Clean Air Status and Trends Network

2010 Annual Report
Clean Air Status and Trends Network

(CASTNET)

2010 Annual Report

Prepared by:

AMEC Environment & Infrastructure, Inc.

formerly known as 

MACTEC Engineering and Consulting, 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 2012
CASTNET Annual Report – 2010 i Table of Contents

Table of Contents

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

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

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

Cross-State Air Pollution Rule ......................................................................................................... 4 

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

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

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

Measurements Recorded at CASTNET Sites .............................................................................. 9 

Atmospheric Mercury Network...................................................................................................11

Modeling Dry Deposition................................................................................................................ 12

Canada Reduces SO 

2Emissions by 46 Percent since 1990............................................... 14 

Sulfur Dioxide and Nitrogen OxidesEmissions ..................................................................... 16 

Chapter 2 Atmospheric Concentrations ..................................................................................................... 19 

 Sulfur Dioxide...................................................................................................................................... 19 

Particulate Sulfate.............................................................................................................................. 21

Ambient Mercury Concentrations Measured at Cherokee Nation, OK ........................23

Total Nitrate......................................................................................................................................... 25 

Particulate Ammonium ................................................................................................................... 27 

Cloud Chemistry Measurements at Clingmans Dome, TN and 

Whiteface Mountain, NY .............................................................................................................. 29 

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

Using PRISM to Estimate Wet Deposition for CASTNET Sites......................................... 32 

Sulfur Deposition ............................................................................................................................... 35 

An Intercomparison of the CASTNET Multi-Layer Model and the

CAPMoN Big-Leaf Model.............................................................................................................. 39 

Nitrogen Deposition.......................................................................................................................... 42 

Estimates of Critical Loads in the Eastern United States...................................................45 

Chapter 4 Ozone Concentrations ................................................................................................................. 48 

 National Ambient Air Quality Standards for Ozone.............................................................49

CASTNET’s Transition to AQS Monitoring...............................................................................50

Eight-Hour Ozone Concentrations..............................................................................................51

W126 Values for 2010 ..................................................................................................................... 55 

Air Quality Concerns in the Upper Green River Basin, WY............................................... 57 

Chapter 5 Data Quality .................................................................................................................................... 61 

Precision................................................................................................................................................ 61

Accuracy................................................................................................................................................. 68 

Precision of Collocated SLAMS and Collocated CASTNET Ozone Measurements...70

Completeness....................................................................................................................................... 71

Results for 2010 Environment Canada Proficiency Testing ............................................ 72 

Summary of Data Quality Results................................................................................................ 74 

References 

Appendix A Locational and Operational Characteristics of CASTNET Sites 

Appendix B Acronyms and Abbreviations 
CASTNET Annual Report – 2010 ii Executive Summary

Executive Summary

The EPA Clean Air Status and Trends Network (CASTNET) provides measured atmospheric 

pollution concentration data in order to evaluate the effectiveness of national and regional 

air pollution control programs. This report presents maps of the geographic distribution of 

sulfur and nitrogen pollutant concentrations, ozone levels, and deposition fluxes measured 

in 2010 and examines trends in air quality since 1990. CASTNET measured rural, regionally 

representative concentrations of sulfur and nitrogen species and ozone at 84 sites in the 

United States and Canada during 2010. 

Key Results through 2010

 Mean annual sulfur dioxide and particulate sulfate concentrations declined 

significantly over the 21-year period 1990 through 2010 and showed a substantive 

decline since 2005. Three-year average sulfur dioxide (SO 

2) levels declined 65 percent 

while sulfate concentrations declined 48 percent at eastern reference sites from

1990 through 2010. SO 

2 concentrations measured at western reference sites were 

reduced by 49 percent over the 15-year period (1996–1999 through 2008–2010). 

Western sulfate levels dropped by 40 percent over the same time period. 

 Total (dry + wet) sulfur deposition measured in the East declined by 55 percent from 

1990 through 2010. Total sulfur deposition aggregated over western reference sites 

fell by 28 percent over the 15 years 1996 through 2010. The 2010 total sulfur flux at 

western sites was higher than the 2009 flux.

 Three-year mean annual concentrations of total nitrate (nitric acid plus particulate 

nitrate) declined 35 percent at eastern sites over the 21-year period. The 2010 

concentration was about 8 percent higher than the 2009 level. Total nitrate levels 

measured at western reference sites dropped by 35 percent over the 15 years.

 Total nitrogen deposition fell by 26 percent from 1990 through 2010 at eastern 

reference sites. Total nitrogen deposition estimated for western reference sites was

reduced by 20 percent over the 15 years. The 2010 total nitrogen flux at western sites 

was higher than the 2009 flux.

 The mean fourth highest daily maximum 8-hour average ozone concentration for 

2010 (69 ppb) was higher than the 2009 value of 64 ppb, which was the lowest in the 

history of the network. Only three eastern and five California sites recorded 

exceedances of the 8-hour standard of 0.075 ppm during the most recent 3-year 

period (2008–2010). 

 Measurements taken during 2010 and historical data collected over the period 

1990-2009 were analyzed relative to data quality objectives and their numerical 

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

confidence and that CASTNET continues to produce information of the highest quality.
CASTNET Annual Report – 2010 1 Chapter 1: CASTNET Overview

Chapter 1: CASTNET Overview

CASTNET is a national, long-term environmental monitoring program designed to provide data for 

assessing trends in air quality, atmospheric deposition, and ecological effects that result from air 

pollutant emission reductions. The Environmental Protection Agency and the National Park Service 

operate the network. CASTNET was established under the 1990 Clean Air Act Amendments and 

continued and expanded the National Dry Deposition Network, which began in 1987. During 2010, the 

network operated 84 monitoring stations throughout the contiguous United States, Alaska, and 

Canada. Measurements collected in 2010 show the continuing decline in sulfur pollutants and a slight 

increase in nitrogen pollutants and ozone concentrations relative to 2009. All pollutants have declined 

significantly over the last 21 years.

Background

The Acid Rain Program (ARP), which was established under Title IV of the 1990 Clean Air 

Act Amendments (CAAA), was promulgated to reduce emissions of sulfur dioxide (SO 

2) and 

nitrogen oxides (NOx) from electric generating units (EGUs). The U.S. Environmental 

Protection Agency (EPA) established the Clean Air Status and Trends Network (CASTNET) 

under Title IX in the CAAA to assess the effectiveness of the ARP and to assess and track 

real-world environmental results as the ARP was implemented and emission reductions 

became effective. The U.S. Congress mandated that CASTNET provide consistent, long-term 

measurements for determining relationships between changes in emissions and 

subsequent changes in air quality, atmospheric deposition, and ecological effects.

The ARP has produced significant reductions in SO 

2 and NOx emissions from EGUs since 

1995. The SO 

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

annually from EGUs and included provisions for trading and banking emission allowances. 

The program has been 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 2010 were about 5.1 million tons.

NOx reductions under the ARP are achieved through a program that applies to a subset of 

coal-fired EGUs by limiting the NOx emission rate based on a unit’s boiler type. Actual NOx

emissions from ARP EGUs in 2010 were about 2.1 million tons. Although the ARP was

responsible for a large portion of these annual NOx reductions, other emission control 

programs have also produced substantive declines in NOx emissions in the eastern United 

States. These programs include the Ozone Transport Commission (OTC) NOx Budget 

(1999–2002) and the NOx State Implementation Plan (SIP) Call/NOx Budget Trading 

Program (NBP), which operated from 2003 through 2008. The NBP placed a cap on total 

NOx emissions from EGUs in the eastern United States during the ozone (O 

3) season 

(May 1 through September 30) when the potential for O 

3 formation is high.
CASTNET Annual Report – 2010 2 Chapter 1: CASTNET Overview

The Clean Air Interstate Rule (CAIR) was promulgated in March 2005 to permanently 

lower SO 

2 and NOx emissions in the eastern United States. The rule was designed to 

continue the progress achieved under the ARP and NBP. CAIR established three compliance 

programs: an annual NOx program, an O 

3 season NOx program, and an annual SO 2 program. 

Although CAIR was remanded back to EPA in 2008, these programs remained in effect 

through 2011. The first phase of the CAIR annual and O 

3 season NOx requirements began in 

2009. The requirement to hold allowances in the SO 

2 program began on January 1, 2010. 

On July 6, 2011, EPA promulgated the Cross-State Air Pollution Rule (CSAPR) to replace 

CAIR. The CSAPR requires 28 states to produce additional reductions in power plant SO 

2

and/or NOx emissions. See the box on page 4 for a summary of the CSAPR (also known as

the Transport Rule).

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

operation in 1987. The 50 NDDN sites were transferred to CASTNET in 1991. During 2010, 

the network included 84 monitoring stations at 82 site locations throughout the contiguous 

United States, Alaska, and Canada. CASTNET is sponsored by EPA and the National Park 

Service (NPS). NPS began its participation in CASTNET in 1994 under an agreement with 

EPA. With the participation of NPS, the network became a national, rather than a primarily 

eastern, network. NPS is responsible for the protection and enhancement of air quality and 

related values in national parks and wilderness areas. Twenty-five CASTNET sites were 

sponsored by NPS during 2010. Additional information on the NPS Air Monitoring program 

can be found on the Web site: http://www.nature.nps.gov/air/monitoring/.

CASTNET stations measure rural, regionally representative concentrations of sulfur and 

nitrogen pollutants and O 

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

distribution of rural pollutants, and estimate dry deposition of pollutants. Dry deposition is 

calculated from CASTNET concentration measurements and modeled deposition velocities 

using the Multi-Layer Model (MLM) or from historical deposition velocity measurements. 

The model requires several meteorological parameters and information on vegetation and 

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

evaluation, e.g., the Community Multiscale Air Quality (CMAQ) modeling system, and to 

provide input to ecological models, such as the Model of Acidification of Groundwater in 

Catchments (MAGIC).

As of October 2010, EPA decided to discontinue meteorological measurements at all but 

four EPA-sponsored CASTNET sites. Meteorological measurements were phased out over 

the fourth quarter as sites were calibrated. NPS sites are continuing meteorological data 

collection. For this report, missing meteorological data were replaced using the standard 

procedure used in previous years of “near site” data replacement. Data were replaced only 

for those EPA sites with designated “near sites.” However, the discontinuance of the 
CASTNET Annual Report – 2010 3 Chapter 1: CASTNET Overview

meteorological measurements requires a long-term method to obtain missing deposition 

velocity/flux data in order to estimate quarterly and annual dry deposition rates. EPA and 

AMEC selected a method based on the results in Bowker et al. (2011). Bowker’s method 

substitutes hour-specific historical averages of deposition velocity (Vd) for missing Vd 

values at specific sites. The substitution procedure was shown to result in long-term, 

unbiased estimates of the annual mean Vd. A variation of Bowker’s method will be applied 

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

December 2010 and also for sites missing historical deposition velocities. The results of the 

updated estimates of dry deposition will be presented in a report that supplements this 

2010 Annual Report. The current MLM modeling approach that was used to estimate dry 

deposition fluxes in this report will continue to be used for the four EPA sites with 

continued meteorological measurements and all NPS-sponsored sites.

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

of precipitation data instead of using cubic inverse distance weighting (IDW). PRISM uses 

terrain elevation, slope, and aspect and climatic measurements to estimate precipitation on 

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

estimated for the PRISM grid. These two grids were merged in order to estimate pollutant 

wet deposition rates. Wet deposition rates for CASTNET sites were estimated from four 

nearby gridded values. Chapter 3 summarizes the new methods used to estimate wet

deposition for CASTNET.

This report summarizes CASTNET monitoring activities and the resulting concentration 

and deposition data collected over the 21-year period from 1990 through 2010. Additional 

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

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

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

site provides a complete archive of concentration and deposition data for all EPA- and NPSsponsored CASTNET sites. Fully validated data are available approximately 10 months 

following collection. NPS data collected during 2010 from the NPS-sponsored CASTNET 

sites and the NPS Gaseous Pollutant Monitoring Program are available in the NPS Annual 

Data Summary 2010 (Ray, 2011) and the 2010 Data Quality Assurance NPS Gaseous 

Pollutant Monitoring Program (NPS, 2011).
CASTNET Annual Report – 2010 4 Chapter 1: CASTNET Overview

Cross-State Air Pollution Rule 

The Cross-State Air Pollution Rule (CSAPR), also known as the Transport Rule, was promulgated 

on July 6, 2011 to improve air quality by reducing power plant emissions that contribute to pollution 

in downwind states. CSAPR replaces CAIR, which was issued in March 2005 and remanded back 

to EPA in December 2008. CSAPR was finalized under the good neighbor provision of the Clean 

Air Act and represents a continuation of the EPA emission reduction strategies that began with the 

1990 CAAA via the ARP and continued under the NBP and CAIR. The rule requires significant 

reductions of SO 

2 and NOx emissions that contribute to O 3 and fine particulate matter (PM2.5) 

pollution in downwind states (EPA, 2011a; 2011b).

In a related action, EPA finalized a supplemental rulemaking on December 15, 2011 to require five 

states (Iowa, Michigan, Missouri, Oklahoma, and Wisconsin) to institute summer NOx reductions 

under the CSAPR ozone season program. The CSAPR requires 28 states (Figure 1a) in the 

eastern United States to reduce annual power plant SO 

2 and NOx emissions and/or ozone season 

NOx emissions that cross state lines and contribute to O 

3 and PM2.5 pollution. EPA expects that by 

2014, the CSAPR and other state and EPA actions will reduce power plant SO 

2 emissions by 

73 percent and NOx emissions by 54 percent from 2005 levels. 

Figure 1a CSAPR Covered States

Source: EPA (2011b)

On December 30, 2011 the United States Court of Appeals for the District of Columbia Circuit 

issued its ruling to stay the CSAPR pending judicial review. This decision is not a ruling on the 

merits of the CSAPR. EPA will continue to administer CAIR pending the Court’s resolution of the 

petitions for review.

States controlled for both fine particles (annual SO 

2 and NOx) and ozone (ozone season NOx) (20 States)

States controlled for fine particles only (annual SO 

2 and NOx) (3 States)

States controlled for ozone only (ozone season NOx) (5 States)

States not covered by the Cross-State Air Pollution Rule
CASTNET Annual Report – 2010 5 Chapter 1: CASTNET Overview

Cooperating Networks

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

National Atmospheric Deposition Program (NADP) operates: 

National Trends Network (NTN), which includes 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 triplicate passive ammonia 

(NH 

3) samplers at about 46 NTN locations. AMoN, in operation for three 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 25 locations in the continental United States, Canada, Hawaii, and Taiwan in 

order to estimate dry and total mercury deposition. See the discussion on page 11.

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

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

Canadian Air and Precipitation Monitoring Network (CAPMoN) operates 

33 measurement sites throughout Canada and one in the United States. CASTNET 

and CAPMoN both operate filter pack samplers in Ontario, Canada. CAPMoN 

operates a wet deposition sampler at Pennsylvania State University. Visit CAPMoN’s 

Web site at http://www.ec.gc.ca/rs-mn/default.asp?lang=En&n=752ce271-1.

Interagency Monitoring of Protected Visual Environments (IMPROVE) measures 

aerosol pollutants near more than 20 CASTNET sites. IMPROVE measures 

particulate air pollutants that affect visibility. For more information on IMPROVE, 

see http://vista.cira.colostate.edu/IMPROVE/.

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

needed to assess the effectiveness of promulgated emission control programs under 

several different mandates including the Government Performance and Results Act, the 

National Acid Precipitation Assessment Program (NAPAP), Title IX of the CAAA, and the 

Canada – United States Air Quality Agreement.

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

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

CASTNET sites. These co-sponsors provide in-kind services that support the overall 

operation of a site, including site operation, land use, or both. All of the sites added during 

recent expansion of the network (since 2001) have an associated co-sponsor. Three 

CASTNET monitoring sites are located on tribal lands: Cherokee Nation in eastern 
CASTNET Annual Report – 2010 6 Chapter 1: CASTNET Overview

Oklahoma (CHE185), Alabama-Coushatta in eastern Texas (ALC188), and Santee Sioux in 

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

measurement devices at some CASTNET sites.

Locations of Monitoring Sites

Figure 1-1 shows the locations of CASTNET monitoring sites during 2010. Eighty-four sites 

were operational at 82 distinct locations. Two CASTNET sites ceased sampling activities. 

Lykens, OH (LYK123) closed during October. Filter pack sampling at Converse Station, CA 

(CON186) ended with the last sampling week of 2010. Most CASTNET sites are located in 

rural or remote locations away from pollutant emission sources and heavily populated 

areas. Appendix A provides the location of each site by state and includes information on 

start date, latitude, longitude, elevation, and the parameters measured. For the purposes of 

this report, CASTNET sites are called “western” or “eastern” depending on whether they 

are west or east of 100 degrees west longitude. In general, sample flow rates are set to 

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

lower pollutant concentrations generally found in the western United States.

Abington, CT (ABT147)
CASTNET Annual Report – 2010 7 Chapter 1: CASTNET Overview

Figure 1-1 CASTNET Sites Operational during 2010

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 similar to those used by EPA in its National Air Quality and Emissions Trends

Report (2000). The criteria include site longevity and data completeness. Because of the 

closing of the site at LYK123, OH in October 2010, the site at Ann Arbor, MI (ANA115) was 

selected to replace LYK123 as an eastern reference site. ANA115 has been operating since 

June 1988. Trends in pollutant concentrations measured at CASTNET western reference 
CASTNET Annual Report – 2010 8 Chapter 1: CASTNET Overview

sites (Figure 1-3) are also presented. The 

17 western reference sites have been 

operating since at least 1996.

The data from the 34 eastern reference 

sites were aggregated and then presented 

using box plots for the 21-year period 

1990 through 2010. Data from the 17 

western reference sites were aggregated 

and then presented using box plots for 

the period 1996 through 2010.

Figure 1-2 CASTNET Eastern

Reference Sites

Figure 1-3 CASTNET Western

Reference Sites

Sand Mountain, AL (SND152) 
CASTNET Annual Report – 2010 9 Chapter 1: CASTNET Overview

Measurements Recorded at CASTNET Sites

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

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

measurement of weekly (Tuesday to Tuesday) average concentrations was selected as the 

basic sampling strategy. Over the course of the seven days, air is drawn at a controlled flow 

rate through an open-face 3-stage filter pack (Figure 1-4) mounted atop a 10-meter tower 

to collect air pollutants in the form of gases and particles. The first stage of the filter pack 

encloses a Teflon filter, the second a nylon filter, and the third holds two potassium 

carbonate (K 

2CO 3)-impregnated cellulose filters. The filter pack is changed out each 

Tuesday and shipped to the analytical chemistry laboratory for analysis.

