2020
Clean Air Status and Trends Network
Five Year Network Assessment
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Clean Air Markets Division
Office of Atmospheric Programs
US Environmental Protection Agency
7/1/2020
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Table of Contents
1. Introduction 2
A. Purpose 2
B. CASTN ET Objectives 2
C. Network Overview 4
D. CASTN ET Partners 6
E. CASTNET Ozone Monitoring Program 7
F. Network Modifications for Regulatory Ozone Monitoring 10
2. Monitoring Results 11
A. Ambient Ozone Concentrations 11
B. W126 12
C. Ozone Trends 13
3. Quality assurance 15
A. Overview 15
B. Precision 15
C. Bias 17
D. Accuracy 18
1. Semi-Annual Site Visits 18
2. Independent PE Results 19
E. Completeness 20
4. Precursor Measurements and Meteorology 20
A. NOy Monitoring 20
B. CASTNET meteorology 20
5. Summary 22
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Five Year Network Assessment
1. Introduction
A. Purpose
Monitoring agencies that submit data to the U.S. Environmental Protection Agency (EPA) for regulatory
purposes are required to conduct an assessment of their air quality surveillance system once every five
years. This network assessment was performed to ensure the Clean Air Status and Trends Network
(CASTNET) meets the requirements in 40 CFR Part 58.10(d). The purpose of the assessment is to
determine, at a minimum, whether the network meets the monitoring agencies' objectives. The focus of
this assessment is on the CASTNET monitoring program from 2015 to 2019. The assessment includes a
review of the network's effectiveness in reporting trends and regional concentrations of 03, S02, CO,
and oxides of nitrogen and recommendations to improve network performance as CASTNET adapts to
meet agency objectives.
B. CASTNET Objectives
CASTNET is a long-term monitoring network designed to measure trends in regional measurements of
acidic pollutants, base cations, chloride, and ambient 03. The Environmental Protection Agency - Clean
Air Markets Division (EPA), the National Park Service - Air Resources Division (NPS), and the Bureau of
Land Management - Wyoming State Office (BLM-WSO) collaboratively manage and operate CASTNET. In
addition to EPA, NPS, and BLM-WSO, numerous other participants including Tribes, other federal
agencies, states, private landowners, and universities provide network support. CASTNET monitors
provide critically important, regionally representative data used to provide air quality trends, estimate
background 03 concentrations, and evaluate air quality models largely in the absence of local emissions
(Brown-Steiner et al., 2018; Kerr et al., 2019; Reider et al., 2018; Travis and Jacob, 2019). Additionally,
CASTNET 03 data are used to evaluate the effectiveness of national and regional emission reduction
control programs, gauge compliance with National Ambient Air Quality Standards (NAAQS), and provide
input into regional air quality and total deposition models. Lastly, CASTNET 03 data are also used to
assess impacts from stratospheric intrusions and wildfires (Hogrefe et al., 2018; Itahashi et al., 2020; and
Lin et al., 2015).
CASTNET currently operates 98 monitoring stations throughout the contiguous United States, Alaska,
and Canada. EPA operates 63 CASTNET monitoring stations, NPS operates 30 CASTNET stations, and
BLM-WSO operates five CASTNET stations. Thirty years of consistent, long-term measurements reported
by CASTNET demonstrate reductions in 03, nitrogen, and sulfur concentrations throughout the United
States. Additionally, continuous trace-level gas monitoring for sulfur dioxide (S02), nitrogen oxide/total
reactive oxides of nitrogen (NO/NOy), and carbon monoxide (CO) is ongoing at three, eight, and three
CASTNET sites, respectively.
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CASTNET's three operating agencies, EPA, IMPS, and BLM-WSO coordinate their resources to fulfill the
following goals:
• monitor the status and trends in regional air quality and atmospheric deposition;
• provide information on the contribution of atmospheric pollution to ecosystem conditions; and
• provide measurements for validating and improving atmospheric models
Each operating agency also utilizes CASTNET to fulfill their own monitoring objectives. Specific examples
are described below.
CASTNET data are used by EPA to evaluate the effectiveness of national and regional emission reduction
programs and to determine compliance with the 03 NAAQS. EPA uses these consistent, long-term
measurements for determining relationships between changes in emissions and subsequent changes in
air quality, atmospheric deposition, and ecological effects. Under Title IV of the Clean Air Act
Amendments (CAAA), the Acid Rain Program (ARP) was promulgated to reduce emissions of sulfur
dioxide (S02) and nitrogen oxides (NOx) from electric generating units (EGUs). A critical component of
the CAAA required the EPA to conduct research, monitoring, and analysis of air pollutant trends and
their effects on terrestrial and aquatic ecosystems. CASTNET data support this legislative requirement.
