Clean Air Status and Trends Network 

(CASTNET)

2007 Annual Report
Clean Air Status and Trends Network 

(CASTNET) 

2007 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. 

December 2008
CASTNET Annual Report – 2007 - i - Table of Contents 

Table of Contents 

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

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

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

 Network Partners .......................................................................................................................... 2

AIRNow: EPA-Sponsored CASTNET Sites Began Participation in May 2007..................... 4 

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

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

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

 Calculating Dry Deposition........................................................................................................... 8

Results for 2007 Laboratory Intercomparison Studies............................................................ 9 

 SO 

2 and NOx Emissions .............................................................................................................. 10

 Significant Events during 2007 ................................................................................................ 13 

Chapter 2 Atmospheric Concentrations ......................................................................................14 

Sulfur Dioxide............................................................................................................................. 14 

 Particulate Sulfate ....................................................................................................................... 16 

Total Nitrate................................................................................................................................ 17 

 Particulate Ammonium ............................................................................................................... 18 

Cherokee Nation Clean Air Program...................................................................................... 20 

Chapter 3 Atmospheric Deposition ..............................................................................................22 

Sulfur Deposition ........................................................................................................................ 23 

Nitrogen Deposition.................................................................................................................... 27 

Rocky Mountain Atmospheric Nitrogen and Sulfur (RoMANS) Study............................... 31 

Chapter 4 Ozone Concentrations .................................................................................................33 

CASTNET Ozone Concentrations .............................................................................................. 34 

National Ambient Air Quality Standards for Ozone ............................................................. 36 

Nitrogen Oxides and Ozone........................................................................................................ 39 

Ozone Episode In Acadia National Park, ME (ACA416) during 2007 ................................ 41 

Chapter 5 Data Quality .................................................................................................................43 

Analysis of CASTNET Laboratory and Field Blanks............................................................ 44 

 Precision...................................................................................................................................... 46 

Comparison of CASTNET and AQS Methods For Determining Precision......................... 51 

Accuracy ..................................................................................................................................... 52 

Completeness .............................................................................................................................. 54 

Conclusion .................................................................................................................................. 55 

References .....................................................................................................................................R-1 

Appendix A Locational and Operational Characteristics of CASTNET Sites ..........................A-1 

Appendix B Acronyms and Abbreviations ...................................................................................B-1 
CASTNET Annual Report – 2007 - ii - Executive Summary 

Executive Summary 

 

This report summarizes Clean Air Status and Trends Network (CASTNET) data collected during 

2007 and examines trends in air quality since 1990. The report also discusses monitoring 

activities, network changes and upgrades, and data quality. CASTNET began collecting 

measurements in 1991 with the incorporation of 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 in order to evaluate the effectiveness of 

national and regional air pollution control programs. 

Key Results through 2007 

 Mean annual sulfur dioxide and particulate sulfate concentrations declined significantly 

over the 18-year period 1990 through 2007. Sulfur dioxide levels declined 40 percent 

while sulfate concentrations declined 26 percent. 

 Total sulfur deposition declined by 34 percent from 1990 through 2007. 

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

by approximately 20 percent over the 18-year period. 

 Total nitrogen deposition declined by 17 percent from 1990 through 2007. 

 Precipitation-weighted mean sulfur and nitrogen concentrations in precipitation show an 

overall decline over the last 18 years with small increases in 2005, 2006, and 2007. 

 Ozone levels measured in 2007 continue to corroborate a downward trend that began 

after a peak in 2002. The median fourth highest daily maximum 8-hour average ozone 

concentration for 2007 (75 ppb) was the third lowest in the history of the network. For 

the most recent 3-year period (2005 through 2007), only three eastern and four California 

sites recorded exceedances of the current 8-hour standard (0.08 ppm). The recent decline 

in rural ozone and nitrate levels has been attributed to the documented decline in nitrogen 

oxides emissions that began in 2000. 

 Routine quality assurance activities include laboratory intercomparison studies under 

the direction of the U.S. Geological Survey and National Water Research Institute of 

Canada. Measurements taken during 2007 and historical data collected over the period 

1990 through 2007 were analyzed relative to data quality objectives (DQO) and their 

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

high degree of confidence. 
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 1

Chapter 1: CASTNET Overview 

Background 

The United States 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). Congress recognized the 

need to track real-world environmental results as the ARP was implemented and emission 

reductions became effective. Congress mandated that CASTNET provide data for determining 

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

atmospheric deposition, and ecological effects. 

The ARP has produced significant reductions in SO2 and NOx emissions from electric generating 

units (EGUs) since 1995. 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 was implemented in 2003. The NBP 

places a cap on total NOx emissions from EGUs in 20 states and the District of Columbia during 

the ozone season (May 1 through September 30) when the potential for photochemical ozone 

formation is high. The EPA relies on CASTNET and other long-term monitoring networks to 

measure pollutant concentrations used to assess the effectiveness of emission control programs. 

CASTNET started as the National Dry Deposition Network (NDDN), which began operation in 

1987. The 50 NDDN sites were transferred to CASTNET in 1991. CASTNET provides a longterm data record with more than 35 sites in operation since the late 1980s. The network, as of 

December 2007, included 87 monitoring stations at 84 site locations throughout the continental 

United States and Alaska. 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. 

The dry deposition estimates are calculated from CASTNET concentration measurements and 

modeled deposition velocities. The model requires several meteorological parameters and 

information on vegetation and land use specific to each site. 

CASTNET is a national air quality monitoring network that provides data for determining 

trends in atmospheric nitrogen, sulfur, and ozone and for determining relationships between 

emissions, air quality, deposition, and ecological effects. CASTNET began operation in 1991 

to provide long-term monitoring of air quality in rural areas. As of December 2007, the 

network operated 87 monitoring stations throughout the continental United States and Alaska. 
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 2

This report summarizes CASTNET/NDDN monitoring activities and the resulting concentration 

and deposition data collected over the 18-year period from 1990 through 2007. 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. Fully validated data are available 

approximately 10 months following collection. 

Network Partners 

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

participation in CASTNET in 1994 under an agreement with EPA. NPS is responsible for the 

protection and enhancement of air quality and related values in national parks and wilderness 

areas. The CASTNET sites sponsored by NPS as of December 2007 numbered 27. Additional 

information on the NPS Air Monitoring program can be found on the Web site: 

http://www.nature.nps.gov/air/monitoring/. In addition to EPA and NPS, CASTNET monitors air 

quality and deposition in cooperation with other national networks. 

 National Atmospheric Deposition Program/National Trends Network (NADP/NTN)

operates 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. 

Additional information on NADP/NTN can be found on its Web site: 

http://nadp.sws.uiuc.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. Additional information on CAPMoN can be found on its 

Web site: http://www.msc.ec.gc.ca/capmon/index_e.cfm. 

 Interagency Monitoring of Protected Visual Environments (IMPROVE) measures 

aerosol pollutants near more than 30 CASTNET sites. IMPROVE measures particulate air 

pollutants that affect visibility. Additional information on IMPROVE can be found on its 

Web site: http://vista.cira.colostate.edu/improve. 

 Mercury Deposition Network (MDN) samplers are operated at multiple CASTNET sites. 

MDN samplers measure mercury in precipitation. Additional information on MDN can be 

found on its Web site: http://nadp.sws.uiuc.edu/mdn/. 

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 
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 3

different mandates including the Program Assessment Rating Tool (PART), 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 an in-kind service that supports the overall operation of a site 

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

the network (since 2001) have an associated co-sponsor. The CASTNET 2006 Annual Report 

(MACTEC, 2007a) presented a list of the co-sponsors and the sites they sponsor. Three 

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

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

Nebraska (SAN189). Tribal air quality monitoring programs are discussed in Chapter 2. State 

agencies also operate special purpose air pollutant measurement devices at some 

CASTNET sites. 

As of December 2007, CASTNET sites in rural California ranged from 

Lassen Volcanic National Park (LAV410) in the north to Joshua Tree 

National Monument (JOT403) in the south and included Converse Station 

(CON186), Death Valley National Monument (DEV412), Pinnacles 

National Monument (PIN414), Sequoia National Park (SEK430), and 

Yosemite National Park (YOS404). 
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 4

AIRNow: EPA-Sponsored CASTNET Sites 

Began Participation in May 2007 

Site ID Site Name AQS Code 

ARE128 Arendtsville, PA 42-001-9991 

CND125 Candor, NC 37-123-9991 

CON186 Converse Station, CA 06-071-9991 

HWF187 Huntington Wildlife Forest, NY 36-031-9991 

LRL117 Laurel Hill State Park, PA 42-111-9991 

PED108 Prince Edward, VA 51-147-9991 

PND165 Pinedale, WY 56-035-9991 

SPD111 Speedwell, TN 47-025-9991 

UVL124 Unionville, MI 26-157-9991 

WSP144 Washington’s Crossing, NJ 34-021-9991 

Notes: AQS is EPA’s Air Quality System for archiving air quality data. NPS-sponsored 

CASTNET sites have been providing ozone data to AIRNow since May 2002. 

AIRNow is a Web site (http://www.airnow.gov) designed to allow the public to view air 

quality data for a specific area in “real-time.” The EPA, the National Oceanic and 

Atmospheric Administration (NOAA), NPS, tribal, state, and local agencies developed the 

AIRNow Web site to provide the public with easy access to national air quality information. 

The Web site offers daily forecasts of Air Quality Index (AQI) as well as maps of real-time 

AQI conditions for over 300 cities across the United States. It also provides links to more 

detailed state and local air quality Web sites. 

The air quality data used in these maps and to generate forecasts are collected using either 

federal reference or equivalent monitoring techniques approved by EPA. Since the 

information needed to make the maps must be as “real-time” as possible, the data are 

displayed as soon as practical after the end of each hour. Although some preliminary data 

quality assessments are performed, the data are not fully verified and validated through the 

quality assurance procedures monitoring organizations use to officially submit and certify 

data on the EPA Air Quality System (AQS). Therefore, data are used on the AIRNow Web 

site only for the purpose of reporting the AQI. 

At the end of 2007, 10 EPA-sponsored CASTNET sites were sending 1-hour ozone data to 

the AIRNow database. The 10 sites were also transmitting 1-hour measurements of 

temperature, wind speed, wind direction, relative humidity, and solar radiation. 

Approximately 30 EPA-sponsored CASTNET sites will be participating in AIRNow by the 

end of 2008, and all sites will be participating by the end of 2009. All NPS-sponsored 

CASTNET sites are participating. 
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 5

Locations of Monitoring Sites 

Figure 1-1 shows the locations of CASTNET monitoring sites as of December 2007. Eightyseven sites were operational at 84 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 and operational characteristics of each site by state including 

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 2007 
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 6

CASTNET Reference Sites 

Chapters 2 through 4 present maps 

illustrating the magnitude of pollutant 

concentrations and deposition fluxes 

across the United States. In addition, 

measurements from 34 CASTNET 

reference sites (Figure 1-2) were 

analyzed for each pollutant in order to 

determine trends in concentrations and 

trends 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. EPA’s procedures to 

interpolate and extrapolate quarterly 

mean data were also used to replace 

missing or invalid data. In chapters 2 

through 4, the data from the 34 reference 

sites were aggregated and then presented 

using box plots for the period 1990-2007. 

Measurements Collected at CASTNET Sites 

CASTNET was designed primarily to measure trends in 

seasonal and annual average concentrations and 

depositions over many years. Consequently, 

measurement of 7-day average (Tuesday to Tuesday) 

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, 

three-stage filter pack (Figure 1-3) mounted atop a 10-

meter tower to collect air pollutants in the form of gases 

and particles. The first stage of the filter pack encloses a 

Teflon® filter, the second a nylon filter, and the third 

holds two potassium carbonate (K2CO3)-impregnated 

cellulose filters. The filter pack is changed out each 

Tuesday and shipped to the analytical chemistry 

laboratory for analysis.

CASTNET Ambient Measurements 

 Sulfur dioxide (SO2) 

 Particulate sulfate (SO2-

4 ) 

 Particulate nitrate (NO3) 

 Nitric acid (HNO3) 

 Particulate ammonium (NH+ 

4 ) 

 Particulate calcium (Ca2+) 

 Particulate sodium (Na +) 

 Particulate magnesium (Mg2+) 

 Particulate potassium (K +) 

 Particulate chloride (Cl -) 

 Ozone (O3) 

 Meteorological variables and 

information on land use and 

vegetation 

Figure 1-2 CASTNET Reference Sites 
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 7

Figure 1-3 Three-Stage Filter Pack 

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). Teflon® filter extracts are also 

analyzed for ammonium (NH+ 

4 ) by the automated indephenol method with the Bran+Luebbe 

AutoAnalyzerTM 3. The filter extracts are additionally 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 3000 Dual View spectrometer. The 

cellulose filter extracts are analyzed for SO2 as sulfate using IC. The nylon filter extracts are 

analyzed via IC for HNO3 and for SO2. The SO2 concentrations from the cellulose and nylon 

filters are summed to obtain total SO2. 

CASTNET also measures ozone concentrations, one of the major components of smog, and 

meteorological parameters, which are used to estimate deposition velocities. The meteorological 

parameters measured at each site include temperature, solar radiation, relative humidity, 

precipitation, wind speed and direction, standard deviation of wind direction (sigma theta), and 

surface wetness. These meteorological measurements are used to gauge the transport of air 

pollutants and as input to the Multi-Layer Model (MLM), a numerical model used for estimating 

dry deposition values. The ozone and meteorological measurements are recorded continuously 

and archived as hourly averages. 

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 (2) 

Gaseous 

• SO 

2

Nylon 

Gaseous 

• HNO 

3 • SO 2 

Teflon®

Particulate 

• SO2-

4 • NO3 • NH+ 4 • K +

• Ca2+ • Mg2+ • Na + • Cl-
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 8

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

Multi-Layer Model (MLM

Flux =

1/Vd = 1

11 1

s b cut b a soil soil

a

rr r r r r

r

+

+

+

+

+

+

,

VxC d

Calculating 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). 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 described by Meyers et al. (1998) and Finkelstein et al. (2000). 

The data used in the MLM to estimate dry deposition are derived from meteorological 

measurements and pollutant concentrations taken at the site together with an estimation of the 

vegetation leaf-out and leaf area index (LAI). 

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

and illustrates the data that are required as model input. An improved version of the MLM 

(Schwede, 2006) was delivered to MACTEC by EPA’s Office of Research and Development 

(ORD) during 2006. This version includes changes to the soil moisture factor, which affects the 

stomatal and soil resistances, and to the radiation algorithm, which also affects the stomatal 

resistance. Deposition velocities and fluxes for the entire network through 2007 were calculated 

using the updated model. 

Figure 1-4 Multi-Layer Model 

Resistances 

ra = aerodynamic 

ra, soil = aerodynamic near soil 

rb = boundary layer in thin layer at surface 

rcut = cuticular 

rs = stomatal 

rsoil = soil 
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 9

Results for 2007 Laboratory Intercomparison Studies 

The MACTEC laboratory is one of eight laboratories participating in the U.S. Geological Survey 

(USGS) interlaboratory comparison program (http://bqs.usgs.gov/precip/interlab_data.htm). The 

laboratory receives four samples for chemical analysis from USGS every two weeks. The samples 

are a mix of 44 synthetically prepared samples, 8 deionized water samples, and 52 natural wet 

deposition samples. The laboratory reported the eight CASTNET parameters for a total of 104 

samples during 2007. Results for the 44 synthetically prepared samples are depicted in the figure 

below, which presents results as percent recoveries of the median value for all laboratories. As 

shown in Figure 1-A, only 12 of 352 reported values were more than 10 percent from their 

corresponding median. All values shown are within two standard deviations of their 

corresponding median. 

