Clean Air Status and Trends Network 

(CASTNET)

2006 Annual Report
Clean Air Status and Trends Network 

(CASTNET) 

2006 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. 

November 2007
CASTNET Annual Report – 2006 - i - Table of Contents 

Table of Contents

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

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

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

 Partnerships................................................................................................................................... 2 

 Twenty Years of Network Monitoring.......................................................................................... 4 

NPS Air Monitoring Program ....................................................................................................... 5 

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

CASTNET Monitoring at Long Term Ecological Research Sites ................................................ 7 

Measurements Collected at CASTNET Sites................................................................................ 9 

 Calculating Dry Deposition......................................................................................................... 10 

 CASTNET Reference Sites......................................................................................................... 11 

 SO 

2 and NOx Emissions .............................................................................................................. 12 

Significant Events during 2006................................................................................................... 14 

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

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

 Particulate Sulfate ....................................................................................................................... 17 

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

Air Quality Concerns in the Four Corners Region...................................................................... 21 

Particulate Ammonium ............................................................................................................... 22 

Chapter 3 Atmospheric Deposition ........................................................................................................... 24 

Sulfur Deposition ........................................................................................................................ 25 

Uncertainties in Estimates of Dry Nitrogen Deposition.............................................................. 28 

Nitrogen Deposition.................................................................................................................... 30

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

National Ambient Air Quality Standard for Ozone..................................................................... 35 

Elevated Ozone Concentrations in Atlanta during 2006 ............................................................. 39

Chapter 5 Data Quality .............................................................................................................................. 41 

Precision...................................................................................................................................... 42 

Accuracy ..................................................................................................................................... 47 

Completeness .............................................................................................................................. 49 

 Laboratory Intercomparison Studies ........................................................................................... 50 

Conclusion .................................................................................................................................. 51 

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

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

Appendix B Acronyms and Abbreviations................................................................................................. B-1 
CASTNET Annual Report – 2006 - ii - Executive Summary 

Executive Summary 

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

2006 and examines trends in air quality and deposition on regional and national scales. 

CASTNET began operation in 1991 with the incorporation of the National Dry Deposition 

Network (NDDN), which had been in existence 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 2006 

♦ Mean annual sulfur dioxide and particulate sulfate concentrations have declined 

significantly over the 17-year period 1990 through 2006. Sulfur dioxide levels have 

declined 37 percent while sulfate concentrations have declined 25 percent. 

♦ Total sulfur deposition has declined by 28 percent from 1990 to 2006. 

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

declined by approximately 18 percent over the 17-year period. 

♦ Total nitrogen deposition has declined by 12 percent from 1990 to 2006. 

♦ Ozone levels measured in 2006 continue to show a downward trend that began after a 

peak in 2002. The median fourth highest daily maximum 8-hour ozone concentration for 

2006 (72 ppb) was the second lowest in the history of the network. For the most recent 3-

year period (2004-2006), only two eastern and four California sites recorded exceedances 

of the 8-hour standard. This 3-year period is notable as it had the fewest exceedances (six 

total) in the history of the network. The recent decline in rural ozone and nitrate levels 

has been attributed to the documented decline in nitrogen oxides emissions as well as 

changes in weather conditions. 
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 1

Chapter 1: CASTNET Overview 

Santee Sioux, NE (SAN189) 

Background 

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

Trends Network (CASTNET) to provide data for determining relationships between changes in 

emissions and any subsequent changes in air quality, atmospheric deposition, and ecological 

effects. The rural monitoring network was mandated by the 1990 Clean Air Act Amendments 

(CAAA) to assess the effectiveness of requirements promulgated to reduce emissions of sulfur 

dioxide (SO2) and nitrogen oxides (NOx) as Congress recognized the need to track real-world 

environmental results as the Acid Rain Program was implemented. 

Under the CAAA, the Acid Rain Program has produced significant reductions in SO2 and NOx

emissions from electric generating plants 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, NOx State 

Implementation Plan (SIP) Call, and NOx Budget Trading Program. EPA relies on CASTNET 

and other long-term monitoring networks to generate the data and information used to assess the 

effectiveness of these emission control programs under several different mandates including the 

Government Performance and Results Act, The National Acid Precipitation Assessment Program 

(NAPAP), Title IX of the CAAA, and the United States – Canada Air Quality Agreement. This 

report summarizes CASTNET monitoring activities and the resulting concentration and 

deposition data collected over the 17-year period from 1990 through 2006. 

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

relationships between emissions, air quality, 

deposition, and ecological effects. 
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 2

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 “Data” link on EPA’s CASTNET Web page. The Web site provides 

archives of the concentration and deposition data. Fully validated data are available 

approximately 10 months following collection. 

 

Partnerships 

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 2006 numbered 27. In addition to 

EPA and NPS, the principal sponsors, CASTNET operates in partnership with other rural longterm monitoring 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. 

♦ 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. 

♦ Interagency Monitoring of Protected Visual Environments (IMPROVE) measures 

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

air pollutants that affect visibility and visual air quality. 

♦ AIRNow is a Web site designed to allow the public to view air quality data for a specific 

area in real-time. Ten CASTNET sites send 1-hour ozone data to the AIRNow database. 

The data have not gone through the full quality assurance process and, therefore, should 

not be used for publication or regulation. 

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

universities, and government agencies play a role in sponsoring individual CASTNET sites. 

These groups, or co-sponsors, provide an in-kind service that supports the operation of a site. 

The types of assistance offered may include site operation, land use, or both. Table 1-1 lists all 

current co-sponsors for the network. All of the sites added during the recent expansion of the 

network (since 2001) have an associated co-sponsor. Some of the co-sponsors listed in Table 1-1 

have maintained long-term relationships with CASTNET sites. For example, Pennsylvania State 

University, co-sponsor of PSU106, PA, and the University of Michigan, co-sponsor of ANA115, 

MI, have been involved with the network since the late 1980s. 
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 3

Three CASTNET monitoring sites are 

located on tribal lands (Figure 1-1) 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). State agencies also operate 

special purpose air pollutant measurement 

devices at some CASTNET sites. 

Table 1-1 Co-Sponsors for CASTNET 

 Site Co-sponsor 

ALC188, TX Alabama-Coushatta Tribe of Texas 

ANA115, MI University of Michigan 

CAD150, AR United States Army 

CHE185, OK Cherokee Nation 

CON186, CA U.S. Department of Agriculture-Forest Service Pacific Southwest 

Research Laboratory 

EGB181/ 

EGB281, ON Environment Canada 

EVE419, FL Environmental Protection Agency 

HWF187, NY State University of New York 

IRL141, FL St. Johns River Water Management District 

KNZ184, KS Konza Prairie Long Term Ecological Research Program 

PND165, WY U. S. Department of Interior-Bureau of Land Management 

PSU106, PA Pennsylvania State University 

SAN189, NE Santee Sioux Tribe of Nebraska 

SND152, AL Tennessee Valley Authority 

Figure 1-1 Native American Participation in 

CASTNET Operations 
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 4

Twenty Years of Network Monitoring 

CASTNET was designed primarily to measure seasonal and annual 

average concentrations and depositions over many years. 

Consequently, measurements of weekly average concentrations were 

selected as the basic sampling strategy. An open-face, three-stage 

filter pack was employed to measure gaseous and particulate sulfur 

and nitrogen pollutants as well as concentrations of other pollutant 

species. The filter pack technology and sampling protocol have been 

used consistently over the 20 years, providing a comparable data set 

each year and allowing for the analysis of long-term trends. Any 

future sampling methods will be examined with respect to 

comparability with the filter pack so the information obtained will 

continue to be available for long-term trends in pollutant species. 

As of December 2006, the network 

included 87 monitoring stations at 

84 site locations throughout the 

continental United States, Alaska, 

and Canada. CASTNET sites 

measure rural, regionally 

representative concentrations of 

sulfur and nitrogen species and 

ozone in order to detect and 

quantify trends, define the spatial 

distribution of rural pollutants, and 

estimate dry deposition fluxes. The 

goal of estimating dry deposition 

also requires the measurement of 

several meteorological parameters 

and information on vegetation and 

land use. 

CASTNET has its origins with the National Dry Deposition Network (NDDN), which 

was established in 1986 and began operation in 1987. Many of the original NDDN sites 

are still operational after 20 years and provide useful information on trends in air quality. 

The site at Pennsylvania State University, PA (PSU106) began collecting samples in 

January 1987 and is the longest running site. 

0

2

4

6

8

10

12

14

16

18

19871988198919901991199219931994199519961997199819992000200120022003200420052006

Concentration (μg/m3

)

Annual Mean SO2 Conc

3 Year Mean SO2 Conc 3-Year Mean SO2 Conc

Annual Mean SO2 Conc

20-Year Trend in SO 

2 Concentrations (μg/m3

) 

at PSU106, PA

Note: Annual mean SO 

2

 concentrations have declined by 39 percent 

at PSU106, PA over the 20 years. 

PSU106, PA 
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 5

NPS Air Monitoring Program 

The NPS Air Resources Division administers an extensive air monitoring program that measures 

air pollution levels in national parks. The purpose of the program is to establish current air 

quality conditions and assess long-term trends of air pollutants that affect park resources. These 

data are also used to determine compliance with the National Ambient Air Quality Standards 

(NAAQS) and to assess the effectiveness of national and regional air pollution control programs. 

Measuring air pollution levels in parks is an essential part of the NPS air resource management 

program that provides vital information to Congress, air pollution control agencies, academia, 

and the public (NPS, 2007). 

The NPS air monitoring program consists of an extensive network of air monitoring stations in 

almost 70 national parks across the country. Twenty-seven of these sites are also CASTNET 

sites. Several NPS sites have been in operation for over 20 years. The program has three primary 

components: visibility (IMPROVE), gaseous pollutants (mainly ozone), and atmospheric 

deposition (wet - NADP/NTN, NADP/MDN and dry - CASTNET). Meteorological monitoring 

is also conducted at most locations to aid in the interpretation of measured air pollution levels. In 

addition to long-term monitoring, NPS is involved in many special studies that tend to be shortterm and more intensive in nature. These studies are often initiated to explore specific local and 

regional research topics, such as identifying sources of air pollution or assessing the potential 

risk to natural resources in parks (NPS, 2007). 

Assessing how air quality is changing is a prime function of the NPS air monitoring program. 

Monitoring data on visibility, ozone, and atmospheric deposition show that air pollution is 

affecting some park resources nationwide. Consistent with the Government Performance and 

Results Act, NPS has established the air quality goal of stable or improved air quality in 70 

percent of reporting park areas by September 30, 2008. An area meets the goal if it does not 

show statistically significant deterioration in any of the performance indicators (NPS, 2007). 