The NPS uses CASTNET monitoring data to assess environmental conditions and trends in 03, sulfur, and
nitrogen deposition, and also to assess compliance with the 03 NAAQS. Coupled with special studies
data, this information allows the NPS to understand how air pollutants are currently impacting park air
quality and air quality related values (AQRVs). These data help the NPS and the public understand which
parks are at highest risk for impacts, and where conditions of park air quality and AQRVs are declining or
improving. Specifically, ambient measurements of 03, NOx, and S02 concentrations, deposition, and
effects on visibility, soils, waters, and plants are critical components of periodic assessments. For
example, 03 and vegetation data from Sequoia and Yosemite National Parks have been used to
document the concentrations at which 03 pollution causes damage to Ponderosa pine trees.
The BLM-WSO uses CASTNET data to identify air quality concerns and evaluate air strategy
effectiveness. These data also fulfill air monitoring commitments in Resource Management Plans
(RMPs) and Records of Decisions (RODs). Lastly, CASTNET data provide necessary information to assess
existing conditions, impacts of federal actions, and long-term trends in air quality and deposition on BLM
land.
While these CASTNET monitoring objectives go beyond the scope of this assessment, they are provided
here in brief to illustrate the utility and breadth of the data generated by CASTNET. In this assessment
we provide an overview of the CASTNET monitoring program, the sponsoring agencies' objectives for
the regulatory monitoring program, trends and annual results, quality assurance metrics, and the future
outlook for the program.
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From the CASTNET QAPP version 9.3:
Project Objective
Required Data
DQO
Estimate dry deposition
fluxes
Ambient concentration data for
sulfur species, nitrogen species and
O3 along with meteorological
parameters and
information on vegetation and land
use. CMAQ calculations of
unmeasured nitrogen species,
including nitrous acid (HONO),
nitrogen pentoxide (N2O5), nitric
oxide (NO), nitrogen dioxide (NO2),
organic nitrate (NTR), peroxyacyl
nitrate (PAN), aromatic PANs
(OPAN), and C3 and higher PANs
(PANX)
No standards or standard methods are available
to determine the accuracy of the CMAQ
deposition model. However, model evaluation
and intercomparison studies indicate that
TDep/CMAQ model simulates higher dry
deposition rates than MLM calculations. However,
year-by-year changes in aggregated deposition
rates were comparable for both modeling
systems and changes in SO2 and NOx-related
pollutants were comparable to changes in SO2
and NOx emissions. In order to better assess
model performance, the model output will have
to be compared to independent, multi-year flux
measurements.
Detect and quantify
seasonal and annual
trends in concentrations
and dry deposition
fluxes for sulfur species,
nitrogen species, and O3
10-year record of ambient
concentration and deposition data
To detect a minimum annual trend of 1.0 percent
in the concentration of selected measured and/or
modeled chemical species with 10 years of data
at a given site in the United States region with a
statistical confidence of 95 percent.
Define the spatial
distribution of
pollutants
Ambient concentration data for
sulfur species, nitrogen species and
O3 collected over a large number of
sites that constitute sufficient
geographic coverage. Gridded
CMAQ-modeled
concentrations of sulfur species,
nitrogen species, O3 and other
pollutants.
Spatial distributions of nationwide SO2, SCV2,
total nitrate, NH4+ and other pollutant
concentrations are produced by combining
CMAQ simulations with measured
concentrations over a specified (e.g., 12 km) grid
system.
C. Network Overview
CASTNET was established under the 1990 Clean Air Act Amendments, expanding the National Dry
Deposition Network (NDDN), which began in 1987. NPS began its participation with CASTNET in 1994
under an agreement with EPA. With the involvement of NPS, the network became a national, rather
than a primarily eastern, network. BLM-WSO began participation in CASTNET in late 2012, with
additional monitoring coverage provided in Wyoming. CASTNET was designed to provide accountability
for emission reduction programs by reporting trends in pollutant concentrations and acidic deposition.
To meet those goals, CASTNET site locations were selected in rural areas to provide regionally
representative concentrations and estimates of dry deposition fluxes. CASTNET has historically used the
Multi-Layer Model (MLM) to estimate dry deposition fluxes using measured concentrations, on-site
meteorology and site characteristics, including land use and vegetation, as input.
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In 2011, EPA-sponsored CASTNET sites discontinued on-site meteorological measurements at all except
five sites. Beginning with 2014 measurements the new total deposition (TDep) hybrid approach
(Schwede and Lear, 2014), which incorporates CMAQ output with air quality monitoring data, was used
for spatial analyses of dry and total deposition. The TDep approach is documented on EPA's total
deposition TDep FTP site
(ftp://newftp.epa.gov/castnet/tdep/Total_Deposition_Documentation_current.pdf). In summary, dry
deposition is determined as the product of the atmospheric concentration and the deposition velocity.
The deposition velocity is modeled in CMAQ using the electrical resistance paradigm where resistances
are defined along pathways from the atmosphere to the vegetation or surface and act in series and
parallel. Beginning in 2015, the TDep approach became the primary method to produce deposition
gridded surfaces. The CASTNET filterpack measurements provide weekly concentrations of gaseous
sulfur dioxide (S02) and nitric acid (HN03), and particulate sulfate (S042 ), nitrate (N03~), ammonium
(NH4+), base cations (Ca2+, K+, Mg2+, Na+), and chloride (CI ). A single laboratory, operated under contract
to the EPA, analyzes the filterpack samples for all CASTNET sites.
