CASTNET

Clean Air Status and Trends Network

2015 Annual Report
Clean Air Status and Trends Network

(CASTNET)

2015 Annual Report

Prepared by:

Amec Foster Wheeler Environment & Infrastructure, Inc.

Prepared for:

U.S. Environmental Protection Agency

Office of Air and Radiation

Clean Air Markets Division

Washington, DC

EPA Contract No. EP-W-16-015

September 2017
CASTNET 2015 Annual Report

i Table of Contents

TABLE OF CONT ENTS

Table of Contents

Executive Summary..........................................................................................................................iii

Chapter 1 CASTNET Update .......................................................................................................1

Introduction...................................................................................................................1

Locations of Monitoring Sites........................................................................................2

Measurements Recorded at CASTNET Sites ...............................................................4

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

Quality Assurance Program..........................................................................................5

Estimating Dry, Wet, and Total Deposition....................................................................6

Building Tribal Partnerships with CASTNET Monitoring Sites .......................................6

Chapter 2 Ozone Concentrations..............................................................................................10

2008 National Ambient Air Quality Standards for Ozone.............................................11

Eight-hour Ozone Concentrations...............................................................................13

Chapter 3 Evaluating Elevated Ozone Concentrations Measured at Washington

Crossing, New Jersey ...........................................................................................16

Chapter 4 Nitrogen Pollutant Concentrations..........................................................................24

Total Nitrate Concentrations .......................................................................................24

Particulate Ammonium Concentrations.......................................................................26

Chapter 5 Update on the Ammonia Monitoring Network.........................................................29

Chapter 6 Sulfur Pollutant Concentrations..............................................................................34

Sulfur Dioxide Concentrations ....................................................................................34

Particulate Sulfate Concentrations..............................................................................36

Chapter 7 Continuous Trace-level Gas Concentrations..........................................................39

Continuous Trace-level NOy and Ozone .....................................................................44

Continuous Trace-level NOy and Filter Pack Total Nitrate Concentrations ..................45

Chapter 8 Deployment and Evaluation of the Monitor for Aerosols and Gases at 

Beltsville, Maryland ...............................................................................................48

Comparison of MARGA, Continuous Sulfur Dioxide Analyzer, and CASTNET

Filter Pack ...............................................................................................................48

Comparison of Co-located MARGA Units ...................................................................49

Integrated MARGA Data and Comparison with Filter Pack Data.................................50

Chapter 9 Atmospheric Deposition of Nitrogen.......................................................................53
CASTNET 2015 Annual Report

ii Table of Contents

Table of Contents (continued)

Chapter 10 Cloud Water, Filter Pack Pollutant, and Ammonia Concentrations at

Whiteface Mountain, New York.............................................................................59

Cloud Water Measurements .......................................................................................61

Filter Pack Measurements ..........................................................................................64

Ammonia Measurements ...........................................................................................66

Chapter 11 Atmospheric Deposition of Sulfur, Base Cations, and Chloride ...........................70

Sulfur Deposition ........................................................................................................70

Deposition of Base Cations and Chloride....................................................................70

Chapter 12 Improved Forest Health in the Eastern United States............................................74

Methods......................................................................................................................74

Results .......................................................................................................................75

References ...................................................................................................................................77

Appendix A Locational and Operational Characteristics of CASTNET Sites

Appendix B Acronyms and Abbreviations

Cranberry, NC (PNF126)
iii Executive Summary

50

60

70

80

90

100

0

2

4

6

8

10

12

19901991199219931994199519961997199819992000200120022003200420052006200720082009201020112012201320142015

Ozone Concentration (ppb)

Filter Pack Concentration (μg/m3)

Total NO3

SO2

Ozone

SO2

Total NO3

-

Ozone

Figure E-1 Trends in O 

3, Total NO3, and 

SO 

2 at Eastern Reference Sites

Executive Summary

Executive Summary

The EPA Clean Air Status and Trends Network (CASTNET) measures concentrations of atmospheric pollutants across 

the United States. The primary objectives of the network are to determine compliance with ozone National Ambient Air 

Quality Standards and to provide data to evaluate the effectiveness of national and regional air pollution control 

programs. CASTNET data are also used to provide input to the National Atmospheric Deposition Program’s Total 

Deposition Hybrid Method for calculating total deposition and evaluating regional air quality models. This report 

presents maps of 2015 ozone levels, nitrogen and sulfur pollutant concentrations, and deposition fluxes and examines 

trends in air quality over the 26-year period from 1990 through 2015. In 2015, CASTNET measured rural, regionally 

representative concentrations of nitrogen and sulfur species at 95 monitoring stations at 93 locations and ozone levels 

at 80 locations.

Key Results and Highlights through 2015

The median fourth highest daily maximum 8-hour average (DM8A) ozone (O 

3) concentration for 2015 

at the eastern CASTNET reference sites (see Appendix A for designated reference sites) was 63.5

parts per billion (ppb) or, equivalently, 0.0635 parts per million (ppm). This is a slight increase from the 

2014 median of 62.5 ppb, which was the lowest in the history of the network. At the western 

CASTNET reference sites, the median was 66 ppb. Three-year averages of fourth highest DM8A O 

3

concentrations exceeded the 2008 8-hour National Ambient Air Quality Standard (NAAQS) of 

0.075 ppm at two California CASTNET sites during the most recent 3-year period (2013–2015). For 

2015, the same two CASTNET sites in California measured fourth highest DM8A O 

3 concentrations 

greater than 0.075 ppm. Three-year averages of fourth highest DM8A O 

3 concentrations have been 

reduced by 25 percent at the eastern reference sites since 1990–1992 and by 9 percent at the 

western reference sites since 1996–1998. Figure E-1 and Table E-1 summarize the long-term 

changes in measured concentrations. 

Regional O 

3 concentrations are influenced by 

nitrogen oxides (NOx) emissions. Federal, state,

and local NOx control programs have resulted in 

substantive reductions in emissions. For example, 

NOx emissions declined by 79 percent over the 

26 year period, 1990 through 2015, at regulated 

electric generating units (EGUs) in the East.

Three-year mean annual concentrations of total 

nitrate (NO3), which is comprised of nitric acid 

(HNO 

3) plus particulate NO3, declined 46 percent 

at the eastern reference sites over the 26-year 
CASTNET 2015 Annual Report

iv Executive Summary

period. Three-year mean annual total NO3 levels measured at the western reference sites dropped by 

30 percent over the 20-year period. 

Mean annual sulfur dioxide (SO 

2) concentrations measured at the eastern reference sites have 

declined significantly over the 26-year period, 1990 through 2015. Three-year mean annual SO 

2 levels 

at eastern sites decreased 83 percent. SO 

2 concentrations measured at the western reference sites 

declined by 47 percent over the 20 years from 1996 through 2015. 

The percent reduction in SO 

2 concentrations at the eastern reference sites is consistent with the 

reduction in regulated eastern EGU SO 

2 emissions (86 percent).

Table E-1 Trends in Aggregated Western and Eastern O 

3, Total NO3, and SO 2

Pollutant Concentrations

Pollutant

Western Reference Sites Eastern Reference Sites Percent Changed

1996–1998 2013–2015 1990–1992 2013–2015 West East

O 

3 (ppb) 74 67 85 63 -9 -25

Total NO3 (μg/m3 ) 1.0 0.7 3.0 1.6 -30 -46

SO 

2 (μg/m3 ) 0.6 0.3 8.8 1.4 -47 -83

The original intent of CASTNET was to measure pollutant concentrations to estimate trends in sulfur 

and nitrogen pollutants. Currently, the focus also includes demonstrating compliance of rural, regional 

O 

3 concentrations with the NAAQS. The network also features measurements of trace-level gases 

and speciated nitrogen pollutants. Additionally, CASTNET supports the National Atmospheric 

Deposition Program’s Ammonia Monitoring Network with operation of ammonia samplers at 68 

CASTNET sites. This report provides information on CASTNET pollutant measurements and 

estimated deposition fluxes and other topics such as forest health in the eastern United States and 

cloud water and filter pack pollutant concentrations measured on Whiteface Mountain, New York.
1 Chapter 1: CASTNET Update 

Chapter 1 

Chapter 1 

CASTNET Update 

The Clean Air Status and Trends Network (CASTNET) is a nationwide air quality monitoring network that has been 

operating for more than 25 years. The network performs long-term measurements of air pollutant concentrations in 

rural areas across the United States to determine compliance with ozone National Ambient Air Quality Standards and 

to evaluate the effectiveness of national and regional emission control programs. CASTNET was designed to identify 

trends in rural ozone, nitrogen, and sulfur concentrations and deposition rates of nitrogen and sulfur pollutants. 

CASTNET data are used for air quality program and strategy evaluation and to provide input for operation and 

evaluation of regional air quality models such as the National Atmospheric Deposition Program’s Total Deposition 

Hybrid Method. CASTNET is managed and operated by the U.S. Environmental Protection Agency in cooperation with 

the National Park Service and other federal, state, and local partners. In 2015, the network operated 95 monitoring 

stations throughout the contiguous United States, Alaska, and Canada. CASTNET data show a 26-year decline in 

ozone, nitrogen, and sulfur pollutant concentrations. 

Introduction

The U.S. Congress established the Acid Rain Program (ARP) in 1990 under Title IV of the Clean Air 

Act Amendments (CAAA). The ARP was enacted to reduce emissions of sulfur dioxide (SO 

2) and 

nitrogen oxides (NOx) from electric generating units (EGUs) and has produced significant reductions 

in emissions since 1995. Since then, other air pollution control programs have been instituted to 

reduce emissions of SO 

2 and NOx. Examples of these control programs include the following: 

 1990 Ozone (O 

3) Transport Commission NOx Budget Program 

 1998 NOx State Implementation Plan Call 

 2003 NOx Budget Trading Program 

 2009 Clear Air Interstate Rule 

 2011 Cross-State Air Pollution Rule (CSAPR) 

On January 1, 2015, implementation of the CSAPR Phase I began. The CSAPR was promulgated in 

2011, requiring 28 states in the eastern half of the United States to significantly improve air quality by 

reducing EGU emissions that cross state lines and contribute to ground-level O 

3 and fine particulate 

matter (PM2.5) in downwind states. CSAPR was enacted under the Clean Air Act’s good neighbor 

provision to address regional interstate transport of O 

3 and PM2.5 pollution for the 1997 National 

Ambient Air Quality Standards (NAAQS) and the 2006 PM2.5 NAAQS. The rule requires reductions in 

annual SO 

2, annual NOx, and O 3 season NOx emissions. 
CASTNET 2015 Annual Report

2 Chapter 1: CASTNET Update

In 2015, a small footprint site was added to tribal lands for the Nez Perce Tribe in Idaho (NPT006). A small 

footprint seasonal site was also established at the summit of Whiteface Mountain, NY (WFM007) to operate 

simultaneously with a cloud water collector operated by the New York State Department of Environmental 

Conservation (NYSDEC). WFM007 also operated during the summer of 2016 and will operate during the 

summer of 2017.

The emission reductions achieved under these programs have resulted in a substantive improvement 

in air quality as demonstrated by air pollutant concentrations measured by CASTNET and other 

cooperating networks. By 2015, EGUs required to comply with the ARP and/or CSAPR had reduced 

their SO 

2 emissions to 2.2 million short tons and NOx emissions to about 1.4 million short tons, 

decreases of 86 and 79 percent, respectively, from 1990 levels.

Congress mandated in the 1990 CAAA that the U.S. Environmental Protection Agency (EPA) provide 

consistent, long-term measurements for determining relationships between changes in emissions and 

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

operated 95 monitoring stations in 2015 throughout the contiguous United States, Alaska, and 

Canada. EPA and the National Park Service (NPS) are the primary sponsors of CASTNET. NPS 

began its participation in 1994 and operated 25 sites during 2015. The Bureau of Land ManagementWyoming State Office (BLM) operated five sites in Wyoming.

This report summarizes CASTNET monitoring and the resulting concentration and deposition data 

collected over the 26-year period from 1990 through 2015. Additional information, previous annual 

reports, other CASTNET documents, and the CASTNET database can be found on the EPA 

CASTNET website, https://www.epa.gov/castnet/. The website provides a complete archive of 

concentration and deposition data for all CASTNET sites.

Locations of Monitoring Sites

The locations of the CASTNET monitoring sites that were operational during 2015 are depicted in 

Figure 1-1. Ninety-five monitoring sites were operated at 93 distinct locations. To estimate precision 

across the network, co-located sites were operated at Mackville, KY (MCK131/231) and Rocky 

Mountain National Park, CO (ROM406/206) during 2015. The ROM406/206 pair ensures consistency 

between EPA (ROM206) and NPS (ROM406). Of the two Rocky Mountain monitoring sites, ROM406 

is specified as the regulatory monitoring site for O 

3. Appendix A provides the location of each site and 

includes information on start date, latitude, longitude, elevation, identification of the nearby National 

Atmospheric Deposition Program (NADP) site, land use, terrain type, operating agency, and if the site

is a reference site used for trends. Two new sites (NPT006, ID and WFM007, NY) were added to the 

network during 2015.
CASTNET 2015 Annual Report

3 Chapter 1: CASTNET Update

Figure 1-1 CASTNET Sites Operational during 2015

Beaufort, NC (BFT142)
CASTNET 2015 Annual Report

4 Chapter 1: CASTNET Update

Measurements Recorded at CASTNET Sites

All CASTNET sites measure weekly ambient concentrations of acidic pollutants, base cations, and 

chloride (Cl-

) using a 3-stage filter pack with a controlled flow rate (Amec Foster Wheeler, 2014). 

Gaseous pollutant concentrations include SO 

2 and nitric acid (HNO 3). Particulate concentrations 

include sulfate (SO2-

4 ), nitrate (NO3), ammonium (NH+ 4 ), magnesium (Mg2+ ), calcium (Ca2+ ), potassium 

(K+ ), sodium (Na+ ), and Cl-

. The filter pack is exchanged each Tuesday and shipped to the analytical 

chemistry laboratory for analysis. Ambient temperature is measured at 9 meters (m) at all sites in part 

to enable conversion of concentrations to local conditions.

Most CASTNET sites include a temperature-controlled shelter and continuous O 

3 monitoring system. 

The O 

3 inlet and filter pack are located atop a 10-m tower. Some CASTNET sites also measure tracelevel SO 

2, carbon monoxide (CO), and nitrogen oxide/total reactive oxides of nitrogen (NO/NOy). In 

2015, meteorological parameters were measured at 6 EPA-, 25 NPS-, and 5 BLM-sponsored 

CASTNET sites. Measured meteorological parameters include 2-m temperature, wind speed and 

direction, standard deviation of the wind direction, solar radiation, relative humidity, precipitation, and 

surface wetness (at select sites).

Cooperating Networks

CASTNET monitors air quality and deposition in cooperation with other national and international 

networks. EPA uses CASTNET and these other long-term national networks to assess the 

effectiveness of emission control programs.

NADP operates: 

• National Trends Network (NTN), which includes about 269 monitoring stations with wet 

deposition samplers to measure the concentrations and deposition rates of air pollutants 

removed from the atmosphere by precipitation. NTN operates wet deposition samplers at or 

near virtually every CASTNET site.

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

3) samplers at 

103 sites with 68 of the AMoN sites at or near CASTNET locations. AMoN, in operation since 

2007, provides information on 2-week integrated NH 

3 concentrations.

• Mercury Deposition Network (MDN), which operates 113 samplers to measure mercury (Hg) in 

precipitation. MDN samplers are operated at several CASTNET sites.

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

gaseous oxidized, particulate-bound, and elemental Hg at 24 locations in the continental 

United States, Canada, Hawaii, and Taiwan in order to estimate dry and total Hg deposition.

NADP’s website gives detailed information on each of these sub-networks: 

http://nadp.isws.illinois.edu/.
CASTNET 2015 Annual Report

5 Chapter 1: CASTNET Update

Canadian Air and Precipitation Monitoring Network (CAPMoN) operates 33 measurement sites 

throughout Canada and one in the United States. CASTNET and CAPMoN both operate filter pack 

samplers in Egbert, Ontario, Canada. CAPMoN operates a wet deposition sampler at the 

Pennsylvania State University CASTNET site (PSU106, PA).

EPA’s National Core Monitoring (NCore) reports on particulate matter (PM) mass; PM species; O 

3, 

SO 

2, NO/NOy, and CO concentrations; and meteorological parameters at approximately 80 sites. Five 

rural NCore sites are co-located with CASTNET sites.

BLM’s Wyoming Air Resources Monitoring System (WARMS) operates eight sites and the 

meteorological system at one of the CASTNET sites, Pinedale (PND165), in Wyoming. Five of the 

WARMS sites are CASTNET-protocol sites (http://www.blmwarms.net/) and include meteorological 

measurements. Two sites include O 

3 monitoring.

Interagency Monitoring of Protected Visual Environments (IMPROVE) measures speciated 

aerosol pollutants that affect visibility near more than 20 CASTNET sites. For more information on 

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

Quality Assurance Program

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

of known and documented quality in order to meet CASTNET objectives. The QA program also 

ensures intra-network consistency and comparability and the delivery of data that are reproducible 

and comparable with data from other monitoring networks. The 2015 QA program elements are 

documented in the CASTNET Quality Assurance Project Plan (QAPP; Amec Foster Wheeler, 2014). 

The QAPP includes standards and policies for all components of project operation, from site selection 

through final data reporting, with appendices that provide standard operating procedures for 

CASTNET operations. 