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

http://www2.nature.nps.gov/air/monitoring/.
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 6

Locations of Monitoring Sites 

Figure 1-2 shows the locations of CASTNET monitoring sites as of December 2006. 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 types of measurements taken at each site. 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 lower 

pollutant concentrations in the western United States. 

Figure 1-2 CASTNET Sites as of December 2006 
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 7

CASTNET Monitoring at Long Term Ecological Research Sites 

The National Science Foundation’s (NSF) Long Term Ecological Research (LTER) Program 

was established in 1980 to support research on long-term ecological phenomena across a wide 

range of geographical scales. The network consists of 26 field sites representing diverse 

ecosystems in continental North America, the Caribbean, the Pacific, and Antarctica. The sites 

represent most of the Earth’s major ecosystems and include forests, grasslands, deserts, urban 

areas, tundra, agricultural systems, freshwater lakes, coastal estuaries and salt marshes, coral 

reefs, and coastal ocean zones. Disturbances often shape ecosystems by periodically reorganizing 

or destroying them, providing opportunities for significant changes in 

plant and animal populations and communities. Disturbances include, 

for example, hurricanes, land use changes, changes in climate and 

atmospheric chemistry, or forest harvesting (NSF, 2005). 

Three CASTNET monitoring systems are located on LTER field sites. 

CASTNET provided early support to LTER by establishing sites at 

Coweeta, NC (COW137) in 1987 and Hubbard Brook, NH (WST109) 

in 1988. The third LTER CASTNET site was established at Konza 

Prairie, KS (KNZ184) in 2002. CASTNET monitors are also located 

near other LTER sites, e.g., Florida Coastal Everglades, Baltimore 

Ecosystem Study, Niwot Ridge in Colorado, and North Temperate 

Lakes in Wisconsin. 

The Coweeta LTER research program centers on 

the effects of disturbances and environmental 

gradients on biogeochemical cycling and the 

underlying watershed ecosystem processes that 

regulate and respond to those cycles. It now 

represents one of the longest continuous 

environmental studies in North America 

(LTER, 2004a). This LTER research project is a 

continuing long-term study concentrating on 

recovery from phenomena that are of major 

consequence in the southern Appalachians, such as 

drought, weather (Figure 1-3), and altered 

atmospheric deposition. 

The Hubbard Brook Experimental Forest was established by the U.S. Department of Agriculture 

Forest Service, Northeastern Research Station in 1955 as a major center for hydrologic research 

in New England. In 1963, the Hubbard Brook Ecosystem Study was initiated in order to use the 

small watershed approach at Hubbard Brook to study linkages between hydrologic processes and 

nutrient flux and cycling in response to natural and human disturbances such as air pollution, 

COW137, NC 

Figure 1-3 Annual Mean Temperature 

(°C) and Precipitation (mm) Data for 

COW137, NC 

9

10

11

12

13

14

15

19901991199219931994199519961997199819992000200120022003200420052006

Temperature (oC)

50

100

150

200

250

Precipitation (mm)

Temperature

Precipitation
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 8

forest cutting, land-use changes, increases in insect populations, and climatic factors 

(LTER, 2004b). 

The Konza Prairie Biological Station was founded in 1971. The 

Konza Prairie LTER Program is a comprehensive, 

interdisciplinary research program designed to provide an 

understanding of ecological processes in mesic grasslands, 

particularly tallgrass prairie, and to contribute to conceptual and 

theoretical advances in the field of ecology (LTER, 2004c). Some 

current areas of research include grassland ecology and the 

effects of fire, grazing, and climatic variability on the structure 

and function of mesic grass-land ecosystems. 

CASTNET supports the three LTER sites by providing information 

on air pollutant concentrations, dry deposition fluxes of pollutants 

to the ecosystems, and meteorological conditions. These data 

contribute to an assessment of disturbances related to air pollution, 

atmospheric deposition, and climate change. 

 LONG TERM ECOLOGICAL RESEARCH NETWORK 

 

 LTER Network Office 

 1 University of New Mexico, 

 UNM Biology Department, MSC03-2020 

 Albuquerque, NM 87131-0001 

 Ph 505.277.2597 Fax 505.277.2541 

 Email Office@lternet.edu Web www.lternet.edu

KNZ184, KS

WST109, NH
CASTNET Annual Report – 2006 9 Chapter 1: CASTNET Overview 

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 weekly average concentrations was 

selected as the basic sampling strategy. Over the course 

of the week, air is drawn at a controlled flow rate 

through a three-stage filter pack (Figure 1-4) mounted 

atop a 10 meter tower to collect air pollutants in the 

form of gases and particles. The first stage of the filter 

pack encloses a Teflon® filter, the second a nylon filter, 

and the third holds two potassium carbonate (K2CO3)-

impregnated cellulose filters. The filter pack is changed 

out each Tuesday and shipped to the analytical 

chemistry laboratory for analysis. 

Figure 1-4 Three-Stage Filter Pack 

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 

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 

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 – 2006 Chapter 1: CASTNET Overview 10

CASTNET measures concentrations of sulfur in the form of sulfur dioxide (SO2) and sulfate 

(SO2-

4 ) and nitrogen as nitrate (NO3), nitric acid (HNO3), and ammonium (NH+ 4 ). In addition, it 

measures concentrations of chloride (Cl -), calcium (Ca2+), sodium (Na+ ), magnesium (Mg2+), and 

potassium (K+ ). Sulfate, NH+ 

4 , NO3, Cl -, and the earth metals are collected on the Teflon® filter. 

The nylon filter collects HNO3 and some SO2. The cellulose filters collect the remaining SO2. 

CASTNET also measures ozone, one of the major components of smog. In addition to the air 

pollutants, CASTNET sites record meteorological measurements of 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 mathematical model 

used for estimating dry deposition to ecosystems in the atmospheric boundary layer. The ozone 

and meteorological measurements are recorded continuously and archived as hourly averages. 

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 are calculated for each hour of each year using the 

MLM. The MLM was 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 1-5 shows the relationships among the various resistances 

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

version of the MLM (Schwede, 2006) was delivered by EPA’s Office of Research and 

Development (ORD) to MACTEC 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. All deposition velocities and fluxes for the entire 

network were recalculated using the updated model. 

Indian River Lagoon, FL (IRL141) 
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 11

Multi-Layer Model (MLM)

Flux =

1/Vd = 

ra = turbulence

ra, soil = turbulence near soil

rb = thin layer at surface

rcut = cuticular

rs = stomatal

rsoil = soil

1

11 1

s b cut b a soil soil

a

rr r r r r

r

+

+

+

+

+

+

,

V d C x

rb

ra

rsoil

Temp, RH, 

SR, LAI

Wind Speed, 

Sigma Theta

Wetness,

Precip

ra, soil

rb

rcut

rs

 Figure 1-6 CASTNET Reference Sites 

Figure 1-5 Multi-Layer Model 

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 1-6) 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 and are used for determining long-term 

trends. 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. In chapters 2 through 4, the data 

from the 34 reference sites were aggregated and 

then presented using box plots for the period 

1990 through 2006. 
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 12

SO2 and NOx Emissions 

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

Amendments, requires major reductions of sulfur dioxide (SO2) and nitrogen oxides (NOx) 

emissions from the electric power industry. Under the Acid Rain Program, SO 

2 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 electric generating units in the contiguous United States. The 

program began in 1995 and is being phased in 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. 

Using 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 Clean Air Act 

Amendments of 1990 allocate 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. 

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. No nationwide 

cap has been placed on NOx emissions. Other NOx emission control programs have resulted in 

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

control programs are the Ozone Transport Commission NOx Budget (1999-2002) and the NOx

State Implementation Plan Call, which began in 2003 and will continue through 2007. 

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 2005, the SO2 Phase II 

requirements affected 3,391 operating units; the Phase II NOx requirements applied to 989 of 

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

(EPA, 2005a). 

The recently promulgated Clean Air Interstate Rule (CAIR) (EPA, 2007b) will establish regional 

caps on SO2 and NOx emissions for affected generating units. Annual SO2 emissions will be 

capped at 3.7 million tons in 2010 and NOx emissions will be capped at 1.5 million tons in 2009. 

CASTNET will be used to assess baseline and program milestones. 

Figure 1-7 presents state-by-state total annual SO2 emissions for Phase I and Phase II electric 

utility plants for four years (1990, 1995, 2000, and 2006). The heaviest emissions occurred in the 

eastern United States with major SO2 sources centered around the Ohio River Valley. The most 

significant reduction in SO2 emissions occurred in 1995 when the ARP began. Many of the states 

with the highest SO2 emissions realized the largest reductions in emissions. For example, Ohio 
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 13

emitted 2,212 tons in 2000. This total dropped to 962 tons in 2006, a 57 percent reduction. 

Annual NOx emissions by state are depicted in Figure 1-8 for the same four years. States with the 

higher NOx emissions also produced significant declines. 

Figure 1-7 Annual Utility SO2 Emissions (Phase I and Phase II Plants only) 

Figure 1-8 Annual Utility NOx Emissions (Phase I and Phase II Plants only) 

Annual Emissions of Sulfur Dioxide

(thousand short tons)

2,300

1990

1995

2000

2006

Annual Emissions of Sulfur Dioxide

(thousand short tons)

2,300

1990

1995

2000

2006

Annual Emissions of Nitrogen Oxides

(thousand short tons)

540

1990

1995

2000

2006

Annual Emissions of Nitrogen Oxides

(thousand short tons)

540

1990

1995

2000

2006
CASTNET Annual Report – 2006 Chapter 1: CASTNET Overview 14

Significant Events during 2006 

January 

Element DataSystemTM replaced Chemical Laboratory Analysis and Scheduling System 

(CLASSTM) as the laboratory information management system used for CASTNET. 

March 

EPA approved the CASTNET Quality Assurance Project Plan (QAPP), Revision 3.0 on 

March 6. 

EPA and the Tennessee Valley Authority began operating collocated ozone monitors at the 

EPA-sponsored CASTNET site Cadiz, KY (CDZ171). 

A field study began at three CASTNET sites to determine the size and distribution of 

particulate nitrate collected by the CASTNET open-face filter pack sampling system. 

July 

A new EPA-sponsored site, Santee Sioux, NE (SAN189), began collecting samples on 

July 5. The site is operated in cooperation with the Santee Sioux Tribe, part of the 

Great Sioux Nation. 

