Clean Air Status and Trends Network

(CASTNET)

2009 Annual Report

Prepared by:

MACTEC Engineering and Consulting, Inc.

Prepared for:

U.S. Environmental Protection Agency

Office of Air and Radiation

Clean Air Markets Division

Washington, D.C.

EPA Contract No. EP‐W‐09‐028

February 2011
CASTNET Annual Report – 2009 i Table of Contents

Table of Contents

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

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

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

 Cooperating Networks ......................................................................................... 3 

Locations of Monitoring Sites .............................................................................. 4 

 CASTNET Reference Sites .................................................................................. 5 

 Measurements Collected at CASTNET Sites ........................................................ 6 

One-Hour Sulfur Dioxide Concentrations Measured at Beltsville, MD and 

 Cherokee Nation, OK ................................................................................................. 8

 Modeling Dry Deposition .................................................................................... 9 

Atmospheric Mercury Network ............................................................................... 10

Sulfur Dioxide and Nitrogen OxidesEmissions .................................................. 12 

2009 Significant Events ...................................................................................... 14

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

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

Spatial Variability of Western Nitrogen Concentrations .............................................. 18 

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

Ten-Year Changes in SO 

2 and Total Nitrate Concentrations ........................................ 23 

 Total Nitrate ...................................................................................................... 26 

 Particulate Ammonium ..................................................................................... 28 

Trends in Sulfur Pollutants and Mercury Deposition .................................................. 30 

Chapter 3 Atmospheric Deposition .................................................................... 34 

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

Trends in Pollutants in Cloud Water and a Comparison to 

Trends in Sulfate and Nitrate in Precipitation ........................................................... 39

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

Estimates of Critical Loads for the Shenandoah Region ............................................... 45 

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

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

 Eight-Hour Ozone Concentrations ..................................................................... 49 

W126 Values for 2009 ........................................................................................ 53 

Changes in Ozone Design Values ............................................................................ 54

Chapter 5 Data Quality ................................................................................... 56 

Precision ........................................................................................................... 56 

Historical Precision of Daily Maximum 8-Hour Average O 

3 Concentrations ................... 62

 Accuracy ........................................................................................................... 66 

 Completeness .................................................................................................... 68 

Results for 2009 Environment Canada Proficiency Testing .......................................... 69 

Summary of Data Quality Results ...................................................................... 70 

References 

Appendix A Locational and Operational Characteristics of CASTNET Sites 

Appendix B Acronyms and Abbreviations 
CASTNET Annual Report – 2009 ii Executive Summary

Executive Summary

The EPA Clean Air Status and Trends Network (CASTNET) provides real-world air 

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

air pollution control programs. This report examines trends in air quality since 1990 and 

presents maps of the geographic distribution of ambient pollutant concentrations and 

deposition fluxes measured in 2009. CASTNET began collecting measurements in 1991 

with the incorporation of 50 sites from the National Dry Deposition Network (NDDN), 

which had been in operation since 1987. CASTNET measures rural, regionally 

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

United States and Canada. 

Key Results through 2009

 CASTNET pollutant measurements collected in 2009 showed the best air quality in 

the history of the network. 

 Mean annual sulfur dioxide and particulate sulfate concentrations declined 

significantly over the 20-year period 1990 through 2009 and showed a sharp decline 

since 2005. Sulfur dioxide levels declined 56 percent while sulfate concentrations 

declined 41 percent at eastern reference sites. 

 Total (dry + wet) sulfur deposition measured in the East declined by 46 percent from 

1990 through 2009. 

 Mean annual concentrations of total nitrate (nitric acid plus particulate nitrate) 

declined 30 percent over the 20-year period. 

 Total nitrogen deposition declined by 26 percent from 1990 through 2009 at eastern 

reference sites. 

 Ozone levels measured in 2009 continue to show a downward trend. The aggregated 

mean fourth highest daily maximum 8-hour average ozone concentration for 2009 

(64 ppb) was the lowest in the history of the network. Only four eastern and five 

California sites recorded exceedances of the 8-hour standard of 0.075 ppm during the 

most recent 3-year period (2007–2009). During 2009, no eastern site recorded a 

fourth highest daily maximum 8-hour average ozone concentration above 75 ppb. 

 Measurements taken during 2009 and historical data collected over the period 

1990–2008 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 – 2009 1 Chapter 1: CASTNET Overview 

Chapter 1: CASTNET Overview 

Background 

The Environmental Protection Agency (EPA) established the Clean Air Status and Trends 

Network (CASTNET) under a provision in the 1990 Clean Air Act Amendments (CAAA) 

to assess the effectiveness of the Acid Rain Program (ARP), which was promulgated to 

reduce emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from electric generating 

units (EGUs). The U.S. Congress recognized the need to assess and track real-world 

environmental results as the ARP was implemented and emission reductions became 

effective. Congress mandated that CASTNET provide consistent, long-term measurements 

for determining relationships between changes in emissions and subsequent changes in air 

quality, atmospheric deposition, and ecological effects. 

The ARP has produced significant reductions in SO2 and NOx emissions from EGUs since 

1995. The SO2 program set a permanent cap on the total amount of SO2 emissions allowed 

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

The program has been phased in with the final 2010 SO2 emissions cap set at 8.95 million 

tons, a level about one-half of the total SO2 emissions from the power sector in 1980. Actual 

SO2 emissions in 2009 from ARP regulated sources were about 5.7 million tons. 

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

EGUs by limiting the NOx emission rate. More recent NOx emission control programs 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 season (May 1 through September 30) when the 

CASTNET is a national long-term environmental monitoring program operated by the 

Environmental Protection Agency and the National Park Service. The network was 

designed to provide data for assessing trends in air quality, atmospheric deposition, and 

ecological effects that result from air pollutant emission reduction regulations. CASTNET 

was established under the 1990 Clean Air Act Amendments and was developed from the 

National Dry Deposition Network, which began in 1987. As of December 2009, the network 

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

Canada. Measurements collected in 2009 show the continuing improvement in air quality 

with significant declines in sulfur and nitrogen pollutants and ozone concentrations 

measured throughout the network.
CASTNET Annual Report – 2009 2 Chapter 1: CASTNET Overview 

potential for ozone formation is high. Actual NOx emissions from ARP-regulated EGUs in 

2009 were about 2.0 million tons. 

The Clean Air Interstate Rule (CAIR), as promulgated in March 2005, was intended to 

permanently lower SO2 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 ozone season NOx program, and 

an annual SO2 program. Although CAIR was remanded back to EPA in 2008, these 

programs remain in effect while EPA works to develop a replacement rule. The first phase 

of the CAIR annual and ozone season NOx requirements began in 2009. The requirement to 

hold allowances in the SO2 program began on January 1, 2010. On July 6, 2010, EPA 

proposed the Transport Rule, which, if finalized as proposed, will replace the CAIR. The 

Transport Rule, as proposed, would require 31 states and the District of Columbia to 

achieve additional reductions in power plant SO2 and NOx emissions. EPA further expects 

that by 2014, the Transport Rule and other state and EPA actions would reduce power plant 

SO2 emissions by 71 percent and NOx emissions by 52 percent from 2005 levels. 

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. 

The network, as of December 2009, included 84 monitoring stations at 82 site locations 

throughout the contiguous United States, Alaska, and Canada. CASTNET stations measure 

rural, regionally representative concentrations of sulfur and nitrogen pollutants and ozone 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, which are estimated using 

the Multi-Layer Model (MLM). 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 may also provide input to ecological models, such as the Model of 

Acidification of Groundwater in Catchments (MAGIC). 

This report summarizes CASTNET monitoring activities and the resulting concentration 

and deposition data collected over the 20-year period from 1990 through 2009. 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 

National Park Service (NPS)-sponsored CASTNET sites. Fully validated data are available 

approximately 10 months following collection. NPS data collected during 2009 from the 

NPS-sponsored CASTNET sites and the NPS Gaseous Pollutant Monitoring Program are 

available in the NPS 2009 Annual Data Summary Report (Ray, 2010) and the NPS 2009 

Data Quality Assurance Report (NPS, 2010). 
CASTNET Annual Report – 2009 3 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), a pilot network, which operates triplicate 

passive ammonia (NH 

3) samplers at about 20 locations. AMoN, in operation for two 

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 20 locations in the United States and Canada in order to estimate dry and total 

mercury deposition. 

Visit NADP’s Web site for more information: http://nadp.isws.illinois.edu/. 

 Canadian Air and Precipitation Monitoring Network (CAPMoN) operates 

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

CAPMoN both operate samplers at monitoring stations in Ontario, Canada and at 

Pennsylvania State University. Visit CAPMoN’s Web site 

http://www.ec.gc.ca/rs-mn/default.asp?lang=En&n=752ce271-1 for more information. 

 Interagency Monitoring of Protected Visual Environments (IMPROVE) measures 

aerosol pollutants at or near more than 30 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 United 

States – Canada 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 

Oklahoma (CHE185), Alabama-Coushatta in eastern Texas (ALC188), and Santee Sioux in 
CASTNET Annual Report – 2009 4 Chapter 1: CASTNET Overview 

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 as of December 2009. 

Eighty-four sites were operational at 82 distinct locations. Most CASTNET sites are located 

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

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

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

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

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

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

lower pollutant concentrations generally found in the western United States. 

Figure 1-1 CASTNET Sites as of December 2009 
CASTNET Annual Report – 2009 5 Chapter 1: CASTNET Overview 

CASTNET Reference Sites 

Maps of pollutant concentrations and 

deposition fluxes across the United States 

are presented in Chapters 2 through 4. 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 in 

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. Trends in pollutant 

concentrations measured at CASTNET 

western reference 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 in 

Chapters 2 through 4 using box plots for the 

20-year period 1990 through 2009. Data from 

the 17 western reference sites were aggregated 

and then presented using box plots for the 

period 1996 through 2009. 

Figure 1-2 CASTNET Eastern 

Reference Sites 

Figure 1-3 CASTNET Western 

Reference Sites 

Speedwell, TN (SPD111) 
CASTNET Annual Report – 2009 6 Chapter 1: CASTNET Overview 

Measurements Collected at CASTNET Sites 

CASTNET was designed primarily to measure 

trends in seasonal and annual average 

concentrations and to model depositions over 

many years. Consequently, measurement of 

weekly average concentrations was selected as 

the basic sampling strategy. Over the course of 

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 carbonateimpregnated 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 

CASTNET Ambient Measurements 

h Sulfur species:

Sulfur dioxide (SO 

2) 

Particulate sulfate (SO2-

4 ) 

h Nitrogen species:

Particulate nitrate (NO3) 

Nitric acid (HNO 

3) 

Particulate ammonium (NH +

4) 

h Metal Cations: 

Particulate calcium (Ca2+

 ) 

Particulate sodium (Na+ ) 

Particulate magnesium (Mg2+

 ) 

Particulate potassium (K+ ) 

h Particulate chloride (Cl-) 

h Ozone (O 

3) 

h Meteorological variables 

h 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 

 Teflon® Spacers 

Cellulose = Gaseous: SO 

2

Nylon = Gaseous: HNO 

3, SO 2

Teflon = Particulate: SO2-

4 , NO3, NH +4 , K+ , Ca2+ , Mg2+ , Na+ , Cl-
CASTNET Annual Report – 2009 7 Chapter 1: CASTNET Overview 

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

MACTEC 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 SO2-4 using IC. 

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

3) as nitrogen and for SO 2

as SO2-

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

total SO 

2. 

CASTNET also measures hourly ozone (O 

3) 

concentrations, one of the major components 

of smog. In addition to the air pollutant 

concentrations, CASTNET sites collect hourly 

meteorological measurements, which are used as 

input to the MLM, a numerical model used for 

estimating dry deposition to ecosystems from the 

atmospheric boundary layer. The meteorological 

data are also used by other investigators for 

hydrogeochemical and ecosystem modeling. 

The O 

3 and meteorological measurements are 

recorded continuously and archived as 

hourly averages. 

CASTNET Meteorological Measurements 

h Temperature:

Temperature (at 2 and 9 meters) 

Delta temperature (difference 

 between 2 and 9 meters) 

h Relative Humidity

h Solar Radiation

h Winds:

Speed 

Direction 

Sigma theta (standard deviation of 

 direction) 

h Precipitation

h Surface Wetness 

Edgar Evins State Park, TN (ESP127) 
CASTNET Annual Report – 2009 8 Chapter 1: CASTNET Overview 

One-Hour Sulfur Dioxide Concentrations Measured at 

Beltsville, MD and Cherokee Nation, OK 

Hourly average trace-level concentrations of carbon monoxide (CO), total reactive oxides of nitrogen (NOy), 

and SO 

2 have been collected at the Beltsville, MD (BEL116) CASTNET site since spring 2005. The continuous 

monitors were initiated to establish baseline concentrations of these trace-level pollutants and to help analyze 

chemical and meteorological measurements. The Cherokee Nation established an NCore monitoring station in 

Stilwell, OK (CHE185) in July 2008, which also measures trace-level pollutants. 

On June 2, 2010, EPA strengthened the primary National Ambient Air Quality Standards (NAAQS) for SO 

2. EPA 

replaced the existing annual and 24-hour primary SO 

2 standards with a new 1-hour SO 2 standard set at 

75 parts per billion (ppb) to better protect public health by reducing human exposure to high short-term 

(5 minutes to 24 hours) concentrations of SO 

2. The new standard is based on the 3-year average of the 99th

percentile of the yearly distribution of daily maximum 1-hour SO 

2 concentrations. For more information on the 

revised standard visit the EPA Web site http://www.epa.gov/air/sulfurdioxide/pdfs/20100602fs.pdf. 

Assessing Compliance with the New Standard 

SO 

2 measurements are collected on an hourly basis. For compliance purposes: 

 The daily maximum 1-hour SO 

2 concentration is recorded for each day. 

 For each year, the 99th percentile of the yearly distribution of daily maximum 1-hour values is calculated. 

 These annual 99th percentile values are averaged over rolling 3-year periods. 

 If the 3-year average of these 99th percentile values exceeds 75 ppb, the area is designated as a 

“nonattainment area” and is held accountable for reaching the attainment status. 

EPA estimates that, based on 2007–2009 data, 59 of 249 monitored counties would have exceeded the new 

SO 

2 NAAQS. EPA plans to use 2009 to 2011 or later data to designate areas as nonattainment. EPA is 

scheduled to make final determination for area designations no later than June 2012. 

2009 SO 

2 Concentration Data from BEL116, MD and CHE185, OK 

SO 

2 concentration data from CASTNET sites BEL116, MD and CHE185, OK can be used to evaluate the 

compliance assessment procedure. Figure 1a presents the cumulative frequency distribution of daily 

maximum 1-hour SO 

2 concentrations for 2009 for BEL116, and Figure 1b presents SO 2 data from CHE185. The 

99th percentile values are below the 75 ppb NAAQS. The figures present data from 2009, only. Compliance 

with the NAAQS must be evaluated with 3-year averages of 99th percentile values. 

Figure 1a BEL116 Cumulative Frequency for 2009 Figure 1b CHE185 Cumulative Frequency for 2009 

 

0

2

4

6

8

10

12

14

16

18

20

0.1%0.2%0.5%1.0%2.0%5.0%

10.0%20.0%30.0%40.0%50.0%60.0%70.0%90.0%95.0%96.0%98.0%99.0%99.8%

100.0%

Concentration (ppb)

0

2

4

6

8

10

12

14

16

18

20

0.1%0.2%0.5%1.0%2.0%5.0%

10.0%20.0%30.0%40.0%50.0%60.0%70.0%90.0%95.0%96.0%98.0%99.0%99.8%

100.0%

Concentration (ppb)
CASTNET Annual Report – 2009 9 Chapter 1: CASTNET Overview 

ra = turbulence

ra, soil = turbulence near soil

rb = thin layer at surface

rcut = cuticular

rs = stomatal

rsoil = soil rb

ra

rsoil

Temp, RH, 

SR, LAI

Wind Speed, 

Sigma Theta

Wetness,

Precip

ra, soil

rb

rcut

rs

 1 1 1

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 deposition velocity 

(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 deposition 

velocity, 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. 

Figure 1-5 Multi-Layer Model 

Flux = C x Vd

1/Vd = + ra

 rs + rb +

 rcut + rb

+

 ra, soil + rsoil

Resistances 

ra = aerodynamic 

ra, soil = aerodynamic near soil 

rb = boundary layer in thin layer at surface 

rcut = cuticular 

rs = stomatal 

rsoil = soil 

1 
CASTNET Annual Report – 2009 10 Chapter 1: CASTNET Overview 

Atmospheric Mercury Network 

Mercury (Hg) is commonly emitted to the atmosphere through power generation from coal-fired power 

plants, the single largest source of anthropogenic Hg in the United States. Other sources include incineration 

of waste containing products with Hg, combustion of coal and petroleum, metal reprocessing facilities, cement 

kilns, and natural sources such as volcanoes. Atmospheric deposition is the dominant source of Hg to surface 

waters and wetlands over most of the globe. Methylmercury (MeHg) is produced from ionic Hg by 

microorganisms in aquatic systems. It is not readily eliminated from organisms, and it is biomagnified in 

aquatic food chains. Because fish and animals can accumulate MeHg faster than they eliminate it, fish and 

animals consume higher concentrations of Hg at each successive level of the food chain. Small environmental 

concentrations of MeHg can thus readily accumulate to potentially harmful concentrations in fish, fish-eating 

wildlife, and humans. 