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

CASTNET measurements and supporting activities. Routine assessment and analysis help guarantee 

the production of high-quality data and information to meet project objectives. Measurements taken 

during 2015 and historical data collected over the period 1990 through 2014 were analyzed relative to 

DQI and their associated metrics. Results from these analyses are available in quarterly and annual 

QA reports posted on the EPA CASTNET website: https://www.epa.gov/castnet/. Selected analyses 

are summarized in callout boxes labeled, “QA Program Summary,” throughout this report.
CASTNET 2015 Annual Report

6 Chapter 1: CASTNET Update

Estimating Dry, Wet, and Total Deposition

Total deposition was assessed using the NADP’s Total Deposition Hybrid Method (TDEP; EPA, 

2015c; Schwede and Lear, 2014), which combines data from established ambient monitoring 

networks and chemical-transport models. To estimate dry deposition, ambient measurement data 

from CASTNET and other networks were merged with dry deposition rates and flux output from the 

Community Multiscale Air Quality (CMAQ) modeling system. Wet deposition estimates were derived 

from precipitation chemistry measurements and precipitation amounts from the Parameter-elevation 

Regressions on Independent Slopes Model (PRISM). Dry and wet deposition fluxes were added to 

obtain the estimates of total deposition that are discussed in Chapters 9 and 11.

Building Tribal Partnerships with CASTNET Monitoring Sites

EPA partners with Cherokee Nation in Oklahoma, the Alabama-Coushatta Tribe in Texas, the Santee 

Sioux Tribe in Nebraska, Kickapoo Tribe in Kansas, the Red Lake Band of Chippewa Indians in 

Minnesota, and the Nez Perce Tribe in Idaho to operate CASTNET sites on tribal lands. In 2012, 

CASTNET developed a small footprint monitoring station that does not require a temperaturecontrolled shelter. The new type of monitoring station includes a 10-m sampling tower, filter pack 

sampling system, and data acquisition system (Figure 1-2), which can be operated off the grid. The 

new site design has produced cost savings and increased efficiency in terms of installation and 

power. The advent of the small footprint monitoring station has opened opportunities for new partners, 

such as the Nez Perce Tribe in Idaho, to participate in the CASTNET monitoring program. EPA may 

support additional tribal groups that are interested in establishing small footprint sites in order to 

assess sulfur and nitrogen inputs to their surrounding ecosystems.

Figure 1-2 Small Footprint Site Operated by Nez Perce Tribe in Idaho (NPT006)

Tower with solar panel and 

equipment box

Interior of equipment box Filter pack sampling enclosure and 

wind turbine
CASTNET 2015 Annual Report

7 Chapter 1: CASTNET Update

The operation of CASTNET monitoring sites on tribal lands provides benefits to both the tribes and 

EPA. CASTNET measurements provide data on atmospheric pollutants and pollutant deposition to 

areas important to tribes. The measurements help the tribes understand the environmental effects of 

prescribed burns, wild fires, and nearby point or urban sources. When a CASTNET site is operated by 

a tribe, the equipment, training, QA, and data reporting are managed and funded by EPA, which 

provides the tribe with assurance that there will be no significant maintenance or repair costs and

allows the tribe to begin or continue to build monitoring capacity.

For EPA, partnerships are important for operating a consistent, stable, long-term monitoring network. 

Current tribal sites fill in spatial gaps in the network within the central and northwest United States 

(Figure 1-3), and tribes provide much needed support for operations and infrastructure. Many tribal 

sites operate co-located monitoring instruments on behalf of NCore, NTN, MDN, AMNet, and AMoN. 

The Cherokee, Alabama-Coushatta, and Santee Sioux tribal sites provide regulatory O 

3 data to 

AIRNow and EPA’s Air Quality System (AQS).

Figure 1-3 Map of CASTNET Tribal Sites in 2015
CASTNET 2015 Annual Report

8 Chapter 1: CASTNET Update

QA PROGRAM SUMMARY

Quality Assurance Program Results

Laboratory Intercomparison Results

During 2015, the Amec Foster Wheeler CASTNET laboratory participated in the Environment Canada 

(ECAN) Proficiency Testing (PT) Program for Inorganic Environmental Substances (http://www.ec.gc.ca/inrenwri/default.asp?lang=En&n=7A20877C-1). The ECAN PT program conforms to the requirements of the 

American Association for Laboratory Accreditation (A2LA). The program meets the International 

Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 17043:2010 

conformity assessment – general requirements for proficiency testing with scope of accreditation 2867.01. 

The Amec Foster Wheeler laboratory is one of 32 laboratories that participated in the 2015 Rain and Soft 

Waters round robin studies. The PT study samples consist of natural waters supplied by the National 

Laboratory for Environmental Testing. The CASTNET laboratory receives 10 samples of mixed rain and 

Canadian Shield waters for chemical analysis from ECAN every 6 months. The laboratory reported the eight 

CASTNET parameters for samples in two studies (study codes 0106 and 0107) during 2015. 

The results reported by the 32 laboratories are evaluated for systematic bias and precision. Systematic bias 

is assessed using the Youden (1969) non-parametric analysis, while precision is calculated using algorithm 

A from the ISO standard 13528 (ISO, 2005). 

Individual sample results are flagged based on the robust standard deviation obtained from the ISO 13528 

computation (ISO, 2005). Samples within two standard deviations of the assigned (median laboratory) value 

are not flagged; samples between two and three standard deviations are given a warning flag; and samples 

greater than three standard deviations from the assigned value are flagged as above the action limit 

(remedial action is required). Laboratory results are considered systematically biased when individual 

parameters are ranked by the Youden analysis to be consistently and significantly higher or lower than the 

assigned value without regard to flagged results. 

The CASTNET laboratory’s 2015 ECAN results for the eight CASTNET parameters are presented in 

Table 1-a. The overall laboratory performance rating was “Very Good” for both studies.

Cherokee Nation (Stilwell), OK (CHE185)
CASTNET 2015 Annual Report

9 Chapter 1: CASTNET Update

QA PROGRAM SUMMARY

Table 1-a Amec Foster Wheeler Results for Studies 0106 and 0107

Test

Parameter

Analytical

Method

Reference

Method

Laboratory Performance Rating*

Very Good: Study 0106

Summer 2015

Very Good: Study 0107

Winter 2015

Ammonia AC EPA Method 350.1 Bias could not be assessed† -0.8 percent | 0.0085

Calcium ICP-OES EPA Method 6010 Ideal Ideal

Chloride IC EPA Method 300.0 0.7 percent | 0.0180 Ideal

Magnesium ICP-OES EPA Method 6010 Ideal Ideal

Nitrate + Nitrite IC EPA Method 300.0 Ideal Ideal

Potassium ICP-OES EPA Method 6010 Ideal Ideal

Sodium ICP-OES EPA Method 6010 Ideal Ideal

Sulfate IC EPA Method 300.0 Ideal 1.6 percent | 0.0214

Note: *Expressed as bias percent slope (percent deviation of test results from assigned values) | y-intercept. Ideal slope = 1 | yintercept = 0. Any result not 1 | 0 is reported as biased by ECAN. The CASTNET laboratory is accredited by A2LA to ISO/IEC 

17025:2005 for a scope that includes the study parameters for methods used by the laboratory. The laboratory’s proficiency 

testing plan requires action for individual test results that are greater than three standard deviations from the assigned value, 

bias 5 percent or higher for a single parameter, three or more biased results of any magnitude in a single study, or a 

consecutive study result indicating bias of any magnitude for a given parameter.

†

Seven of the 10 assigned values were below the laboratory limit of quantitation.

AC=automated colorimetry

ICP-OES=inductively coupled plasma-optical emission spectrometry

IC=ion chromatography

Source: Environment Canada (2015; 2016)

The overall laboratory rating indicates a percent score as described in Table 1-b. The five-year historical 

laboratory rating is listed by ECAN as “Very Good.”

Table 1-b Laboratory Performance Rating from 2011–2015

Laboratory Performance Rating

Rating Percent Score*

Very Good 0 – 5

Good > 5 – 12.5

Fair > 12.5 – 30

Poor > 30

*Sum of parameters biased and results flagged

Source: Environment Canada (2015; 2016)
10 Chapter 2: Ozone Concentrations

Chapter 2

Chapter 2

Ozone Concentrations

CASTNET is the principal network for monitoring rural, ground-level ozone concentrations in the United States and 

producing critical information on geographic patterns in rural ozone levels. Ozone data measured at 80 CASTNET sites 

from 2013 through 2015 were evaluated with respect to the National Ambient Air Quality Standards (EPA, 2008) and 

used to calculate design values, which are defined in Title 40 Code of Federal Regulations Part 50 Appendix U (EPA, 

2015a). Maps of 3-year averages of fourth highest daily maximum 8-hour average (DM8A) ozone concentrations for 

2013 through 2015 and fourth highest DM8A ozone concentrations for 2015 are presented. Trends in fourth highest 

DM8A ozone concentrations for eastern and western reference sites are shown using box plots.

In 2015, hourly average concentrations were measured at 80 CASTNET sites. These data are 

archived in the CASTNET database and delivered routinely to the EPA AQS. Data from these sites, 

with the exception of Howland, ME (HOW191) and the co-located sites at MCK231, KY and ROM206, 

CO, which are designated as non-regulatory, are used to calculate fourth highest DM8A 

concentrations when three years of Title 40 Code of Federal Regulations (CFR) Part 58-compliant 

data become available. CASTNET measurements provide information that is essential for evaluating 

rural O 

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

trends and geographic patterns in regional O 

3.

Sample of Heading Three

The information presented in this chapter includes maps and trends in the annual fourth highest 

DM8A O 

3 concentrations measured at CASTNET sites. Additional maps of O 3 concentrations from the 

NPS Air Atlas can be viewed at http://nature.nps.gov/air/maps/airatlas/index.cfm and 

http://nature.nps.gov/air/data/products/parks/index.cfm. 

Measurements from 34 eastern and 16 western reference sites (see Appendix A) were analyzed to 

determine trends in O 

3 concentrations. These sites were also used to show trends in ambient nitrogen 

and sulfur concentrations (Chapters 4 and 6). The eastern reference sites have been reporting 

CASTNET measurements since at least 1990 and the western reference sites since at least 1996.

A design value is a statistic that describes the air quality status of a given area relative to the concentration 

values required by the NAAQS. Design values change as each new 3-year period of monitored ozone 

concentrations becomes available. Design values are used to classify nonattainment areas, assess progress 

towards meeting the NAAQS, and develop control strategies to achieve the NAAQS. For example, if 3-year 

averages of 2013–2015 fourth highest DM8A ozone concentrations are used for purposes of attainment 

designation and exceed 0.075 parts per million (ppm), then ozone concentrations will have to be reduced to 

0.075 ppm or below to achieve the 2008 ozone NAAQS. Designated criteria pollutant nonattainment areas 

are provided on the EPA website: https://www.epa.gov/green-book.
CASTNET 2015 Annual Report

11 Chapter 2: Ozone Concentrations

2008 National Ambient Air Quality Standards for Ozone

Ozone1

Primary Standard Secondary Standard

Level Averaging Time Level Averaging Time

0.075 ppm1 8-hour2 0.075 ppm1 8-hour2

Note: 1 The NAAQS was revised from 0.075 ppm to 0.070 ppm on October 15, 2015. 

2 To attain this standard, the 3-year average of the fourth highest DM8A O 

3 concentrations measured at each monitor within a 

specified area must not exceed 0.075 ppm or 75 parts per billion (ppb) in practice (effective May 27, 2008; EPA, 2008). O 

3

concentrations are commonly presented in units of ppb.

The primary O 

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

protect public welfare and the environment. Both O 

3 NAAQS are set at a level of 0.075 ppm averaged 

over eight hours for the annual fourth highest value.

For the primary and secondary standards, EPA uses the pollutant indicator (O 

3), forms (fourth highest 

daily maximum averaged across three consecutive years), and averaging time (eight hours; EPA, 

2008). The secondary standard is equivalent, based on the W126 index, to a level of protection of 

17 ppm-hour or lower averaged over three years (EPA, 2008). The W126 index is a weighted index 

designed to reflect the cumulative exposures that can damage plants and trees during the 

consecutive three months in the growing season when daytime O 

3 concentrations are the highest and 

plant growth and production are most affected (Lefohn and Runeckles, 1987). 

The EPA and other federal, tribal, state, and local agencies measure O 

3 concentrations on an hourly 

basis through national and local monitoring programs. Amec Foster Wheeler followed EPA 

procedures (2015a) to estimate O 

3 design values and 2015 fourth highest DM8A O 3 concentrations 

at CASTNET sites. Measurements potentially affected by exceptional events were not removed 

when calculating these estimates. “Exceptional events are unusual or naturally occurring events that 

can affect air quality but are not reasonably controllable using techniques that state, tribal, or local air 

agencies may implement in order to attain and maintain the NAAQS” (EPA, 2016). The Exceptional 

Events Rule was updated on October 3, 2016 (epa.gov/air-quality-analysis/exceptional-events-ruleand-guidance) and provides the requirements for excluding air quality data from regulatory 

decisions if the data are affected by events outside an agency’s control, such as a wildfire or 

stratospheric intrusion. 

CASTNET O 

3 data are used to gauge compliance with the NAAQS at EPA-, NPS- and BLMsponsored sites that were 40 CFR Part 58 compliant for the years 2013 through 2015.
CASTNET 2015 Annual Report

12 Chapter 2: Ozone Concentrations

QA PROGRAM SUMMARY

Quality Assurance Program Results

Ozone Concentrations

Ozone quality control (QC) criteria are set forth in 40 CFR Part 58, Appendix A (EPA, 2013). Figure 2-a 

presents summary statistics of critical criteria measurements at O 

3 sites for measurements collected during 

2015. All data associated with QC checks that failed to meet the established criteria were invalidated unless 

the cause of failure was documented to have no effect on ambient data collection. QC failures for EPAsponsored sites are addressed in quarterly QA reports, which can be found on the EPA CASTNET website 

at https://java.epa.gov/castnet/documents.do.

Figure 2-a 2015 Quality Control Check Summary

Note: QA program results for NPS-sponsored sites measuring O 

3 may be accessed at 

http://www.nature.nps.gov/air/monitoring/network.cfm

Prince Edward, VA (PED108)

ABT147 ALC188ALH157ANA115

ARE128

ASH135

BEL116

BFT142

BVL130

BWR139

CAD150

CDR119

CDZ171

CKT136

CND125

CNT169

COW137

CTH110

CVL151

DCP114

ESP127

GAS153

GTH161

HOX148 HWF187 IRL141 KEF112 LRL117 MCK131 MCK231

MKG113

OXF122

PAL190

PAR107

PED108

PND165

PNF126

PRK134

PSU106

QAK172

ROM206

SAL133

SAN189

SND152

SPD111

STK138

SUM156

UVL124

VIN140

VPI120WSP144WST109

2015 Single Point, Span, and Zero Results: Percent Passing

2015 Single Point

2015 Span

2015 Zero
CASTNET 2015 Annual Report

13 Chapter 2: Ozone Concentrations

Eight-hour Ozone Concentrations

Regulatory-compliant procedures were used to measure all CASTNET O 

3 data for 2013 through 2015 

and were used to calculate fourth highest DM8A O 

3 concentrations and to gauge compliance with the 

NAAQS, with the exception of sites HOW191, ME; MCK231, KY; and ROM206, CO. HOW191 does 

not meet regulatory siting criteria. MCK231 and ROM206 are co-located sites used solely for QA 

purposes and are designated as “NAAQS excluded.” O 

3 concentrations were not included on the 

maps in this chapter if the 3-year average was not available because of incomplete data; these sites 

are shown as dots with no value. Three-year averages of the fourth highest DM8A O 

3 concentrations 

for 2013 through 2015 are presented in Figure 2-1. Two California CASTNET sites measured fourth 

highest DM8A O 

3 concentrations above the 2008 NAAQS. The highest O 3 concentration of 89 parts 

per billion (ppb) was sampled at the Sequoia and Kings Canyon National Parks, CA (SEK430) site. 

The highest eastern concentration (71 ppb) was measured at Washington Crossing, NJ (WSP144). 

Table 2-1 lists sites with 2013–2015 O 

3 design values greater than 75 ppb.

Figure 2-1 Three-year Averages of Fourth Highest DM8A O 

3 Concentrations for 

2013–2015 
CASTNET 2015 Annual Report

14 Chapter 2: Ozone Concentrations

Table 2-1 Sites with Design Values for 2013–2015 greater than 75 ppb

Site ID State Sponsor 3-year Average

SEK430 California NPS 89

JOT403 California NPS 86

All valid 2015 O 

3 concentrations that meet regulatory requirements are shown in Figure 2-2. During 

2015, WSP144, NJ measured a fourth highest DM8A O 

3 concentration of 75 ppb. Two California sites 

measured fourth highest DM8A O 

3 concentrations greater than 75 ppb.

Figure 2-2 Fourth Highest DM8A O 

3 Concentrations for 2015

Figure 2-3 provides box plots depicting trends in 1-year mean, median, and annual distribution of 

fourth highest DM8A O 

3 concentrations from the eastern reference sites (right side) for 1990 through 

2015 and for the western reference sites (left side) for 1996 through 2015. The reference sites were 

selected for their long-term data record and consistent performance. The eastern O 

3 data show an 

overall decline since 2002. There was an increase in fourth highest DM8A O 

3 concentrations from the 

2009 minimum through 2012, followed by a decrease in 2013. The 2015 aggregate median data 

point, 63.5 ppb, is slightly higher than the 2014 value of 62.5 ppb, the lowest level in the history of 

the network.
CASTNET 2015 Annual Report

15 Chapter 2: Ozone Concentrations

The western O 

3 data also show an increase in fourth highest DM8A O 3 concentrations from the 2009 

minimum through 2012, followed by a decrease in 2013. The 2015 median of the fourth highest DM8A 

O 

3 concentrations for the western reference sites was 66 ppb.