Figure 1 shows the locations of all CASTNET monitoring sites. Circles represent sites operating a
filterpack and a continuous 03 monitor. Continuous 03 concentrations are measured at 86 sites. Squares
represent sites with trace gas monitors operated by EPA or NPS. Sites at Bondville, IL (BVL130), Acadia
National Park, ME (ACA416), Great Smoky Mountains National Park - Look Rock, TN (GRS420), Cherokee
Nation, OK (CHE185), and Underhill, VT (UND002) are NCore sites with trace gas NO/NOy, S02, and CO.
Additionally, there are ten sites that operate without a walk-in shelter, represented by a triangle on the
map in Figure 1 (e.g., Small Footprint sites). Forty CASTNET sites measure hourly meteorological
parameters including all NPS sites, all BLM-WSO sites, and five EPA sites. Additional information and
data from the CASTNET monitoring program can be found on the CASTNET website at
https://www.epa.gov/castnet.
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Figure 1. Map of CASTNET sites (May 2020). Green shapes represent EPA-sponsored sites. Orange shapes
represent NPS-sponsored sites. The purple diamond represents a co-located pair of NPS-sponsored ozone
and filterpack monitoring and EPA-sponsored ozone, filterpack, and trace-level gas monitoring. Blue
shapes represent BLM-Wyoming State Office-sponsored sites. National Core network (NCore) sites are
identified with a large red circle.
D. CASTNET Partners
In addition to EPA, NPS, and BLM-WSO, numerous other participants including Tribes, other federal
agencies, states, private landowners, and universities provide network support (Table 1). CASTNET
partners may provide local operational support, space for shelters and equipment, or scientific
expertise. The EPA contractor, Wood Environment & Infrastructure Solutions, Inc. (Wood), manages the
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day-to-day operations for the EPA-sponsored sites while the NPS and BLM contractor, Air Resource
Specialists, Inc. (ARS), manages the operations for the remaining sites.
Table 1. CASTNET Program Partners
Program Partners
Federal
State/Local/Tribal
University
Allegheny National Forest (NF)
Alabama-Coushatta Tribe of Texas
Auburn University Alabama Agricultural
Apalachicola NF
Cedar Creek State Park WV Division of
Experiment Station
Environment Canada
Natural Resources
Cornell University, Ecology & Evolutionary
Gunnison NF
Cherokee Nation
Biology
Holly Springs NF
Cumberland St. Forest VA
KS State University (KSU) Division of
Hubbard Brook Experimental Forest
Department of Forestry
Biology/Konza Prairie Long-term ecological
Medicine Bow-Routt NF
Deer Creek State Park OH Dept. of
research (LTER)
Monongahela NF
Natural Resources (DNR)
Miami University Institute for the
Nantahala NF
Edgar Evans State Park TN Dept. of
Environment & Sustainability
National Park Service (NPS)
Environmental Conservation (DEC)
Ouachita Baptist University School of
United States Army Engineering
Laurel Hill State Park PA Dept. of
Natural Sciences
District/Louisville
Conservation & Natural Resources
Pennsylvania State University (PSU) Fruit
US Department of Agriculture (USDA)
(DCNR)
Research & Extension Center
Natural Resources Conservation
ME Dept. of Environmental
PSU Department of Meteorology
Service (NRCS)
Protection (ME DEP)
Proctor Maple Research Center (UVM)
USDA Agricultural Research Center
Maurice K. Goddard State Park (PA
Purdue University Department of
(ARS)
DCNR)
Agronomy
USDA Southern Research Station
New York DEC
State University of NY (SUNY) ESF
Coweeta Hydrological Lab
ND Department of Health
Adirondack Ecology Center
US Fish & Wildlife Service
NYS Energy Research & Development
Texas A&M Agrilife Research & Extension
USDA- Forest Service (FS) Timber &
Authority
Center
Watershed Lab
Rocky Mountain Biological Laboratory
University of GA, College of Agriculture &
USDA-FS Forestry Sciences Laboratory
Santee Sioux Tribe of Nebraska
Environmental Science
USDA-FS Rocky Mountain Research
St. Johns River Water Management
University of IL, Illinois State Water Survey
Station
District
University of Maine Plant, Soil &
USDA-FS Toecane District
SD Dept. of Environmental & Natural
Environmental Science
US Department of Interior (DOI)-
Resources (SD DENR)
University of MD Department of
Bureau of Land Management
Vermont DEC
Atmospheric & Oceanic Science
White Mountain NF
Washington Crossing State Park (NJ
University of Michigan School of Natural
DEP)
Resources
University of NC Institute of Marine
Sciences
VA Tech Department of Plant Pathology,
Physiology & Weed Science
E. CASTNET Ozone Monitoring Program
CASTNET operates 98 monitoring sites throughout the US and Canada and 86 of those sites measure
ground-level, continuous 03 following the regulations in the 40 Code of Federal Regulations (CFR) Part
58 and EPA's Quality Assurance Handbook for Air Pollution Measurement Systems: "Volume II: Ambient
Air Quality Monitoring Program" (US EPA, 2017). CASTNET 03 monitors are located in 38 states with at
least one CASTNET 03 monitor in each of the ten EPA Regions. Four CASTNET 03 sites are located on
Tribal Lands including Santee Sioux, NE (SAN189), Cherokee Nation, OK (CHE185), Alabama-Coushatta,
TX (ALC188), and Nez Perce, ID (NPT006).