Despite the widespread Hg pollution and its serious health risks, a comprehensive and integrated mercury 

monitoring network does not exist. However, considerable progress has been made in the design of a national 

mercury monitoring program. In fall 2009, the NADP established the Atmospheric Mercury Network (AMNet) 

to measure Hg species concentrations in ambient air. Data from the AMNet are used to estimate dry 

deposition, assess regulatory emission reduction programs, evaluate atmospheric models, and estimate longterm trends. The NADP’s MDN has been monitoring wet deposition of Hg at 113 sites, including approximately 

25 sites that measure MeHg in precipitation, since 1996. The measurement of wet deposition at MDN sites 

and the modeled dry deposition estimates at AMNet sites will enable improved estimates of total Hg 

deposition. All AMNet sites operate a Tekran analyzer, which provides a continuous analysis of gaseous 

oxidized Hg, elemental Hg, and particulate-bound Hg. Chemical analysis is based on cold vapor atomic 

fluorescence spectroscopy.

AMNet Goals 

The NADP established three major goals for AMNet, including determining the status and trends in 

concentrations of atmospheric Hg fractions (gaseous oxidized, particulate-bound, and elemental) in select 

locations; offering high-quality meteorological measurements to estimate dry and total deposition of 

atmospheric Hg to aquatic ecosystems and other areas of interest on the local, regional, and global scale; and 

providing data for atmospheric Hg model development, validation, and improvement (NADP/AMNet, 2010). 

Figure 1c shows the 20 AMNet sites that are currently operating in the United States and Canada. Most sites 

are collocated with MDN sites and include meteorological measurements. Dual sampling systems are operated 

at Beltsville, MD (AMNet sites MD98 and MD99: collocated with NADP/MDN site MD99 and CASTNET site 

BEL116), Grand Bay National Estuarine Research Reserve (NERR), MS (AMNet sites MS12 and MS99: collocated 

with NADP/MDN site MS12) and Rochester, NY (AMNet sites NY43 and NY95: collocated with NADP/MDN sites 

NY43 and NY95, respectively). 

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). The Web site discusses the sampling system 

and related information. It includes a network description, a list of network participants, site information, and 

network contacts. Mercury concentration data will be posted on the Web site when they are available. 
CASTNET Annual Report – 2009 11 Chapter 1: CASTNET Overview 

Figure 1c Atmospheric Mercury Network Sites 

Table 1a Atmospheric Mercury Network Site Locations and Sponsors 

Site Site ID Name Location Latitude Longitude Organization

1 MD99 

MD98 

Beltsville 

Beltsville Collocated 

Maryland 39.0284 -76.8172 NOAA/EPA 

2 MD08 Piney Reservoir Maryland 39.7053 -79.0122 State MD 

3 MS12 

MS99 

Grand Bay NERR 

Grand Bay NERR Collocated 

Mississippi 30.4294 -88.4277 NOAA 

4 NJ05 Brigantine New Jersey 39.4020 -74.3790 State NJ 

5 NJ32 Chester New Jersey 40.7876 -74.6763 State NJ 

6 NJ54 Elizabeth Lab New Jersey 40.6414 -74.2084 State NJ 

7 NJ30 New Brunswick New Jersey 40.4728 -74.4225 State NJ 

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

9 NY06 NYC New York 40.7146 -74.0058 State NY 

10 NY43 Rochester New York 43.1544 -77.6160 Clarkson U. 

11 NY95 Rochester Collocated New York 43.1544 -77.6160 State NY 

12 OH02 Athens Ohio 39.3000 -82.1167 Ohio U. 

13 OK99 Stilwell Oklahoma 35.7514 -94.6717 Cherokee Nation 

14 VT99 Underhill Vermont 44.5283 -72.8689 Ecosystems Res. Gr. 

15 WV99 Canaan Valley Institute West Virginia 39.0636 -79.4222 NOAA 

16 UT96 Antelope Island Utah 41.0467 -112.0248 U Utah 

17 UT97 Salt Lake City Utah 40.7118 -111.9609 State Utah 

18 NH06 Thompson Farm New Hampshire 43.1100 -70.9500 U New Hampshire 

19 NU15 Alert Nunavut 54.0000 -90.0000 Env. Canada 

20 NS02 Kejimkujik Nova Scotia 44.4300 -65.2100 Env. Canada 

Note: Collocated CASTNET sites are highlighted in yellow. 

Source: NADP/AMNet, 2010
CASTNET Annual Report – 2009 12 Chapter 1: CASTNET Overview 

Sulfur Dioxide and Nitrogen Oxides Emissions 

Title IV of the 1990 CAAA established the ARP, which 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 recently assessed the effects of the ARP and related programs in its 

2009 progress report (EPA, 2010a). The report summarizes 2009 compliance with the ARP 

and reported progress toward achieving environmental goals.

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 

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 O 

3 formation is high. The CAIR NOx ozone season 

trading program replaced the NBP in 2009. The CAIR annual NOx controls were also 

instituted in 2009. Although CAIR was remanded back to EPA in 2008, the CAIR 

programs continue in effect temporarily. If the Transport Rule is finalized as proposed, the 

CAIR programs would continue in effect through the completion of all 2011 control period 

activities before being replaced. 

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 2009, the SO 

2 Phase II requirements affected 3,572 operating units. The Phase II NOx

requirements applied to 960 of those operating units that exceed 25 megawatts and burned 

coal between 1990 and 1995 (EPA, 2010a). Under the ARP’s emission reduction 

requirements, total SO 

2 emissions from ARP sources were 5.7 million tons in 2009; total 

NOx emissions from ARP sources were 2.0 million tons. 
CASTNET Annual Report – 2009 13 Chapter 1: CASTNET Overview 

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

2 emissions for Phase I and Phase II electric 

utility plants for five years (1990, 1995, 2000, 2005, and 2009). 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 with the highest SO 

2 emissions in 1990 

realized the largest reductions. Annual NOx emissions by state are depicted in Figure 1-7 for 

the same five years. States with the highest NOx emissions also had the most significant 

declines. Every state east of the Mississippi River experienced a significant decline in annual 

NOx emissions from 2000 to 2009. For example, NOx emissions from power plants in more 

than 20 eastern states declined by at least 65 percent from 2000 to 2009. Also, EGU NOx

emissions in Texas and Missouri declined by 65 percent or more from 2000 to 2009. 

Figure 1-6 Annual Utility SO 

2 Emissions (Phase I and Phase II Plants only) 

Figure 1-7 Annual Utility NOx Emissions (Phase I and Phase II Plants only) 

Annual Emissions of Sulfur Dioxide

1990

1995

2000

2005

2009

Scale: Largest bar equals 

2,212 thousand short 

tons (Ohio, 1990).

Annual Emissions of Nitrogen Oxides

Scale: Largest bar equals 

534 thousand short 

tons (Ohio, 1990).

1990

1995

2000

2005

2009
CASTNET Annual Report – 2009 14 Chapter 1: CASTNET Overview 

2009 Significant Events 

January 

The span for O 

3 multipoint calibrations changed from 400 ppb to 450 ppb. The checkpoints are now 

0 ppb, 450 ppb, 300 ppb, 200 ppb, 100 ppb, and 60 ppb. 

As of January 2009, the field calibration accuracy criterion for relative humidity changed to 

± 10 percent for all relative humidity measurements.

March 

MACTEC tied for first place with two of 36 other laboratories in the intercomparison proficiency test 

(PT) Study 0093 with Environmental Canada. For the U.S. Geological Survey 2008 Interlaboratory 

Comparison samples, MACTEC achieved the target value or within less than 5 percent for all of 

the 144 samples analyzed.

May 

Shelter temperature data from EPA-sponsored CASTNET sites were included in the publically-available 

CASTNET database hosted by EPA. 

July 

The Campbell Scientific CR3000 data logger rollout was completed. With the exception of the 

Cherokee Nation, OK (CHE185) site, all EPA-sponsored CASTNET sites operate CR3000 data 

loggers. The CHE185, OK site will continue to operate an Environmental Systems Corporation 

data logger, which is also used at all NPS-sponsored CASTNET sites. 

August 

The CASTNET IV contract (EP-W-09-028) officially began on August 11, 2009. 

By early August 2009, all EPA-sponsored CASTNET sites that measure O 

3 (see Appendix A) were 

uploading hourly O 

3 and meteorological data to AIRNow. NPS-sponsored CASTNET sites have 

been included in AIRNow since 2004. 

Beginning on August 11, 2009, precision measurement criteria for filter pack concentrations changed 

from a mean absolute relative percent difference (MARPD) of 5.0 percent for all collocated 

CASTNET filter pack measurements (except for particulate NH+ 

4, which had a criterion of 

10.0 percent MARPD) to a single MARPD value of 20.0 percent. 

NPS produced a biannual CASTNET newsletter called The Monitor that contained 

information about some of the changes and important events affecting CASTNET operations. 

Recently, NPS replaced The Monitor by publishing an e-mail newsbrief called “Eye on Air 

Quality,” which will be published each April, July, and October. This newsbrief can be found 

at http://ard-request.air-resource.com/project/documents.aspx. For an electronic library of 

previous newsletters, go to http://www.nature.nps.gov/air/Pubs/theMonitor.cfm. 
CASTNET Annual Report – 2009 15 Chapter 2: Atmospheric Concentrations 

Chapter 2: Atmospheric Concentrations 

Maps of annual mean concentrations of SO 

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

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

(MACTEC 2009a; 2009c; 2010a; 2010c). The concentration shading used in the maps was 

prepared using an algorithm based on inverse distance weighting and the scales specified on 

each map. In addition, trends in concentrations over the 20-year period (1990 through 2009) 

were derived from the 34 eastern CASTNET reference sites (Figure 1-2) and are presented 

using box plots. Trends in annual mean concentrations were also estimated from 

measurements aggregated over the 17 western CASTNET reference sites (Figure 1-3) for 

the period 1996 through 2009. 

Sulfur Dioxide 

Sulfur dioxide is a 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. As discussed in Chapter 1, SO 

2 is a criteria pollutant regulated by the NAAQS. To be 

compliant under the new SO 

2 NAAQS, the 3-year average of the 99th percentile of the yearly 

distribution of daily maximum 1-hour SO 

2 concentrations must be at 75 ppb 

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

2 gas reacts in the atmosphere to form 

various acidic compounds including sulfuric acid (H 

2SO 4), a significant contributor to acid 

rain, and ammonium sulfate [(NH 

4)

 

2SO 4], a major component of fine particle matter (PM2.5). 

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

contributes to degradation of atmospheric visibility. 

Weekly average concentrations of SO 

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

, and four earth metals 

were measured at 84 CASTNET monitoring stations using 3-stage filter packs. Measured 

annual mean concentrations of SO 

2 and SO2-4 have decreased steadily since 1990 with a sharp 

decline since 2005. Concentrations of total nitrate (HNO 

3 + NO3) showed little change from 

1990 until 2000 when they began to decline in response to NOx emission reduction 

programs. 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 2009 and for 

17 western reference sites for the period 1996 through 2009 using box plots for each year. All 

four parameters measured at the eastern sites declined over the 20-year period. SO 

2

concentrations measured at the 17 western reference sites also declined over the 14 years. 

Concentrations of the other three pollutants measured at the western sites did not change 

significantly but did decline in 2009.
CASTNET Annual Report – 2009 16 Chapter 2: Atmospheric Concentrations 

Annual mean SO 

2 concentrations recorded in 2009 are presented in Figure 2-1. Only two 

sites in the eastern United States measured mean concentrations greater than 5.0 μg/m3 of 

air. The sites are Quaker City, OH (QAK172), which measured the highest concentration in 

the network with a value of 8.3 μg/m3 , and Laurel Hill State Park, PA (LRL117), which 

measured a concentration of 5.5 μg/m3 . SO 

2 concentrations recorded in 2009 were 

significantly lower than SO 

2 levels measured in previous years. For example, in 2008, 

concentrations greater than 5.0 μg/m3 were measured in a six-state region centered on the 

Ohio River Valley. Only two western sites (Figure 1-1) measured a 2009 annual mean SO 

2

concentration equal to 1.0 μg/m3 . The western sites are Palo Duro Canyon State Park, TX 

(PAL190) and Theodore Roosevelt National Park, ND (THR422). 

Figure 2-1 Annual Mean SO 

2 Concentrations (μg/m3 ) for 2009 

Figure 2-2 shows box plots of annual mean SO 

2 concentrations aggregated over the 

34 eastern reference sites from 1990 through 2009 (right side) and the 17 western reference 

sites from 1996 through 2009 (left side). The y-axes on the western and eastern plots have 

different scales since concentrations measured at the western CASTNET sites are generally 

much lower than concentrations measured at the eastern sites. 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 2009. 

Three-year mean concentrations for 1990–1992 and 2007–2009 were 8.9 μg/m3 and 

3.9 μg/m3 , respectively. This change constitutes a 56 percent reduction in 3-year mean SO 

2

concentrations between the two time periods. The 2009 mean level of 2.7 μg/m3 was the 
CASTNET Annual Report – 2009 17 Chapter 2: Atmospheric Concentrations 

lowest mean value measured by the eastern reference sites in the history of the network and 

represents a dramatic decline over the past four 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 2007–2009 were 0.61 μg/m3 and 0.34 μg/m3 , respectively. This change 

constitutes a 44 percent reduction in 3-year mean SO 

2 concentrations over the 14 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 

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

19961997199819992000200120022003200420052006200720082009

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

12

16

20

19901991199219931994199519961997199819992000200120022003200420052006200720082009

Concentration (μg/m3)

Georgia Station, GA (GAS153)
CASTNET Annual Report – 2009 18 Chapter 2: Atmospheric Concentrations 

Spatial Variability of Western Nitrogen Concentrations 

CASTNET measures weekly concentrations of the nitrogen containing compounds HNO 

3, NH +4, and NO3. The 

dry deposition of nitrogen to natural areas is known to acidify soils, induce biological species changes, and act 

as a fertilizer that helps promote the growth of exotic plant species. In the western United States, CASTNET 

stations are typically 100-200 miles apart, often with mountainous terrain between sites. Although the 

concentrations and deposition fluxes of nitrogen compounds in the West are generally much lower than in 

the East, western desert and alpine ecosystems are sensitive to changes in nitrogen deposition. 

Taking into account the complexity and variability of western landscapes and terrain, how dense a network of 

sites is needed in the West to adequately capture the rural spatial variability of the dry chemistry species? An 

independent NPS program consisting of a network of 18-20 portable O 

3 monitoring systems (POMS) that 

operate during the summer months mostly at western locations provides some insight into this question. 

Four of the POMS have masts and enough solar power to run CASTNET filter pack systems. Weekly samples 

are taken on the CASTNET schedule. The filter packs are prepared and analyzed by the CASTNET laboratory to 

obtain results comparable to those from nearby CASTNET stations. The POMS season runs from May through 

September to correspond with higher O 

3 levels. The POMS sites with CASTNET filter pack systems provide 

“fill-in” measurements to help gauge areas that may need a permanent monitor. 

Table 2a Comparison of Total Nitrogen (as N) Concentrations (μg/m3 ) at POMS 

and Nearby CASTNET Sites 

 Miles Mean Mean Max Max 

SITE ID Site Years Distance Total_N % Diff Total_N % Diff 

CAV100 Carlsbad Caverns National Park, NM 

(POMS) 4 0 1.2 4.1 

BBE401 Big Bend National Park, TX (CASTNET) 5 214 1.0 -16.5 5.9 44 

DET100 Devils Tower National Monument, WY 

(POMS) 2 0 0.6 0.9 

WNC429 Wind Cave National Park, SD (CASTNET) 5 93 0.8 19 1.4 56 

DIN100 Dinosaur National Monument, CO & UT 

(POMS) 5 0 0.7 1.4 

CAN407 Canyonlands National Park, UT 

(CASTNET) 5 140 0.7 5 1.0 -29 

JOT100 Joshua Tree National Park (Pinto 

Wells), CA (POMS) 4 0 1.6 3.5 

JOT403 Joshua Tree National Park, CA 

(CASTNET) 5 57 2.0 55 3.4 -7 

CON186 Converse Station, CA (CASTNET) 5 88 2.2 94 4.2 50 

ROM406 Rocky Mountain National Park, CO 

(NPS) (CASTNET primary site) 5 0 0.6 1.3 

ROM206 Rocky Mountain National Park, CO 

(EPA) (CASTNET collocated site) 5 0 0.6 1.1 

Source: J.D. Ray 
CASTNET Annual Report – 2009 19 Chapter 2: Atmospheric Concentrations 

Figure 2a Western CASTNET Sites and POMS Locations Table 2a compares the mean and maximum 

total nitrogen (Total_N) content of the HNO 

3, 

NH +

4, and NO3 concentrations measured at four 

POMS and nearby CASTNET sites, plus the 

CASTNET collocated site pair at Rocky 

Mountain National Park, CO 

(ROM406/ROM206) during the 5-month 

sampling season. Both samplers at 

ROM406/ROM206, CO, where the separation 

between sampling sites is about 50 feet, 

measured a mean concentration of 0.6 μg/m3 

. 

The recent (Malm and Collett, 2009) Rocky 

Mountain Atmospheric Nitrogen and Sulfur 

(RoMANS) Study, a research study of highelevation ecosystems performed over two 

6-week time periods, showed that large 

episodic events accounted for much of the wet 

and dry nitrogen deposition in and near 

Rocky Mountain National Park. Table 2a also 

shows that the distances between the POMS 

and their nearby CASTNET sites do not explain 

the differences in 5-month Total_N concentrations. For example, concentrations measured at Dinosaur 

National Monument and Canyonlands National Park show little difference (5 percent) in mean Total_N. On the 

other hand, the two sites in Joshua Tree National Park and Converse Station in southern California measured 

the highest concentrations and showed a significant increasing east to west gradient in concentrations toward 

Los Angeles over about the same distance. 

A map of western CASTNET and POMS sites is presented in Figure 2a. The POMS and nearby CASTNET site 

groups are shown with the related groups linked with yellow lines. 