Figure 1-4 Three-Stage Filter Pack

Cellulose = Gaseous: sulfur dioxide

Nylon = Gaseous: nitric acid, sulfur dioxide

Teflon = Particulate: sulfate, nitrate, ammonium, sodium, 

calcium, magnesium, potassium, and chloride

CASTNET Ambient Measurements

 Sulfur species:

Sulfur dioxide

Particulate sulfate

 Nitrogen species:

Particulate nitrate

Nitric acid

Particulate ammonium

 Metal cations:

Particulate calcium

Particulate sodium

Particulate magnesium

Particulate potassium

 Particulate chloride

 Ozone

 Meteorological variables 

 Information on land use and vegetation

Quick Disconnect

Two

Whatman Nylon Teflon

Filters Filter Filter Shipping Cap 

(removed during sampling)

Teflon Spacers

Direction of Air Flow

Quick Disconnect

Two

Whatman Nylon Teflon

Filters Filter Filter Shipping Cap 

(removed during sampling)

Teflon Spacers

Direction of Air Flow

Quick Disconnect

Two

Whatman Nylon Teflon

Filters Filter Filter Shipping Cap 

(removed during sampling)

Teflon Spacers

Direction of Air Flow

Two 

Cellulose Nylon Teflon

Filters Filter Filter

TeflonSpacers
CASTNET Annual Report – 2010 10 Chapter 1: CASTNET Overview

CASTNET Meteorological Measurements

 Temperature:

Temperature (at 2 and 9 meters)

Delta temperature (difference between 

2 and 9 meters)

 Relative Humidity

 Solar Radiation

 Winds:

Speed

Direction

Sigma theta (standard deviation 

of direction)

 Precipitation

 Surface Wetness

The filter packs are prepared, loaded, shipped, received, extracted, and analyzed at the 

AMEC Gainesville, FL laboratory. Following receipt from the field, exposed Teflon filters 

and blanks are extracted and then analyzed for sulfate (SO2-

4 ), nitrate (NO3), and 

concentrations of chloride (Cl-) by micromembrane-suppressed ion chromatography (IC) 

and also for ammonium (NH+ 

4 ) by the automated indophenol method with the 

Bran+Luebbe AutoAnalyzer 3. Additionally, Teflon filter extracts are analyzed for calcium 

(Ca2+), sodium (Na+ ), magnesium (Mg2+), and potassium (K+ ) by inductively coupled 

plasma-atomic emission spectrometry (ICP-AES) using a Perkin Elmer Optima 7300 Dual 

View spectrometer. The cellulose filter extracts are analyzed for SO 

2 as sulfate using IC. The 

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

3) as nitrogen and for SO 2 as 

sulfate. The SO 

2 concentrations from the cellulose and nylon filters are summed to obtain 

total SO 

2. 

CASTNET also measures hourly O 

3 concentrations, 

one of the major components of smog. Ozone 

measurements are recorded continually and 

archived as hourly averages. In addition to the air 

pollutant concentrations, CASTNET sites collected 

hourly meteorological measurements, which are 

used as input to the MLM, a numerical model used 

for estimating dry deposition to ecosystems in the 

atmospheric boundary layer. As of October 2010, 

EPA decided to discontinue meteorological 

measurements at all but four EPA-sponsored 

sites. The four sites continuing meteorological 

measurements are Beltsville, MD (BEL116); 

Bondville, IL (BVL130); Palo Duro State Park, TX 

(PAL190); and CHE185, OK. Meteorological 

measurements are continuing at all NPS sites. For the discontinued sites, measurements 

were phased out by the end of 2010 as sites were calibrated. The meteorological 

measurements are recorded continuously and archived as hourly averages.
CASTNET Annual Report – 2010 11 Chapter 1: CASTNET Overview

Atmospheric Mercury Network 

Figure 1b shows 22 of the 25 AMNet sites that are currently operating in the continental United 

States, Canada, Hawaii, and Taiwan. Most sites are collocated with MDN sites and include 

meteorological measurements. Table 1a lists the AMNet sites, their locations, and sponsoring 

organizations. Information about AMNet can be found on the NADP Web site 

(http://nadp.isws.illinois.edu/amn). 

Figure 1b Atmospheric Mercury Network Sites as of December 2011 

Table 1a Atmospheric Mercury Network Site Locations and Sponsors

Site ID Name Location Latitude Longitude Organization

AL19 Birmingham Alabama 33.55303 -86.8148 SEARCH

CA48 Elkhorn Slough California 36.81000 -121.780 UC Santa Cruz

FL96 Pensacola Florida 30.43671 -87.2571 SEARCH

GA40 Yorkville Georgia 33.92833 -85.0456 SEARCH

HI00 Mauna Loa Hawaii 19.5400 -155.5800 NOAA/EPA

MD08 Piney Reservoir Maryland 39.7053 -79.0122 State MD

MD99

MD98

Beltsville

Beltsville Collocated Maryland 39.0284 -76.8172 NOAA/EPA

MS12

MS99

Grand Bay NERR

Grand Bay NERR Collocated Mississippi 30.4294 -88.4277 NOAA

NH06 Thompson Farm New Hampshire 43.1100 -70.9500 U New Hampshire

NJ05 Brigantine New Jersey 39.4020 -74.3790 State NJ

NS01 Kejimkujik Nova Scotia 44.4300 -65.2100 Env. Canada

NU15 Alert Nunavut 54.0000 -90.0000 Env. Canada

NY06 NYC New York 40.7146 -74.0058 State NY

NY20 Huntington Wildlife Forest New York 43.9731 -74.2232 Clarkson U.

NY95 Rochester New York 43.1544 -77.6160 State NY

OH02 Athens Ohio 39.3000 -82.1167 Ohio U.

OK99 Stilwell Oklahoma 35.7514 -94.6717 Cherokee Nation

PA13 Allegheny Portage Railroad

National Historic Site

Pennsylvania 40.45971 -78.5618 NOAA, PDEP & NPS

TW00 Mt. Lulin Taiwan 25.4700 120.8730 National Central U & 

EPA (Taiwan)

UT96 Antelope Island Utah 41.0467 -112.0248 U Utah

UT97 Salt Lake City Utah 40.7118 -111.9609 State Utah

VT99 Underhill Vermont 44.5283 -72.8689 Ecosystems Res. Gr.

WV99 Canaan Valley Institute West Virginia 39.0636 -79.4222 NOAA

Note: Collocated CASTNET sites are highlighted in yellow. Source: NADP/AMNet, 2010
CASTNET Annual Report – 2010 12 Chapter 1: CASTNET Overview

Modeling Dry Deposition

Dry deposition processes are modeled as resistances to deposition. The original network 

design was based on the assumption that dry deposition or flux could be estimated as the 

linear product of measured pollutant concentration (C) and modeled Vd. The equation to 

estimate flux is shown in Figure 1-5. Measured atmospheric concentrations are calculated 

based on the mass of each analyte in each filter extract and the volume of air sampled. The 

rate of deposition of a pollutant, also known as Vd, is influenced by meteorological 

conditions, vegetation, and atmospheric and plant chemistry. The deposition velocity 

values for each site are calculated for each hour of each year using the MLM. The MLM is 

summarized by Meyers et al. (1998) and Finkelstein et al. (2000). The data used in the 

MLM to estimate dry deposition are derived from meteorological measurements and 

pollutant concentrations taken at the site together with an estimation of the vegetation 

leaf-out and leaf area index (LAI).

The schematic of the MLM in Figure 1-5 shows the relationships among the various 

resistances and illustrates the meteorological and other data that are required as model 

input. The deposition velocities and fluxes presented in Chapter 3 were calculated using 

the MLM. The decision to discontinue meteorological measurements at all but four EPA 

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

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

hourly Vd values for missing MLM simulations. The substitution method will be applied 

to the CASTNET database and reported in a supplemental report. 

Arendtsville, PA (ARE128)
CASTNET Annual Report – 2010 13 Chapter 1: CASTNET Overview

Figure 1-5 Multi-Layer Model

Resistances 

ra = aerodynamic

ra, soil = aerodynamic near soil

rb = boundary layer in thin layer at surface

rcut = cuticular

rs = stomatal

rsoil = soil

Prince Edward, VA (PED108)

1 1 1

1

Flux = C x Vd

1/Vd = + ra

rs + rb 

+ rcut + rb +

 ra, soil + rsoil
CASTNET Annual Report – 2010 14 Chapter 1: CASTNET Overview

Canada Reduces SO 

2 Emissions by 46 Percent since 1990 

The Canada – United States Air Quality Agreement (AQA) and its Acid Rain Annex, which was 

negotiated in 1991, committed both countries to reduce emissions of SO 

2 and NOx. These 

pollutants are precursors to acid rain. The Ozone Annex was added in 2000 to address 

transboundary levels of ozone. This annex committed both countries to reduce emissions of NOx

and volatile organic compounds (VOCs) in a defined region known at the Pollutant Emission 

Management Area (PEMA), which includes central and southern Ontario, southern Quebec, 

18 U.S. states (Connecticut, Delaware, Illinois, Indiana, Kentucky, Maine, Maryland, 

Massachusetts, Michigan, New Hampshire, New York, New Jersey, Ohio, Pennsylvania, Rhode 

Island, Vermont, West Virginia, and Wisconsin) and the District of Columbia. The AQA and related 

national emission reduction programs have resulted in substantive declines in emissions of SO 

2, 

NOx, and VOCs. The results are summarized in the Canada – United States Air Quality Agreement 

Progress Report 2010 (International Joint Commission, 2011).

Canada’s total SO 

2 emissions in 2008 were 1.7 million tonnes (1 tonne = 1.1 tons), which represent 

a 46 percent decrease from the 1990 emission level and a 63 percent reduction from 1980 

(Figure 1c). Canada has also met its commitment to reduce NOx emissions from metal smelting 

operations, power plants, and major combustion sources by 100,000 tonnes. Total NOx emissions 

declined from 2.5 million tonnes in 1990 to 2.1 million tonnes in 2008. Canada is continuing to 

develop programs to further reduce NOx emissions nationwide. The United States’ emission 

reductions are discussed elsewhere in this report.

Figure 1c Total Canadian Emissions of SO 

2 (1980–2008) 

Source: International Joint Commission (2011)

The Ozone Annex was developed to reduce concentrations of ozone pollution in the PEMA around 

the Canadian – United States border. Strategies for reducing emissions of NOx and VOCs 

discussed in the Annex indicate that Canadian emissions of NOx and VOCs in the PEMA will have 

been reduced by 34 percent and 29 percent, respectively, by 2010 from 1990 levels. Figure 1d 

shows Canadian NOx and VOCs emissions for 1990 and approximate emissions for 2010. Figure 

1e illustrates emissions of U.S. NOx and VOCs for PEMA states for 1990 and 2010. NOx emissions 

declined by 53 percent and VOCs emissions by 41 percent (International Joint Commission, 2011). 

Canada and the United States developed procedures for providing notification when potential new 

sources of transboundary air pollution are identified and when existing sources are modified. The 

purpose is to mitigate any significant transboundary pollution before the new or modified sources 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1980 1984 1988 1992 1996 2000 2004 2006 2008

Emissions (million tonnes)
CASTNET Annual Report – 2010 15 Chapter 1: CASTNET Overview

begin operation. Since this agreement was initiated, Canada has issued 55 notifications to the 

United States and the United States issued 61 to Canada. 

The AQA also includes consultation, cooperation, and research on other matters:

 Reporting emissions

 Reporting air quality measurements taken within 500 km of the border

 Studying health and exposure 

 Tracking public health 

The CASTNET site at Egbert, ON (EGB181) is located within the PEMA. Canadian and United 

States emission reductions have resulted in improved air quality at this site. Figure 1f provides 

trends in annual mean SO 

2 and total nitrate concentrations from 1995 through 2010. Annual mean 

SO 

2 concentrations have been reduced by 59 percent based on 3-year average concentrations 

from 1995–1997 to 2008–2010. Total nitrate concentrations declined by 39 percent.

Figure 1d Canadian NOx and VOCs PEMA Figure 1e U.S. NOx and VOCs PEMA

 Emissions and Projections Emissions

 

Source: International Joint Commission (2011) Source: International Joint Commission (2011)

Figure 1f Trends in Annual SO 

2 and Total Nitrate Concentrations for Egbert, ON

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Concentration (ug/m3)

Sulfur Dioxide

Total Nitrate

0

1

2

3

4

5

6

7

8

9

10

Nitrogen Oxides VOC

Emissions (Million Tons)

1990

2010

0

1

2

3

4

5

6

7

8

9

10

Nitrogen Oxides VOC

Emissions (Million Tons)

1990

2010
CASTNET Annual Report – 2010 16 Chapter 1: CASTNET Overview

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, 2011a) on the effects of the ARP and related programs. The 

2010 EPA report summarizes 2010 emission and compliance data, market analyses, and 

environmental and human health results. 

Under the ARP, SO 

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

permanent cap on the total amount of SO 

2 that may be emitted annually by all regulated 

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

incremental reductions in SO 

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

8.95 million tons, a level equal to about one-half of the emissions from the power sector in 

1980. Use of a market-based cap and trade mechanism to reduce SO 

2 emissions allows 

flexibility for individual combustion units to select their own methods of compliance. 

Currently, one allowance provides a regulated unit limited authorization to emit one ton of 

SO 

2. The CAAA allocated allowances to regulated units based on historic fuel consumption 

and specific emission rates prior to the start of the program. The total allowances allocated 

for each year equal the SO 

2 emission cap. The program encourages early reductions by 

allowing sources to bank unused allowances in one year and use them in a later year.

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

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

2 emission 

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

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

season. Two prominent control programs are the OTC NOx Budget (1999–2002) and the 

NOx SIP Call/NBP, which began in 2003 and continued through 2008. The NBP placed a cap 

on total NOx emissions from EGUs during the ozone season (May 1 through September 30) 

when the potential for ozone formation is high. The CAIR NOx programs include annual and 

ozone season control programs. CAIR was designed to further SO 

2 and NOx emission 

reductions. The CAIR NOx ozone season trading program replaced the NBP in 2009. 

Although CAIR was remanded back to EPA in 2008, the CAIR programs remain in effect 

until the CSAPR becomes effective. The CSAPR requires significant reductions in SO 

2 and 

NOx emissions that contribute to ozone and PM2.5 pollution in downwind states. EPA has 

set emission budgets for each state. See the summary of CSAPR on page 4. 

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

2 and NOx. Phase I applied 

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

2 emissions and 

from 1996 through 1999 for NOx emissions. Phase II for both pollutants began in 2000. In 

2010, the Phase II ARP SO 

2 requirements affected 3,613 operating units. The Phase II ARP 

NOx requirements applied to 956 of those operating units that exceed 25 megawatts and 
CASTNET Annual Report – 2010 17 Chapter 1: CASTNET Overview

burned coal between 1990 and 1995 (EPA, 2011a). The CAIR NOx ozone season program 

affected 3,309 units in 2010, and the NOx and SO 

2 annual programs affected 3,349 units. 

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

2 emissions from 

affected sources were 5.1 million tons in 2010; total NOx emissions from NBP and CAIR

sources were 599,000 tons.

Figure 1-6 presents state-by-state total annual SO 

2 emissions for CAIR and ARP electric 

utility plants for five years (1990, 1995, 2000, 2005, and 2010). The most significant 

reduction in SO 

2 emissions occurred in and near the Ohio River Valley where many coalfired power plants are located. Many of the states east of the Mississippi River realized 

reduced SO 

2 emissions by more than 50 percent over the 21 years; the larger emitting 

states (Illinois, Indiana, Kentucky, Ohio, Pennsylvania, and West Virginia) reduced SO 

2

emissions by more than 65 percent. Annual NOx emissions by state are depicted in 

Figure 1-7 for the same five years. States with the highest NOx emissions also produced the 

most significant declines. Every state east of the Mississippi River experienced a significant 

decline in annual NOx emissions from 2000 to 2010. For example, NOx emissions from 

Indiana EGUs declined by 67 percent from 2000 to 2010.

Horton Station, VA (VPI120)
CASTNET Annual Report – 2010 18 Chapter 1: CASTNET Overview

Figure 1-6 Annual SO 

2 Emissions (for CAIR and ARP Sources) 

Figure 1-7 Annual NOx Emissions (for CAIR and ARP Sources) 

Annual Emissions of Sulfur Dioxide

 

Scale: Largest bar equals 

2,212 thousand short 

tons (Ohio, 1990).

 

1990

1995

2000

2005

2010

Annual Emissions of Nitrogen Oxides

 

Scale: Largest bar equals 

534 thousand short 

tons (Ohio, 1990).

 

1990

1995

2000

2005

2010
CASTNET Annual Report – 2010 19 Chapter 2: Atmospheric Concentrations

Chapter 2: Atmospheric Concentrations

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

2, SO2-4 , HNO 3, 

NO3, NH+ 

4 , Cl-

, and 4 earth metals at 84 CASTNET monitoring stations. Trends in mean annual SO 

2, 

SO2-

4 , total NO3, and NH+ 4 concentrations aggregated over the 34 eastern reference sites are shown 

over the period 1990 through 2010 and for 17 western reference sites for the period 1996 through 

2010 using box plots for each year. All four parameters measured at the eastern sites declined over 

the 21-year period although total nitrate and NH+ 

4 concentrations increased slightly in 2010 relative to 

2009. Measured annual mean concentrations of SO 

2 and SO2-4 have decreased steadily with a 

significant reduction since 2005. Concentrations of total nitrate showed little change from 1990 until 

2000 when they began to drop in response to NOx emission reduction programs. SO 

2, total nitrate, and 

NH+ 

4 concentrations measured at the 17 western reference sites declined over the 15 years. 

This chapter presents maps of 2010 annual mean concentrations of SO 

2, SO2-4 , total nitrate 

(HNO 

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

(MACTEC, 2010a; 2010d; 2011a; 2011c). Trends in annual mean concentrations over the 

21-year period (1990 through 2010) are presented using box plots derived from 

measurements from the 34 CASTNET eastern reference sites (Figure 1-2). Trends in annual 

mean concentrations were also estimated from data from 17 CASTNET western reference 

sites (Figure 1-3) for the period 1996 through 2010. This chapter also includes information 

on ambient mercury concentrations measured at CHE185, OK, and it compares cloud 

chemistry measurements collected at Clingmans Dome, TN and Whiteface Mountain, NY.

Sulfur Dioxide

Sulfur dioxide is a major air pollutant that affects human health and plant communities. It 

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

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

2 in the United States. 

Some industrial facilities also emit SO 

2. To be compliant under the SO 2 National Ambient Air 

Quality Standard (NAAQS), the 3-year average of the 99th percentile of the yearly 

distribution of daily maximum 1-hour SO 

2 concentrations must be at or below 75 parts per 

billion (ppb) [215 micrograms per cubic meter (μg/m3 ) of air]. SO 

2 gas reacts in the 

atmosphere to form various acidic compounds including sulfuric acid (H 

2SO 4), a significant 

contributor to acid rain, and ammonium sulfate [(NH 

4)

 

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

Fine particles are also regulated by the NAAQS since PM2.5 adversely affects health and 

contributes to degradation of atmospheric visibility.

Figure 2-1 shows annual mean SO 

2 concentrations measured in 2010. Three sites in the 

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

highest concentration (7.3 μg/m3 ) was measured at Quaker City, OH (QAK172). The 

other two sites with levels above 5.0 μg/m3 were Horton Station, VA (VPI120) and 
CASTNET Annual Report – 2010 20 Chapter 2: Atmospheric Concentrations

Oxford,OH (OXF122). SO 

2 concentrations recorded in 2010 were somewhat lower than SO 2

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

SO 

2 concentrations measured in 2010 at 11 monitoring stations in Indiana, Ohio, and 

Pennsylvania was 4.3 μg/m3 versus a 3-state average of 4.7 μg/m3 in 2009. Only two 

western sites (Figure 1-1) measured a 2010 annual mean SO 

2 concentration equal to 

1.0 μg/m3 . The western sites are PAL190, TX and Theodore Roosevelt National 

Park, ND (THR422).