 Figure 1-A Results from Intercomparison of Synthetic and Natural Samples 

 

Additionally, precision results for the 52 natural wet deposition samples were reported as absolute 

percent differences for replicate analyses. By parameter, less than 5 percent of approximately 260 

paired natural wet deposition sample data points exceeded 10 percent difference. These results are 

presented in the USGS report (USGS, 2008). 

The laboratory generally participates in two to three studies each year through the National Water 

Research Institute (NWRI) of Canada’s Proficiency Testing QA Program 

(http://www.ec.gc.ca/inre-nwri/Default.asp?lang=En&n=7A20877C-1) in addition to 

interlaboratory comparison studies for the USGS. MACTEC’s laboratory was rated free of 

systemic bias for all eight parameters for the two NWRI 2007 studies and was ranked number six 

out of 36 participating laboratories at the end of 2007 (Environment Canada, 2007). 

75

80

85

90

95

100

105

110

115

120

125

0 5 10 15 20 25 30 35 40 45

Sample Number

% Recovery

CA

MG

NA

K

NH4

CL

NO3

SO4

Ca2+

Mg2+

Na+

K+

NH4

+

ClNO3

-

SO4

2-
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 10

SO2 and NOx Emissions 

The ARP, established under Title IV of the 1990 CAAA, was designed to reduce the adverse 

effects of acid deposition by requiring major reductions of sulfur dioxide (SO2) and nitrogen 

oxides (NOx) emissions from the electric power industry. EPA recently assessed the effects of the 

ARP and related programs in its 2007 progress report (EPA, 2009). The 2007 report summarized 

2007 compliance with the ARP and reported progress toward achieving environmental goals. 

Under the ARP, SO2 reductions are achieved using a cap and trade program that sets a 

permanent cap on the total amount of SO2 that may be emitted by all regulated EGUs in the 

contiguous United States. The program began in 1995 and is underway with the final 2010 SO2

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 SO2 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 SO2. 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 SO2

emission cap. The program encourages early reductions by allowing sources to bank unused 

allowances in one year and use them in a later year. 

Unlike the cap and trade program used to reduce SO2 emissions, 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 ARP, and no nationwide cap has been placed on NOx

emissions under the ARP. Other NOx emission control programs have resulted in significant 

reductions in NOx emissions during the ozone season (see Chapter 4). Two prominent control 

programs are the OTC NOx Budget (1999-2002) and the NOx SIP Call/NBP, which began 

in 2003. 

The ARP is comprised of two phases for the reduction of SO2 and NOx. Phase I applied primarily 

to the largest coal-fired electric generation sources from 1995 through 1999 for SO2 and from 

1996 through 1999 for NOx. Phase II for both pollutants began in 2000. In 2007, the SO2 Phase II 

requirements affected 3,536 operating units. The Phase II NOx requirements applied to 978 of 

those operating units that exceed 25 megawatts and burned coal between 1990 and 1995 

(EPA, 2009). Under the ARP’s emission reduction requirements, total SO2 emissions from 

ARP sources were 8.9 million tons in 2007; total NOx emissions from ARP sources were 

3.3 million tons. 

 

Figure 1-5 presents state-by-state total annual SO2 emissions for Phase I and Phase II EGUs for 

four years (1990, 1995, 2000, and 2007). The most significant reduction in SO2 emissions 

occurred in 1995 in the Ohio River Valley where a large concentration of coal-fired power plants 

are located. Many of the states with the highest SO2 emissions realized the largest reductions. 

The six states with the greatest annual reductions include Ohio, which has decreased emissions 
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 11

by 1.3 million tons since 1990, Illinois, Indiana, Missouri, Tennessee, and West Virginia. Annual 

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

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

River experienced a significant decline in annual NOx emissions from 2000 through 2007. 

Coweeta, NC (COW137) 
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 12

Figure 1-5 Annual Utility SO2 Emissions (Phase I and Phase II Plants only) 

Figure 1-6 Annual Utility NOx Emissions (Phase I and Phase II Plants only) 

Annual Emissions of Sulfur Dioxide

1990

1995

2000

2007

Scale: Largest bar equals 

2,200 thousand short 

tons (Ohio, 1990).

Annual Emissions of Sulfur Dioxide

1990

1995

2000

2007

Scale: Largest bar equals 

2,200 thousand short 

tons (Ohio, 1990).

Annual Emissions of Sulfur Dioxide

1990

1995

2000

2007

Scale: Largest bar equals 

2,200 thousand short 

tons (Ohio, 1990).

Annual Emissions of Nitrogen Oxides

1990

1995

2000

2007

Scale: Largest bar equals 

534 thousand short 

tons (Ohio, 1990).

Annual Emissions of Nitrogen Oxides

1990

1995

2000

2007

Scale: Largest bar equals 

534 thousand short 

tons (Ohio, 1990).

Annual Emissions of Nitrogen Oxides

1990

1995

2000

2007

Scale: Largest bar equals 

534 thousand short 

tons (Ohio, 1990).
CASTNET Annual Report – 2007 Chapter 1: CASTNET Overview 13

Significant Events during 2007 

January 

EPA-sponsored sites began every-other-day zero/span/precision auto-calibration checks of the ozone analyzer. 

Daily data submittals started including NPS meteorological data. 

February

An independent audit of CASTNET data operations was performed. Overall, CASTNET data operations are 

managed satisfactorily. Only minor changes were recommended. 

April

A new EPA-sponsored site, Palo Duro Canyon State Park, TX (PAL190), began collecting samples on April 24. 

The site is operated in cooperation with the Texas Agricultural Experiment Station and Palo Duro 

Canyon State Park. 

Ozone sampling was discontinued at the two solar-powered CASTNET sites, Claryville, NY (CAT175) and 

Lye Brook, VT (LYE145). 

The MLM, the deposition model used for estimating dry deposition at CASTNET sites, was run to simulate dry 

deposition for the entire CASTNET database. The results were archived in the official EPA CASTNET 

database. 

May

Atmospheric sampling at the LYE145, VT site was discontinued, and the site was closed. 

Ten EPA-sponsored CASTNET sites began uploading ozone data to AIRNow. 

July

The Campbell Scientific, Inc. Model CR3000 data logger was selected as the most suitable model for upgrading 

the data collection instrumentation at all EPA-sponsored sites. 

EPA approved the CASTNET Quality Assurance Project Plan (QAPP) Revision 4.0. (MACTEC, 2007d) 

http://www.epa.gov/castnet/docs.html.

August

All EPA-sponsored CASTNET sites had laptop computers by August. 

September

The annual “floor-to-ceiling” CASTNET property audit was conducted by EPA and the Defense Contract 

Management Agency at MACTEC’s Gainesville, FL facility. The property management system was 

approved by the audit. 

EPA conducted a routine audit of CASTNET financial records. Management of CASTNET finances was deemed 

satisfactory.

October

Proper removal and disposal of the mercury thermometers at the EPA-sponsored sites began. A third party 

certified mercury disposal company was contracted for proper disposal. 

December

Dry deposition and ozone sampling were discontinued at North Cascades National Park, WA (NCS415). Dry 

deposition sampling was terminated at Death Valley National Monument, CA (DEV412). Sampling 

ended as of December 31. 

Day-night sampling was discontinued at Egbert, Ontario (EGB281).
CASTNET Annual Report – 2007 Chapter 2: Atmospheric Concentrations 14

Chapter 2: Atmospheric Concentrations 

Separate maps depicting the annual mean concentrations of sulfur dioxide (SO2), particulate 

sulfate (SO2-

4 ), total nitrate (HNO3 + NO3), and ammonium (NH+ 4 ) are presented in this chapter. 

Additional maps are provided in CASTNET Quarterly Reports (MACTEC 2007b, 2007e, 2008a, 

2008c). The concentration shading in the maps in this chapter was prepared using an algorithm 

based on inverse distance cubed weighting with a radius of influence of 500 kilometers (km). In 

addition, trends in concentrations over the 18-year period (1990-2007) were derived from the 34 

CASTNET reference sites (Figure 1-2) and are presented using box plots. 

Sulfur Dioxide 

Sulfur dioxide (SO2) is a gaseous pollutant emitted through the combustion of coal, oil, and other 

fossil fuels that contain sulfur. EGUs are the largest source of SO2 in the United States. SO2 

emissions are controlled by the ARP, and ambient concentrations are regulated by the National 

Ambient Air Quality Standards (NAAQS), which are designed to protect human health and 

welfare. The gas reacts in the atmosphere to form various acidic compounds including sulfuric 

acid (H2SO4), a significant contributor to acid rain, and ammonium sulfate [(NH4)

 

2SO4], a major 

component of fine particle matter (PM2.5). Fine particles are regulated as a criteria pollutant. 

They are harmful to human health and also contribute to degradation of atmospheric visibility 

and visual quality. 

Annual mean SO2 concentrations measured in 2007 are presented in Figure 2-1. Locations on the 

map with no value had insufficient data to estimate annual mean concentrations. For 2007, Palo 

Duro State Park, TX (PAL190) and Lye Brook, VT (LYE145) had insufficient data. PAL190, 

TX began filter pack sampling at the end of April, and sampling at LYE145, VT was terminated 

during May. A 7-state region centered on the Ohio River Valley in the eastern United States 

recorded mean concentrations greater than or equal to 5.0 micrograms per cubic meter (μg/m3

) of 

air. The region covers western Virginia, Kentucky, Indiana, Ohio, Pennsylvania, New Jersey, 

and Maryland. A concentration of 5.3 μg/m3 was observed at Sand Mountain, AL (SND152). 

Two sites recorded concentrations in excess of 10.0 μg/m3

 – Quaker City, OH (QAK172) with a 

value of 13.2 μg/m3

, the highest in the network, and Laurel Hill State Park, PA (LRL117) with a 

level of 10.1 μg/m3. Only three western sites (i.e., sites west of 100 degrees west longitude) 

measured an annual mean SO2 concentration greater than 1.0 μg/m3

. 

Three-stage filter packs are used to measure 7-day average concentrations of sulfur dioxide 

(SO2), sulfate (SO2-

4 ), nitric acid (HNO3), nitrate (NO3), ammonium (NH+ 4 ), chloride (Cl -), and 

several earth metals at 87 monitoring stations. Since 1990, measured concentrations of sulfur 

species have decreased significantly. Concentrations of nitrogen species remained relatively 

steady from 1990 until 2000 when they began to show a slight decline. Trends in mean annual 

SO2, SO2-

4 , total nitrate, and NH+ 4 concentrations are shown over the period 1990 through 2007 

using box plots for each year. All four parameters have declined over the 18-year period. 
CASTNET Annual Report – 2007 Chapter 2: Atmospheric Concentrations 15

Figure 2-2 shows a downward trend in annual mean SO 

2 concentrations aggregated over the 34 

reference sites (Figure 1-2) from 1990-2007. The most significant reduction in ambient SO2 was 

recorded in 1995 at the beginning of the ARP. Three-year mean concentrations for 1990-1992 

and 2005-2007 were 8.9 μg/m3 and 5.3 μg/m3, respectively. This change constitutes a 40 percent 

reduction in 3-year mean SO2 concentrations between the two time periods. The 2007 mean level 

was the lowest value measured by the reference sites in the history of the network. 

Figure 2-1 Annual Mean SO2 Concentrations (μg/m3) for 2007 

Figure 2-2 Trend in Annual Mean SO2 Concentrations (μg/m3

) 

Note: All trend diagrams are based on data collected at 34 eastern reference sites. 

0

4

8

12

16

20

199019911992199319941995199619971998199920002001200220032004200520062007

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

199019911992199319941995199619971998199920002001200220032004200520062007

Concentration (

μg/m3

)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2007 Chapter 2: Atmospheric Concentrations 16

Particulate Sulfate 

Particulate sulfate (SO2-

4 ) is formed in the atmosphere through the transformation of SO2 via both 

gas and aqueous (cloud) phase reactions. It typically exists in the atmosphere as ammonium 

sulfate [(NH4)

 

2SO4], a major component of PM2.5, which is regulated by the NAAQS. A map of 

annual mean particulate SO2-

4 concentrations measured during 2007 is provided in Figure 2-3. 

The map shows a large region of concentrations greater than or equal to 4.0 μg/m3. The region 

extends from Kentucky northeastward to Pennsylvania and also southeastward to Georgia. 

Concentration levels were fairly uniform throughout the region. Crockett, KY (CKT136) and 

Shenandoah National Park, VA (SHN418) were the two sites within this region that measured 

concentrations below 4.0 μg/m3

. The highest concentration of 5.0 μg/m3 was measured at 

QAK172, OH. Sulfate concentrations greater than or equal to 1.0 μg/m3 were measured in the 

southwestern United States at four sites in California, two in Arizona, and at Big Bend National 

Park, TX (BBE401). The western monitoring station at Theodore Roosevelt National Park, ND 

(THR422) measured a SO2-

4 concentration of 1.1 μg/m3

. 

Box plots of annual mean SO2-

4 concentrations from the eastern reference sites from 1990-2007 

are given in Figure 2-4 to illustrate the trend in atmospheric SO2-

4 over the 18-year period. The 

figure shows a significant decline in SO2-

4 with some interannual variability. The difference 

between 3-year means from 1990-1992 to 2005-2007 is 26 percent, a change from 5.4 μg/m3 to 

4.0 μg/m3, respectively. The 2007 mean level was slightly higher than 2006 value. The annual 

mean concentrations for 2006 and 2007 were the two lowest in the history of the network. 

Figure 2-3 Annual Mean SO2-

4 Concentrations (μg/m3

) for 2007 
CASTNET Annual Report – 2007 Chapter 2: Atmospheric Concentrations 17

Figure 2-4 Trend in Annual Mean SO2-

4 Concentrations (μg/m3

) 

Total Nitrate 

Nitric acid (HNO3) and particulate nitrate (NO3) are formed from emissions of oxides of nitrogen 

(NOx) produced through the combustion of fossil fuels. Mobile sources are the largest 

contributors of NOx emissions in the United States. Nitric acid exists as a gas and is a contributor 

to acid rain. Nitrate is a component of fine particles in the form of ammonium nitrate (NH4NO3) 

and as larger particles, e.g., sodium nitrate (NaNO3) or calcium nitrate [Ca(NO3)

 

2]. Figure 2-5 

shows a map of annual mean total nitrate (NO3 + HNO3) concentrations for 2007. Data on total 

nitrate levels are discussed in this chapter because total nitrate is more representative of a 

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

the individual constituents are thought to include measurement uncertainties (Lavery et al., 2009). 

The map of 2007 total nitrate concentrations shows three regions with mean concentrations 

greater than or equal to 3.0 μg/m3

. The largest region covers portions of five states in the 

Midwest, including Illinois, Indiana, Kentucky, Ohio, and Michigan. The second, smaller region 

covers southeastern Pennsylvania, New Jersey, and one site in Maryland. Three sites in 

California constitute the third region. Sequoia National Park, CA (SEK430) measured the highest 

concentration (4.7 μg/m3) in the network. 

Box plots of total nitrate levels for the period 1990-2007 are given in Figure 2-6. These data 

were aggregated from the 34 eastern reference sites. The data show no trend in mean 

concentrations until 2000 when nitrate levels began a slow decline in response to NOx emission 

control programs that were implemented in the eastern United States. See also the discussion in 

Chapter 4 about ozone season NOx emissions. The difference between 3-year means from 

1990-1992 to 2005-2007 is 20 percent, a change from 3.1 μg/m3 to 2.5 μg/m3

, respectively. The 

2007 mean level was slightly higher than the 2006 value. 

