2016

Clean Air Status and Trends Network

2017

ANNUAL

REPORT

CASTNET
Clean Air Status and Trends Network

(CASTNET)

2017 Annual Report

Prepared by:

Wood Environment & Infrastructure Solutions, 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

August 2019
CASTNET 2017 Annual Report

i Table of Contents

TABLE OF CONT ENTS

Table of Contents

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

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

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

Related Monitoring Networks ................................................................................................................. 2

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

Measurements Recorded at CASTNET Sites ...................................................................................... 3

Quality Assurance Program...................................................................................................................... 4

Estimating Dry, Wet, and Total Deposition ........................................................................................ 4

Chapter 2 Ozone Concentrations.....................................................................................................6

Eight-hour Ozone Concentrations......................................................................................................... 7

Chapter 3 Stratospheric Ozone Intrusions....................................................................................10

Examples of Possible Stratospheric Ozone Intrusion -

Colorado and Nearby States............................................................................................................11

Additional Examples of Stratospheric Ozone Intrusion –

Ute Indian Tribe, Utah and Thunder Basin, Wyoming............................................................22

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

Total Nitrate Concentrations..................................................................................................................25

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

Continuous Trace-level NOy and Filter Pack Total Nitrate Concentrations..........................29

Chapter 5 Update on the Ammonia Monitoring Network ..........................................................32

Chapter 6 Sulfur Pollutant Concentrations ..................................................................................35

Sulfur Dioxide Concentrations...............................................................................................................35

Particulate Sulfate Concentrations.......................................................................................................37

Sulfur Dioxide Concentrations at Basin, WY ....................................................................................39

Chapter 7 Atmospheric Deposition of Nitrogen ..........................................................................41

Chapter 8 Effects of California Wildfires on Air Quality ............................................................47

Northern California Wildfires.................................................................................................................47

Southern California Wildfires.................................................................................................................50

Chapter 9 Atmospheric Deposition of Sulfur, Base Cations, and Chloride ...............................56

Sulfur Deposition........................................................................................................................................56

Deposition of Base Cations and Chloride .........................................................................................56

References ........................................................................................................................................60

Appendix A Locational and Operational Characteristics of CASTNET Sites

Appendix B Acronyms and Abbreviations
ii Executive Summary

Executive Summary

The EPA Clean Air Status and Trends Network (CASTNET) monitors atmospheric pollutant concentrations 

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 and 

dry deposition. This report presents maps of 2017 ozone levels, nitrogen and sulfur pollutant 

concentrations, and deposition fluxes. Additionally, it provides box plots to show trends in air quality over 

the 28-year period from 1990 through 2017. In 2017, CASTNET measured rural, regionally representative 

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

at 82 locations.

Key Results and Highlights through 2017

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

3) concentration for 2017 at 

the eastern CASTNET reference sites (see Appendix A for designated reference sites) was 62 parts per 

billion (ppb). This is a decrease from the 2016 median of 65 ppb. At the western CASTNET reference sites, 

the median was 67 ppb, an increase from the 2016 value of 65 ppb. Three-year averages of fourth highest 

DM8A O 

3 concentrations exceeded the 2015 8-hour National Ambient Air Quality Standard (NAAQS) of 

0.070 parts per million (ppm) or, equivalently, 70 ppb, at three CASTNET sites in California, one in New 

Jersey, one in Connecticut, and one in Utah during the most recent 3-year period (2015–2017). During 

2017, fourth highest DM8A O 

3 concentrations greater than 70 ppb were observed at the same six CASTNET 

sites in California, New Jersey, Connecticut, and Utah that measured the highest concentrations in 

2015-2017. Three-year averages of fourth highest 

DM8A O 

3 concentrations have been reduced by 

24 percent at the eastern reference sites since 1990–

1992 and by 10 percent at the western reference 

sites since 1996–1998. Figure E-1 and Table E-1 

summarize the long-term changes in measured 

concentrations.