Use of a routine laboratory control sample (LCS) with all analyses was implemented as an 

additional quality assurance/quality control mechanism to monitor for potential sample 

handling artifacts and possible analyte loss between extractions. 

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

Defense Contract Management Agency at MACTEC’s Gainesville, FL location. The 

property management system was approved by the audit. 

MACTEC conducted a two-day site operator refresher training course for 10 site 

operators/backup operators from seven sites. The training session took place in 

Somerset, PA and at the nearby CASTNET site Laurel Hill State Park, PA (LRL117). 

August 

New Thermo Fisher Scientific Model 49i ozone analyzers were purchased for replacement 

of the older analyzers. 

September 

Field calibrations began utilizing portable relative humidity chambers instead of aqueous 

saturated salts to verify the accuracy of the relative humidity sensors. 

Statistical analyses were completed to support acceptance testing of the nylon filters. 

October 

Sponsorship of the CASTNET site at Indian River Lagoon, FL (IRL141) passed from the 

St. Johns River Water Management District to EPA. 

The draft of the CASTNET QAPP, Revision 4.0 was submitted to EPA. 

November 

Version 2.5 of the Multi-Layer Model (MLM) was received from EPA. 
 

CASTNET Annual Report – 2006 Chapter 2: Atmospheric Concentrations 15

Chapter 2: 

Atmospheric Concentrations 

The geographic distribution and magnitude of annual mean concentrations across the United 

States are presented for sulfur dioxide (SO2), sulfate (SO2-

4 ), total nitrate (HNO3 + NO3), and 

ammonium (NH+ 

4 ) concentrations. A map is provided for each pollutant. Maps of other filter 

pack measurements are provided in CASTNET quarterly reports (MACTEC, 2006b; 2006d; 

2007a; 2007c). The concentration shading in the figures 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 concentration data from the 34 CASTNET reference sites 

(Figure 1-6) are presented using box plots for each year of the 17-year period from 1990 

through 2006. 

Sulfur Dioxide 

Figure 2-1 presents annual mean sulfur dioxide (SO2) concentrations for 2006. The map shows a 

large region in the eastern United States with concentrations greater than or equal to 

5.0 micrograms per cubic meter (μg/m3

). The region extends from western Kentucky 

northeastward along the Ohio River Valley to Pennsylvania and to New Jersey, Maryland, and 

Virginia. The Ohio River Valley is the major SO2 emission source region (see Figure 1-7) in the 

United States. A concentration of 5.5 μg/m3

 was observed at Georgia Station, GA (GAS153). 

Two sites in West Virginia – Parsons (PAR107) and Cedar Creek State Park (CDR119) – 

recorded concentrations slightly less than 5.0 μg/m3. The single highest SO2 concentration 

(13.1 μg/m3

) was measured in eastern Ohio at Quaker City (QAK172). Only two western sites 

(i.e., sites west of 100 degrees west longitude) measured an annual mean SO2 concentration 

greater than 1.0 μg/m3

. These sites were Theodore Roosevelt National Park, ND (THR422) and 

Petrified Forest National Park, AZ (PET427). 

Three-stage filter packs are used to measure concentrations of sulfur dioxide (SO2), sulfate 

(SO2-

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

earth metals. 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 discussed in this chapter. 

Georgia Station, GA (GAS153)
 

CASTNET Annual Report – 2006 Chapter 2: Atmospheric Concentrations 16

Figure 2-2 provides box plots that show the 1990-2006 trend in annual mean SO2 concentrations 

aggregated over the 34 reference sites. The diagram shows a significant downward trend with 

small interannual changes. Three-year means for 1990-1992 and 2004-2006 were 9.0 μg/m3

 and 

5.6 μg/m3

, respectively. This change demonstrates a significant reduction of 37 percent in 3-year 

mean SO2 concentrations for the two time periods. Annual mean SO2 concentrations measured in 

2006 were the lowest measured by CASTNET reference sites. 

Figure 2-1 Annual Mean SO2 Concentrations (μg/m3) for 2006 

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

) 

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

0

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CASTNET Annual Report – 2006 Chapter 2: Atmospheric Concentrations 17

Particulate Sulfate 

Sulfate is formed in the atmosphere through the transformation of SO2, which is emitted directly 

from sources burning fossil fuels. Sulfate is formed via both gas and aqueous (cloud) phase 

reactions. Since SO2 can be transported by the winds away from the emission source, 

meteorological conditions and atmospheric chemical constituents determine the path by which 

sulfate is formed. Sulfur dioxide gas can have a lifetime of up to a week in the atmosphere before 

it is subsequently oxidized by hydroxyl (OH) radicals to form sulfur trioxide. In the presence of 

water vapor, sulfur trioxide quickly forms sulfuric acid (H2SO4). In turn, H2SO4 reacts with 

particles to form particulate sulfate. These gas phase reactions are relatively slow, approximately 

equivalent to a SO2 conversion rate of 1.0 percent per hour. 

On the other hand, SO2 is a soluble gas and can dissolve in cloud droplets during transport if they 

are present. These droplets act as cloud condensation nuclei for sulfate formation. As SO2 is 

dissolved in cloud droplets, it is oxidized by hydrogen peroxide to form sulfate. Dissipating 

clouds produce ambient particulate SO2-

4 . 

Sulfate can also be formed directly in stack gases before release to the atmosphere. This type of 

production results in a direct emission of SO2-

4 . However, a typical emission rate of SO2-4 is 

significantly smaller than an emission rate of SO2. 

Sulfate concentrations were generally lower in 2006 than in 2005. A map of annual mean sulfate 

(SO2-

4 ) concentrations for 2006 is presented in Figure 2-3. The map shows a complex region of 

concentrations greater than or equal to 4.0 μg/m3. The region extends from Georgia northward to 

Kentucky and along the Ohio River Valley to Pennsylvania, Maryland, Virginia, and central 

North Carolina. In 2006, the Quaker City, OH (QAK172) site recorded the highest concentration 

of 4.7 μg/m3

. Sulfate concentrations greater than or equal to 1.0 μg/m3 were measured at three 

sites in California – Pinnacles National Monument (PIN414), Sequoia National Park (SEK430), 

and Joshua Tree National Monument (JOT403) – and along the southern tier to Chiricahua 

National Monument, AZ (CHA467) and Big Bend National Park, TX (BBE401). The CASTNET 

site in North Dakota (THR422) measured a SO2-

4 level of 1.1 μg/m3

. 

Figure 2-4 provides box plots of annual mean SO2-

4 data from 1990 through 2006 for the 34 

CASTNET eastern reference sites. Overall, the figure depicts a significant reduction in SO2-

4 over 

the last 17 years with some interannual changes. The 2006 mean level was lower than the 2005 

value. The difference between 3-year means from 1990-1992 to 2004-2006 is 25 percent, a 

change from 5.4 μg/m3

 to 4.1 μg/m3, respectively. Over the 2005 to 2006 time period CASTNET 

recorded the largest single year drop in sulfate concentrations. 
 

CASTNET Annual Report – 2006 Chapter 2: Atmospheric Concentrations 18

Figure 2-3 Annual Mean SO2-

4 Concentrations (μg/m3

) for 2006 

Figure 2-4 Trend in Annual Mean SO2-

4 Concentrations (μg/m3

) 

2

3

4

5

6

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CASTNET Annual Report – 2006 Chapter 2: Atmospheric Concentrations 19

Total Nitrate 

A map of annual mean total nitrate concentrations for 2006 is given in Figure 2-5. Total nitrate 

is defined as the sum of gaseous nitric acid (HNO3) and particulate nitrate (NO3). Measurement 

of total nitrate concentrations measures the response to changes in nitrogen oxides (NOx) 

emissions more accurately than measurement of either of its constituents alone. Also, 

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

See the call out box in Chapter 3 (page 28) for a discussion of the uncertainties in CASTNET 

HNO3 and NO3 measurements. 

The map in Figure 2-5 shows a complex pattern of total nitrate concentrations in the eastern 

United States. Most CASTNET monitors recorded values greater than 2.0 μg/m3. The geographic 

variability in concentrations is caused by changes in the elevation and location of the monitoring 

stations and the associated differences in deposition velocities, inversion heights, and exposure to 

fresh NOx emissions. The highest annual mean total nitrate values were recorded in Illinois, 

Indiana, and Ohio. Three CASTNET sites in California measured total nitrate concentrations 

above 3.0 μg/m3

. Two sites – Converse Station (CON186) and JOT403 – are located downwind 

of the Los Angeles Basin. The third California site, SEK430, is located in the Sierra Nevada 

Mountains adjacent to the agricultural region known as the Central Valley. 

Box plots of annual total nitrate values from the 34 CASTNET reference sites are provided in 

Figure 2-6. The data show a decline in total nitrate from 2000 to 2006. The 2006 mean 

concentration was the lowest in the history of the network. The overall trend depicts an 18 

percent reduction in total nitrate from 1990 to 2006. 
 

CASTNET Annual Report – 2006 20 Chapter 2: Atmospheric Concentrations 

Figure 2-5 Annual Mean Total Nitrate (NO3 + HNO3) Concentrations (μg/m3) for 2006 

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

0

2

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CASTNET Annual Report – 2006 21 Chapter 2: Atmospheric Concentrations 

Air Quality Concerns in the Four Corners Region

The Four Corners region of Colorado, New 

Mexico, Arizona, and Utah is experiencing 

significant energy development including new 

oil and gas wells, more power plants, and 

an increasing population. The Bureau of Land 

Management has expressed concern that Class I 

areas in the region may experience a deterioration 

in visibility and air quality in addition to an 

increase in ozone (O3) concentrations as a result 

of the growth in energy development. This led to 

the formation of a multi-stakeholder task force to 

focus on air quality in the region (NM 

Environment Dept., 2007). 

The states of Colorado and New Mexico convened the task force in 2005. The efforts of this 

task force are shared by federal, tribal, state, and local air quality management agencies in 

the Four Corners region. The public is also participating, and NPS has been actively 

involved since 2005. The task force is assessing existing conditions and developing options 

for improving air quality in the area. The goal is to create options that will consider 

accommodating energy growth and the associated economic development without 

compromising air quality (NM Environment Dept., 2007). For more information on the task 

force, visit http://www.nmenv.state.nm.us/aqb/4C/index.html. 