2

3

4

5

6

7

8

199019911992199319941995199619971998199920002001200220032004200520062007

Concentration (

μg/m3

)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

2

3

4

5

6

7

8

199019911992199319941995199619971998199920002001200220032004200520062007

Concentration (

μg/m3

)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2007 Chapter 2: Atmospheric Concentrations 18

Figure 2-5 Annual Mean Total Nitrate (NO3 + HNO3) Concentrations (μg/m3) for 2007 

Figure 2-6 Trend in Annual Total Nitrate (NO3 + HNO3) Concentrations (μg/m3

) 

Particulate Ammonium 

Particulate ammonium (NH+ 

4 ) is formed when ammonia (NH3) reacts with SO2-4 and NO3 particles 

in the atmosphere. Ammonia is formed through combustion processes and volatilization of farm 

animal wastes and fertilizers. Annual mean NH+ 

4 concentrations for 2007 are depicted in 

Figure 2-7. No monitors measured annual mean concentrations above 2.0 μg/m3

. The data show 

a flat geographic distribution across the eastern United States with most sites reporting values 

greater than or equal to 1.0 μg/m3

. The NH+ 

4 concentrations measured at western sites were low 

0

2

4

6

8

199019911992199319941995199619971998199920002001200220032004200520062007

Concentration (

μg/m3

)

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

199019911992199319941995199619971998199920002001200220032004200520062007

Concentration (

μg/m3

)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2007 Chapter 2: Atmospheric Concentrations 19

with only one site (SEK430, CA) measuring a concentration greater than or equal to 1.0 μg/m3. 

Box plots of NH+ 

4 concentrations are provided in Figure 2-8. The trend diagram shows a 

21 percent reduction in mean NH+ 

4 levels from 1990-1992 to 2005-2007. The 1990-1992 mean 

concentration was 1.8 μg/m3

, and the 2005-2007 value was 1.4 μg/m3. The 2007 mean value was 

slightly higher than the 2006 level. This is consistent with the 2007 SO2-

4 and total nitrate 

concentrations, whose values were higher in 2007 than in 2006, and is also consistent with the 

relationship between SO2-

4 and NH+ 4 as constituents of ammonium sulfate and the relationship 

between particulate NO3 and NH+ 

4 as components of ammonium nitrate. 

Figure 2-7 Annual Mean NH+ 

4 Concentrations (μg/m3

) for 2007 

Figure 2-8 Trend in Annual Mean NH+ 

4 Concentrations (μg/m3

) 

0

1

2

3

4

199019911992199319941995199619971998199920002001200220032004200520062007

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

199019911992199319941995199619971998199920002001200220032004200520062007

Concentration (

μg/m3

)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2007 Chapter 2: Atmospheric Concentrations 20

Cherokee Nation Clean Air Program 

Three CASTNET monitoring sites are located on tribal 

lands as a result of a unique cooperative effort among 

EPA Headquarters, EPA Regions, tribal governments, 

and the Inter-Tribal Environmental Council (ITEC). 

This collaborative effort has resulted in monitoring 

sites operating on the tribal lands of the Cherokee 

Nation in eastern Oklahoma (CHE185), the AlabamaCoushatta in eastern Texas (ALC188), and the Santee 

Sioux in northern Nebraska (SAN189). Figure 2-A 

depicts the three tribal CASTNET monitoring sites.

The Cherokee Nation operates the most extensive air 

quality monitoring network among tribal organizations. 

The Cherokee Nation, acting through its Cherokee 

Nation Environmental Programs (CNEP) office, has 

been active in ambient air quality monitoring since 1996. The Cherokee Nation currently 

operates a network of five PM2.5 monitors and various criteria pollutant monitors at five 

stations in northeastern Oklahoma (Figure 2-B) with four of the sites on lands within its 

jurisdictional boundary (green border). The Cherokee Nation also operates meteorological 

instruments at each of its five stations in Oklahoma. 

Figure 2-B Cherokee Nation Monitoring Sites in Northeastern Oklahoma 

Site ID AQS No. Site Name

C2 40-021-9002 Tahlequah 

C4 40-001-9009 Stilwell 

C5 40-071-9010 Newkirk

C6 40-097-9014 Pryor 

C7 40-135-9015 Marble City 

Figure 2-A Native American 

Participation in CASTNET 

Operations 
CASTNET Annual Report – 2007 Chapter 2: Atmospheric Concentrations 21

During the past five years, the CNEP has been increasing its participation in EPA, regional, 

and national monitoring programs and projects. The CNEP operates a CASTNET monitor at 

its Stilwell station (CHE185), an IMPROVE monitor at its Newkirk station, MDN samplers at 

its Stilwell and Newkirk stations, and passive ammonia and passive ozone samplers at its 

Stilwell station. The CNEP participated in the Midwest Ammonia Monitoring Project, 

operating an ammonia sampler at its Stilwell station from 2003 to 2006, and it has operated a 

continuous ammonia analyzer at that station since 2003 (Cherokee Nation, 2008). 

In the future, the CNEP will continue criteria pollutant and ammonia monitoring at its 

stations in Oklahoma and continue monitoring as a part of IMPROVE, CASTNET, and 

MDN. CNEP plans to add monitoring as part of the NADP Atmospheric Mercury Initiative 

and the NTN. The Cherokee Nation also plans to add trace gas analyzers to the Stilwell 

station and has requested an NCore designation (http://epa.gov/ttnamti1/ncore/index.html) 

for Stilwell (Cherokee Nation, 2008). 

The Cherokee Nation will use the data from these projects to assess the threat of air 

pollutants to the health of tribal members. The data will also be used to assess trends in air 

quality and to investigate the interrelationships among the measured air quality parameters 

such as those depicted in Figure 2-C, a time series of O 

3, SO 2, and HNO 3 concentrations over 

the period February 27, 2007 through November 27, 2007. 

Figure 2-C Time Series of Weekly HNO 

3, SO 2, and O 3 at CHE185, OK during March 

through November 2007 

20

25

30

35

40

45

50

55

60

65

70

2/27/073/13/073/27/074/10/074/24/07

5/8/07

5/22/07

6/5/07

6/19/077/10/077/24/07

8/7/07

8/21/07

9/4/07

9/18/0710/2/07

10/16/0710/30/0711/13/0711/27/07

Concentration [O3 (ppb)]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Concentration [HNO3, SO2 (μg/m3)]

Daily Max

HNO3

SO2

Weekly Mean of Daily Max 8-Hr Avg O3

HNO3 (filter pack)

SO2 (filter pack)
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 22

Chapter 3: Atmospheric Deposition 

CASTNET and its predecessor network (NDDN) were designed to provide estimates of the 

dry deposition of sulfur and nitrogen pollutants across the United States. CASTNET uses a 

hybrid approach to estimate dry deposition by combining measured pollutant concentrations 

and modeled deposition velocities. The MLM is the computer model used to calculate hourly 

deposition velocities for each monitoring site based on meteorological measurements and 

information on the surrounding vegetation. Total deposition is the sum of estimated dry 

deposition and measured wet deposition. Since 1990, total sulfur deposition has declined 

significantly. The data show a 34 percent reduction in 3-year mean sulfur fluxes over 

the period from 1990-1992 to 2005-2007. Total nitrogen deposition declined by 17 percent 

over the same period. Dry deposition is responsible for approximately 20 to 80 percent of 

total deposition depending on location and climate. The percentage is higher in major 

source regions. 

Gaseous and particulate sulfur and nitrogen pollutants are deposited into ecosystems 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 ecosystems based on measured meteorological 

and other environmental conditions. Flux values are estimated as the product of measured 

concentration data and MLM-modeled 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, and total nitrogen deposition decreased during 2007 

while dry nitrogen deposition increased slightly over 2006. Precipitation-weighted mean 

concentrations of total atmospheric sulfur have declined over the past 18 years. Nitrogen 

concentrations in precipitation have declined slowly since 1998. 

Dry deposition processes were simulated using the MLM (Figure 1-4) as described by 

Meyers et al. (1998) and Finkelstein et al. (2000). The most recent version of the MLM 

(Schwede, 2006) was provided to MACTEC in 2006. For this report, the MLM was run using 

CASTNET filter pack concentrations and meteorological measurements with information on 

land use, vegetation, and surface conditions to calculate deposition velocities for sulfur dioxide 

(SO2), nitric acid (HNO3), ozone (O3), and the particles sulfate (SO2-

4 ), nitrate (NO3), and 

ammonium (NH+ 

4 ). The deposition velocities were assumed to be identical for all particle species. 

Deposition velocity values were calculated for each pollutant species for each hour with valid 

meteorological data for each CASTNET site for the entire period 1990 through 2007. For a 

deposition velocity to be estimated, measurements of temperature, solar radiation, relative 

humidity, wind speed, and standard deviation of the wind direction (sigma theta) must all be 

valid for the hour. 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 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 
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 23

or extrapolation using valid annual estimates. For example, if an intermediate year was missing, 

the value for that year was interpolated from adjacent years. If a 2007 value was missing, the 

2007 value was assumed equal to the value for 2006. 

Sulfur Deposition 

MLM simulations were run separately for sulfur dioxide (SO2) and sulfate (SO2-

4 ). The model 

calculations were summed to obtain estimates of dry sulfur deposition [as sulfur (S)]. A map of 

dry sulfur deposition fluxes for 2007 is provided in Figure 3-1. The magnitude of a deposition 

rate is illustrated by the size of the circle. The map depicts five CASTNET sites with fluxes 

greater than or equal to 5.0 kilograms per hectare per year (kg/ha/yr). These sites are located in 

Kentucky, Indiana, Ohio, and Pennsylvania. Locations on the map with no value had insufficient 

data to calculate fluxes. The highest deposition rate was estimated for Quaker City, OH 

(QAK172) with a flux of 10.3 kg/ha/yr. The highest dry sulfur deposition rates coincided 

with the major SO2 source region (Figure 1-5) and declined sharply 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. 

Wet deposition values used to estimate total deposition represent 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 (when responsibility for wet deposition 

monitoring activities at CASTNET sites was transferred to NADP/NTN), 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. 

Figure 3-2 provides a map of estimates of total sulfur deposition. 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. Two regions were observed with total (dry + wet) sulfur deposition 

greater than 10.0 kg/ha/yr. The larger region extended from eastern Ohio and northern West 

Virginia across Pennsylvania to New Jersey. The second region encompassed Vincennes, IN 

(VIN140) and Mackville, KY (MCK131). The highest total sulfur deposition rate was estimated 

for QAK172, OH with a value of 21.0 kg/ha/yr. Sulfur deposition at western sites was less than 

2.0 kg/ha/yr. 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 MCK131, KY to less than 20 percent at sites in New England. 
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 24

Figure 3-1 Dry Sulfur (SO2 + SO2-

4 ) Deposition (kg/ha/yr) for 2007 

Figure 3-2 Total (Dry + Wet) Sulfur Deposition (kg/ha/yr) for 2007 

2.2 3.6

2.6

1.6

4.8

4.7

2.1

2.7

1.8

2.5

0.3

3.9

0.3

0.3

0.2

1.5

0.6

1.4

2.7

1.9

5.5

0.7

0.6

2.6 2.7

0.6

0.2

0.3

1.0

1.9

0.7

2.6

0.9

4.9

0.2

1.4

0.6

3.2

0.2

0.2

0.2

0.2

0.9

0.4

0.3

2.7

0.8

0.1

4.0

5.6 0.3

0.1

3.2

0.9

0.3

0.2

1.9

6.2

0.2 10.3

2.3

0.7

0.6

2.2

1.9

2.7

5.0

3.2

0.3

Site not pictured:

DEN417, AK 0.1

Deposition

10

5

1

7.3

7.6 7.0

7.9

0.6

7.5

12.0

6.1

5.4

5.3

6.1

7.1

12.4

4.2

1.4

7.9

5.5

6.4

4.3

0.5

1.3

3.9

5.5

6.3

8.6

3.6

9.9

0.6

6.3

4.8

6.2

0.8

0.6

0.6

0.7

3.8

4.7

0.7

12.0

3.9

0.5

9.8

10.7 1.2

1.2

9.9 11.3

6.2

1.6

0.4

0.6

6.2

13.3

0.9 21.0

7.8

2.8

0.8

7.6

7.0

6.1

6.1

12.3

8.0

1.2

11.2

5.6

0.7

0.5

Site not pictured:

DEN417, AK 0.2

Total Deposition

16

8

1.6

Dry Deposition

Wet Deposition

7.3

7.6 7.0

7.9

0.6

7.5

12.0

6.1

5.4

5.3

6.1

7.1

12.4

4.2

1.4

7.9

5.5

6.4

4.3

0.5

1.3

3.9

5.5

6.3

8.6

3.6

9.9

0.6

6.3

4.8

6.2

0.8

0.6

0.6

0.7

3.8

4.7

0.7

12.0

3.9

0.5

9.8

10.7 1.2

1.2

9.9 11.3

6.2

1.6

0.4

0.6

6.2

13.3

0.9 21.0

7.8

2.8

0.8

7.6

7.0

6.1

6.1

12.3

8.0

1.2

11.2

5.6

0.7

0.5

Site not pictured:

DEN417, AK 0.2

Total Deposition

16

8

1.6

Dry Deposition

Wet Deposition
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 25

Figure 3-3 presents box plots that show the trend in dry sulfur deposition (as S), and Figure 3-4 

shows the trend in annual total (dry + wet) sulfur deposition (as S) over the 18 years, 1990-2007. 

The box plots were based on data obtained from the 34 eastern CASTNET reference sites 

(Figure 1-2). Sulfur deposition at these sites declined significantly over the 18 years. Figure 3-5 

presents estimates of trends in dry, wet, and total deposition of sulfur (as S) on the same diagram. 

The trend line for precipitation-weighted mean sulfur concentrations in precipitation shows a 

decline over the last 18 years with small increases in 2005 through 2007. 

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 reflect changes in SO2 emissions. The year 2003 

experienced relatively high total sulfur deposition even though the sulfur concentration in 

precipitation was relatively low. Above average precipitation produced the relatively high total 

flux of sulfur. Overall, total sulfur deposition declined from a 1990-1992 mean of 13.2 kg/ha/yr 

to a 2005-2007 mean of 8.7 kg/ha/yr, a 34 percent reduction. 

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

Note: All trend diagrams are based on data collected at 34 eastern reference sites. 

0

2

4

6

8

10

12

199019911992199319941995199619971998199920002001200220032004200520062007

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

199019911992199319941995199619971998199920002001200220032004200520062007

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 26

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

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

Georgia Station, GA (GAS153) 

0

5

10

15

20

25

199019911992199319941995199619971998199920002001200220032004200520062007

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

199019911992199319941995199619971998199920002001200220032004200520062007

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

14

16

199019911992199319941995199619971998199920002001200220032004200520062007

Deposition (kg/ha/yr)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Concentration (mg/l)

Wet Deposition

Dry Deposition

Concentration in Precipitation

Total Deposition
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 27

Nitrogen Deposition 

Nitrogen deposition, as estimated from CASTNET filter packs, is comprised of fluxes of nitric 

acid (HNO3) + nitrate (NO3) + ammonium (NH+ 

4 ). Dry fluxes of nitrogen [as nitrogen (N)] for 

2007 are shown in Figure 3-6. This map was constructed by summing the individual MLM 

simulations for the three species. Almost all of the CASTNET sites in the eastern United States 

had estimated dry nitrogen deposition rates greater than 1.0 kg/ha/yr. The higher deposition rates 

were estimated for the Midwestern and Mid-Atlantic States. Deposition rates greater than or 

equal to 2.0 kg/ha/yr were also calculated for sites in North Carolina, Georgia, Alabama, and at 

Great Smoky Mountains National Park, TN (GRS420). In the East, the nitrogen fluxes were 

fairly uniform geographically, reflecting a wide distribution of nitrogen oxides (NOx) sources 

(such as motor vehicles). The values at the western sites ranged from 0.2 kg/ha/yr at Mount 

Rainier National Park, WA (MOR409) and Glacier National Park, MT (GLR468) to 3.0 kg/ha/yr 

at SEK430, CA. Locations on the map with no value had insufficient data to calculate fluxes. 