Figure 2-3 Trends in Fourth Highest DM8A O 

3 Concentrations

Western Reference Sites Eastern Reference Sites

Palo Duro Canyon State Park, TX (PAL190)
16 Chapter 3: Evaluating Elevated Ozone Concentrations Measured at 

Washington Crossing, New Jersey

Chapter 3

Chapter 3

Evaluating Elevated Ozone Concentrations Measured at Washington Crossing, New Jersey

CASTNET has operated a monitoring site at Washington Crossing, NJ (WSP144) since December 1988. The site 

frequently measures ozone concentrations that are higher than those measured at most CASTNET sites located in the 

eastern United States. Over the past two years, the annual fourth highest DM8A ozone concentrations exceeded 

70 ppb. Elevated concentrations at this site were typically associated with regional episodes when high concentrations 

extend throughout the Mid-Atlantic and Northeast urban corridor.

CASTNET began operating a monitoring site at WSP144 in 1988 as part of the National Dry 

Deposition Network. The site (see Figure 1-1 for location and Appendix A for additional site details) is 

situated in Washington Crossing State Park approximately 1,000 feet northeast of the Delaware River. 

In addition to monitoring weekly pollutant concentrations, the O 

3 monitoring at the site helps the state 

of New Jersey meet the minimum required number of O 

3 monitoring sites within the Trenton-Ewing 

metropolitan statistical area, thereby fulfilling 40 CFR Part 58 Appendix D monitoring network design 

requirements. The monitoring site lies about 35 miles northeast of Philadelphia and 60 miles 

southwest of New York City—within the Mid-Atlantic and Northeast urban corridor. This broad region 

experiences some of the highest O 

3 concentrations in the eastern United States. While not as high as 

concentrations measured at some relatively nearby urban regulatory monitoring sites, which were 

designated nonattainment based on the 2008 O 

3 NAAQS, historical fourth highest DM8A O 3

concentrations at WSP144 have exceeded the 2008 O 

3 NAAQS of 75 ppb (Table 3-1).

Table 3-1 Fourth Highest DM8A O 

3 Concentrations Measured at WSP144

Year Concentration (ppb)

2015 75

2014 71

2013 69

2012 83

2011 80

2010 82

EPA works with state and tribal partners to understand air quality dynamics (i.e., the complex 

relationship between air quality, emissions, and weather) in order to reduce O 

3 concentrations in 

areas that exceed the O 

3 NAAQS1

. While not the focus of gauging the specific effects of emission 

 1 For a list of the areas that have been designated as nonattainment, see the EPA Green Book 

https://www.epa.gov/green-book. To review O 

3 trends at these and other monitors, view 

https://www.epa.gov/air-trends/air-quality-design-values.
CASTNET 2015 Annual Report

17 Chapter 3: Evaluating Elevated Ozone Concentrations Measured at 

Washington Crossing, New Jersey

reduction programs, such as CSAPR2, CASTNET sites are well positioned for evaluating regional 

impacts of changes in O 

3 concentrations. Comprehensive Air Quality Model with Extensions (CAMx)

modeling3 indicated that, across all of the CASTNET sites, WSP144 received the highest percentage 

of contributions from upwind states totaling 65 percent of its projected 2017 O 

3 design value. New 

Jersey in-state emissions also affected this site. 

As with most CASTNET sites, WSP144 was sited to be representative of a region and minimally 

impacted by local sources. Washington Crossing’s immediate environs are characterized by a mix of 

parks, woods, and agriculture. Comparisons of O 

3 measurements collected at the WSP144 monitor 

with data from other regulatory O 

3 monitors within the Mid-Atlantic region suggest that the site does 

provide regionally representative measurements. Figure 3-1 displays coefficient of determination (R2

) 

values from a least squares linear regression comparison of the WSP144 DM8A values with DM8A 

values from surrounding regulatory O 

3 monitors within 800 kilometers (km) for the May 1 through 

October 31, 2015 O 

3 season. The color scale highlights the spatial extent where R2 is highest (at or 

above 0.70) indicating that these sites often experience similar patterns of O 

3 concentrations. This 

area of strongest correlation spans the Eastern Seaboard from Maryland to Long Island, including

central and western New Jersey. The strong correlation over this region is likely a result of similar 

emission and weather patterns. Nearby sites that exhibit a substantially weaker correlation suggest 

they are impacted by different emissions and weather conditions.

Washington Crossing (State Park), NJ (WSP144)

 2 Clean Air Act section 110(a)(2)(D)(i)(I).

3 For the CSAPR update, the modeled 2017 base case design values were based on a projected scenario 

where a number of EGUs (including several in Pennsylvania) were modeled to have particularly high NOx

emissions because they were not using selective catalytic reduction (SCR) controls. As a result of the final 

CSAPR update and the Pennsylvania reasonably achievable control technology rule, both of which are effective 

in 2017, NOx control regulations are expected to result in these units operating SCR controls in 2017.
CASTNET 2015 Annual Report

18 Chapter 3: Evaluating Elevated Ozone Concentrations Measured at 

Washington Crossing, New Jersey

Figure 3-1 Regional Site-Level DM8A Comparisons with WSP144 (May 1–October 31, 2015)

A comparison of the coefficients of determination between WSP144 DM8A values and DM8A values 

from other O 

3 monitors within the region is shown in Figure 3-2. The coefficient of determination for 

each monitor was calculated as a function of distance from WSP144. Typically, a monitor within 

140 km of WSP144 had a coefficient of determination greater than or equal to 0.70.4 A high coefficient 

was also calculated for a monitor located in Carroll County, Maryland (AQS number 24-013-0001),

which is 209 km southwest of WSP144. Alternatively, some nearby sites did not meet the 0.70 

criterion. For example, a site located in Philadelphia, PA (AQS number 42-101-0004), which is 39 km

southwest of WSP144, had a DM8A R2 of 0.63 indicating that the Philadelphia monitor may be 

affected by both regional and local O 

3 pollution.

 4 A linear regression equation was fit to all sites within 250 km of WSP144, and a formula for the expected 

coefficient of determination was calculated as a function of distance, i.e., y = -(0.00213238)*(distance) + 1.
CASTNET 2015 Annual Report

19 Chapter 3: Evaluating Elevated Ozone Concentrations Measured at 

Washington Crossing, New Jersey

Figure 3-2 Regional Site-Level DM8A Comparisons with WSP144 by Distance

(May 1–October 31, 2015)

While Figure 3-2 displays correlation values for each monitoring site, DM8A data on individual days 

have to be analyzed to in order to understand the variable nature of weather patterns and the 

correlation with proximal monitors. The correlation patterns were different on an episode-by-episode 

basis. In any particular high DM8A episode, the correlation patterns varied depending on

meteorological events. The eight days when DM8A levels exceeded 70 ppb during the 2015 O 

3

season are listed in Table 3-2. Elevated temperatures are known to play an important role (Camalier 

et al., 2007) in O 

3 formation. Therefore, maximum 1-hour temperature values and their percentiles are 

listed to help evaluate any effects temperature may have had on O 

3 on these days. The temperature 

measurements and statistics illustrate that four out of eight high DM8A episodes occurred when the 

maximum 1-hour temperature was within the 95th percentile for the 2015 O 

3 season. These results

support the claim that elevated temperature is an important factor during these episodes, but they do 

not explain elevated O 

3 on moderate temperature days (e.g., May 8) or explain why other days with 

high temperatures (i.e., within the 95th percentile) did not have DM8A values above 70 ppb. Regional 

DM8A values measured at regulatory monitors on these eight days were then analyzed to better 

understand elevated O 

3 patterns.
CASTNET 2015 Annual Report

20 Chapter 3: Evaluating Elevated Ozone Concentrations Measured at 

Washington Crossing, New Jersey

Table 3-2 DM8A Levels Exceeding 70 ppb at WSP144 with Temperature and HYSPLIT 24-Hour 

Back Trajectory Distances

Date

2015 DM8A 

Concentration 

(ppb)

2015 DM8A 

Concentration 

Rank

Maximum 

1-Hour 

Temperature 

(Celsius)

Maximum 

1-Hour 

Temperature 

Percentile

Average 

HYSPLIT 

Straight Line

Distance (km)

Average 

HYSPLIT 

Curved Path 

Distance (km)

5/08/2015 81 1 28.4 71 168 185

7/29/2015 81 1 32.6 99 146 174

6/11/2015 79 3 32.2 96 340 361

9/17/2015 75 4 29.1 77 116 145

9/18/2015 75 4 28.6 72 143 156

9/08/2015 74 6 33.1 99 244 307

9/02/2015 72 7 31.8 94 63 111

6/12/2015 71 8 32.2 96 193 261

Figure 3-3 shows regional maps with DM8A concentrations at regulatory sites during the eight days 

when WSP144 reported DM8A concentrations above 70 ppb. The patterns of high O 

3 concentrations 

demonstrate the widespread regional nature of elevated DM8A values and show that the patterns 

varied from episode to episode. On some days (e.g., June 11), the region of elevated O 

3

concentrations extended from Annapolis, MD to the Connecticut/Rhode Island border along the MidAtlantic to Northeast urban corridor, a distance of approximately 550 km. On other days, (e.g., 

July 29), the region of elevated O 

3 concentrations was much smaller, extending only from central New 

Jersey to Connecticut. These patterns mirror the correlation map (Figure 3-1) in that O 

3 levels 

measured in central New Jersey were consistently correlated while concentrations measured farther 

away showed weaker correlation. In short, the episodic patterns of high O 

3 concentrations were 

related to emission patterns and synoptic scale weather events and to meteorological parameters, 

such as ambient temperature, relative humidity, wind speed, and wind direction that affect air mass 

and pollutant transport.

To better understand the effect of air mass transport on the episodic high regional O 

3 concentrations, 

the Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT; Stein et al., 2015) was 

run for the eight days. The air masses affecting WSP144 on high O 

3 days were tracked backward in 

time over 24-hour periods. The trajectories show the air mass movement pattern and distance 

covered over the 24-hour period preceding its arrival at the measurement site, recognizing the 

simulated 24-hour air masses would have traveled and evolved over even longer times and distances. 

These 24-hour back trajectories were based on 10-, 50-, and 150-m starting elevation heights and 

were driven by North American Regional Reanalysis meteorological conditions. The trajectories are 

shown in Figure 3-3, while the distances between the origin of the trajectory and WSP144 are shown 
CASTNET 2015 Annual Report

21 Chapter 3: Evaluating Elevated Ozone Concentrations Measured at 

Washington Crossing, New Jersey

in Table 3-2.

5 The two distance categories are the straight line distance and the curved 

path distance.

6

All 24-hour trajectories, except for two, pass through upwind states outside New Jersey. The 24-hour 

trajectory for the May 8 episode tracks back to the New Jersey coastline, while the September 2 

episode tracks back within New Jersey. Incidentally, the September 2 episode not only had the 

shortest back trajectory distances for both the straight line and curved path measurements, but also 

had an origin located to the northeast of WSP144. This was the only trajectory that did not arrive from 

the southwesterly direction and was the only one of the eight that did not have an average 24-hour 

back trajectory straight line distance in excess of 100 km, suggesting that O 

3 transport was important 

in all but this one episode. 

In conclusion, seasonal O 

3 measurements at WSP144 were highly correlated with O 3 measurements 

from regulatory monitors in the surrounding region. The correlation patterns were also evident for high 

O 

3 episodes when the DM8A values exceeded 70 ppb. These analyses, in addition to the site’s 

central location within the region and fulfillment of CASTNET siting criteria, indicate that WSP144 

provides regionally representative measurements. HYSPLIT model runs for eight elevated O 

3 days in 

2015 show that air masses were transported to WSP144 most frequently from upwind states from a 

southwesterly direction. The HYSPLIT model runs support the CAMx modeling results that indicate 

both upwind state and in-state contributions affect O 

3 concentrations measured at WSP144. More 

research and analysis are needed to understand the effects of meteorology and local and transported 

emissions for all observed elevated O 

3 events.

 5 The distances between the receptor and the origin of the air mass 24 hours prior were measured and 

averaged over the three starting elevations for each date.

6 The straight line distance is the “as the crow flies” distance between the monitor and the location of the start of 

the trajectory; straight line distance was used by Camalier et al. (2007) as an important factor to understand O 

3

transport events. The curved path distance is the sum of the distance covered by the air mass over the 24-hour 

time period.
CASTNET 2015 Annual Report

22 Chapter 3: Evaluating Elevated Ozone Concentrations Measured at 

Washington Crossing, New Jersey

Figure 3-3 Daily Maximum O 

3 Concentrations Measured at WSP144 and Nearby Regulatory

Sites with HYSPLIT 24-hour Back Trajectories when WSP144 Exceeded 70 ppb

(1 of 2)

May 8, 2015 – 81 ppb July 29, 2015 – 81 ppb

June 11, 2015 – 79 ppb September 17, 2015 – 75 ppb
CASTNET 2015 Annual Report

23 Chapter 3: Evaluating Elevated Ozone Concentrations Measured at 

Washington Crossing, New Jersey

Figure 3-3 Daily Maximum O 

3 Concentrations Measured at WSP144 and Nearby Regulatory

Sites with HYSPLIT 24-hour Back Trajectories when WSP144 Exceeded 70 ppb

(2 of 2)

September 18, 2015 – 75 ppb September 8, 2015 – 74 ppb

September 2, 2015 – 72 ppb June 12, 2015 – 71 ppb
24 Chapter 4: Nitrogen Pollutant Concentrations

Chapter 4

Chapter 4

Nitrogen Pollutant Concentrations

Weekly average concentrations of nitric acid, particulate nitrate, and particulate ammonium were measured using 

3-stage filter packs at 95 CASTNET monitoring stations during 2015. Box pots are used to illustrate trends in annual 

total nitrate (nitric acid plus nitrate) and ammonium concentrations aggregated over 34 eastern and 16 western 

reference sites. The nitrogen pollutants measured at the eastern reference sites declined over the 26-year period from 

1990 through 2015. Mean annual concentrations of total nitrate were reduced by 46 percent from 1990 through 2015. 

Ammonium concentrations declined by 57 percent. Total nitrate and ammonium concentrations measured at the 

western reference sites were reduced by 30 percent and 23 percent, respectively, over the 20-year period 1996 

through 2015.

Annual mean concentrations of total NO3 (HNO 

3 + NO3) and NH+ 4 for 2015 are presented in two maps 

in this chapter. Additional maps of 2015 quarterly mean concentrations are provided in CASTNET 

quarterly data reports (Amec Foster Wheeler, 2015a; 2015b; 2016a; 2016b; 2016c). Trends in 

annual mean concentrations over the 26-year period (1990 through 2015) were derived from 

measurements from the eastern reference sites and for the period 1996 through 2015 from data 

measured at western reference sites. See Appendix A for the designated reference sites.

Total Nitrate Concentrations

Mean total NO3 concentrations measured in 2015 are presented in the map in Figure 4-1. To illustrate 

trends, Figure 4-2 provides box plots of total NO3 levels for the eastern and western reference sites 

through 2015. Each box presents the mean and median concentrations and the 10th, 25th, 75th, and 

90th percentiles for that year. The data shown on the right side of the figure were aggregated from the 

eastern reference sites. The data show no trend in mean concentrations until 2000 when total NO3

levels began to decline in response to NOx emission controls. Total NO3 levels measured at the 

eastern reference sites were reduced by 50 percent from a mean value of 3.1 micrograms per cubic 

meter (μg/m3 ) of air in 2000 to a mean value of 1.6 μg/m3 in 2015. Over the history of the network, 

3-year mean levels declined from 3.0 μg/m3 for 1990–1992 to 1.6 μg/m3 for 2013–2015, producing a 

46 percent reduction in total NO3. 

The left side of Figure 4-2 shows data aggregated from the western reference sites. Total NO3 levels 

declined from 1.1 μg/m3 to 0.7 μg/m3 from 2000 through 2015, a 36 percent reduction. The 

3-year mean total NO3 concentration for 2013–2015 was 30 percent lower than the corresponding 

1996–1998 level. The 3-year mean concentration was 1.0 μg/m3 for 1996–1998 and 0.7 μg/m3 for 

2013–2015. 
CASTNET 2015 Annual Report

25 Chapter 4: Nitrogen Pollutant Concentrations

Figure 4-1 Annual Mean Total NO3 Concentrations for 2015

Figure 4-2 Trends in Annual Mean Total NO3 Concentrations

Western Reference Sites Eastern Reference Sites

Total nitrate concentrations measured at the eastern reference sites declined by 46 percent since 1990. 

Total nitrate levels measured at the western reference sites were reduced by 30 percent since 1996. Total 

nitrate concentrations measured at the eastern reference sites were two to three times higher than 

concentrations measured at western reference sites.
CASTNET 2015 Annual Report

26 Chapter 4: Nitrogen Pollutant Concentrations

Figure 4-3 illustrates the trend in NOx emissions from regulated EGUs operating from 1990 through 

2015 in six eastern states: Illinois, Indiana, Kentucky, Ohio, Pennsylvania, and West Virginia. The 26-

year decline in aggregated EGU emissions in these states was 82 percent.

Figure 4-3 Trend in Annual Composite NOx Emissions from Regulated EGUs Operating in 

Illinois, Indiana, Kentucky, Ohio, Pennsylvania, and West Virginia

Source: EPA (2017)

Particulate Ammonium Concentrations

A map of 2015 mean particulate NH+ 

4 concentrations is provided in Figure 4-4. Figure 4-5 shows box 

plots of NH+ 

4 concentrations. The trend diagram for the eastern reference sites (right side) shows a 

reduction in mean NH+ 

4 levels from 1990–1992 to 2013–2015. The 1990–1992 mean concentration 

was 1.8 μg/m3 , and the 2013–2015 value was 0.8 μg/m3 , a 57 percent decline. Similar to total NO3, 

the eastern NH+ 

4 concentrations began to decline in 2000 and have been reduced from 1.6 μg/m3 in 

2000 to 0.6 μg/m3 in 2015. The western reference sites show a decline from 0.3 μg/m3 in 1996–1998 

to 0.2 μg/m3 in 2013–2015, a 23 percent reduction.