Data from the CASTNET site at Mesa Verde National Park, CO (MEV405), which is 

located in the Four Corners region, provide information on changes in sulfur and nitrogen 

pollutants and ozone over the period 1995 through 2006. The figure shows annual mean 

concentrations of sulfur dioxide (SO2) 

and total nitrate (NO3) and fourth 

highest daily maximum 8-hour average 

ozone (O3) concentrations for the 

12 years. The data suggest increases in 

total nitrate concentrations and an 

overall decline in SO2. However, SO2

values have increased over the last two 

years. Three-year average fourth 

highest daily maximum 8-hour average 

O3 concentrations increased by about 

10 percent from 1995 to 2006. 

Mesa Verde National Park, CO (MEV405) 

0.0

0.2

0.4

0.6

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1.2

1.4

199519961997199819992000200120022003200420052006

Concentration (μg/m3

)

40

50

60

70

80

90

O3 Concentration (ppb)

SO2

Total NO3

Ozone

SO2

Total NO3

-

Ozone

Trend in Pollutant Concentrations at MEV405, CO 
 

CASTNET Annual Report – 2006 22 Chapter 2: Atmospheric Concentrations 

Particulate Ammonium 

A map of annual mean ammonium (NH+ 

4 ) concentrations is presented in Figure 2-7. The map 

shows no annual concentrations greater than 2.0 μg/m3

. The NH+ 

4 concentrations measured at 

western sites were low with all sites measuring concentrations below 1.0 μg/m3

. The trend 

diagram for annual mean NH+ 

4 values is provided in Figure 2-8. The box plots show a 20 percent 

decline in 3-year mean NH+ 

4 concentrations from 1990 to 2006 (1.79 μg/m3

 to 1.43 μg/m3). The 

2006 mean was the lowest in CASTNET history. It is important to note that these mean values 

were based only on data from the 34 CASTNET reference sites (Figure 1-6), which include only 

one site west of the Mississippi River. Additional studies using wet deposition data from 

NADP/NTN have shown an increase in NH+ 

4 concentrations in the Great Plains where the 

geographic coverage of CASTNET sites is less in extent (i.e., fewer sites) than the coverage 

provided by NTN (NADP, 2003). 

Figure 2-7 Annual Mean NH+ 

4 Concentrations (μg/m3

) for 2006 
 

CASTNET Annual Report – 2006 23 Chapter 2: Atmospheric Concentrations 

Figure 2-8 Trend in Annual Mean NH+ 

4 Concentrations (μg/m3

) 

Petrified Forest National Park (PET427), AZ 

0

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CASTNET Annual Report – 2006 24 Chapter 3: Atmospheric Deposition

Chapter 3: 

Atmospheric Deposition 

One of the principal objectives of CASTNET is to provide estimates of the dry deposition 

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

approach to estimating dry deposition by combining measured pollutant concentrations and 

modeled deposition velocities. The Multi-Layer Model (MLM) is the computer model used 

to calculate site-by-site deposition velocities 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 28 percent reduction in 3-year mean sulfur fluxes over 

the period from 1990-1992 to 2004-2006. Total nitrate deposition declined by 12 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 aerosol sulfur and nitrogen pollutants are deposited into ecosystems through dry 

and wet atmospheric processes. One of the most critical objectives of CASTNET is to estimate 

the rate, or flux, of dry deposition from measured meteorological and other environmental 

conditions. Flux values are estimated as the product of measured concentration data and MLMmodeled dry deposition velocities. For this report, wet deposition measurements were obtained 

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

deposition. Dry sulfur and nitrogen deposition rates decreased slightly during 2006 while total 

deposition stayed about the same. Precipitation-weighted mean concentrations in precipitation of 

total atmospheric sulfur have declined over the past 17 years. Nitrogen concentrations in 

precipitation have declined slowly since 1996. 

Dry deposition processes were simulated using the MLM (Figure 1-5) as described by 

Meyers et al. (1998) and Finkelstein et al. (2000). An improved 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 

Denali National Park, AK (DEN417) 
CASTNET Annual Report – 2006 25 Chapter 3: Atmospheric Deposition

(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 2006. For a 

deposition velocity to be estimated, 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 is not available for a specific site, the results are not included on maps 

presented in this chapter. For trends analyses, missing values are replaced by interpolation or 

extrapolation using valid annual estimates. 

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)]. Figure 3-1 

provides a map of estimates of dry sulfur deposition for 2006. The magnitude of a deposition rate 

is illustrated by the size of the circle. The map shows a narrow region with fluxes greater than 

5.0 kilograms per hectare per year (kg/ha/yr) centered around and downwind of the Ohio River 

Valley from southern Indiana and central Kentucky to eastern 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 11.0 kg/ha/yr. The highest dry sulfur 

deposition rates were coincidental to the major SO2 source region (Figure 1-7) 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. 

Figure 3-1 Dry Sulfur (SO2 + SO2-

4 ) Deposition (as S) (kg/ha/yr) for 2006 

5.4

3.5 2.3 2.3

2.2

3.2

2.0

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1.6

2.7

1.7

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0.7

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2.3

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0.2

Site not pictured:

DEN417, AK 0.2

Deposition

10

5

1
CASTNET Annual Report – 2006 26 Chapter 3: Atmospheric Deposition

Wet deposition values used to estimate total deposition represent a combination of historical 

CASTNET wet deposition data with 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 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. A region with total (dry + wet) sulfur deposition (kg/ha/yr) greater than 10.0 kg/ha/yr 

extended from southwestern Indiana and central Kentucky along the Ohio River Valley and into 

Ontario, Canada. Sites in Virginia, Tennessee, and Alabama also had total sulfur fluxes greater 

than or equal to 10.0 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 Oxford, OH (OXF122) to less than 25 percent at sites in New England. 

Figure 3-2 Total (Dry + Wet) Sulfur Deposition (as S) (kg/ha/yr) for 2006 

Total Deposition

16

8

1.6

Dry Deposition Dry Deposition

Wet Deposition

1.2

12.5

13.6

8.6

7.6

7.8

10.0 10.2

6.5

5.5

4.7

6.3

7.5

11.7

3.4

1.7

7.2

8.1

4.5

0.9

1.3

4.6

9.9

7.2

7.1

5.0

12.6

0.8

10.2

8.5

9.1

0.7

0.9

0.9

4.5

5.9

4.5

3.8

1.2

12.8

3.5

0.6

10.1

10.6

12.4 1.1

1.2

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12.5

1.5

0.6

0.6

8.5

3.0

14.0

1.1 20.2

1.0

7.7

6.7

5.2

1.2

6.5

15.4

1.9

10.0

0.8

0.4

Site not pictured:

DEN417, AK 0.4

Total Deposition

16

8

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Dry Deposition Dry Deposition

Wet Deposition

1.2

12.5

13.6

8.6

7.6

7.8

10.0 10.2

6.5

5.5

4.7

6.3

7.5

11.7

3.4

1.7

7.2

8.1

4.5

0.9

1.3

4.6

9.9

7.2

7.1

5.0

12.6

0.8

10.2

8.5

9.1

0.7

0.9

0.9

4.5

5.9

4.5

3.8

1.2

12.8

3.5

0.6

10.1

10.6

12.4 1.1

1.2

1.7

12.5

1.5

0.6

0.6

8.5

3.0

14.0

1.1 20.2

1.0

7.7

6.7

5.2

1.2

6.5

15.4

1.9

10.0

0.8

0.4

Site not pictured:

DEN417, AK 0.4
CASTNET Annual Report – 2006 27 Chapter 3: Atmospheric Deposition

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 

17 years, 1990 through 2006. The box plots 

were based on data obtained from the 34 

eastern CASTNET reference sites 

(Figure 1-6). Sulfur deposition at these sites 

declined significantly over the 17 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 

continuing decrease over the last several 

years with small increases in 2005 and 2006. 

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. 

Nonetheless, total sulfur deposition 

declined from a 1990-1992 mean of 

13.2 kg/ha/yr to a 2004-2006 mean of 9.5, 

a 28 percent reduction. 

Figure 3-3 Trend in Dry Sulfur Deposition 

(kg/ha/yr) 

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) 

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

reference sites. 

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CASTNET Annual Report – 2006 28 Chapter 3: Atmospheric Deposition

Uncertainties in Estimates of Dry Nitrogen Deposition 

EPA’s National Exposure Research 

Laboratory (NERL), which is part of the 

Office of Research and Development (ORD), 

recently sponsored a short, warm-season 

study to investigate the measurement 

uncertainties for CASTNET nitrogen species. 

The study examined the size distribution of 

CASTNET particle measurements through 

additional sampling in 2006 at three MidAtlantic CASTNET sites: Beltsville, MD 

(BEL116), Arendtsville, PA (ARE128), and 

Blackwater Wildlife Refuge, MD (BWR139). 

Weekly concentrations were measured with 

and without cyclone particle size separators 

for six measurement cycles over the period 

March 28, 2006 through August 22, 2006. 

BEL116 utilized cyclones with three different 

size cut off points: 10.0, 5.0, and 2.5 

micrometers (μm). The other two sites 

operated cyclones with a 2.5 μm cut off. 

The results are summarized in the three bar 

charts shown to the right. Mean, maximum, 

and minimum concentrations from the six 

samples are provided for ammonium (NH+ 

4 ), 

nitric acid (HNO3), and nitrate (NO3) for each 

sampling configuration. The measurements 

show that the NH+ 

4 concentrations at the three 

sites were not affected by the cyclones and 

existed as small (< 2.5 μm) particles. NH+ 

4

results were very similar to those of SO2-

4 (not 

shown) because during the warm season (i.e., 

the time of the study) most NH+ 

4 exists as 

ammonium sulfate [(NH 

4)

 

2SO2-4 ]. However, 

HNO3 and particulate NO3 concentrations 

were affected by the presence of cyclones. 

Higher HNO3 concentrations were measured 

0.0

1.0

2.0

3.0

4.0

ARE128

ARE128 (2.5)

BEL116

BEL116 (10.0)

BEL116 (5.0)

BEL116 (2.5)

BWR139

BWR139 (2.5)

Concentration (μg/m3

)

Mean Maximum Minimum

0.0

0.5

1.0

1.5

2.0

2.5

ARE128

ARE128 (2.5)

BEL116

BEL116 (10.0)

BEL116 (5.0)

BEL116 (2.5)

BWR139

BWR139 (2.5)

Concentration (μg/m3

)

Mean Maximum Minimum

0.0

1.0

2.0

3.0

4.0

5.0

ARE128

ARE128 (2.5)

BEL116

BEL116 (10.0)

BEL116 (5.0)

BEL116 (2.5)

BWR139

BWR139 (2.5)

Concentration (μg/m3

)

Mean Maximum Minimum

Ammonium Special Study Results 

Nitric Acid Special Study Results 

Nitrate Special Study Results 
CASTNET Annual Report – 2006 29 Chapter 3: Atmospheric Deposition

with the standard CASTNET open-face filter packs. Considerably higher particulate NO3

concentrations were measured with the open face configurations, and the concentrations 

decreased substantially as the cyclone cut-off points decreased (Lavery et al., 2007). 