Converse Station, CA (CON186)

Source: J.D. Ray
CASTNET Annual Report – 2009 20 Chapter 2: Atmospheric Concentrations 

Figure 2b presents a broad statistical 

picture of the western Total_N data by 

comparing the POMS data with data from 

additional nearby CASTNET sites. In 

Figure 2b, statistical distributions of all the 

weekly filter data for total nitrogen are 

represented using box and whisker plots. 

The 10th, 25th, 50th, 75th and 90th percentiles 

are shown for each site. The sites are 

organized in site groups for comparison. 

The diamonds represent the confidence 

interval about the mean and are useful for 

comparing to nearby sites to see if the 

means are different. If the diamonds 

overlap, the mean values are not 

statistically significant. 

Gradients in total nitrogen were observed 

at the western sites. The concentration 

gradient increases from east to west and 

also from north to south. The southern 

California sites measured the highest total 

nitrogen concentrations among the 

western sites. The Carlsbad Caverns, NM 

and two Texas sites observed higher 

nitrogen levels than the remaining sites. 

The data suggest that there are strong 

gradients associated with major source 

areas from Los Angeles along the southern 

tier to Texas that may not be fully 

represented spatially by the current 

network. The sites within a broad area of Utah, Colorado, Wyoming, and the Dakotas recorded lower total 

nitrogen concentrations with similar means. Most of the monitors in this region had concentrations below 

0.8 μg/m3 and are probably fairly well represented by the widely spaced network in those areas of the 

West. No attempt was made in this analysis to account for elevation differences. 

More information can be gained by analyzing the other dry chemistry compounds measured on the filters 

along with the high weekly concentrations that indicate episodes. The Devils Tower and Dinosaur National 

Monuments filter pack sampler data do not add much new information to the network, despite oil and gas 

development near Dinosaur National Monument. There was also a question whether pollution from the Salt 

Lake City area was transported to the park. Since the nitrogen species and O 

3 concentrations are not 

elevated, any impact from those source areas to Dinosaur National Monument is not detectable with the 

POMS O 

3 and filter pack systems. The Pinto Wells site in Joshua Tree National Park, CA and the Carlsbad 

Caverns, NM site, however, do indicate transport from pollution source areas. 

Carlsbad, NM

Devils Tower, WY

Dinosaur, UT

Joshua Tree, CA

Source: J.D. Ray

Figure 2b Comparison of POMS and Nearby 

CASTNET Site Total Nitrogen 

Concentrations (μg/m3 ) 

SITE_ID
CASTNET Annual Report – 2009 21 Chapter 2: Atmospheric Concentrations 

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 typically exists in the atmosphere as (NH 4)

 

2SO 4, a 

major component of PM2.5. A map of annual mean particulate SO2-

4 concentrations 

measured during 2009 is given in Figure 2-3. 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. SO2-

4 concentrations greater than or equal to 

1.0 μg/m3 in the western region were measured at two sites in California, two in Texas, and 

the monitoring station at THR422, ND. 

Figure 2-3 Annual Mean SO2-

4 Concentrations (μg/m3 ) for 2009 

Box plots of annual mean SO2-

4 concentrations from the 34 eastern reference sites from 

1990 through 2009 are presented in the right half of Figure 2-4. The figure shows a substantial 

decline in SO2-

4 with some interannual variability. Like SO 2, sulfate concentrations declined 

significantly from 2005 through 2009. The difference between 3-year means from 1990–1992 

to 2007–2009 represents a 41 percent reduction in SO2-

4 from 5.4 μg/m3 to 3.2 μg/m3 

, 

respectively. The 2009 mean SO2-

4 level of 2.6 μg/m3 was the lowest in the history of 

the network. 

 
CASTNET Annual Report – 2009 22 Chapter 2: Atmospheric Concentrations 

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

boxes show no real change in annual mean SO2-

4 concentrations aggregated over the 17 sites, 

although the 2009 aggregated mean concentration was the lowest recorded. Mean SO2-

4

concentrations measured at the western sites were lower than mean values measured at 

eastern sites. 

Figure 2-4 Trend in Annual Mean SO2-

4 Concentrations (μg/m3 ) 

Western Reference Sites Eastern Reference Sites 

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

19961997199819992000200120022003200420052006200720082009

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

19901991199219931994199519961997199819992000200120022003200420052006200720082009

Concentration (μg/m3)

Palo Duro Canyon State Park, TX (PAL190) 
CASTNET Annual Report – 2009 23 Chapter 2: Atmospheric Concentrations 

Ten-Year Changes in SO 

2 and Total Nitrate Concentrations 

Emission reductions of SO 

2 and NOx resulting from the implementation of the ARP in 1995, the NBP in 1999, 

and the CAIR in 2009, resulted in a dramatic improvement in air quality in the eastern United States. SO 

2 and 

NOx emissions are shown in Figures 1-6 and 1-7. Three-year mean SO 

2 emissions from ARP units declined from 

13.2 million tons in 1997–1999 to about 7.4 million tons in 2007–2009, a 44 percent reduction. 

Aggregated 3-year mean SO 

2 concentrations declined approximately 42 percent over that period. More 

recently, SO 

2 emissions declined from 10.2 million tons in 2005 to 5.7 million tons in 2009. This significant 

decrease in emissions was produced mostly from the installation of SO 

2 emission control technology because 

of early compliance planning to meet CAIR requirements (EPA, 2010a). 

NOx emissions from ARP units declined from a 1997–1999 mean of 5.2 tons per year to a 2007–2009 mean of 

2.8 tons per year, a 46 percent reduction. Three-year mean total NO3 concentrations declined from 3.1 to 

2.1 μg/m3 , a 33 percent reduction over that period. The drop in NOx emissions from 2008 to 2009 was the 

largest single year decline in the history of the ARP. This large reduction in emissions can be primarily 

attributed to compliance with the annual and O 

3 season CAIR programs for NOx, which began in 2009 

(EPA, 2010a). 

Beaufort, NC (BFT142) 
CASTNET Annual Report – 2009 24 Chapter 2: Atmospheric Concentrations 

The maps shown in Figures 2c and 2d show 3-year mean SO 

2 concentrations for 1997–1999 and 

2007–2009, respectively. The maps show a significant reduction in SO 

2 over the 10 years. Both the magnitude 

and geographic extent of high concentrations declined. For example, the 3-year mean SO 

2 concentration 

measured at Quaker City, OH (QAK172) declined from 12.5 μg/m3 to 8.1 μg/m3 

. 

Figure 2c Three-year Mean SO 

2 Concentrations (μg/m3 ) for 1997–1999

Figure 2d Three-year Mean SO 

2 Concentrations (μg/m3 ) for 2007–2009 
CASTNET Annual Report – 2009 25 Chapter 2: Atmospheric Concentrations 

Maps of 3-year mean total nitrate concentrations are depicted in Figures 2e and 2f for 1997–1999 and 

2007–2009. Again, both the magnitude and geographic extent of the high concentrations have been reduced. 

Figure 2e Three-year Mean Total Nitrate Concentrations (μg/m3 ) for 1997–1999 

Figure 2f Three-year Mean Total Nitrate Concentrations (μg/m3 ) for 2007–2009 
CASTNET Annual Report – 2009 26 Chapter 2: Atmospheric Concentrations 

Total Nitrate 

Total nitrate is the sum of HNO 

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

emitted during combustion of fossil fuels. Transportation sources (e.g., automobiles and 

trucks) are the largest contributors of NOx emissions in the United States. Nitric acid exists as 

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

ammonium nitrate and as larger particles, e.g., sodium nitrate or calcium nitrate. Figure 2-5 

provides a map of annual mean total nitrate concentrations for 2009. This chapter discusses 

data on total nitrate levels 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. The map of 2009 total nitrate concentrations shows three 

regions with mean concentrations greater than or equal to 2.5 μg/m3 . The largest region 

includes the four states of Illinois, Indiana, Ohio, and Michigan. The area around 

Arendtsville, PA (ARE128) is the second region. The ARE128 monitor measured a 

concentration of 2.6 μg/m3 . Two sites, Sequoia National Park (SEK430) and Converse 

Station (CON186), in California constitute the third region. Salamonie Reservoir, IN 

(SAL133) measured the highest concentration (3.6 μg/m3 ) in the network. 

Box plots of total nitrate levels for 1990 through 2009 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 

3.0 μg/m3 to 2.1 μg/m3 from 1990–1992 to 2007–2009, producing a 30 percent reduction in 

total nitrate. Total nitrate levels measured at the eastern reference sites declined from a mean 

value of 3.1 μg/m3 in 2000 to a mean value of 1.8 μg/m3 in 2009, which is the lowest in the 

history of CASTNET. 

 

Cranberry, NC (PNF126)
CASTNET Annual Report – 2009 27 Chapter 2: Atmospheric Concentrations 

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

3) Concentrations (μg/m3 ) for 2009 

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 2007–2009 was lower than the corresponding 

1996–1998 level, and the two mean concentrations for 2008 and 2009 were the lowest over 

the 14-year period. 

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

3) Concentrations (μg/m3 ) 

Western Reference Sites Eastern Reference Sites 

 

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

19961997199819992000200120022003200420052006200720082009

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

19901991199219931994199519961997199819992000200120022003200420052006200720082009

Concentration (μg/m3)
CASTNET Annual Report – 2009 28 Chapter 2: Atmospheric Concentrations 

Particulate Ammonium 

Particulate ammonium is formed when NH 

3 reacts with other gases and particles in the 

atmosphere including HNO 

3, particulate SO2-4 , and particulate NO3. A map of annual mean 

NH +

4 concentrations for 2009 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 about half the sites reporting values greater than or 

equal to 1.0 μg/m3 . Higher concentrations were measured in the agricultural Midwest, 

Pennsylvania, 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 

. 

 Sumatra, FL (SUM156)
CASTNET Annual Report – 2009 29 Chapter 2: Atmospheric Concentrations 

Figure 2-7 Annual Mean NH +

4 Concentrations (μg/m3 ) for 2009 

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 2007–2009. 

The 1990–1992 mean concentration was 1.8 μg/m3 , and the 2007–2009 value was 

1.2 μg/m3 , a 34 percent decline. The box plots characterizing the western sites show no 

significant change. 

Figure 2-8 Trend in Annual Mean NH +

4 Concentrations (μg/m3 ) 

 Western Reference Sites Eastern Reference Sites 

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0.0

0.1

0.2

0.3

0.4

0.5

0.6

19961997199819992000200120022003200420052006200720082009

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

19901991199219931994199519961997199819992000200120022003200420052006200720082009

Concentration (μg/m3)
CASTNET Annual Report – 2009 30 Chapter 2: Atmospheric Concentrations 

Trends in Sulfur Pollutants and Mercury Deposition 

EGUs and other sources, such as incinerators and gold mines, emit significant quantities of Hg. These 

emissions produce concentrations of Hg in the atmosphere and contribute to subsequent Hg deposition, 

which leads to contamination of sensitive ecosystems. In some ecosystems, excess sulfur contributes to 

increased Hg methylation—the transformation of Hg into MeHg, a highly toxic form of Hg associated with a 

range of adverse effects in humans and animals. EPA is currently developing air toxics emissions standards for 

power plants under Section 112 of the CAAA, which gives EPA the authority to regulate power plant Hg 

emissions by establishing performance standards or maximum achievable control technology (MACT). 

A recent study investigated if a secondary benefit of reduced Hg in precipitation accompanied the decline in 

SO 

2 and NOx emissions and the related decline in ambient sulfur and nitrogen concentrations and depositions. 

The study examined the relationships between CASTNET measurements of SO 

2 and particulate SO2-4

concentrations and NADP’s MDN measurements of concentrations of Hg in precipitation. The study also 

investigated long-term trends in the three parameters. Eighteen collocated MDN and CASTNET sites were 

identified (Figure 2g), and data sets were developed by matching weekly samples. Selected sites were divided 

into three groups consisting of eastern, western, and “other” sites (Table 2b), which were outside of or 

located on the periphery of the eastern and western groups. Analyses were performed on all sites individually 

and for each regional group. 

Figure 2g CASTNET/MDN Collocated Site Pairs 

Figure 2h depicts 2009 annual mean concentrations of Hg in precipitation from all MDN sites. Annual mean 

SO 

2 and SO2-4 concentrations for 2009 are shown in Figures 2-1 and 2-3. Figure 2h shows relatively high Hg 

concentrations in precipitation were measured in the western states and southern Florida while Figures 2-1 

and 2-3 show elevated ambient SO 

2 and SO2-4 concentrations were observed in and near the major SO 2 source 

region in the Ohio River Valley.
CASTNET Annual Report – 2009 31 Chapter 2: Atmospheric Concentrations 

Figure 2h 2009 Mercury Concentrations in Precipitation* [nanograms per liter (ng/L)] 

Three primary approaches were used. The initial approach included correlation analyses and stepwise 

regression to determine relationships between Hg (as a dependent variable) and SO2-

4 and SO 2 (as independent 

variables). Then, long-term trends in Hg, SO 

2, and SO2-4 were investigated by nonparametric trend analyses 

using the Mann-Kendall test and also by parametric trend analyses using regression analysis. Since the data 

sets were not normally distributed even after log-transformation of the data, nonparametric analysis was 

applied. Regression approaches require normally distributed data for technical validity. Nonparametric tests 

do not rely on/are insensitive to assumptions about the data distribution making them more rigorous than 

parametric tests. However, nonparametric procedures are less likely to observe a trend even if a trend exists. 

Correlation of Hg with SO 

2 and SO2-4

The first approach investigated whether Hg concentrations were correlated with either SO 

2 or SO2-4

concentrations. These analyses were performed for the entire data set and for individual stations. The results 

are summarized in Table 2b. 

Taken as a group, the eastern sites exhibited significant positive correlation between SO2-

4 and Hg but 

significant negative correlation between SO 

2 and Hg. Most of the individual sites exhibited similar 

relationships. The correlations were interpreted as significant at the 0.10 level of significance. All 10 of the 

eastern sites showed a positive correlation between Hg and SO2-

4 ; 7 of the 10 exhibited a negative correlation 

between Hg and SO 

2. The western sites positively correlated both SO2-4 and SO 2 with Hg, though results varied 

for individual sites. The “other sites” were similar to the eastern group. 
CASTNET Annual Report – 2009 32 Chapter 2: Atmospheric Concentrations 

Table 2b Correlations between LogTransformed Hg and SO2-

4 and SO 2

Site 

SO2-

4 SO 2

Sign α Sign α N 

All + <0.001 - <0.001 3140

Eastern Sites 

ACA416/ME98 + <0.001 - <0.001 375

ARE128/PA00 + <0.001 - 0.09 278

BEL116/MD99 + 0.001 96

BVL130/IL11 + <0.001 - 0.002 311

CAT175/NY68 + <0.001 - 0.02 133

CND125/NC26 + 0.01 61

HWF187/NY20 + <0.001 - <0.001 259

MAC426/KY10 + <0.001 - 0.002 234

SHN418/VA28 + <0.001 - 0.03 200

WST109/NH02 + 0.07 39

Eastern + <0.001 - <0.001 1986

Western Sites 

CON186/CA94 + 0.03 + 0.006 26

GLR468/MT05 + 0.08 109

MEV405/CO99 + <0.001 147

SEK430/CA75 + <0.001 61

YEL408/WY08 + 0.006 21

Western + <0.001 + <0.001 364

Other Sites 

CHE185/OK99 173

EGB181/ON07 + <0.001 - <0.001 300

EVE419/FL11 310

Table 2c Individual Stations, Mann- 

 Kendall, SO2-

4 Trends 

Station N P 

Significant 

trend? 

Eastern Sites 

ACA416/ME98 514 0.000 Decreasing

ARE128/PA00 411 0.002 Decreasing

BEL116/MD99 168 0.009 Decreasing

BVL130/IL11 464 0.001 Decreasing

CAT175/NY68 195 0.003 Decreasing

CND125/NC26 84 0.010 Increasing

HWF187/NY20 339 0.000 Decreasing

MAC426/KY10 323 0.337 

SHN418/VA28 292 0.751 

WST109/NH02 50 0.001 Decreasing

Western Sites 

CON186/CA94 139 0.095 Decreasing

GLR468/MT05 260 0.024 Decreasing

MEV405/CO99 310 0.806 

SEK430/CA75 179 0.044 Decreasing

YEL408/WY08 277 0.626 

Other Sites 

CHE185/OK99 84 0.013 Decreasing

EVE419/FL11 446 0.075 Decreasing

The positive correlation between Hg and SO2-

4 is of 

particular concern because SO2-

4 -reducing bacteria play a 

primary role in methylation of Hg to MeHg in the 

environment. Although atmospheric-ecosystem chemical 

reactions are complex, scientists (e.g., Jeremiason et al., 

2006; Gilmour et al., 1992; Hines and Brezonik, 2004) 

have shown through laboratory and field studies that the 

addition of SO2-

4 to experimental wetlands has increased 

MeHg concentrations.

Trends in Hg, SO 

2, and SO2-4

The Mann-Kendall test is a nonparametric statistical test 

that was used to analyze trends in the three parameters 

for individual monitoring sites as well as in the four 

groups of sites (i.e., eastern, western, other, and all sites). 

The individual results showed downward trends for SO 

2

and SO2-

4 at most of the sites and downward trends for Hg 

at two sites (HWF187/NY20, NY and WST109/NH02, NH). 

The results for the four groups showed only a downward 

trend for SO2-

4 measured at western sites. Table 2c 

summarizes the Mann-Kendall trend analyses for SO2-

4 for 

the individual monitoring sites. Seven eastern, three 

western, and two other sites exhibited statistically 

significant decreasing trends. The Mann-Kendall test also 

showed significant declines in SO 

2 concentrations at 15 of 

17 sites. 

The parametric trend analyses used regression techniques 

to gauge trends in the eastern and western subsets after 

the statistical elimination of annual cycles in the raw data. 