0

500

1000

1500

2000

2500

1990 1995 2000 2005 2010 2015

Thousand Short Tons
CASTNET 2015 Annual Report

27 Chapter 4: Nitrogen Pollutant Concentrations

QA PROGRAM SUMMARY

Figure 4-a Historical and 2015 Precision Results for 

Atmospheric Concentrations and Laboratory 

Replicate Samples

0

5

10

15

20

25

30

Sulfur

Dioxide

Sulfate

Nitric Acid

Nitrate

Ammonium

Calcium

Potassium

Magnesium

Sodium

Chloride

MARPD

Overall - 1990 through 2014

Mackville, KY - 2015

Rocky Mtn, CO - 2015

Laboratory Replicates - 2015

Figure 4-b Historical and 2015 Percent Completeness of 

Measurements (black bars are 1990–2014 for long-term data 

and 2013–2014 for trace-level gas data)

65 70 75 80 85 90 95 100

Atmospheric Concentrations

Meteorological Parameters

Filter Pack Flow

CO

NOYDIF

NO

NOY

SO2

Ozone

Percent Completeness

Historical (1990-2014)

2015

Continuous

Parameters

40 CFR Part 50 Requirement DQI Measurement Criterion

SO2

NOy

NOyDIF

O3

Quality Assurance Program Results

Precision of Filter Pack Measurements

Historical (1990 through 2014) mean absolute relative percent difference (MARPD) data for all 11 co-located 

site pairs operated over the history of the network are provided in the bar chart in Figure 4-a. The 2015 data 

for the current co-located sites at MCK131/231 and ROM406/206 are also provided. The precision criterion is 

a MARPD of 20 percent. Historical and 2015 measurements met the criterion for each analyte. 

The 2015 analytical precision results for 

10 measurements are presented in Figure 4-a. 

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 with the original concentration 

results. The laboratory precision data met the 

20 percent measurement criterion.

Data 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.

The historical results and the results 

for 2015 are given in Figure 4-b. 

Historical results for trace-level gas 

measurements represent data from 

2013–2014. The completeness 

criterion was met for atmospheric 

(filter pack) and O 

3 concentrations, 

filter pack flow, and meteorological 

measurements. Completeness of 

trace-level gas measurements 

(Chapter 7) met the completeness 

requirements of 40 CFR Part 50 

(EPA, 2015b), except for CO. The 

substandard completeness for CO 

was caused by calibration drift in 

August and sample pump failure and 

replacement in November.
CASTNET 2015 Annual Report

28 Chapter 4: Nitrogen Pollutant Concentrations

Figure 4-4 Annual Mean NH+ 

4 Concentrations for 2015

Figure 4-5 Trends in Annual Mean NH+ 

4 Concentrations

Western Reference Sites Eastern Reference Sites
29 Chapter 5: Update on the Ammonia Monitoring Network

Chapter 5

Chapter 5

Update on the Ammonia Monitoring Network

The Ammonia Monitoring Network (AMoN) operates passive ammonia samplers at 103 locations, 68 of which are 

located at CASTNET sites. AMoN is managed by the NADP. The network has been in operation since 2007 and 

provides information on 2-week integrated ammonia concentrations. Like other NADP networks, the goal of AMoN is to 

operate a long-term (i.e., for several decades), spatially diverse network with consistent measurements at as many as 

300 sites, covering all sensitive ecoregions of the continental United States.

Reduced nitrogen (NH 

3 + NH+ 4 ) is an important component to total nitrogen deposition. Wet deposition 

of NH+ 

4 measured by NTN has been increasing in many areas of the United States over the past 

10 years; however, until 2007, gaseous NH 

3 concentrations were not routinely measured (Li et al., 

2016). NH 

3 is the most prevalent alkaline gas in the atmosphere and is released into the air from a 

variety of agricultural sources (animal waste, fertilizer application, and agricultural burning), biological 

sources, gas and oil production and processing, and combustion. Agriculture is by far the largest 

source, producing approximately 80 percent of gaseous NH 

3 emissions (EPA National Emissions 

Inventory, 2014a). Although NH 

3 has beneficial applications, such as NH 3-based fertilizer use, it can 

have a negative effect on the environment when it reacts with acidic ions such as SO2-

4 and NO3 to 

form PM2.5, which contributes to negative impacts on human health and visibility degradation. 

Atmospheric deposition of reduced nitrogen also contributes to eutrophication of sensitive 

ecosystems, decreases in species diversity, and increases in invasive species.

Ambient concentrations of NH 

3 have shown little to no decrease despite significant reductions in NOx

emissions nationwide. Butler et al. (2016) showed that NH 

3 concentrations measured at 18 of the 

longest operating AMoN sites have increased over large regions of the United States. Figure 5-1 

shows the 18 AMoN sites that have been operating since 2010, with most sites operating since 2007. 

The sites were allocated into regions with similar seasonal trends and ambient concentrations. The 

regions and sites are listed in Table 5-1. The map also shows nearby CASTNET and wet deposition 

sites (NTN or AIRMoN).
CASTNET 2015 Annual Report

30 Chapter 5: Update on the Ammonia Monitoring Network

Figure 5-1 AMoN Sites Located within Five Study Regions

Laurel Hill State Park, PA (LRL117)
CASTNET 2015 Annual Report

31 Chapter 5: Update on the Ammonia Monitoring Network

Table 5-1 Long-term AMoN Sites Located within Five Study Regions

Region AMoN Site

AMoN Start 

Date

Nearby 

CASTNET Site

Nearby NTN 

or AIRMoN 

Site

Northeast (NE) NY16 10/2009 CAT175, NY NY68

NY67 10/2007 CTH110, NY NY67

PA00 10/2009 ARE128, PA PA00

MD08 08/2010 LRL117, PA MD08

Midwest (MW) IL11 10/2007 BVL130, IL IL11

IN99 10/2007 SAL133, IN IN41

MI96 10/2007 ANA115, MI MI52

MN18 10/2007 VOY413, MN MN18

OH02 10/2007 QAK172, OH OH49

OH27 10/2007 OXF122, OH OH09

Southeast (SE) NC06 04/2010 BFT142, NC NC06

NC30 06/2008 CND125, NC NC36

SC05 10/2007 COW137, NC SC05

Mountain West 

(MT)

CO13 10/2007 ROM406, CO CO22

NM98 10/2007 MEV405, CO CO99

NM99 10/2007 CAN407, UT CO99

Oklahoma-Texas 

(O-T)

OK99 10/2007 CHE185, OK AR27

TX43 10/2007 PAL190, TX TX43

The average increases in NH 

3 concentrations for the 18 sites over a 9-year period was 7 percent per 

year (95 percent confidence interval). Figures 5-2 and 5-3 show regressions based on the natural 

logarithm (ln) of observed annual NH 

3 concentrations (2007 through 2015) for (a) the 18 long-term 

AMoN sites and (b) the 5 regions. Points are the individual annual concentrations from each site used 

in the analysis. There was a similar increase in NH+ 

4 in precipitation at NTN and AIRMoN sites over 

the same period. Butler et al. (2016) found an increase of 5 percent per year (95 percent confidence 

interval) in NH+ 

4 in precipitation at the co-located or nearby wet deposition sites. Significant reductions 

in CASTNET-measured ambient total NO3 concentrations, which follow trends in NOx emissions, do 

not translate into reductions in total nitrogen because of increases in ambient NH 

3.
CASTNET 2015 Annual Report

32 Chapter 5: Update on the Ammonia Monitoring Network

Figure 5-2 Regression of Natural Log (In) of NH 

3 from 18 Individual AMoN Sites

Source: Butler et al. (2016)

Figure 5-3 Regression of Natural Log (In) of NH 

3 from Regional Aggregates

Source: Butler et al. (2016)

ln (NH3) ln (NH3)
CASTNET 2015 Annual Report

33 Chapter 5: Update on the Ammonia Monitoring Network

Average annual NH 

3 concentrations for 2015 for the 66 sites that met completeness requirements are 

mapped in Figure 5-4 and show a wide range of concentrations from a low of 0.3 μg/m3 at several 

northern sites to a high of 13.0 μg/m3 in northern Utah (UT01), which measured the highest annual 

average. The Utah monitoring site is located on a Utah State University research farm, and small 

quantities of livestock are often present (Martin and Baasandorj, 2016). The farm is situated in the 

Cache Valley, an area with frequent stagnant weather. The next highest concentrations in the West 

ranged from 3.2 to 4.6 μg/m3 at six sites west of the Mississippi River. The highest concentration east 

of the Mississippi was observed at Beaufort, NC (NC06) with a mean concentration of 4.9 μg/m3 . High 

local NH 

3 concentrations are largely influenced by agricultural operations (e.g., hog and cattle feeding 

and waste and crop fertilization and production).

Figure 5-4 Annual Mean NH 

3 Concentrations at AMoN sites for 2015
34 Chapter 6: Sulfur Pollutant Concentrations

Chapter 6

Chapter 6

Sulfur Pollutant Concentrations

Weekly average concentrations of sulfur dioxide and particulate sulfate were measured using 3-stage filter packs at 

95 CASTNET monitoring stations during 2015. Annual sulfur dioxide and sulfate concentrations were aggregated over 

34 eastern and 16 western reference sites in order to estimate trends, which are depicted using box plots. The sulfur 

dioxide pollutants measured at the eastern reference sites declined by 83 percent over the 26-year period from 1990 

through 2015. Particulate sulfate concentrations were reduced by 66 percent. Measured concentrations of sulfur 

dioxide and sulfate have decreased steadily since 2005. Sulfur dioxide and sulfate concentrations measured at the 

western reference sites have decreased by 47 and 25 percent, respectively, over the 20-year period 1996 

through 2015.

Annual mean concentrations of SO 

2 and SO2-4 for 2015 are presented in this chapter. Additional maps 

of 2015 quarterly mean concentrations are provided in CASTNET quarterly data reports (Amec Foster 

Wheeler, 2015a; 2015b; 2016a; 2016b; 2016c). Trends in annual mean concentrations over the 26-

year period (1990 through 2015) were derived from measurements from the eastern reference sites 

and for the period 1996 through 2015 from data measured at western reference sites. See Appendix 

A for descriptions of the designated reference sites.

Sulfur Dioxide Concentrations

During review of concentrations measured from May through December 2015, it was determined that 

a positive bias for SO 

2 concentrations measured by the impregnated cellulose filters had been 

introduced through the use of a potassium carbonate reagent from a new vendor. Data analysis and 

field testing showed that the bias was present following filter exposure to ambient air and had the 

largest effect on low concentration samples. Samples collected during the May through December 

2015 period were adjusted by decreasing all concentrations by 0.16 micrograms per milliliter before 

total microgram and atmospheric concentration calculations were applied. The adjustment was 

determined by reviewing the 5th, 10th, and 25th percentiles of the affected time period compared with 

unaffected (prior) years. All percentiles gave approximately the same increase during the affected 

time period, showing this to be a positive, constant bias as opposed to a percentage increase. 

Affected data were flagged to indicate the adjustment was made.

Annual mean SO 

2 concentrations are shown in Figure 6-1 for 2015. Annual mean concentrations were 

highest in the Midwest and East near and downwind of the Ohio River. Box plots of annual SO 

2

concentrations aggregated over the eastern reference sites from 1990 through 2015 (right side) and 

the western reference sites from 1996 through 2015 (left side) are depicted in Figure 6-2. The y-axes 

on the eastern and western plots have different scales because concentrations measured at the 

western sites were much lower than those measured at the eastern sites. Three-year mean 
CASTNET 2015 Annual Report

35 Chapter 6: Sulfur Pollutant Concentrations

concentrations for the eastern reference sites for 1990–1992 and 2013–2015 were 8.8 μg/m3 and 1.4

μg/m3 , respectively. This change constitutes an 83 percent reduction in 3-year mean SO 

2

concentrations between the two periods. The 2015 mean level of 1.2 μg/m3 was the lowest 

concentration measured by the eastern reference sites in the history of the network and represents a 

significant decline from the 2005 mean concentration of 6.0 μg/m3 

.

The box plots for the western reference sites indicate a decline in annual mean SO 

2 concentrations 

aggregated over the 16 sites. Three-year mean SO 

2 concentrations for 1996–1998 and 2013–2015 

were 0.6 μg/m3 and 0.3 μg/m3 , respectively. This change constitutes a 47 percent reduction in 3-year 

mean SO 

2 concentrations at the western reference sites over the 20 years.

Figure 6-1 Annual Mean SO 

2 Concentrations for 2015

The 2015 average sulfur dioxide concentration for the eastern reference sites was 1.2 μg/m3 . The eastern 

sulfur dioxide data show a substantive decline since 1997. The reduction (83 percent) in sulfur dioxide 

concentrations over the period 1990 through 2015 is consistent with the reduction (86 percent) in sulfur 

dioxide emissions from EGUs operating in the eastern United States, suggesting an approximately linear 

relationship between concentrations and emissions.
CASTNET 2015 Annual Report

36 Chapter 6: Sulfur Pollutant Concentrations

Figure 6-2 Trends in Annual Mean SO 

2 Concentrations

Western Reference Sites Eastern Reference Sites

Figure 6-3 Trend in Annual Composite SO 

2 Emissions from Regulated EGUs Operating in 

Illinois, Indiana, Kentucky, Ohio, Pennsylvania, and West Virginia

Source: EPA (2017)

Figure 6-3 illustrates the trend in SO 

2 emissions from regulated EGUs operating from 1990 through 

2015 in six eastern states: Illinois, Indiana, Kentucky, Ohio, Pennsylvania, and West Virginia. The 26-

year decline in aggregated EGU emissions in these states was 89 percent.

Particulate Sulfate Concentrations

Figure 6-4 shows a map of 2015 annual mean particulate SO2-

4 concentrations. Figure 6-5 provides 

box plot of annual SO2-

4 concentrations from the eastern reference sites (right side) and western 

reference sites. The figure shows a substantial decline in SO2-

4 over the 26 years for the eastern 

reference sites. Of particular interest, concentrations declined rapidly from 2005 through 2015. The 

difference between 3-year means from 1990–1992 to 2013–2015 represents a 66 percent reduction in 

SO2-

4 from 5.4 μg/m3 to 1.8 μg/m3 , respectively. The 2015 mean SO2-4 level of 1.6 μg/m3 for the eastern 

reference sites was the lowest in the history of the network.

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

1990 1995 2000 2005 2010 2015

Thousand Short Tons
CASTNET 2015 Annual Report

37 Chapter 6: Sulfur Pollutant Concentrations

The box plot for the western reference sites are provided on the left side of Figure 6-5. The data show 

a 25 percent reduction in annual mean SO2-

4 concentrations aggregated over the 16 sites with 1996–

1998 and 2013–2015 concentrations of 0.8 μg/m3 

 and 0.6 μg/m3 , respectively.

Figure 6-4 Annual Mean SO2-

4 Concentrations for 2015

The 2015 average sulfate concentration for the eastern reference sites was 1.6 μg/m3 , the lowest level in the 

history of the network. The eastern sulfate data show a substantive decline since 2005. Sulfate 

concentrations declined more slowly than sulfur dioxide concentrations at the eastern reference sites. 

Western sulfate concentrations were lower and decreased at a slower rate than concentrations measured at 

the eastern sites.
CASTNET 2015 Annual Report

38 Chapter 6: Sulfur Pollutant Concentrations

Figure 6-5 Trends in Annual Mean SO2-

4 Concentrations

Western Reference Sites Eastern Reference Sites

Sumatra, FL (SUM156)
39 Chapter 7: Continuous Trace-level Gas Concentrations

Chapter 7

Chapter 7

Continuous Trace-level Gas Concentrations

Continuous, trace-level, gaseous, air quality monitors were run at eight CASTNET sites during 2015. EPA operated six 

of the monitors, and NPS operated two. The measurements at these sites were performed to (1) provide data to 

understand atmospheric processes such as ozone and fine particulate matter formation, (2) generate data for regional 

model input and evaluation, and (3) support EPA NCore monitoring. Total reactive oxides of nitrogen were measured 

at all eight sites, sulfur dioxide at four sites, and carbon monoxide at two sites. Total reactive oxides of nitrogen 

concentrations were highest at the Beltsville, MD site and lowest at the high-elevation, mountainous sites.

Continuous trace-level gas analyzers were deployed at six EPA and two NPS CASTNET sites during 

2015. NO/NOy measurements were also taken by other federal, tribal, and state agencies at 

CASTNET or nearby sites in Maine, North Dakota, Oklahoma, and South Dakota. Table 7-1 lists the 

site locations, start dates, and the trace-level gas parameters measured at each CASTNET site. 

These data were sampled continuously and archived as 1-hour values.