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Eighty-six CASTNET sites collect ambient 03 concentrations, reported as hourly averages, using a dual
cell, ultraviolet photometric analyzer. Eighty-five of the eighty-six CASTNET 03 monitoring analyzers
meet the ambient monitoring and quality assurance requirements of Title 40, Code of Federal
Regulations (CFR) Part 58 Appendices A, C, D and E. The ozone analyzer at Duke Forest, NC (DUK008)
does not meet the siting criteria requirements from Appendix E of Part 58 because it has an inlet height
of 44 meters. Additional information about CASTNET siting criteria can be found in the Quality
Assurance Project Plan version 9.3 at http://iava.epa.gov/castnet/documents.do (Wood, 2019).
Figure 2 shows images of the typical configuration of a CASTNET site with the full suite of monitoring
equipment including the temperature-controlled shelter and a 10-m tipping tower. The 03 inlet is
located within the rain shield at the top of the 10-m tipping tower which also houses the CASTNET filter
pack. Two NPS-sponsored CASTNET sites, Wind Cave National Park, SD (WNC429) and Theodore
Roosevelt National Monument, ND (THR422), have 03 inlet heights at 3.35 m and 12.2 meters,
respectively. The 03 monitors at WNC429 and THR422 are managed by the respective state agencies.
Ambient temperature is measured at every CASTNET site.
CASTNET 03 analyzers, site transfer standards, data loggers, and computers are located within a
temperature-controlled shelter. The datalogger can be operated remotely to run manual quality
assurance (QC) checks, review status flags, or recover missing data. Each on-site transfer standard has
been verified against a National Institute of Standards and Technology (NIST) -traceable Level II transfer
standard.
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Figure 2. (a) CASTNET monitoring site
Pinedale, WY (PND165)
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Figure 2. (b) Ozone instrumentation, PC, and data logger inside a CASTNETshelter
Palo Duro, TX (PAL190)
F. Network Modifications for Regulatory Ozone Monitoring
The National Park Service established their regulatory 03 monitoring program prior to 1990. While the
NPS-sponsored 03 monitoring program was designed to meet 03 monitoring regulations from the
beginning, the EPA-sponsored 03 monitoring program was not. All EPA-sponsored 03 monitors were
upgraded by 2011 to comply with the requirements in 40 CFR Part 58. EPA replaced the existing 03
analyzers with a pair of Thermo Scientific™ Model 49i analyzers, where one analyzer has an onboard 03-
generator for use as an on-site transfer standard. CASTNET uses the monitoring quality objectives from
the Quality Assurance Handbook for Air Pollution Measurement Systems, Volume II, Appendix D (US
EPA, 2017) to ensure that the highest quality data are being submitted to EPA's Air Quality System (AQS)
database. The upgrade of the EPA-sponsored 03 analyzers resulted in an improved ability to evaluate
the quality of the ambient data. On-going improvements to site equipment and infrastructure are
posted to the individual CASTNET site information pages (https://www.epa.gov/castnet/castnet-site-
locations).
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Prior to being used for regulatory monitoring, zero, span, and precision (ZSP) checks of the 03 analyzer
at EPA-sponsored sites were performed every week; now all sites perform ZSP checks daily. In addition
to the daily ZSP QC checks, technicians perform semi-annual audits at each CASTNET site. During these
semi-annual visits, technicians audit the on-site analyzer, reverify the on-site transfer standard, calibrate
the on-site analyzer to the traveling transfer standard (Level 2) as needed, and verify the responses of
the data logger and shelter temperature probe with NIST-traceable standards. All on-site 03 transfer
standards at CASTNET sites are NIST-traceable at Level 3. Audit results are used to perform the final
validation on the hourly 03 data and validated data are submitted to the sponsoring agency.
As required by 40 CFR Part 58 Appendix A, an annual Performance Evaluation (PE) is conducted at each
CASTNET 03 site by an independent auditor. For most CASTNET sites the independent auditor is
Environmental Engineering & Measurement Services (EE&MS); however, some states act as an
independent auditor and perform PEs at CASTNET sites.
The validated hourly 03 concentrations are submitted monthly to AQS by the sponsoring agency's
contractor. Additionally, the daily 1-point precision checks are submitted quarterly to AQS for each site.