Figure 3-7 presents a map of total nitrogen deposition (as N) for 2007. The map was constructed 

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

shows that all but a few of the eastern sites recorded deposition rates greater than 5.0 kg/ha/yr. 

No values above 10.0 kg/ha/yr were observed in 2007. The highest total nitrogen flux 

(9.8 kg/ha/yr) was estimated for QAK172, OH. Fluxes with values less than 5.0 kg/ha/yr in the 

eastern United States were estimated for New England, upstate New York, and West Virginia. In 

the southeastern United States, sites in North Carolina, Tennessee, Florida, Arkansas, and 

Mississippi had estimated values below 5.0 kg/ha/yr. The values at the western sites ranged from 

1.1 kg/ha/yr at Lassen Volcanic National Park, CA (LAV410) to 4.1 kg/ha/yr in southern 

California at Converse Station (CON186) and Joshua Tree National Monument (JOT403). The 

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

contributions of dry sulfur deposition. Dry nitrogen deposition typically contributed less than 

50 percent of total deposition in the East. Interestingly, dry nitrogen deposition contributed more 

than half of total nitrogen deposition at sites in California, a region with elevated concentrations 

of nitrogen species and limited rainfall. The CASTNET site at QAK172 in eastern Ohio 

measured both the highest total sulfur and total nitrogen deposition rates. 

Figure 3-8 presents box plots that were constructed from data from the 34 reference sites. They 

show the 18-year trend in dry nitrogen deposition (as N). Figure 3-9 shows the trend in annual 

total (dry + wet) nitrogen deposition over the period 1990 through 2007. The box plots in 

Figure 3-8 show a consistent decline in dry nitrogen deposition beginning in 1999. Three-year 

mean fluxes declined from 2.2 kg/ha/yr in 1990-1992 to 1.7 kg/ha/yr in 2005-2007, a 17 percent 

decline. Total nitrogen deposition (Figure 3-9) is more variable because the annual wet and total 

fluxes depend on the amount of precipitation. The figure suggests that total flux has decreased 

since 1996 with a significant decline over the last four years. Again, the total flux in 2003 was 

relatively high because of unusually high precipitation. 
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 28

Estimates of trends in wet, dry, and total deposition of atmospheric nitrogen (as N) are presented 

in Figure 3-10. The trend in precipitation-weighted mean nitrogen concentrations in precipitation 

is also provided on the same figure. The trend line for precipitation-weighted mean nitrogen 

concentrations in precipitation shows a downward trend since 1998 with a slight increase over 

the last three years. This trend line reflects the effect of changes in NOx emissions. Total nitrogen 

deposition declined 17 percent over the 18 years. 

Centennial, WY (CNT169) 
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 29

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

4 ) Deposition (kg/ha/yr) for 2007 

Figure 3-7 Total (Dry + Wet) Nitrogen Deposition (kg/ha/yr) for 2007 

2.8 2.4

1.7

0.8

1.9

2.7

1.2

2.2

1.6

2.2

2.6

2.4

0.3

0.3

0.8

1.5

0.6

1.5

1.5

1.7

3.1

0.5

1.1

1.7 2.4

0.8

0.9

0.7

1.6

2.0

0.7

1.5

1.2

2.4

1.1

1.1

0.6

2.5

0.2

0.7

0.9

0.4

1.1

0.3

2.6

1.2

1.6

0.5

2.6

2.6 1.1

0.2

1.3

1.4

1.1

0.5

1.5

1.9

0.8 3.1

2.7

1.4

3.0

1.8

2.1

1.9

2.1

1.9

1.3

Site not pictured:

DEN417, AK 0.1

Deposition

3.0

1.5

0.3

6.4

6.3 6.3

6.4

4.1

4.8

8.0

5.8

3.6

5.3

5.1

7.7

9.3

3.4

1.9

6.4

3.8

5.8

4.5

1.9

3.1

5.6

5.6

4.8

7.2

4.1

6.6

2.2

5.2

4.7

5.2

1.3

1.8

1.8

1.7

6.3

4.5

4.1

8.3

7.0

1.1

8.4

7.4 2.8

1.6

7.2 6.8

5.2

2.8

1.3

1.5

5.0

7.4

2.9 9.8

8.9

7.1

3.9

6.3

5.8

9.2

6.6

8.4

5.7

4.0

8.4

4.9

1.5

2.1

Site not pictured:

DEN417, AK 0.2

Total Deposition

10

5

1

Dry Deposition

Wet Deposition

6.4

6.3 6.3

6.4

4.1

4.8

8.0

5.8

3.6

5.3

5.1

7.7

9.3

3.4

1.9

6.4

3.8

5.8

4.5

1.9

3.1

5.6

5.6

4.8

7.2

4.1

6.6

2.2

5.2

4.7

5.2

1.3

1.8

1.8

1.7

6.3

4.5

4.1

8.3

7.0

1.1

8.4

7.4 2.8

1.6

7.2 6.8

5.2

2.8

1.3

1.5

5.0

7.4

2.9 9.8

8.9

7.1

3.9

6.3

5.8

9.2

6.6

8.4

5.7

4.0

8.4

4.9

1.5

2.1

Site not pictured:

DEN417, AK 0.2

Total Deposition

10

5

1

Dry Deposition

Wet Deposition
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 30

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

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

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

0

1

2

3

4

5

6

199019911992199319941995199619971998199920002001200220032004200520062007

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

199019911992199319941995199619971998199920002001200220032004200520062007

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

4

6

8

10

12

14

199019911992199319941995199619971998199920002001200220032004200520062007

Deposition (kg/ha/yr)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

4

6

8

10

12

14

199019911992199319941995199619971998199920002001200220032004200520062007

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

199019911992199319941995199619971998199920002001200220032004200520062007

Deposition (kg/ha/yr)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Concentration (mg/l)

Wet Deposition

Dry Deposition

Concentration in Precipitation

Total Deposition
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 31

Figure 3-A Time Series of Wet Nitrogen 

Deposition Values at ROM406, CO 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

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

Wet Deposition (kg/ha) Wet Deposition (kg/ha/yr) 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

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

Wet Deposition (kg/ha) Wet Deposition (kg/ha/yr)

Rocky Mountain Atmospheric Nitrogen and Sulfur 

(RoMANS) Study 

Rocky Mountain National Park (RMNP) is home to several sensitive, high-elevation 

ecosystems that are subject to wet and dry deposition of nitrogen and sulfur pollutants. 

Evidence suggests the park is experiencing changes in ecosystem function and surface water 

chemistry in addition to visibility degradation as a result of atmospheric pollutants. 

Consequently, the park has specified a goal for a critical load of 1.5 kg/ha/yr of wet nitrogen 

deposition (Baron, 2006). This deposition rate is considered protective of park ecosystems. 

To obtain a better understanding of the effects of air pollution on RMNP, NPS is sponsoring 

the Rocky Mountain Atmospheric Nitrogen and Sulfur (RoMANS) Study. The objectives of 

the study are to improve understanding of visibility degradation, improve understanding of 

atmospheric wet and dry deposition to park ecosystems, provide assessment of sources and 

contributors to air pollution, and increase knowledge about specific pollutants contributing 

to air quality problems. The cornerstone of the study was a large field campaign and 

modeling analysis that was conducted during the spring and summer of 2006 

(RoMANS, 2006). 

NPS has sponsored standard CASTNET measurements of sulfur and nitrogen pollutants and 

O 

3 near RMNP since 1994, and NADP/NTN measurements of wet nitrogen and sulfur 

deposition have been collected since 1980. Figure 3-A shows a time series of wet nitrogen 

deposition fluxes from 1995-2007. These data were interpolated from NADP/NTN 

measurements for the location of the NPS-sponsored CASTNET site (ROM406) based on 

methodology discussed in the sulfur deposition section on page 23. It should be noted that 

NPS policy on nitrogen deposition is based on measurements from the NADP Loch Vale 

site (CO98). Figure 3-A shows that annual wet nitrogen deposition rates have been 

consistently higher than the critical 

load of 1.5 kg/ha/yr. The values 

ranged from about 1.6 to 3.5 kg/ha/yr. 

The peak value in 2004 was produced 

by unusually wet weather. 

The spring (3/26-4/28) and summer 

(7/6-8/11) 2006 RoMANS field 

studies included precipitation 

chemistry and gaseous and particulate 

pollutant and meteorological 

measurements at 12 to 13 sites in and 

near RMNP. The core RoMANS site
CASTNET Annual Report – 2007 Chapter 3: Atmospheric Deposition 32

was collocated with CASTNET and 

IMPROVE measurements at the 

EPA/NPS collocated site 

(ROM406/206). Recognizing that 

CASTNET and NADP/NTN 

measurements do not represent all 

nitrogen inputs to deposition, 

RoMANS sponsored measurement 

and modeling studies of previously 

unquantified nitrogen species that 

contribute to wet and dry deposition. 

The RoMANS investigators 

concluded that wet organic nitrate 

(ON) and ambient levels of 

ammonia (NH 

3), total reactive 

oxides of nitrogen (NOy), and other 

nitrogen species contributed to the 

nitrogen deposition budget. Barna 

and his colleagues (2008) performed 

Comprehensive Air Quality Model 

with Extensions (CAMx) model 

calculations, which estimated that 

the deposition of NH 

3, NOy, and ON 

is almost double the quantity of dry 

nitrogen deposition as currently 

estimated by CASTNET. Figures 

3-B and 3-C show pie charts of 

nitrogen deposition based on 

measurements collected during the 

spring and summer field studies. The two figures illustrate the contributions of wet ON and 

dry NH 

3 deposition to the nitrogen budget. These two parameters are not measured by 

NADP/NTN and CASTNET. Dry deposition of all species was less than wet deposition 

during both the spring and summer field campaigns. Wet deposition of nitrogen more than 

doubled during the summer while sulfur deposition increased by approximately 50 percent. 

ON in the particle and gas phase was not measured in RoMANS and was not included in the 

two figures. 

Ongoing work includes continuing analyses of the spring and summer field measurements 

and the identification of meteorological conditions and source types and regions that 

contribute to deposition in RMNP. 

Figure 3-B Relative contributions of individual 

N deposition pathways to total measured N 

deposition at the RoMANS Core Site during the 

spring campaign

Spring

NO3 wet

23.76%

NH3

13.75% NH4 dry

3.15%

ON wet

16.65%

NH4 wet

34.11%

HNO3

7.49%

NO3 dry

1.08%

Summer

NO3 wet

27.83%

NH3

16.48%

NH4 dry

1.41%

NH4 wet

34.24%

ON wet

12.33%

NO3 dry

0.20%

HNO3

7.50%

Figure 3-C Relative contributions of individual 

N deposition pathways to total measured N 

deposition at the RoMANS Core Site during the 

summer campaign
CASTNET Annual Report – 2007 Chapter 4: Ozone Concentrations 33

Chapter 4: Ozone Concentrations 

Most CASTNET sites operate an ozone (O3) analyzer that measures hourly concentrations. 

While CASTNET is not a compliance network, the data collected provide information on 

geographic patterns in regional O3 and the extent to which rural areas potentially exceed 

concentration values mandated by the NAAQS. The 8-hour O3 standard is a useful measure for 

assessing status and trends in rural O3 in order to gauge the success of EPA emission reduction 

programs such as the NOx SIP Call. The analyses presented in this section provide maps and 

examine trends in the fourth highest daily maximum 8-hour average O3 concentrations. 

On March 12, 2008, EPA revised the NAAQS for ground-level O3. EPA revised the 8-hour 

primary standard to 0.075 ppm (EPA, 2008b). The previous standard, set in 1997 (EPA, 1997), 

was 0.08 ppm. Because O 

3 is measured out to three decimal places, the standard effectively 

became 0.084 ppm (84 ppb) as a result of rounding. The 1997 ozone NAAQS was exceeded if 

the fourth highest daily maximum ozone concentration was 0.08 ppm (85 ppb) or higher. EPA 

also revised the secondary standard to the 0.075 ppm (75 ppb) level making it identical to the 

primary standard. A primary standard is designed to protect human health while a secondary 

standard is designed to protect public welfare including protection against visibility impairment 

and damage to sensitive ecosystems and forests. An area will meet the new standard if the design 

value, or the 3-year average of the annual fourth highest daily maximum 8-hour average 

concentrations, is less than or equal to 0.075 ppm. EPA will issue final designations of 

attainment, nonattainment, and unclassifiable areas no later than March 2010 unless there is 

insufficient information to make these designation decisions. In that case, EPA will issue 

designations no later than March 2011. These designations will be made using O 

3 measurements 

from 2006-2008 or later. This chapter presents O 

3 concentrations in terms of both standards, 0.08 

ppm and 0.075 ppm. More information on trends in ozone concentrations and ozone monitoring 

requirements can be found in the EPA (2008a) report on National Air Quality Status and Trends 

through 2007. 

CASTNET is the primary monitoring network for measuring rural, ground-level ozone (O3) 

concentrations in the United States. The network provides information critical for the 

evaluation of the NAAQS, which are written in terms of fourth highest daily maximum 

8-hour average O3 concentrations (EPA, 1997). These annual fourth highest daily 

maximum concentrations are averaged over 3-year periods to determine potential areas 

of nonattainment. 

During early 2008, EPA revised the ozone NAAQS to 0.075 parts per million (ppm; 

EPA, 2008b). Consequently, the 2007 and 2005-2007 O3 concentrations were analyzed in 

terms of both the current and the revised standards. Nearly half (24) of CASTNET monitoring 

stations in the eastern United States and five sites in California would have recorded 

exceedances of the new standard for the period 2005-2007. 
CASTNET Annual Report – 2007 Chapter 4: Ozone Concentrations 34

At ground level, O3 is an air pollutant that can cause harmful effects to the human respiratory 

system as well as damage to vegetation and ecosystems. Ground-level ozone is formed in the 

troposphere when volatile organic compounds (VOCs) and NO 

x react in the presence of sunlight. 

Emissions from motor vehicle exhaust, EGUs, industrial facilities, chemical solvents, and 

gasoline vapors are the major anthropogenic sources of NO 

x and VOCs. Also, emissions of 

biogenic VOCs from trees and other vegetation contribute to O3 formation in rural areas 

(EPA, 2008d). Emission reduction strategies for VOCs have been successful in reducing high O3

concentrations in and downwind of urban areas. NO 

x emission reductions were mandated by the 

ARP, NOx SIP Call, and other NO 

x emission control programs to reduce O3 concentrations. 

These reductions will continue to be required in order to achieve additional declines in 8-hour 

average O3 levels. 

The concentration shading for the figures used in this chapter was prepared based on rural ozone 

concentration data using an algorithm with inverse distance cubed weighting with a radius of 

influence of 500 km. Consequently, concentration estimates for areas with limited monitoring 

site coverage, such as much of Texas, and concentrations depicted for urban areas, such as 

Atlanta, are approximations. Additional maps of ozone concentrations can be viewed on the Web 

site for the NPS Air Atlas (http://science.nature.nps.gov/AirAtlas/AirAtlas0105/viewer.htm). 