The results are summarized in Figure 2i and Table 2d. The 

results show that in the west, SO2-

4 increased from 2002 to 

2008 at approximately 2.8 percent/year, while in the east, 

no statistically significant trend was observed. However, 

the apparent, but not significant, trend in the east was a 

decline of 0.7 percent/year. The trend in SO2-

4

concentrations measured at the CASTNET eastern 

reference sites shown in Figure 2-4 gives a 41 percent 

reduction in SO2-

4 over 20 years. Figure 2i and Table 2d also 

show that SO 

2 decreased in both the eastern and western 

regions at a rate of 2.3 percent/year in the east and 

7.6 percent/year in the west, and that Hg declined at a rate 

of 1.1 percent/year in the east and 5.2 percent/year in 

the west. 
CASTNET Annual Report – 2009 33 Chapter 2: Atmospheric Concentrations 

Figure 2i Trends in SO2-

4 , SO 2, and Hg Table 2d Parametric Trend Results 

Summary 

Hg was positively correlated with SO2-

4 at 14 of 18 sites evaluated. Exceptions were SEK430/CA75, CA; 

YEL408/WY08, WY; EVE419/FL11, FL; and CHE185/OK99, OK. Hg was also positively correlated with SO2-

4 in the 

eastern and western groups. Overall, this study demonstrated a positive correlation between SO2-

4

concentrations and Hg in precipitation. 

Hg was negatively correlated with SO 

2 at 7 of the 10 eastern sites but was positively correlated with SO 2 in the 

west. This was related to the seasonal cycle of SO 

2 emissions near the western sites where SO 2 is higher in the 

summer months because of increased demand for electrical power. 

A strong declining trend for SO 

2 was demonstrated consistently by both the nonparametric and parametric 

analyses. No significant declining trend for SO2-

4 in either the eastern or western subsets was demonstrated by 

the parametric analyses while, in contrast, the Mann-Kendall nonparametric procedure identified significant 

decreasing trends for SO2-

4 at a number of sites. Because Hg and SO2-4 are strongly correlated, similar trends 

would be expected for Hg and SO2-

4 ; however, the trends at individual stations and within regional subsets 

were not consistent. The nonparametric procedure indicates SO2-

4 concentrations were generally declining 

while decreasing trends in Hg were observed at relatively few stations. Parametric trend analysis reveals 

significant decreases for Hg in both the western and eastern subsets, but these clear trends were not 

observed for SO2-

4 in the regional subsets. These results may indicate that geographic differences have not 

been correctly accounted for in the regional analyses. 

The trends analyses demonstrated that both Hg and SO 

2 have declined at individual monitoring stations and 

the eastern and western subsets. However, the results do not demonstrate that a secondary benefit of a 

reduction in Hg concentrations accompanied the declines in SO 

2 concentrations. 

 Region Direction 

Rate 

(%/year) 

SO2-

4

Eastern Not significant -0.7±0.5 

Western Increasing 2.8±0.8 

SO 

2

Eastern Decreasing -2.3±0.7 

Western Decreasing -7.6±1.1 

Hg Eastern Decreasing -1.1±0.6 

Western Decreasing -5.2±2.3 

0

2

4

6

8

10

12

14

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1998 2000 2002 2004 2006 2008 2010

Hg (ng/L) 

SOX (μg/m3)

Year

SO4 east SO4 west SO2 east SO2 west Hg east Hg west

Apparent SO4 east trend is not significant.

SO2-

4 east SO2-4 west SO 2 east SO 2 west
CASTNET Annual Report – 2009 34 Chapter 3: Atmospheric Deposition

Chapter 3: Atmospheric Deposition

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

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

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

Flux values are estimated as the product of measured concentration data and MLMmodeled dry deposition velocities. Wet deposition measurements were obtained from 

NADP/NTN and combined with CASTNET dry deposition data to estimate total 

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

during 2009 at the eastern reference sites. These same parameters, with the exception of 

total sulfur deposition, declined during 2009 at the western reference sites. Precipitationweighted mean concentrations of atmospheric sulfur declined over the past 20 years at the 

eastern sites, and mean concentrations of atmospheric nitrogen in precipitation have 

declined slowly since 1998 at the same sites. 

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

Meyers et al. (1998) and Finkelstein et al. (2000). The MLM was run using CASTNET filter 

pack concentrations and meteorological measurements, with information on percent leaf-out 

of nearby vegetation, LAI, and surface conditions, to 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 2009. Valid measurements of temperature, solar radiation, 

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

pollutants across the United States. CASTNET 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 vegetation within 1 kilometer (km) of each site. Dry deposition is then

calculated as the product of the modeled deposition velocity and measured pollutant

concentration. Total deposition is the sum of estimated dry deposition and measured wet

deposition. Three‐year mean total sulfur fluxes aggregated over the eastern reference sites

declined by 46 percent over the period from 1990–1992 to 2007–2009. Three‐year mean

total sulfur deposition estimated for 17 western sites declined by about a third over the

period 1996–1998 to 2007–2009. Total nitrogen deposition estimates declined 26 percent for

the eastern reference sites and 21 percent at the western reference sites. Dry deposition is

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

source regions.
CASTNET Annual Report – 2009 35 Chapter 3: Atmospheric Deposition

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. Aggregation rules 

for CASTNET require three valid quarters for the calculation of an annual value. If an 

annual value was not available for a specific site, the results were not included on the maps 

presented in this chapter. Sites with no deposition estimates are shown as dots with no value. 

For trends analyses, missing values were replaced by interpolation or extrapolation using 

valid annual estimates. 

Sulfur Deposition

The MLM was run separately for SO 

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

obtain the 2009 estimates of dry sulfur deposition (as S) shown in the map in Figure 3-1. 

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

only seven CASTNET sites in four 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, and Pennsylvania. The geographic extent of elevated deposition rates was significantly 

smaller than in 2008. The highest 2009 deposition rate was estimated for QAK172, OH with 

a flux of 5.4 kg/ha/yr compared to a flux of 8.9 kg/ha/yr in 2008. The highest dry sulfur 

deposition rates coincided with the major SO 

2 source region (Figure 1-6) and declined with 

distance. The dry deposition rates for the western sites were all less than 1.0 kg/ha/yr with 

the majority of sites less than 0.5 kg/ha/yr. 

Figure 3‐1  Dry Sulfur (SO  

2 + SO2‐4 ) Deposition (kg/ha/yr) for 2009

3.3

3.1

0.9

1.7

0.2

2.3

1.2 1.8 0.9

1.4

1.0

0.4

1.1

2.5

1.4

3.5

0.4

0.5

2.4 1.9

0.5

0.2

1.1

0.2

2.1

1.0

0.2

0.4

1.5

0.6

2.7

0.5

0.8

0.4

1.7

0.1

0.2

0.1

0.4

0.6

0.2

0.6

0.3

1.5

0.6

0.1

2.5

2.6

3.0 0.2

0.1

2.0

0.3

0.1

0.9

0.5

3.9

0.2 5.4

1.8

0.6

0.5

1.2

1.3

0.5

0.3

1.6

3.9

0.3

2.4

0.2

2.4

0.2

0.1

Site not pictured:

DEN417, AK 0.1

Deposition

6.0

3.0

0.6
CASTNET Annual Report – 2009 36 Chapter 3: Atmospheric Deposition

Wet deposition values used to estimate total deposition were based on a combination of 

historical CASTNET wet deposition data and NADP/NTN wet deposition data. For 

CASTNET sites where wet concentrations were measured prior to January 1999, those 

values were used in the data set. For sites where no wet concentrations were measured and 

for all sites after January 1999, values were obtained from a grid of concentration estimates 

derived from available NADP/NTN sites by using an inverse distance weighting function. 

Estimated concentrations were multiplied by the precipitation measured at the CASTNET 

sites to obtain estimates of wet deposition. 

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

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

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

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

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

states of Illinois, Indiana, Kentucky, Ohio, Pennsylvania, and Maryland had total (dry + 

wet) sulfur deposition fluxes greater than 7.0 kg/ha/yr. The highest total sulfur deposition 

rate was estimated for QAK172, OH with a value of 9.7 kg/ha/yr. The total sulfur 

deposition value was 17.2 kg/ha/yr for this site in 2008. Total sulfur deposition at western 

sites was less than or equal to 1.7 kg/ha/yr, which was the estimate for Centennial, WY 

(CNT169). 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 QAK172, OH to less than 20 percent at Indian River 

Lagoon, FL (IRL141). 

Figure 3‐2  Total (Dry + Wet) Sulfur Deposition (kg/ha/yr) for 2009

5.1

4.3

5.4 5.1

7.3

5.6

7.8

8.1

4.5

0.5

4.5

3.4

4.9

7.2

5.1

8.9

1.8

1.0

7.9 8.2

4.7

0.6

5.3

0.9

5.7

4.2

1.7

4.2

5.3

4.1

6.5

5.9

5.1

3.5

4.2

0.8

0.6

0.9

2.3

2.9

2.8

3.4

0.4

7.3

4.0

0.4

5.8

6.4

7.2 1.5

0.9

5.3

0.5

0.6

4.6

2.4

8.0

1.4 9.7

6.1

2.1

1.1

5.1

4.8

4.1

1.0

4.0

9.6

1.7

5.4

1.2

6.8

3.0

0.4

Site not pictured:

DEN417, AK 0.4

Total Deposition

10

5

1

Dry Deposition

Wet Deposition
CASTNET Annual Report – 2009 37 Chapter 3: Atmospheric Deposition

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

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

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

from the 34 CASTNET eastern reference sites (Figure 1-2) for the period 1990 through 2009 

and from the 17 western reference sites (Figure 1-3) for 1996 through 2009. 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 2007–2009 mean of 7.1 kg/ha/yr, a 46 percent reduction. Total sulfur deposition 

declined at the western reference sites from a 1996–1998 mean of 1.2 kg/ha/yr to a 

2007–2009 mean of 0.8 kg/ha/yr, a 31 percent reduction. 

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

Western Reference Sites Eastern Reference Sites 

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

Western Reference Sites Eastern Reference Sites 

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

0.0

0.2

0.4

0.6

0.8

19961997199819992000200120022003200420052006200720082009

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

19901991199219931994199519961997199819992000200120022003200420052006200720082009

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

19961997199819992000200120022003200420052006200720082009

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

19901991199219931994199519961997199819992000200120022003200420052006200720082009

Deposition (kg/ha/yr)
CASTNET Annual Report – 2009 38 Chapter 3: Atmospheric Deposition

Figure 3-5 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 3-5. The solid 

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

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

more representative of changes in SO 

2 emissions. The trend line for precipitation-weighted 

mean sulfur concentrations in precipitation shows a decline in 2008 and 2009 after small 

increases during the previous three years. 

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

Precipitation [milligrams per liter (mg/l)]

Eastern Reference Sites

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

2

4

6

8

10

12

14

16

19901991199219931994199519961997199819992000200120022003200420052006200720082009

Deposition (kg/ha/yr)

Concentration (mg/l)

Wet Deposition

Dry Deposition

Total Deposition

Concentration in Precipitation

Laurel Hill State Park, PA (LRL117)
CASTNET Annual Report – 2009 39 Chapter 3: Atmospheric Deposition

Trends in Pollutants in Cloud Water and a Comparison to  

Trends in Sulfate and Nitrate in Precipitation

The CASTNET Mountain Acid Deposition Program (MADPro) has been monitoring cloud water and its chemical

constituents at Clingmans Dome, TN (CLD303) in the Great Smoky Mountains National Park (Figure 3a) since

1994. High‐elevation (typically higher than 800 meters) ecosystems such as Clingmans Dome are subject to

substantial levels of acid deposition from clouds that originate in polluted areas and contain high

concentrations of dissolved acidic ions. High levels of acid deposition result from frequent cloud immersion,

orographically enhanced precipitation, high wind speeds, and the large leaf areas typical of mountain

tree species.

Figure 3a  Great Smoky Mountains National Park Monitoring Sites

Sulfur and nitrogen species concentrations and total (dry + wet) deposition fluxes have declined steadily over

the last 10 to 15 years at the lower elevation CASTNET sites in the eastern United States. Changes in cloud

water concentrations and deposition rates based on CLD303 estimates (MACTEC, 2010d) have been more

sporadic yet also show a decline.

Recently, significant reductions in sulfur and

nitrogen species have been measured at CLD303,

perhaps associated with emission reductions at

Tennessee Valley Authority (TVA) power plants

during the period from 2007 through 2009 (TVA,

2010). Mean seasonal (June through September)

reductions in SO2‐

4 and NO‐ 3 cloud water

concentrations were 60.4 percent and

49.3 percent, respectively, between 2007 and 2009

(MACTEC, 2010d). SO  

2 emissions from all TVA

plants declined by 47 percent from 2007 to 2009.

The NOx emission reduction over the same period

was 70 percent (TVA, 2010; Figure 3b).

Because of the substantial decline in both mean

SO2‐

4 and mean NO‐ 3 cloud water concentrations,

Close Up View of Cloud Collector

GRS420

TN11

CLD303
CASTNET Annual Report – 2009 40 Chapter 3: Atmospheric Deposition

precipitation concentration data were

obtained from the nearby NADP/NTN site at

Elkmont, TN (TN11) to assess whether the 2009

mean seasonal (June through September) SO2‐

4

and NO‐ 

3 precipitation concentrations exhibited

a similar reduction. The data were compared

over the same period from 2000 through 2007

and 2009.

Figures 3c and 3d show mean seasonal cloud

water and precipitation concentrations for SO2‐

4

and NO‐ 

3 , respectively. Cloud water

concentrations are plotted on the left y‐axis,

and precipitation concentrations are plotted on

the right y‐axis. No data are available for 2008

because cloud sampling was not performed

during that year. Both figures show that the

large drop in the 2009 cloud water SO2‐

4 and NO‐ 3

concentrations was mirrored by a decline in

precipitation SO2‐

4 and NO‐ 3 concentrations. The

60 percent decrease in 2009 cloud water SO2‐

4

concentrations from 2007 concentrations is

paralleled by a 55 percent decrease in

precipitation SO2‐

4 concentrations. The

49 percent decrease in 2009 cloud water NO‐ 

3

concentrations is tracked by a 40 percent

decrease in 2009 precipitation NO‐ 

3

concentrations.

SO2‐

4 and NO‐ 3 mean seasonal cloud water and

wet deposition fluxes as kilograms per hectare

(kg/ha) for 2000 through 2007 and 2009 are

shown in Figures 3e and 3f. Cloud water

deposition rates are plotted on the left y‐axis

and precipitation fluxes are plotted on the right

y‐axis. Deposition fluxes for both species

basically follow the same pattern starting in

2004. The main exception is that the

precipitation SO2‐

4 deposition value for 2009

decreased with respect to the 2007 value but

was higher than the 2008 deposition rate. The

NO‐ 

3 depositions for both cloud and

precipitation deposition show increases in 2009

with respect to the 2007 value. The increases in

the cloud water SO2‐

4 and NO‐ 3 and the wet NO‐ 3

deposition are opposite to the decrease in

seasonal concentrations of these parameters.

0

2

4

6

8

10

12

14

16

18

20

0

50

100

150

200

250

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Microequivalents/liter ‐ Precipitation Concentration

Microequivalents/liter ‐ Cloud Water Concentration

Cloud Nitrate

Precipitation

Nitrate

Figure 3d  Mean Seasonal Cloud Water and Mean

Seasonal Precipitation Nitrate

Concentrations, 2000–2009

Figure 3b  Sulfur Dioxide and Nitrogen Oxides

Emissions at TVA Power Plants

0

100

200

300

400

500

600

700

800

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Thousands of Tons

Sulfur

Dioxide

Nitrogen

Oxides

0

10

20

30

40

50

60

0

50

100

150

200

250

300

350

400

450

500

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Microequivalents/liter ‐ Precipitation Concentration

Microequivalents/liter ‐ Cloud Water Concentration

Cloud Sulfate

Precipitation

Sulfate

Figure 3c  Mean Seasonal Cloud Water and Mean

Seasonal Precipitation Sulfate

Concentrations, 2000–2009
CASTNET Annual Report – 2009 41 Chapter 3: Atmospheric Deposition

The increases in cloud water deposition for 2009

have been attributed to higher water deposition

(Lovett, 2010). Water deposition for 2009 was

9.1 centimeters per month (cm/month) versus

3.5 cm/month for 2007 (MACTEC, 2010d). The

year 2007 was characterized by drought.

Although substantial decreases were observed in

both cloud water and precipitation SO2‐

4 and NO‐ 3

concentrations since 2007, these decreases were

not accompanied by decreases in cloud water

and wet deposition. Consequently, sensitive

high‐elevation ecosystems of the Smoky

Mountains may continue to experience damage

from acid deposition and acidification because

of poorly buffered soils and other ecological

factors. Therefore, it is important to continue

cloud water collection and calculation of SO2‐

4

and NO‐ 

3 deposition fluxes at the

Clingmans Dome site.

View from the Clingmans Dome Tower Clingmans Dome Tower

0

1

2

3

4

5

6

7

0

10

20

30

40

50

60

70

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Wet Deposition (kg/ha)

Cloud Water Deposition (kg/ha)

Cloud Water

Nitrate

WetNitrate

Figure 3f  Mean Seasonal Cloud Water and Wet

Nitrate Deposition Estimates,  

2000‐2009

0

2

4

6

8

10

12

14

0

10

20

30

40

50

60

70

80

90

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Wet Deposition (kg/ha)

Cloud Water Deposition (kg/ha)

Cloud Water

Sulfate

Wet Sulfate

Figure 3e  Mean Seasonal Cloud Water and Wet

Sulfate Deposition Estimates, 2000‐2009
CASTNET Annual Report – 2009 42 Chapter 3: Atmospheric Deposition

Nitrogen Deposition

Figure 3-6 illustrates dry fluxes of nitrogen (as N) for 2009. 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. QAK172, OH and ARE128, PA were 

the only two eastern sites with deposition rates greater than 2.0 kg/ha/yr. Three monitoring 

sites in California had fluxes greater than or equal to 2.0 kg/ha/yr. The highest value was 

calculated for the CASTNET site in Sequoia National Park (SEK430) with a dry nitrogen 

flux of 2.4 kg/ha/yr. Locations on the map with no value had insufficient data to 

calculate fluxes. 