PE results are submitted to AQS routinely by the designated independent auditor. A subset of the
CASTNET partners act as the principal quality assurance organizations (PQAOs) - a unique role where
states and Tribes collect and own 03 data at CASTNET sites. This subset includes Acadia National Park,
ME (ACA416) submitted by Maine Department of Environmental Protection, Wind Cave National Park,
SD (WNC429) submitted by South Dakota Department of Environment and Natural Resources, Cherokee
Nation, OK (CHE185) submitted by Cherokee Nation Clean Air Program, and Theodore Roosevelt
National Monument, ND (THR422) submitted by North Dakota Department of Health.
2. Monitoring Results
A. Ambient Ozone Concentrations
CASTNET data provide an assessment tool for quantifying the improvements in air quality due to
regional and national emission reduction programs (e.g., the NOx Budget Trading Program, Clean Air
Interstate Rule, Cross State Air Pollution Rule, and the Cross State Air Pollution Rule Update).
CASTNET sites measure ambient 03 concentrations for the entire year, which extends beyond the
required 03 season for many states. Ozone concentrations from CASTNET are used to gauge compliance
with the primary NAAQS. Design values are used to designate and classify nonattainment areas, as well
as to assess progress towards meeting the NAAQS. The design values are based on the 3-year average of
the fourth highest daily maximum 8-hour average. Figure 4 depicts the 2016-2018 fourth highest daily
maximum 8-hour 03 average for all sites that met the completeness criteria (40 CFR Part 50, Appendix I).
Ozone concentrations are not included (shown as dots with no value) if the 3-year average was not
available because of incomplete data. In this map, exceptional event-impacted data are not excluded. In
2016-2018, eight sites exceeded the primary 03 standard of 70 ppb.
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Figure 4. Map of2016-2018 fourth highest daily maximum 8-hour ozone average
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B. W126
CASTNET also provides a unique dataset for evaluating the secondary NAAQS, which protect against
vegetation-related effects and other deleterious impacts to public welfare. The secondary 03 NAAQS is
currently set equal to the primary NAAQS. While the secondary NAAQS is currently set equal to the
primary NAAQS, the W126 index is often used to relate vegetation losses, such as reduced crop yield,
foliar injury, and decreased biomass accumulation, with 03 exposure. The W126 index is a cumulative
metric that sums weighted hourly 03 concentrations during the 03 season. The W126 is reported as the
maximum weighted monthly average during three consecutive months in the growing season when
daytime 03 concentrations are the highest and plant growth is most likely to be affected. CASTNET sites
are in rural areas and often in sensitive ecosystems where vegetation related effects are significant.
Figure 5 shows the W126 values from CASTNET sites in 2018.
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Figure 5. Maximum W126 value for 2018
Site not pictured:
DEN417, AK 1
C. Ozone Trends
For the purpose of reporting long-term regional trends, CASTNET sites are labeled as "western" or
"eastern" depending on whether they are west or east of 100 degrees west longitude (Figure 6). Eastern
long-term sites have been operating since at least 1990, while Western long-term sites have been
operating since at least 1996.
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Figure 6. CASTNET Western and Eastern Reference Sites
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Figure 7 shows the trends in ambient fourth highest eight hour daily maximum 03 concentrations from
1990-2018 (eastern sites) and 1996-2018 (western sites). The fourth highest eight hour daily maximum
03 data from the 34 Eastern reference sites show substantial reductions in concentrations since 2002.
The Eastern reference sites realized a 25% reduction between 2000-2002 and 2016-2018. In 2018, the
median fourth highest daily maximum 8-hour average for the Eastern reference sites was 64 ppb. The
western reference sites do not show the same dramatic reductions in 03 concentrations. There was a 9%
reduction in 03 concentrations as measured by the Western reference sites between 2000-2002 and
2016-2018. In 2018, the median fourth highest daily maximum 8-hour average was 69 ppb at the 16
western reference sites.
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Figure 7. Annual trends in fourth highest eight hour daily maximum ozone concentrations from the
eastern (left) arid western (right) CASTNETsites.
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3. Quality assurance
A, Overview
The purpose of the CASTNET quality assurance (QA) program is to ensure that all reported data are of
known and documented quality in order to meet the CASTNET objectives and to be reproducible and
comparable with data from other monitoring networks. The CASTNET OA program is managed by an
independent QA Manager and Project QA Supervisor. The QA manager routinely performs internal
systems audits, reviews concentration and audit data, and prepares QA reports to management.
The CASTNET QAPP revision 9.3 (Wood, 2019) is comprehensive and covers all aspects of the monitoring
program. The QAPP is reviewed and updated by the contractor annually. Details on field, data, and
laboratory operations, training, SOPs, system audits, and reporting are examples of information that can
be found in the QAPP (http://iava.epa.gov/castnet/documents.do).
CASTNET data quality indicators including precision, accuracy, bias, and completeness are reported by
the AMP256 QA Data Quality Indicator Report made available from the U.S. EPA Office of Air Quality
Planning and Standards' (OAQPS) Air Quality System (AQS). The AMP256 report provides annual
summaries of 1-point QC check and PE audit results to determine whether each analyzer being used for
comparison against the NAAQS meets the precision, accuracy, bias, and completeness requirements
from 40 CFR Part 58 Appendix A.