Federal, state, and local NOx control programs have 

resulted in substantive reductions in emissions. NOx

emissions declined by 89 percent over the 28-year 

period, 1990 through 2017, at regulated electric 

generating units (EGUs) in the East. Three-year 

Figure E-1 Trends in O 

3, Total NO3, and SO 2

at Eastern Reference Sites

50

60

70

80

90

100

0

2

4

6

8

10

12

1990199119921993199419951996199719981999200020012002200320042005200620072008200920102011201220132014201520162017

Ozone Concentration (ppb)

Filter Pack Concentration (μg/m3)

Total NO3

SO2

Ozone

SO2

Total NO3

-

O3
CASTNET 2017 Annual Report 

 iii Executive Summary 

mean annual concentrations of total nitrate (NO3), which is comprised of nitric acid plus particulate NO3, 

declined 51 percent at the eastern reference sites over the 28-year period. Three-year mean annual total 

NO3 levels measured at the western reference sites dropped by 37 percent over the 21-year period. 

Mean annual sulfur dioxide (SO 

2) concentrations measured at the eastern reference sites have declined 

significantly over the 28-year period, 1990 through 2017. Three-year mean annual SO 

2 levels at eastern 

sites decreased 89 percent reflecting the reduction in regulated eastern EGU SO 

2 emissions (96 percent). 

SO 

2 concentrations measured at the western reference sites declined by 45 percent over the 21 years from 

1996 through 2017. 

Table E-1 Trends in Aggregated Western and Eastern O 

3, Total NO3, and SO 2 Pollutant 

Concentrations 

Western Reference Sites Eastern Reference Sites Percent Changed 

Pollutant 1996–1998 2014–2016 1990–1992 2014–2016 West East 

O 

3 (ppb) 74 66 85 64 -10 -24 

Total NO3 (μg/m3 ) 1.0 0.6 3.0 1.5 -37 -51 

SO 

2 (μg/m3 ) 0.6 0.3 8.8 0.9 -45 -89 

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

nitrogen pollutants. Currently, the focus also includes measuring rural, regional O 

3 concentrations. 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 65 CASTNET sites. This report provides information on CASTNET 

pollutant measurements, estimated deposition fluxes, and other topics that can be addressed using 

CASTNET data, such as the contribution of stratospheric O 

3 to ground-level concentrations in the 

Intermountain West and the effects of the California wildfires in October and December 2017 on air quality. 
1 Chapter 1: CASTNET Update

Chapter 1

CASTNET Update

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

began operating in 1991. The network provides 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 data are used to provide input to regional air quality models in order to calculate atmospheric 

pollutant deposition. CASTNET is managed and operated by the U.S. Environmental Protection Agency in 

cooperation with the National Park Service and other federal, state, tribal, and local partners. In 2017, 

the network operated 96 monitoring stations throughout the contiguous United States, Alaska, and 

Canada. CASTNET data show a 28-year decline in ozone concentrations and in nitrogen and sulfur 

pollutant levels and deposition rates.

Introduction

The Clean Air Status and Trends Network (CASTNET) operated 96 monitoring stations throughout the 

contiguous United States, Alaska, and Canada in 2017. Ninety-five of 96 sites included filter pack systems 

to sample weekly concentrations of acid gases and aerosols. Eighty-two sites included continuous ozone 

(O 

3) analyzers. The U.S. Environmental Protection Agency (EPA) and the National Park Service (NPS) are the 

primary sponsors of CASTNET. NPS began its participation in 1994 and operated 26 sites during 2017. The 

Bureau of Land Management-Wyoming State Office (BLM) operated five sites in Wyoming. 

CASTNET depends on contributions from many organizations (EPA, 2018a) including Native American 

tribes, state and federal government agencies, and universities. These participants sponsor individual 

CASTNET sites and provide in-kind services that support the overall performance of the network. For 

example, partners operate and repair site instruments, change weekly filter packs, and perform general site 

maintenance. Many partners provide the land for the CASTNET site. Others, such as universities, provide 

their expertise in air quality monitoring, which is invaluable for improving CASTNET monitoring capabilities 

and ensuring that CASTNET collects data that are of value to the scientific research community.