Figure 3‐6  Dry Nitrogen (HNO  

3 + NO‐ 3 + NH+4 ) Deposition (kg/ha/yr) for 2009

A map of total nitrogen deposition (as N) for 2009 is given in Figure 3-7. The map was 

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

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

greater than 5.0 kg/ha/yr. No values above 10.0 kg/ha/yr were estimated for 2009. 

Vincennes, IN (VIN140) recorded the highest total nitrogen flux (8.8 kg/ha/yr). The values 

at the western sites ranged from 0.8 kg/ha/yr at Mount Rainier National Park, WA 

(MOR409) to 4.2 kg/ha/yr in California at SEK430. The contributions of dry nitrogen 

deposition to total nitrogen were lower than the corresponding contributions of dry sulfur 

deposition. Dry nitrogen 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 

0.8 1.3 1.5

1.1

1.3

0.4 1.9

1.0

1.7

2.0

1.1

0.4

1.2

1.4

1.3

2.2

0.3

0.8

1.5 1.3

0.5

0.7

1.3

0.6

1.4

1.1

0.5

0.4

1.0

0.8

1.8

0.8

0.7

0.4

1.2

0.2

0.7

0.3

0.4

0.7

0.2

0.5

2.1

0.8

1.3

0.3

0.8

1.8

1.7 0.9

0.2

0.9

0.9

0.3

0.9

0.8

1.4

0.6 2.1

1.9

1.1

2.4

0.9

1.5

0.4

0.6

1.0

1.8

0.4

1.4

0.8

1.6

0.1

0.6

Site not pictured:

DEN417, AK 0.1

Deposition

3.0

1.5

0.3
CASTNET Annual Report – 2009 43 Chapter 3: Atmospheric Deposition

nitrogen deposition at four of five monitoring sites in California, a region with elevated 

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

85 percent of the total nitrogen deposition at Joshua Tree National Park, CA (JOT403). 

Figure 3‐7  Total (Dry + Wet) Nitrogen Deposition (kg/ha/yr) for 2009

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

sites over the period 1990 through 2009 (right side) and 17 western reference sites for 1996 

through 2009 (left side). The box plots show the trends in dry nitrogen deposition (as N). 

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

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

1.5 kg/ha/yr for 2007–2009, a 32 percent reduction over the 20 years. The box plots for the 

western sites show a smaller 20 percent 

decline, a change from a mean of 

0.9 kg/ha/yr for 1996–1998 to 

0.7 kg/ha/yr for 2007–2009. 

Total Deposition

10

5

1

Dry Deposition

Wet Deposition

5.8

3.4

4.2

4.9 5.5

7.6 5.7

4.6

3.1

4.9

2.9

5.2

6.8

6.3

7.9

1.8

1.8

7.6 6.9

5.1

1.6

5.9

2.0

5.6

4.9

3.1

4.3

5.6

5.6

5.4

6.3

5.0

3.9

3.8

1.7

1.6

1.7

2.5

4.3

3.4

2.9

2.5

5.0

7.4

1.0

3.6 6.2

5.9 2.3

0.8

6.6 3.8

1.1

1.3

4.6

4.9

5.3

3.0 5.2 7.4

5.6

4.2

5.0

7.7

4.1

2.2

4.1

8.8

3.6

4.9

3.5

6.1

3.3

1.5

Site not pictured:

DEN417, AK 0.2

Rocky Mountain National Park, CO (ROM206)
CASTNET Annual Report – 2009 44 Chapter 3: Atmospheric Deposition

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

Western Reference Sites Eastern Reference Sites 

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

(right side) and western (left side) reference sites. Total nitrogen deposition aggregated over 

the eastern sites is more variable because the annual wet and total fluxes depend on the 

amount of precipitation. The figure suggests that total flux in the east 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. Estimates of trends in wet, dry, and total deposition 

of atmospheric nitrogen (as N) are presented in Figure 3-10 for the eastern reference sites 

only. The trend line (dotted) for precipitation-weighted mean nitrogen concentrations in 

precipitation shows a slow downward trend since 1998 with a drop in 2009. 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 26 percent in the East 

over the 20 years and 21 percent for the aggregated western sites over the 14-year period. 

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

Western Reference Sites Eastern Reference Sites 

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

19961997199819992000200120022003200420052006200720082009

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

19901991199219931994199519961997199819992000200120022003200420052006200720082009

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

19961997199819992000200120022003200420052006200720082009

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

19901991199219931994199519961997199819992000200120022003200420052006200720082009

Deposition (kg/ha/yr)
CASTNET Annual Report – 2009 45 Chapter 3: Atmospheric Deposition

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

Precipitation (mg/l)

 Eastern Reference Sites

Estimates of Critical Loads for the Shenandoah Region

The mid‐Appalachian Mountains in the eastern United States have been strongly impacted by

acidic deposition. Many of the small forested watersheds, particularly on ridge and mountaintops, have soils

and surface waters that are unable to buffer the acidity from pollutant deposition, causing the ecosystems to

acidify. As a result, the health of some forest trees and aquatic biota has declined, and some species, such as

brook trout, have been lost in the small streams in these watersheds.

The critical load approach is an emerging assessment tool that can be used to determine the degree to which

acidic deposition may be affecting ecological health. 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). 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. A critical load exceedance is the measure of

pollutant exposure above the critical load and indicates the ecosystem is exposed to damaging levels of

pollutants.

Building on previous surface water modeling of this region (EPA, 2008a), this critical load study examines

22 streams in the Shenandoah National Park and George, Jefferson, and Monongahela National Forests. Three

CASTNET monitoring sites, Parsons, WV (PAR107); Shenandoah National Park, VA – Big Meadows (SHN418);

and Horton Station, VA (VPI120), are located near the streams. For this particular analysis, the critical load

represents the deposition load of sulfur to which a stream could be subjected and still have an acid

neutralizing capacity (ANC) of 50 microequivalents per liter or higher, which would support a healthy aquatic

ecosystem. Critical loads of total sulfur deposition are expressed in terms of ionic charge balance as

milliequivalents per square meter per year.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0

2

4

6

8

10

12

19901991199219931994199519961997199819992000200120022003200420052006200720082009

Deposition (kg/ha/yr)

Concentration (mg/l)

Wet Deposition

Dry Deposition

Total Deposition

Concentration in Precipitation
CASTNET Annual Report – 2009 46 Chapter 3: Atmospheric Deposition

Critical loads were calculated for the 22 streams using the Steady‐State Water Chemistry Model (Dupont et al.,

2005). Catchment weathering rates were derived from the Model of Acidification of Groundwater in

Catchments (MAGIC; Cosby et al., 1985). CASTNET (dry) and NADP/NTN (wet) deposition data were used to

determine total base cation, Cl‐

, and sulfur deposition each watershed received. Dry deposition was estimated

from data from the CASTNET sites closest to the streams. Wet deposition was estimated from NADP/NTN data

collected at sites within 0.2 km of the three CASTNET sites. The 22 streams for which critical loads were

calculated represent small trout streams that are highly sensitive to acidic deposition and should not be

considered characteristic of all streams across this region (Sullivan et al., 2008).

For the period from 2006 through 2008, 60 percent of the modeled streams received levels of sulfur

deposition that exceeded the critical load (Figure 3g). While reductions in acidic deposition over the past

decade have helped to improve water quality and support healthy aquatic ecosystems, many of the highly

sensitive trout streams in the mid‐Appalachian Mountains are still at risk from acidification at current

deposition levels.

Figure 3g  Stream Exceedances of Modeled Critical Loads for Total Sulfur Deposition for the

Period 2006–2008

Source: EPA Clean Air Markets Division

Critical Load Exceedances  

2006–2008
CASTNET Annual Report – 2009 47 Chapter 4: Ozone Concentrations 

Chapter 4: Ozone Concentrations 

All but three CASTNET 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 O 3

concentrations in context of the O 

3 NAAQS. The analyses presented in this chapter provide 

maps of 8-hour average O 

3 concentrations and W126 values and examine trends in the 

annual fourth highest daily maximum 8-hour average O 

3 concentrations using data derived 

from O 

3 concentration measurements at CASTNET sites. The concentration shading for the 

figures used in this chapter was prepared using the scales shown on each map. 

Additional maps of O 

3 concentrations can be viewed on the Web site for the NPS Air Atlas 

(http://science.nature.nps.gov/AirAtlas/AirAtlas0105/viewer.htm). The Air Atlas 

incorporates all O 

3 monitoring reported to AQS plus the CASTNET O 3 measurements. 

Ground-level O 

3 is formed in the lower atmosphere when volatile organic compounds 

(VOCs) and NOx react in the presence of sunlight and is the main component of smog. It is 

an air pollutant that can cause a number of human respiratory effects and damage to 

vegetation and ecosystems. 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 is mainly a summertime pollutant because sunlight and hot weather 

accelerate its formation. O 

3 concentrations typically increase during daylight hours and peak 

in the late afternoon after temperature and sunlight intensity have peaked. Concentrations 

typically fall in the evening although many rural sites measure peak O 

3 concentrations after 

sunset. O 

3 concentrations are generally higher in the warmer months but can vary by region 

of country. 

CASTNET is the principal network 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. Even so, the 

network provides data that can be evaluated in the context of the ozone NAAQS levels. In 

this report, O 

3 measurements collected during 2007–2009 are evaluated with respect to both 

the 1997 O 

3 standard of 0.08 parts per million (ppm) and the 2008 standard of 0.075 ppm. 

On January 6, 2010, EPA initiated a revision of the NAAQS for O 

3. EPA proposed 

lowering the primary standard to a level within the range of 0.060 to 0.070 ppm and 

establishing a secondary standard based on the measure W126 within a range of 

7 to 15 ppm-hours. EPA plans to promulgate final standards by July 2011.
CASTNET Annual Report – 2009 48 Chapter 4: Ozone Concentrations 

The 8-hour average O 

3 standard can be used to assess the status and trends in rural O 3 levels 

in order to gauge the success of EPA emission reduction programs such as the NOx SIP 

Call/NBP and now CAIR. On May 1, 2009, the CAIR NOx O 

3 season trading program 

began, replacing the NBP in states covered by the new program and requiring further NOx

emission reductions from the power sector. Implementation of the NOx emission reductions 

that were mandated by the ARP and other NOx emission control programs have contributed 

to reductions in O 

3 concentrations, especially in rural areas with elevated 8-hour average 

levels, because regional transport of pollutants contributes to O 

3 formation.VOC emission 

reduction strategies also have been successful in reducing higher, short-term O 

3

concentrations in and downwind of urban areas. 

The current O 

3 NAAQS are summarized in the box on the following page. On January 6, 

2010, EPA proposed revising the NAAQS for O 

3 by lowering the primary standard to a level 

between 0.060 and 0.070 ppm based on the 3-year average of the fourth highest daily 

maximum 8-hour average concentration. The primary standard is designed to protect public 

health, including the health of at-risk populations. EPA also proposed establishing a 

secondary standard based on the measure W126 within a range of 7 to 15 ppm-hours. The 

secondary standard is designed to protect public welfare and the environment, including 

sensitive vegetation and ecosystems. W126 is a cumulative metric that sums weighted 

hourly O 

3 concentrations during daylight 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. EPA plans to promulgate final standards by 

the end of July 2011.

Unionville, MI (UVL124)
CASTNET Annual Report – 2009 49 Chapter 4: Ozone Concentrations 

National Ambient Air Quality Standards for Ozone 

Primary Standards Secondary Standards 

Level Averaging Time Level Averaging Time 

Ozone1 (Current) 0.075 ppm 8-hour2

 0.075 ppm 8-hour2

0.08 ppm 8-hour3 0.08 ppm 8-hour3

Notes: 1 On January 6, 2010, EPA announced that it is revising the O 

3 primary and secondary standards 

(EPA, 2010c). 

2

 To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour average 

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). 

3

 (a) To attain this standard, the 3-year average of the fourth-highest daily maximum 8-hour 

 average O 

3 concentrations measured at each monitor within an area must not exceed 

 0.08 ppm (EPA, 1997) or 84 ppb in practice (effective 1997). 

 (b) The 1997 standard—and the implementation rules for the standard—will remain in place for 

implementation purposes as EPA undertakes rulemaking to address the transition to the revised 

O 

3 standard. 

Eight-Hour Ozone Concentrations 

Figure 4-1 presents 3-year averages of the fourth highest daily maximum 8-hour average O 

3

concentrations for 2007–2009. During this period, 3-year averages of the fourth highest daily 

maximum 8-hour average O 

3 concentrations were greater than or equal to 85 ppb at four 

sites in California, including Yosemite National Park (YOS404), Sequoia National Park 

(SEK430), Converse Station (CON186), and Joshua Tree National Park (JOT403). 

Four eastern and five California CASTNET sites measured concentrations above the 

2008 NAAQS of 0.075 ppm. The four eastern sites include Washington’s Crossing, NJ 

(WSP144); Beltsville, MD (BEL116); Blackwater National Wildlife Refuge, MD 

(BWR139); and Great Smoky Mountains National Park, TN (GRS420). The California 

sites include Pinnacles National Monument (PIN414) in addition to the four sites listed 

previously. Three-year average concentrations were generally lower in 2007–2009 than in 

2006–2008. 

The 2007–2009 3-year average of fourth highest daily maximum 8-hour average 

O 

3 concentrations constitute the current design values for achieving the 8-hour O 3 NAAQS. 

For example, the value of 107 ppb at Converse Station, CA (CON186) 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. 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 are typically used to classify 

nonattainment areas, assess progress towards meeting the NAAQS, and develop control 

strategies to achieve the NAAQS. Design values change as each new 3-year database of 

monitored O 

3 concentrations become available.
CASTNET Annual Report – 2009 50 Chapter 4: Ozone Concentrations 

Figure 4-1 Three-year Average of Fourth Highest Daily Maximum 8-Hour Average 

 Ozone Concentrations (ppb) for 2007–2009 

Figure 4-2 Three-year Average of Fourth Highest Daily Maximum 8-Hour Average 

 Ozone Concentrations (ppb) for 2000–2002 
CASTNET Annual Report – 2009 51 Chapter 4: Ozone Concentrations 

Three-year averages of the fourth highest daily maximum 8-hour average O 

3 concentrations 

for 2000–2002 are presented in Figure 4-2 for comparison to 2007–2009 design values. The 

period 2000–2002 was selected because the NOx SIP Call/NBP began in 2003 for the 

eastern United States. The O 

3 data for these two 3-year periods illustrates the effectiveness of 

the emission reduction programs. Based on the 1997 NAAQS level (84 ppb), 19 eastern and 

3 California sites recorded 3-year averages greater than or equal to 85 ppb during 2000–

2002. The regions with elevated 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. Using the 2008 standard level (75 ppb), 

the map in Figure 4-2 shows that most of the eastern sites measured O 

3 concentrations 

greater than 75 ppb. Five monitors in California and the site at Rocky Mountain National 

Park (ROM406) also recorded concentrations greater than 75 ppb. The period 2007–2009 

represents a significant improvement in air quality and is one of the best periods of air 

quality since the inception of CASTNET in terms of having the fewest sites with 

exceedances of the 8-hour O 

3 NAAQS. 

Measurements of 8-hour average O 

3 concentrations during 2009 (Figure 4-3) were lower 

than 2008 and considerably lower than concentrations measured during the period 

2000–2002. No eastern CASTNET sites measured concentrations above 75 ppb. Four sites 

in California measured 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, CA measured the highest value 

(108 ppb) in the network during 2009. The year 2009 is the first year when no eastern 

CASTNET sites measured 8-hour average O 

3 concentrations greater than 75 ppb. 

Laurel Hill State Park, PA (LRL117)
CASTNET Annual Report – 2009 52 Chapter 4: Ozone Concentrations 

Figure 4-3 Fourth Highest Daily Maximum 8-Hour Average Ozone Concentrations 

 (ppb) for 2009 

Figure 4-4 provides box plots depicting trends in fourth highest daily maximum 8-hour 

average 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 a reduction since 2002. 

The mean aggregated value (64 ppb) for the eastern reference sites for 2009 was the lowest 

in the history of the network. It represents 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 2007–2009 average of the fourth highest daily maximum 8-hour average 

O 

3 concentrations for the western reference sites was 70 ppb. 

Figure 4-4 Trend in Fourth Highest Daily Maximum 8-Hour Average Ozone 

 Concentrations (ppb) 

Western Reference Sites Eastern Reference Sites 

50

70

90

110

19961997199819992000200120022003200420052006200720082009

Concentration (ppb)

50

70

90

110

19901991199219931994199519961997199819992000200120022003200420052006200720082009

Concentration (ppb)
CASTNET Annual Report – 2009 53 Chapter 4: Ozone Concentrations 

W126 Values for 2009 

The W126 O 

3 exposure index is a biologically-based cumulative exposure index (Lefohn 

and Runeckles, 1987) that was described and used in the EPA’s Ozone Criteria Documents 

(EPA, 1996; 2006) for both characterizing O 

3 trends and relating vegetation yield reduction 

losses with O 

3 exposure. W126 is a seasonal metric derived 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, 2010c). Many researchers (e.g., Musselman et al., 2006) concluded that both peak 

concentrations and cumulative effects are important in relating vegetation damage to 

atmospheric O 

3. W126 characterizes 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 

 Where: 

 n = number of hours with valid O 

3 concentrations. 

The exposure unit is ppm-hours. For this report, W126 exposures were calculated using O 

3

values measured during the 12 hours from 8:00 a.m. to 8:00 p.m. in the months of May 

through September of each year. 