Figure 2-1 Annual Mean SO 

2 Concentrations (μg/m3 ) for 2010

Figure 2-2 shows box plots of annual mean SO 

2 concentrations aggregated over the 

34 eastern reference sites from 1990 through 2010 (right side) and the 17 western 

reference sites from 1996 through 2010 (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. A sharp decline was also 

recorded from 2005 through 2010, although the rate of decline was lower in 2010 relative 

to the previous four years. Three-year mean concentrations for the eastern reference sites 

for 1990–1992 and 2008–2010 were 8.9 μg/m3 and 3.2 μg/m3 , respectively. This change 

constitutes a 65 percent reduction in 3-year mean SO 

2 concentrations between the two 

periods. The 2010 mean level of 2.6 μg/m3 was the lowest mean value measured by the 
CASTNET Annual Report – 2010 21 Chapter 2: Atmospheric Concentrations

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

over the past five 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 2008–2010 were 0.61 μg/m3 and 0.31 μg/m3 , respectively. This change 

constitutes a 49 percent reduction in 3-year mean SO 

2 concentrations over the 15 years. 

The aggregated mean SO 

2 concentrations from the western reference sites were 

typically more than 10 times lower than mean concentrations measured at the eastern 

reference sites.

Figure 2-2 Trend in Annual Mean SO 

2 Concentrations (μg/m3 ) 

Western Reference Sites Eastern Reference Sites

Particulate Sulfate

Particulate SO2-

4 is formed in the atmosphere through the transformation of SO 2 via both gas 

and aqueous (cloud) phase reactions. SO2-

4 exists in the atmosphere typically as (NH 4)

 

2SO 4, 

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

4

concentrations measured during 2010. The map shows a fairly uniform distribution of SO2-

4

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

2 source region 

in the vicinity of the Ohio River Valley. CASTNET monitors in five contiguous states 

(Kentucky, Indiana, Ohio, Pennsylvania, and West Virginia) plus the monitor at Sand 

Mountain, AL (SND152) measured sulfate concentrations greater than or equal to 

3.0 μg/m3 

. Sulfate concentrations greater than or equal to 1.0 μg/m3 in the western region 

were measured at Big Bend National Park, TX (BBE401); PAL190, TX; THR422, ND; and 

Sequoia National Park, CA (SEK430).

0.0

0.5

1.0

1.5

199619971998199920002001200220032004200520062007200820092010

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

4

8

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16

20

1990199119921993

199419951996199719981999200020012002200320042005200620072008

20092010

Concentration (μg/m3)

90th Percentile

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Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2010 22 Chapter 2: Atmospheric Concentrations

Figure 2-3 Annual Mean SO2-

4 Concentrations (μg/m3 ) for 2010

The right side of Figure 2-4 provides 1990–2010 box plots of annual mean SO2-

4

concentrations from the 34 eastern reference sites. The figure shows a substantial decline 

in SO2-

4 . Like SO 2, sulfate concentrations declined significantly from 2005 through 2010. The 

difference between 3-year means from 1990–1992 to 2008–2010 represents a 48 percent 

reduction in SO2-

4 from 5.4 μg/m3 to 2.8 μg/m3 , respectively. The mean SO2-4 level of 

2.6 μg/m3 , which was measured for both 2009 and 2010, 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 40 percent reduction in annual mean SO2-

4 concentrations aggregated 

over the 17 sites, with 1996–1998 and 2008–2010 concentrations of 0.80 μg/m3 and 

0.48 μg/m3 , respectively. The 2010 aggregated mean concentration (0.6 μg/m3 ) was the 

lowest recorded.
CASTNET Annual Report – 2010 23 Chapter 2: Atmospheric Concentrations

Figure 2-4 Trend in Annual Mean SO2-

4 Concentrations (μg/m3 ) 

Western Reference Sites Eastern Reference Sites

Ambient Mercury Concentrations Measured at Cherokee Nation, OK 

The Cherokee Nation operates AMNet and MDN monitoring systems at its CASTNET site 

(CHE185) in Oklahoma. Ambient air concentrations of gaseous elemental mercury (GEM), 

gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM2.5) with a mean 

aerodynamic diameter of less than 2.5 micrometers (μm) are measured with a Tekran 2537A, 

1130, and 1135 speciation measurement system using AMNet protocols (NADP, 2011a). The 

system operates on a 3-hour cycle (NADP, 2011b). GEM concentrations are measured every 

five minutes during the first two hours of the cycle. Integrated 2-hour concentrations of GOM and 

PBM2.5 are also collected during the first two hours. During the third hour, the PBM2.5 and GOM 

samples are analyzed sequentially. No air sampling occurs during the third hour. Additionally, 

CHE185 incorporates a MDN sampler to collect Hg in precipitation in order to measure wet 

deposition of Hg (NADP/MDN, 2012). 

GEM, GOM, and PBM2.5 concentration data were assembled from the AMNet database for the 

year 2010. Concentration data were obtained as 2-hour values every three hours. Table 2a 

summarizes the 2010 measurements. Note the different reporting units. The mean GEM value of 

1.40 nanograms per cubic meter of air (ng/m3

) was 300 to 600 times higher than the respective 

means for GOM with 2.18 picograms per cubic meter of air (pg/m3

) and PBM2.5 with 4.22 pg/m3. 

GEM values showed a relatively small range and little variability. GOM in the air is reactive and can 

be removed from the atmosphere quickly by dry and wet deposition. GEM and PBM2.5 are relatively 

inert and have a low propensity for deposition. 

Table 2a 2010 Annual Atmospheric Mercury Species Concentrations

GEM (ng/m3) GOM (pg/m3) PBM2.5 (pg/m3)

Minimum 0.64 0.00 0.00

Mean 1.40 2.18 4.22

Median 1.42 0.74 3.33

Maximum 2.13 52.52 30.64

Standard Deviation 0.18 4.00 3.47

Count 2468 2325 2251

Time series of weekly average concentrations are shown in Figure 2a. The lowest GOM 

concentrations were measured in May through July and the highest in early spring. This was 

unexpected given the reactivity of this species and its hypothesized production through the 

oxidation of ozone. Figure 2b shows the average diurnal cycle of GOM for 2010. The figure shows 

0.0

0.5

1.0

1.5

2.0

199619971998199920002001200220032004200520062007200820092010

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

1

2

3

4

5

6

7

8

199019911992199319941995199619971998199920002001200220032004200520062007200820092010

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2010 24 Chapter 2: Atmospheric Concentrations

a distinct diurnal cycle with minimum values during the night and early morning and a peak in the 

afternoon. The figure suggests the production of GOM is related to daytime meteorological 

conditions (mixing after sunrise, temperature, and sunlight intensity) and perhaps atmospheric 

photochemistry, although the data in Figure 2a indicate GOM concentrations are lowest during the 

time of year with most active photochemical conditions.

Annual average GEM, GOM, and PBM2.5 concentration values were used to estimate the annual 

dry deposition flux of total ambient Hg for CHE185. Dry fluxes of the individual Hg constituents 

were approximated based on annual Vd of 0.1, 2.0, and 0.1 centimeters per second (cm/sec) for 

the respective species. The deposition velocity values were selected from the scientific literature 

(e.g., Castro et al., 2011; Lindberg and Stratton, 1998). Annual fluxes were calculated as the 

product (Figure 1-5) of annual concentrations and deposition velocity values. Total Hg flux, 

45.60 micrograms per square meter per year (μg/m2

/yr) was estimated as the sum of the fluxes for 

the three species. Even though the Vd for GOM is roughly 20 times higher than Vd values for the 

other two species, the dry flux of GEM contributed the largest fraction of total dry deposition 

because the GEM concentration was greater than 300 to 600 times higher than the other two 

species concentrations. However, given the bi-directional deposition/evasion of GEM, the GEM 

deposition velocity is likely a high estimate at 0.1 cm/s. While 44.10 μg/m2

/yr is a good estimate 

of the total GEM deposition, net GEM deposition would likely be less due to evasion back into 

the atmosphere.

Table 2b Estimates of 2010 Dry and Total Deposition of Atmospheric Mercury Species

Species Annual Average Concentration (pg/m3) Vd (cm/sec) Flux (μg/m2/yr)

GEM 1,400.00 0.1 44.10

GOM 2.18 2.0 1.37

PBM2.5 4.22 0.1 0.13

Total 45.60

Hg in Precipitation 12.30

Total Dry + Wet 57.89

The annual wet flux of 12.30 μg/m2/yr of Hg for the CHE185 site was obtained from the 

NADP/MDN (NADP/MDN, 2012b) Web site. The total Hg deposition for 2010 was 57.89 μg/m2

/yr. 

The dry flux was about four times higher than the wet flux at the eastern Oklahoma site.

Figure 2a Time Series of Weekly PBM2.5, GOM, Figure 2b Average Diurnal Cycle of GOM

and GEM Concentrations for 2010 for 2010

0

1

2

3

4

5

6

0123456789

1011121314151617181920212223

Concentration (pg/m3)

Hour

GOM

0

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5

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4

6

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10

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1/22/102/12/10

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7/9/10

7/30/108/20/109/10/1010/1/10

10/22/1011/12/10

12/3/10

12/24/10

Concentration of GEM (ng/m3)

Concentrations of GOM and PBM2.5 (pg/m3)

PBM

GOM

GEM

2.5
CASTNET Annual Report – 2010 25 Chapter 2: Atmospheric Concentrations

Total Nitrate

Total nitrate is the sum of HNO 

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

emitted when fossil fuels combust at high temperatures. Transportation sources 

(e.g., automobiles and trucks) are the largest contributors of NOx emissions in the United 

States and produce more than 50 percent of nationwide NOx emissions. NOx is also emitted 

from EGUs and industrial processes. A natural source of NOx is lightning in the atmosphere. 

Nitric acid exists as a gas and contributes to acid deposition. Nitrate 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. This chapter discusses data on total nitrate rather than HNO 

3 and 

NO3 individually because of the uncertainty in individual species measurements and 

because total nitrate is more representative of a response to changes in NOx emissions than 

either of its constituents alone. A map of annual mean total nitrate concentrations for 2010 

is given in Figure 2-5. The map shows a 3-state region with mean concentrations greater 

than or equal to 3.0 μg/m3 . The region includes the states of Illinois, Indiana, and Ohio. 

Three sites, SEK430, CON186, and Joshua Tree National Park (JOT403) in California 

measured annual mean total nitrate concentrations greater than 2.0 μg/m3 . Salamonie 

Reservoir, IN (SAL133) measured the highest concentration (4.0 μg/m3 ) in the network.

Box plots of total nitrate levels for eastern 

sites for the years 1990 through 2010 are 

given in Figure 2-6. 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 nitrate levels began to decline in 

response to NOx emission control programs. 

Three-year mean levels declined from 

3.1 μg/m3 to 2.0 μg/m3 from 1990–1992 to 

2008–2010, producing a 35 percent 

reduction in total nitrate. Total nitrate levels 

measured at the eastern reference sites 

declined from a mean value of 3.2 μg/m3 in 

2000 to a mean value of 2.0 μg/m3 in 2010.

Salamonie Reservoir, IN (SAL133)
CASTNET Annual Report – 2010 26 Chapter 2: Atmospheric Concentrations

Figure 2-5 Annual Mean Total Nitrate (NO3 + HNO 

3) Concentrations (μg/m3 ) for 2010

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

3-year mean total nitrate concentration for 2008–2010 was 35 percent lower than the 

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

1.0 μg/m3 , respectively. The two mean concentrations for 2009 and 2010 were the lowest 

over the 15-year period.

Figure 2-6 Trend in Annual Total Nitrate (NO3 + HNO 

3) Concentrations (μg/m3 ) 

Western Reference Sites Eastern Reference Sites

0.0

0.5

1.0

1.5

2.0

2.5

3.0

199619971998199920002001200220032004200520062007200820092010

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

5

6

7

8

199019911992199319941995199619971998199920002001200220032004200520062007200820092010

Concentration (μg/m3)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2010 27 Chapter 2: Atmospheric Concentrations

Particulate Ammonium

Particulate ammonium is formed when gaseous NH 

3 reacts with other gases and particles 

in the atmosphere including HNO 

3, particulate SO2-4 , and particulate NO3. Particulate 

ammonium exists in the atmosphere typically as (NH 

4)

 

2SO 4 and NH 4NO 3. A map of annual 

mean NH+ 

4 concentrations for 2010 is depicted in Figure 2-7. No monitors measured 

concentrations above 2.0 μg/m3 . The data show a flat geographic distribution across the 

central and eastern United States with more than half the sites reporting values less than or 

equal to 1.0 μg/m3 . Higher concentrations were measured in Illinois, Indiana, Ohio, 

Kentucky, and northern Alabama. The NH +

4 concentrations measured at western sites were 

low with all sites measuring concentrations less than 1.0 μg/m3 . 

Figure 2-7 Annual Mean NH+ 

4 Concentrations (μg/m3 ) for 2010

Box plots of NH+ 

4 concentrations are provided in Figure 2-8. The trend diagram for the 

eastern sites shows a reduction in mean NH+ 

4 levels from 1990–1992 to 2008–2010. 

The 1990–1992 mean concentration was 1.8 μg/m3 , and the 2008–2010 value was 

1.1 μg/m3 , a 39 percent decline. The box plots characterizing the western sites show no 

significant change.
CASTNET Annual Report – 2010 28 Chapter 2: Atmospheric Concentrations

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

199619971998199920002001200220032004200520062007200820092010

Concentration (μg/m3)

90th Percentile

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Median

Mean

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90th Percentile

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Mean

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4

199019911992199319941995199619971998199920002001200220032004200520062007200820092010

Concentration (μg/m3)

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Mean

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10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

Speedwell, TN (SPD111)
CASTNET Annual Report – 2010 29 Chapter 2: Atmospheric Concentrations

Cloud Chemistry Measurements at Clingmans Dome, TN and 

Whiteface Mountain, NY 

The Mountain Acid Deposition Program (MADPro) was initiated in 1993 to measure cloud water 

and its chemical constituents and their trends at mountaintop sites in the eastern United States. 

MADPro’s main objective is to update the cloud water concentration and deposition data 

collected in the Appalachian Mountains during the National Acid Precipitation Assessment Program

in the 1980s. MADPro measurements were conducted from 1994 through 1999 during the warm 

season (May through October) at three mountaintop sampling stations located at Whiteface 

Mountain, NY (WFM300); Clingmans Dome, TN (CLD303); and Whitetop Mountain, VA. A mobile 

manual sampling station also was operated at two locations in the Catskill Mountains in New York 

during 1995, 1997, and 1998. Measurements during the 2000 and 2001 sampling seasons were 

collected from WFM300 and CLD303. Beginning with the 2002 sampling season, cloud water 

measurements have been collected under CASTNET solely from the site at CLD303. The CLD303 

site did not operate in 2008 because funding was unavailable. For the 2009 and 2010 sampling 

seasons, CLD303 was operated under the direction and funding of EPA and the Tennessee Valley 

Authority with infrastructure support provided by NPS. Since 2001, the WFM300 site has been 

operated by New York State Department of Environmental Conservation. 

Data from CLD303 and WFM300 are available for the years 2000 through 2009 and are presented 

herein. Data from CLD303 for 2010 are presented also. Cloud liquid water content (LWC) and 

sulfate and nitrate concentrations for both sites are shown in Figures 2c through 2e, respectively.

LWC is the measure of the mass of water in a cloud and is typically expressed as grams per cubic 

meter (g/m3 ) of air. LWC varies significantly with the cloud type. Figure 2c shows that seasonal 

LWC values at WFM300 were consistently higher than the seasonal LWC values at CLD303. The 

10-year average (2000–2009) value of 0.528 g/m3 at WFM300 was 46 percent higher than the 

10-year average value of 0.285 g/m3 at CLD303. Higher LWC values usually result in greater water 

deposition to surrounding ecosystems. Cloud pollutant deposition depends on pollutant 

concentrations in cloud water, LWC, and several other parameters.

Figure 2c Seasonal Cloud Liquid Water Content (g/m3 ) Measured at CLD303 and WFM300

Cloud deposition estimates for CLD303 were provided by MACTEC (2011d). Sulfate and nitrate 

deposition estimates calculated using the CLOUD model declined by 82 percent and 78 percent,

respectively. Cloud water deposition estimates are not currently available for WFM300. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

g/m3

CLD303 

LWC

WFM300 

LWC
CASTNET Annual Report – 2010 30 Chapter 2: Atmospheric Concentrations

However, Aleskic et al. (2009) presents data for 1994 through 2006. They estimated sulfate and 

nitrate depositions declined by 10 percent and 17 percent, respectively, at WFM300 over that 

time period. 

Seasonal sulfate concentrations from 2000 through 2010 are presented in Figure 2d. Yearly 

variations in concentrations at both sites were considerable. Sulfate concentrations were 

consistently higher at CLD303 with a 10-year average seasonal concentration of 

359.46 microequivalents per liter (μeq/L) compared to 208.85 μeq/L at WFM300. Sulfate 

concentrations were 42.9 percent higher at CLD303. Both sites experienced an overall downward 

trend from 2000 through 2009. The 2000–2009 decrease in sulfate concentrations was 

60.8 percent at CLD303 and 38.1 percent at WFM300. 

Figure 2d Seasonal Sulfate Concentrations (μeq/L) Measured at CLD303 and WFM300

Seasonal nitrate concentrations for 2000 through 2010 are presented in Figure 2e. Nitrate 

concentrations were consistently higher at CLD303 with a 10-year average seasonal concentration

of 139.71 μeq/L at CLD303 versus 83.76 μeq/L at WFM300. The CLD303 average was 40 percent 

higher than the WFM300 level. Overall downward trends were observed at both sites from 

2000 through 2009. The decrease in nitrate concentrations from 2000–2009 was 58.4 percent at 

CLD303 and 34.0 percent at WFM300. 

Figure 2e Seasonal Nitrate Concentrations (μeq/L) Measured at CLD303 and WFM300

0

50

100

150

200

250

300

350

400

450

500

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Microequivalents/L

CLD303 

Sulfate

WFM300 

Sulfate

0

50

100

150

200

250

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Microequivalents/L

CLD303 

Nitrate

WFM300 

Nitrate
CASTNET Annual Report – 2010 31 Chapter 3: Atmospheric Deposition

Chapter 3: Atmospheric Deposition

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

across the United States. CASTNET primarily uses a hybrid measurement-modeling approach to 

estimate dry deposition. The hybrid approach combines measured pollutant concentrations and 

modeled deposition velocities. The MLM is used to calculate hourly deposition velocities for each 

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

1 km of each site. Dry deposition is calculated as the product of the modeled deposition velocity and 

measured pollutant concentration. In order to improve estimates of wet deposition, the Parameterelevation Regressions on Independent Slopes Model (PRISM) was selected 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 total sulfur fluxes 

aggregated over the eastern reference sites declined by 55 percent over the period from 1990–1992 

to 2008–2010. Three-year mean total sulfur deposition estimated for 17 western sites declined by 

28 percent over the period 1996–1998 to 2008–2010. Total nitrogen deposition estimates declined 

32 percent for the eastern reference sites and 20 percent at the western reference sites. Dry 

deposition is responsible for a significant percentage of total deposition, especially in major emission 

source regions.