Although limited in both temporal and spatial coverage, analysis of the study results 

provides new information on the measurement of nitrogen species using the standard 

CASTNET sampling protocol. The CASTNET open-face filter pack measures particles with 

a range of sizes from more than 10.0 μm to less than 2.5 μm. The larger particles are 

comprised of salts and trace metals, which are produced by sea spray and soil dust. Sodium 

nitrate (NaNO3) and calcium nitrate [Ca(NO3)2] constitute a significant fraction of the coarse 

particles. These nitrate particles are produced by the reaction of HNO3 with sodium chloride 

(NaCl) and calcium carbonate (CaCO3) in ambient air, on the Teflon® filter, or both. The 

fine particles are comprised of ammonium nitrate (NH4NO3), which is produced by the 

reaction of ammonia (NH3) with HNO3. 

A comparison of CASTNET measurements with data from other studies suggests that the 

right combination of atmospheric conditions (e.g., temperature, relative humidity, and solar 

radiation) causes the volatilization and conversion of NH4NO3 to HNO3 and NH3 during the 

CASTNET weekly sampling period. These reactions result in a decrease in NO3 and an 

increase in HNO3 on the filters. These NO3 → HNO3 concentration changes were 

corroborated by Teflon® and nylon filter measurements that were part of the Midwest 

Ammonia Monitoring Project (Sweet et al., 2005). The relative loss of NO3 and gain of 

HNO3 are unknown and vary significantly based on the geographic location of the 

monitoring site and on the season (Ames and Malm, 2001). 

One of the goals of CASTNET is to estimate dry deposition of particles and gases. The 

collection of coarse nitrate particles complicates the calculation of deposition velocities. The 

coarse particles are affected by the meteorological and biochemical processes that are 

simulated by the MLM. However, these particles are also subject to gravitational settling, 

which is not simulated. The net effect is likely an underestimation of the dry deposition of 

nitrate particles. Since dry deposition is directly proportional to the concentration of the 

pollutant, an uncertainty in the concentration measurement produces an uncertainty in the 

flux estimate. In the case of nitrogen species, an overestimation of HNO3 or NO3 affects 

the modeled dry nitrogen deposition. Since HNO3 deposition velocities are significantly 

higher than that of NO3, bias in the HNO3 concentration is amplified in the total nitrogen 

deposition reported. 
CASTNET Annual Report – 2006 30 Chapter 3: Atmospheric Deposition

Nitrogen Deposition 

Figure 3-6 presents a map of dry fluxes of nitrogen [as nitrogen (N)] for 2006. Nitrogen fluxes 

are comprised of nitric acid (HNO3) + nitrate (NO3) + ammonium (NH+ 

4 ). 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. Other than the relatively low deposition rates at Cedar Creek State Park, WV 

(CDR119) and Coweeta, NC (COW137), the nitrogen fluxes were fairly uniform geographically, 

reflecting a wide distribution of nitrogen oxides (NOx) sources (such as motor vehicles). The flux 

values ranged from 0.4 kg/ha/yr in upstate New York to 3.1 kg/ha/yr in eastern Ohio. The values 

at the western sites ranged from 0.2 kg/ha/yr at Mount Rainier National Park, WA (MOR409) to 

3.0 kg/ha/yr at Sequoia National Park, CA (SEK430). Locations on the map with no value had 

insufficient data to calculate fluxes. 

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

4 ) Deposition (as N) (kg/ha/yr) for 2006 

2.3

2.4 2.7 1.3

1.8

2.3

0.8

1.8

2.6

1.5

0.7

1.5

1.3

1.6

2.7

0.5

1.2

1.7

1.0

0.9

0.8

1.9

2.5

1.9

0.8

1.6

2.9

1.0

1.1

1.2

2.4

0.8

0.9

0.3

0.6

1.0

0.4

0.7

1.9

1.3

1.8

0.4

2.4

1.7

2.5 1.1

0.2

0.3

1.3

1.3

1.0

0.5

1.8

1.0

1.8

0.8 3.1

3.0

1.4

1.9

1.0

1.0

1.3

2.0

0.6

1.9

0.3

0.6

Site not pictured:

DEN417, AK 0.1

Deposition

3.0

1.5

0.3
CASTNET Annual Report – 2006 31 Chapter 3: Atmospheric Deposition

Figure 3-7 presents a map of total nitrogen deposition (as N) for 2006. The map was constructed 

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

shows that a large majority 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 2006. The highest total nitrogen 

flux (9.2 kg/ha/yr) was estimated for Vincennes, IN (VIN140) and Quaker City, OH (QAK172). 

Fluxes with values less than 5.0 kg/ha/yr in the eastern United States were estimated for New 

England, upstate New York, Minnesota, Florida, and Texas. The values at the western sites 

ranged from 1.2 kg/ha/yr at Yosemite National Park, CA (YOS404), Pinedale, WY (PND165), 

and MOR409, WA to 5.1 kg/ha/yr in southern California at Converse Station (CON186). 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, OH had the distinction 

of measuring both the highest total sulfur and total nitrogen deposition rates. 

Figure 3-7 Total (Dry + Wet) Nitrogen Deposition (as N) (kg/ha/yr) for 2006 

Total Deposition

10

5

1

Dry Deposition

Wet Deposition

5.1

8.2

8.1

5.5

5.1

6.3

8.4 8.8

5.7

4.1

4.3

5.4

8.6

7.5

2.8

2.4

4.6

7.8

5.3

2.3

2.6

7.2

7.7

6.4

5.0

5.7

8.4

2.4

8.4

6.6

6.8

2.0

2.4

1.9

3.6

7.7

4.0

2.8

4.4

7.8

6.5

1.3

9.0

7.8

8.7 2.6

1.2

2.4

6.9

2.6

1.3

1.2

6.9

5.9

7.0

3.0 9.2

3.9

6.0

8.1

4.2

2.8

6.0

9.2

3.8

6.5

1.4

1.2

Site not pictured:

DEN417, AK 0.2

Total Deposition

10

5

1

Dry Deposition

Wet Deposition

5.1

8.2

8.1

5.5

5.1

6.3

8.4 8.8

5.7

4.1

4.3

5.4

8.6

7.5

2.8

2.4

4.6

7.8

5.3

2.3

2.6

7.2

7.7

6.4

5.0

5.7

8.4

2.4

8.4

6.6

6.8

2.0

2.4

1.9

3.6

7.7

4.0

2.8

4.4

7.8

6.5

1.3

9.0

7.8

8.7 2.6

1.2

2.4

6.9

2.6

1.3

1.2

6.9

5.9

7.0

3.0 9.2

3.9

6.0

8.1

4.2

2.8

6.0

9.2

3.8

6.5

1.4

1.2

Site not pictured:

DEN417, AK 0.2
CASTNET Annual Report – 2006 32 Chapter 3: Atmospheric Deposition

Figure 3-8 presents box plots that show the 

17-year trend in dry nitrogen deposition 

(as N), and Figure 3-9 shows the trend in 

annual total (dry + wet) nitrogen deposition 

over the 17 years, 1990 through 2006. The 

box plots in Figure 3-8 show little overall 

trend although the data suggest a decline in 

dry nitrogen deposition since 1999. 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 perhaps 1996. 

Again, the total flux in 2003 was relatively 

high because of unusually high precipitation. 

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 

slight downward trend since 1998. This 

trend line reflects the effect of changes in 

NOx emissions. Total nitrogen deposition 

declined 12 percent over the 17 years. 

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

2

4

6

8

10

12

19901991199219931994199519961997199819992000200120022003200420052006

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

0

1

2

3

4

5

6

19901991199219931994199519961997199819992000200120022003200420052006

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

19901991199219931994199519961997199819992000200120022003200420052006

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

19901991199219931994199519961997199819992000200120022003200420052006

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

19901991199219931994199519961997199819992000200120022003200420052006

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 – 2006 Chapter 4: Ozone Concentrations 33

Chapter 4: 

Ozone Concentrations

Most CASTNET sites operate an ozone (O 

3) analyzer that provides information on 

ozone concentrations. Ozone data are recorded and archived as hourly averages. While 

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

geographic patterns in regional O 

3 and the extent to which rural areas potentially exceed 

concentration values mandated by the National Ambient Air Quality Standards (NAAQS). The 

8-hour O 

3 standard is a useful metric for assessing status and trends in rural O 3 in order to gauge 

the success of EPA emission reduction programs such as the NOx Budget Program. The analyses 

presented in this section provide maps and examine trends in the fourth highest daily maximum 

8-hour average O 

3 concentrations. 

Ozone is an allotrope of oxygen (made up of three oxygen atoms). At ground level, it is an air 

pollutant that can cause harmful effects on the human respiratory system as well as damage to 

vegetation and ecosystems. Ozone is formed in the troposphere when volatile organic 

compounds (VOC), nitrogen oxides (NOx), and carbon monoxide (CO) react in the presence of 

sunlight. Ozone is a result of the photolysis of nitrogen dioxide (NO 

2): 

 NO 

2 + hv → NO + O (4-1) 

 O + O 

2 → O 3 + M (4-2) 

Where hv represents incoming solar radiation and M represents any body [e.g., nitrogen (N 

2) or 

oxygen (O 

2) molecules] present during the reaction to absorb energy. Reaction 4-2 is the only 

CASTNET provides the primary means for monitoring rural, ground-level ozone (O 

3) 

concentrations in the United States. Fourth highest daily maximum 8-hour average O 

3

concentrations represent the critical metric for evaluating compliance with National 

Ambient Air Quality Standards (NAAQS). These annual fourth highest daily maximum 

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

nonattainment. If the average ozone concentration for an area exceeds 0.08 ppm or 

reaches 85 ppb in practice, the area is designated as a “nonattainment area.” For the 

most recent 3-year period (2004-2006), only two eastern and four California sites 

recorded exceedances of the 8-hour standard. This represents the fewest number of sites 

with exceedances over a 3-year period in the history of the network. 

Gothic, CO (GTH161) 
CASTNET Annual Report – 2006 Chapter 4: Ozone Concentrations 34

significant source of ozone in the atmosphere. Once formed, O 

3 reacts with nitric oxide (NO) to 

regenerate NO 

2: 

 O 

3 + NO → NO 2 + O 2 (4-3) 

Measured O 

3 concentrations frequently exceed those predicted by Reactions 4-1 through 4-3. 