In 2017, ozone monitoring was initiated at Chaco Canyon, NM (CHC432). This site is the sole ozone-only 

CASTNET site in the network. It is the first CASTNET site in New Mexico. Filter pack sampling and an enhanced 

nitrogen oxide/total reactive oxides of nitrogen (NO/NOy) system, which permits speciation of reactive nitrogen 

components, began operating at Duke Forest, NC (DUK008). The seasonal small footprint site at the summit of 

Whiteface Mountain, NY (WFM007) was operated by the New York State Department of Environmental 

Conservation during the summer of 2017. Trace-level gas measurements were discontinued at Beltsville, MD 

(BEL116).
CASTNET 2017 Annual Report

2 Chapter 1: CASTNET Update

The U.S. Congress established the Acid Rain Program (ARP) in 1990 to reduce emissions of sulfur dioxide 

(SO 

2) and nitrogen oxides (NOx) from electric generating units (EGUs). CASTNET was created to assess the 

effectiveness of the ARP by providing consistent, long-term measurements for determining relationships 

between changes in emissions and changes in air quality, atmospheric deposition, and ecological effects. 

Related Monitoring Networks

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

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

effectiveness of emission control programs. These networks include the National Atmospheric Deposition 

Program (NADP; http://nadp.slh.wisc.edu/) and its affiliated networks: 

• National Trends Network (NTN)

• Ammonia Monitoring Network (AMoN)

• Mercury Deposition Network (MDN)

• Atmospheric Mercury Network (AMNet).

Other cooperating networks include: 

• Canadian Air and Precipitation Monitoring Network (CAPMoN)

(https://www.canada.ca/en/environment-climate-change/services/air-pollution/monitoring-networksdata/canadian-air-precipitation.html)

• EPA’s National Core Monitoring (NCore)

(https://www3.epa.gov/ttnamti1/ncore.html)

• BLM’s Wyoming Air Resources Monitoring System (WARMS)

(https://www.blmwarms.net/) 

• Interagency Monitoring of Protected Visual Environments (IMPROVE) 

(http://vista.cira.colostate.edu/IMPROVE/)

Locations of Monitoring Sites

A map of CASTNET monitoring sites is given in Figure 1-1. Ninety-six monitoring sites were operated at 

94 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 2017. 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. The location of each site 

and information on start date, latitude, longitude, elevation, identification of the nearby NADP site, land 

use, terrain type, operating agency, and if the site is a reference site used for trends are given in 

Appendix A.
CASTNET 2017 Annual Report

3 Chapter 1: CASTNET Update

Figure 1-1 CASTNET Sites Operational during 2017

Measurements Recorded at CASTNET Sites

During 2017, all but one CASTNET site measured weekly ambient concentrations of acidic pollutants, base 

cations, and chloride (Cl-

) using a 3-stage filter pack with a controlled flow rate (Wood, 2017). Gaseous 

pollutant concentrations include nitric acid (HNO 

3) and SO 2. Particulate concentrations include nitrate 

(NO3), ammonium (NH+ 

4 ), sulfate (SO2-4 ), 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 also include a temperature-controlled shelter and continuous O 

3 monitoring system. 

O 

3 concentrations were measured at 82 sites. The O 3 inlet and filter pack are located atop a 10-m tower. 

Some CASTNET sites also measure trace-level SO 

2, carbon monoxide (CO), and NO/NOy. In 2017, 

meteorological parameters were measured at 6 EPA-, 26 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).
CASTNET 2017 Annual Report

4 Chapter 1: CASTNET Update

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 intranetwork consistency and comparability and the delivery of data that are reproducible and comparable with 

data from other monitoring networks. The 2017 QA program elements are documented in the CASTNET 

Quality Assurance Project Plan (QAPP; Wood, 2017). 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 2017 and 

historical data collected over the period 1990 through 2016 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/.

The Wood CASTNET laboratory regularly participates in interlaboratory comparison studies and proficiency 

testing (PT) programs such as the Environment Canada (ECAN) PT Program for Inorganic Environmental 

Substances (http://www.ec.gc.ca/inre-nwri/default.asp?lang=En&n=7A20877C-1). The ECAN PT program

conforms to the requirements of the American Association for Laboratory Accreditation. The program 

meets the International Organization for Standardization (ISO)/International Electrotechnical Commission 

17043:2010 conformity assessment – general requirements for proficiency testing with scope of 

accreditation 2867.01. The results reported 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). CASTNET laboratory results from the PT studies are 

summarized in the QA annual reports posted to the EPA CASTNET website: https://www.epa.gov/castnet/.