B. Precision
Ozone precision is reported as the 90 percent confidence limit (CL) of the coefficient of variation (CV) as
measured by the 1-point QC checks (40 CFR Part 58, Appendix A 4.1.2). The 1-pt QC check is the
difference between a known 03 concentration and the response of the 03 analyzer. For a site to meet
the acceptance criterion, the 90% CL of the CV must be < 7.1%. The analyzer is challenged with 60 ppb
of 03 during the daily 1-point QC check, which is considered representative of the ambient
concentrations measured within the network. Results from the 1-point QC checks are loaded into AQS
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quarterly. Additional data review is required for sites that do not meet the 7.1% criterion. Figure 9
reports the precision estimates for all CASTNET sites from 2015 to 2019 in the form of box plots from
the AMP256 report. In 2015, 2016, 2018, and 2019 all sites met the 7.1% acceptance criterion. In 2017,
all sites except Rocky Mountain National Park (ROM406, CO, 8.67%), Canyonlands National Park
(CAN407, UT, 12.72%), and Sequoia and Kings Canyon National Park (SEK430, CA, 8.20%) met the
acceptance criteria.
Figure 9. Box Plot showing all CASTNET ozone precision estimates for 2015-2019. The median values
(line) are shown for each year.
Ozons One Point QC Check Precision Estimate for All CASTNET Sites
12
10
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Figure 10. Relative percent difference between expected response and analyzer response for the 1-point
QC checks at all CASTNET sites. The median values (line) are shown for each year.
Ozone One Point QC Check Mean Relative Percent Difference for All CASTNET Sites
4
t 0
a
-1
¦u
a.
c
-3
-4
-5
2015
2016
2017
2018
2019
C. Bias
The bias estimate is also calculated using results from the 1-point QC checks. A site is required to meet a
95 percent CL of the absolute bias estimate (40 CFR Part 58 App A sec 4.1.3). A site meets the
acceptance criterion if the absolute bias is < 7.1%.
A positive or negative direction is assigned to the bias estimate when the signs of both the 25th and 75th
percentiles of the percent differences for each site are in the same direction. No direction is assigned if
the percentiles are of different signs. Signed bias results, by site, for 2015 through 2019 are shown in
Table 2. Sites have orange font if the bias estimate was positive, blue font if the bias estimate was
negative, and black font if the bias estimate had no sign. Each site met the 7.1% acceptance criterion for
2015 through 2019. Table 2 reports 1-point QC check bias results evaluated from the AMP256 report.
17 | Page
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Table 2. The bias estimate calculated from one-point QC checks for 2015 through 2019.
Ozone One Point QC Bias for Each CAST NET Site
Site ID
2015
2016
2017
201B
2019
Site ID
2015
2016
2017
2018
2019
BiasSgn
ABT147
1.571
1.172
0.720
1.384
IRL141
1.600
1082
1.453
0.9B1
1037
¦
Aiciaa
1185
1.577
1.314
1.260
1-B.3S
JOT403
2.282
1810
0.858
0.943
0.802
¦+
ALU 157
1.366
0.B39
1.284
1378
1595
KEF112
1.104
1253
¦ v-
ANA115
0.B53
i.948
2.466
LAV410
D.959
1.459
1.339
1.556
1-169
AREL28
1.D6Q
L.1B3
1.137
1.731
1940
LRL117
0.751
0.742
0.923
1051
1035
ASH 13 5
1.614
0.758
1.786
1844
1539
MAC42G
1.851
1568
1.075
1681
1975
BASGOl
1.249
1.550
1.467
1526
2.025
MCK131
2.222
0.814
1.016
1764
1.071
BBE401
2.044
1.793
1.073
1-057
2.144
MCK231
2.554
1347
0.B22
1893
1.140
BE LI 16
4.BG9
1.024
1.175
1.152
1145
MEV405
1.053
3-086
1.612
1-300
1.529
BFT142
1.604
1.514
1.230
0.953
2.364
MKG113
0.929
0.721
1.D29
1615
1190
BVL130
2.993
L657
1.110
1602
1888
MEC602
0.953
0.755
2.207
2.841
1741
BWR139
3.Z30
2.154
2.630
2.102
1529
HPT006
0.590
0.925
1.038
CAD150
1.D76
1.206
1.274
1481
1614
QXF122
2.995
2.352
2.469
1628
1.605
CAN407
2.489
1374
3.254
0.572
1479
PAL190
2.495
1200
1.35B
1610
1.422
CDR119
2-B40
2.143
1.390
1383
1401
PAR107
1.692
1.374
1,272
0.910
1.109
CDZ171
0.754