Table 7-1 Continuous, Trace-level Gas Monitoring Stations Operated at CASTNET Sites 

during 2015

Site Location Start Dates Measurements

Beltsville, MD (BEL116) March 2005 NO/NOy and SO 

2

Mammoth Cave National Park, KY (MAC426)* May 2009 NO/NOy, SO 

2, and CO

Bondville, IL (BVL130) July 2012 NO/NOy, SO 

2, and CO

Huntington Wildlife Forest, NY (HWF187) November 2012 NO/NOy

Pinedale, WY (PND165) May 2013 NO/NOy

Cranberry, NC (PNF126) October 2013 NO/NOy

Rocky Mountain National Park, CO (ROM206) October 2013 NO/NOy

Great Smoky Mountains National Park (GRS420)* November 2014 NO/NOy and SO 

2

Note: *Operated by NPS

NOy is defined as NOx [NO + nitrogen dioxide (NO 

2)] plus NOz [HNO 3, nitrous acid (HONO), 

peroxyacetyl nitrate, peroxyproyl nitrate, other organic nitrates, and nitrite]. NOy consists of reactive 

gases that are considered precursors of O 

3 and PM2.5. NOy is measured by conversion to NO using a 

thermal catalytic converter and measurement of the NO by chemiluminescence. Continuous SO 

2 is 

measured using ultraviolet fluorescence, and CO is measured by gas filter correlation. Data are 

used to assess the effectiveness of emission reductions, to understand O 

3 and PM2.5 formation 

processes, for model input and evaluation, and for comparison with the weekly integrated CASTNET 

filter pack measurements.
CASTNET 2015 Annual Report

40 Chapter 7: Continuous Trace-level Gas Concentrations

Figure 7-1 provides a map of the CASTNET continuous trace-level gas monitoring locations. All EPA 

sites were operated according to the CASTNET QAPP Appendix 11, “Procuring, Installing, and 

Operating NCore Air Monitoring Equipment at CASTNET Sites” (Amec Foster Wheeler, 2014). NPS 

sites were operated according to their QA protocols. The CASTNET QAPP and all related appendices 

can be accessed at https://java.epa.gov/castnet/documents.do.

Figure 7-1 CASTNET NOy Monitoring Network in 2015

Rocky Mountain National Park, CO (ROM206)
CASTNET 2015 Annual Report

41 Chapter 7: Continuous Trace-level Gas Concentrations

QA PROGRAM SUMMARY

Quality Assurance Program Results

Continuous Trace-level Gas Concentrations

The precision of continuous trace-level gas concentration measurements was estimated based on the 

acceptance criteria enumerated in 40 CFR Part 58 Appendix A (EPA, 2013) for NOy (including the criteria 

pollutant NO 

2), SO 2, and CO. Figure 7-a presents summary statistics of critical criteria measurements at 

EPA-sponsored trace-level gas monitoring sites collected during 2015. All data associated with QC checks 

that failed to meet the established criteria were invalidated unless the cause of failure was documented to 

have no effect on ambient data collection. QC failures for EPA-sponsored sites are addressed in quarterly 

QA reports, which can be found on the EPA CASTNET website at 

https://java.epa.gov/castnet/documents.do.

Figure 7-a 2015 Quality Control Check Summary

Note: QA program results for NPS-sponsored sites measuring trace-level gas may be accessed at 

http://www.nature.nps.gov/air/monitoring/network.cfm

Figures 7-2 through 7-4 and Figure 7-6 present 2015 annual average hourly composite diurnal profiles 

of SO 

2, NOy, and O 3 for BEL116, BVL130, MAC426, and GRS420, respectively. Figures 7-5 and 7-7 

through 7-9 show the 2015 annual average hourly composite diurnal profiles of NOy and O 

3 for 

HWF187, PNF126, PND165, and ROM206. The profiles in Figures 7-2 through 7-9 were constructed 

by averaging all values from the same hour for their respective time periods. Different scales were 

used for y-axes. SO 

2 and NOy are plotted against the left y-axis, and O 3 is plotted against the right

y-axis in the eight diagrams. The figures illustrate that differences in geography, terrain, and elevation 

affect the evolution of photochemically reactive pollutants in the boundary layer.

BEL116

BVL130

HWF187

PND165

PNF126

ROM206

2015 Single Point, Span, and Zero Results: Percent Passing

2015 NOy Single Point

2015 NOy Span

2015 NOy Zero

2015 SO2 Single Point

2015 SO2 Span

2015 SO2 Zero

2015 CO Single Point

2015 CO Span

2015 CO Zero

NOy

NOy

NOy

SO2

SO2

SO2
CASTNET 2015 Annual Report

42 Chapter 7: Continuous Trace-level Gas Concentrations

Figure 7-2 BEL116, MD 1-hour Mean Figure 7-3 BVL130, IL 1-hour Mean

Concentrations of SO 

2, NOy, Concentrations of SO 2, NOy, 

and O 

3 for 2015 and O 3 for 2015

Figure 7-4 MAC426, KY 1-hour Mean Figure 7-5 HWF187, NY 1-hour Mean

Concentrations of SO 

2, NOy, Concentrations of NOy

and O 

3 for 2015 and O 3 for 2015

0

10

20

30

40

50

60

0

2

4

6

8

10

12

14

16

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Concentration - Ozone (ppb)

Concentration - SO2/NOy (ppb)

Hour

SO2

NOY

Ozone

0

10

20

30

40

50

60

0

1

2

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4

5

6

7

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Concentration - Ozone (ppb)

Concentration - SO2/NOy (ppb)

Hour

SO2

NOY

Ozone

0

10

20

30

40

50

60

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Concentration - Ozone (ppb)

Concentration - SO2/NOy (ppb)

Hour

SO2

NOY

Ozone

0

10

20

30

40

50

60

0.0

0.2

0.4

0.6

0.8

1.0

1.2

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1.8

2.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Concentration - Ozone (ppb)

Concentration - NOy (ppb)

Hour

NOY

Ozone

SO 

2

NOy

Ozone

SO 

2

NOy

Ozone

NOy

Ozone

SO 

2

NOy

Ozone
CASTNET 2015 Annual Report

43 Chapter 7: Continuous Trace-level Gas Concentrations

Figure 7-6 GRS420, TN 1-hour Mean Figure 7-7 PNF126, NC 1-hour Mean

Concentrations of SO 

2, NOy, Concentrations of NOy

and O 

3 for 2015 and O 3 for 2015

Figure 7-8 PND165, WY 1-hour Mean Figure 7-9 ROM206, CO 1-hour Mean

Concentrations of NOy Concentrations of NOy

and O 

3 for 2015 and O 3 for 2015

0

10

20

30

40

50

60

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Concentration - Ozone (ppb)

Concentration - SO2/NOy (ppb)

Hour

SO2

NOY

Ozone

0

10

20

30

40

50

60

0.0

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Concentration - Ozone (ppb)

Concentration - NOy (ppb)

Hour

NOY

Ozone

0

10

20

30

40

50

60

0.0

0.2

0.4

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1.0

1.2

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2.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Concentration - Ozone (ppb)

Concentration - NOy (ppb)

Hour

NOY

Ozone

0

10

20

30

40

50

60

0.0

0.2

0.4

0.6

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1.0

1.2

1.4

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1.8

2.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Concentration - Ozone (ppb)

Concentration - NOy (ppb)

Hour

NOY

Ozone

NOy

Ozone

NOy

Ozone

NOy

Ozone

SO 

2

NOy

Ozone
CASTNET 2015 Annual Report

44 Chapter 7: Continuous Trace-level Gas Concentrations

Table 7-2 Summary of 2015 Minimum and Maximum Values from Diurnal Charts

Site Location*

Site Elevation 

(meters)

NOy (ppb) O 

3 (ppb)

Minimum Maximum Minimum Maximum

BEL116, MD 47 5.9 13.7 15 42

BVL130, IL 213 3.1 5.5 20 41

MAC426, KY 243 2.3 3.5 23 39

HWF187, NY 497 0.7 1.1 15 35

GRS420, TN 793 1.7 2.1 32 41

PNF126, NC 1,216 0.7 1.0 37 43

PND165, WY 2,386 0.6 0.9 42 49

ROM206, CO 2,742 0.8 1.7 43 50

Note: * Sites are listed in order of elevation

Continuous Trace-level NOy and Ozone

The minimum and maximum mean composite NOy and O 

3 concentrations shown in Figures 7-2 

through 7-9 are summarized in Table 7-2. The sites are listed in order of elevation. The highest NOy

concentrations were measured at BEL116, which has numerous, nearby mobile-source NOx

emissions. Low NOy values (less than 2.0 ppb) were recorded at the five rural sites in New York,

Tennessee, North Carolina, Wyoming, and Colorado. These rural NOy values are similar to Edgerton 

et al. (2016) NOy measurements at Coweeta, NC. The eight profiles in Figures 7-2 through 7-9 

illustrate the loss of O 

3 during the late afternoon and nighttime hours and its production during daylight 

hours. The diurnal change was less pronounced at the six rural sites. The BEL116 site observed the 

largest nighttime loss and subsequent highest daytime production, an average difference of about 

27 ppb of O 

3 for 2015. The BVL130 data illustrate a typical diurnal relationship between NOy and O 3

concentrations with high NOy concentrations associated with vehicular NOx emissions in the morning 

and evening and elevated O 

3 concentrations in the afternoon. The data from the rural MAC426 site 

also show a diurnal evolution with the highest NOy concentrations in the morning and the evening and 

highest O 

3 concentrations in the afternoon. The MAC426 data appear similar to measurements made 

at an urban site.

O 

3 concentrations were also measured at HWF187, GRS420, PNF126, PND165, and ROM206 during 

2015 (Table 7-2). Of these sites, the highest O 

3 concentration (50 ppb) was measured at ROM206. 

The concentrations measured at the three sites with the highest elevations show less 24-hour 

variation than the other five sites because of the absence of nighttime O 

3 loss/depletion mechanisms 

near the high-elevation monitors (Talbot et al., 2005). In particular, nighttime dry deposition is small 

because shallow boundary layers typically do not form at these high-elevation sites and scavenging is 

diminished because fresh nitric oxide is not available to react with O 

3. The ROM206 data suggest a 

large production or transport of NOy around sunset. There are no significant NOx sources in the 
CASTNET 2015 Annual Report

45 Chapter 7: Continuous Trace-level Gas Concentrations

immediate area. The increase in NOy is likely a result of the transformation of polluted air masses 

from the Front Range Urban Corridor and the frequent late afternoon upslope flow from the east 

(Baumann et al., 1997).

Continuous Trace-level NOy and Filter Pack Total Nitrate Concentrations

HNO 

3 and particulate NO3 are measured on CASTNET filter packs, and the sum is reported as total 

NO3. Because HNO 

3 and particulate NO3 are measured as components of NOy, NOy concentrations 

should always be higher than total NO3 levels (i.e., the ratio of NOy to total NO3 should always be 

greater than 1.0). A comparison of weekly mean continuous NOy concentrations with filter pack total 

NO3 levels at BVL130, PNF126, and PND165 for 2015 was used to evaluate the measurements 

(Figures 7-10 through 7-12). The NOy concentrations were consistently higher than the total NO3

levels, as expected. The results are similar for the other five sites. The weekly total NO3

concentrations, the average weekly NOy levels, and their ratios are listed in Table 7-3. These were 

calculated as the average of all valid weekly filter pack concentrations, the average of mean NOy

values matching the run time of the weekly filter packs, and the average of the ratios calculated for 

each week. Weekly NOy levels were higher than the weekly total NO3 concentrations with ratios of 

NOy to total NO3 varying from 2.40 at PNF126 to 10.87 at BEL116. The highest concentration 

(0.98 ppb) of total NO3 was measured at BVL130.

Bondville, IL (BVL130)
CASTNET 2015 Annual Report

46 Chapter 7: Continuous Trace-level Gas Concentrations

Figure 7-10 Comparison of BVL130, IL Weekly Mean Continuous Trace-level NOy and 

Filter Pack Total NO3 Concentrations

Figure 7-11 Comparison of PNF126, NC Weekly Mean Continuous Trace-level NOy and 

Filter Pack Total NO3 Concentrations

0.0

2.0

4.0

6.0

8.0

10.0

12/28/14

1/10/151/25/15

2/8/15

2/22/15

3/8/15

3/22/15

4/5/15

4/19/15

5/3/15

5/17/155/31/156/14/156/28/157/12/157/26/15

8/9/15

8/23/15

9/6/15

9/20/1510/4/15

10/18/15

11/1/15

11/14/1511/29/1512/13/15

Concentration (ppb)

Filter Pack Total NO3

Mean Trace NOy

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

12/28/14

1/11/151/25/15

2/8/15

2/22/15

3/8/15

3/22/15

4/5/15

4/19/15

5/3/15

5/17/155/31/156/14/156/28/157/12/157/26/15

8/9/15

8/23/15

9/6/15

9/20/1510/4/15

10/19/15

11/1/15

11/15/1511/29/1512/13/15

Concentration (ppb)

Filter Pack Total NO3

Mean Trace NOy

Filter Pack Total NO3

Mean Continuous NOy

Filter Pack Total NO3

Mean Continuous NOy
CASTNET 2015 Annual Report

47 Chapter 7: Continuous Trace-level Gas Concentrations

Figure 7-12 Comparison of PND165, WY Weekly Mean Continuous Trace-level NOy and 

Filter Pack Total NO3 Concentrations

Table 7-3 Summary of Total NO3 and NOy Measurements for 2015

Site Location Total NO3 (ppb) NOy (ppb) Ratio

BEL116, MD 0.76 8.01 10.87

BVL130, IL 0.98 4.39 5.03

MAC426, KY 0.70 2.79 4.34

HWF187, NY 0.21 0.94 3.58

GRS420, TN 0.45 1.97 4.93

PNF126, NC 0.33 0.81 2.40

PND165, WY 0.14 0.75 5.43

ROM206, CO 0.20 1.20 6.59

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

12/28/14

1/12/151/25/15

2/8/15

2/22/15

3/8/15

3/22/15

4/5/15

4/19/15

5/3/15

5/17/155/31/156/14/156/28/157/12/157/26/15

8/9/15

8/23/15

9/6/15

9/20/1510/4/15

10/25/15

11/8/15

11/22/15

12/7/15

12/20/15

Concentration (ppb)

Filter Pack Total NO3

Mean Trace NOy

Filter Pack Total NO3

Mean Continuous NOy
48 Chapter 8: Deployment and Evaluation of the

MARGA at Beltsville, Maryland

Chapter 8

Chapter 8

Deployment and Evaluation of the

Monitor for Aerosols and Gases at Beltsville, Maryland

The EPA deployed two separate, co-located Monitor for AeRosols and GAses (MARGA) 1S systems at its CASTNET 

Beltsville, MD site between August 2014 and May 2015. The measurement campaign produced eight months of valid 

hourly data for soluble gases (nitric acid, nitrous acid, sulfur dioxide, and ammonia) and particles (nitrate, sulfate, 

ammonium, calcium, magnesium, potassium, and sodium). The gas and particulate concentration measurements were 

used to evaluate the performance of the MARGA instrument systems based on (1) filter pack concentration data and 

(2) data from a conventional pulsed fluorescence sulfur dioxide analyzer. The measurements were also used to 

intercompare the performance of the two MARGAs.

Comparison of MARGA, Continuous Sulfur Dioxide Analyzer, and CASTNET Filter Pack

To evaluate MARGA measurements, SO 

2 data from the two MARGA systems were compared with 

the already operational hourly SO 

2 measurements from a co-located federal equivalent method 

pulsed fluorescence SO 

2 analyzer (PFA). No additional provisions were made to eliminate variables 

that could lead to measurement differences (i.e., different inlet heights or more frequent calibrations 

to the analyzer). Thus, the hourly PFA SO 

2 measurements were compared as a check and not as a

performance verification. The agreement of one species (SO 

2) in the suite of co-located

measurements is not a validation of all species measured with the MARGA but does provide 

additional confidence in overall MARGA performance (Rumsey et al., 2014; McKernan et al., 2011). It 

is important to note the differences between the two methods. Inlet height for the MARGA was 

sampled at either 5 or 3 m (inlets were changed midway through the evaluation period); however, this 

had little effect for SO 

2 recovery. The PFA inlet was at 10 m. Additionally, the MARGA samples gases 

semi-continuously every hour via absorption into solution using a wet, rotating denuder with 

subsequent analysis via on-line ion chromatography. The PFA is an optical emission method for 

continuous SO 

2 measurement integrated up to hourly resolution.

The PFA SO 

2 data required a linear baseline adjustment to correct for drift (evident from calibration 

data). After the correction, the MARPD between the hourly PFA and the MARGA SO 

2 concentrations 

was 32 percent for concentrations exceeding the MARGA method detection limit (MDL) of greater 

than or equal to 0.05 μg/m3 . Filtering the data for higher values (greater than or equal to 1.33 μg/m3 ) 

improved the MARPD to 22 percent. This analysis indicates that scatter in low-level concentrations is 

an issue in the co-location and suggests the limit of comparison between the two methods should be 

set higher than the MARGA MDL. A traditional linear regression was run with the MARGA SO 

2

concentrations as the independent variable and gave a slope of 0.950 ± 0.007 and y-intercept of 

0.022 ± 0.023 with a root mean square error of 1.30 and an R2 of 0.790. Hourly SO 

2 concentrations 

averaged between the two MARGA units and the PFA are shown in Figure 8-1.
CASTNET 2015 Annual Report

49 Chapter 8: Deployment and Evaluation of the

MARGA at Beltsville, Maryland

Figure 8-1 Time-series of Hourly SO 

2 Concentrations Based on an Average of Two MARGA 

Units and PFA from July 30, 2014 through May 4, 2015

Comparison of Co-located MARGA Units

The sampling campaign with the MARGAs was conducted for eight months between July 30, 2014 

and May 4, 2015, with two identical MARGA units (MU1 and MU2) running in the same shelter. The 

performance metrics of the hourly MARGA measurements are shown in Table 8-1. Sampling during 

this time resulted in a high percentage of valid data for all species (89–92 percent for MU1 and 

87–93 percent for MU2) with an overall time coverage of 82–84 percent valid data from at least one 

instrument. Overall performance statistics between the co-located MARGA units (Table 8-1) were 

excellent with MARPD values less than or equal to 15 percent for aerosol species (NH+ 

4 , NO3, SO2-4 ), 

values less than or equal to 20 percent for gaseous species (HONO, SO 

2), and values less than or 

equal to 25 percent for gaseous HNO 

3. An exception to this was gaseous NH 3 with a MARPD of 39

percent. Linear regression slopes were near an expected value of 1: within ± 5 percent for HNO 

3, NH+ 4 , 

NO3, and SO2-

4 ; within ± 15 percent for HONO; and within ± 18 percent for SO 2. The slope for NH 3 was 

30 percent less than 1, indicating a bias between the instruments. Comparison of base cation data,

Ca2+, Na+ , Mg2+

, and K+ , was affected by low ambient levels nearing detection limits and required

blank corrections.