Estimating Dry, Wet, and Total Deposition

Total deposition was assessed using the NADP’s Total Deposition Hybrid Method (EPA, 2015c; Schwede 

and Lear, 2014), which is a hybrid approach that combines data from established ambient monitoring 

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

data from CASTNET were merged with dry deposition 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). NADP-measured precipitation and chemistry data and precipitation amounts from 

PRISM were merged. Dry and wet deposition fluxes were added to obtain the estimates of total deposition 

that are discussed in Chapters 7 and 9.
6 Chapter 2: Ozone Concentrations

Chapter 2

Ozone Concentrations

CASTNET is the primary air quality network for monitoring rural, ground-level ozone concentrations in 

the United States. The network produces ozone data for evaluation of compliance with the National 

Ambient Air Quality Standards (NAAQS; EPA, 2015b) plus information on geographic patterns in 

regional ozone levels. Ozone data measured at 80 CASTNET sites from 2015 through 2017 were 

evaluated with respect to the NAAQS and used to calculate design values following the requirements 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 2015 through 2017 and fourth 

highest DM8A ozone concentrations for 2017 are presented. Trends in fourth highest DM8A ozone 

concentrations for eastern and western reference sites are shown. For the 2015 through 2017 period, 

three California sites, two eastern sites, and one site in Utah measured concentrations greater than the 

0.070 parts per million NAAQS.

CASTNET O 

3 data are based on strict QA/quality control procedures specified by the CASTNET QA Program.

Hourly average concentrations were measured at 82 CASTNET sites in 2017. These data are delivered 

routinely to the EPA Air Quality System (AQS) database (epa.gov/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 O 

3 concentrations when three 

years of Title 40 Code of Federal Regulations (CFR) Part 58-compliant data become available. CASTNET 

measurements provide information for evaluating rural O 

3 concentrations for O 3 NAAQS compliance and 

for presenting information on trends and geographic patterns in regional O 

3.

The primary O 

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

public welfare and the environment. Both the primary and secondary O 

3 NAAQS are set at a level of 0.070 

parts per million (ppm; EPA, 2015b). 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 0.070 ppm or 70 

parts per billion (ppb) in practice.

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

3 concentrations through 

national and local monitoring programs. Wood followed EPA procedures (2015a) to estimate O 

3 design 

values and 2017 fourth highest DM8A O 

3 concentrations at CASTNET sites. Measurements potentially 

affected by exceptional events, i.e., unusual or natural events that can affect air quality but are not 

reasonably controllable, were not removed when calculating these estimates. The Exceptional Events Rule 

(EPA, 2016) was updated on October 3, 2016 (https://www.epa.gov/air-quality-analysis/exceptional-eventsrule-and-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 wildfires or stratospheric intrusion. 
CASTNET 2017 Annual Report

7 Chapter 2: Ozone Concentrations

Eight-hour Ozone Concentrations

All CASTNET sites, with the exception of HOW191, ME; MCK231, KY; and ROM206, CO, are compliant under 

40 CFR Part 58 and may be used for NAAQS compliance. 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.” Three-year averages of the fourth highest DM8A O 

3 concentrations for 2015 through 2017 are 

presented in Figure 2-1. Ozone concentrations were not included on the map if the 3-year average was not 

available because of incomplete data; these sites are shown as dots with no value. Three California 

CASTNET sites, two eastern sites, and one site in Utah measured fourth highest DM8A O 

3 concentrations 

above the NAAQS of 0.070 ppm (EPA, 2015b). The highest 3-year design value of 89 ppb was sampled at 

the Sequoia and Kings Canyon National Parks, CA (SEK430) site. The highest 3-year eastern concentration 

(73 ppb) was measured at Washington Crossing, NJ (WSP144) and Abington, CT (ABT147). Table 2-1 lists 

sites with 2015–2017 O 

3 design values greater than 70 ppb.