0.61B
1.140
1574
1.664
PED108
3.066
2.137
1.392
0.843
0.768
CHA467
1.01S
2.419
0.927
1.944
2.797
PET427
1.990
2.679
0.882
0.659
0.728
CHC432
3.304
2.608
PIN 414
1.279
1.745
O.B68
1915
1763
CKT136
1.538
0.734
0.677
0.649
0.576
PND165
2.755
2.173
1552
1711
CN0125
1.045
2.076
1.282
2-202
1362
PNF126
2.613
2.021
1.B67
1.353
0.807
CNT169
2-611
1-312
2.144
0.775
0.804
PRK134
1.856
2.128
1.410
1773
1.315
COW137
2.093
1.735
1
1105
1407
P5U106
2.128
0.611
0.612
1.713
1434
CRM435
1.218
QAK172
1.220
1.340
2.511
2.BB9
2.624
CTH110
2.223
1.103
1.408
1587
1551
R0M206
1.063
0.856
1.746
1.977
2.554
CVL151
0.6G7
0.662
2.003
1-656
1193
R0M406
2.503
1170
3.161
100D
1.086
DCP114
1.687
1.247
1.597
1893
2.203
SAL133
1.381
1.040
1.700
4.024
1.950
0EM417
1.628
1.224
1.626
1.344
1651
SAN189
0.744
D.951
1.955
1.648
2.747
OEV412
0.749
SEK430
1.274
0.830
2.621
1-562
1.334
DIN431
2.419
3.7BB
1.381
1725
1568
SHN418
1.781
1.723
1.851
1.138
2.067
ESP127
1-399
0.969
1.249
1.888
1.991
SM0152
2.085
1267
2.978
2.669
2.493
GAS 153
0.557
0.511
2.129
2-319
19B9
SPD111
1.772
0.922
0.821
0.961
0.907
GLR460
1-523
1.62B
1.159
0.757
0.BB8
STK138
0.821
0.882
1.307
1702
1-147
GRB411
2.083
1.717
3 350
2-017
0.917
SUM156
1.373
1194
2.232
0.956
0.819
GRC474
1.128
2.144
2.3U
0.797
1324
UVL124
0.816
0 854
1.579
1063
1.075
GRS42G
2.388
1.256
0.473
0.967
1.194
VIN140
1.130
3.154
1.473
1435
2.053
GRT434
2.292
V0Y413
0.833
0.967
0.579
O.BQD
1727
GTH1G1
1.069
3-229
2.480
1.994
1.915
VPI120
1.015
0.97D
O.B13
1591
1.689
HOX148
G.BB4
1.307
1.483
1.554
1645
WSP144
1.455
1728
1.733
2.146
2.442
HWF187
1.018
0.9B1
0.906
1698
0.921
WST109
1.208
1.013
0.605
0.939
1341
D.
Accuracy
1.
Semi-Annual Site Visits
Approxi
mately every six months, technicians managed by the Field Operati
ons Manager perform semi-
annual performance checks to the on-site analyzer and reverify the on-site transfer standard, calibrate
the on-site analyzer to the traveling transfer standard (Level 2) as needed, and verify the data logger and
the shelter temperature probe using NIST traceable standards. These results are used to perform final
validation on the hourly 0
3 data.
18 | Page
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2. Independent PE Results
The Audit Agency performs annual PEs in accordance with 40 CFR Part 58 Appendix A Section 3.2.2 and
EPA's Quality Assurance Handbook for Air Pollution Measurement Systems: Volume II and submits these
results to AQS on a quarterly basis (US EPA, 2017). The auditor is required to select audit levels that
bracket 80 percent of the ambient data; however, the audit levels do not need to be consecutive. For
levels 1 and 2 (which includes the range of 4 to 19 ppb), the acceptance criteria are ±1.5 ppb difference
or ± 15.1 percent difference, whichever is greater. The acceptance criteria for levels 3-10 are ±15.1
percent difference. PE audit mean bias estimates from the AMP256 report are displayed in the box plots
shown in Figure 11.
Figure 11. Estimated Bias in 03 concentrations from PE Audit Results for All CASTNET Sites for years 2015
through 2019
Ozone PE Audit Mean Bias of All CASTNET Sites
10
CO
c -5
T
I
-10 •
-15
-20
2015 2016 2017 2018 2019
The bias is estimated from the PE values for the years 2015 through 2019 and displayed in Figure 11. The
median for all sites are within ±2.5% for all years.
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E. Completeness
Completeness is a measure of the amount of valid data obtained from a measurement system compared
to the amount that was expected to be obtained under correct, normal conditions. For comparison with
the NAAQS for a given 3-year period, a site must meet two completeness criteria: having at least 75
percent valid data for each of the three ozone seasons comprising the 3-year period and having at least
90 percent valid data for all three ozone seasons combined. For the 2017-2019 time period, 74 out of
82 (90 percent) CASTNET 03 sites met both completeness criteria.