CASTNET hourly O 

3 concentrations were used to calculate W126 values for 2009. First, 

the daily index value for each site was calculated. The daily index values were then used to 

calculate the monthly index value 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. 

)126.0exp(44031

1

i

i

c

w

+ − =

∑=

=

n

i

ii W cw

1

126
CASTNET Annual Report – 2009 54 Chapter 4: Ozone Concentrations 

W126 levels for 2009 are presented in Figure 4-5. The higher W126 values were measured 

in California and at western sites in high terrain. O 

3 concentrations measured at high 

elevation sites (MACTEC, 2003) typically show a flat diurnal pattern with little variability 

from hour to hour because dry deposition and scavenging of O 

3 by nitric oxide are low at 

night. Nighttime dry deposition is low because shallow boundary layers typically do not 

form at elevated sites, and scavenging is low because little fresh NO 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 2009 

Changes in Ozone Design Values 

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 are typically used to classify nonattainment areas, assess progress towards meeting the 

NAAQS, and develop control strategies to achieve the NAAQS. Because the NAAQS for O 

3 are written in terms 

of the 3-year average of the fourth highest daily maximum 8-hour average concentrations, design values 

change as each new 3-year database of monitored O 

3 concentrations becomes available. 

CASTNET was not designed to operate as a network for demonstration of compliance with O 

3 NAAQS. 

However, the network provides data that can be evaluated in the context of the O 

3 NAAQS, and the data can 

be used to provide reasonable estimates of design values. 

Figures 4-1 and 4-2 in this chapter provide maps of design values for 2007–2009 and 2000–2002, respectively. 

For example, the design value for Beltsville, MD (BEL116) for 2007–2009 was 79 ppb. This value would have to 

be reduced to 75 ppb to achieve the level of the 2008 standard. 

No sites in the East measured design values greater than or equal to 85 ppb in 2009 or 2007–2009 though four 

sites had values above 75 ppb. Figure 4a provides a time series of annual design values from 2000–2002 

through 2007–2009 for three eastern CASTNET sites, which are Abington, CT (ABT147); Beaufort, NC (BFT142); 
CASTNET Annual Report – 2009 55 Chapter 4: Ozone Concentrations 

and Bondville, IL (BVL130), and Figure 4b shows design values for three western CASTNET sites, Grand Canyon 

National Park, AZ (GRC474); Sequoia National Park, CA (SEK430); and Converse Station, CA (CON186). These 

two figures demonstrate the progress made toward achieving the 2008 NAAQS level of 0.08 ppm (84 ppb in 

practice). ABT147, CT began the 10-year period with a 3-year design value of 99 ppb. The 2007–2009 value 

was 76 ppb. The other two eastern sites in Figure 4a are below the 2008 NAAQS level. Figure 4a shows design 

values above 100 ppb for the two California sites. The design value measured at CON186, CA decreased from 

128 ppb in 2001–2003 to 107 ppb in 2007–2009. The site at the Grand Canyon shows a level below the 

2008 standard. 

Figure 4a Trends in Ozone Design Values

Western Sites Eastern Sites 

Figure 4b was constructed by taking the difference (in ppb) between the design values at each CASTNET site 

from 2000–2002 to 2007–2009. Negative values indicate an improvement in O 

3 air quality. The map shows 

widespread reduction in O 

3 concentrations in the eastern United States. In fact, every eastern site shows a 

reduction ranging from -2 to -25 ppb. The figure shows a mixture of some increasing and many decreasing O 

3

values at the western sites. 

Figure 4b Change (ppb) in Ozone Design Values 2000–2002 to 2007–2009 

60

80

100

120

140

160

2000-022001-032002-042003-052004-062005-072006-082007-09

Concentration (ppb)

CON186, CA

GRC474, AZ

SEK402/430, CA

84 ppb

75 ppb

60

80

100

120

140

160

2000-022001-032002-042003-052004-062005-072006-082007-09

Concentration (ppb)

ABT147, CT

BFT142, NC

BVL130, IL

84 ppb

75 ppb
CASTNET Annual Report – 2009 56 Chapter 5: Data Quality 

Chapter 5: Data 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 2009 QA 

program elements are documented in the CASTNET Quality Assurance Project Plan 

(QAPP), Revision 4.1 (MACTEC, 2008). 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 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 2009 data were analyzed and compared with historical 

data collected during the period 1990 through 2008. The information in this report is 

supplemented by analyses that are discussed in quarterly CASTNET Quality Assurance 

Reports (MACTEC, 2009b; 2009d; 2009e; 2010b). 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 mean absolute relative percent 

difference (MARPD). Table 5-1 lists the precision and accuracy criteria for laboratory filter 

pack measurements. Precision DQI criteria for filter pack concentrations changed during 

third quarter 2009. Previously, DQI criteria required a MARPD of 5.0 percent for all 

collocated CASTNET filter pack measurements, except for NH +

4 , which had a criterion of 

10.0 percent MARPD. Beginning on August 11, 2009, the DQI criteria were changed to a 

Data quality indicators (DQI) such as precision, accuracy, and completeness are used to 

assess CASTNET measurements and supporting activities. Routine assessment helps 

guarantee the production of high-quality information to meet project objectives. 

Measurements taken during 2009 and historical data collected over the period 1990–2008 

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.
CASTNET Annual Report – 2009 57 Chapter 5: Data Quality 

single MARPD value of 20.0 percent. Since the new criterion became effective during the 

middle of the third quarter, this report uses the old criteria to evaluate filter pack precision. 

Figure 5-1 Historical and 2009 Precision Data for Atmospheric Concentrations 

Table 5-1 Data Quality Indicator Criteria for CASTNET Laboratory Measurements 

Through 

08-10-09 

Precision1

Beginning

08-11-09 

Precision1 Accuracy2

Nominal 

Reporting Limits 

Analyte Medium Method (MARPD) (MARPD) (%) mg/l μg/Filter 

Ammonium (NH +

4 ) F AC 10 20 90 – 110 0.020*

 0.5 

Sodium (Na+ ) F ICP-AES 5 20 95 – 105 0.005 0.125 

Potassium (K+ ) F ICP-AES 5 20 95 – 105 0.006 0.15 

Magnesium (Mg2+) F ICP-AES 5 20 95 – 105 0.003 0.075 

Calcium (Ca2+) F ICP-AES 5 20 95 – 105 0.006 0.15 

Chloride (Cl-) F IC 5 20 95 – 105 0.020 0.5 

Nitrate (NO3) F IC 5 20 95 – 105 0.008*

 0.2 

Sulfate (SO2-

4 ) F IC 5 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. 

F = filter pack samples

AC = automated colorimetry

ICP-AES = inductively coupled plasma-atomic emission spectrometry

IC = ion chromatography

MARPD = mean absolute relative percent difference

L = liter 

μg/Filter = micrograms per filter

* = as nitrogen 

0

5

10

15

20

25

30

12345

MARPD

Overall - 1990 through 2008

Mackville, KY - 2009

Rocky Mtn, CO - 2009

SO2 SO4

2- HNO3 NO3- NH4+
CASTNET Annual Report – 2009 58 Chapter 5: Data Quality 

Historical (1990 through 2008) data for all 11 collocated site pairs operated over the 

history of the network and the 2009 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 Cl-

 are 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 for SO2-

4 met the 5 percent precision criterion for 

the CASTNET filter pack measurements shown in Table 5-1. The historical results for SO 

2

and HNO 

3 were above the 5 percent criterion but are considered reasonable. The results for 

NH +

4 met the goal of 10 percent. The results for NO3 were significantly above the 5 percent 

goal. Historically, the precision of NO3 measurements has been consistently worse than for 

the other analytes, possibly because NO3 and HNO 

3 concentrations include sampling 

artifacts. The historical precision data met the new DQI criterion of 20 percent MARPD. 

MARPD values were not calculated for SO 

2, HNO 3, and total NO3 for the fourth quarter for 

MCK131/231. These data are labeled “NC” in Table 5-2. Beginning on October 20, 2009, 

with approval from EPA, MACTEC changed the analysis protocol for the nylon filter 

fraction analyses for the remainder of 2009 to perform a short study. Samples for weeks 

42 through 52 were extracted using the Teflon filter extraction fluid (deionized water) and 

included automated colorimetry (AC) analyses for NH 

3 + NH +4 (as N) in addition to the 

regular IC analyses for SO 

2 as SO2-4 and HNO 3 as N. Because of this change in extraction 

fluid, the three parameters were invalidated and precision was not calculated. 

The 2009 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. Three parameters (SO 

2, SO2-4 , and NH +4 ) met the MARPD precision criteria. The 

results for HNO 

3 were above the 5 percent criterion but are considered reasonable. The 2009 

data for MCK131/231 met the new criterion of 20 percent. The 2009 results for 

ROM406/206 showed better precision than historical results for SO2-

4 , HNO 3, and NH +4 . The 

results for SO2-

4 and NH +4 met their respective DQI criteria. The 5 percent precision criterion 

for NO3 was not met at either site during 2009. All the parameters met the new 20 percent 

criterion. The MARPD results were higher for ROM406/206 because of lower 

concentrations measured at this site. Overall, the filter pack precision results for 2009 were 

about the same as the 2008 results. 

Historical and 2009 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 did 

not meet the DQI criterion of 5 percent. The 2009 precision results show that Na+ and Clmet the 5 percent precision criterion at MCK131/231, and Cl-

 met the 5 percent MARPD 

criterion at ROM406/206. The overall results for four of five ions were reasonable, and they 

met the new DQI criterion of 20 percent MARPD. 
CASTNET Annual Report – 2009 59 Chapter 5: Data Quality 

The MARPD values for K+ exceeded the former and new DQI criteria at both sites. Several 

pairs of K+ concentrations were different by as much as a factor of 2.0 during the year. The 

poor results are currently being investigated. The 2009 precision results were about the same 

as for 2008. 

Table 5-2 summarizes 2009 precision results by quarter for the two sets of collocated sites. 

See the 2009 Quarterly Data Reports (MACTEC, 2009a; 2009c; 2010a; 2010c) and QA 

Quarterly Reports (MACTEC, 2009b; 2009d; 2009e; 2010b) for discussions of quarterly 

precision data. 

Figure 5-2 Historical and 2009 Precision Data for Cation and Cl- Concentrations 

Table 5-2 Collocated Precision Results (MARPD) for 2009 Filter Pack Data by Quarter 

Site Pairs SO2-

4 NO3 NH +4 Ca2+ Mg2+ Na+ K+ ClHNO 3 SO 2 Total NO3

 MCK131/231, KY 

Quarter 1 1.80 3.77 1.65 8.84 4.97 2.89 14.76 5.79 2.95 1.78 2.78 

Quarter 2 2.52 9.89 2.49 6.17 6.62 3.53 6.09 1.59 4.50 3.23 5.54 

Quarter 3 1.44 10.47 1.75 5.81 6.54 3.35 22.95 0.25 6.41 6.99 5.52 

Quarter 4 2.69 6.61 2.88 8.07 6.96 5.89 24.59 2.72 NC NC NC 

2009 2.11 7.68 2.19 7.22 6.27 3.92 17.10 2.59 4.62 4.00 4.61 

 ROM406/ROM206, CO 

Quarter 1 4.15 15.16 3.57 9.42 6.55 9.57 16.43 2.12 5.96 8.61 5.61 

Quarter 2 5.29 9.69 5.35 5.08 6.82 5.47 17.36 5.51 7.16 16.06 4.94 

Quarter 3 1.92 11.89 3.90 8.21 8.07 5.09 21.47 5.85 4.95 3.54 3.34 

Quarter 4 6.01 21.83 6.80 12.03 9.86 8.26 46.70 4.20 9.62 8.31 10.61 

2009 4.34 14.64 4.90 8.68 7.82 7.09 25.49 4.42 6.92 9.13 6.12 

0

5

10

15

20

25

30

12345

Overall - 2000 through 2008

Mackville, KY - 2009

Rocky Mtn, CO - 2009

MARPD

Ca2+ K+ Mg2+ Na+ Cl-
CASTNET Annual Report – 2009 60 Chapter 5: Data Quality 

The 2009 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 

5 percent measurement criterion listed in Table 5-1. 

Figure 5-3 Precision Results for Laboratory Replicate Samples (2009) 

Ozone Concentrations 

CASTNET QA procedures for the EPA-sponsored O 

3 analyzers are different from the 

EPA QA requirements for State and Local Monitoring Stations (SLAMS) monitoring 

(EPA, 2010b; 2008b). The operation of the collocated EPA and NPS O 

3 analyzers at 

ROM406/206, CO 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, KY and ROM406/206, CO during 2009. 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 2009. The data show the historical and 2009 results met the 

10 percent criterion. 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 – 2009 61 Chapter 5: Data Quality 

Table 5-3 Data Quality Indicator Criteria for CASTNET Field Measurements 

Measurement Criteria*

Parameter Method MAD/MARPD Precision Accuracy 

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 

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†

Ambient 

Temperature Platinum RTD MAD ± 1.0°C ± 0.5°C 

Delta Temperature Platinum RTD MAD ± 0.5°C ± 0.5°C 

Surface Wetness Conductivity Bridge MAD Undefined Undefined 

Ozone UV Absorbance MARPD ± 10% (of reading) ± 10% 

Filter Pack Flow Mass Flow Controller MARPD ± 10% ± 5% 

Note: Mean absolute difference (MAD) is the precision measure for difference criteria such as wind speed. 

 MARPD is the precision measure for percentage criteria such as relative humidity. 

 °C = degrees Celsius 

 m/s = meters per second 

 RTD = resistance-temperature device 

 UV = ultraviolet * 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 2009 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 2008

Mackville, KY - 2009

Rocky Mtn, CO - 2009

MARPD
CASTNET Annual Report – 2009 62 Chapter 5: Data Quality 

Table 5-4 Collocated Precision Results (MARPD) for 2009 Ozone Concentrations 

 by Quarter 

Sampling Period MCK131/231, KY ROM406/206, CO 

Quarter 1 4.47 3.28 

Quarter 2 1.40 3.92 

Quarter 3 1.13 4.90 

Quarter 4 1.38 7.09 

2009 2.09 4.80 

Historical Precision of Daily Maximum 8-Hour Average O 

3 Concentrations 

Precision is a DQI for CASTNET continuous measurements including O 

3 values. Precision is calculated as the 

MARPD of hourly concentration data from collocated sites. The O 

3 precision goal is a MARPD of less than or 

equal to 10 percent. CASTNET sites at Mackville, KY (MCK131/231) and Rocky Mountain National Park, CO 

(ROM406/206) currently host collocated O 

3 analyzers. Nine other CASTNET sites have run collocated O 3

analyzers over the history of the network. MCK131/231 has operated collocated analyzers since 1992 and 

ROM406/206 since 2001. MCK131 is considered the primary site of the MCK131/231 pair. ROM406 is 

considered the primary site of the Rocky Mountain pair and is operated on behalf of NPS. The two sites at 

Rocky Mountain National Park are operated independently. ROM206 is operated on behalf of EPA. 

The two Rocky Mountain sites are served by different site operators and calibration technicians, have 

operated different instruments, and currently have different sampling configurations. Until recently, ROM206 

did not meet the EPA SLAMS monitoring requirements, as described in 40 CFR Part 58, Appendix A 

(EPA, 2010b). ROM406 met the site configuration requirements 

but not all of the contemporaneous operational requirements 

including frequency of calibrations. In any event, the operation 

of the collocated NPS and EPA O 

3 analyzers at ROM406/206 

provides an opportunity to evaluate the precision of 

independent systems. A discussion of the precision of 2009 

hourly O 

3 concentrations is presented in this chapter. Both sites 

met the 10 percent precision goal for each quarter of 2009 

(Table 5-4). Another useful measure of precision is the MARPD 

of daily maximum 8-hour average (DM8A) O 

3 concentrations 

since these data are used to gauge compliance with the NAAQS. 

The NAAQS for O 

3 are written in terms of the 3-year average of 

the fourth-highest daily maximum 8-hour average O 

3

concentration. Table 5a presents MARPD values based on DM8A 

O 

3 levels for the two collocated sites over the period 2002 

through 2009. The MARPD values were within 10 percent except 

for the 2007 ROM406/206 results. The cause of the problems in 

2007 was not determined. During 2007, zero/span/ precision 

(z/s/p) checks and calibration results were acceptable. Audit 

results from an independent audit were also acceptable, 

although the problems began shortly after completion of the 

site audit. 

Table 5a Ozone MARPD Values Based 

on DM8A Levels 

Year x y MARPD

MCK131/231, KY

2002 43.7 44.5 3.3

2003 41.9 41.5 1.5

2004 40.2 40.3 1.3

2005 43.7 43.5 2.0

2006 47.9 45.8 5.8

2007 47.4 47.1 3.4

2008 39.8 39.2 6.4

2009 40.0 39.7 1.9

ROM406/206, CO 

2002 56.4 53.5 5.1

2003 54.5 51.0 6.4

2004 52.0 48.0 8.1

2005 49.9 50.0 5.9

2006 53.7 51.0 6.4

2007 53.0 47.8 10.8

2008 51.5 51.1 1.8

2009 51.2 48.3 5.8

Note: x is the mean DM8A value for the primary 

site and y for the collocated site. 
CASTNET Annual Report – 2009 63 Chapter 5: Data Quality 

Table 5b provides overall MARPD values based on hourly O 

3 concentrations for every collocated monitoring 

site over the history of the network. The MARPD values for the entire period for MCK131/231 and 

ROM406/206 are 1.3 percent and 3.5 percent. 