Other reactions involving carbon-containing species enhance the production of O 

3 by 

producing free radicals that oxidize NO to NO 

2 and subsequently to O 3 via Reaction 4-2. In the 

“background” troposphere, O 

3 concentrations are produced by methane and CO oxidation, 

transport from the stratosphere, and very long-range transport. In more polluted locations, the 

chemical reactions of VOC such as alkanes, alkenes, and aromatic hydrocarbons and NOx

dominate over methane and CO chemistry. Biogenic VOC emissions from trees and other 

vegetation contribute to O 

3 formation in rural areas (EPA, 2007c). Volatile organic compound 

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

3

concentrations in and downwind of urban areas. However, as discussed in this chapter, NOx

emission reductions that were mandated by the Acid Rain Program and other NOx emission 

control programs were necessary to reduce O 

3 concentrations, especially elevated 8-hour 

levels in rural areas. These reductions will continue to be required for additional declines in 

8-hour O 

3 levels. 

During the period 2004-2006, 3-year averages of the fourth highest daily maximum 8-hour 

average O 

3 concentrations were greater than or equal to 85 parts per billion (ppb) at four sites in 

California, one in New Jersey, and one in Maryland. Measurements of 8-hour concentrations 

during 2006 were somewhat lower than 2005 and were considerably lower than concentrations 

measured over the period 1990-2002. Following a peak in ozone levels in 2002, there has been a 

downward trend that has continued through 2006. 

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

concentration data using an algorithm 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). 

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

3

concentrations for 2004-2006. Two eastern and four California sites measured 3-year average 

concentrations greater than or equal to 85 ppb. These 3-year average concentrations constitute 

current design values for achieving the 8-hour NAAQS for O 

3. For example, the estimated value 

of 117 ppb at Converse Station, CA (CON186) would have to be reduced to 84 ppb to achieve 

the standard. Design values change as a new 3-year database becomes available. 
CASTNET Annual Report – 2006 Chapter 4: Ozone Concentrations 35

National Ambient Air Quality Standard for Ozone 

8-Hour Ozone Standard 

To better protect public health, EPA (1997) revised its national air quality standards for 

ozone in 1997, establishing an 8-hour standard. The 8-hour standard is 0.08 parts per 

million (ppm). An area meets the standard if the 3-year average of the annual fourth 

highest daily maximum 8-hour average concentration is less than or equal to 0.08 ppm. 

The 3-year average reduces the influence that meteorological conditions have on the 

extent and magnitude of ozone formation. Recently, EPA proposed strengthening the 

8-hour standard to a level within the range of 0.07 ppm to 0.075 ppm (EPA, 2007a). For 

more information on the 8-hour ozone standard and ozone nonattainment areas in the 

United States, visit http://www.epa.gov/ozonedesignations/. 

What Does It Mean? 

EPA collects ozone data on an hourly basis. Essentially, 8-hour average ozone 

concentrations at a monitor cannot exceed 0.08 ppm more than three days during the 

ozone season. For compliance purposes: 

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

♦ The daily maximum 8-hour average is recorded 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 fourth highest daily 8-hour average exceeds 0.084 ppm (0.085 rounds up to 

0.09 thus exceeding the standard criteria) or reaches 85 ppb in practice, the area is 

designated as a “nonattainment area” and is held accountable for reaching the 

attainment status. 

For comparison to current data, 3-year averages of the fourth highest daily maximum 8-hour 

average O 

3 concentrations for 2001-2003 are presented in Figure 4-2. Sixteen eastern and the 

same four California sites recorded 3-year averages greater than or equal to 85 ppb. The regions 

with elevated concentrations were located along the East Coast from northern Virginia to Maine 

and in Pennsylvania, Ohio, and Michigan. A value of 92 ppb was measured at Great Smoky 

Mountains National Park, TN (GRS420). The period 2004-2006 represents a significant 

improvement in air quality. In fact, the period 2004-2006 represents the best period of air quality 

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

8-hour O 

3NAAQS. 
CASTNET Annual Report – 2006 Chapter 4: Ozone Concentrations 36

 . 

Figure 4-1 Fourth Highest Daily Maximum 8-Hour Ozone Concentrations (ppb) for 

2004-2006

Figure 4-2 Fourth Highest Daily Maximum 8-Hour Ozone Concentrations (ppb) for 

2001-2003
CASTNET Annual Report – 2006 Chapter 4: Ozone Concentrations 37

Fourth highest daily maximum 8-hour O 

3 concentrations for 2006 are shown in Figure 4-3. 

Concentrations greater than or equal to 85 ppb were recorded at three sites in the East including 

GRS420, TN; Georgia Station, GA (GAS153); and Beltsville, MD (BEL116). In addition, four 

California CASTNET sites monitored exceedances of the 8-hour ozone NAAQS. The California 

sites include CON186, Joshua Tree National Monument (JOT403), Sequoia National Park 

(SEK430), and Yosemite National Park (YOS404). The 8-hour level of 108 ppb at CON186, CA 

was the highest value that was measured in the network during 2006. 

Figure 4-3 Fourth Highest Daily Maximum 8-Hour Ozone Concentrations (ppb) for 2006

Since 2002, CASTNET ozone measurements have shown a significant decline in annual fourth 

highest daily maximum 8-hour average concentrations throughout the 34 eastern reference sites 

(see Figure 1-6) used to track trends in concentrations. During 2002, 24 reference sites measured 

fourth highest daily maximum 8-hour O 

3 concentrations greater than or equal to 85 ppb. The 

number of reference sites with exceedances was reduced to six in 2003, followed by a further 

reduction to none in 2004, and five in 2005. As shown in Figure 4-3, two eastern reference sites 

(BEL116, MD and GAS153, GA) recorded elevated 8-hour concentrations in 2006. 

Figure 4-4 shows the 17-year trend in 8-hour O 

3 concentrations aggregated over the 34 reference 

stations. The box plots show a significant decline from 2002 to 2004. The 2005 data show a 

small increase, and the 2006 data show a decrease from 2005. Median values for the aggregated 

measurements for the five years 2002 through 2006 were 88 ppb, 79 ppb, 70 ppb, 76 ppb, and 

72 ppb, respectively. 
CASTNET Annual Report – 2006 Chapter 4: Ozone Concentrations 38

Figure 4-4 Trend in Fourth Highest Daily Figure 4-5 Trend in Ozone Season Mean 

Maximum 8-Hour Ozone Concentrations (ppb) Total Nitrate Concentrations (μg/m3 ) 

The median O 

3 concentration for 2006 was the second lowest in the 17-year period. Additional 

information on ozone measurements can be found in the NPS annual reports on O 

3 monitoring 

and O 

3 trends (http://www2.nature.nps.gov/air/Monitoring/network.cfm#procedures). 

The CASTNET 2005 Annual Report (MACTEC, 2006a) discussed the relationship between 

changes in O 

3 concentrations and changes in emissions of nitrogen oxides (NOx). EPA (2005b, 

2006) attributed the decline in NOx emissions to several effective control programs: 

♦ Mobile source controls, 

♦ Volatile Organic Compound Reasonably Available Control Technology and Maximum 

Available Control Technology, 

♦ New Source Review, 

♦ Acid Rain Program, 

♦ Ozone Transport Commission NOx Budget (1999-2002), and 

♦ NOx State Implementation Plan Call (2003-current). 

These programs continue to produce reductions in NOx emissions in the eastern United States. 

The trend in CASTNET measurements of total nitrate concentrations (Figure 4-5) shows a 

relationship between the decline in ozone and a decline in atmospheric nitrogen. The box plots 

show a significant reduction in total nitrate concentrations during the ozone season since 1999. 

The ozone season varies between states but is typically from the beginning of April through the 

end of September or October in the eastern United States. 

 

60

80

100

120

19901991199219931994199519961997199819992000200120022003200420052006

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

19901991199219931994199519961997199819992000200120022003200420052006

Concentration (ppb)

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

19901991199219931994199519961997199819992000200120022003200420052006

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

19901991199219931994199519961997199819992000200120022003200420052006

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 – 2006 Chapter 4: Ozone Concentrations 39

Elevated Ozone Concentrations in Atlanta during 2006 

The CASTNET site at Georgia Station, GA (GAS153) recorded its 2006 fourth highest daily 

maximum ozone (O 

3) concentration of 92 ppb on June 9, 2006. This site had not recorded an 

exceedance since 2002. The GAS153 O 

3 level was part of an ozone episode that occurred in the 

greater Atlanta region over the period June 8 through June 11, 2006. Daily maximum 1-hour O 

3

concentrations for State and Local Ambient Monitoring Stations (SLAMS) ozone sites operated 

by the State of Georgia and for the GAS153 and Sand Mountain, AL (SND152) CASTNET sites 

are presented by four maps, one for each day from June 8 through June 11, 2006. The Atlanta 

Metropolitan Statistical Area, which consists of 20 counties, is shown on the four maps using 

gray outlines. The SLAMS data provide information on urban O 

3 concentrations, which 

supplement the rural CASTNET measurements. The use of both SLAMS and CASTNET data 

provides a detailed depiction of the geographic pattern of ozone levels in the greater Atlanta area. 

The use of daily maximum 1-hour O 

3 levels illustrates the evolution and extent of the episode 

over the four days. The circles on the maps designate the two CASTNET sites. 

Daily Maximum 1-Hour O3 Concentrations for CASTNET and SLAMS 

Daily Maximum 1-Hour

Ozone Concentration

100

85

80

75

70

60

Athens

Columbus Columbus

Savannah Savannah

72

83

82

75 74

71

67 81

69 Atlanta Atlanta

Augusta Augusta

Macon

87

68

72

88

65

71

74

78

67

75

68

71

77

70

64

Athens

Columbus Columbus

Savannah Savannah Savannah

77

86 75

82

80 107

101

81

Atlanta Atlanta

Augusta Augusta

Macon

73

76

78

99

67

69

80

73

76

80

71

67

81

68

84

77

Columbus Columbus

Savannah Savannah

Athens

83 73

101

100

104 92

108

85

109

Atlanta Atlanta

Augusta Augusta

Macon

83

78

92

111

81

91

90

80

80

96

88

80

95

81

Savannah Savannah Savannah

Columbus Columbus

Athens

83

72

80 77

80

70

73

67

Atlanta Atlanta

Augusta Augusta

Macon

78

46

71

83

72

61

93

62

73

73

69

78

87

65

84

June 8 June 9

June 10 June 11
CASTNET Annual Report – 2006 Chapter 4: Ozone Concentrations 40

The beginning of the episode was signaled by ozone concentrations in excess of 80 ppb in 

southeast Atlanta on June 8. The levels increased on June 9 with concentrations as high as 

107 ppb at the McDonough SLAMS site southeast of downtown Atlanta. The peak 1-hour value 

at GAS153 on June 9 was 101 ppb. 