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

through dry and wet atmospheric processes. A principal objective of CASTNET is to 

estimate the rate, or flux, of dry deposition from the atmosphere to sensitive ecosystems. 

Flux values are estimated as the product of measured concentration data and modeled dry 

deposition velocities. Wet deposition measurements were obtained from NADP/NTN. Wet 

deposition values used to estimate total deposition were based on a grid of PRISM-modeled 

precipitation amounts and gridded values of pollutant concentrations in precipitation. The 

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

were interpolated to CASTNET sites and combined with CASTNET dry deposition data to 

estimate total deposition. Dry sulfur, total sulfur, and total nitrogen deposition decreased 

during 2010 at the eastern reference sites. Dry nitrogen deposition increased slightly in 

2010 relative to 2009. The four deposition rates remained constant or increased slightly in 

2010 at the western reference sites. Precipitation-weighted mean concentrations of 

atmospheric sulfur declined over the past 21 years at the eastern sites, and mean 

concentrations of atmospheric nitrogen in precipitation have declined slowly since 1998 at 

the same sites. However, nitrogen in precipitation increased slightly in 2010.

The MLM (Figure 1-5) simulates dry deposition processes. The MLM was summarized by 

Meyers et al. (1998) and Finkelstein et al. (2000). For this report, the MLM was run using 

CASTNET filter pack concentrations and meteorological measurements together with 

information on percent leaf-out of nearby vegetation, LAI, and surface conditions to 
CASTNET Annual Report – 2010 32 Chapter 3: Atmospheric Deposition

calculate deposition velocities for SO 

2, HNO 3, O 3, and the particles, SO2-4 , NO3, and NH+ 4 . The 

deposition velocities were assumed to be identical for all particle species. Deposition 

velocity values were calculated for each of the pollutant species for each hour with valid 

meteorological data for each CASTNET site for the entire period 1990 through 2010. Valid 

measurements of temperature, solar radiation, relative humidity, wind speed, and standard 

deviation of the wind direction (sigma theta) must all be available for the hour in order to 

estimate deposition velocity. Currently, the CASTNET measurement criteria for calculation 

of hourly fluxes and weekly concentrations/fluxes are approximately 70 percent 

completeness. In future reports, a substitution method developed by Bowker et al. (2011)

will be used to complete missing Vd data by substituting long-term averages. This will 

result in nearly 100 percent data completeness. Updated dry and total deposition estimates 

will be presented in a separate report that supplements this 2010 Annual Report.

Using PRISM to Estimate Wet Deposition for CASTNET Sites

NADP/NTN operates wet deposition samplers to measure concentrations of pollutants in 

precipitation and to estimate the deposition rate of air pollutants removed by precipitation. 

NADP/NTN operates more than 200 monitoring sites across the United States, including 

monitoring stations at or near virtually every CASTNET site. Historically, wet deposition 

values were obtained from a grid of concentration and precipitation estimates derived 

from available NADP/NTN sites by using an inverse distance weighting (IDW) algorithm. 

Estimated concentrations were multiplied by the precipitation rates to obtain gridded 

estimates of wet deposition. Gridded wet deposition values were then interpolated to 

CASTNET sites. Utilization of this method resulted in precipitation and wet deposition rates 

equivalent to measured NADP/NTN values. Despite the equivalent precipitation and wet 

deposition values at NADP/NTN sites, the historical IDW approach produces interpolation 

errors, especially in regions with variable terrain and low geographic site density. The IDW 

approach is based simply on distances between measurements and does not include 

important data such as terrain elevation and slope and meteorological measurements.

PRISM was selected to produce grids of precipitation because the model provides more 

accurate estimates of annual precipitation values in areas with complex terrain. PRISM 

uses point precipitation measurements, a digital elevation model (DEM), and other 

geographic data to estimate annual, monthly, and event-based precipitation data. PRISM 

incorporates elevation, slope, and aspect ratio in the DEM with measurements of 

precipitation and temperature from several data sources to estimate precipitation on a 

4-km grid resolution. Point precipitation measurement data used for PRISM are obtained 

from the National Weather Service Cooperative stations across the United States, Natural 

Resources Conservation Service snow telemetry (SNOTEL) sites in western states, and 

other local networks. 
CASTNET Annual Report – 2010 33 Chapter 3: Atmospheric Deposition

Unlike statistical models such as IDW and kriging, PRISM is not a system of equations but a 

coordinated set of rules, decisions, and calculations designed to emulate the decisionmaking process a climatologist uses when creating a map of climatic data. The model was 

developed and is operated by the PRISM Climate Group at Oregon State University. The 

Climate Group obtains, screens, and validates the model input data and produces the 

gridded precipitation values. In order to utilize the NADP/NTN measurements, the PRISM 

gridded data are adjusted to the point measurements from NADP. The first step in the 

adjustment process is to create a weighting factor (WF) grid where the center point is the 

relative difference between NADP/NTN and PRISM, and the influence of a point is inverse 

distance weighted up to 20 km. Each precipitation grid cell is adjusted by:

≤ 20 km from NADP/NTN site: PRISM value + (WF*NADP value)

> 20 km from NADP/NTN site: PRISM value

The adjustment process results in CASTNET estimates equal to NADP/NTN measurements 

at collocated sites (EPA, 2011e). The second step is to produce concentration in 

precipitation values for the PRISM grid. The concentration values are multiplied by the 

precipitation rates to obtain gridded wet deposition fluxes. The final step is to interpolate 

the gridded wet deposition fluxes to CASTNET monitoring locations.

The PRISM results indicate a significant improvement over the IDW estimates. Figure 3-1 

presents scatter plots that show NADP/NTN measured precipitation rates compared to 

estimated IDW values in the left plot and measured NADP/NTN rates compared to 

estimated PRISM values in the right diagram. In Figure 3-1 the mean absolute relative 

percent difference (MARPD) calculated using a bootstrapping method for PRISM and 

NADP/NTN precipitation data is 2.5 percent (right panel) versus a MARPD of 39 percent 

between IDW-derived grid and NADP/NTN precipitation values. Figure 3-2 shows the 

PRISM precipitation grid for 2009, and Figure 3-3 depicts the relative percent difference 

(RPD) between grids creating using PRISM and NADP/NTN precipitation for 2007–2009. 

Figure 3-1 Comparison to NADP Measurements

Measured NADP vs. Estimated IDW 1999–2009 Measured NADP vs. Estimated PRISM 1999–2009
CASTNET Annual Report – 2010 34 Chapter 3: Atmospheric Deposition

Figure 3-2 Resulting PRISM Precipitation Grid for 2009

Source: EPA Clean Air Markets Division (2011e)

Figure 3-3 Relative Percent Difference in PRISM and NADP Precipitation Grids

0 

20

41

60

80

100

120

141

160

180

200

Precip

(cm)

PRISM - NADP

Avg RPD (%)

< – 100 

-99 – -80

-79 – -60

-59 – -40

-39 – -20

-19 – 0 

1 – 20

21 – 40 

41 – 60

61 – 80

81 – 100

● NTN

Source: EPA Clean Air Markets Division (2011e)
CASTNET Annual Report – 2010 35 Chapter 3: Atmospheric Deposition

Sulfur Deposition

The MLM was run separately for SO 

2 and SO2-4 . The model calculations were summed to 

obtain the 2010 estimates of dry sulfur deposition (as S) shown on the map in Figure 3-4. 

The magnitude of a deposition rate is illustrated by the size of the circle. The map shows 

only seven CASTNET sites in five states with estimated fluxes greater than or equal to 

3.0 kilograms per hectare per year (kg/ha/yr). These sites are located in Indiana, Kentucky, 

Ohio, Pennsylvania, and Virginia. The geographic extent of elevated deposition rates was 

about the same as 2009. The 3-state geographic average dry S deposition rate for Indiana, 

Ohio, and Pennsylvania was 2.9 kg/ha/yr compared to an average flux of 3.4 kg/ha/yr in 

2009. The highest 2010 deposition rate was estimated for QAK172, OH with a flux of 

5.5 kg/ha/yr compared to a flux of 5.4 kg/ha/yr in 2009. The highest estimated dry sulfur 

deposition rates coincided with the major SO 

2 source region (Figures 1-6 and 2-1) and 

declined with distance. The dry deposition rates estimated for the western sites were all 

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

Figure 3-4 Dry Sulfur (SO 

2 + SO2-4 ) Deposition (kg/ha/yr) for 2010

1.8 1.0

2.2

1.0

1.5

1.9

1.5

5.5

3.1

2.1

2.4

0.2

0.7

0.3

1.3

2.1

3.2

0.3

0.5

2.2 1.5

0.5

0.2

0.9

0.2

0.8

2.0

0.2

0.5

1.5

0.7

0.9

0.6

1.8

0.1

0.2

0.1

0.6

0.2

0.8

0.3

1.4

0.5

0.1

1.8

2.1

3.1 0.2

0.7

2.0

0.4

0.2

0.2

1.0

0.5

4.1

0.2

2.0

0.6

0.4

1.4

1.2

0.5

0.3

1.2

3.7

0.3

3.0

0.2

2.1

0.1

0.2

Site not pictured:

DEN417, AK 0.1

Deposition

6.0

3.0

0.6
CASTNET Annual Report – 2010 36 Chapter 3: Atmospheric Deposition

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

precipitation. The concentrations were multiplied by precipitation rates to estimate wet 

fluxes. Wet flux data were interpolated to CASTNET sites.

A map of estimates of total sulfur deposition is given in Figure 3-5. The map was 

constructed by adding dry and wet deposition. The diameters of the circles in the figure 

illustrate the magnitude of total sulfur deposition and also the relative contributions from 

wet and dry deposition. The dark shading (blue) signifies the percent wet deposition, and 

the light shading (tan) shows the percent dry deposition. Only three CASTNET sites in the 

states of Indiana, Ohio, and Pennsylvania had total (dry + wet) sulfur deposition fluxes 

greater than 7.0 kg/ha/yr in 2010. The highest total sulfur deposition rate was estimated for 

QAK172, OH with a value of 9.8 kg/ha/yr. The total sulfur deposition value for this site was 

9.7 kg/ha/yr in 2009 and 17.2 kg/ha/yr in 2008. Total sulfur deposition at western sites 

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

contribution of dry deposition was much more significant in and near major source regions. 

For example, the contribution of dry sulfur deposition ranged from about half of total sulfur 

deposition at Mackville, KY (MCK131) to approximately a third at Beaufort, NC (BFT142).

Figure 3-5 Total (Dry + Wet) Sulfur Deposition (kg/ha/yr) for 2010

6.5

0.6

5.8

4.2

4.5

4.5

5.9

5.4

4.9 3.8

4.1

3.3

3.3

4.2

6.1

6.7

1.7

1.0

5.3 4.6

3.0

0.5

3.9

1.0

2.9

5.2

1.4

3.3

5.0

4.3

3.6

4.3

0.7

1.3

1.0

2.9

2.1

3.0

0.5

5.6

2.8

0.7

4.5

5.6

5.6 1.0

2.0

6.1

1.0

0.6

0.6

3.9

2.7

7.4

0.9 9.8

5.4

2.3

1.1

4.9

5.0

3.3

1.2

3.9

8.0

1.5

5.9

1.4

4.4

2.6

1.0

Site not pictured:

DEN417, AK 0.2

Total Deposition

10

5

1

Dry Deposition

Wet Deposition
CASTNET Annual Report – 2010 37 Chapter 3: Atmospheric Deposition

Figure 3-6 presents box plots that show the trend in dry sulfur deposition (as S) for the 

eastern (right side) and western (left side) reference sites. Figure 3-7 shows the trend in 

annual total (dry + wet) sulfur deposition (as S). The box plots were based on data 

obtained from the 34 CASTNET eastern reference sites (Figure 1-2) for the period 1990 

through 2010 and from the 17 western reference sites (Figure 1-3) for 1996 through 2010. 

The y-axes on the two figures have different scales. 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 13.2 kg/ha/yr to a 2008–2010 mean of 6.0 kg/ha/yr, a 55 percent reduction. Total 

sulfur deposition declined at the western reference sites from a 1996–1998 mean of 

1.2 kg/ha/yr to a 2008–2010 mean of 0.8 kg/ha/yr, a 28 percent reduction. The total sulfur 

flux for the western reference sites was somewhat higher in 2010 than in 2009.

Figure 3-6 Trend in Dry Sulfur Deposition (kg/ha/yr)

Western Reference Sites Eastern Reference Sites

Figure 3-7 Trend in Total Sulfur Deposition (kg/ha/yr)

Western Reference Sites Eastern Reference Sites

0.0

0.2

0.4

0.6

0.8

199619971998199920002001200220032004200520062007200820092010

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0.0

0.5

1.0

1.5

2.0

2.5

3.0

199619971998199920002001200220032004200520062007200820092010

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

5

10

15

20

25

199019911992199319941995199619971998199920002001200220032004200520062007200820092010

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

2

4

6

8

10

12

199019911992199319941995199619971998199920002001200220032004200520062007200820092010

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2010 38 Chapter 3: Atmospheric Deposition

Figure 3-8 presents estimates of trends in dry, wet, and total deposition of sulfur (as S) for 

the 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 3-8. The solid 

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

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

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

have all declined significantly.

Figure 3-8 Trend in Sulfur Deposition (kg/ha/yr) with Concentrations in Precipitation 

[milligrams per liter (mg/l)]

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

2

4

6

8

10

12

14

16

19901991199219931994199519961997199819992000

200120022003200420052006200720082009

2010

Deposition (kg/ha/yr)

Concentration (mg/l)

Wet Deposition

Dry Deposition

Total Deposition

Concentration in Precipitation

Crockett, KY (CKT136)
CASTNET Annual Report – 2010 39 Chapter 3: Atmospheric Deposition

An Intercomparison of the CASTNET Multi-Layer Model and 

the CAPMoN Big-Leaf Model 

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

CAPMoN, which operates 33 monitoring sites in Canada and one in the United States. CASTNET 

and CAPMoN both operate filter pack samplers at Egbert, ON. Both networks combine modeled 

dry Vd with measured air pollutant concentrations to estimate dry deposition fluxes. CASTNET uses 

the MLM to simulate hourly values of Vd, and CAPMoN uses the Big-Leaf Model (BLM). The MLM 

uses measured meteorological conditions as model input. The BLM and other inferential models, 

which are used primarily as modules within regional chemical transport models, use meteorological 

input data produced by numerical weather prediction models. In particular, the BLM obtains 

meteorological input data from the Global Environmental Multiscale model. Recent studies (e.g., 

Flechard et al., 2011) have shown that flux estimates produced by inferential models can be 

inconsistent because of differences in model-calculated Vd values. 

Schwede and colleagues at EPA and Environment Canada (Schwede et al., 2011) recently 

intercompared the MLM and BLM “to identify those model inputs and model algorithms that are 

responsible for the differences in the dry deposition velocity predictions of the gaseous trace 

species O 

3, SO 2, and HNO 3.” This section summarizes the work of Schwede and her colleagues. 

Their work is thoroughly described in the paper in Atmospheric Environment (Volume 45, Issue 6, 

February 2011).

Measured pollutant concentrations, modeled Vd values, and estimates of dry deposition flux from 

the joint CAPMoN and CASTNET site at EGB181, ON were compared for the period 2002–2007. 

The results, which are shown in Figure 3a, depict reasonably good agreement between SO 

2 and 

HNO 

3 concentrations measured at the two monitoring stations. The median percent differences 

between the two networks were -4.2 percent for SO 

2 and -18.5 percent for HNO 3 (CAPMoN values 

are higher than CASTNET values). However, the estimated fluxes and Vd values were significantly 

different. The CAPMoN fluxes exceeded the CASTNET values with median percent differences of -

53.6 percent for SO 

2 and -46.6 percent for HNO 3. These large differences were primarily the result 

of differences in Vd values with higher values produced by the BLM. The median percent 

differences for Vd were -49.3 percent for SO 

2 and -34.7 percent for HNO 3. 

Estimates of deposition velocities from both the MLM and BLM were evaluated using field 

measurements (Meyers et al., 1998; Finkelstein et al., 2000), and the results showed good 

agreement between modeled values and field data. However, it was not possible to directly 

compare the MLM results with the BLM results because of averaging time and other limitations. 

Both models have been used to provide estimates of the dry deposition of sulfur and nitrogen 

pollutants for ecological and critical loads assessments. The choice of model could have significant 

implications in estimating sulfur and nitrogen pollutant budgets, leading to different conclusions on 

the ecological assessments. For example, the use of the BLM could result in higher estimates of 

deposition relative to critical loads.

To identify the key model input data and model algorithms that affect Vd estimates, the BLM was 

inserted into the MLM framework so that measured meteorological data could be used as input to 

both models. The two models were run for four CASTNET sites that represent different land uses 

and climates. The four sites include BVL130, IL; SND152, AL; Sumatra, FL (SUM156); and 

EGB181, ON (see Figure 1-1 and Appendix A for site locations). After running the models for 

baseline conditions, the models were modified incrementally to allow the assessment of the 

contribution of each model algorithm to differences in modeled values of Vd. Table 3a summarizes 

the model runs.
CASTNET Annual Report – 2010 40 Chapter 3: Atmospheric Deposition

Figure 3a Comparison of Weekly Values of Measured SO 

2 and HNO 3 Concentrations, Estimated 

SO 

2 and HNO 3 Fluxes, and Modeled SO 2 and HNO 3 Deposition Velocities at EGB181, ON 

(Schwede et al., 2011)

Reprinted with permission from EPA

Table 3a Model Configurations (Schwede et al., 2011)

Run Name Description

Base Both MLM and BLM run in their native configuration using on-site meteorology from CASTNET

LAI As with the Base run except MLM was modified to use the LAI from BLM

PP As with the LAI run except BLM was modified to use the MLM plant parameters

Ra As with the PP run except BLM was modified to use the MLM aerodynamic resistance

Fv As with the Ra run except BLM was modified to use the MLM vapor pressure deficit function in the 

stomatal resistance calculation

Rs As with the Fv run except BLM was modified to use the MLM water stress function in the stomatal 

resistance calculation

Rac As with the Rs run except MLM was modified to use the BLM in-canopy aerodynamic resistance

Rg As with the Rac run except MLM was modified to use the BLM ground (soil) resistance

Rcut As with the Rg run except MLM was modified to use the BLM cuticular resistance

Reprinted with permission from EPA

Notes: PP = plant parameters

Ra = aerodynamic resistance

Fv = vapor pressure deficit function

Rs = water stress function

Rac = in-canopy aerodynamic resistance

Rg = ground (soil) resistance

Rcut = cuticular resistance

Concentration (ug/m3 ) 
CASTNET Annual Report – 2010 41 Chapter 3: Atmospheric Deposition

Figure 3b illustrates the distributions of SO 

2 Vd values for the four sites based on the model 

configurations listed in Table 3a. The box indicates the 25-75 percentile range, the X indicates the 

mean, the horizontal line indicates the median, the whiskers show the 5-95 percentile range, and 

the circles depict outliers. The effects of the different model configurations can be seen by 

comparing an individual configuration (e.g., Rcut for cuticular resistance) to the base case. For 

example, changing the MLM code to use the BLM value for the cuticular resistance had a 

significant effect in that much higher values of Vd were estimated for the MLM.