Table 5b Overall Ozone MARPD Values for all Collocated Sites 

Site 

Annual 

MARPD 

Years

Collocated Site 

Annual 

MARPD 

Years

Collocated 

ALH157/257, IL 0.7 1 MCK131/231, KY* 1.3 17

ARE128/228, PA 1.2 6 PAR107/207, WV 0.4 1

ASH135/235, ME 1.5 7 RCK163/263, ID 0.9 3

CHA167/267, AZ 0.9 3 ROM406/206, CO* 3.5 9

DCP114/214, OH 2.4 3 SUM156/256, FL 2.7 4

GAS153/253, GA 0.6 1

Note: *Currently active collocated sites 

Another way of evaluating the collocated data is to 

compare fourth-highest DM8A O 

3 concentrations for 

both collocated sites for each year. Figure 5a presents 

these results for MCK131/231 for 2000 through 2009. 

Figure 5b presents the results for ROM406/206 for 

2001 through 2009. The figures also depict the 2008 

and 1997 NAAQS of 75 ppb and 85 ppb. The results 

from the two analyzers at MCK131/231 track well. In 

particular, both analyzers showed NAAQS 

exceedances for the same years. The worst years for 

precision were 2006 and 2008 when data 

completeness was lower than normal. 

Although the fourth highest values for the 

ROM406/206 site pair were generally close, the 

results from ROM206 were generally lower. At times 

ROM406 showed exceedances, and ROM206 did not. 

For example, ROM406 measured a fourth-highest 

DM8A value of 76 ppb in 2006, and ROM206 recorded 

73 ppb. These results led to the analysis of the 

instruments, sampling systems, and data processing 

procedures at the two Rocky Mountain monitoring 

sites. Historically, ROM206 was not operated for 

regulatory monitoring purposes and did not meet 

SLAMS requirements. The ROM206 system did not 

include a second O 

3 analyzer as a transfer standard. 

In 2010, the O 

3 sampling system at ROM206 was 

replaced with a system that uses a separate site 

analyzer and site transfer standard and is 

otherwise configured to comply with 40 CFR Part 

58 requirements.

Figure 5a Fourth Highest Daily Maximum 

8-Hour Average Concentration (ppb) 

MCK131/231, KY 

Figure 5b Fourth Highest Daily Maximum 

8-Hour Average Concentration (ppb) 

ROM406/206, CO 

60

65

70

75

80

85

90

95

100

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

4th Highest Daily Maximum 8-hr Average Concentration (ppb)

MCK131

MCK231

60

65

70

75

80

85

90

95

100

2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

4th Highest Daily Maximum 8-hr Average Concentration (ppb)

ROM406

ROM206
CASTNET Annual Report – 2009 64 Chapter 5: Data Quality 

The review of the ROM406/206 sampling systems also showed that differences in O 

3 sampling trains may 

explain the consistent differences in concentrations. The ROM406 sampling line has a diameter of 1/4 inch and 

does not utilize an inline filter and knockout bottle. The ROM206 sampling train includes an inline filter and 

knockout bottle. Until recently, the ROM206 sampling line had a diameter of 3/8 inch. The wider sampling line 

combined with the inline filter and knockout bottle resulted in a longer residence time in the ROM206 O 

3

system, which could result in collection of some O 

3 in the sampling system, perhaps resulting in lower 

concentrations measured by the analyzer. The 3/8-inch tubing was subsequently replaced with 1/4 inch 

tubing. The concentration differences might also be explained by differences in the altitude of the location 

where the O 

3 analyzers are certified, even though all analyzers utilize an altitude compensation algorithm. The 

ROM406 analyzer and transfer standards are certified at an altitude near the actual location of the monitoring 

system while the ROM206 analyzer standards are certified at a location near sea level, a difference of 

approximately 2,700 meters. 

The assessment of the ROM406/206 O 

3 measurements and sampling systems showed the importance of 

collocated measurements and replacing older equipment when failure rates increase. Records indicate that 

use of older equipment results in extended periods of lost data and requires more significant efforts to 

troubleshoot, maintain, and replace instruments than any other cause. O 

3 measurements are sensitive to 

sampling train differences (materials, integrity, and length). To maximize precision of O 

3 measurements, sites 

must be standardized for sampling train materials, configuration, and minimum residence time, and sampling 

train integrity must be properly maintained and routinely monitored for leaks, moisture, and cleanliness.

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 2008) at all collocated site pairs 

and 2009 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. Table 5-5 gives precision 

data by quarter for 2009 for the two collocated sites. The results show excellent instrument 

precision. All but five 2009 annual and quarterly precision results met the DQI criteria. The 

third and fourth quarter and annual MARPD values for solar radiation at MCK131/231 

did not meet the 10 percent goal. The first quarter MARPD value for relative humidity and 

the fourth quarter MARPD value for solar radiation at ROM406/206 did not meet the 

10 percent criterion. 
CASTNET Annual Report – 2009 65 Chapter 5: Data Quality 

Figure 5-5 Historical and 2009 Precision Data for Continuous Measurements 

Table 5-5 Collocated Precision Results for 2009 Continuous Measurements by Quarter Site Pairs 

MARPD MAD 

Flow Rate Relative Humidity Sigma Theta Solar Radiation Delta Temperature Precipitation Scalar Wind Speed Temperature Wetness Wind Direction Wind Speed 

MCK131/231, KY

Quarter 1 2.39 3.16 2.58 5.85 0.56 0.03 0.04 0.49 0.04 3.18 0.04 

Quarter 2 0.66 1.17 2.47 7.28 0.32 0.04 0.03 0.27 0.06 3.84 0.03 

Quarter 3 0.07 0.79 3.11 15.30 0.25 0.02 0.02 0.22 0.08 3.52 0.02 

Quarter 4 0.02 2.09 1.77 15.87 0.26 0.02 0.03 0.16 0.06 3.64 0.03 

2009 0.79 1.80 2.48 11.07 0.35 0.03 0.03 0.29 0.06 3.55 0.03 

ROM406/206, CO

Quarter 1 1.00 13.74 2.83 6.40 0.07 0.02 0.14 0.09 0.06 2.06 0.13 

Quarter 2 1.36 7.04 3.89 4.35 0.11 0.04 0.16 0.19 0.08 2.57 0.09 

Quarter 3 2.35 5.42 NC 2.03 0.17 0.02 0.16 0.35 0.06 NC NC 

Quarter 4 2.76 9.90 3.82 10.29 0.09 0.02 0.14 0.26 0.07 1.72 0.13 

2009 1.87 9.03 3.51 5.77 0.11 0.02 0.15 0.22 0.07 2.12 0.11 

Note: NC = not calculated 

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

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 2008

MARPD: Mackville, KY - 2009

MARPD: Rocky Mtn, CO - 2009

MAD: Overall - 1990 through 2008

MAD: Mackville, KY - 2009

MAD: Rocky Mtn, CO - 2009

Flow RateRelativeHumiditySigmaThetaSolarRadiationDeltaTemperaturePrecipitationScalarWind SpeedTemperatureWetnessWindDirectionWind Speed
CASTNET Annual Report – 2009 66 Chapter 5: Data Quality 

Accuracy 

Laboratory Filter Concentrations 

Accuracy of laboratory measurements is assessed through the analysis of reference and 

continuing calibration verification (CCV) samples. Reference samples and CCV are 

procured from independent suppliers and are traceable to the National Institute of Standards 

and Technology (NIST). Reference samples are analyzed at the beginning and end of each 

analytical batch to verify the accuracy and stability of the calibration curve. The target value 

of the CCV solution is set to the midrange of the calibration curve. The CCV 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 2009. 

Table 5-6 Filter Pack Quality Control Summary for 2009 Filter Type Parameter 

Reference 

Sample1 Recovery 

(%R) 

Continuing Calibration 

Verification Samples (%R)

In-Run Replicate2

(RPD) 

 Mean  Standard  Deviation  Count3 Mean  Standard  Deviation  Count3 Mean  Standard  Deviation  Count3

Teflon SO2-

4 97.71 1.05 143 98.81 0.96 689 0.48 0.98 309 

 NO3 99.37 1.03 143 98.83 0.95 689 0.82 1.08 309 

 NH +

4 100.83 2.99 135 99.84 1.36 677 0.56 0.65 317 

 Ca2+ 104.15 3.16 132 100.44 1.01 676 1.57 2.46 310 

 Mg2+ 102.33 1.53 132 100.08 0.77 676 1.94 2.62 310 

 Na+ 100.16 2.03 132 100.20 1.02 676 1.31 1.35 310 

 K+ 98.10 2.87 132 100.06 0.84 676 2.18 2.80 310 

 Cl- 101.97 1.66 143 99.32 1.06 689 0.56 0.76 309 

Nylon SO2-

4 98.82 1.08 139 99.13 1.22 668 2.77 3.17 307 

 NO3 100.15 1.28 139 99.39 1.27 668 1.18 1.69 307 

Cellulose SO2-

4 98.65 1.24 179 99.43 1.14 691 1.31 1.57 336 

Note: % R = percent recovery 

RPD = relative percent difference 

1 Results of reference sample analyses provide accuracy estimates 

2

 Results of replicate analyses provide precision estimates 

3

 Number of QC Samples 
CASTNET Annual Report – 2009 67 Chapter 5: Data Quality 

Continuous Measurements 

Table 5-7 presents field accuracy results for 2009 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 2009 Field Measurements 

Parameter Percent Within Criterion 

Temperature (0°C) 99.1 percent 

Temperature (ambient) 99.1 percent 

Delta Temperature (0°C) 98.8 percent 

Delta Temperature (ambient) 98.1 percent 

Relative Humidity 100.0 percent 

Solar Radiation 91.1 percent 

Wind Direction North 95.7 percent 

Wind Direction South 95.7 percent 

Wind Speed < 5 m/s 100.0 percent 

Wind Speed ≥ 5 m/s 90.4 percent 

Precipitation 100.0 percent 

Wetness (within 0.5 volts) 97.4 percent 

Ozone Slope 98.2 percent 

Ozone Intercept 98.2 percent 

Flow Rate 96.6 percent 

Note: °C = degrees Celsius

 m/s = meters per second 

Vincennes, IN (VIN140) 
CASTNET Annual Report – 2009 68 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. In addition, data aggregation procedures require approximately 70 percent 

completeness for hourly fluxes and weekly concentrations/fluxes. Figure 5-6 presents 

historical (black) and 2009 completeness data for all sites for measured filter concentrations 

and continuous measurements and calculated parameters. The figure shows that the 2009 

direct measurements met the 90 percent completeness goal, and nine continuous 

measurements exceeded 95 percent, including filter pack flow. Atmospheric concentrations 

also exceeded 95 percent. The four parameters derived from model results exceeded 

90 percent completeness for 2009. Completeness results for 2009 are better than historical 

results for all parameters. 

Figure 5-6 Historical and 2009 Percent Completeness of Measurements and Modeled 

Estimates (black bars are 1990–2008) 

65 70 75 80 85 90 95 100

Annually Aggregated Flux Estimates

Annual Mean Deposition Velocities

Annual Mean Atm. Concentrations

Hourly Flux Estimates

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

Model

Results
CASTNET Annual Report – 2009 69 Chapter 5: Data Quality 

Results for 2009 Environment Canada Proficiency Testing 

The MACTEC laboratory is one of 33 laboratories that participated in the Environment Canada (ECAN) 

Proficiency Testing Program for Inorganic Environmental Substances during 2009 

(http://www.ec.gc.ca/inre-nwri/default.asp?lang=En&xml=7A20877C-359F-44C5-9497-EB904806131C). 

Specifically, the MACTEC laboratory participates in the Rain and Soft Waters round robin studies, which 

consist of natural waters supplied by the National Laboratory for Environmental Testing. The laboratory 

receives ten samples of mixed rain and Canadian Shield waters for chemical analysis from ECAN every 

six months. The MACTEC laboratory reported the eight CASTNET parameters for samples in two studies 

(study codes PT 0094 and PT 0095) during 2009. 

The participating laboratories’ reported results are evaluated for systematic bias and precision. Systematic 

bias is evaluated using Youden (1969) non-parametric analysis, while precision is evaluated using 

algorithm A from International Organization for Standardization (ISO) standard 13528 

[ISO 13528:2005(E); ISO, 2005]. 

Individual sample results are flagged based on the robust standard deviation obtained from the ISO 13528 

computation. Sample results within 2 standard deviations of the assigned (median laboratory) value are not 

flagged, sample results between 2 and 3 standard deviations are given a warning flag, and sample results 

greater than 3 standard deviations from the assigned value are flagged as above the action limit (remedial 

action is required). Laboratory results are considered to be systematically biased when individual 

parameters are ranked by the Youden analysis as consistently and significantly higher or lower than the 

assigned value regardless of deviations from the assigned (median laboratory) value. 

The MACTEC laboratory’s 2009 ECAN results (ECAN, 2010) for the eight CASTNET parameters are presented 

in Table 5c below. All eight parameters received the highest possible rating of “ideal,” indicating an 

unbiased score. 

Table 5c MACTEC Results for ECAN Studies PT 0094 and PT 0095 

Test Parameter Analytical Method Reference Method 

Laboratory Performance Rating

PT Study 0094 PT Study 0095

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 Ideal Ideal

Sulfate IC EPA Method 300.0 Ideal Ideal
CASTNET Annual Report – 2009 70 Chapter 5: Data Quality 

Summary of Data Quality Results 

DQI results demonstrate that field and laboratory processes were adequately monitored 

through QA/QC procedures and were generally free of systemic bias during 2009. Precision 

data for SO 

2, SO2-4 , HNO 3, and NH +4 are considered acceptable. Precision data for NO3

analyses of collocated field samples have not met the established criterion due, most likely, 

to the low concentrations generally measured and the uncertain nature of the gas/particle 

equilibrium of the nitrogen species. The precision of K+ filter pack measurements exceeded 

the established criteria and are not considered acceptable. The poor K+ precision is 

under investigation. 

The 2009 precision data for O 

3 concentrations met the 10 percent DQI criterion. The 2009 

O 

3 precision data for ROM406/206 demonstrate the comparability of the two independent 

O 

3 sampling systems. The precision of continuous measurements for 2009 was excellent. All 

but four quarterly and one annual precision result for 2009 met the DQI criteria. 

Accuracy data met the established criteria for field and laboratory parameters. 

Completeness criteria were met for all parameters in 2009. Completeness results for 2009 

are considered excellent and were better than historical results for all parameters. ECAN 

proficiency test studies (0094 and 0095) showed that the MACTEC laboratory performed 

well on the interlaboratory chemical analyses with all parameters rated ideal. 

Alhambra, IL (ALH157)
CASTNET Annual Report – 2009 71 References

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Network (CASTNET) Fourth Quarter 2009 Data Report. Prepared for U.S. 

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Markets Division, Washington, DC. Contract No. EP-W-09-028. Gainesville, FL. 

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Network (CASTNET) Third Quarter 2009 Data Report. Prepared for U.S. Environmental 

Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets Division, 

Washington, DC. Contract No. EP-W-09-028. Gainesville, FL. 

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Monitoring, Clingmans Dome, TN, Great Smoky Mountains National Park, 2009. Prepared 

for U.S. Environmental Protection Agency (EPA), Office of Air and Radiation, 

Clean Air Markets Division, Washington, DC. Contract No. EP-W-09-028. 

Gainesville, FL. http://java.epa.gov/castnet/documents.do (accessed February 

2011). 

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Network (CASTNET) First Quarter 2009 Data Report. Prepared for U.S. Environmental 

Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets Division, 

Washington, DC. Contract No. 68-D-03-052. Gainesville, FL. 
CASTNET Annual Report – 2009 73 References

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Network (CASTNET) First Quarter 2009 Quality Assurance Report. Prepared for U.S. 

Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air 

Markets Division, Washington, DC. Contract No. 68-D-03-052. Gainesville, FL. 

http://java.epa.gov/castnet/documents.do (accessed February 2011). 

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Network (CASTNET) Second Quarter 2009 Data Report. Prepared for U.S. 

Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air 

Markets Division, Washington, DC. Contract No. EP-W-09-028. Gainesville, FL. 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2009d. Clean Air Status and Trends 

Network (CASTNET) Second Quarter 2009 Quality Assurance Report. Prepared for U.S. 

Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air 

Markets Division, Washington, DC. Contract No. 68-D-03-052. Gainesville, FL. 

http://java.epa.gov/castnet/documents.do (accessed February 2011). 

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Network (CASTNET) Third Quarter 2009 Quality Assurance Report. Prepared for U.S. 

Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air 

Markets Division, Washington, DC. Contract No. EP-W-09-028. Gainesville, FL. 

http://java.epa.gov/castnet/documents.do (accessed February 2011). 

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Network (CASTNET) Quality Assurance Project Plan (QAPP) Revision 4.1. Prepared for 

U.S. Environmental Protection Agency (EPA), Office of Air and Radiation, 

Clean Air Markets Division, Washington, DC. Contract No. EP-W-09-028. 

Gainesville, FL. http://java.epa.gov/castnet/documents.do (accessed February 

2011). 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2003. Clean Air Status and Trends 

Network (CASTNET) 2002 Annual Report. Prepared for U.S. Environmental 

Protection Agency (EPA), Research Triangle Park, NC. Contract No. 68-D-03-052. 

Gainesville, FL. 
CASTNET Annual Report – 2009 74 References

References (continued)

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Denver, CO. http://www.nature.nps.gov/air/Pubs/pdf/RoMANS_V1_20100218.pdf 

(accessed November 2010). 

Meyers, T.P., Finkelstein, P., Clarke, J., Ellestad, T.G., and Sims, P.F. 1998. A Multilayer 

Model for Inferring Dry Deposition Using Standard Meteorological Measurements. 

J. Geophys. Res., 103D17:22,645-22,661. 

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and Analysis of the Use of Exposure- and Flux-Based Ozone Indices for Predicting 

Vegetation Effects. Atmos Environ., 40:1869-1888. 

National Atmospheric Deposition Program (NADP)/Atmospheric Mercury Network 

(AMNet). 2010. Atmospheric Mercury Network (AMNet). 