Beltsville, MD (BEL116)
CASTNET 2015 Annual Report

50 Chapter 8: Deployment and Evaluation of the

MARGA at Beltsville, Maryland

Table 8-1. Statistical Comparisons between Hourly Concentrations (μg/m3 ) Measured with

Co-located MARGA Units from July 30, 2014 through May 4, 2015

MARPD (%) Linear Regression

Species Overall N m b R2 

SO 

2 ŧ ¥ (PFA) 32 (22) 4776 0.950 0.022 0.790

HNO 

3 24 2979 1.039 0.011 0.748

HONO 20 2975 0.857 0.088 0.695

NH 

3 39 3076 0.700 0.171 0.519

SO 

2 16 2984 0.820 0.168 0.851

NO3 11 2895 0.952 0.014 0.980

NH+ 

4 15 3068 0.947 -0.055 0.979

SO2-

4 4 2654 0.973 0.106 0.955

Ca2+ 43 3068 0.664 0.069 0.680

Na+ 30 3068 0.596 0.073 0.743

Note: Table includes average MARGA hourly SO 

2 concentrations compared with the PFA hourly SO 2 concentrations

ŧ = comparison of values greater than 1.3 μg/m3 because of baseline correction

¥ = value from average of MU1 and MU2 to compare with PFA SO 

2

N = number of samples

m = slope

b = intercept

R2 = coefficient of determination

The MARGA units show more consistent performance in measuring anion aerosols than gases. This 

was likely due to the higher mobility of gases coupled with high polarity and solubility that led to 

increased adsorption to the inlet. Results not presented here support that hypothesis. 

Integrated MARGA Data and Comparison with Filter Pack Data

To compare the semi-continuous MARGA data with the CASTNET filter pack concentrations, hourly 

MARGA data were integrated into weekly averages. Only weeks where the MARGA collected at least 

100 hours of valid data (i.e., greater than 60 percent) were considered. Time-series plots of the 

integrated MARGA data, PFA SO 

2 data, and the CASTNET filter pack data are shown in Figure 8-2. 

The NH 

3 concentrations were extrapolated into weekly intervals from AMoN two-week concentrations, 

which were measured by a passive sampler at the Beltsville site.
CASTNET 2015 Annual Report

51 Chapter 8: Deployment and Evaluation of the

MARGA at Beltsville, Maryland

Figure 8-2 Time-series of Weekly Concentrations Based on Integrated Averages of the Two 

MARGA Units, CASTNET Filter Pack, and PFA (SO 

2 only) from July 30, 2014 

through May 4, 2015

Statistical comparisons were made for each of the species and are shown in Table 8-2. The 

concentrations measured by the different methods compared very well (i.e., predicted slope within 0.8 

to 1.2 and R2 within 0.8 to 1.2) for SO 

2 (all measurement methods); total NO 3, which is the sum of 

HNO 

3 and NO3; NO3; and NH+ 4 . Surprisingly, the correlation statistics were not as good for SO2-4

(m= 0.74; R2 = 0.61) owing to an observed bias in MARGA concentrations measured from 

September 12, 2014 to November 5, 2014. Further evaluation is needed to assess if this is a sampling 

artifact. Interestingly, NH+ 

4 and SO 2 also appear to be biased high for the MARGA (and the PFA) 

during this period.
CASTNET 2015 Annual Report

52 Chapter 8: Deployment and Evaluation of the

MARGA at Beltsville, Maryland

Table 8-2. Comparison of Concentration (μg/m3 ) Statistics for Integrated Weekly 

Sampling Methods from September 12, 2014 through November 5, 2014

Species

Dependent

Variable

Independent

Variable N

MRPD

(%)

MARPD

(%) m (± σ) b (± σ) R2 

SO 

2 PFA MARGA 37 -5 11 0.933 (0.047) 0.103 (0.105) 0.920

SO 

2 FP MARGA 36 -5 17 0.896 (0.072) 0.160 (0.163) 0.821

SO 

2 FP PFA 39 -5 14 0.942 (0.058) 0.089 (0.137) 0.876

HNO 

3 FP MARGA 31 55 55 0.915 (0.298) 0.444 (0.166) 0.245

NO3 FP MARGA 31 -24 30 0.932 (0.065) -0.154 (0.100) 0.875

Total NO3 FP MARGA 31 12 13 0.857 (0.060) 0.404 (0.120) 0.877

NH 

3 FP MARGA 31 38 43 0.793 (0.193) 0.275 (0.096) 0.368

NH+ 

4 FP MARGA 32 -8 14 0.958 (0.083) -0.044 (0.073) 0.816

SO2-

4 FP MARGA 31 -17 17 0.737 (0.110) 0.193 (0.244) 0.607

Ca2+ FP MARGA 32 -54 57 0.314 (0.060) 0.102 (0.022) 0.473

K

+ FP MARGA 32 90 90 0.354 (0.118) 0.051 (0.003) 0.231

Mg2+ FP MARGA 32 -12 29 0.382 (0.077) 0.026 (0.005) 0.450

Na+ FP MARGA 32 31 45 1.553 (0.133) -0.015 (0.025) 0.821

Note: N = number of samples

m = slope

b = intercept

R2 = coefficient of determination

σ = standard deviation

The filter pack concentrations for HNO 

3 were biased higher than the MARGA concentrations 

[55 percent mean relative percent difference (MRPD)], and there was a low bias of filter pack NO3

concentrations (-24 percent MRPD), which is attributed to the volatilization of ammonium nitrate 

(NH 

4NO 3) from the Teflon filter to the nylon filter in the CASTNET filter pack setup (Sickles and 

Shadwick, 2002; Lavery et al., 2009). The MARGA concentrations do not experience any effects due 

to NH 

4NO 3 volatility as the aerosol is rapidly sampled into the absorbing medium and immediately 

analyzed (Slanina et al., 2001). A comparison of the total NO3 concentrations (12 percent MRPD) 

supports this hypothesis. The comparison with NH 

3 was complicated by the inlet adsorption losses

and is being evaluated further.
53 Chapter 9: Atmospheric Deposition of Nitrogen 

Chapter 9

Chapter 9 

Atmospheric Deposition of Nitrogen

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

States. To assess the status of dry and total deposition for 2015, EPA used NADP’s Total Deposition Hybrid Method to 

estimate dry deposition. The hybrid method combines measured pollutant concentrations with output from the CMAQ 

modeling system and uses precipitation chemistry measurements at NADP/NTN sites and precipitation amounts from 

the Parameter-elevation Regressions on Independent Slopes Model to estimate wet deposition. Total deposition was 

calculated as the sum of estimated dry and wet deposition.

Gaseous and particulate nitrogen pollutants are deposited to the environment through dry and wet 

atmospheric processes. A major goal of CASTNET is to estimate the rate of dry deposition of 

pollutants from the atmosphere to sensitive ecosystems. The NADP TDEP Hybrid Method (EPA, 

2015c; Schwede and Lear, 2014) combines monitoring data with output from the CMAQ modeling 

system (Byun and Schere, 2006) to estimate dry deposition. Air quality measurements were obtained 

from CASTNET and the Southeastern Aerosol Research and Characterization (SEARCH) Network. 

The TDEP method gives priority to using measurement data from air quality monitoring sites when 

available and to CMAQ output in areas where monitoring data are not available. In addition, CMAQ 

provides modeled data for species that are not routinely measured. The TDEP method and its recent 

updates are discussed on the TDEP website 

(http://nadp.isws.illinois.edu/committees/tdep/tdepmaps/). See also the NADP Fact Sheet, “Hybrid 

Approach to Mapping Total Deposition,” (http://nadp.isws.illinois.edu/lib/brochures/tdepsheet.pdf). 

TDEP estimates wet deposition using PRISM to develop a continuous grid of precipitation data. 

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

on a 4-kilometer resolution grid. Pollutant concentrations in precipitation, which were estimated for the 

PRISM grid, were provided by NADP/NTN. The concentration and precipitation grids were merged in 

order to estimate pollutant wet deposition rates. Dry and wet deposition fluxes were added to obtain 

estimates of total deposition, which are presented on the maps in this chapter as kilograms per 

hectare per year (kg ha-1

 yr-1 ). 

Figure 9-1 illustrates TDEP estimates of dry fluxes of nitrogen (N) for 2015. The magnitude of the 

deposition fluxes is illustrated by the shading in the figure legend. A map of total deposition of N for 

2015 is given in Figure 9-2. The percentage of total deposition of N due to dry deposition is shown in 

Figure 9-3. A map of TDEP estimates of total deposition of reduced N species for 2015 is given in 

Figure 9-4. Figure 9-5 gives the percentage of reduced N species from dry deposition. Dry deposition 

of N species that are not routinely measured but are estimated by TDEP is shown in the map in 

Figure 9-6. The percentage of total deposition of N that is not measured directly but is calculated by 
CASTNET 2015 Annual Report 

54 Chapter 9: Atmospheric Deposition of Nitrogen 

TDEP is shown in Figure 9-7. Annual gross deposition of NH 

3 for 2015 is shown in Figure 9-8, and the 

net deposition of NH 

3 is given in Figure 9-9.

Figure 9-1 TDEP Dry Deposition Estimates of N (kg ha-1 yr-1) for 2015

Source: CASTNET/CMAQ/NTN/SEARCH

Figure 9-2 TDEP Total Deposition Estimates of N (kg ha-1

yr-1) for 2015

Source: CASTNET/CMAQ/NTN/SEARCH
CASTNET 2015 Annual Report 

55 Chapter 9: Atmospheric Deposition of Nitrogen 

Figure 9-3 TDEP Percent of Total Deposition of N from Dry Deposition for 2015

Source: CASTNET/CMAQ/NTN/SEARCH

Figure 9-4 TDEP Total Deposition Estimates of Reduced N Species (kg ha-1

yr-1) for 2015

Source: CASTNET/CMAQ/NTN/SEARCH
CASTNET 2015 Annual Report 

56 Chapter 9: Atmospheric Deposition of Nitrogen 

Figure 9-5 TDEP Percent of Total Deposition of Reduced N Species from Dry Deposition 

for 2015

Source: CASTNET/CMAQ/NTN/SEARCH

Figure 9-6 TDEP Dry Deposition Estimates of Unmonitored N Species (kg ha-1 yr-1) for 2015

Source: CASTNET/CMAQ/NTN/SEARCH
CASTNET 2015 Annual Report 

57 Chapter 9: Atmospheric Deposition of Nitrogen 

Figure 9-7 TDEP Percent of Total Deposition of N from Dry Deposition of Unmonitored Species 

for 2015

Source: CASTNET/CMAQ/NTN/SEARCH

Figure 9-8 TDEP Gross Dry Deposition of NH 

3 as N (kg ha-1

yr-1) for 2015

Source: CASTNET/CMAQ/NTN/SEARCH
CASTNET 2015 Annual Report 

58 Chapter 9: Atmospheric Deposition of Nitrogen 

Figure 9-9 TDEP Net Dry Deposition of NH 

3 as N (kg ha-1

yr-1) for 2015

Source: CASTNET/CMAQ/NTN/SEARCH

Pinedale, WY (PND165) 
59 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Chapter 10

Chapter 10

Cloud Water, Filter Pack Pollutant, and Ammonia Concentrations at 

Whiteface Mountain, New York

Environmental measurements have been performed at Whiteface Mountain, NY for more than 75 years. The 

Atmospheric Sciences Research Center currently operates monitoring sites at the Marble Mountain Lodge and the 

Summit Observatory. Measurements at these two sites include seasonal cloud water, cloud water chemistry, 

precipitation chemistry, and filter pack concentration data. EPA operates two CASTNET small footprint, filter pack-only 

sites on Whiteface Mountain: a permanent site near Marble Mountain Lodge (WFM105) and a seasonal (May through 

October) site at the Summit Observatory (WFM007). An AMoN site was installed at Marble Mountain Lodge in late 

2012. Trends in cloud water pollutant concentrations and CASTNET filter pack measurements of pollutant gases and 

aerosols from the Whiteface Mountain sites are presented in this chapter. Trends in ammonia concentrations from 

WFM105 and nearby sites are also shown.

Whiteface Mountain, located in the High Peaks region of the Adirondack Mountains, has a long history 

of atmospheric chemistry measurements. The Marble Mountain Lodge and the Summit Observatory 

are currently operated by the Atmospheric Sciences Research Center (ASRC). Marble Mountain 

Lodge sits on the eastern shoulder of the Whiteface Massif at an elevation of 604 m in a heavily 

wooded area representative of a mixed northern hardwood forest. The Summit Observatory is located 

on the peak of Whiteface Mountain at an elevation of 1,483 m. This location is windswept, above the 

tree line, and comprised primarily of bedrock with patches of thin soils. The summit microclimate is 

one of the coldest in the eastern United States. The short growing season and high winds limit the 

types of vegetation that can survive these harsh conditions (Schwab, Wolfe et al., 2016).

The first organized environmental measurements at Whiteface Mountain began in 1937 as a 

cooperative project between Rensselaer Polytechnic Institute, New York University, the Whiteface 

Mountain Highway Commission, and the Weather Bureau (now the National Weather Service) and 

continued until 1946. This effort provided the initial extensive meteorological data record at this 

location. With support from the Whiteface Mountain Authority, a wide array of meteorological 

instrumentation was operated throughout the late 1950s and early 1960s. The ASRC was established 

in 1961, and Whiteface Mountain became the main field station of the new research center. The 

station has been available for project work for the State University of New York schools, as well as 

other universities and organizations. An existing summit fire tower was replaced in 1970 with the 

current permanent observatory building (Figure 10-1). This structure enabled more extensive and 

long-term measurements at the summit. The ongoing long-term pollutant gas measurements began in 

1973 when the NYSDEC started measuring O 

3 concentrations. Table 10-1 summarizes the timeline of 

the major measurement programs at the two Whiteface Mountain measurement locations (Schwab, 

Wolfe et al., 2016).
CASTNET 2015 Annual Report

60 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Figure 10-1 Whiteface Mountain Summit Observatory

High volume total suspended particle (TSP) aerosol sampling at Whiteface Mountain began in 1975 

and has been continuous at the Summit Observatory since 1978. The aerosol samplers collect TSP 

on cellulose filters and are analyzed for mass, major ions, and, as of 2007, black carbon. The NTN 

has operated a precipitation collection site at the Marble Mountain Lodge site since 1984. It is one of 

six NTN sites currently operating in the Adirondack Park. Many gas and aerosol species are 

incorporated into cloud water and can deposit to the environment either in precipitation or through 

cloud impaction, which usually occurs at elevations above 800 m. For this reason, both precipitation 

and cloud water samples are collected at Whiteface Mountain (Schwab, Casson et al., 2016).
CASTNET 2015 Annual Report

61 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Table 10-1 Timeline of Major Measurement Activity at Whiteface Mountain

Dates* Activity Location Organizations

1937–1946 Temperature measurements Summit Observatory Weather Bureau, RPI, NYU, 

WMHC

1957 Meteorological measurements Summit Observatory ASRC

1963–1978 Aerosol particle number concentration Summit Observatory ASRC and MML

1973 Ozone concentrations Summit Observatory NYSDEC

1975 High volume aerosol samples (TSP) Summit Observatory NYSDOH

1976 Cloud collection (seasonal) Summit Observatory ASRC/EPA/ALSC

1976 Precipitation collection MML ASRC/NYSDEC/NADP

1984 Ozone and other pollutant gases MML NYSDEC/ASRC

1988 Pollutant and ozone precursor gases Summit Observatory ASRC

1999 PM2.5 mass concentration (filter samples and 

continuous)

MML NYSDEC

2001 PM2.5 chemical speciation (filter samples) MML EPA/CSN

1987–1993

2012

Filter pack measurements MML EPA/CASTNET

2012 AMoN NH 

3 Summit Observatory NADP

2015 Filter pack measurements Summit Observatory EPA/CASTNET

Note: *When no end date is given, the specified activity is continuing.

Abbreviations used in Table 10-1:

ALSC – Adirondack Lake Survey Corporation

ASRC – Atmospheric Sciences Research Center, University 

at Albany, State University of New York

CSN – Chemical Speciation Network

MML – Marble Mountain Lodge

Source: Schwab, Wolfe et al. (2016)

NADP – National Atmospheric Deposition Program

NYSDEC – New York State Department of Environmental 

Conservation

NYSDOH – New York State Department of Health

NYU – New York University

RPI – Rensselaer Polytechnic Institute

WMHC – Whiteface Mountain Highway Commission

Cloud Water Measurements

Cloud water collection at the station started in 1976 when the ASRC began a routine program of 

continuous cloud water collection and pH analysis (Schwab, Wolfe et al., 2016). These efforts later 

became a part of the Mountain Cloud Chemistry Project (MCCP), which operated from 1986 to 1989 

and was administered by the Forest Response Program of the National Acid Precipitation and 

Assessment Program (NAPAP) and was sponsored by the EPA. MCCP data were used in the NAPAP 

Integrated Assessment to evaluate the role of airborne chemicals in the changing condition of forests 

(National Science and Technology Council, 2005). 