The study by Schwede et al., (2011) illustrates some of the key differences in MLM and BLM. The 

differences in meteorological input data and the algorithm that simulates aerodynamic resistance 

have a strong influence on Vd for HNO 

3. On the other hand, the differences in Vd estimates for SO 2

and O 

3 are influenced strongly by differences in soil and cuticular resistances. Because these 

resistances have not been well measured or well understood, their formulation has to be 

considered further in both models. While some studies have compared the MLM and BLM Vd

values to field data, the accuracy of the two models is not well understood. Vd values from both 

models will have to be compared to recent, independent, multi-year flux measurements in order to 

assess model accuracy.

Figure 3b Sulfur Dioxide Deposition Velocity Distributions for BVL130, EGB181, SND152, and 

SUM156 for Different Model Configurations (Schwede et al., 2011)

Reprinted with permission from EPA
CASTNET Annual Report – 2010 42 Chapter 3: Atmospheric Deposition

Nitrogen Deposition

Figure 3-9 illustrates dry fluxes of nitrogen (as N) for 2010. These fluxes are the sum of 

fluxes of HNO 

3 + NO3 + NH+ 4 , based on the individual MLM simulations for the three species. 

A majority of CASTNET sites in the eastern United States had estimated dry nitrogen 

deposition rates greater than 1.0 kg/ha/yr. Five eastern and one western site had 

deposition rates greater than or equal to 2.0 kg/ha/yr. QAK172, OH and ARE128, PA had 

estimated dry nitrogen fluxes of 2.5 kg/ha/yr. The highest value for western sites was 

calculated for the CASTNET site in JOT403, CA with a dry nitrogen flux of 2.3 kg/ha/yr. 

Figure 3-9 Dry Nitrogen (HNO 

3 + NO3 + NH+ 4 ) Deposition (kg/ha/yr) for 2010

A map of total nitrogen deposition (as N) for 2010 is given in Figure 3-10. The map was 

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

deposition. The figure shows that a majority of the eastern sites estimated deposition rates 

greater than 5.0 kg/ha/yr. Stockton, IL (STK138) recorded the highest total nitrogen flux 

(8.1 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 5.1 kg/ha/yr. Six other western sites in 

California, Arizona, South Dakota, and North Dakota had estimated total nitrogen fluxes 

greater than 3.0 kg/ha/yr. The contributions of dry nitrogen deposition to total nitrogen 

were lower than the corresponding contributions of dry sulfur deposition. Dry nitrogen 

1.5 1.9

1.6

0.6

1.2

1.7

1.2

2.5

2.0

1.3

1.7

1.8

1.1

0.4

1.3

1.5

2.5

0.3

0.8

1.5 1.6

0.7

0.7

1.4

0.6

1.3

1.4

0.6

0.6

1.1

0.9

0.8

0.6

1.5

0.1

0.6

0.2

0.8

0.2

0.5

2.3

0.8

1.2

0.3

0.9

2.0

1.9 0.8

1.2

1.1

0.9

0.6

0.4

1.1

0.7

1.6

0.6

2.1

1.1

1.6

1.1

1.3

0.6

0.6

1.1

1.8

0.5

1.8

0.7

1.8

0.1

0.2

Site not pictured:

DEN417, AK 0.1

Deposition

3.0

1.5

0.3
CASTNET Annual Report – 2010 43 Chapter 3: Atmospheric Deposition

deposition contributed less than 50 percent of total deposition in the East. On the other 

hand, dry nitrogen deposition contributed more than half of total nitrogen deposition at the 

two monitoring sites downwind of the Los Angeles in California, a region with elevated 

concentrations of nitrogen species and limited rainfall. Dry nitrogen deposition contributed 

70 percent of the total nitrogen deposition at JOT403.

Figure 3-10 Total (Dry + Wet) Nitrogen Deposition (kg/ha/yr) for 2010

Figure 3-11 presents box plots that were constructed using data from the 34 eastern 

reference sites over the period 1990 through 2010 (right side) and 17 western reference 

sites for 1996 through 2010 (left side). The box plots show the trends in dry nitrogen 

deposition (as N). The box plots for the eastern sites in Figure 3-12 show a reduction in dry 

nitrogen deposition beginning in 1999 with a slight increase in 2010. Three-year mean 

fluxes declined from 2.2 kg/ha/yr for 1990–1992 to 1.3 kg/ha/yr for 2008–2010, a 

41 percent reduction over the 21 years. The box plots for the western sites show a smaller 

28 percent decline, a change from a mean of 0.9 kg/ha/yr for 1996–1998 to 0.6 kg/ha/yr 

for 2008–2010.

5.9

3.3

5.4

3.1

4.8

3.4

6.1

5.1

5.5 5.1

4.5

4.1

3.1

4.1

5.9

7.2

2.0

1.5

5.6 5.5

3.5

1.7

5.0

2.2

4.7

5.1

2.8

3.6

5.5

4.8

3.3

4.2

1.5

3.5

2.0

4.4

2.9

2.7

3.2

4.4

5.2

1.4

3.6

6.2

5.0 2.5

5.1

5.2

2.2

2.1

1.3

4.7

5.5

5.1

2.2 5.6

6.9

7.3

3.9

5.0

8.1

3.1

3.1

4.9

6.7

3.6

4.4

4.3

4.9

3.0

1.8

Site not pictured:

DEN417, AK 0.3

Total Deposition

10

5

1

Dry Deposition

Wet Deposition
CASTNET Annual Report – 2010 44 Chapter 3: Atmospheric Deposition

Figure 3-11 Trend in Dry Nitrogen Deposition (kg/ha/yr)

Western Reference Sites Eastern Reference Sites

Figure 3-12 shows the trends in annual total (dry + wet) nitrogen deposition for the 

eastern (right side) and western (left side) reference sites. The figure suggests that total 

nitrogen flux in the eastern United States has decreased since 1996 with a substantial 

decline over the last five years. Total nitrogen flux for the western sites shows a relatively 

flat distribution with an increase in 2010. Estimates of trends in wet, dry, and total 

deposition of atmospheric nitrogen (as N) are presented in Figure 3-13 for the eastern 

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

concentrations in precipitation shows an overall slow downward trend since 1998 with a 

slight increase in 2010. This trend line is reasonably representative of the effect of changes 

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

deposition declined 32 percent in the East over the 21 years and 20 percent for the 

aggregated western sites over the 15-year period.

Figure 3-12 Trend in Total Nitrogen Deposition (kg/ha/yr)

Western Reference Sites Eastern Reference Sites

0.0

0.5

1.0

1.5

2.0

199619971998199920002001200220032004200520062007200820092010

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

5

6

199019911992199319941995199619971998199920002001200220032004200520062007200820092010

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0

1

2

3

4

5

6

199619971998199920002001200220032004200520062007200820092010

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

2

4

6

8

10

12

14

199019911992199319941995199619971998199920002001200220032004200520062007200820092010

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2010 45 Chapter 3: Atmospheric Deposition

Figure 3-13 Trend in Nitrogen Deposition (kg/ha/yr) with Concentrations in 

Precipitation (mg/l)

Estimates of Critical Loads in the Eastern United States 

The critical load approach is an emerging assessment and regulatory tool that is used to determine 

the degree to which acidic deposition may be affecting the ecological health of surface waters. The 

recent EPA (2011d) review of the secondary NAAQS for oxides of nitrogen and oxides of sulfur 

explored the idea of a critical load-based approach for regulating these atmospheric oxides using 

an Aquatic Acidification Index (AAI). A critical load is a quantitative estimate of exposure to one or 

more pollutants below which significant harmful effects on specific sensitive elements of the 

environment do not occur according to present knowledge (Nilsson and Grennfelt, 1988). A 

critical load provides a benchmark against which to assess the extent to which a water body is 

potentially at risk due to current acidic deposition levels. If pollutant exposure is less than the 

critical load, adverse ecological effects (e.g., reduced reproductive success, stunted growth, or 

loss of biological diversity) are not anticipated, and recovery is expected over time if an ecosystem 

has been damaged by past exposure. A critical load exceedance is the measure of pollutant 

exposure above the critical load. This means pollutant exposure is higher than, or “exceeds,” the 

critical load, and the ecosystem continues to be exposed to damaging levels of sulfur and 

nitrogen deposition. 

This critical load assessment examined select lakes and streams within 20 km of 19 CASTNET 

monitoring locations in the eastern United States (Figure 3c). Critical loads were obtained for 

545 lakes and streams from the National Critical Load Database (NCLD), developed in conjunction

with the NADP Critical Loads of Atmospheric Deposition (CLAD) science committee. Critical loads 

were estimated using the Steady-State Water Chemistry (SSWC) mass-balance model, which 

estimates catchment weathering using the F-factor approach and surface water chemistry data 

(Henriksen et al., 2002). The critical load in this study represents the amount of acidic deposition of 

S and N that a stream/lake’s watershed could receive and still maintain an acid neutralizing 

capacity (ANC) of 50 μeq/L or higher, which supports a healthy aquatic ecosystem. Critical loads 

are expressed in terms of ionic charge balance as milliequivalents per square meter per year 

(meq/m2

/yr). Total sulfur and nitrogen deposition were determined for each lake or stream. Wet 

deposition for SO2-

4 , NO3, and NH+ 4 were based on PRISM-corrected NADP estimates. 

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

2

4

6

8

10

12

199019911992199319941995199619971998199920002001200220032004200520062007200820092010

Deposition (kg/ha/yr)

Concentration (mg/l)

Wet Deposition

Dry Deposition

Total Deposition

Concentration in Precipitation
CASTNET Annual Report – 2010 46 Chapter 3: Atmospheric Deposition

Dry deposition of SO 

2, SO2-4 , NO3, HNO 3, and NH+ 4 were based on CASTNET modeled values for 

each CASTNET location. At CASTNET locations with AMoN monitors, dry NH 

3 values were 

calculated using AMoN’s air concentration and a deposition velocity from a 2002 CMAQ model run 

(EPA, 2011c). For CASTNET sites without AMoN monitors, dry NH 

3 was determined using a dry 

NH 

3 to wet NH+ 4 ratio (NH 3/NH+ 4 ) calculated for AMoN monitoring locations and applied to 

CASTNET locations.

Figure 3c shows critical load estimates for surface water acidification for select lakes and streams 

within 20 km of select CASTNET locations. Gray circles indicate CASTNET monitoring locations, 

and red and green symbols represent lakes or streams that exceed or do not exceed, respectively, 

their critical load for an ANC of 50 μeq/L. Graphs represent the 25 percentile critical load and total 

sulfur and nitrogen deposition for years 2008 through 2009 at CASTNET locations. On the graphs, 

dark and light red bars represent the maximum critical load (CLmaxS) of sulfur and the amount 

(CLminN) of nitrogen that is removed by the ecosystem, e.g., through immobilization and/or 

denitrification. Dark to light blue represents total wet and dry inorganic and organic nitrogen and 

sulfur deposition, respectively.

For the period from 2008 through 2009, 29 percent of the 545 modeled lakes and streams received 

levels of S and N deposition that exceeded the critical load (Figure 3c). Figure 3c also shows a 

comparison between the 25 percentile critical load for lakes and streams within 20 km of a 

CASTNET location and total average N and S deposition. For the 19 selected CASTNET locations, 

8 received total S and N deposition greater than the 25 percentile critical load, indicating that at 

least a quarter of the modeled lakes or streams 20 km from the CASTNET locations exceeds total 

S and N deposition. Many of the eight locations, such as Laurel Hill State Park, PA (LRL117); 

Cranberry, NC (PNF126); SND152, AL; Cowetta, NC (COW137); Shenandoah National Park, VA 

(SHN418); OXF122, OH; BEL116, MD; and Pennsylvania State University, PA (PSU106), with 

exceedances are located in the central and southern Appalachian Mountains where many 

streams are still at risk of acidification despite significant reductions of emissions of sulfur and 

nitrogen oxides.

Springtime view of Bluestone River in West Virginia
CASTNET Annual Report – 2010 47 Chapter 3: Atmospheric Deposition

Figure 3c Critical Load Estimates for Select Lakes and Streams

Source: J. Lynch
CASTNET Annual Report – 2010 48 Chapter 4: Ozone Concentrations

Chapter 4: Ozone Concentrations

CASTNET provides the primary platform for monitoring rural, ground-level O 

3 concentrations in the 

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

3 levels. CASTNET was 

not originally designed to operate as a regulatory network. However, as of May 2011, CASTNET O 

3

monitoring systems comply with regulatory monitoring requirements, and data collected during the 

2011 ozone season were submitted to the EPA Air Quality System. In this report, O 

3 measurements 

collected during 2008–2010 were evaluated with respect to both the 1997 O 

3 standard (EPA, 1997) of 

0.08 parts per million (ppm) and the 2008 standard (EPA, 2008a) of 0.075 ppm. For the 2008–2010 

period, only three eastern and four California sites recorded exceedances of the 0.075 ppm standard.

All but 3 of CASTNET’s 84 sites operate an O 

3 analyzer that measures hourly 

concentrations. CASTNET’s geographic coverage of the United States provides data that are 

useful in terms of presenting information on geographic patterns in regional O 

3 and for 

evaluating rural O 

3 concentrations in the context of the O 3 NAAQS. The analyses presented 

in this chapter provide maps and examine trends in the annual fourth highest daily 

maximum 8-hour average (DM8A) O 

3 concentrations measured at CASTNET sites. A map of 

2010 W126 values is also presented. The concentration shading for the figures used in this 

chapter was prepared using the scales shown on each map. The NPS Air Atlas incorporates 

all O 

3 monitoring reported to the EPA Air Quality System (AQS) plus the CASTNET O 3

measurements. Additional maps of O 

3 concentrations can be viewed at 

http://science.nature.nps.gov/AirAtlas/AirAtlas0105/viewer.htm.

Ground-level O 

3 is an air pollutant that can cause a number of human respiratory effects 

and damage to vegetation and ecosystems. It is formed in the lower atmosphere when 

VOCs and NOx react in the presence of sunlight. Ozone is the main component of smog. 

Emissions from motor vehicle exhaust, industrial facilities, chemical solvents, gasoline 

vapors, and power plants are the major anthropogenic sources of NOx and VOCs. Also, 

biogenic VOC emissions from trees and other vegetation contribute to O 

3 formation in rural 

areas. Meteorological conditions play a significant role in O 

3 formation. O 3 concentrations 

are generally higher in the warmer months because sunlight and hot weather accelerate its 

formation, but episodes of high O 

3 concentrations can vary by region of country. For 

example, the highest O 

3 concentrations measured in Maryland occur during summer and 

early fall while the highest O 

3 concentrations measured in Wyoming occur during late 

winter and early spring. See the discussion on page 57 about Wyoming ozone data.

The DM8A O 

3 standard can be used to assess the status and trends in rural O 3 levels in 

order to gauge the success of EPA emission reduction programs such as the NOx SIP 

Call/NBP, CAIR, and now CSAPR. On May 1, 2009, the CAIR NOx ozone season trading 

program began, replacing the NBP in states covered by the new program and requiring 
CASTNET Annual Report – 2010 49 Chapter 4: Ozone Concentrations

further NOx emission reductions from the power sector. Implementation of the NOx

emission reductions that were mandated by the ARP and other NOx emission control 

programs have contributed to reductions in O 

3 concentrations, especially in rural areas 

with elevated DM8A levels, because regional transport of pollutants contributes to O 

3

formation. VOCs emission reduction strategies also have been successful in reducing 

higher, short-term O 

3 concentrations in and downwind of urban areas. 

The O 

3 NAAQS are summarized in the following table. The primary standard is designed to 

protect public health, including the health of at-risk populations. The secondary standard is 

designed to protect public welfare and the environment, including sensitive vegetation 

and ecosystems. Both the primary and secondary O 

3 NAAQS are given in terms of an O 3

concentration of 0.075 ppm and an 8-hour averaging time. O 

3 concentrations collected in 

2010 were also analyzed in terms of W126, a cumulative metric that sums weighted hourly 

O 

3 concentrations during the O 3 season. For this report, W126 was calculated for the months 

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

is the W126 value.

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). 

Rocky Mountain National Park, CO (ROM406/ROM206)
CASTNET Annual Report – 2010 50 Chapter 4: Ozone Concentrations

CASTNET’s Transition to AQS Monitoring 

Background

CASTNET was originally designed to provide information on geographic patterns in regional, rural 

O 

3 levels. However, because of interest in rural O 3 concentrations by the regulatory and scientific 

communities, EPA directed MACTEC, now known as AMEC, to prepare the EPA-sponsored sites 

that measure O 

3 concentrations to meet 40 CFR Part 58 monitoring requirements and to ultimately 

deliver CASTNET O 

3 measurements to AQS for demonstration of compliance with NAAQS.

AMEC began this effort in July 2010, and installed the initial 19 sites prior to October 1, 2010. Data 

from these sites were evaluated, and protocols were refined prior to installing Part 58-compliant 

systems at the remaining sites. The remaining sites were upgraded in 2011. As of May 26, 2011, 

the effort was complete with compliant systems installed at 55 EPA-sponsored sites. In addition, 

NPS operates 24 AQS-compliant sites for a total of 79 AQS-compliant CASTNET sites.

Instrument upgrades were performed to support conversion to regulatory-compliant operating and 

reporting procedures. CASTNET monitoring equipment was upgraded to include new ozone 

analyzers, data loggers with improved data communication, and internal shelter temperature 

sensors. The new data loggers also provide system monitoring data that enable remote 

troubleshooting and optimization of operations. The data logger is programmed to control daily 

zero/span/precision checks for the O 

3 analyzer. Zero air is used for the daily zero checks and as the 

dilution medium for the precision and span checks. The O 

3 concentrations needed for the precision 

(90 ppb) and span (400 ppb) checks are produced by a separate on-site transfer standard.

Traceability of Standards

The on-site transfer standards initially carried Level 4 authority, which is three generations 

removed from a standard reference photometer (SRP) – see Figure 4a, which was taken from the 

EPA (2010c) Technical Assistance Document, Transfer Standards for the Calibration of Air 

Monitoring Analyzers for Ozone. In order to maintain the highest level of authority practical for the 

on-site transfer standards, AMEC subsequently opted to have all of its traveling transfer standards 

verified by a National Institute of Standards and Technology (NIST) SRP. Therefore, the traveling 

transfer standards now carry Level 2 authority. This facilitates the maintenance of Level 3 authority 

for all EPA-sponsored CASTNET on-site standards.

Figure 4a Ambient Air Ozone Traceability Scheme

Source: EPA (2010c)
CASTNET Annual Report – 2010 51 Chapter 4: Ozone Concentrations

Data Collection

All CASTNET continuous measurements, including the O 

3 data and supporting parameters, are 

collected via the data logger, polled remotely, archived at the CASTNET Data Management Center, 

and uploaded to AIRNow every hour. For sites to meet Part 58 monitoring criteria, data will be 

collected in accordance with the EPA Quality Assurance (QA) Handbook, Volume II (EPA, 2008b). 

Multi-point calibrations, which will be performed every six months, will also be in compliance with 

the EPA QA handbook. Ozone transfer standards will be used in accordance with EPA Technical 

Assistance Document, Transfer Standards for the Calibration of Air Monitoring Analyzers for Ozone

(EPA, 2010c).

