inventory

NH 

3 ammonia

NH +

4 particulate ammonium

NH 

4NO 3 ammonium nitrate

(NH 

4)

 

2SO 4 ammonium sulfate

NIST National Institute of Standards and Technology

NLCD National Land Cover Database

NO nitrogen oxide

NO 

2 nitrogen dioxide

NO -

3 particulate nitrate 

NOx nitrogen oxides [nitrogen oxide (NO) + nitrogen dioxide (NO 

2)]

NOy total reactive oxides of nitrogen

NOz HNO 

3, HNO 2, PAN, NTR, and nitrite

NPAP National Performance Audit Program

NPS National Park Service
CASTNET 2012 Annual Report

B-3 Appendix B

List of Acronyms and Abbreviations (continued)

NTN National Trends Network

NTR organic nitrates 

NYSDEC New York State Department of Environmental Conservation

NYSERDA New York State Energy Research and Development Authority

O 

3 ozone

OTC Ozone Transport Commission

PAN peroxyacytyl nitrate

PANx peroxypropionyl nitrate 

PHASE Public Health Air Surveillance Evaluation

PM 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

RH relative humidity

RPD relative percent difference

S sulfur (SO 

2 + SO2-4 )

SEARCH Southeastern Aerosol Research and Characterization Network

SIP State Implementation Plan

SO 

2 sulfur dioxide

SO2-

4 particulate sulfate

SRP standard reference photometer

ΔT temperature difference 

TDEP NADP’s Total Deposition Science Committee

TCEQ Texas Commission on Environmental Quality

TERP Texas Emission Reduction Plan

total NO3 gaseous nitric acid (HNO 

3) + particulate nitrate (NO3)

μeq/L microequivalents per liter

μm micrometer

μg/m3 micrograms per cubic meter

Vd deposition velocity

VOC volatile organic compounds

WARMS Wyoming Air Resources Monitoring System

WRF Weather Research and Forecasting Model
For More Information:

U.S. Environmental Protection Agency

Office of Air and Radiation

Clean Air Markets Division

Washington, DC

On the Web:

CASTNET Home Page:

www.epa.gov/castnet

Clean Air Markets

Division Home Page:

www.epa.gov/airmarkets

EPA Home Page:

www.epa.gov