4. Precursor Measurements and Meteorology
A. NO,, Monitoring
Reactive nitrogen compounds are precursors for both 03 and PM2.5 formation. Total reactive oxidized
nitrogen (NOy) is defined as NOx (NO + N02) plus NOz(PAN, HN03, HN02, PPN, other organic nitrates, and
N02"). EPA and NPS operate eight trace-level continuous NOy analyzers at CASTNET sites (Figure 1). Great
Smokies National Park, TN (GRS420) operated by NPS and Bondville, IL (BVL130) operated by EPA are
also NCore stations. GRS420, TN and BVL130, IL also measure trace S02 and CO as part of the NCore
suite of measurements. The Duke Forest, NC (DUK008) NOy analyzer has been converted to an
"enhanced" NOy analyzer which includes a heated stainless steel converter (TNX), Light Emitting Diode
(LED) converter (NOx) and molybdenum converter (NOx) followed by a sodium chloride denuder. The
sample stream switches between each converter (or no converter for NO) to measure or calculate
speciated reactive nitrogen, including NOy, N02, NOx, TNX, NHX, NO, HN03 and NOz.
Total reactive oxidized nitrogen (NOy) is measured using a thermal molybdenum converter at the inlet to
convert reactive nitrogen species to NO followed by the detection of NO by chemiluminescence. The
EPA-sponsored CASTNET sites with trace NOy each have a Teledyne (API) T200U chemiluminescence
analyzer, 701H zero air system, and a T700U multi-gas calibrator in addition to the typical suite of
CASTNET equipment (e.g., data logger, ozone analyzer, etc.).
Trace NOy is audited twice per year by the CASTNET contractor and audited once every other year by an
independent 3rd party. The ambient data are submitted to AQS monthly and the QC results are
submitted quarterly. Trace-level precision is verified against the acceptance criteria in 40 CFR Part 58
Appendix A. The acceptance criterion is an upper 90 CL for the CV of 10%. The NOy analyzer is
challenged with 15 ppb NO every other day. Efforts to better understand the trace gas methods and
quality control procedures are on-going between CAMD, OAQPS, the EPA Regions, the manufacturer,
and contractors.
B. CASTNET meteorology
All NPS-sponsored and all BLM-WSO-sponsored CASTNET sites include meteorological measurements.
Five EPA-sponsored CASTNET sites: Beltsville, MD (BEL116); Bondville, IL (BVL130); Cherokee Nation
Stilwell, OK (CHE185); Indian River Lagoon, FL (IRL141); and Pinedale, WY (PND165) also collect
meteorological data. The locations of the 40 CASTNET sites reporting meteorological measurements
with regulatory 03 are displayed in Figure 13.
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5. Summary
The CASTNET ozone monitoring program provides critical information to stakeholders and has met its
primary monitoring objectives through consistent, long-term measurements since 1987. The rural
ozone monitors detect regional air quality signals, provide a unique data set for evaluating the effects of
03 on vegetation and ecosystems, are used to evaluate the primary and secondary 03 NAAQS, and used
to evaluate the impacts to 03 formation from wildfire and stratospheric ozone intrusions. Federal land
managers use CASTNET data to assess environmental conditions and risk of air quality impacts on
nationally-recognized sensitive areas and other federal lands. Other stakeholders and participants
include Tribes, States, other federal agencies, and universities who use CASTNET data to evaluate air
quality models and determine human health and environmental risks in their areas.
With thirty years of data from many of its sites, CASTNET has measured a significant reduction in
regional 03 concentrations in the Eastern US in response to emission control programs, allowing policy
makers to assess the effectiveness of these programs for improving air quality. In the coming years,
CASTNET data will allow stakeholders to evaluate the effectiveness of current policies and ongoing
emission reduction programs such as the Cross-State Air Pollution Rule and Cross-State Air Pollution
Update Rule.
Data quality indicators indicate that most CASTNET sites are meeting the network quality assurance
criteria for accuracy, bias, and precision for 2017-2019, the most recent 3-year period available. While
90 percent of CASTNET sites met the completeness criteria for NAAQS attainment decisions for 2017-
2019, many of the sites that did not meet the criteria were due to infrastructure damage from
hurricanes, power failures at the monitoring stations, and analyzer pump failures. Recent efforts to
improve the data capture efficiency throughout the network include back plane replacements.
CASTNET remains committed to improving our understanding of reactive nitrogen and other 03 and
PM2.5 precursors in the ambient environment. Eight monitoring sites already provide continuous NOy
data, and several of these sites also measure continuous S02 and CO. In addition, an enhanced NO/NOy
analyzer has been developed and was deployed at Duke Forest, North Carolina in May 2017 where it
continues to provide hourly measurements of reactive nitrogen concentrations. Expanded use of these
and other continuous monitors will enhance the utility of CASTNET data in model evaluation and
development.
CASTNET has been a stable platform for regional air monitoring for thirty years and the program
continues to evolve within the constraints of budgets, regulatory demands, and agency priorities.
Developing long-term solutions to improve the cost-effectiveness of routine measurements and
leveraging existing and new partnerships has been crucial for the continuity of CASTNET, and these
attributes will continue to be important over the next five years as CASTNET strives to improve data
capture, enhance the types of measurements collected, and expand into areas with limited air quality
monitoring.
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