AQS Data Submission

Data will be uploaded to the AQS data submittal Web application in batch format using text files of 

raw ozone data and measurement accuracy and precision data beginning in 2011. Data will be 

used to determine NAAQS compliance and to assess current and future regulatory programs.

Eight-Hour Ozone Concentrations

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

3 concentrations for 

2008–2010. During this period, 3-year averages of the fourth highest DM8A O 

3

concentrations were greater than 75 ppb at three eastern sites and five sites in California. 

The eastern sites include Washington’s Crossing, NJ (WSP144); BEL116, MD; and Great 

Smoky Mountains National Park, TN (GRS420). The five California sites include Yosemite 

National Park (YOS404), SEK430, Pinnacles National Monument (PIN414), CON186, and 

JOT403. Three California CASTNET sites measured concentrations greater than or 

equal to the 1997 NAAQS of 85 ppb. The California sites include SEK430, CON186, and 

JOT403. Three-year average concentrations were generally lower in 2008–2010 than in 

2007–2009.

The 2008–2010 3-year average of fourth highest DM8A O 

3 concentrations constitutes the 

current design value for achieving the DM8A O 

3 NAAQS. 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 typically used to classify nonattainment areas, assess progress 

towards meeting the NAAQS, and develop control strategies to achieve the NAAQS. For 

example, the value of 106 ppb at CON186 that was measured during 2008–2010 would 

have to be reduced to 84 ppb to achieve the level of the 1997 standard and to 75 ppb to 

achieve the level of the 2008 standard. Similarly, the concentration of 78 ppb at WSP144 

would have to be reduced to 75 ppb to meet the 2008 NAAQS. 
CASTNET Annual Report – 2010 52 Chapter 4: Ozone Concentrations

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

2008–2010

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

2000–2002
CASTNET Annual Report – 2010 53 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 2008–2010 design values shown in 

Figure 4-1. The period 2000–2002 was selected because the NOx SIP Call/NBP began in 

2003 for the eastern United States. The changes in O 

3 concentrations from 2000–2002 to 

2008–2010 illustrate the effectiveness of the emission reduction programs. Based on the 

84 ppb NAAQS level, 19 eastern and 3 California sites recorded 3-year averages greater than 

or equal to 85 ppb during 2000–2002. The regions with elevated DM8A O 

3 concentrations 

were located along the East Coast from northern Virginia to Connecticut, extending almost 

to Maine, and in the central states of Pennsylvania, Ohio, Michigan, Indiana, and Illinois. 

Exceedances were also recorded in Kentucky, Tennessee, and the greater Atlanta area. The 

map in Figure 4-2 shows that most of the eastern sites measured O 

3 concentrations greater 

than 75 ppb, the current standard. Five monitors in California and the site at Rocky 

Mountain National Park (ROM406) also recorded concentrations greater than 75 ppb. The 

period 2008–2010 represents a significant improvement in air quality.

Measurements of fourth highest DM8A O 

3 concentrations during 2010 (Figure 4-3) were 

somewhat higher than 2009 but considerably lower than concentrations measured during 

the period 2000–2002. Seven eastern CASTNET sites measured concentrations above 

75 ppb. These include WSP144; BEL116; GRS420; MK Goddard State Park, PA (MKG113); 

Quaker City, OH; OXF122, OH; and Crockett, KY (CKT136). Four sites in California plus 

ROM406, the primary of the collocated site at Rocky Mountain National Park, measured 

fourth highest DM8A O 

3 concentrations greater than 75 ppb. The four California sites 

include YOS404, SEK430, CON186, and JOT403. Three of the four sites measured 

concentrations above 84 ppb. The monitor at CON186 measured the highest value 

(101 ppb) in the network during 2010.

Washington’s Crossing, NJ

(WSP144)

MK Goddard State Park, PA 

(MKG113)

Beltsville, MD (BEL116) 
CASTNET Annual Report – 2010 54 Chapter 4: Ozone Concentrations

Figure 4-3 Fourth Highest DM8A Ozone Concentrations (ppb) for 2010

Figure 4-4 provides box plots depicting trends in fourth highest DM8A O 

3 concentrations 

from the 34 CASTNET eastern reference sites (right side) and 17 western reference sites 

(left side). The eastern O 

3 data show an overall reduction since 2002. However, the figure 

shows an increase in 2010. The mean aggregated value (69 ppb) for the eastern reference 

sites for 2010 was higher than the 2009 value of 64 ppb, which was the lowest in the 

history of the network. The concentrations in 2009 and 2010 represent a substantive 

improvement over the mean value of 90 ppb measured in 1998. The box plots constructed 

from the aggregated western O 

3 sites show no trend. The 2008–2010 average of the fourth 

highest DM8A O 

3 concentrations for the western reference sites was 69 ppb.

Converse Station, CA (CON186) 
CASTNET Annual Report – 2010 55 Chapter 4: Ozone Concentrations

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

Western Reference Sites Eastern Reference Sites

W126 Values for 2010

The W126 O 

3 exposure index is a biologically-based, cumulative exposure index (Lefohn 

and Runeckles, 1987; EPA, 1996; 2006) for characterizing O 

3 trends and relating vegetation 

yield reduction losses with O 

3 exposure. Many researchers (e.g., Musselman et al., 2006) 

concluded that both peak concentrations and cumulative effects are important in relating 

atmospheric O 

3 to vegetation damage. Consequently, W126 was developed as a seasonal 

metric from the sum of weighted hourly values measured during the 12-hour daylight 

period during the O 

3 season, with the higher hourly concentrations being weighted more 

heavily than lower concentrations (EPA, 2010b). In this report, W126 was used to 

characterize high concentrations over a seasonal, 3-month exposure period.

The W126 index is represented as the sum of all hourly O 

3 concentrations, where each 

measured concentration is weighted by a function, wi, to assign greater emphasis to the 

higher measured concentrations. This weighting function provides a weighting value that is 

unique for each hourly O 

3 concentration. The weighting function, as described by Lefohn

et al. (1988) is:

Hourly Weighting = Equation 4-1 

Where:

wi = weighting value for hourly concentration i, and

 ci = hourly concentration i in ppb.

Each hourly weighting value is multiplied by its corresponding hourly concentration. This 

product is summed over all valid hours to calculate the W126 exposure. Thus, the W126 

exposure is:

 Equation 4-2 

50

70

90

110

199619971998199920002001200220032004200520062007200820092010

Concentration (ppb)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

50

70

90

110

199019911992199319941995199619971998199920002001200220032004200520062007200820092010

Concentration (ppb)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

44031 exp( )126.0-

1

i

i

c

w

+ =

∑=

=

n

i 1

126 ii W cw
CASTNET Annual Report – 2010 56 Chapter 4: Ozone Concentrations

Where:

n = number of hours with valid O 

3 concentrations.

The exposure unit is ppm-hours. For CASTNET W126 exposures are calculated using O 

3

values measured during the 12 hours (n = 12) from 8:00 a.m. to 8:00 p.m. in the months of 

May through September of each year.

CASTNET hourly O 

3 concentrations were used to calculate W126 values for 2010. 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. 

W126 levels for 2010 are presented in Figure 4-5. The higher W126 values were measured 

in California and at western sites in high terrain. The highest W126 value (54.1 ppm-hour) 

was measured at SEK430, CA. Ozone concentrations measured at high elevation sites 

(MACTEC, 2003) typically show low variability from hour to hour because dry deposition 

and scavenging of O 

3 by nitric oxide are low at night, resulting in the persistence of daytime 

values throughout the 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.

Figure 4-5 W126 Values (ppm-hour) for 2010
CASTNET Annual Report – 2010 57 Chapter 4: Ozone Concentrations

Air Quality Concerns in the Upper Green River Basin, WY 

The Upper Green River Basin (UGRB) of southwest Wyoming has experienced substantive growth 

in gas and oil exploration, drilling, and production. Thousands of wells operate in the region of the 

Pinedale Anticline, located near the town of Pinedale, south to the Jonah field. These wells and 

related activities produce emissions of VOCs and NOx to the atmosphere. Consequently, state 

and local government officials and the general population have expressed concern about O 

3 levels, 

regional haze, and sulfur and nitrogen deposition. While this discussion focuses primarily on 

data collected through 2010, some data were available for 2011 and are included to provide 

current context.

Figure 4b Upper Green River Winter Ozone Study 2010 Monitoring Site Locations

Elevated O 

3 concentrations have been recorded 

at air quality monitoring stations (Figure 4b) in 

UGRB during late winter and early spring since 

2005. DM8A O 

3 concentrations above the 75 ppb 

NAAQS were measured in 2005, 2006, 2008, 

and 2011, typically during episodes of from one

to six days. For example, during 2008, DM8A O 

3

concentrations exceeded 75 ppb during six

episodes. The higher DM8A concentrations 

exceeded 100 ppb. The fourth highest DM8A

concentration during the principal period (2005–

2010) was 101 ppb during 2008 at the Boulder, 

WY monitor. Table 4a was prepared by 

Meteorological Solutions, Inc. (MSI, 2010). The 

table lists average and maximum 8-hour O 

3

concentrations for the six years 2005 through 

2010 at monitors operated by the Wyoming 

Department of Environmental Quality (WDEQ)

plus the CASTNET site at Pinedale, WY 

(PND165). 

The PND165 CASTNET site is located northeast 

of the Town of Pinedale on a ridge with an 

elevation about 700 feet higher than the town’s 

elevation in the UGRB. Although the O 

3

concentrations recorded at PND165 were 

generally not as high as those measured in the 

UGRB, hourly concentrations have exceeded 

80 ppb, and DM8A concentrations have exceeded 70 ppb. O 

3 levels measured at the PND165 

ridge site were higher during ozone episodes experienced simultaneously in the basin. Figure 4c 

shows trends in fourth highest DM8A O 

3 concentrations and first quarter DM8A O 3 concentrations

measured at the PND165 site from 1990–2010.

Note: YEL408 and CNT169 locations are shown in 

Figure 1-1.
CASTNET Annual Report – 2010 58 Chapter 4: Ozone Concentrations

Table 4a Eight-Hour Monthly Average and Maximum Ozone by Year (2005–2010) at Selected 

WDEQ and CASTNET Sites

January Average Ozone (ppb) Maximum Ozone (ppb)

Year Jonah Boulder Daniel PND165 Jonah Boulder Daniel PND165

2005 35 NA NA 45 78 NA NA 60

2006 33 41 43 42 49 67 53 56

2007 27 43 40 41 57 71 53 52

2008 29 39 42 44 47 58 56 59

2009 24 34 37 41 52 55 48 51

2010 34 38 39 44 57 69 49 64

February

2005 42 51 NA 50 98 89 NA 79

2006 39 48 49 47 93 71 82 65

2007 29 42 40 43 46 59 57 56

2008 40 54 50 49 102 122 76 82

2009 33 42 43 48 69 67 64 62

2010 44 51 46 46 54 62 52 59

March

2005 40 48 NA 50 58 71 NA 68

2006 44 48 50 51 68 67 71 70

2007 32 44 40 46 44 65 55 60

2008 39 53 50 53 98 102 75 89

2009 39 46 43 49 63 70 67 82

2010 48 53 48 49 55 66 54 63

Note: NA = not available

Source: MSI (2010) 

The time series (Figure 4c) of fourth highest DM8A O 

3 concentrations shows no trend over the 

21 years. However, despite the significant variability, the first quarter daily maxima increased 

over the period. The 2008–2010 average of the DM8A O 

3 concentrations was 66 ppb compared to 

a 1990–1992 average of 57 ppb. The most recent years (2005, 2006, and 2008) with elevated O 

3

concentrations at PND165 correspond to years with elevated first quarter DM8A O 

3 concentrations 

in the UGRB. In short, although O 

3 concentrations measured at PND165 were generally not as 

high as those observed in the basin, O 

3 levels at PND165 increased when pollution accumulated in 

the UGRB.

Figure 4c Trends in Fourth Highest DM8A O 

3 Concentrations and First Quarter DM8A O 3 Levels 

at PND165, WY

40

50

60

70

80

90

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Concentration (ppb)

4th Highest Daily 

Max 8-Hour 

Ozone 

Concentration

Max First Quarter 

Daily Max 8-Hour 

Ozone 

Concentration
CASTNET Annual Report – 2010 59 Chapter 4: Ozone Concentrations

During the winters of 2007 through 2010, WDEQ sponsored a study (MSI, 2010; Environ, 2010) of 

ozone formation and corresponding meteorological conditions in the UGRB called the Upper Green 

River Winter Ozone Study (UGWOS).

Data from the 2007 through 2010 field studies can be obtained from the WDEQ Air Quality 

Division’s monitoring information page: http://deq.state.wy.us/aqd/Monitoring%20Data.asp. The 

field campaigns measured 14 DM8A O 

3 concentrations above the 75 ppb NAAQS in 2008. Thirteen 

exceedances were measured in 2011. The highest hourly concentrations exceeded 150 ppb, and 

the highest DM8A levels exceeded 120 ppb. Natural gas operators on the Anticline have effected 

significant emission reductions since 2008. VOCs emissions were reduced 50 percent and NOx

emissions 75 percent. VOCs emissions have declined by 20 percent and NOx emissions by 

25 percent throughout Sublette County, WY since 2008. The shale gas deposits in the UGRB are 

located in concentrated areas, which makes it possible to consolidate facilities and employ 

additional emission control technologies. However, elevated O 

3 concentrations were measured in 

2011 despite the emission reductions. 

The occurrence of elevated O 

3 concentrations in the late winter and early spring is unusual relative 

to the understanding that ozone is produced during sunny, hot, summertime weather. Data from 

the 2008 UGWOS were analyzed by Environ (2010) to understand the key characteristics of winter 

ozone episodes in the UGRB. Environ scientists (2010) analyzed air quality and meteorological 

measurements and developed a conceptual model of these episodes. The features (Environ, 2010) 

of the winter ozone episodes include:

♦ Presence of a strong, shallow ground-based inversion with warm temperatures aloft associated 

with a 700 millibar ridge. Extensive snow cover reduces daytime heating and maintains the 

inversion, which is typically less than 150 meters deep. The shallow mixed layer traps and 

concentrates pollutants near the surface.

♦ Northwest surface winds during the night and morning hours and southeast winds during 

afternoon, which result in recirculation of pollutants within the basin.

♦ Extensive snow cover that reflects incoming solar radiation and increases flux of ultraviolet (UV) 

radiation to levels comparable to those in midsummer. The increased UV radiation drives the 

photochemical reactions that produce ozone.

♦ High concentrations of precursor emissions produced by well pads and drill rigs in the basin.

♦ No significant transport of precursor emissions or ozone from upwind urban areas or other 

source regions.

O 

3 concentrations measured at PND165 are lower than those measured in basin because the 

CASTNET site elevation is typically above the top of the ground-based inversion. However, O 

3

levels at PND165 are higher than normal during the episodes that occur in the UGRB.

The unusual adverse weather conditions that produce elevated O 

3 concentrations can occur at any 

time in the months of January through March in the UGRB, albeit infrequently. Twenty-seven days 

in the last four years experienced high daily DM8A O 

3 concentrations.

Data from PND165 also provides information on changes in sulfur and nitrogen pollutant 

concentrations over the period 1990 through 2010 (Figures 4d and e). The figures show annual 

and first quarter mean SO 

2 and total nitrate concentrations. SO 2 concentrations have declined over 

the 21 years, and total nitrate concentrations show an overall increase with interannual variability.
CASTNET Annual Report – 2010 60 Chapter 4: Ozone Concentrations

Figure 4d Trend in Annual Mean Atmospheric Concentrations of SO 

2 and Total NO3 

at PND165, WY

Figure 4e Trend in First Quarter Mean Atmospheric Concentrations of SO 

2 and Total NO3 

at PND165, WY

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Concentration (μg/m3)

Sulfur Dioxide

Total Nitrate

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010

Concentration (μg/m3)

Sulfur Dioxide

Total Nitrate
CASTNET Annual Report – 2010 61 Chapter 5: Data Quality

Chapter 5: Data Quality

CASTNET measurements and supporting activities are assessed routinely in order to provide highquality information to meet data quality objectives (DQO). DQO are evaluated using data quality 

indicators (DQI) such as precision, accuracy, and completeness. Measurements taken during 2010 

and historical data collected over the period 1990–2009 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.

The CASTNET QA program was designed to ensure that all reported data are of known and 

documented quality in order to meet CASTNET objectives and to be reproducible and 

comparable with data from other monitoring networks and laboratories. The 2010 QA 

program elements are documented in the CASTNET Quality Assurance Project Plan (QAPP), 

Revision 6.0 (MACTEC, 2010c). The QAPP is comprehensive and includes standards and 

policies for all components of project operation from site selection through final data 

reporting. Sections on field measurements, chemical analysis of field samples, data 

management, and assessments and response actions are included. Standard operating 

procedures (SOPs) for all aspects of CASTNET operations are provided in the appendices.

DQI are quantitative statistics and qualitative descriptors used in interpreting 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 2010 data were analyzed and compared with historical data 

collected during the period 1990 through 2009. The information in this report is 

supplemented by analyses that are discussed in quarterly CASTNET Quality Assurance 

Reports (MACTEC, 2010b; 2010e; 2010f; 2011b). These QA reports are produced four 

times per year with the fourth quarter report including an annual summary.

Precision

Exposed Filter Concentrations

Figure 5-1 provides a bar chart in which the bars represent precision estimates for five 

CASTNET filter pack pollutants. Precision is expressed as MARPD. Table 5-1 lists the 

precision and accuracy criteria for laboratory filter pack measurements.
CASTNET Annual Report – 2010 62 Chapter 5: Data Quality

Figure 5-1 Historical and 2010 Precision Data for Atmospheric Concentrations

Table 5-1 Data Quality Indicator Criteria for CASTNET Laboratory Measurements

Precision1 Accuracy2

Nominal

Reporting Limits

Analyte Method (MARPD) (%) mg/l μg/Filter

Ammonium (NH+ 

4 ) AC 20 90 – 110 0.020* 0.5

Sodium (Na+ ) ICP-AES 20 95 – 105 0.005 0.125

Potassium (K+ ) ICP-AES 20 95 – 105 0.006 0.15

Magnesium (Mg2+) ICP-AES 20 95 – 105 0.003 0.075

Calcium (Ca2+) ICP-AES 20 95 – 105 0.006 0.15

Chloride (Cl-) IC 20 95 – 105 0.020 0.5

Nitrate (NO3) IC 20 95 – 105 0.008* 0.2

Sulfate (SO2-

4 ) IC 20 95 – 105 0.040 1.0

Note: 1 This column lists precision goals for both network precision calculated from collocated filter samples and 

 laboratory precision based on replicate samples. 

2 This column lists laboratory accuracy goals based on reference standards and continuing calibration 

 verification spikes. The criterion is 90-110 percent for ICP-AES reference standards.

AC = automated colorimetry

μg/Filter = micrograms per filter

* = as nitrogen

Historical (1990 through 2009) data for all 11 collocated site pairs operated over the 

history of the network and the 2010 data for the current collocated sites at Mackville, KY 

(MCK131/231) and Rocky Mountain National Park, CO (ROM406/206) are provided in

Figure 5-1. The two sites at Rocky Mountain National Park are operated independently. 