CASTNET Ozone Concentrations 

Figure 4-1 presents 3-year averages of the fourth highest daily maximum 8-hour average O3

concentrations for 2005-2007. During this period, 3-year averages of the fourth highest daily 

maximum 8-hour average O3 concentrations were greater than or equal to 85 ppb at four sites in 

California, one in New Jersey, one in Maryland, and at Great Smoky Mountains National Park, 

TN (GRS420). Figure 4-1 also shows the geographic distribution of fourth highest daily 

maximum 8-hour average O3 concentrations greater than 75 ppb, which exceed the revised 

NAAQS (0.075 ppm). The geographic extent of 8-hour levels greater than the new standard is 

obviously much larger than for values greater than or equal to 85 ppb. Twenty-four eastern and 

five sites in California exceeded the new standard. Although the CASTNET monitors are not 

Part 58 compliant, these 3-year average (2005-2007) concentrations represent current design 

values for the 8-hour NAAQS for O3 as measured at each site. For example, the value of 111 ppb 

at Converse Station, CA (CON186) would have to be reduced to 84 ppb to achieve the old 

standard and to 75 ppb to achieve the new standard. Design values change as a new 3-year 

database becomes available. 
CASTNET Annual Report – 2007 Chapter 4: Ozone Concentrations 35

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

Concentrations (ppb) for 2005-2007 

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

Concentrations (ppb) for 2000-2002 
CASTNET Annual Report – 2007 Chapter 4: Ozone Concentrations 36

National Ambient Air Quality Standards for Ozone 

New 8-Hour Ozone Standards 

EPA’s lowering of the primary and secondary standards to 0.075 ppm (EPA, 2008b) will 

improve protection of public health and sensitive vegetation and ecosystems. An area will 

meet the new standard if the 3-year average of the annual fourth highest daily maximum 

8-hour average concentration is less than or equal to 0.075 ppm. The 3-year average reduces 

the influence that meteorological conditions have on the extent and magnitude of ozone 

formation. For more information on the revised 8-hour ozone standard, visit the EPA Web 

site http://www.epa.gov/air/ozonepollution/pdfs/2008_03_factsheet.pdf. More general 

information on ozone also can be found at http://www.epa.gov/groundlevelozone/. 

Assessing Compliance with the New Standard 

For regulatory purposes, EPA collects ozone measurements on an hourly basis. To comply 

with the standard, daily maximum 8-hour average ozone concentrations at a monitor cannot 

exceed 0.075 ppm during the ozone season. To determine if a monitor is in or out of 

attainment: 

 Hourly ozone measurements are used to compute 8-hour average concentrations. 

 The daily maximum 8-hour average is calculated for each day. 

 For each year, the fourth highest daily maximum concentration is calculated. 

 These annual fourth highest daily maximum concentrations are averaged over 

3-year periods. 

 If the 3-year average of the fourth highest daily maximum 8-hour average O 

3

concentration is greater than 0.075 ppm (or 75 ppb), the area will be designated as a 

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

To comply with the new standard, the 3-year average of the daily maximum 8-hour average 

ozone concentrations at a monitor cannot exceed 0.075 ppm during the ozone season. EPA 

estimates, based on 2004-2006 data, 345 counties would have exceeded the new NAAQS 

(http://www.epa.gov/air/ozonepollution/pdfs/2008_03_monitors_violating_2008.pdf). EPA 

will not designate areas as nonattainment based on these three years but will use 2006-2008 

or later data. EPA is scheduled to make final determinations for designation of areas as 

attainment, nonattainment, or unclassifiable no later than March 2011. For further 

information, see the EPA Fact Sheet for the final revisions to the ozone NAAQS at 

http://www.epa.gov/air/ozonepollution/pdfs/2008_03_factsheet.pdf. 
CASTNET Annual Report – 2007 Chapter 4: Ozone Concentrations 37

For comparison to current design values, 3-year averages of the fourth highest daily maximum 

8-hour average O3 concentrations for 2000-2002 are presented in Figure 4-2. This period was 

selected for comparison because the NOx SIP Call was implemented in 2003. A comparison of O3

data for these two 3-year periods illustrates the effectiveness of the emission reduction programs. 

Using the 84 ppb standard, nineteen eastern and three 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, and Illinois. Exceedances were also 

recorded in Kentucky, Tennessee, and greater Atlanta. Using the 75 ppb standard, the map shows 

that most of the Eastern sites measured concentrations greater than 75 ppb and five monitors in 

California measured concentrations greater than 75 ppb. The period 2005-2007 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 O3

NAAQS. However, sites in California, along the East coast, and at GRS420, TN continued to 

measure elevated ozone levels. 

Measurements of fourth highest daily maximum 8-hour average concentrations during 2007 

(Figure 4-3) were somewhat higher than 2006 but were considerably lower than concentrations 

measured over the period 2000-2002. Six CASTNET monitors in the East measured fourth 

highest daily maximum 8-hour average O3 concentrations greater than or equal to 85 ppb. These 

sites are widely scattered geographically and are located in five states. Peak values of 92 ppb 

were measured in the East at Beltsville, MD (BEL116) and Oxford, OH (OXF122). GRS420, TN 

continued to measure 8-hour average O3 levels above 85 ppb. Five of seven sites in California 

measured fourth highest daily maximum 8-hour average O3 concentrations greater than or equal 

to 85 ppb. The monitor at Joshua Tree National Monument, CA (JOT403) measured the highest 

value (104 ppb) in the network during 2007. Figure 4-3 also depicts the geographic distribution 

of fourth highest daily maximum 8-hour average O3 concentrations greater than 75 ppb. 

The box plots depicting the trend in 

fourth highest daily maximum 

8-hour average O3 concentrations 

from the CASTNET eastern 

reference sites are provided in 

Figure 4-4. The median value 

(75 ppb) from the reference site data 

for 2007 was the third lowest in the 

18-year period. It represents a 

significant improvement over the 

2002 median value of 87.5 ppb and 

the peak median value of 90 ppb that 

was measured in 1998 and 1999. 
CASTNET Annual Report – 2007 Chapter 4: Ozone Concentrations 38

Figure 4-3 Fourth Highest Daily Maximum 8-Hour Average Ozone Concentrations (ppb) 2007 

Figure 4-4 Trend in Fourth Highest Daily Maximum 8-Hour Average Ozone Concentrations (ppb) 

 Eastern United States 

Note: All trend diagrams are based on data collected at 34 eastern reference sites 

60

80

100

120

199019911992199319941995199619971998199920002001200220032004200520062007

Concentration (ppb)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

60

80

100

120

199019911992199319941995199619971998199920002001200220032004200520062007

Concentration (ppb)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2007 Chapter 4: Ozone Concentrations 39

Nitrogen Oxides and Ozone 

EPA recently analyzed changes in NO 

x emissions and O3 concentrations from 1990-2007. The 

findings were published in a report (EPA, 2008c) that summarized 2007 compliance and 

environmental results from the NO 

x SIP Call/NBP. The report discussed the NO x and VOCs 

emission control programs, their effectiveness, subsequent changes in NO 

x and VOCs emissions 

in the eastern United States, and the resulting changes in ozone levels. Figure 4-5, which was 

taken from the NO 

x Budget Trading Program Compliance and Environmental Results (EPA, 

2008c), depicts ozone season NO 

x emissions from all NBP sources in the eastern United States 

from 1990-2007 (see Figure 4-7 for states covered by the NBP). 

Specifically, EPA reported that NBP sources emitted 506,000 tons of NO 

x in 2007, constituting a 

74 percent reduction in ozone season NO 

x emissions from NBP sources from 1990 levels. Most of 

the emission reductions since 1990 were the result of the ARP NO 

x reduction program and other 

federal, state, and local programs. The significant declines after 2000 resulted from the OTC 

trading program, which was in effect between 1999 and 2002, and the NBP, which began in 2003. 

Figure 4-5 Ozone Season NOx Emissions from All NBP Sources 

Source: EPA 

Trends in CASTNET measurements of total nitrate concentrations (Figure 4-6) measured during 

the ozone season (May through September) at the 34 eastern reference sites show almost no 

decline from 1990 through 1999 and a more significant decline thereafter as the emission 

reductions from the OTC and NBP became effective. Three-year median concentrations before 

(2000-2002) and after (2003-2005) the implementation of the NBP were 2.43 μg/m3 and 1.87 

μg/m3 , respectively. The most recent three-year (2005-2007) median value was 1.77 μg/m3 

. 

Figure 4-7, which was taken from the 2008 EPA Report on NBP compliance (EPA, 2008c), 

shows the relationship between NO 

x emission reductions from power industry sources and 

changes in O3 levels at CASTNET and EPA AQS sites in the eastern United States over the 

period 2002-2007. The figure shows declines in concentrations in all NBP states. The figure also 

shows a strong association between regions with greatest emission reductions and downwind 

monitoring sites showing the greatest improvement in ozone levels. 

1,924

1,256

849

609 549 508 506

0

500

1,000

1,500

2,000

2,500

1990 2000 2003 2004 2005 2006 2007

Ozone Season NOx Emissions (thousand tons)
CASTNET Annual Report – 2007 Chapter 4: Ozone Concentrations 40

Figure 4-6 Trend in Ozone Season (May thorough September) Mean Total Nitrate 

 (NO3 + HNO3) Concentrations (μg/m3) – Eastern United States 

Figure 4-7 Reductions in Ozone Season Power Industry NOx Emissions and Changes in 8-Hour 

Ozone, 2002 versus 2007 

Percent Change in Meteorologically Adjusted 

Seasonal 8-hour Ozone 2002 versus 2007 

 Decrease Between 15% and 24% 

 Decrease Between 5% and 15% 

 Decrease Less Than 5% 

 Increase Less Than 5% 

Ozone Season NOx Emissions 

2002 versus 2007 (tons) 

 Increase Less Than 2,000 

 Decrease Less Than 25,000 

 Decrease Between 25,000 and 50,000 

 Decrease Between 50,000 and 75,000 

 Decrease Between 75,000 and 100,000 

Source: EPA 

0

1

2

3

4

5

6

7

199019911992199319941995199619971998199920002001200220032004200520062007

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

199019911992199319941995199619971998199920002001200220032004200520062007

Concentration (

μg/m3

)

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile

90th Percentile

75th Percentile

Median

Mean

25th Percentile

10th Percentile
CASTNET Annual Report – 2007 Chapter 4: Ozone Concentrations 41

Ozone Episode in ACA416, ME during 2007 

Historical CASTNET O 

3 and 

meteorological data show summer high 

pressure systems have occasionally 

produced transport of ozone plumes 

along the East coast from Washington, 

DC all the way to Acadia National 

Park, ME (ACA416). A 3-day ozone 

episode was observed at the park 

during June 25-28, 2007. Surface 

weather conditions are illustrated in 

Figure 4-A, which shows a Bermuda 

high off the East coast that produced 

southwesterly winds over New 

England. A time series of hourly O 

3

concentrations observed at 

ACA416, ME during June 24-July 1, 

2007 is provided in Figure 4-B. Daily 

maximum 8-hour average O 

3

concentrations are also shown. The 

8-hour levels are plotted at 00:00 for 

each day. Figure 4-B shows the gradual 

buildup of O 

3 beginning on June 24 

with a subsequent peak hourly value of 

113 ppb and a peak daily 8-hour 

average value of 92 ppb on June 27. 

The episode was terminated by the 

passage of a cold front on June 28. 

Backward trajectories from the NOAA Hybrid Single-Particle Lagrangian Integrated 

Trajectory (HYSPLIT) Model are provided in Figures 4-C and 4-D for June 27, 2007, 00:00 

Coordinated Universal Time (UTC) [June 26, 2007, 19:00 Eastern Standard Time (EST)] and 

for June 27, 2007, 23:00 UTC (June 27, 2007, 18:00 EST). Trajectories ending at 

ACA416, ME are given for 50 meters (m), 100 m, and 500 m above ground level. The three 

trajectories ending at ACA416, ME at 19:00 EST on June 26 show disparate origins. The 

50 m trajectory originated in eastern Pennsylvania. The other two trajectories originated 

further west. All three trajectories show significant subsidence. Hourly ozone concentrations 

reached 79 ppb at 00:00 EST and 98 ppb at 19:00 EST on June 26. The recording of high 

concentrations late in the day suggests the transport of ozone and its precursors along the 

trajectory pathways. 

Figure 4-A Surface Weather Map for 6/27/07 

Figure 4-B Time Series of Ozone 

Concentrations at ACA416, ME 

0

20

40

60

80

100

120

6/24/07 6/25/07 6/26/07 6/27/07 6/28/07 6/29/07 6/30/07 7/1/07

Concentration (ppb)

Hourly Ozone

Daily Max 8-Hr Avg Ozone

85 ppb
CASTNET Annual Report – 2007 Chapter 4: Ozone Concentrations 42

Figure 4-C Backward Trajectories ending Figure 4-D Backward Trajectories ending 

at June 27, 2007 00:00 UTC at June 27, 2007 23:00 UTC 

Concentrations increased to an hourly value of 113 ppb at 18:00 EST and a daily maximum 

8-hour value of 92 ppb on June 27. These concentrations were the highest measured at 

ACA416, ME during 2007. The 50 m and 100 m trajectories that ended at the park at 

18:00 EST on June 27 both traveled along the East coast from North Carolina and Virginia 

over the Gulf of Maine to ACA416, ME. The transport of ozone and its precursors along 

this heavily populated region resulted in elevated ozone concentrations. Ray and his 

colleagues (Ray et al., 1996) demonstrated that ozone precursors over the Gulf of Maine add 

to the pool of ozone available for transport to coastal locations by daily sea breezes. In 

other words, offshore urban plumes are 

brought inland by sea breezes and 

contribute to ozone measured at the 

national park. The combination of longrange transport and local wind patterns 

resulted in ozone concentrations in excess 

of the national standard at the park, almost 

200 miles from Boston, the nearest major 

population center.

Acadia National Park, ME (ACA416) 

Photo Courtesy NPS 
CASTNET Annual Report – 2007 Chapter 5: Data Quality 43

Chapter 5: Data Quality 

The CASTNET Quality Assurance (QA) program was designed to ensure that all reported data are 

of known and documented quality in order to meet CASTNET objectives and to be reproducible 

and comparable with data from other monitoring networks and laboratories. The 2007 QA program 

elements are documented in the QAPP, Revision 4.0 (MACTEC, 2007d). The QAPP is 

comprehensive and includes standards and policies for all components of project operation from 

site selection through final data reporting. It includes major sections on field measurements, 

chemical analysis of field samples, data management, and assessments and response actions. 

Standard operating procedures are included as appendices. 

Data quality indicators (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 2007 data were analyzed and compared with historical data 

collected during the period 1990-2006. The information in this report is supplemented by analyses 

that are discussed in quarterly CASTNET Quality Assurance Reports (MACTEC, 2007c; 2007f; 

2007g; 2008b). These QA reports are produced four times per year with the fourth quarter report 

including an annual summary. 

Huntington Wildlife Forest, NY (HWF187) 

CASTNET measurements and supporting activities are assessed routinely. This assessment 

assures that CASTNET data meet data quality objectives (DQO). DQO are evaluated using 

data quality indicators (DQI) such as precision, accuracy, and completeness. Measurements 

taken during 2007 and historical data collected over the period 1990-2006 were analyzed 

relative to DQI 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 – 2007 Chapter 5: Data Quality 44

Analysis of CASTNET Laboratory and Field Blanks 

CASTNET utilizes an open-face, three-stage filter pack to measure sulfur and nitrogen species. 

Filter pack handling procedures are designed to minimize handling and transfers and, 

consequently, any opportunities for contamination and misdirection. Sample integrity is 

maintained by ensuring that materials in contact with filters do not affect measured analytes. 