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

3 Concentrations for 

2015–2017

An ozone design value describes the status of a given area with respect to the concentration values required by 

the ozone NAAQS. Design values change as each new 3-year period (e.g., 2015–2017, 2016–2018, etc.) of 

monitored ozone concentrations becomes available. Design values are used to classify nonattainment areas, 

develop control strategies to achieve the NAAQS, and to assess progress towards meeting the NAAQS. For an 

area to achieve the ozone NAAQS, the 3-year average of fourth highest DM8A ozone concentrations cannot 

exceed 0.070 ppm (EPA, 2015b). Designated criteria pollutant nonattainment areas are provided on the EPA 

website: https://www.epa.gov/green-book.
CASTNET 2017 Annual Report

8 Chapter 2: Ozone Concentrations

Table 2-1 Sites with Calculated Design Values for 2015–2017 greater than 70 ppb

Site ID State Sponsor 3-year Average

SEK430 California NPS 89

JOT403 California NPS 87

YOS404 California NPS 75

WSP144 New Jersey EPA 73

ABT147 Connecticut EPA 73

DIN431 Utah NPS 72

Figure 2-2 shows 2017 fourth highest DM8A O 

3 concentrations that met percent completeness criteria. 

During 2017, three California sites, two eastern sites, and one site in Utah measured fourth highest DM8A 

O 

3 concentrations greater than 70 ppb. These are the same six sites with 2015–2017 design values greater 

than 70 ppb.

Figure 2-2 Fourth Highest DM8A O 

3 Concentrations for 2017
CASTNET 2017 Annual Report

9 Chapter 2: Ozone Concentrations

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

highest DM8A O 

3 concentrations from 34 eastern reference sites (top) for 1990 through 2017 and for 16 

western reference sites (bottom) for 1996 through 2017. The reference sites were selected for their longterm data record and consistent performance. These sites were also used to show trends in ambient 

nitrogen and sulfur concentrations (Chapters 4 and 6).

The eastern O 

3 data show an overall decline of 24 percent in fourth highest DM8A O 3 concentrations from 

1990–1992 through 2015–2017. The 2017 median data point, 62 ppb, was the lowest in the history of the 

network.

The western O 

3 data show a reduction of 10 percent in fourth highest DM8A O 3 concentrations from 

1996–1998 through 2015–2017. The 2017 median of the fourth highest DM8A O 

3 concentrations for the 

western reference sites was 67 ppb, a slight increase from the 2016 concentration of 65 ppb.

Figure 2-3 Trends in Fourth Highest DM8A O 

3 Concentrations

Eastern Reference Sites

Western Reference Sites
10 Chapter 3: Stratospheric Ozone Intrusions

Chapter 3

Stratospheric Ozone Intrusions

Certain weather conditions can transport ozone from the stratosphere to ground level at high-elevation 

sites primarily within the Intermountain Region of the western United States. These occasional intrusions 

of ozone-laden stratospheric air can contribute to atypically high ozone concentrations or even 

exceedances of ozone NAAQS. The EPA recently formed a workgroup of scientists and air quality 

managers from local, state, and federal agencies to provide support for identifying ozone stratospheric

intrusions using a combination of model simulations, surface observations, and atmospheric sounding 

and satellite data. Examples of potential stratospheric ozone intrusions are discussed herein. The most 

prominent event occurred in Colorado and nearby states. Others happened at the Uintah and Ouray 

Reservation in the Uinta Basin, UT and in Thunder Basin, WY.

A stratospheric O 

3 intrusion occurs when O 3-laden stratospheric air is transported across the tropopause 

and descends into the troposphere. Over the Intermountain West of the United States, intrusions may 

sporadically reach the ground at high elevations. They are rare at surface sites in the East due to the lower 

elevation. Stratospheric intrusions are identified by low tropopause heights, very low relative and specific 

humidity, low CO concentrations, and high concentrations of O 

3. Stratospheric intrusions that reach ground 

level typically occur in the spring and early summer, commonly follow strong cold fronts, and can extend 

across multiple states. 

The identification of intrusions using only surface monitoring is difficult. Because intrusions are episodic,

transient and localized, surface measurements along with vertical profile data from radiosondes, satellites, 

ozonesondes, and aircraft, if available, with supporting model simulations are needed to help demonstrate 

when intrusions contribute to high O 

3 concentrations at ground level. Satellite data show vertically 

integrated concentrations of O 

3, CO, and humidity. Low concentrations of CO and humidity, in conjunction 

with elevated O 

3 concentrations, give evidence of stratospheric air. 