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(MDN). 2010. Mercury Deposition Network (MDN). http://nadp.isws.illinois.edu/mdn 

(accessed November 2010). 

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UNECE/Nordic Council workshop report, Skokloster, Sweden. March 1988. Nordic 

Council of Ministers: Copenhagen. 

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Resource Data Series, NPS/NRPC/ARD/NRDS−2010/086. National Park Service, 

Denver, CO. http://www.nature.nps.gov/air/Pubs/pdf/ads/2009/GPMP-XX.pdf 

(accessed November 2010). 

Sullivan, T. J., Cosby, B.J., Webb, J.R., Dennis, R.L., Bulger, A.J., and Deviney, F.A., Jr. 

2008. Streamwater Acid-base Chemistry and Critical Loads of Atmospheric Sulfur 

Deposition in Shenandoah National Park, Virginia. Environ. Monit. Assess., 137:85–99. 
CASTNET Annual Report – 2009 75 References

References (continued)

Tennessee Valley Authority. 2010. SO 

2 and NOx Emissions at TVA Power Plants.

http://www.tva.com/environment/air/index.htm (accessed November 2010). 

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Emission, Compliance, and Market Data. http://www.epa.gov/airmarkets/progress/ 

ARP09_3.html (accessed November 2010). 

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Assurance Requirements for State and Local Air Monitoring Stations (SLAMS), Special 

Purpose Monitors (SPMs), and Prevention of Significant Deterioration (PSD) Air Monitoring. 

40 CFR Part 58. 

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for Ground-Level Ozone. http://www.epa.gov/air/ozonepollution/actions.html 

(accessed November 2010). 

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2008 Environmental Results. http://www.epa.gov/airmarkets/progress/ARP_3.html 

(accessed November 2010). 

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Standards for Ozone; Final Rule. Federal Register 73, No. 60 (March). EPA-HQOAR-2005-0172. 

U.S. Environmental Protection Agency (EPA). 2006. Air Quality Criteria for Ozone and 

Related Photochemical Oxidants. Report No. EPA/600/R-05/004af. ORD, Research 

Triangle Park, NC. 

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Trends Report, 1998. Report No. EPA-454-R-00-003. OAQPS, Research Triangle 

Park, NC 27711. 

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for Ozone. 40 CFR Part 50. 

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Related Photochemical Oxidants. EPA Report No. EPA/600/P-93/004bF. ORD, 

Research Triangle Park, NC. 
CASTNET Annual Report – 2009 76 References

References (continued)

Youden, W.J. (Ku, H.H., ed). 1969. Precision Measurement and Calibration. NBS Special 

Publication 300-Volume 1. U.S. Government Printing Office, Washington, DC. 
CASTNET Annual Report – 2009 Appendix A 

Appendix A 

Locational and Operational Characteristics of CASTNET Sites
CASTNET Annual Report – 2009 ~ 1 ~ 

Table A-1 Locational and Operational Characteristics of CASTNET Sites Site ID Site Name 

Start date 

Latitude (°N) 

Longitude (°W) 

Elevation (m) 

Deposition Filter 

Pack 1 

Ozone and 

Meteorology 2 

Nearby NADP Site 

Primary Land Use 

Alabama SND152 Sand Mountain 12/27/88 34.2894 85.9704 352 • • AL99 Agri. 

Alaska 

DEN417 Denali National Park 10/06/98 63.7258 148.9633 661 • • AK03 Forested 

Arizona 

CHA467 Chiricahua National Monument 04/25/89 32.0092 109.3892 1570 • • AZ98 Range GRC474 Grand Canyon National Park 05/16/89 36.0597 112.1822 2073 • • AZ03 Forested PET427 Petrified Forest National Park 09/24/02 34.8225 109.8919 1723 • • AZ97 Desert 

Arkansas 

CAD150 Caddo Valley 10/04/88 34.1792 93.0989 71 • • AR03 Forested 

California 

CON186 Converse Station 06/17/03 34.1941 116.9130 1837 • • CA94 Agri./Forested JOT403 Joshua Tree National Monument 02/16/95 34.0714 116.3906 1244 • • CA67 Desert LAV410 Lassen Volcanic National Park 07/25/95 40.5403 121.5764 1756 • • CA96 Forested PIN414 Pinnacles National Monument 05/16/95 36.4850 121.1556 335 • • CA66 Forested SEK430 Sequoia National Park 04/07/05 36.4894 118.8269 457 • • CA75 Forested YOS404 Yosemite National Park 09/25/95 37.7133 119.7061 1605 • • CA99 Forested 

Colorado 

GTH161 Gothic 05/16/89 38.9573 106.9854 2926 • • CO10 Range MEV405 Mesa Verde National Park 01/10/95 37.1983 108.4903 2165 • • CO99 Forested ROM206 Rocky Mountain National Park 07/03/01 40.2778 105.5453 2743 • • CO98 Forested ROM406 Rocky Mountain National Park 12/20/94 40.2778 105.5453 2743 • • CO98 Forested 
CASTNET Annual Report – 2009 ~ 2 ~ 

Table A-1 Locational and Operational Characteristics of CASTNET Sites Site ID Site Name 

Start date 

Latitude (°N) 

Longitude (°W) 

Elevation (m) 

Deposition Filter 

Pack 1 

Ozone and 

Meteorology 2 

Nearby NADP Site 

Primary Land Use 

Connecticut ABT147 Abington 12/28/93 41.8402 72.0111 209 • • CT15 Urban-Agri. 

Florida 

EVE419 Everglades National Park 10/06/98 25.3911 80.6806 2 • • 4 FL11 Swamp IRL141 Indian River Lagoon 07/09/01 30.1065 80.4554 2 • • FL99 Beach SUM156 Sumatra 12/28/88 30.1065 84.9938 14 • • FL23 Forested 

Georgia 

GAS153 Georgia Station 06/28/88 33.1812 84.4100 270 • • GA41 Agri. 

Illinois 

ALH157 Alhambra 06/28/88 38.8690 89.6229 164 • • IL46 Agri. BVL130 Bondville 02/09/88 40.0520 88.3725 212 • • IL11 Agri. STK138 Stockton 12/28/93 42.2872 89.9998 274 • • IL18 Agri. 

Indiana 

SAL133 Salamonie Reservoir 06/28/88 40.8164 85.6608 250 • • IN20 Agri. VIN140 Vincennes 08/04/87 38.7406 87.4844 134 • • IN22 Agri. 

Kansas 

KNZ184 Konza Prairie 03/26/02 39.1021 96.6096 348 • • KS31 Prairie 

Kentucky 

CDZ171 Cadiz 10/01/93 36.7841 87.8500 189 • • KY99 Agri. CKT136 Crockett 08/24/93 37.9211 83.0658 455 • • KY35 Agri. MAC426 Mammoth Cave National Park 07/24/02 37.1319 86.1478 243 • • KY10 Agri./Forested MCK131 Mackville 07/31/90 37.7044 85.0483 353 • • KY03 Agri. 
CASTNET Annual Report – 2009 ~ 3 ~ 

Table A-1 Locational and Operational Characteristics of CASTNET Sites Site ID Site Name 

Start date 

Latitude (°N) 

Longitude (°W) 

Elevation (m) 

Deposition Filter 

Pack 1 

Ozone and 

Meteorology 2 

Nearby NADP Site 

Primary Land Use 

Maine ACA416 Acadia National Park 12/01/98 44.3769 68.2608 158 • • ME98 Forested CASH135 Ashland 12/20/88 46.6039 68.4142 235 • • ME00 Agri. HOW132 Howland 11/24/92 45.2158 68.7085 69 • • ME09 Forested 

Maryland 

BEL116 Beltsville 11/01/88 39.0283 76.8175 46 • • MD99 Urban-Agri. BWR139 Blackwater National Wildlife Refuge 07/04/95 38.4448 76.1115 4 • • MD15 Forest-Marsh C

Michigan 

ANA115 Ann Arbor 06/28/88 42.4164 83.9019 267 • • MI52 Forested HOX148 Hoxeyville 10/31/00 44.1809 85.7390 298 • • MI53 Forested UVL124 Unionville 06/28/88 43.6139 83.3597 201 • • MI51 Agri. 

Minnesota 

VOY413 Voyageurs National Park 06/13/96 48.4128 92.8292 429 • • MN32 Forested 

Mississippi 

CVL151 Coffeeville 12/27/88 34.0028 89.7989 134 • • MS30 Forested 

Montana 

GLR468 Glacier National Park 12/27/88 48.5103 113.9956 976 • • MT05 Forested C

Nebraska 

SAN189 Santee Sioux 07/05/06 42.8292 97.8541 429 • • SD99 Agri. 

Nevada 

GRB411 Great Basin National Park 05/16/95 39.0053 114.2158 2060 • • NV05 Forested C

New Hampshire 

WST109 Woodstock 12/27/88 43.9446 71.7008 258 • • NH02 Forested C
CASTNET Annual Report – 2009 ~ 4 ~ 

Table A-1 Locational and Operational Characteristics of CASTNET Sites Site ID Site Name 

Start date 

Latitude (°N) 

Longitude (°W) 

Elevation (m) 

Deposition Filter 

Pack 1 

Ozone and 

Meteorology 2 

Nearby NADP Site 

Primary Land Use 

New Jersey WSP144 Washington’s Crossing 12/27/88 40.3133 74.8726 61 • • NJ99 Urban-Agri. R

New York 

CAT175 Claryville 05/10/94 41.9423 74.5519 765 • • 4, 5 NY68 Forested CoCTH110 Connecticut Hill 09/28/87 42.4010 76.6535 515 • • NY67 Forested RHWF187 Huntington Wildlife Forest 05/28/02 43.9732 74.2232 502 • • NY20 Forested Co

North Carolina 

BFT142 Beaufort 12/28/93 34.8843 76.6213 2 • • NC06 Agri. CND125 Candor 09/25/90 35.2643 79.8382 198 • • NC36 Forested RCOW137 Coweeta 11/04/87 35.0605 83.4302 686 • • NC25 Forested CoPNF126 Cranberry 12/27/88 36.1040 82.0448 1250 • • NC45 Forested Mou

North Dakota 

THR422 Theodore Roosevelt National Park 10/06/98 46.8947 103.3778 850 • • ND00 Range R

Ohio 

DCP114 Deer Creek State Park 09/28/88 39.6358 83.2600 267 • • OH54 Agri. RLYK123 Lykens 01/10/89 40.9169 82.9981 303 • • Agri. OXF122 Oxford 08/18/87 39.5314 84.7231 284 • • OH09 Agri. RQAK172 Quaker City 09/28/93 39.9431 81.3378 372 • • OH49 Agri. R

Oklahoma 

CHE185 Cherokee Nation 04/02/02 35.7507 94.6700 299 • • AR27 Agri. R

Ontario 

EGB181 Egbert, Ontario 12/27/94 44.2317 79.7840 251 • • 4 NY10 Agri. R
CASTNET Annual Report – 2009 ~ 5 ~ 

Table A-1 Locational and Operational Characteristics of CASTNET Sites Site ID Site Name 

Start date 

Latitude (°N) 

Longitude (°W) 

Elevation (m) 

Deposition Filter 

Pack 1 

Ozone and 

Meteorology 2 

Nearby NADP Site 

Primary Land Use 

Pennsylvania ARE128 Arendtsville 06/28/88 39.9231 77.3078 269 • • PA00 Agri. RKEF112 Kane Experimental Forest 01/03/89 41.5981 78.7683 622 • • PA29 Forested RLRL117 Laurel Hill State Park 12/15/87 39.9883 79.2522 615 • • MD08 Forested CoMKG113 M.K. Goddard State Park 01/12/88 41.4250 80.1447 384 • • NY10 Forested RPSU106 Penn. State University 01/06/87 40.7209 77.9316 376 • • PA42 Agri. R

South Dakota 

WNC429 Wind Cave National Park 11/18/03 43.5578 103.4839 1292 • • SD04 Prairie R

Tennessee 

ESP127 Edgar Evins State Park 03/22/88 36.0389 85.7330 302 • • KY10 Forested RGRS420 Great Smoky Mountains National Park 10/06/98 35.6331 83.9422 793 • • TN11 Forested CoSPD111 Speedwell 06/12/89 36.4698 83.8265 361 • • TN04 Agri. R

Texas 

ALC188 Alabama-Coushatta 04/02/04 30.7017 94.6742 101 • • TX10 Forested RBBE401 Big Bend National Park 07/18/95 29.3022 103.1772 1052 • • TX04 Forested CoPAL190 Palo Duro Canyon State Park 04/24/07 34.8803 101.6649 1050 • • TX43 Prairie Co

Utah 

CAN407 Canyonlands National Park 01/24/95 38.4586 109.8211 1809 • • UT09 Desert Co

Virginia 

PED108 Prince Edward 11/03/87 37.1653 78.3070 150 • • VA24 Forested RSHN418 Shenandoah National Park 06/28/88 38.5231 78.4347 1073 • • VA28 Forested MouVPI120 Horton Station 06/02/87 37.3300 80.5573 920 • • VA13 Forested Mou
CASTNET Annual Report – 2009 ~ 6 ~ 

Table A-1 Locational and Operational Characteristics of CASTNET Sites Site ID Site Name 

Start date 

Latitude (°N) 

Longitude (°W) 

Elevation (m) 

Deposition Filter 

Pack 1 

Ozone and 

Meteorology 2 

Nearby NADP Site 

Primary Land Use 

Washington MOR409 Mount Rainier National Park 08/29/95 46.7583 122.1244 415 • • WA99 Forested Co

West Virginia 

CDR119 Cedar Creek State Park 11/10/87 38.8794 80.8478 234 • • WV05 Forested CoPAR107 Parsons 01/19/88 39.0906 79.6614 510 • • WV18 Forested Co

Wisconsin 

PRK134 Perkinstown 09/27/88 45.2066 90.5972 472 • • WI35 Agri. R

Wyoming 

CNT169 Centennial 08/19/91 41.3722 106.2422 3178 • • WY95 Range CoPND165 Pinedale 12/27/88 42.9214 109.7900 2388 • • WY06 Range RYEL408 Yellowstone National Park 06/26/96 44.5597 110.4006 2400 • • WY08 Forested R

1. The dry deposition filters are analyzed for the following constituents: 

Teflon = SO

2-

4 , NO

- 

3, NH

 +

4 , Cl -, K+ , Na+ , Mg2+, Ca2+

Nylon = SO

2-

4 , NO

- 

3 (reported as HNO3) 

Cellulose = SO

2-

4 (reported as SO2) 

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. O3 not measured 5. Solar-powered site • Indicates current monitoring 100 and 200 series = EPA-Operated Sites 400 series = NPS-Operated Sites
CASTNET Annual Report – 2009 Appendix B 

Appendix B 

Acronyms and Abbreviations
CASTNET Annual Report – 2009 ~ 1 ~ Appendix B 

List of Acronyms and Abbreviations 

AC automated colorimetry 

ANC acid neutralizing capacity 

AMNet Atmospheric Mercury Network 

AMoN Ammonia Monitoring Network 

ARP Acid Rain Program 

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 

CLD303 MADPro cloud water monitoring site at Clingmans Dome, TN 

CMAQ Community Multiscale Air Quality modeling system 

cm/month centimeters per month 

DM8A daily maximum 8-hour average 

DQI data quality indicator 

ECAN Environment Canada 

EGU electric generating unit 

EPA U.S. Environmental Protection Agency 

H2SO4 sulfuric acid 

Hg mercury 

HNO 

3 nitric acid 

IC ion chromatography 

ICP-AES inductively coupled plasma-atomic emission spectrometry 

IMPROVE Interagency Monitoring of Protected Visual Environments 

ISO International Organization for Standardization 

K+ particulate potassium ion 

kg/ha kilograms per hectare 

kg/ha/yr kilograms per hectare per year 

km kilometer 

LAI leaf area index 

lpm liters per minute 

MACT maximum achievable control technology 

MACTEC MACTEC Engineering and Consulting, 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 

MeHg methylmercury 
CASTNET Annual Report – 2009 ~ 2 ~ Appendix B 

List of Acronyms and Abbreviations (continued) 

Mg2+

 particulate magnesium ion 

mg/l milligrams per liter 

MLM Multi-Layer Model 

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 

NDDN National Dry Deposition Network 

NERR National Estuarine Research Reserve 

ng/L nanograms per liter 

NH 

3 ammonia 

NH +

4 particulate ammonium 

(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)] 

NOy total reactive oxides of nitrogen 

NPS National Park Service 

NTN National Trends Network 

O 

3 ozone 

OTC Ozone Transport Commission 

PM2.5 fine particle matter 

POMS portable ozone monitoring system 

ppb parts per billion 

ppm parts per million 

PT proficiency test 

QAPP Quality Assurance Project Plan 

RoMANS Rocky Mountain Atmospheric Nitrogen and Sulfur study

S sulfur 

SIP State Implementation Plan 

SLAMS State and Local Monitoring Stations 

SO 

2 sulfur dioxide 

SO2-

4 particulate sulfate 

TN11 NADP wet deposition sampling site at Elkmont, TN

total NO3 gaseous nitric acid (HNO 

3) + particulate nitrate (NO3) 

Total_N total nitrogen content of HNO 

3, NH +4 , and NO3

TVA Tennessee Valley Authority 

μg/m3 micrograms per cubic meter 

V 

d deposition velocity 

VOCs volatile organic compounds 

z/s/p zero/span/precision 
For More Information:

U.S. Environmental Environmental Protection Protection Agency

Office of Air and Radiation Radiation

Clean Air M kt ar e s Di i i v s on

Washington, DC

On the Web:

CASTNET CASTNET Home Page: Cl Ai M kt EPA H P

www.epa.gov/castnet

Clean Air Markets

Division Home Page:

www.epa.gov/airmarkets

EPA Home Page:

www.epa.gov