Backward trajectories from the National Oceanic and Atmospheric Administration (NOAA) 

Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) Model for June 9, 2006 are 

provided. Trajectories ending at GAS153 are given for 100 meters (m), 500 m, and 2000 m 

above ground level (AGL). The three trajectories on June 9 indicate advection through the area 

of highest ozone concentrations and an area of dense, low-level precursor emissions. 

Concentrations increased on June 10 with several daily maximum values in excess of 90 ppb and 

six stations with levels greater than or equal to 100 ppb. Backward trajectories are also provided 

for this day. High ozone values were widespread on June 10 because of air mass subsidence and 

higher temperatures. The 100 m trajectory ending at GAS153 originated at an elevation of almost 

2000 m 24 hours earlier. Evidently, a localized high ozone event was observed on June 9, and the 

episode became more widespread on June 10 with prevalent high pressure and warm 

temperatures favorable to production of high ozone concentrations. Daily maximum hourly O 

3

concentrations were significantly lower on June 11 with only two values above 85 ppb. 

Trajectory June 9, 2006 Trajectory June 10, 2006 

. 
CASTNET Annual Report – 2006 Chapter 5: Data Quality 41

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 

QA program elements are documented in the Quality Assurance Project Plan (QAPP), 

Revision 3.0 (MACTEC, 2005). 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 2006 data were analyzed and compared with 

historical data collected during the period 1990-2005. The information in this report is 

supplemented by analyses that are discussed in quarterly CASTNET Quality Assurance Reports 

(e.g., MACTEC, 2007b). These QA reports are produced four times per year with the fourth 

quarter report including an annual summary. 

Claryville, NY (CAT175) 

CASTNET measurements are assessed routinely in terms of providing high-quality 

information to meet CASTNET objectives. The CASTNET quality assurance program is 

based on specified data quality objectives, which are evaluated using data quality indicators 

(DQI). Measurements taken during 2006 and historical data collected over the period 

1990-2005 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 data of the highest quality. 
CASTNET Annual Report – 2006 Chapter 5: Data Quality 42

Figure 5-1 Historical and 2006 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) for both the historical (1990–2005) data for all collocated site pairs and the 2006 data 

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

Park, CO (ROM406/206). Trace cations and chloride are excluded from this figure but are shown 

later in Figure 5-2. The historical results vary from about 4 percent for particulate sulfate (SO2-

4 ) 

to about 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., 2007). 

The 2006 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 2006 results for 

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

4 , HNO3, and NO3; the 

0

2

4

6

8

10

12

14

16

12345

MARPD

Overall - 1990 to 2005

Mackville, KY - 2006

Rocky Mtn, CO - 2006

SO2 SO4

2- HNO3 NO3-

 NH4

+
CASTNET Annual Report – 2006 Chapter 5: Data Quality 43

results for SO2-

4 , HNO3, and NH+ 4 met the DQI criteria. Overall, the filter pack precision results for 

2006 showed some improvement over 2005. 

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 0.005 mg/L 

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

Calcium (Ca2+) ICP-AES 5 95 - 105 0.003 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 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 to 95 – 105 percent for continuing calibration verification spikes at 

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

 

 ICP-AES = inductively coupled plasma-atomic emission spectrometry 

 MARPD = mean absolute relative percent difference 

 N = as nitrogen 

For more information on analytical methods and associated precision and accuracy criteria, see the CASTNET QAPP – Revision 3.0 

(MACTEC, 2005). 

Precision statistics for 2006 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, except for Cl –

 , which had a MARPD of approximately 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 2006 precision results show that Na + and Cl – met the 

precision criterion at MCK131/231. The MARPD for Cl –

 calculated for ROM406/206 met the 

criterion for 2006. The 2006 results are similar to the precision data for 2005. 

Table 5-2 summarizes 2005 precision results by quarter for the two sets of collocated sites. See 

the 2006 Quarterly Data Reports (MACTEC, 2006b; 2006d; 2007c; 2007a; ) and QA Quarterly 

Reports (MACTEC, 2006c; 2006e; 2006f; 2007b; ) for discussions of quarterly precision data. 
CASTNET Annual Report – 2006 Chapter 5: Data Quality 44

Figure 5-2 Historical and 2006 Precision Data for Cation and Cl –

 Concentrations 

Table 5-2 Collocated Precision Results for 2006 Filter Pack Data by Quarter (MARPD) 

Site Pairs SO2-

4 NO3 NH+ 

4 Ca2+ Mg2+ Na + K + HNO3 SO2

Total 

NO3 Cl-

 MCK 131/231, KY

Quarter 1 3.22 8.71 3.86 12.17 10.49 6.40 10.30 2.90 2.98 4.74 3.46 

Quarter 2 0.96 7.49 1.70 4.79 5.20 4.28 11.16 3.90 1.81 3.82 2.06 

Quarter 3 1.35 6.36 1.62 2.86 2.33 3.03 1.85 2.37 2.35 2.26 0.46 

Quarter 4 1.96 6.30 4.03 8.33 6.61 4.00 5.09 2.74 2.86 2.89 5.05 

2006 1.87 7.21 2.80 7.03 6.16 4.43 7.10 2.98 2.50 3.43 2.76 

 ROM 406/206, CO

Quarter 1 4.68 3.64 2.43 7.52 8.07 6.38 9.21 4.55 8.64 2.63 1.32 

Quarter 2 5.66 8.03 4.84 5.86 11.10 6.17 14.89 2.36 4.97 3.56 1.14 

Quarter 3 3.70 12.13 3.09 5.89 5.28 10.74 7.56 2.54 13.00 1.65 1.76 

Quarter 4 2.89 13.26 15.16 5.99 7.44 9.15 11.69 4.63 11.78 2.11 2.89 

2006 4.23 9.26 6.38 6.31 7.97 8.11 10.84 3.52 9.60 2.49 1.78 

Note: Shaded values exceed DQI criterion 

The 2006 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 2005

Mackville, KY - 2006

Rocky Mtn, CO - 2006

MARPD

Ca2+ K+ Mg2+ Na+ Cl-
CASTNET Annual Report – 2006 Chapter 5: Data Quality 45

Figure 5-3 Precision Results for Laboratory Replicate Samples (2006) 

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. 

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 – 2006 46 Chapter 5: Data Quality 

Ozone Concentrations 

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

EPA QA requirements for SLAMS monitoring (EPA, 1998). The QA procedures for the O3

analyzers at the NPS-sponsored sites also do not meet the SLAMS requirements. While NPS 

utilizes the appropriate procedures and equipment, the NPS sites are calibrated twice per year 

versus the SLAMS requirements of four times per year. In any event, the operation of the 

collocated 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 2006. In addition, quarterly historical 

precision statistics were compiled for all collocated sites. Quarterly precision results are 

summarized in Figure 5-4. The data show the historical results met the 10 percent criterion. The 

2006 precision data also met the 10 percent criterion for the second and third quarters for both 

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

the first quarter. The high MARPD was caused by unusually low O 

3 concentrations. Many hourly 

levels were less than 10 ppb. MARPD was not calculated for MCK131/231 fourth quarter O 

3 

data. The MCK131 analyzer exhibited significant drift that began near the end of October, and 

the data are considered suspect and are undergoing additional review. 

Figure 5-4 Historical and 2006 Precision Data by Quarter for Ozone Concentrations 

Accuracy 

0

5

10

15

20

25

30

First

Quarter

Second

Quarter

Third

Quarter

Fourth

Quarter

Overall - 1990 to 2005

Mackville, KY - 2006

Rocky Mtn, CO - 2006

MARPD
CASTNET Annual Report – 2006 Chapter 5: Data Quality 47

Accuracy 

Laboratory Filter Concentrations 

Accuracy of laboratory measurements is assessed through the analysis of reference samples and 

continuing calibration verification (CCV) samples. 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 2006, the CCV were analyzed every tenth sample 

to verify that instrument calibration had not drifted beyond established limits. Table 5-4 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 2006. Table 5-4 

also lists the precision results that were shown in Figure 5-3. 

Table 5-4 Filter Pack QC Summary for 2006 

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 98.50 1.67 155 99.74 1.21 781 0.48 0.57 322

 NO3 101.81 1.07 155 99.10 1.21 781 1.21 1.48 322

 NH+ 

4 100.71 1.73 153 99.48 1.73 751 0.58 0.63 266

 Ca2+103.02 2.73 194 100.72 1.10 797 1.58 2.34 324

 Mg2+101.61 1.85 194 99.96 0.74 797 2.48 2.95 324

 Na+94.43 1.69 194 100.23 1.00 797 2.38 4.51 324

 K+100.79 2.44 194 100.50 0.98 797 3.40 3.29 324

 Cl- 100.97 1.39 155 99.25 2.41 781 1.17 1.48 322

Nylon SO2-

4 101.01 1.45 144 99.90 1.99 730 2.12 2.82 305

NO3 99.95 1.46 144 99.53 1.96 730 0.52 0.88 306

Cellulose SO2-

4 101.46 0.59 182 99.16 0.80 740 3.75 5.87 257

Note: % R = percent recovery 

 RPD = relative percent difference 

1

 Results of reference sample analyses provide accuracy estimates 

2 Results of replicate analyses provide precision estimates 3

 Number of QC Samples 
CASTNET Annual Report – 2006 Chapter 5: Data Quality 48

Continuous Measurements 

Table 5-5 presents field accuracy results for 2006 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 

72.8 percent, solar radiation at 88.1 percent, wind direction north at 88.0 percent, and wind 

direction south at 88.9 percent. However, these results did not adversely affect data collection 

because data are not considered invalid unless criteria are exceeded by more than their own 

magnitude. Using the two times standard, the four parameters ranged from 92.6 percent to 98.0 

percent frequency. 