The MCCP was followed several years later by the Mountain Acid Deposition Program (MADPro),

which officially operated under CASTNET from 1993 through 2011 during the warm seasons. 

MADPro’s two main objectives were to develop cloud water measurement systems geared towards 

operating in a network setting and update the Appalachian Mountains cloud water concentrations and 

deposition data collected by NAPAP during MCCP. MADPro consisted of three main sampling 

stations: Whiteface Mountain, NY; Whitetop Mountain, which was located in the Mount Rogers 

National Recreation Area in Virginia; and Clingmans Dome, which was located in the Great Smoky
CASTNET 2015 Annual Report

62 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Mountains National Park in Tennessee. Of these three sites, Whiteface Mountain is the only site still 

in operation today. Whitetop Mountain closed after the 1999 season, and the Clingmans Dome site 

closed after the 2011 season. After the 2000 season, Whiteface Mountain cloud water collection 

activities have been sponsored by the NYSDEC and the New York State Energy Research and 

Development Authority, and the site, WFM300, is operated by the Adirondack Lake Survey 

Corporation. The particle volume monitor, which is used to determine the presence of a cloud as well 

as the liquid water content of the cloud, is shown in Figure 10-2. The cloud water collector is shown 

in Figure 10-3. Figure 10-4 shows concentrations in microequivalents per liter (μeq/L) of the major 

ions, SO2-

4 , NO3, and NH+ 4 , in cloud water from 2001 through 2015. SO2-4 , NO3, and NH+ 4 all show 

downward trends.

Figure 10-2 Particle Volume Monitor
CASTNET 2015 Annual Report

63 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Figure 10-3 Cloud Water Collector

Figure 10-4 Cloud Water SO2-

4 , NO3, and NH+ 4 Concentrations

0

50

100

150

200

250

300

350

400

200120022003200420052006200720082009201020112012201320142015

μeq/L

Whiteface

Sulfate

Whiteface

Nitrate

Whiteface

Ammonium
CASTNET 2015 Annual Report

64 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Filter Pack Measurements

A CASTNET small footprint filter pack site 

(WFM105) was installed near the NTN 

precipitation collector at Marble Mountain 

Lodge in 2012 (Figure 10-5). Dry deposition 

data estimated from filter pack concentrations 

and wet deposition data calculated from the 

precipitation concentrations allow for estimation 

of total deposition at this location. A CASTNET 

filter pack (WFM007) was also installed at the 

Summit Observatory in 2015 (Figure 10-6). 

Total deposition values can now be estimated 

for the summit location as well, albeit with NTN 

precipitation data from the Marble Mountain 

Lodge site. Ideally, data from an NTN collector 

at the summit would be preferable. Precipitation 

collection at the summit would enable direct 

comparison of total deposition between the two 

locations. Even without the NTN collector at the 

summit site, comparison of dry deposition 

values between the two CASTNET filter pack 

sites is still useful for discerning the contribution 

from cloud water to total deposition. 

Figures 10-7, 10-8, and 10-9 compare filter 

pack concentrations from the summit 

(WFM007) and lodge (WFM105) locations for 

SO2-

4 , NO3, and NH+ 4 ; Ca2+

; and Mg2+, Na+ , and 

K+ 

. The weekly concentrations measured at 

WFM105 and WFM007 during the summer of 

2015 were reasonably comparable.

Figure 10-5 Small Footprint CASTNET Site 

(WFM105) near the Marble 

Mountain Lodge

Figure 10-6 CASTNET Filter Pack (WFM007) 

at the Summit Observatory
CASTNET 2015 Annual Report

65 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Figure 10-7 Filter Pack SO2-

4 , NO3, and NH+ 4 Concentrations for WFM007 and WFM105

Figure 10-8 Filter Pack Ca2+ Concentrations for WFM007 and WFM105

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

6/2/156/9/15

6/16/156/23/156/30/15

7/7/15

7/14/157/21/157/28/15

8/4/15

8/11/158/18/158/25/15

9/1/159/8/15

9/15/15

μg/m3

Sulfate - WFM007

Sulfate - WFM105

Nitrate - WFM007

Nitrate - WFM105

Ammonium - WFM007

Ammonium - WFM105

0.00

0.05

0.10

0.15

0.20

0.25

0.30

6/2/156/9/15

6/16/156/23/156/30/15

7/7/15

7/14/157/21/157/28/15

8/4/15

8/11/158/18/158/25/15

9/1/159/8/15

9/15/15

μg/m3

Calcium - WFM007

Calcium - WFM105
CASTNET 2015 Annual Report

66 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Figure 10-9 Filter Pack Mg2+, Na+ , and K+ Concentrations for WFM007 and WFM105

Including the two sites at Whiteface Mountain, six CASTNET filter pack systems were operated in 

New York along with one filter pack in Vermont and one in New Hampshire (Figure 1-1 and 

Appendix A). Figures 10-10, 10-11, and 10-12 present time series of weekly NH+ 

4 , SO2-4 , and total NO3

concentrations measured during the summer of 2015 at the eight sites. Lower concentrations were 

measured at the more remote sites and higher concentrations at the downstate sites at CTH110, NY

and CAT175, NY.

Ammonia Measurements

AMoN began monitoring biweekly NH 

3 gas concentrations using passive samplers in 

2007 (Chapter 5). An AMoN station was installed at the Marble Mountain Lodge site in late 2012. 

Currently, there are nine AMoN sites in upstate New York and one site each in Vermont and New 

Hampshire. Figure 10-13 shows biweekly NH 

3 concentrations for five New York sites, one Vermont

site, and one New Hampshire site. Trend lines were plotted for the Marble Mountain Lodge (NY98) 

and Ithaca, NY (NY67/CTH110) sites to illustrate a slight increase in NH 

3 concentrations since 

November 2012. Figure 10-14 shows time series of 2-week NH 

3 concentrations measured at 

NY67/CTH110 and the Cary Institute (NY16) from October 2007 through February 2016. Linear 

regression suggest slight increases in NH 

3 concentrations.

Figure 10-15 compares AMoN ambient NH 

3 concentrations (μg/m3 ) and NTN precipitation NH+ 4

concentrations [milligrams per liter (mg/L)] measured at Marble Mountain Lodge over three years. 

Ambient NH 

3 concentrations were not correlated with NH+ 4 concentrations in precipitation.

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

6/2/156/9/15

6/16/156/23/156/30/15

7/7/15

7/14/15

7/21/157/28/15

8/4/15

8/11/158/18/158/25/15

9/1/159/8/15

9/15/15

μg/m3

Magnesium - WFM007

Magnesium - WFM105

Sodium - WFM007

Sodium - WFM105

Potassium - WFM007

Potassium - WFM105
CASTNET 2015 Annual Report

67 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Figure 10-10 Time Series of Weekly Filter Pack NH+ 

4 Concentrations Measured at Eight 

CASTNET Sites

Figure 10-11 Time Series of Weekly Filter Pack SO2-

4 Concentrations Measured at Eight 

CASTNET Sites

0.0

0.5

1.0

1.5

2.0

2.5

6/2/156/9/15

6/16/156/23/156/30/15

7/7/15

7/14/157/21/157/28/15

8/4/15

8/11/158/18/158/25/15

9/1/159/8/15

9/15/159/22/15

μg/m3

WFM105

WFM007

CAT175

HWF187

NIC001

UND002

WST109

CTH110

0.0

1.0

2.0

3.0

4.0

5.0

6/2/156/9/15

6/16/156/23/156/30/15

7/7/15

7/14/157/21/157/28/15

8/4/15

8/11/158/18/158/25/15

9/1/159/8/15

9/15/159/22/15

μg/m3

WFM105

WFM007

CAT175

HWF187

NIC001

UND002

WST109

CTH110
CASTNET 2015 Annual Report

68 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Figure 10-12 Time Series of Weekly Filter Pack Total NO3 Concentrations Measured at Eight 

CASTNET Sites

Figure 10-13 Time Series of Biweekly NH3 Concentrations measured at Seven AMoN Sites

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

6/2/156/9/15

6/16/156/23/156/30/15

7/7/15

7/14/157/21/157/28/15

8/4/15

8/11/158/18/158/25/15

9/1/159/8/15

9/15/159/22/15

μg/m3

WFM105

WFM007

CAT175

HWF187

NIC001

UND002

WST109

CTH110

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

11/20/12

2/20/135/20/138/20/13

11/20/13

2/20/145/20/148/20/14

11/20/14

2/20/155/20/158/20/15

11/20/15

2/20/165/20/16

μg/m3

NY98

NY20

NY94

VT99

NY67

NH02

NY16

Linear

(NY98)

Linear

(NY67)
CASTNET 2015 Annual Report

69 Chapter 10: Cloud Water, Filter Pack Pollutant, and Ammonia 

Concentrations at Whiteface Mountain, NY

Figure 10-14 Time Series of Biweekly NH3 Concentrations at AMoN Sites NY67 and NY16

Figure 10-15 Comparison of AMoN Ambient NH 

3 Concentrations (left axis) and NTN 

Precipitation NH+ 

4 Concentrations (right axis) at Marble Mountain Lodge

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

10/30/07

3/30/088/30/081/30/096/30/09

11/30/09

4/30/10

9/30/102/28/117/31/11

12/31/11

5/31/12

10/31/12

3/31/138/31/131/31/146/30/14

11/30/14

4/30/159/30/152/29/16

μg/m3

NY67

NY16

Linear

(NY67)

Linear

(NY16)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

11/20/12

1/20/133/20/135/20/137/20/139/20/13

11/20/13

1/20/143/20/145/20/147/20/149/20/14

11/20/14

1/20/153/20/155/20/157/20/159/20/15

11/20/15

μg/m

mg/L

3

AMoN

NTN
70 Chapter 11: Atmospheric Deposition of Sulfur, 

Base Cations, and Chloride

Chapter 11

Chapter 11

Atmospheric Deposition of Sulfur, Base Cations, and Chloride

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

chloride across the United States. EPA used the NADP Total Deposition Hybrid Method to assess the status of dry and 

wet deposition for 2015. Total deposition was calculated as the sum of estimated dry and wet deposition.

Gaseous and particulate sulfur pollutants, base cations, and chloride are deposited to the 

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

estimate the rate of dry deposition of these compounds from the atmosphere to sensitive ecosystems. 

The NADP TDEP Hybrid Method (EPA, 2015c; Schwede and Lear, 2014) was used to estimate dry 

and wet deposition. Dry and wet deposition rates were summed to obtain total deposition across the 

United States.

Sulfur Deposition

Figure 11-1 shows a map of TDEP dry deposition rates of sulfur (S) for 2015. Estimated rates of total 

deposition of sulfur for 2015 are given in Figure 11-2. The magnitude of the deposition fluxes is 

illustrated by the shading in the figure legends. The percentage of total deposition of S due to dry 

deposition is shown in Figure 11-3.

Deposition of Base Cations and Chloride

A map of 2015 estimated total deposition of base cations (Ca2+

 , K+ , Mg2+ , and Na+ ) is given in 

Figure 11-4. Figure 11-5 provides estimated rates for 2015 for total deposition of Cl-

.

Sand Mountain, AL (SND152)
CASTNET 2015 Annual Report 

71 Chapter 11: Atmospheric Deposition of Sulfur, 

Base Cations, and Chloride 

Figure 11-1 TDEP Dry Deposition Estimates of S (kg ha-1yr-1) for 2015

Source: CASTNET/CMAQ/NTN/SEARCH

Figure 11-2 TDEP Total Deposition Estimates of S (kg ha-1

yr-1) for 2015

Source: CASTNET/CMAQ/NTN/SEARCH
CASTNET 2015 Annual Report 

72 Chapter 11: Atmospheric Deposition of Sulfur, 

Base Cations, and Chloride 

Figure 11-3 TDEP Percent of Total Deposition of S from Dry Deposition for 2015

Source: CASTNET/CMAQ/NTN/SEARCH

Figure 11-4 TDEP Total Deposition Estimates of the Base Cations Ca2+

 , K+ , Mg2+ , Na+ 

 

(kiloequivalents per hectare per year) for 2015

Source: CASTNET/CMAQ/NTN/SEARCH
CASTNET 2015 Annual Report 

73 Chapter 11: Atmospheric Deposition of Sulfur, 

Base Cations, and Chloride 

Figure 11-5 TDEP Total Deposition Estimates of Total Cl-(kg ha-1yr-1) for 2015

Source: CASTNET/CMAQ/NTN/SEARCH

Speedwell, TN (SPD111) 
74 Chapter 12: Improved Forest Health in the Eastern United States

Chapter 12

Chapter 12

Improved Forest Health in the Eastern United States

Reduced power plant and industrial combustion emissions implemented under the Clean Air Act have resulted in 

improved air quality and health of forest ecosystems over the last 30 years. CASTNET data and data from other 

monitoring programs, which are archived and documented in the EPA AQS, were used to assess the extent to which 

ambient concentrations of ozone and total nitrate have declined and to understand the corresponding changes in forest 

ecosystem health. Ambient ozone can enter a plant through stomatal pores in leaves and cause significant cellular 

damage that may compromise the plant’s photosynthetic capacity leading to reduced growth and/or reproduction of 

plants. Ozone stress also increases the susceptibility of plants to disease, insects, fungus, and other environmental 

stresses (e.g., harsh weather). Ozone is formed primarily from photochemical reactions between volatile organic 

compounds and nitrogen oxides. Thus, reducing nitrogen oxides emissions would in turn lower ground-level ozone 

concentrations and help to improve forest ecosystem health. This chapter discusses how nitrogen oxides emission 

reduction programs have resulted in improved air quality, decreased ozone levels, and improved forest health.

Methods

Ground-level O 

3 has been shown in numerous studies to have a strong effect on the health of many 

plants, including a variety of commercial and ecologically important forest tree species throughout the 

United States. (Ollinger et al., 1997; EPA, 2007). Assessing the impact of ground-level O 

3 on forests 

in the eastern United States involved analyzing the risk to tree species from ambient O 

3

concentrations and accounting for the prevalence of those species within a forest. As a way to 

quantify the risk to particular trees, concentration-response (C-R) functions, which relate O 

3 exposure 

to tree response, were calculated (EPA, 2014b). Tree seedling C-R functions were determined by 

exposing tree seedlings to different O 

3 levels and measuring reductions in growth as “biomass loss.” 

In areas where certain species dominate the forest community, the biomass loss from O 

3 can be 

significant. In this analysis, biomass loss was used as an indicator for the effects of O 

3 on the forest 

ecosystem. 

Common tree species in the eastern United States that are sensitive to O 

3 include black cherry 

(Prunus serotina), yellow poplar (Liriodendron tulipifera), sugar maple (Acer saccharum), eastern 

white pine (Pinus strobus), Virginia pine (Pinus virginiana), red maple (Acer rubrum), and quaking 

aspen (Populus trenuloides; Chappelka and Samuelson, 1998). To estimate the biomass loss for 

forest ecosystems across the eastern United States, the biomass loss for each of the seven tree 

species was calculated using the 3-month, 12-hour W126 exposure metric (https://www.epa.gov/airquality-analysis/ozone-w126-index) at each monitoring site, along with each tree’s individual C-R 

functions. The W126 exposure metric is a biologically based, cumulative exposure index that places a 

greater weight on the higher hourly O 

3 concentrations (Heck and Cowling, 1997). The W126 exposure 

metric was calculated using O 

3 data from the rural CASTNET and urban AQS networks and averaged 
CASTNET 2015 Annual Report

75 Chapter 12: Improved Forest Health in the Eastern United States

over a 3-year period to mitigate the effect of variations in meteorological and soil moisture conditions. 

The biomass loss estimate for each species was then multiplied by its prevalence in the forest 

community using the U.S. Department of Agriculture’s Forest Service IV index of tree abundance 

calculated from Forest Inventory and Analysis measurements (Prasad and Iverson, 2003). This 

analysis compared two time periods, 2000–2002 (e.g., beginning of NOx Budget Trading Program) 

and 2012–2014 (most recent modeling years) and demonstrates the benefit of decreasing O 

3

concentrations to forest ecosystems.

Results

Ambient Total Nitrate and Ozone Concentrations

Based on eastern reference site data from 1989 through 2014, the annual mean total NO3

concentration declined 45 percent from 3.1 ppb to 1.7 ppb in the eastern United States. (Figure 12-1 

and Chapter 4). Rural DM8A O 

3 concentrations decreased from 85 ppb to 66 ppb (22 percent) from 

2000–2002 to 2012–2014 in the eastern United States (Figure 12-2 and Figure 2-3 in Chapter 2).

Figure 12-1 Annual Mean Total NO3 Concentrations, 1989–1991 versus 2012–2014

Source: https://www3.epa.gov/castnet/maps/airconc.html

Figure 12-2 Three-year Average of Fourth Highest DM8A O 

3 Concentrations, 2000–2002 versus 

2012–2014

Source: https://www3.epa.gov/castnet/maps/airconc.html
CASTNET 2015 Annual Report

76 Chapter 12: Improved Forest Health in the Eastern United States

Effects of Decreased Ozone Pollution on Forest Biomass

Forest areas with significant biomass loss (EPA, 2007)1 because of O 

3 exposure decreased from 

33 percent to 5 percent from 2000–2002 to 2012–2014 (Figure 12-3) for all seven tree species that 

were modeled across their ranges in the eastern United States. Of these, black cherry and yellow 

poplar are the most sensitive to O 

3. Comparing data from 2000–2002 versus 2012–2014, total land 

area in the region with significant biomass loss decreased from 15.3 to 5.1 percent for black cherry 

and from 3.1 to 0.0 percent for yellow poplar. Significant biomass loss for the remaining five species 

(red maple, sugar maple, quaking aspen, Virginia pine, and eastern white pine) is zero. This is in 

contrast to the period 2000–2002, when 34 percent of range loss could be attributed to O 

3 exposure. 