ROM206 is operated on behalf of EPA and ROM406 on behalf of NPS. Trace cations and Clare excluded from Figure 5-1 but are shown later in Figure 5-2. 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 2010 met the 20 percent criterion 

for the CASTNET filter pack measurements shown in Table 5-1. The precision of NO3

0

5

10

15

20

25

30

1 2 3 4 5

MARPD

Overall - 1990 through 2009

Mackville, KY - 2010

Rocky Mtn, CO - 2010

SO2 SO4

2- HNO3 NO3- NH4+
CASTNET Annual Report – 2010 63 Chapter 5: Data Quality

measurements was consistently worse than for the other analytes, possibly because NO3

concentrations include sampling artifacts. 

The 2010 precision results shown in Figure 5-1 indicate that the MARPD data for 

MCK131/231 were lower than (i.e., more precise) than the historical results for all 

parameters and met the criterion of 20 percent. The 2010 results for ROM406/206 showed 

better precision than historical results for NO3. All the parameters met the 20 percent 

criterion. The MARPD results were higher for ROM406/206 because of lower 

concentrations measured at this site. Overall, the filter pack precision estimates for 2010 

were somewhat higher than the 2009 results.

Historical and 2010 precision statistics for four metal cations and Cl- are summarized in 

Figure 5-2. The historical MARPD statistics for both MCK131/231 and ROM406/206 met 

the DQI criterion of 20 percent. The 2010 precision results for four of five ions were 

reasonable, and met the criterion. The MARPD values for K+ exceeded the DQI criterion at 

ROM406/206. The 2010 precision results were about the same as for 2009.

Possible sources for the suspected intermittent K+ contamination of filters used for filter 

packs and filter blanks were investigated during 2010. Audits of all stages of handling 

procedures (e.g., filter pack packing and unpacking, extraction and analysis, and 

impregnation of the potassium carbonate filters) indicated that SOPs are followed, and 

contamination is not likely to be introduced by current handling methods. Additionally, the 

reagents used were checked and verified to be stored properly. Recent investigations have 

included verifying that adequate time is allowed for drying freshly impregnated cellulose 

filters and reviewing sample intake coding. Review of the intake coding procedures 

resulted in clarification of the defining rule for coding filters as being excessively wet. A 

filter with visible wetness is to be coded as showing “excessive wetness.” The room where 

filters are handled is now isolated during handling procedures. Only filter packs and filter 

blanks have showed evidence of contamination. Method blanks, filter acceptance test 

results, and all other quality control (QC) samples remained within established limits. The 

next investigation involved the potential for volatization and migration of the cellulose 

filter impregnation solution (K 

2CO 3) during shipping in periods of hot weather. The 

contamination correlates with high temperatures in the shipping tube and primarily occurs 

during the warmest months and in warmer regions. AMEC has evaluated other carbonates 

for impregnation of the cellulose filters. Carbonate salts completely free of target analytes 

are cost-prohibitive. 
CASTNET Annual Report – 2010 64 Chapter 5: Data Quality

Table 5-2 summarizes 2010 precision results by quarter for the two sets of collocated sites. 

See the 2010 Quarterly Data Reports (MACTEC, 2010a; 2010d; 2011a; 2011c) and QA 

Quarterly Reports (MACTEC, 2010b; 2010e; 2010f; 2011b) for discussions of quarterly 

precision data.

Figure 5-2 Historical and 2010 Precision Data for Cation and Cl- Concentrations

Table 5-2 Collocated Precision Results (MARPD) for 2010 Filter Pack Data by Quarter

Site Pairs SO2-

4 NO3 NH+ 4 Ca2+ Mg2+ Na+ K+ Cl- HNO 3 SO 2

Total 

NO3

MCK131/231, KY

Quarter 1 3.62 7.23 4.65 9.32 9.18 6.06 18.43 9.38 6.08 3.80 5.99

Quarter 2 2.52 13.57 2.54 6.38 4.91 4.82 15.96 0.90 4.15 4.42 4.78

Quarter 3 2.80 6.68 2.77 6.19 5.61 5.39 21.87 0.00 7.80 6.36 5.65

Quarter 4 3.49 7.21 3.31 10.01 8.87 6.98 8.77 4.81 5.12 2.74 5.16

2010 3.11 8.67 3.32 7.97 7.14 5.81 16.26 3.77 5.79 4.33 5.40

ROM406/ROM206, CO

Quarter 1 14.15 12.93 16.92 16.49 16.76 15.36 27.85 3.09 12.01 10.12 12.83

Quarter 2 5.69 8.03 6.40 10.91 12.11 9.65 20.43 6.61 7.83 8.58 5.12

Quarter 3 4.80 9.06 3.83 5.47 7.76 6.72 19.33 3.18 14.25 7.72 11.33

Quarter 4 4.11 16.07 6.88 9.32 7.50 9.57 19.79 3.08 11.29 6.37 11.59

2010 7.19 11.52 8.51 10.55 11.03 10.32 21.85 3.99 11.34 8.20 10.22

The 2010 analytical precision results for eight analytes and the three filter types are 

presented in Figure 5-3. The results were based on analysis of 5 percent of the samples that 

were randomly selected for replication in each batch. The results of in-run replicate 

analyses were compared to the original concentration results. The laboratory precision 

data met the measurement criterion listed in Table 5-1.

0

5

10

15

20

25

30

1 2 3 4 5

Overall - 2000 through 2009

Mackville, KY - 2010

Rocky Mtn, CO - 2010

MARPD

Ca2+ K+ Mg2+ Na+ Cl-
CASTNET Annual Report – 2010 65 Chapter 5: Data Quality

Figure 5-3 Precision Results for Laboratory Replicate Samples (2010)

Ozone Concentrations

CASTNET was originally designed to report on regional, rural O 

3 levels. The QA procedures 

for the EPA-sponsored O 

3 analyzers at CASTNET sites have differed from the EPA QA 

requirements for State and Local Monitoring Stations (SLAMS) monitoring (EPA, 2010a; 

2008a), including ozone monitoring in 2010. EPA directed MACTEC, now known as AMEC,

to prepare EPA-sponsored ozone monitoring to meet 40 CFR Part 58 monitoring 

requirements. This effort was completed in 2011. In any event, despite differences in QA 

procedures, operation of the collocated EPA and NPS O 

3 analyzers at ROM406/206 

provides an opportunity to evaluate the precision of the independent systems. Table 5-3 

provides the DQI criteria for the CASTNET continuous measurements including O 

3. The 

precision criterion for the collocated O 

3 data is 10 percent.

MARPD statistics were calculated from hourly O 

3 measurements obtained from the 

collocated sites MCK131/231 and ROM406/206 during 2010. In addition, quarterly 

historical precision statistics were compiled for all collocated sites. Quarterly precision 

results are summarized in Figure 5-4. Table 5-4 provides precision results for O 

3

concentrations by quarter for 2010. The data show the historical and 2010 results met the 

10 percent criterion. However, the 2010 results (MACTEC, 2011b) show a consistent bias 

between ROM406 and ROM206 O 

3 measurements with ROM406 measuring higher 

concentrations by about 4 ppb. The historical data are based on the operation of 

11 collocated site pairs over the history of the network.

0

1

2

3

4

5

1 2 3 4 5 6 7 8 9 10 11

contribute to SO2

MARPD

for In-Run Replicates

Teflon

SO4

2-

Nylon

SO4

2-

Nylon

NO3

-

Teflon

NH4

+

Cellulose 

SO4

2-

Teflon

NO3

-

Teflon

Na+

Teflon

K+

Teflon

Mg2+

Teflon

Ca2+

Teflon

ClHNO3
CASTNET Annual Report – 2010 66 Chapter 5: Data Quality

Table 5-3 Data Quality Indicator Criteria for CASTNET Field Measurements

Measurement Criteria*

Parameter Method MARPD/MAD Precision Accuracy

Filter Pack Flow Mass Flow Controller MARPD ± 10% ± 5%

Ozone UV Absorbance MARPD ± 10% (of reading) ± 10%

Ambient Temperature Platinum RTD MAD ± 1.0°C ± 0.5°C

Wind Speed Anemometer MAD ± 0.5 m/s

The greater of ± 0.5 m/s for 

winds < 5 m/s or ± 5% for 

winds ≥ 5 m/s

Wind Direction Wind Vane MAD ± 5° ± 5°

Sigma Theta Wind Vane MAD Undefined Undefined

Delta Temperature Platinum RTD MAD ± 0.5°C ± 0.5°C

Relative Humidity Thin Film Capacitor MARPD ± 10% (of full scale) ± 10 percent

Solar Radiation Pyranometer MARPD ± 10% (of reading taken 

at local noon) ± 10%

Precipitation Tipping Bucket Rain 

Gauge MARPD ± 10% (of reading) ± 0.05 inch†

Surface Wetness Conductivity Bridge MAD Undefined Undefined

Note: MARPD is the precision measure for percentage criteria such as relative humidity.

Mean absolute difference (MAD) is the precision measure for difference criteria such as wind speed.

°C = degrees Celsius

m/s = meters per second

RTD = resistance-temperature device

* Precision criteria apply to collocated instruments, and accuracy criteria apply to calibration of instruments

 † For target value of 0.50 inch

Figure 5-4 Historical and 2010 Precision Data by Quarter for Ozone Concentrations

0

5

10

15

20

25

30

First

Quarter

Second

Quarter

Third

Quarter

Fourth

Quarter

Overall - 1990 through 2009

Mackville, KY - 2010

Rocky Mtn, CO - 2010

MARPD
CASTNET Annual Report – 2010 67 Chapter 5: Data Quality

Table 5-4 Collocated Precision Results (MARPD) for 2010 Ozone Concentrations 

by Quarter 

Sampling Period MCK131/231, KY ROM406/206, CO

Quarter 1 1.13 7.87

Quarter 2 1.60 7.63

Quarter 3 0.69 6.03

Quarter 4 0.82 8.57

2010 1.06 7.53

Continuous Measurements

Precision criteria for the continuous measurements are listed in Table 5-3. Figure 5-5 

provides precision results for historical data (1990 through 2009) at all collocated site 

pairs and 2010 precision statistics for the two current collocated site pairs. Precision was 

calculated in terms of the MARPD or mean absolute difference (MAD) of hourly 

measurements. All historical precision results met the DQI criteria. However, relative 

humidity and solar radiation measurements taken at ROM406/206 and wind direction data 

from MCK131/231 did not meet the precision goals during 2010. Table 5-5 gives precision 

data by quarter for 2010 for the two collocated sites. The results show overall good 

instrument precision. All but 11 (of 110) 2010 annual and quarterly precision results met 

the DQI criteria. The first, second, and fourth quarter and annual MARPD values for relative 

humidity and the first two quarters and annual MARPD values for solar radiation at 

ROM406/206 did not meet the 10 percent criterion. All four quarters plus the annual MAD 

values for wind direction at MCK131/231 did not meet the 5-degree goal. 

Figure 5-5 Historical and 2010 Precision Data for Continuous Measurements

Note: The left y-axis indicates MARPD for these parameters with percentage criteria (Table 5-3) and MAD for wind direction only.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0

2

4

6

8

10

12

14

16

Flow Rate

Relative

Humidity

Sigma

Theta

Solar

Radiation

Delta

Temperature

Precipitation

Scalar

Wind Speed

Temperature

Wetness

Wind

Direction

Wind Speed

MAD

MARPD / MAD for Wind Direction

MARPD: Overall - 1990 through 2009

MARPD: Mackville, KY - 2010

MARPD: Rocky Mtn, CO - 2010

MAD: Overall - 1990 through 2009

MAD: Mackville, KY - 2010

MAD: Rocky Mtn, CO - 2010

Flow RateRelativeHumiditySigmaThetaSolar

Radiation

DeltaTemperaturePrecipitation

ScalarWind Speed

TemperatureWetness

Wind

DirectionWind Speed

MARPD or MAD for Wind Direction
CASTNET Annual Report – 2010 68 Chapter 5: Data Quality

Table 5-5 Collocated Precision Results for 2010 Continuous Measurements by Quarter Site Pairs

MARPD MAD

Flow RateRelative HumiditySigma ThetaSolar RadiationDelta TemperaturePrecipitationScalar Wind SpeedTemperatureWetnessWind DirectionWind Speed

MCK131/231, KY

Quarter 1 0.01 4.14 1.95 6.00 0.29 0.02 0.05 0.16 0.05 5.53 0.05

Quarter 2 0.00 1.02 2.69 1.38 0.25 0.02 0.04 0.21 0.07 6.17 0.04

Quarter 3 0.00 1.05 4.16 4.49 0.15 0.02 0.03 0.11 0.03 6.14 0.03

Quarter 4 0.47 3.65 2.47 7.48 0.13 0.03 0.04 0.10 0.06 5.77 0.04

2010 0.12 2.46 2.82 4.84 0.21 0.02 0.04 0.14 0.05 5.90 0.04

ROM406/206, CO

Quarter 1 1.51 13.89 4.06 12.07 0.15 0.02 0.09 0.18 NC 3.66 0.08

Quarter 2 0.72 16.89 3.25 12.22 0.19 0.05 0.07 0.12 0.06 4.84 0.05

Quarter 3 1.79 NC 4.36 8.89 NC 0.01 0.07 0.22 NC 3.51 0.06

Quarter 4 8.39 12.84 4.55 8.85 NC 0.04 0.08 0.24 0.12 2.72 0.07

2010 3.10 14.54 4.06 10.51 0.17 0.03 0.08 0.19 0.09 3.68 0.07

Note: NC = not calculated

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 were analyzed every tenth sample to verify that

instrument calibration had not drifted beyond established limits. Table 5-6 presents the 

percent recoveries and standard deviations for reference samples and CCV relative 

to target concentrations. The table shows that the DQI goals (see Table 5-1) were met 

in 2010.
CASTNET Annual Report – 2010 69 Chapter 5: Data Quality

Table 5-6 Filter Pack Quality Control Summary for 2010 Filter Type Parameter

Reference

Sample1 Recovery

(%R)

Continuing Calibration 

Verification Samples (%R)

In-Run Replicate2

(RPD)

MeanStandard

Deviation

Count3MeanStandard

Deviation

Count3MeanStandard

Deviation

Count3

Teflon SO2-

4 97.54 1.19 149 99.86 1.17 720 0.49 0.71 338

NO3 99.77 1.19 149 100.44 1.21 720 0.99 1.91 338

NH+ 

4 99.76 1.66 136 98.69 1.39 714 0.68 0.89 357

Ca2+ 99.83 2.80 139 100.20 1.09 725 1.04 1.15 343

Mg2+ 102.53 2.18 139 99.95 0.94 725 1.40 1.64 343

Na+ 100.97 2.17 139 100.07 1.12 725 1.13 1.50 343

K+ 97.83 2.87 139 100.04 0.93 725 1.75 1.93 343

Cl- 103.09 1.10 149 100.27 1.34 720 2.33 4.44 335

Nylon SO2-

4 98.01 1.15 156 99.74 1.10 725 2.50 2.55 345

NO3 100.37 1.13 156 100.52 1.09 725 1.07 1.47 345

Cellulose SO2-

4 99.04 1.58 176 100.32 1.49 680 1.13 1.58 337

Note: % R = percent recovery

1 Results of reference sample analyses provide accuracy estimates

2 Results of replicate analyses provide precision estimates

3 Number of QC Samples

Continuous Measurements

Table 5-7 presents field accuracy results for 2010 based on instrument challenges 

performed using independent reference standards during site calibration visits. CASTNET 

sites were calibrated every six months with NIST-traceable standards. The calibration 

results were evaluated using the accuracy criteria listed in Table 5-3. Each parameter was 

within its criterion with at least 90 percent frequency.

Table 5-7 Accuracy Results for 2010 Field Measurements

Parameter Percent Within Criterion Parameter Percent Within Criterion

Flow Rate 98.9 percent Temperature (ambient) 100.0 percent

Ozone Slope 97.7 percent Delta Temperature (0°C) 100.0 percent

Ozone Intercept 100.0 percent Delta Temperature (ambient) 100.0 percent

Wind Speed < 5 m/s 100.0 percent *Relative Humidity 98.9 percent

Wind Speed ≥ 5 m/s 98.9 percent Precipitation 95.5 percent

Wind Direction North 96.6 percent *Solar Radiation 90.9 percent

Wind Direction South 96.6 percent Wetness (w/in 0.5 volts) 100.0 percent

Temperature (0°C) 100.0 percent

Note: °C = degrees Celsius

 m/s = meters per second

 * = Per CASTNET project protocols, data are flagged as “suspect” but still considered valid if the calibration criterion is 

not exceeded by more than its magnitude (i.e., if within 2x the criterion).
CASTNET Annual Report – 2010 70 Chapter 5: Data Quality

Precision of Collocated SLAMS and Collocated 

CASTNET Ozone Measurements 

Precision is a DQI for CASTNET ozone measurements. Precision is calculated as the MARPD of 

concentration measurements from collocated sites. Collocated O 

3 analyzers are operated currently 

at MCK131/231, KY and ROM406/206, CO. MCK131/231 has operated collocated analyzers since 

1992 and ROM406/206 since 2001. Nine other CASTNET sites have run collocated O 

3 analyzers 

over the history of the network. Thermo Scientific 49i ozone analyzers are currently operated at all 

four collocated monitoring stations.

The SLAMS network consists of approximately 4,000 monitoring stations across the United States 

whose measurement systems are largely determined by the needs of State and local air pollution 

control agencies to meet their respective SIP requirements. Several collocated SLAMS monitoring 

sites were operated from 1992 through 2009. Two of these collocated ozone monitoring sites were 

selected for analysis based on length of collocation and availability of recent data. These sites were 

collocated during the period 2006 through 2009. One Thermo Scientific 49i analyzer and one 

Dasibi 1008 analyzer were operated at site 36-103-0009, located in Suffolk County, NY, from 

2006 through 2009. Two API 400 ozone analyzers were operated at site 29-189-0014, located in 

St. Louis County, MO, in 2008 and 2009.

The normal precision DQI is the MARPD of hourly O 

3 data. For this analysis, the precision measure 

is the MARPD of DM8A concentrations from the two CASTNET and two SLAMS sites. Table 5a 

presents quarterly MARPD results calculated from the available data from the four sites. The site 

average MARPD values are also presented. The best precision was realized at the St. Louis 

County and Mackville collocated sites. Both sites operate identical analyzers at the respective sites 

and are serviced by the same operator. The Suffolk County site operates two different analyzers. 

The CASTNET monitors at ROM406/206 are run independently and serviced by two different 

operators. Precision of hourly ozone concentrations discussed elsewhere in this chapter shows a 

consistent bias between ROM406 and ROM206 hourly O 

3 data with ROM406 sampling higher 

concentrations by about 4 ppb. Similarly, this analysis found a consistent 4 to 5 ppb bias between 

the collocated Suffolk County analyzers.