These materials must be traceable to enable documentation of their contact with a sample. 

These goals are supported by documented chain of custody and evaluation of filter acceptance 

tests, laboratory blanks, and field (trip) blanks. 

Two laboratory blanks are prepared each week while the filter packs are being prepared for the 

field. Each blank contains a filter from the same lot as the filters used in preparation of the 

field filter packs. The filters selected for the laboratory blanks are placed directly into the 

extraction bottles, bypassing the handing involved in loading and unloading the filter packs. 

These filters then are extracted and analyzed to provide an indication of the effects of filter 

handling within the laboratory. 

Field blanks are prepared quarterly for 

each CASTNET site. The filter packs 

used for the field blanks contain a 

nonstandard connection that cannot 

be installed in the sampling system in 

the field. The filter pack with the 

field blank is prepared and loaded 

according to standard CASTNET 

procedures. The field blanks are 

clearly identified and labeled to 

prohibit their removal from the 

resealable plastic bag used for 

shipping. When the field blank filter 

is returned from a site, it is unpacked, 

extracted, and analyzed following 

procedures for exposed filters. The 

field blank samples indicate the 

potential for contamination from the 

filter pack loading, shipping, and 

unloading processes. 

Figure 5-A Summary of Filter Blanks for 2007 

0 3 6 9 12 15

CALCIUM,

TEFLON

CHLORIDE,

TEFLON

MAGNESIUM,

TEFLON

NH3+NH4 AS

N, TEFLON

NO3 AS N,

NYLON

NO3 AS N,

TEFLON

POTASSIUM,

TEFLON

SO4, NYLON

SO4, TEFLON

SO4,

CELLULOSE

SODIUM,

TEFLON

Total Micrograms

Mean Concentration

Field Blanks - Max

Field Blanks - Mean

Lab Blanks - Max

Lab Blanks - Mean

Reporting Limit

46.5 

59.0 
CASTNET Annual Report – 2007 Chapter 5: Data Quality 45

The results of the field and laboratory blanks for 2007 are summarized in Figure 5-A and Table 

5-A. All results were within the two times reporting limit criterion, with the exception of two 

cellulose field blank results and one Teflon® field blank result. For example, 340 Teflon® field 

blanks were analyzed for NO3 and only 13 values were above the 0.20 microgram (μg) 

reporting limit, and the highest value was 0.423 μg. The cellulose filters are impregnated with 

potassium carbonate (K 

2CO 3) and undergo additional handling. Consequently, about a third of 

the cellulose field and laboratory blanks tested positive for small amounts of sulfate. However, 

no SO2-

4 values exceeded three times the reporting limit and only two exceeded two times the 

limit. In short, very few field and laboratory blanks showed significant contamination, and the 

magnitude of concentrations above detection was small. 

Table 5-A Evaluation of 2007 Field and Laboratory Filter Blanks

Parameter Name 

Reporting 

Limit 

Total μg 

Total 

Number 

Number >

Reporting 

Limit 

Average 

Total μg 

Average 

Absolute 

Deviation 

Maximum 

Total μg 

FIELD BLANKS 

Teflon®- NH+ 

4 -N 0.500 341 1 0.500 0.000 0.515 

Teflon®- NO3-N 0.200 340 13 0.203 0.006 0.423 

Teflon®- SO2-

4 1.000 340 0 1.000 0.000 1.000 

Cl- 0.500 340 1 0.507 0.013 2.790 

Ca2+ 0.075 340 26 0.106 0.037 0.251 

Mg2+ 0.075 340 0 0.075 0.000 0.075 

Na + 0.125 340 4 0.130 0.009 1.230 

K + 0.125 340 4 0.135 0.012 0.177 

Nylon - NO3-N 0.200 342 0 0.200 0.000 0.200 

Nylon - SO2-

4 1.000 342 33 1.012 0.021 2.000 

Cellulose - SO2-

4 2.000 358 138 2.313 0.405 4.550* 

LABORATORY BLANKS 

Teflon®- NH+ 

4 -N 0.500 96 0 0.500 0.000 0.500 

Teflon®- NO3-N 0.200 92 3 0.205 0.010 0.468 

Teflon®- SO2-

4 1.000 92 0 1.000 0.000 1.000 

Cl- 0.500 92 0 0.500 0.000 0.500 

Ca2+ 0.075 92 3 0.101 0.037 0.520 

Mg2+ 0.075 92 0 0.075 0.000 0.075 

Na + 0.125 92 0 0.125 0.000 0.125 

K + 0.125 92 0 0.132 0.010 0.150 

Nylon - NO3-N 0.200 96 0 0.200 0.000 0.500 

Nylon - SO2-

4 1.000 96 3 1.006 0.012 1.300 

Cellulose - SO2-

4 2.000 96 32 2.250 0.344 4.050 

 Note: Average is defined as average of total number of blanks 

 Maximum is highest value from total number of blanks 

 * A second blank had a value of 4.250 μg compared to the reporting limit of 2.00 μg. No other blanks exceeded the limit. 
CASTNET Annual Report – 2007 Chapter 5: Data Quality 46

Figure 5-1 Historical and 2007 Precision Data for Atmospheric Concentrations 

Precision 

Exposed Filter Concentrations 

Figure 5-1 provides a bar chart in which the bars represent precision estimates for five CASTNET 

analytes. Precision is defined as the mean absolute relative percent difference (MARPD). 

 Equation 5-1 

Where: 

S1 = The value for the primary sampler 

S2 = The value for the collocated sampler 

k = The number of pairs of valid data 

Historical (1990-2006) data for all 11 collocated site pairs operated over the history of the network 

and the 2007 data for the current collocated sites at Mackville, KY (MCK131/231) and Rocky 

Mountain National Park, CO (ROM406/206) are provided. 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 chloride are excluded from this figure but are shown later in 

Figure 5-2. The historical results vary from just over 4 percent for particulate sulfate (SO2-

4 ) to more 

than 12 percent for particulate nitrate (NO3). The historical MARPD for SO2-

4 met the criterion for 

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

dioxide (SO2) and nitric acid (HNO3) were above the 5 percent criterion but are considered 

reasonable. The results for ammonium (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 concentrations are the lowest 

of all the pollutants, and nitrate species include sampling artifacts (Lavery et al., 2009). 

0

2

4

6

8

10

12

14

16

12345

MARPD

Overall - 1990 to 2006

Mackville, KY - 2007

Rocky Mtn, CO - 2007

SO2 SO4

2- HNO3 NO3-

 NH4

+

200

21

1 21

1

x

SS

SS

k

MARPD j

k

j

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

− = ∑=
CASTNET Annual Report – 2007 Chapter 5: Data Quality 47

The 2007 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 five parameters. Four 

parameters (SO2, SO2-

4 , HNO3, and NH+ 4 ) met precision criteria. The 2007 results for ROM406/206 

showed greater precision than historical results for HNO3, NO3, and NH+ 

4 ; the results for HNO3 and 

NH+ 

4 met the DQI criteria. The precision criterion for NO3 was not met at either site during 2007. 

The precision results for SO2, SO2-

4 , HNO3, and NH+ 4 for MCK131/231 are considered acceptable. 

The precision results were worse for ROM406/206 because of lower concentrations measured at 

this site. Overall, the filter pack precision results for 2007 were about the same as the 2006 results. 

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

Precision1 Accuracy2 Nominal 

Analyte Method (MARPD) (%) Reporting Limits 

Ammonium (NH +

4 ) Automated colorimetry 10 90 - 110 0.020 mg-N/L 

Sodium (Na +) ICP-AES 5 95 - 105 0.005 mg/L 

Potassium (K +) ICP-AES 5 95 - 105 0.006* mg/L 

Magnesium (Mg2+) ICP-AES 5 95 - 105 0.003 mg/L 

Calcium (Ca2+) ICP-AES 5 95 - 105 0.006* mg/L 

Chloride (Cl-) Ion chromatography 5 95 - 105 0.020 mg/L 

Nitrate (NO3) Ion chromatography 5 95 - 105 0.008 mg-N/L 

Sulfate (SO2-

4 ) Ion chromatography 5 95 - 105 0.040 mg/L 

Note: 1 This column lists precision goals for both network precision calculated from collocated filter samples and laboratory precision based on 

replicate samples. The goal for the ICP-AES precision relative percent difference (RPD) criterion changed from 10 percent to 5 percent at the 

onset of the current contract beginning on July 30, 2003. The precision criterion is applied as described below: 

 QC conditions: (v1 = initial response; v2 = replicate response; RL = nominal reporting limit) 

 Condition 1: if (v1 or v2 < RL and the absolute value of (v1 – v2) < RL) = OK 

 Condition 2: if (v1 – v2) < RL and v1 < 5 x RL) = OK 

 Condition 3: if (v1 > 5*RL and RPD < 5%) = OK 

 Status: one of the conditions is OK = Precision QC Passes 

2

 This column lists laboratory accuracy goals based on reference standards and continuing calibration verification spikes. The goal for the 

ICP-AES accuracy criterion changed from 90 – 110 percent recovery to 95 – 105 percent for continuing calibration verification spikes 

beginning on July 30, 2003. The criterion remains 90 – 110 percent for ICP-AES reference standards. 

 * Effective September 26, 2007, changed to 0.006 mg/L from 0.003 mg/L for calcium and 0.005 mg/L for potassium 

 MARPD = See Equation 5-1 

 

 Percent recovery = Equation 5-2 

 

 RPD = Equation 5-3 

 

 For more information on the analytical methods and associated precision and accuracy criteria used for 2007, see the CASTNET QAPP – 

Revision 4.0 (MACTEC, 2007d). 

Historical and 2007 precision statistics for four cations and chloride (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. As discussed in earlier CASTNET reports (e.g., MACTEC, 2004 

and MACTEC, 2003), the very high historical MARPD for sodium (Na +) was the result of sample 

bottle contamination. These bottles are no longer purchased. Also, acceptance testing of the 

Teflon® filters was instituted for the trace cations and Cl- in 2003. The 2007 precision results show 

that Na + met the precision criterion at MCK131/231. The 2007 results are somewhat worse than the 

precision data for 2006. 

100

2

1 x S

S

200

21

21 x SS

SS

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

−
CASTNET Annual Report – 2007 Chapter 5: Data Quality 48

Table 5-2 summarizes 2007 precision results by quarter for the two sets of collocated sites. See the 

2007 Quarterly Data Reports (MACTEC, 2007b; 2007e; 2008c; 2008a) and QA Quarterly Reports 

(MACTEC, 2007c; 2007f; 2007g; 2008b) for discussions of quarterly precision data. 

Figure 5-2 Historical and 2007 Precision Data for Cation and Cl- Concentrations 

Table 5-2 Collocated Precision Results (MARPD) for 2007 Filter Pack Data by Quarter 

Site Pairs SO2-

4 NO3 NH+ 

4 Ca2+ Mg2+ Na + K + HNO3 SO2

Total 

NO3 Cl-

 MCK 131/231, KY 

Quarter 1 3.76 7.24 6.19 12.72 9.04 8.77 6.57 5.43 3.31 5.78 20.70 

Quarter 2 1.67 8.71 1.94 6.15 6.85 3.34 8.37 3.59 1.39 4.17 2.13 

Quarter 3 3.40 11.25 3.02 5.95 5.31 5.15 6.94 2.39 1.42 2.91 1.06 

Quarter 4 1.27 3.99 2.10 4.86 3.57 3.01 10.87 5.16 3.27 3.62 5.64 

2007 2.53 7.80 3.31 7.42 6.19 5.07 8.19 4.14 2.34 4.12 7.38 

 ROM 406/206, CO 

Quarter 1 7.96 9.15 4.64 6.38 5.82 4.42 12.94 3.44 10.70 4.22 16.64 

Quarter 2 3.96 6.60 3.82 4.07 7.17 6.01 7.94 2.29 3.65 2.16 2.95 

Quarter 3 9.41 11.85 8.48 10.04 13.08 10.25 14.84 7.22 10.53 7.40 5.55 

Quarter 4 8.24 18.02 7.55 9.55 8.76 13.15 10.54 4.44 6.48 6.84 5.71 

2007 7.39 11.41 6.12 7.51 8.71 8.46 11.56 4.35 7.84 5.16 7.71 

The 2007 analytical precision results for five 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 

results of the original concentrations. The laboratory precision data met the 5 percent measurement 

criterion listed in Table 5-1. 

0

2

4

6

8

10

12

14

16

18

20

12345

Overall - 2000 to 2006

Mackville, KY - 2007

Rocky Mtn, CO - 2007

MARPD

Ca2+ K+ Mg2+ Na+ Cl-
CASTNET Annual Report – 2007 Chapter 5: Data Quality 49

Ozone Concentrations 

CASTNET QA procedures for the EPA-sponsored ozone (O3) analyzers are different from the EPA 

QA requirements for State and Local Monitoring Stations (SLAMS) monitoring (EPA, 1998). The 

QA procedures for the O3 analyzers at the NPS-sponsored CASTNET sites also do not meet the 

SLAMS requirements. In any event, the operation of the collocated EPA and NPS O3 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 O3. The 

precision criterion for the collocated O3 data is 10 percent. 

MARPD statistics were calculated from hourly O3 measurements obtained from the collocated sites 

MCK131/231, KY and ROM406/206, CO during 2007. 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 O3 concentrations by quarter for 2007. The data 

show the historical 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. The 2007 precision data also 

met the 10 percent criterion for the first and second quarters for both MCK131/231 and 

ROM406/206. The results for MCK131/231 did not meet the criterion during the fourth quarter. 

The poor results for this quarter were the result of sampling problems and periods of very low 

concentrations. The two analyzers at ROM406/206 did not meet the precision criterion during the 

third and fourth quarters. The hourly O3 data from ROM406 were consistently higher than the data 

from ROM206 during the third quarter. The reason for this systematic difference has not been 

determined despite acceptable automatic calibration results and an acceptable external audit. The 

imprecision of the O3 data during the fourth quarter was caused by sampling train problems. 

Figure 5-3 Precision Results for Laboratory Replicate Samples (2007) 

0

1

2

3

4

5

1 2 3 4 5 6 7 8 9 10 11

contribute to SO 2

Mean Absolute Relative Percent

Difference 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

Cl-
CASTNET Annual Report – 2007 Chapter 5: Data Quality 50

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

Measurement Criteria* 

Parameter Method Precision Accuracy 

Wind Speed Anemometer ± 0.5 m/s The greater of ± 0.5 m/s for winds < 

5 m/s or ± 5% for winds ≥ 5 m/s 

Wind Direction Wind Vane ± 5° ± 5° 

Sigma Theta Wind Vane Undefined Undefined 

Relative Humidity Thin Film Capacitor ± 10% (of full scale) ± 5%, relative humidity > 85% 

± 20%, relative humidity ≤ 85% 

Solar Radiation Pyranometer ± 10% (of reading taken 

at local noon) 

± 10% 

Precipitation Tipping Bucket Rain 

Gauge 

± 10% (of reading) ± 0.05 inch† 

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

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

Surface Wetness Conductivity Bridge Undefined Undefined 

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

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

Note: °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 2007 Precision Data by Quarter for Ozone Concentrations 

Table 5-4 Collocated Precision Results (MARPD) for 2007 Ozone Concentrations by Quarter 

Sampling Period MCK131/231, KY ROM406/206, CO 

Quarter 1 0.80 3.52 

Quarter 2 1.16 7.81 

Quarter 3 3.25 16.84 

Quarter 4 12.18 11.78 

 2007 4.35 9.91 

0

5

10

15

20

25

30

First

Quarter

Second

Quarter

Third

Quarter

Fourth

Quarter

Overall - 1990 to 2006

Mackville, KY - 2007

Rocky Mtn, CO - 2007

MARPD
CASTNET Annual Report – 2007 Chapter 5: Data Quality 51

Comparison of CASTNET and AQS Methods For Determining Precision 

The precision results shown in Table 5-2 for the collocated sampling systems at 

MCK131/231, KY and ROM406/206, CO were recalculated for the four quarters of 2007 using 

the methods suggested in the “Air Quality System Precision and Accuracy Summarization 

Formulas” (EPA, 2004). The results of the recalculations for ozone and six filter pack 

measurements are shown in Table 5-B. 