While this modeled improvement in forest health cannot be exclusively attributed to Clean Air Actproduced NOx emission reduction programs, e.g., NOx Budget Trading Program and Clean Air 

Interstate Rule, it is likely that NOx emission reduction programs and the corresponding decreases in 

O 

3 air quality contributed to the reduction in forest biomass loss.

Figure 12-3 Estimated Black Cherry, Yellow Poplar, Sugar Maple, Eastern White Pine, Virginia 

Pine, Red Maple, and Quaking Aspen Biomass Loss due to O 

3 Exposure, 

2000–2002 versus 2012–20014

Max = 23% Max = 11%

2000–2002 2012–2014

Source: Lynch et al., 2016

 

1 Areas with more than 2 percent biomass loss are defined here as significant based on a consensus workshop on O 

3 effects, which 

reported that a 2 percent annual biomass loss causes harm because of the potential for compounding effects over multiple years as shortterm negative effects on seedlings affect long-term forest health (Heck and Cowling, 1997).
77 References

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CASTNET 2015 Annual Report

Appendix A

Appendix A

Locational and Operational Characteristics of CASTNET Sites
CASTNET 2015 Annual Report

A-1 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites (1 of 3)

Site ID Site Name Start date Latitude Longitude Elevation

Nearby

NADP

Land 

Use Terrain Agency Reference

ABT147, CT Abington 12/28/1993 41.8405 -72.0104 202 CT15 Urban/A

gric

Rolling EPA

ACA416, ME Acadia NP 12/1/1998 44.3771 -68.2608 158 ME98 Forest Rolling NPS

ALC188, TX AlabamaCoushatta4/6/2004 30.7016 -94.674 105 TX10 Prairie Rolling EPA

ALH157, IL Alhambra 6/28/1988 38.869 -89.6228 164 IL46 Agric Flat EPA E

ANA115, MI Ann Arbor 6/28/1988 42.4166 -83.9022 266 MI52 Forest Flat EPA E

ARE128, PA Arendtsville 6/28/1988 39.9232 -77.3079 266 PA00 Agric Rolling EPA E

ASH135, ME Ashland 12/20/1988 46.6038 -68.4132 231 ME00 Agric Flat EPA E

BAS601, WY Basin 11/6/2012 44.28 -108.0411 1242 MT00 Prairie Rolling BLM

BBE401, TX Big Bend NP 7/18/1995 29.3027 -103.1778 1052 TX04 Forest Complex NPS W

BEL116, MD Beltsville 11/1/1988 39.0282 -76.8171 47 MD99 Range Flat EPA E

BFT142, NC Beaufort 12/28/1993 34.8847 -76.6207 5 NC06 Agric Flat EPA

BUF603, WY Buffalo 11/6/2012 44.1442 -106.1089 1324 WY99 Prairie Rolling BLM

BVL130, IL Bondville 2/9/1988 40.052 -88.3725 213 IL11 Agric Flat EPA E

BWR139, MD Blackwater NWR 7/4/1995 38.445 -76.1113 1 MD15 Forest/

Marsh

Coastal EPA

CAD150, AR Caddo Valley 10/4/1988 34.1793 -93.0988 78 AR03 Forest Complex EPA E

CAN407, UT Canyonlands NP 1/24/1995 38.4583 -109.8213 1809 UT09 Desert Complex NPS W

CAT175, NY Claryville 5/10/1994 41.9423 -74.552 754 NY68 Forest Complex EPA

CDR119, WV Cedar Creek 11/10/1987 38.8795 -80.8477 240 WV05 Forest Complex EPA E

CDZ171, KY Cadiz 1/5/1999 36.7841 -87.8502 190 KY99 Agric Rolling EPA

CHA467, AZ Chiricahua NM 4/25/1989 32.0094 -109.3891 1570 AZ98 Range Complex NPS W

CHE185, OK Cherokee Nation 4/2/2002 35.7508 -94.6698 305 AR27 Agric Rolling EPA

CKT136, KY Crockett 8/24/1993 37.9215 -83.0663 376 KY35 Agric Rolling EPA

CND125, NC Candor 9/25/1990 35.2633 -79.8375 172 NC36 Forest Rolling EPA E

CNT169, WY Centennial 5/9/1989 41.3645 -106.24 3175 WY95 Forest Complex EPA W

COW005, NC Coweeta 

Screwdriver 

Knob

11/18/2014 35.0469 -83.4531 960 NC25 Forest Complex EPA

COW137, NC Coweeta 11/4/1987 35.0605 -83.4303 683 NC25 Forest Complex EPA E

CTH110, NY Connecticut Hill 9/29/1987 42.4009 -76.6535 511 NY67 Forest Rolling EPA E

CVL151, MS Coffeeville 12/27/1988 34.0027 -89.7992 138 MS30 Forest Rolling EPA

DCP114, OH Deer Creek 9/28/1988 39.6359 -83.2606 264 OH54 Agric Rolling EPA E

DEN417, AK Denali NP 10/6/1998 63.7232 -148.9676 661 AK03 Forest Complex NPS

DIN431, UT Dinosaur NM 11/20/2013 40.4373 -109.3046 1463 CO15 Desert Complex NPS

EGB181, ON Egbert 12/27/1994 44.2311 -79.7831 227 NY10 Agric Rolling EPA

ESP127, TN Edgar Evins 3/22/1988 36.0389 -85.7331 302 KY10 Forest Rolling EPA E

EVE419, FL Everglades NP 10/6/1998 25.3912 -80.6808 2 FL11 Forest/

Marsh

Flat NPS

FOR605, WY Fortification 

Creek

5/21/2013 44.3395 -105.9198 1408 WY99 Prairie Rolling BLM

GAS153, GA Georgia Station 6/28/1988 33.1812 -84.4101 265 GA41 Agric Rolling EPA E

GLR468, MT Glacier NP 12/27/1988 48.5103 -113.9968 976 MT05 Forest Complex NPS W

GRB411, NV Great Basin NP 5/16/1995 39.0051 -114.2159 2060 NV05 Forest Complex NPS W

GRC474, AZ Grand Canyon 

NP

5/16/1989 36.0586 -112.1836 2073 AZ03 Forest Rolling NPS W

GRS420, TN Great Smoky 

NP- Look Rock

10/16/1998 35.6335 -83.9416 793 TN11 Forest Complex NPS
CASTNET 2015 Annual Report

A-2 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites (2 of 3)

Site ID Site Name Start date Latitude Longitude Elevation

Nearby 

NADP

Land 

Use Terrain Agency Reference

GTH161, CO Gothic 5/16/1989 38.9563 -106.9859 2915 CO10 Range Complex EPA W

HOW191, ME Howland 

Ameriflux

9/27/2011 45.204 -68.74 68 ME09 Forest Rolling EPA

HOX148, MI Hoxeyville 10/31/2000 44.1809 -85.739 297 MI53 Forest Flat EPA

HWF187, NY Huntington 

Wildlife Forest

5/28/2002 43.973 -74.2233 497 NY20 Forest Complex EPA

IRL141, FL Indian River 

Lagoon

7/9/2001 27.8492 -80.4556 2 FL99 Coastal/

Marsh

Flat/

Water

EPA

JOT403, CA Joshua Tree NP 2/16/1995 34.0696 -116.3889 1244 CA67 Desert Complex NPS W

KEF112, PA Kane 

Experimental 

Forest

1/3/1989 41.5981 -78.7679 618 PA29 Forest Rolling EPA E

KIC003, KS Kickapoo Tribe 2/18/2014 39.8539 -95.6578 367 KS97 Prairie Rolling EPA

KNZ184, KS Konza Prairie 3/26/2002 39.1022 -96.6096 346 KS31 Prairie Flat EPA

LAV410, CA Lassen Volcanic 

NP

7/25/1995 40.54 -121.5765 1756 CA96 Forest Complex NPS W

LRL117, PA Laurel Hill 12/15/1987 39.9883 -79.2516 609 PA83 Forest Complex EPA E

MAC426, KY Mammoth Cave 

NP

7/24/2002 37.1314 -86.1481 243 KY10 Agric Rolling NPS

MCK131, KY Mackville 7/31/1990 37.7047 -85.0487 293 KY03 Agric Rolling EPA E

MCK231, KY Mackville 

Co-located

12/29/1992 37.7047 -85.0487 293 KY03 Agric Rolling EPA

MEV405, CO Mesa Verde NP 1/10/1995 37.1984 -108.4905 2165 CO99 Forest Complex NPS W

MKG113, PA M.K. Goddard 1/12/1988 41.4268 -80.1452 377 NY10 Forest Rolling EPA E

NEC602, WY Newcastle 11/7/2012 43.873 -104.1919 1468 WY99 Prairie Rolling BLM

NIC001, NY Nicks Lake 11/20/2012 43.6805 -74.9891 525 NY29 Forest Rolling NYSDEC

NPT006, ID Nez Perce Tribe 12/15/2015 46.2756 -116.0216 945 Forest Rolling EPA

OXF122, OH Oxford 8/18/1987 39.5311 -84.7235 284 OH09 Agric Rolling EPA E

PAL190, TX Palo Duro SP 4/24/2007 34.8806 -101.6647 1053 TX02 Prairie Complex EPA

PAR107, WV Parsons 1/19/1988 39.0904 -79.6617 510 WV18 Forest Complex EPA E

PED108, VA Prince Edward 11/3/1987 37.1652 -78.3071 149 VA24 Forest Rolling EPA E

PET427, AZ Petrified Forest 9/12/2002 34.8225 -109.8925 1723 AZ97 Desert Flat NPS

PIN414, CA Pinnacles NM 5/16/1995 36.4832 -121.1569 335 CA66 Forest Complex NPS W

PND165, WY Pinedale 12/27/1988 42.929 -109.7878 2386 WY06 Forest Rolling EPA W

PNF126, NC Cranberry 12/27/1988 36.1054 -82.045 1216 NC45 Forest MountaintopEPA E

PRK134, WI Perkinstown 9/27/1988 45.2065 -90.5972 462 WI35 Agric Rolling EPA E

PSU106, PA Penn State 1/6/1987 40.7209 -77.9318 364 PA42 Agric Rolling EPA E

QAK172, OH Quaker City 1/5/1999 39.9427 -81.3379 371 OH49 Agric Rolling EPA

RED004, MN Red Lake Nation 8/26/2014 47.8638 -94.8352 372 MN16 Forest Flat EPA

ROM206, CO Rocky Mountain 

NP Co-located

7/3/2001 40.2781 -105.5456 2742 CO19 Forest Complex EPA

ROM406, CO Rocky Mountain 

NP

12/20/1994 40.2781 -105.5456 2743 CO19 Forest Complex NPS W

SAL133, IN Salamonie 

Reservoir

6/28/1988 40.816 -85.6614 250 IN20 Agric Flat EPA E

SAN189, NE Santee Sioux 7/5/2006 42.8292 -97.8541 434 SD99 Range Rolling EPA

SEK430, CA Sequoia & Kings 

Canyon NP - Ash 

Mountain

4/7/2005 36.4895 -118.8292 510 CA75 Forest MountaintopNPS
CASTNET 2015 Annual Report

A-3 Appendix A

Table A-1 Locational and Operational Characteristics of CASTNET Sites (3 of 3)

Site ID Site Name Start date Latitude Longitude Elevation

Nearby 

NADP

Land 

Use Terrain Agency Reference

SHE604, WY Sheridan 11/6/2012 44.93 -106.85 1115 MT00 Prairie Rolling BLM

SHN418, VA Shenandoah NP 

- Big Meadows

6/28/1988 38.5231 -78.4347 1073 VA28 Forest MountaintopNPS E

SND152, AL Sand Mountain 12/27/1988 34.289 -85.9701 349 AL99 Agric Rolling EPA E

SPD111, TN Speedwell 6/12/1989 36.4698 -83.8265 361 TN04 Agric Rolling EPA E

STK138, IL Stockton 12/28/1993 42.2872 -90 281 IL18 Agric Rolling EPA

SUM156, FL Sumatra 12/27/1988 30.1102 -84.9904 16 FL23 Forest Flat EPA E

THR422, ND Theodore 

Roosevelt NP

10/6/1998 46.8948 -103.3777 850 ND00 Forest Rolling NPS

UND002, VT Underhill 11/13/2012 44.5283 -72.8688 399 VT99 Forest Complex EPA

UVL124, MI Unionville 6/28/1988 43.6136 -83.3599 202 MI51 Agric Flat EPA E

VIN140, IN Vincennes 8/4/1987 38.7408 -87.4849 136 IN22 Agric Rolling EPA E

VOY413, MN Voyageurs NP 6/13/1996 48.4125 -92.8292 429 MN32 Forest Rolling NPS

VPI120, VA Horton Station 6/2/1987 37.3298 -80.5575 920 VA13 Agric MountaintopEPA E

WFM007, NY Whiteface 

Mountain Summit

6/4/2015 44.36608 -73.90312 1415 NY98 Forest Complex EPA

WFM105, NY Whiteface 

Mountain

11/20/2012 44.39 -73.86 570 NY98 Forest Complex NYSDEC

WNC429, SD Wind Cave NP 11/20/2003 43.5576 -103.4839 1292 SD04 Prairie Rolling NPS

WSP144, NJ Washington

Crossing

12/27/1988 40.3123 -74.8727 59 NJ99 Range Rolling EPA E

WST109, NH Woodstock 12/27/1988 43.9445 -71.7008 255 NH02 Forest Complex EPA E

YEL408, WY Yellowstone NP 6/26/1996 44.5654 -110.4003 2400 WY08 Agric Rolling NPS W

YOS404, CA Yosemite NP -

Turtleback Dome

9/25/1995 37.7133 -119.7062 1605 CA99 Forest Complex NPS W

Note: NM = National Monument

NP = National Park

NWR = National Wildlife Reserve

SP = State Park

E = eastern reference site

W = western reference site
CASTNET 2015 Annual Report

Appendix B

Appendix B

Acronyms and Abbreviations
CASTNET 2015 Annual Report

B-1 Appendix B

List of Acronyms and Abbreviations

A2LA American Association for Laboratory Accreditation

Amec Foster Wheeler Amec Foster Wheeler Environment & Infrastructure, Inc.

AMNet Atmospheric Mercury Network

AMoN Ammonia Monitoring Network 

AQS EPA’s Air Quality System

ARP Acid Rain Program

ASRC Atmospheric Sciences Research Center

BLM Bureau of Land Management

C-R concentation-response

Ca2+ particulate calcium 

CAAA Clean Air Act Amendments

CAMx Comprehensive Air Quality Model with Extensions

CAPMoN Canadian Air and Precipitation Monitoring Network

CASTNET Clean Air Status and Trends Network

CFR Code of Federal Regulations

Cl- particulate chloride 

CMAQ Community Multiscale Air Quality Modeling System

CO carbon monoxide

CSAPR Cross-State Air Pollution Rule

DM8A daily maximum 8-hour average

DQI data quality indicator

ECAN Environment Canada

EGUs electric generating units

EPA U.S. Environmental Protection Agency

Hg mercury

HNO 

3 nitric acid

HONO nitrous acid

HYSPLIT Hybrid Single Particle Lagrangian Integrated Trajectory model

IEC International Electrotechnical Commission

IMPROVE Interagency Monitoring of Protected Visual Environments

ISO International Organization for Standardization

K+ particulate potassium

kg ha-1 yr-1 kilograms per hectare per year

km kilometer

m meter

MADPro Mountain Acid Deposition Program

MARGA Monitor for Aerosols and Gases in Ambient Air

MARPD mean absolute relative percent difference
CASTNET 2015 Annual Report

B-2 Appendix B

List of Acronyms and Abbreviations (continued)

MCCP Mountain Cloud Chemistry Project

MDL method detection limit

MDN Mercury Deposition Network

Mg2+

 particulate magnesium 

mg/L milligrams per liter

MRPD mean relative percent difference

MU MARGA unit

N nitrogen

Na+ particulate sodium 

NAAQS National Ambient Air Quality Standards

NADP National Atmospheric Deposition Program

NAPAP National Acid Precipitation and Assessment Program

NCore EPA’s National Core Monitoring

NH 

3 ammonia

NH +

4 particulate ammonium

NH 

4NO 3 ammonium nitrate

NO nitrogen oxide

NO 

2 nitrogen dioxide

NO -

3 particulate nitrate 

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

2)]

NOy total reactive oxides of nitrogen

NOz HNO 

3, nitrous oxide, peroxyacetyl nitrate, peroxyproyl nitrate, other organic 

nitrates, and nitrite

NPS National Park Service

NTN National Trends Network

NYSDEC New York State Department of Environmental Conservation

O 

3 ozone

PFA pulsed fluorescence analyzer 

PM particulate matter

PM2.5 fine particulate matter

ppb parts per billion

ppm parts per million

PRISM Parameter-elevation Regressions on Independent Slopes Model

PT proficiency testing

QA quality assurance

QAPP Quality Assurance Project Plan

QC quality control
CASTNET 2015 Annual Report

B-3 Appendix B

List of Acronyms and Abbreviations (continued)

R2 coefficient of determination

S sulfur

SCR selective catalytic reduction 

SEARCH Southeastern Aerosol Research and Characterization Network

SO 

2 sulfur dioxide

SO2-

4 particulate sulfate

TDEP NADP Total Deposition Hybrid Method

total NO3 gaseous nitric acid (HNO 

3) + particulate nitrate (NO3)

TSP total suspended particles

USDA U.S. Department of Agriculture

μeq/L microequivalents per liter

μg/m3 micrograms per cubic meter

WARMS Wyoming Air Resources Monitoring System