Table 5a Precision (MARPD) of DM8A O 

3 Concentrations Measured at CASTNET and SLAMS Sites

Year Quarter MCK131/231 ROM406/206 36-103-0009

POC 1 and 2

29-189-0014

POC 1 and 2

2006 2 2.4 1.8 10.9

2006 3 7.0 2.7 8.9

2006 4 9.5 15.4

2007 1 0.7 4.1 6.6

2007 2 1.0 8.1 5.2

2007 3 3.5 16.2 3.5

2007 4 8.0 12.0 4.7

2008 1 7.4 1.6

2008 2 6.1 1.1 9.6 0.9

2008 3 2.5 12.2 0.7

2008 4 5.5 1.8 19.6 0.8

2009 1 4.7 4.5 15.2

2009 2 1.3 4.8 13.5 1.3

2009 3 1.0 6.3 13.7 1.8

2009 4 1.2 7.7 17.4 3.0

Average 3.8 5.6 11.2 1.4

Note: Site 36-103-0009 is located in Suffolk County, NY

Site 29-189-0014 is located in St. Louis County, MO

POC 1 is primary site and POC 2 is secondary site
CASTNET Annual Report – 2010 71 Chapter 5: Data Quality

Completeness

Completeness is defined as the percentage of valid data points obtained from a 

measurement system relative to total possible data points. The CASTNET measurement 

criterion for completeness requires a minimum completeness of 90 percent for every 

measurement for each quarter. Figure 5-6 presents historical (black) and 2010 

completeness data for all sites for measured filter concentrations and continuous 

measurements. The figure shows that the 2010 direct measurements met the 90 percent 

completeness goal, and nine meteorological measurements and filter pack flow exceeded 

95 percent. Atmospheric concentrations also exceeded 95 percent.

Figure 5-6 Historical and 2010 Percent Completeness of Measurements (black bars are 

 1990–2009) 

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

Meteorological

Parameters

DQI Measurement Criterion

Connecticut Hill, NY (CTH110) 
CASTNET Annual Report – 2010 72 Chapter 5: Data Quality

Results for 2010 Environment Canada Proficiency Testing 

During 2010, the MACTEC (now known as AMEC) CASTNET laboratory participated in the 

Environment Canada (ECAN) Proficiency Testing Program for Inorganic Environmental 

Substances (http://www.ec.gc.ca/inre-nwri/default.asp?lang=En&n=7A20877C-1). Specifically, the 

laboratory is one of 41 laboratories that participated in the 2010 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 0096 and 0097) during 2010. 

The results reported by the 41 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 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 2010 ECAN (2011) results for the eight CASTNET parameters are 

presented in Table 5b. All eight parameters received the highest possible rating of “Ideal” indicating 

an unbiased score with the exception of sodium in study 0096. There were no individual results 

flagged for study 0096, indicating that each result was within the study’s acceptance limits. The 

score was based solely on the bias ranking. The high sodium bias found by study 0096 as 

compared with the ideal results for that parameter in study 0097 is illustrated in Figure 5a. 

Table 5b MACTEC (AMEC) Results for Studies 0096 and 0097

Test

Parameter

Analytical

Method

Reference

Method

Laboratory Performance Rating

Study 0096

Summer 2010

Study 0097

Winter 2010

Ammonia AC EPA Method 350.1 Ideal Ideal

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 Biased High Ideal

Sulfate IC EPA Method 300.0 Ideal Ideal

Source: Environment Canada (2011)
CASTNET Annual Report – 2010 73 Chapter 5: Data Quality

Figure 5a Sodium Results Comparison – Studies 0096 and 0097

The overall laboratory performance rating was “Good” for each study conducted during 2010. This 

is the highest possible overall laboratory rating, indicating a percent score (the sum of parameters 

biased and results flagged) of 5.0 or less as described in Table 5c. 

Table 5c Laboratory Performance Rating

Laboratory Performance Rating

Rating % Score*

Good 0 - 5

Satisfactory > 5 - 12.5

Moderate > 12.5 - 30

Poor > 30

*Sum of Parameters Biased & Results Flagged

Source: Environment Canada (2011)

The report for study 0097 included a summary of the laboratory’s performance over the last five 

years. The laboratory achieved the highest possible rating for each of the 10 studies conducted 

during that period. These results are listed below in Table 5d. 

Table 5d Five-Year Historical Laboratory Performance

Study No. / Date Score*

No. 0088, Summer 2006 0.5

No. 0089, Winter 2006 1.0

No. 0090, Summer 2007 1.0

No. 0091, Winter 2007 3.0

No. 0092, Summer 2008 0.0

No. 0093, Winter 2008 0.0

No. 0094, Summer 2009 5.0

No. 0095, Winter 2009 2.0

No. 0096, Summer 2010 5.0

No. 0097, Winter 2010 0.0

5-Year Median 1.0

Note: * Percent score (sum of parameters biased and results flagged)

Source: Environment Canada (2011)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0097 Sodium 

Laboratory Results

0097 Sodium 

Assigned Values

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0096 Sodium 

Laboratory Results

0096 Sodium 

Assigned Values
CASTNET Annual Report – 2010 74 Chapter 5: Data Quality

Summary of Data Quality Results

DQI results demonstrate that CASTNET field and laboratory processes were adequately 

monitored through QA/QC procedures and were generally free of systemic bias during 

2010 with the exception of O 

3 measurements at ROM406/206, CO. The O 3 concentrations 

measured at ROM406 were consistently higher than ROM206 by about 4 ppb. This bias 

is under investigation.

Historical and 2010 precision data for filter pack parameters are considered acceptable, 

except for K+ . The precision of K+ filter pack measurements exceeded the established 

criterion at ROM406/206 during the first two quarters of the year and are not considered 

acceptable. The poor K+ precision continues to be investigated. 

The 2010 precision data for O 

3 concentrations met the 10 percent DQI criterion. The 

precision of continuous measurements for 2010 was good. Ninety-nine of 110 quarterly 

and annual precision results for 2010 met the DQI criteria.

Accuracy data met the established criteria for field and laboratory parameters. 

Completeness criteria were met for all parameters in 2010. Completeness results for 2010 

are considered excellent. ECAN proficiency test studies (0096 and 0097) showed that the 

CASTNET laboratory performed well on the interlaboratory chemical analyses with all but 

one parameter rated ideal.

Mackville, KY (MCK131/231) 
CASTNET Annual Report – 2010 75 References

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CASTNET Annual Report – 2010 Appendix A

Appendix A

Locational and Operational Characteristics of 

CASTNET Sites 
CASTNET Annual Report – 2010 1 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

Alabama

SND152 Sand Mountain 12/27/88 34.2894 85.9704 352 • • AL99 Agri. Rolling Y EPA

Alaska

DEN417 Denali National Park 10/06/98 63.7258 148.9633 661 • • AK03 Forested Complex N NPS

Arizona

CHA467 Chiricahua National Monument 04/25/89 32.0092 109.3892 1570 • • AZ98 Range Complex N NPS

GRC474 Grand Canyon National Park 05/16/89 36.0597 112.1822 2073 • • AZ03 Forested Complex M NPS

PET427 Petrified Forest National Park 09/24/02 34.8225 109.8919 1723 • • AZ97 Desert Flat Y NPS

Arkansas

CAD150 Caddo Valley 10/04/88 34.1792 93.0989 71 • • AR03 Forested Rolling N EPA

California

CON186 Converse Station 06/17/03 34.1941 116.9130 1837 • • CA94 Agri./Forested Complex N EPA

JOT403 Joshua Tree National Monument 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 Annual Report – 2010 2 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

Florida

EVE419 Everglades National Park 10/06/98 25.3911 80.6806 2 • •4 FL11 Swamp Flat Y NPS

IRL141 Indian River Lagoon 07/09/01 30.1065 80.4554 2 • • FL99 Beach Flat Y EPA

SUM156 Sumatra 12/28/88 30.1065 84.9938 14 • • FL23 Forested Flat Y EPA

Georgia

GAS153 Georgia Station 06/28/88 33.1812 84.4100 270 • • GA41 Agri. Rolling M EPA

Illinois

ALH157 Alhambra 06/28/88 38.8690 89.6229 164 • • IL46 Agri. Flat Y EPA

BVL130 Bondville 02/09/88 40.0520 88.3725 212 • • IL11 Agri. Flat Y EPA

STK138 Stockton 12/28/93 42.2872 89.9998 274 • • IL18 Agri. Rolling M EPA

Indiana

SAL133 Salamonie Reservoir 06/28/88 40.8164 85.6608 250 • • IN20 Agri. Flat Y EPA

VIN140 Vincennes 08/04/87 38.7406 87.4844 134 • • IN22 Agri. Rolling M EPA

Kansas

KNZ184 Konza Prairie 03/26/02 39.1021 96.6096 348 • • KS31 Prairie Flat Y EPA

Kentucky

CDZ171 Cadiz 10/01/93 36.7841 87.8500 189 • • KY99 Agri. Rolling M EPA

CKT136 Crockett 08/24/93 37.9211 83.0658 455 • • KY35 Agri. Rolling Y EPA

MAC426 Mammoth Cave National Park 07/24/02 37.1319 86.1478 243 • • KY10 Agri./Forested Rolling M NPS

MCK131 Mackville 07/31/90 37.7044 85.0483 353 • • KY03 Agri. Rolling M EPA

Maine

ACA416 Acadia National Park 12/01/98 44.3769 68.2608 158 • • ME98 Forested Complex M NPS

ASH135 Ashland 12/20/88 46.6039 68.4142 235 • • ME00 Agri. Flat Y EPA

HOW132 Howland 11/24/92 45.2158 68.7085 69 • • ME09 Forested Rolling Y EPA
CASTNET Annual Report – 2010 3 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

Maryland

BEL116 Beltsville 11/01/88 39.0283 76.8175 46 • • MD99 Urban-Agri. Flat N EPA

BWR139 Blackwater National Wildlife Refuge 07/04/95 38.4448 76.1115 4 • • MD15 Forest-Marsh Coastal M EPA

Michigan

ANA115 Ann Arbor 06/28/88 42.4164 83.9019 267 • • MI52 Forested Flat M EPA

HOX148 Hoxeyville 10/31/00 44.1809 85.7390 298 • • MI53 Forested Flat Y EPA

UVL124 Unionville 06/28/88 43.6139 83.3597 201 • • MI51 Agri. Flat Y EPA

Minnesota

VOY413 Voyageurs National Park 06/13/96 48.4128 92.8292 429 • • MN32 Forested Rolling M NPS

Mississippi

CVL151 Coffeeville 12/27/88 34.0028 89.7989 134 • • MS30 Forested Rolling M EPA

Montana

GLR468 Glacier National Park 12/27/88 48.5103 113.9956 976 • • MT05 Forested Complex N NPS

Nebraska

SAN189 Santee Sioux 07/05/06 42.8292 97.8541 429 • • SD99 Agri. Rolling N EPA

Nevada

GRB411 Great Basin National Park 05/16/95 39.0053 114.2158 2060 • • NV05 Forested Complex M NPS

New Hampshire

WST109 Woodstock 12/27/88 43.9446 71.7008 258 • • NH02 Forested Complex N EPA

New Jersey

WSP144 Washington’s Crossing 12/27/88 40.3133 74.8726 61 • • NJ99 Urban-Agri. Rolling M EPA

New York

CAT175 Claryville 05/10/94 41.9423 74.5519 765 • •4, 5 NY68 Forested Complex N EPA

CTH110 Connecticut Hill 09/28/87 42.4010 76.6535 515 • • NY67 Forested Rolling N EPA

HWF187 Huntington Wildlife Forest 05/28/02 43.9732 74.2232 502 • • NY20 Forested Complex N EPA
CASTNET Annual Report – 2010 4 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

North Carolina

BFT142 Beaufort 12/28/93 34.8843 76.6213 2 • • NC06 Agri. Flat Y EPA

CND125 Candor 09/25/90 35.2643 79.8382 198 • • NC36 Forested Rolling M EPA

COW137 Coweeta 11/04/87 35.0605 83.4302 686 • • NC25 Forested Complex N EPA

PNF126 Cranberry 12/27/88 36.1040 82.0448 1250 • • NC45 Forested Mountaintop M EPA

North Dakota

THR422 Theodore Roosevelt National Park 10/06/98 46.8947 103.3778 850 • • ND00 Range Rolling Y NPS

Ohio

DCP114 Deer Creek State Park 09/28/88 39.6358 83.2600 267 • • OH54 Agri. Rolling Y EPA

LYK123 Lykens 01/10/89 40.9169 82.9981 303 • • Agri. Flat M 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 Annual Report – 2010 5 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

Tennessee

ESP127 Edgar Evins State Park 03/22/88 36.0389 85.7330 302 • • KY10 Forested Rolling N EPA

GRS420 Great Smoky Mountains National Park 10/06/98 35.6331 83.9422 793 • • TN11 Forested Complex N NPS

SPD111 Speedwell 06/12/89 36.4698 83.8265 361 • • TN04 Agri. Rolling Y EPA

Texas

ALC188 Alabama-Coushatta 04/02/04 30.4210 94.4045 101 • • TX10 Forested Rolling Y EPA

BBE401 Big Bend National Park 07/18/95 29.3022 103.1772 1052 • • TX04 Forested Complex M NPS

PAL190 Palo Duro Canyon State Park 04/24/07 34.8803 101.6649 1050 • • TX43 Prairie Complex M EPA

Utah

CAN407 Canyonlands National Park 01/24/95 38.4586 109.8211 1809 • • UT09 Desert Complex M NPS

Virginia

PED108 Prince Edward 11/03/87 37.1653 78.3070 150 • • VA24 Forested Rolling M EPA

SHN418 Shenandoah National Park 06/28/88 38.5231 78.4347 1073 • • VA28 Forested Mountaintop M NPS

VPI120 Horton Station 06/02/87 37.3300 80.5573 920 • • VA13 Forested Mountaintop N EPA

Washington

MOR409 Mount Rainier National Park 08/29/95 46.7583 122.1244 415 • • WA99 Forested Complex N NPS

West Virginia

CDR119 Cedar Creek State Park 11/10/87 38.8794 80.8478 234 • • WV05 Forested Complex N EPA

PAR107 Parsons 01/19/88 39.0906 79.6614 510 • • WV18 Forested Complex N EPA

Wisconsin

PRK134 Perkinstown 09/27/88 45.2066 90.5972 472 • • WI35 Agri. Rolling M EPA
CASTNET Annual Report – 2010 6 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name

Start dateLatitude (°N)Longitude (°W)Elevation (m)Deposition Filter Pack1 Ozone and Meteorology2 Nearby NADP SitePrimary Land UseTerrainRepresentative to the MLM3 Sponsor

Wyoming

CNT169 Centennial 08/19/91 41.3722 106.2422 3178 • • WY95 Range Complex M EPA

PND165 Pinedale 12/27/88 42.9214 109.7900 2388 • • WY06 Range Rolling M EPA

YEL408 Yellowstone National Park 06/26/96 44.5597 110.4006 2400 • • WY08 Forested Rolling N NPS

1. The dry deposition filters are analyzed for the following constituents:

Teflon = SO2-

4 , NO3, NH +4 , Cl -, K +, Na +, Mg2+ , Ca2+ 

Nylon = SO2-

4 , NO3 (reported as HNO 3) 

Cellulose = SO2-

4 (reported as SO 2) 

2. Meteorological sensors: temperature, delta temperature, relative humidity, solar radiation, 

vector wind speed, scalar wind speed, wind direction, sigma theta, surface wetness, and 

precipitation via tipping bucket rain gauge. 

3. N = No; Y = Yes; M = Marginal.

4. O 

3 not measured.

5. Solar-powered site.

• Indicates current monitoring.

100 and 200 series = EPA – Operated Sites

400 series = NPS – Operated Sites 
CASTNET Annual Report – 2010 Appendix B 

Appendix B 

Acronyms and Abbreviations
CASTNET Annual Report – 2010 1 Appendix B

List of Acronyms and Abbreviations

AAI Aquatic Acidification Index

AMNet Atmospheric Mercury Network

AMoN Ammonia Monitoring Network 

ANC acid neutralizing capacity

AQA Canada – United States Air Quality Agreement

AQS EPA’s Air Quality System

ARP Acid Rain Program

BLM Big-Leaf Model

C concentration

Ca2+

 particulate calcium ion

CAAA Clean Air Act Amendments

CAIR Clean Air Interstate Rule

CAPMoN Canadian Air and Precipitation Monitoring Network

CASTNET Clean Air Status and Trends Network

CCV continuing calibration verification samples

Cl- particulate chloride ion

CLAD NADP Critical Loads of Atmospheric Deposition science committee

CLD303 Clingmans Dome, TN cloud sampling site

CLmaxS maximum critical load of sulfur

CLminN critical load of nitrogen removed by the ecosystem

cm centimeter

cm/sec centimeters per second

CMAQ Community Multiscale Air Quality Modeling System

CSAPR Cross-State Air Pollution Rule

DEM digital elevation model

DM8A daily maximum 8-hour average

DQI data quality indicator

DQO data quality objective

ECAN Environment Canada 

EGU electric generating unit

EPA U.S. Environmental Protection Agency

g/m3 grams per cubic meter

GEM gaseous elemental mercury

GOM gaseous oxidized mercury

H 

2SO 4 sulfuric acid

HNO 

3 nitric acid

IC ion chromatography

ICP-AES inductively coupled plasma-atomic emission spectrometry

IDW inverse distance weighting

IMPROVE Interagency Monitoring of Protected Visual Environments
CASTNET Annual Report – 2010 2 Appendix B

List of Acronyms and Abbreviations (continued)

ISO International Organization for Standardization

K+ particulate potassium ion

K 

2CO 3 potassium carbonate

kg/ha/yr kilograms per hectare per year

km kilometer

LAI leaf area index

lpm liters per minute

LWC liquid water content

MACTEC MACTEC Engineering and Consulting, Inc., now known as AMEC E&I, Inc.

MAD mean absolute difference

MADPro Mountain Acid Deposition Program

MAGIC Model of Acidification of Groundwater in Catchments

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

MSI Meteorological Solutions, Inc.

N nitrogen

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

NCLD National Critical Load Database

NDDN National Dry Deposition Network

ng/m3 nanograms per cubic meter

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

NO -

3 particulate nitrate

NOx nitrogen oxides [nitric oxide (NO) + nitrogen dioxide (NO 

2)]

NPS National Park Service

NTN National Trends Network

O 

3 ozone

OTC Ozone Transport Commission

PBM2.5 particulate-bound mercury with aerodynamic diameter < 2.5 μm
CASTNET Annual Report – 2010 3 Appendix B

List of Acronyms and Abbreviations (continued)

PEMA Pollutant Emission Management Area

pg/m3 picograms per cubic meter

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

RPD relative percent difference

S sulfur

SIP State Implementation Plan

SLAMS State and Local Monitoring Stations

SNOTEL snow telemetry (from National Resources Conservation Service)

SO 

2 sulfur dioxide

SO2-

4 particulate sulfate

SOP standard operating procedure

SRP standard reference photometer

SSWC Steady-State Water Chemistry

total NO3 gaseous nitric acid (HNO 

3) + particulate nitrate (NO3) 

μeq/L microequivalents per liter

μm micrometer

μg/m3 micrograms per cubic meter

UGRB Upper Green River Basin, WY

UGWOS Upper Green River Winter Ozone Study

UV ultraviolet

Vd deposition velocity

VOCs volatile organic compounds

WDEQ Wyoming Department of Environmental Quality

WF weighting factor

WFM300 Whiteface Mountain, NY cloud sampling site
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