Table 5-B Precision Results for Ozone and Filter Pack Analytes Using AQS Methods 

O 

3 SO2-4 NO3 NH+ 4 HNO 3 SO 2 Total NO3

MRPD Std. Dev. MRPD Std. Dev. MRPD Std. Dev. MRPD Std. Dev. MRPD Std. Dev. MRPD Std. Dev. MRPD Std. Dev. 

MCK131/231, KY Quarter 1 -0.56 3.22 -1.67 7.15 -4.59 8.79 -0.86 9.00 0.37 8.78 -0.60 4.35 -2.25 7.39 

Quarter 2 0.08 1.76 -1.54 1.31 -8.64 5.27 -1.56 1.82 -2.60 4.12 -0.77 1.38 -3.96 3.28 

Quarter 3 -0.80 3.97 -3.31 8.32 -9.15 12.37 -3.00 8.15 -1.12 2.70 0.96 1.63 -2.02 3.37 

Quarter 4 12.11 24.34 -0.05 1.89 -2.15 4.59 0.34 2.82 0.83 5.60 0.20 4.22 -0.34 4.23 

Average 2.71 -1.64 -6.13 -1.27 -0.63 -0.05 -2.14 

ROM406/206, CO 

Quarter 1 2.92 4.23 -4.22 18.89 1.01 13.42 1.74 6.68 -0.44 5.34 -1.29 14.36 0.19 5.81 

Quarter 2 7.42 7.84 3.67 2.60 -0.38 9.17 3.46 3.21 2.08 1.84 2.94 4.83 1.56 2.28 

Quarter 3 16.23 9.44 1.42 16.07 3.50 17.79 0.24 16.32 -2.80 15.28 -2.23 17.83 -1.77 15.50 

Quarter 4 11.78 9.66 4.80 9.34 14.52 24.28 5.15 7.43 4.00 5.90 1.81 9.26 6.66 6.49 

Average 9.59 1.42 4.66 2.64 0.71 0.31 1.66 

The principal difference in method is that AQS precision is based on the mean of relative 

percent difference values, i.e., MRPD, while CASTNET utilizes MARPD. AQS also requires 

the calculation of the standard deviation of the relative percent difference (RPD) values. 

MRPD is the mean of the RPD values shown in Equation 5-3. 

The precision criterion for O 

3 concentrations is 10 percent. The AQS O 3 precision data are 

consistent with the CASTNET MARPD data. Results for fourth quarter for MCK131/231 and 

for third and fourth quarter for ROM406/206 exceeded the criterion. As expected, the 

calculated AQS precision values for the six sets of filter pack concentrations are lower than the 

corresponding MARPD values. The MARPD criterion is more stringent than MRPD. The filter 

pack precision criterion is 10 percent for NH+ 

4 and 5 percent for the other pollutants. The AQS 

precision data indicate the precision criteria would have been met for all parameters except 

particulate nitrate for two quarters at MCK131/231 and one quarter at ROM406/206 and total 

nitrate at ROM406/206 for the fourth quarter. 
CASTNET Annual Report – 2007 Chapter 5: Data Quality 52

Accuracy 

Laboratory Filter Concentrations 

Accuracy of laboratory measurements is assessed through the analysis of reference samples and 

continuing calibration verification (CCV) spikes. Reference samples and CCV are procured from 

independent suppliers and are National Institute of Standards and Technology (NIST) traceable. 

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. In 2007, the CCV were analyzed every tenth sample to verify 

that instrument calibration had not drifted beyond established limits. Table 5-5 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 2007. Table 5-5 corroborates the 

precision results that were shown in Figure 5-3. 

Table 5-5 Filter Pack Quality Control Summary for 2007 

Reference 

Sample1 Recovery 

(%R) 

Continuing Calibration 

Verification Samples (%R) 

In-Run Replicate2

(RPD) 

 Filter Type  Parameter 

 Mean  Std. Dev.  Count3 Mean  Std. Dev.  Count3 Mean  Std. Dev.  Count3

Teflon® SO2-

4 99.79 1.64 154 100.11 1.07 809 0.60 1.65 332 

 NO3 101.71 1.40 154 99.68 1.14 809 1.20 1.98 332 

 NH+ 

4 100.08 2.41 118 99.82 2.09 649 0.63 1.04 269 

 Ca2+ 100.42 2.91 140 100.75 1.28 805 2.02 3.33 334 

 Mg2+ 100.70 1.16 140 99.99 0.89 805 1.99 2.11 334 

 Na+ 97.01 2.11 140 100.12 1.15 805 1.59 2.02 334 

 K+ 98.04 2.91 140 100.21 1.09 805 3.09 3.79 334 

 Cl- 100.82 1.50 154 99.83 2.19 809 1.31 1.91 332 

Nylon SO2-

4 100.61 1.32 151 98.90 2.11 798 1.27 1.65 331 

 NO3 99.47 1.73 151 98.76 2.50 798 0.80 1.45 331 

Cellulose SO2-

4 101.64 1.51 187 99.28 1.71 809 3.16 5.33 274 

 Note: % R = percent recovery (see Equation 5-2) 

 RPD = relative percent difference (see Equation 5-3) 

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 – 2007 Chapter 5: Data Quality 53

Continuous Measurements 

Table 5-6 presents field accuracy results for 2007 based on instrument challenges performed using 

independent reference standards during site calibration visits. CASTNET sites were calibrated 

every six months with NIST-traceable standards. The calibration results were evaluated using the 

accuracy criteria listed in Table 5-3. Each parameter was within its criterion with at least 90 percent 

frequency with the exception of high (> 85 percent) relative humidity at 88.2 percent, solar 

radiation at 89.1 percent, and wind direction north at 88.0 percent. However, these results did not 

adversely affect data collection because data are not considered invalid unless criteria are exceeded 

by more than two times the criterion. Using the two times standard, the three parameters passed 

with 95.0, 96.4, and 97.4 percent frequency, respectively. 

Table 5-6 Accuracy Results for 2007 Field Measurements 

Parameter Percent Within Criterion 

Temperature (0°C) 98.3 percent 

Temperature (ambient) 97.4 percent 

Delta Temperature (0°C) 99.1 percent 

Delta Temperature (ambient) 98.0 percent 

*Relative Humidity > 85% 88.2 percent 

Relative Humidity ≤ 50% 98.3 percent 

*Solar Radiation 89.1 percent 

*Wind Direction North 88.0 percent 

Wind Direction South 95.7 percent 

Wind Speed < 5 m/s 100.0 percent 

Wind Speed ≥ 5 m/s 94.8 percent 

Precipitation 100.0 percent 

Wetness (w/in 0.5 volts) 100.0 percent 

Ozone Slope 98.1 percent 

Ozone Intercept 99.1 percent 

Flow Rate 95.8 percent 

Note: °C = degrees Celsius 

 m/s = meters per second 

 * Per CASTNET project protocols, data are flagged as “suspect” (S) but still considered valid if the calibration 

criterion is not exceeded by more than its magnitude (i.e., if within 2x the criterion). The percent within 2x 

criterion for these parameters ranged from 95.0 percent to 97.4 percent. 
CASTNET Annual Report – 2007 Chapter 5: Data Quality 54

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 5-5 presents historical (black) and 2007 

completeness data for all sites for measured filter concentrations and continuous measurements and 

calculated parameters. The figure shows that the 2007 direct measurements met the 90 percent 

completeness goal and exceeded 95 percent for six continuous measurements, including filter pack 

flow, plus annual mean concentrations. The four parameters derived from model results exceeded 

83 percent completeness for 2007. Completeness results for 2007 are better than historical results 

for all parameters except atmospheric concentrations, temperature, and precipitation. 

Figure 5-5 Historical and 2007 Percent Completeness of Measurements and Modeled Estimates 

(black bars are 1990-2006) 

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 – 2007 Chapter 5: Data Quality 55

Conclusion 

DQI results demonstrate that field and laboratory processes were adequately monitored through 

QA/QC procedures and were generally free of systemic bias during 2007. Accuracy data met the 

established criteria for field and laboratory parameters with the exception of relative humidity > 85 

percent, solar radiation, and wind direction north. However, since the criterion was exceeded by a 

value less than its own magnitude, the associated continuous data collected are considered valid. 

Precision data for SO 

2, SO2-4 , HNO 3, and ammonium are considered acceptable. Precision data for 

nitrate 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 nitrate species. 

The 2007 precision data for ozone concentrations also met the 10 percent criterion for the first and 

second quarters for both MCK131/231 and ROM406/206. The results for MCK131/231 did not 

meet the criterion during the fourth quarter, and ROM406/206 data did not meet the precision 

criterion during the third and fourth quarters. The imprecise results for these three quarters were the 

result of various sampling problems and periods of very low concentrations. Completeness criteria 

were met for all parameters in 2007. Completeness results for 2007 were generally better than 

historical results. 

Horton Station, VA (VPI120) 
CASTNET Annual Report – 2007 References R-1

References 

Barna, M.G., Rodriquez, M.A., Moore, T., Malm, W.C., Gebhart, K.A., and Schichtel, B.A. 

2008. Predicting Dry Deposition of Total Nitrogen at Rocky Mountain National Park, 

Extended Abstract # 107, A&WMA’s Aerosol & Atmospheric Optics: Visual Air Quality 

and Radiation Conference, April 28 – May 2, 2008, Moab, UT. 

Baron, J.S. 2006. Hindcasting nitrogen deposition to determine an ecological critical load. 

Ecological Applications, 16(2), 433-439. 

Cherokee Nation. 2008. History of the Cherokee Nation Clean Air Program. Cherokee Nation. 

Environment Canada. 2007. Water Science and Technology Directorate. Proficiency Testing 

Program. Rain and Soft Waters PT Studies 0089 and 0090 Report. Prepared for 

MACTEC Engineering and Consulting, Inc. September 12. 

Finkelstein, P.L., Ellestad, T.G., Clarke, J.F., Meyers, T.P., Schwede, D.B., Hebert, E.O., and 

Neal, J.A. 2000. Ozone and Sulfur Dioxide Dry Deposition to Forests: Observations and 

Model Evaluation. J. Geophys. Res. 105:D12:15,365-15,377. 

Lavery, T.F., Rogers, C.M., Baumgardner, R., and Mishoe, K.P. 2009. Intercomparison of 

CASTNET NO3 and HNO 

3 Measurements with Data from Other Monitoring Programs. 

Journal of Air & Waste Management Association (JAWMA). 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2008a. Clean Air Status and Trends 

Network (CASTNET) Fourth Quarter 2007 Data Report. Prepared for U.S. 

Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets 

Division, Washington, D.C. Contract No. 68-D-03-052. Gainesville, FL. 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2008b. Clean Air Status and Trends 

Network (CASTNET) Fourth Quarter 2007 Quality Assurance Report with 2007 Annual 

Summary. Prepared for U.S. Environmental Protection Agency (EPA), Office of Air and 

Radiation, Clean Air Markets Division, Washington, D.C. Contract No. 68-D-03-052., 

Gainesville, FL. 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2008c. Clean Air Status and Trends 

Network (CASTNET) Third Quarter 2007 Data Report. Prepared for U.S. Environmental 

Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets Division, 

Washington, D.C. Contract No. 68-D-03-052., Gainesville, FL. 
CASTNET Annual Report – 2007 References R-2

References (continued) 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2007a. Clean Air Status and Trends 

Network (CASTNET) 2006 Annual Report. Prepared for U.S. Environmental Protection 

Agency (EPA), Office of Air and Radiation, Clean Air Markets Division, Washington, 

D.C. Contract No. 68-D-03-052. Gainesville, FL. 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2007b. Clean Air Status and Trends 

Network (CASTNET) First Quarter 2007 Data Report. Prepared for U.S. Environmental 

Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets Division, 

Washington, D.C. Contract No. 68-D-03-052. Gainesville, FL. 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2007c. Clean Air Status and Trends 

Network (CASTNET) First Quarter 2007 Quality Assurance Report. Prepared for U.S. 

Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets 

Division, Washington, D.C. Contract No. 68-D-03-052. Gainesville, FL. 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2007d. Clean Air Status and Trends 

Network (CASTNET) Quality Assurance Project Plan (QAPP), Revision 4.0. Prepared 

for U.S. Environmental Protection Agency (EPA), Research Triangle Park, NC, Contract 

No. 68-D-98-112. Gainesville, FL. 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2007e. Clean Air Status and Trends 

Network (CASTNET) Second Quarter 2007 Data Report. Prepared for U.S. 

Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets 

Division, Washington, D.C. Contract No. 68-D-03-052. Gainesville, FL. 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2007f. Clean Air Status and Trends 

Network (CASTNET) Second Quarter 2007 Quality Assurance Report. Prepared for U.S. 

Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets 

Division, Washington, D.C. Contract No. 68-D-03-052. Gainesville, FL. 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2007g. Clean Air Status and Trends 

Network (CASTNET) Third Quarter 2007 Quality Assurance Report. Prepared for U.S. 

Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets 

Division, Washington, D.C. Contract No. 68-D-03-052. Gainesville, FL. 

MACTEC Engineering and Consulting, Inc. (MACTEC). 2004. Clean Air Status and Trends 

Network (CASTNET) 2002 Quality Assurance Report. Prepared for U.S. Environmental 

Protection Agency (EPA), Research Triangle Park, NC, Contract No. 68-D-03-052. 

Gainesville, FL. 
CASTNET Annual Report – 2007 References R-3

References (continued) 

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. 

Ray, J.D., Heavner, R.L., Flores, M., and Michaelsen, C.W. 1996. Surface level measurements 

of ozone and precursors at coastal and offshore locations in the Gulf of Maine. J. 

Geophys. Res. 101:D22:29,005-29,011. 

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. 

Rocky Mountain Atmospheric Nitrogen and Sulfur (RoMANS) Study Brochure. 2006. 

http://vista.cira.colostate.edu/improve/Publications/GrayLit/022_ROMANSbrocure/RoM

ANSBrochure.pdf (accessed November 3, 2008). 

Schwede, D.B. 2006. A Comparison of the Deposition Velocity Estimates from the CASTNET 

and CAPMoN Networks. (Working paper). Research Triangle Park, NC. 

U.S. Environmental Protection Agency (EPA). 2009. Acid Rain and Related Programs 2007 

Progress Report. January 2009. EPA 430-K-08-010. 

U.S. Environmental Protection Agency (EPA). 2008a. National Air Quality Status and Trends 

through 2007. November 2008. EPA-454/R-08-006. 

U.S. Environmental Protection Agency (EPA). 2008b. National Ambient Air Quality Standards 

for Ozone; Final Rule, Federal Register 73, no. 60 (March 2008). EPA-HQ-OAR-2005-

0172. 

U.S. Environmental Protection Agency (EPA). 2008c. NOx Budget Trading Program. 2007 

Program Compliance and Environmental Results. December 2008. EPA 430-R-07-009. 

U.S. Environmental Protection Agency (EPA). 2008d. Vegetation and Air Quality. 

http://www.epa.gov/heatisland/strategies/level3_vegairquality.html (accessed August 10, 

2008). 