Table 5-5 Accuracy Results for 2006 Field Measurements 

Parameter Percent Within Criterion 

Temperature (0°C) 99.0 percent 

Temperature (ambient) 98.1 percent 

Delta Temperature (0°C) 100.0 percent 

Delta Temperature (ambient) 97.1 percent 

*Relative Humidity > 85% 72.8 percent 

Relative Humidity ≤ 50% 94.3 percent 

*Solar Radiation 88.1 percent 

*Wind Direction North 88.0 percent 

*Wind Direction South 88.9 percent 

Wind Speed < 5 m/s 100.0 percent 

Wind Speed ≥ 5 m/s 95.3 percent 

Precipitation 99.1 percent 

Wetness (w/in 0.5 volts) 100.0 percent 

Ozone Slope 96.8 percent 

Ozone Intercept 100.0 percent 

Flow Rate 97.1 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 92.6 percent to 98 percent. 
CASTNET Annual Report – 2006 Chapter 5: Data Quality 49

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 and 2006 

completeness data for all sites for measured filter concentrations, continuous measurements, and 

calculated parameters. The figure shows that the 2006 direct measurements met the 90 percent 

goal. The 2006 results show that data completeness exceeded 95 percent for six continuous 

measurements, including filter pack flow. The four parameters derived from model results 

exceeded 83 percent completeness for 2006. Completeness results for 2006 are better than 

historical results for all parameters except sigma theta and ozone. 

Figure 5-5 Historical and 2006 Percent Completeness of Measurements and Modeled 

Estimates (black bars are 1990-2005) 

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 – 2006 Chapter 5: Data Quality 50

Laboratory Intercomparison Studies

The MACTEC laboratory is one of eight laboratories that participate in the U.S. Geological 

Survey (USGS) interlaboratory comparison program. 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. MACTEC 

reported the eight CASTNET parameters for 104 samples during 2006. Results for only the 44 

synthetically prepared samples are depicted in the figure below as the absolute percent difference 

of the MACTEC value from the median value for all laboratories. 

Results from Intercomparison of 44 Synthetic Samples 

 

As shown above, a single sodium result was very high. Internal investigation led to the 

conclusion that this sample had been contaminated. The probable timing and mechanism for the 

contamination event are indeterminate. The results for the eight ultra-pure deionized water 

samples analyzed during the year were all below the laboratory reporting limits for 

every parameter. 

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

absolute percent differences for replicate analyses. By parameter, less than 4 percent of 

approximately 208 paired natural wet deposition sample data points exceeded 10 percent 

difference. These results are presented in the USGS report (USGS, 2006). 

MACTEC generally participates in two to three studies by the National Water Research Institute 

(NWRI) of Canada’s Proficiency Testing QA Program each year in addition to interlaboratory 

comparison studies for the USGS. MACTEC’s laboratory was rated free of systemic bias for all 

eight parameters for the three NWRI 2006 studies. MACTEC was ranked number six out of 37 

participating laboratories at the end of 2006 (Environment Canada, 2006). 
CASTNET Annual Report – 2006 Chapter 5: Data Quality 51

Conclusion 

DQI results demonstrate that field and laboratory processes were adequately monitored through 

QA/QC procedures and were generally free of systemic bias during 2006. Accuracy data met the 

established criteria for field and laboratory parameters with the exception of relative humidity 

> 85 percent, solar radiation, wind direction north, and wind direction south. 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 sulfur constituents, HNO3, 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 unpredictable 

nature of the gas-particle equilibrium of the nitrate species. 

Precision data for ozone concentrations measured at ROM406/206 did not meet the 10 percent 

goal during the first quarter because of unusually low levels. MARPD was not calculated for 

MCK131/231 fourth quarter ozone data because the analyzer at MCK131 exhibited significant 

drift that began near the end of October. Results for other quarters are considered acceptable. 

Completeness criteria were met for all parameters in 2006. Completeness results for 2006 were 

generally better than historical results. 

Rocky Mountain National Park, CO (ROM406/206) 
CASTNET Annual Report – 2006 References R-1

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Concentrations Measured by IMPROVE and the CDN. Atmospheric Environment. 

Vol. 35(5) 905-916. 

Environment Canada. 2006. Water Science and Technology Directorate. Proficiency Testing 

Program. Rain and Soft Waters PT Studies 0087, 0088, and 0089. 

http://www.nwri.ca/nlet-lnee/ptp-pec/prof-e.html (accessed November 1, 2007). 

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3 Measurements with Data from Other 

Monitoring Programs. Journal of Air & Waste Management Association (JAWMA). 

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CASTNET Annual Report – 2006 References R-3

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2007). 

National Science Foundation (NSF). 2005. Translating Science for Society: Broader Impacts of 

NSF’s Long-Term Ecological Research Program. http://intranet.lternet.edu/archives/ 

documents/ Publications/brochures/nsf0533.pdf (accessed June 5, 2007). 

New Mexico Environment Department. 2007. Four Corners Air Quality Task Force. 

http://www.nmenv.state.nm.us/aqb/4C/ (accessed June 1, 2007). 

Schwede, D.B. 2006. A Comparison of the Deposition Velocity Estimates from the CASTNET 

and CAPMoN Networks. (Working paper). Research Triangle Park, NC. 

Sweet, C., Caughey, M., and Gay, D. 2005. Midwest Ammonia Monitoring Project, Summary 

for October 2003 through November 2004. http://www.ladco.org/reports/rpo/ 

MWRPOprojects/Monitoring/YearOneReport(Final).pdf (accessed November 5, 2007). 

U.S. Environmental Protection Agency (EPA). 2007a. 8-Hour Ground–level Ozone 

Designations. http://www.epa.gov/ozonedesignations/ (accessed March 1, 2007). 
CASTNET Annual Report – 2006 References R-4

References (continued) 

U.S. Environmental Protection Agency (EPA). 2007b. Clean Air Interstate Rule. 

http://www.epa.gov/cair/index.html (accessed March 1, 2007). 

U.S. Environmental Protection Agency (EPA). 2007c. Vegetation and Air Quality. 

http://www.epa.gov/heatisland/strategies/level3_vegairquality.html (accessed 

December 6, 2007). 

U.S. Environmental Protection Agency (EPA). 2006. NOx Budget Trading Program. 2005 

Program Compliance and Environmental Results. EPA 430-R-06-013. September 2006. 

U.S. Environmental Protection Agency (EPA). 2005a. Acid Rain Program 2004 Progress 

Report. EPA 430-R-05-012. October 2005. 

U.S. Environmental Protection Agency (EPA). 2005b. Evaluating Ozone Control Programs in 

the Eastern United States. EPA 454-K-05-001. August 2005. 

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). 2006. Office of Water Quality. Branch of Quality Systems. 

Interlaboratory Comparison Program http://bqs.usgs.gov/precip/new/frontpage_home.htm 

(accessed November 1, 2007). 
AAppppeennddiixxAA

Locational and Operational Characteristics of CASTNET Sites
CASTNET Annual Report – 2006 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 

Terrain 

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 – 2006 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 

Terrain 

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 – 2006 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 

Terrain 

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 – 2006 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 

Terrain 

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 – 2006 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 

Terrain 

New York CAT175 Claryville 05/10/94 41.9423 74.5519 765 • •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 •6 •4 Agri. R
CASTNET Annual Report – 2006 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 

Terrain 

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 CUtah CAN407 Canyonlands National Park 01/24/95 38.4586 109.8211 1809 • • Desert CVermont LYE145 Lye Brook 03/30/94 43.0510 73.0613 730 • •5 ◊ Forested M
CASTNET Annual Report – 2006 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 

Terrain 

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 – 2006 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 sites. 6. Composite filter pack, day filter pack, and night filter pack. 

• 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 – 2006 B-1 Appendix B

List of Acronyms and Abbreviations

AGL above ground level 

ARP Acid Rain Program 

BLM Bureau of Land Management 

°C degrees Celsius 

Ca2+

 particulate calcium ion 

CAAA Clean Air Act Amendments 

CaCO 

3 calcium carbonate 

CAIR Clean Air Interstate Rule 

Ca(NO 

3)

 

2 calcium nitrate 

CAPMoN Canadian Air and Precipitation Monitoring Network 

CASTNET Clean Air Status and Trends Network 

CCV continuing calibration verification samples 

Cl- particulate chloride ion 

CLASSTM Chemical Laboratory Analysis and Scheduling System 

CO carbon monoxide 

DQI data quality indicator 

EPA U.S. Environmental Protection Agency 

H 

2SO 4 sulfuric acid 

HNO 

3 nitric acid 

HYSPLIT HYbrid Single-Particle Lagrangian Integrated Trajectory 

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 

LCS laboratory control sample 

lpm liters per minute 

LTER Long Term Ecological Research Program 

m meter 

MACTEC MACTEC Engineering and Consulting, Inc. 

MARPD mean absolute relative percent difference 

Mg2+

 particulate magnesium ion 

mg/l milligrams per liter 

MLM Multi-Layer Model 

mm millimeter 

MSA Metropolitan Statistical Area 

N nitrogen 

N 

2 nitrogen gas in atmosphere 
CASTNET Annual Report – 2006 Appendix B B-2 

List of Acronyms and Abbreviations (continued) 

Na+ particulate sodium ion 

NAAQS National Ambient Air Quality Standards 

NaCl sodium chloride 

NADP/NTN National Atmospheric Deposition Program/National Trends Network 

NADP/MDN National Atmospheric Deposition Program/Mercury Deposition Network 

NaNO 

3 sodium nitrate 

NAPAP National Acid Precipitation Assessment Program 

NERL National Exposure Research Laboratory 

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 nitric oxide 

NO 

2 nitrogen dioxide 

NO -

3 particulate nitrate 

NOx nitrogen oxides [nitric oxide (NO) + nitrogen dioxide (NO 

2)] 

NOAA National Oceanic and Atmospheric Administration 

NPS National Park Service 

NSF National Science Foundation 

NWRI Environment Canada’s National Water Research Institute 

O 

2 oxygen gas in atmosphere 

O 

3 ozone 

OH hydroxyl 

ORD EPA’s Office of Research and Development 

OTC Ozone Transport Commission 

ppb parts per billion 

ppm parts per million 

QA quality assurance 

QAPP Quality Assurance Project Plan 

QC quality control 

S sulfur 

SIP State Implementation Plan 

SLAMS State and Local Monitoring Stations 

SO 

2 sulfur dioxide 

SO2-

4 particulate sulfate 

total NO3 gaseous nitric acid (HNO 

3) + particulate nitrate (NO3) 

μg/m3 micrograms per cubic meter
CASTNET Annual Report – 2006 Appendix B B-3 

List of Acronyms and Abbreviations (continued) 

μm micrometer 

USGS U.S. Geological Survey 

V 

d deposition velocity 

VOC 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: Clean Air Markets EPA Home Page:

Division Home Page:

www.epa.gov/castnet www.epa.gov/airmarkets www.epa.gov