U.S. Environmental Protection Agency (EPA). 2004. Air Quality System Precision and 

Accuracy Summarization Formulas. 
CASTNET Annual Report – 2007 References R-4

References (continued) 

U.S. Environmental Protection Agency (EPA). 2000. National Air Quality and Emissions 

Trends Report, 1998. EPA-454-R-00-003. OAQPS, RTP, NC 27711. 

 

U.S. Environmental Protection Agency (EPA). 1998. Quality Assurance Requirements for State 

and Local Air Monitoring Stations (SLAMS). 40 CFR 50, Appendix L. 

U.S. Environmental Protection Agency (EPA). 1997. National Ambient Air Quality Standards 

for Ozone. 40 CFR 50. 

U.S. Geological Survey (USGS). 2008. Office of Water Quality. Branch of Quality Systems. 

Interlaboratory Comparison Program http://bqs.usgs.gov/precip/new/frontpage_home.htm 

(accessed July 28, 2008). 
CASTNET Annual Report – 2007 Appendix A 

AAppppeennddiixxAA

Locational and Operational Characteristics of CASTNET Sites
CASTNET Annual Report – 2007 Appendix A 
CASTNET Annual Report – 2007 A-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 

Pack1 

Ozone and 

Meteorology2 

Wet Deposition 

Primary Land Use 

Alabama SND152 Sand Mountain 12/27/88 34.2894 85.9704 352 • • Agri. RAlaska DEN417 Denali National Park 10/06/98 63.7258 148.9633 661 • • Forested CArizona CHA467 Chiricahua National Monument 04/25/89 32.0092 109.3892 1570 • • ◊ Range CGRC474 Grand Canyon National Park 05/16/89 36.0597 112.1822 2073 • • Forested CPET427 Petrified Forest National Park 09/24/02 34.8225 109.8919 1723 • • Desert FlArkansas CAD150 Caddo Valley 10/04/88 34.1792 93.0989 71 • • Forested RCalifornia CON186 Converse Station 06/17/03 34.1941 116.9130 1837 • • Agri./Forested CDEV412 Death Valley National Monument 02/21/95 36.5092 116.8481 125 • • Desert CJOT403 Joshua Tree National Monument 02/16/95 34.0714 116.3906 1244 • • Desert CLAV410 Lassen Volcanic National Park 07/25/95 40.5403 121.5764 1756 • • Forested CPIN414 Pinnacles National Monument 05/16/95 36.4850 121.1556 335 • • Forested CSEK430 Sequoia National Park 04/07/05 36.4894 118.8269 457 • • Forested MYOS404 Yosemite National Park 09/25/95 37.7133 119.7061 1605 • • Forested C
CASTNET Annual Report – 2007 A-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 

Pack1 

Ozone and 

Meteorology2 

Wet Deposition 

Primary Land Use 

Colorado GTH161 Gothic 05/16/89 38.9573 106.9854 2926 • • ◊ Range CMEV405 Mesa Verde National Park 01/10/95 37.1983 108.4903 2165 • • Forested CROM206 Rocky Mountain National Park 07/03/01 40.2778 105.5453 2743 • • Forested CROM406 Rocky Mountain National Park 12/20/94 40.2778 105.5453 2743 • • Forested CConnecticut ABT147 Abington 12/28/93 41.8402 72.0111 209 • • ◊ Urban-Agri. RFlorida EVE419 Everglades National Park 10/06/98 25.3911 80.6806 2 • •4 Swamp FIRL141 Indian River Lagoon 07/09/01 30.1065 80.4554 2 • • Beach FSUM156 Sumatra 12/28/88 30.1065 84.9938 14 • • ◊ Forested FGeorgia GAS153 Georgia Station 06/28/88 33.1812 84.4100 270 • • Agri. RIllinois ALH157 Alhambra 06/28/88 38.8690 89.6229 164 • • ◊ Agri. FBVL130 Bondville 02/09/88 40.0520 88.3725 212 • • Agri. FSTK138 Stockton 12/28/93 42.2872 89.9998 274 • • Agri. R
CASTNET Annual Report – 2007 A-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 

Pack1 

Ozone and 

Meteorology2 

Wet Deposition 

Primary Land Use 

Indiana SAL133 Salamonie Reservoir 06/28/88 40.8164 85.6608 250 • • Agri. FlVIN140 Vincennes 08/04/87 38.7406 87.4844 134 • • Agri. RKansas KNZ184 Konza Prairie 03/26/02 39.1021 96.6096 348 • • Prairie FlKentucky CDZ171 Cadiz 10/01/93 36.7841 87.8500 189 • • Agri. RCKT136 Crockett 08/24/93 37.9211 83.0658 455 • • Agri. RMAC426 Mammoth Cave National Park 07/24/02 37.1319 86.1478 243 • • Agri./Forested RMCK131 Mackville 07/31/90 37.7044 85.0483 353 • • Agri. RMaine ACA416 Acadia National Park 12/01/98 44.3769 68.2608 158 • • Forested CASH135 Ashland 12/20/88 46.6039 68.4142 235 • • Agri. FlHOW132 Howland 11/24/92 45.2158 68.7085 69 • • Forested RMaryland BEL116 Beltsville 11/01/88 39.0283 76.8175 46 • • Urban-Agri. FlBWR139 Blackwater National Wildlife Refuge 07/04/95 38.4448 76.1115 4 • • Forest-Marsh C
CASTNET Annual Report – 2007 A-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 

Pack1 

Ozone and 

Meteorology2 

Wet Deposition 

Primary Land Use 

Ti

Michigan ANA115 Ann Arbor 06/28/88 42.4164 83.9019 267 • • ◊ Forested FlHOX148 Hoxeyville 10/31/00 44.1809 85.7390 298 • • Forested FlUVL124 Unionville 06/28/88 43.6139 83.3597 201 • • ◊ Agri. FlMinnesota VOY413 Voyageurs National Park 06/13/96 48.4128 92.8292 429 • • Forested RMississippi CVL151 Coffeeville 12/27/88 34.0028 89.7989 134 • • Forested RMontana GLR468 Glacier National Park 12/27/88 48.5103 113.9956 976 • • Forested CNebraska SAN189 Santee Sioux 07/05/06 42.8292 97.8541 429 • • Agri. RNevada GRB411 Great Basin National Park 05/16/95 39.0053 114.2158 2060 • • Forested CNew Hampshire WST109 Woodstock 12/27/88 43.9446 71.7008 258 • • Forested CNew Jersey WSP144 Washington’s Crossing 12/27/88 40.3133 74.8726 61 • • Urban-Agri. R
CASTNET Annual Report – 2007 A-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 

Pack1 

Ozone and 

Meteorology2 

Wet Deposition 

Primary Land Use 

Ti

New York CAT175 Claryville 05/10/94 41.9423 74.5519 765 • •4, 5 Forested CCTH110 Connecticut Hill 09/28/87 42.4010 76.6535 515 • • ◊ Forested RHWF187 Huntington Wildlife Forest 05/28/02 43.9732 74.2232 502 • • Forested CNorth Carolina BFT142 Beaufort 12/28/93 34.8843 76.6213 2 • • ◊ Agri. FlCND125 Candor 09/25/90 35.2643 79.8382 198 • • ◊ Forested RCOW137 Coweeta 11/04/87 35.0605 83.4302 686 • • Forested CPNF126 Cranberry 12/27/88 36.1040 82.0448 1250 • • ◊ Forested MNorth Dakota THR422 Theodore Roosevelt National Park 10/06/98 46.8947 103.3778 850 • • Range ROhio DCP114 Deer Creek State Park 09/28/88 39.6358 83.2600 267 • • ◊ Agri. RLYK123 Lykens 01/10/89 40.9169 82.9981 303 • • ◊ Agri. FlOXF122 Oxford 08/18/87 39.5314 84.7231 284 • • Agri. RQAK172 Quaker City 09/28/93 39.9431 81.3378 372 • • Agri. ROklahoma CHE185 Cherokee Nation 04/02/02 35.7507 94.6700 299 • • Agri. ROntario EGB181 Egbert, Ontario 12/27/94 44.2317 79.7840 251 • •4 Agri. R
CASTNET Annual Report – 2007 A-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 

Pack1 

Ozone and 

Meteorology2 

Wet Deposition 

Primary Land Use 

PennsylvaniaARE128 Arendtsville 06/28/88 39.9231 77.3078 269 • • ◊ Agri. RKEF112 Kane Experimental Forest 01/03/89 41.5981 78.7683 622 • • Forested RLRL117 Laurel Hill State Park 12/15/87 39.9883 79.2522 615 • • Forested CMKG113 M.K. Goddard State Park 01/12/88 41.4250 80.1447 384 • • Forested RPSU106 Penn. State University 01/06/87 40.7209 77.9316 376 • • Agri. RSouth Dakota WNC429 Wind Cave National Park 11/18/03 43.5578 103.4839 1292 • • Prairie RTennessee ESP127 Edgar Evins State Park 03/22/88 36.0389 85.7330 302 • • Forested RGRS420 Great Smoky Mountains National Park 10/06/98 35.6331 83.9422 793 • • Forested CSPD111 Speedwell 06/12/89 36.4698 83.8265 361 • • ◊ Agri. RTexas ALC188 Alabama-Coushatta 04/02/04 30.4210 94.4045 101 • • Forested RBBE401 Big Bend National Park 07/18/95 29.3022 103.1772 1052 • • Forested CPAL190 Palo Duro Canyon State Park 04/24/07 34.8803 101.6649 1050 • • Prairie CUtah CAN407 Canyonlands National Park 01/24/95 38.4586 109.8211 1809 • • Desert C
CASTNET Annual Report – 2007 A-7 

Table A-1 Locational and Operational Characteristics of CASTNET Sites

Site ID Site Name 

Start date 

Latitude (°N) 

Longitude (°W) 

Elevation (m) 

Deposition Filter 

Pack1 

Ozone and 

Meteorology2 

Wet Deposition 

Primary Land Use 

Virginia PED108 Prince Edward 11/03/87 37.1653 78.3070 150 • • ◊ Forested RSHN418 Shenandoah National Park 06/28/88 38.5231 78.4347 1073 • • Forested MVPI120 Horton Station 06/02/87 37.3300 80.5573 920 • • Forested MWashington MOR409 Mount Rainier National Park 08/29/95 46.7583 122.1244 415 • • Forested CNCS415 North Cascades National Park 02/14/96 48.5397 121.4472 109 • • Forested CWest Virginia CDR119 Cedar Creek State Park 11/10/87 38.8794 80.8478 234 • • ◊ Forested CPAR107 Parsons 01/19/88 39.0906 79.6614 510 • • Forested CWisconsin PRK134 Perkinstown 09/27/88 45.2066 90.5972 472 • • ◊ Agri. RWyoming CNT169 Centennial 08/19/91 41.3722 106.2422 3178 • • Range CPND165 Pinedale 12/27/88 42.9214 109.7900 2388 • • Range RYEL408 Yellowstone National Park 06/26/96 44.5597 110.4006 2400 • • Forested R
CASTNET Annual Report – 2007 A-8 

1. The dry deposition filters are analyzed for the following constituents: 

Teflon® = SO2-4 , NO- 3, NH+ 4 , Cl -, K +, Na +, Mg2+, Ca2+

Nylon = SO2-4 , NO- 3 (reported as HNO3)

Cellulose = SO2-4 (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. 

◊ Indicates discontinued monitoring. 

 Measurements were discontinued at the variou

including: 

(1) rotate limited number of instrum(2) redundant measurements (e.g.(3) funding limitations. 

100 and 200 series = EPA – Operated Sites 400 series = NPS – Operated Sites 
AAppppeennddiixxBB

Acronyms and Abbreviations

CASTNET Annual Report – 2007 B-1 Appendix B 

List of Acronyms and Abbreviations

AQI Air Quality Index 

AQS EPA’s Air Quality System 

ARP Acid Rain Program 

Ca2+

 particulate calcium ion 

CAAA Clean Air Act Amendments 

CAMx Comprehensive Air Quality Model with Extensions 

Ca(NO 

3)

 

2 calcium nitrate 

CAPMoN Canadian Air and Precipitation Monitoring Network 

CASTNET Clean Air Status and Trends Network 

CCV continuing calibration verification spikes 

Cl- particulate chloride ion 

CNEP Cherokee Nation Environmental Programs 

DQI data quality indicator 

DQO data quality objectives 

EGU electric generating unit 

EPA U.S. Environmental Protection Agency 

EST Eastern Standard Time 

H 

2SO 4 sulfuric acid 

HNO 

3 nitric acid 

HYSPLIT Hybrid Single-Particle Lagrangian Integrated Trajectory 

IC ion chromatography 

ICP-AES inductively coupled plasma-atomic emission spectrometry 

IMPROVE Interagency Monitoring of Protected Visual Environments 

ITEC Inter-Tribal Environmental Council 

K+ particulate potassium ion 

K 

2CO 3 potassium carbonate 

kg/ha/yr kilograms per hectare per year 

km kilometer 

LAI leaf area index 

lpm liters per minute 

m meters 

MACTEC MACTEC Engineering and Consulting, Inc. 

MARPD mean absolute relative percent difference 

MDN Mercury Deposition Network 

Mg2+

 particulate magnesium ion 

mg/l milligrams per liter 

MLM Multi-Layer Model 

MRPD mean of relative percent difference 
CASTNET Annual Report – 2007 B-2 Appendix B 

List of Acronyms and Abbreviations (continued) 

N nitrogen 

Na+ particulate sodium ion 

NAAQS National Ambient Air Quality Standards 

NADP National Atmospheric Deposition Program 

NaNO 

3 sodium nitrate 

NAPAP National Acid Precipitation Assessment Program 

NBP NOx National Budget Trading Program 

NDDN National Dry Deposition Network 

NH 

3 ammonia 

NH +

4 particulate ammonium 

NH 

4NO 3 ammonium nitrate 

(NH 

4)

 

2SO 4 ammonium sulfate 

NIST National Institute of Standards and Technology 

NO -

3 particulate nitrate 

NOy total reactive oxides of nitrogen 

NOx nitrogen oxides [nitric oxide (NO) + nitrogen dioxide (NO 

2)] 

NOAA National Oceanic and Atmospheric Administration 

NPS National Park Service 

NTN National Trends Network 

NWRI Environment Canada’s National Water Research Institute 

O 

3 ozone 

ON organic nitrate 

ORD EPA’s Office of Research and Development 

OTC Ozone Transport Commission 

PART Program Assessment Rating Tool 

PM2.5 fine particle matter 

ppb parts per billion 

ppm parts per million 

QA quality assurance 

QAPP Quality Assurance Project Plan 

QC quality control 

RMNP Rocky Mountain National Park 

RoMANS Rocky Mountain Atmospheric Nitrogen and Sulfur Study 

RPD relative percent difference 

S sulfur 

SIP State Implementation Plan 

SLAMS State and Local Monitoring Stations 

SO 

2 sulfur dioxide 

SO2-

4 particulate sulfate 
CASTNET Annual Report – 2007 B-3 Appendix B 

List of Acronyms and Abbreviations (continued) 

total NO3 gaseous nitric acid (HNO 

3) + particulate nitrate (NO3) 

μg microgram 

μg/m3 micrograms per cubic meter 

USGS U.S. Geological Survey 

UTC Coordinated Universal Time 

V 

d deposition velocity 

VOCs volatile organic compounds 
For More Information

U.S. Environmental Protection Agency

Office of Air and Radiation

Clean Air Markets Division

Washington, D.C.

On the Web: 

CASTNET Home Page:

www.epa.gov/castnet

Clean Air Markets

Division Home Page:

www.epa.gov/airmarkets

EPA Home Page:

www.epa.gov