The EPA recently formed a workgroup of scientists and air quality managers from local, state, and federal 

agencies to provide support for identifying stratospheric intrusions. The group is called the EPA Ozone 

Stratospheric Intrusion Workgroup (SI Workgroup; Kaldunski et al., 2017). Examples of supporting analyses 

undertaken by the SI Workgroup to-date include:

• Analyses of O 

3 concentrations collected over surrounding regions;

• Descriptions of weather conditions, including an analysis of rapid spatial changes in 500 or 300 

millibar (mb) flows, e.g., for sharp troughs;

• Presentation of Geostationary Operational Environmental Satellite System (GOES) satellite data 

including column-average O 

3, CO, relative humidity, and other parameters and their ratios;
CASTNET 2017 Annual Report

11 Chapter 3: Stratospheric Ozone Intrusions

• Estimates of tropopause height, isentropic potential vorticity (PV), and potential temperature;

• Presentation of upper air radiosonde data;

• Trajectory analyses, light detection and ranging, O 

3 profiles, satellite total column measurements, 

and atmospheric chemistry model simulations; and 

• Regional model, e.g., Realtime Air Quality Modeling System (RAQMS) simulations (see the example 

in Figure 3-11).

The analyses included in this chapter are not intended to represent an exceptional events demonstration or 

to serve as examples of how to satisfy the Exceptional Events Rule criteria. For information about the 

Exceptional Events Rule and related guidance on stratospheric intrusion events, see 

https://www.epa.gov/air-quality-analysis/final-2016-exceptional-events-rule-supporting-guidancedocuments-updated-faqs.

Examples of Possible Stratospheric Ozone Intrusion – Colorado and Nearby 

States

The SI Workgroup identified a possible stratospheric intrusion event over parts of the Intermountain West 

on April 22–23, 2017. A detailed analysis of the April 2017 stratospheric intrusion event in Colorado and 

nearby states is presented first. The Ute Indian Tribe of the Uintah and Ouray Reservation documented 

high O 

3 concentrations influenced by stratospheric intrusion over the period June 8–15, 2015 in Uinta 

Basin, UT. The State of Wyoming Department of Environmental Quality/Air Quality Division (WDEQ/AQI)

described a stratospheric intrusion event on June 6, 2012. These latter two examples are summarized at the 

end of the chapter.

Table 3-1 summarizes DM8A O 

3 concentrations measured over parts of the Intermountain West during the 

period of the suspected stratospheric intrusion on April 22, 2017. This table supplements the DM8A O 

3

concentrations displayed on the map in Figure 3-1.

Table 3-1 2017 DM8A O 

3 Concentrations 

Date O 

3 (ppb) Site* State

April 22 82 Gothic (GTH161) Colorado

80 Navajo Lake New Mexico

75 Centennial (CNT169) Wyoming

73 Bloomfield New Mexico

73 Ute 1 Colorado

72 Coyote Ranger New Mexico

72 Mesa Verde National Park (MEV405) Colorado

71 Ute 3 Colorado

71 Substation New Mexico

Note: *CASTNET site identification is included in parentheses. Ute 1 and Ute 3 ozone analyzers were operated by Southern Ute Indian Tribe 

in La Plata County, CO.
CASTNET 2017 Annual Report 

12 Chapter 3: Stratospheric Ozone Intrusions 

Figure 3-1 DM8A O 

3 Concentrations (ppb) April 22, 2017 

Source: EPA (2018b) 

Figure 3-2 shows time series of hourly O 

3 concentrations measured at three CASTNET sites (GTH161, 

CNT169, and MEV405) over the period April 21–23, 2017. The time series show hourly O 

3 values at GTH161 

exceeded 85 ppb on April 22, 2017. 
CASTNET 2017 Annual Report 

13 Chapter 3: Stratospheric Ozone Intrusions 

Figure 3-2 Time Series of Hourly O 

3 Concentrations Measured at CASTNET Sites in Colorado 

and Wyoming 

The Colorado Department of Public Health and the Environment (CDPHE) requested that CASTNET 

managers append informational stratospheric intrusion flags (“IO”) to the hourly O 

3 values in EPA’s AQS to 

indicate to data users that these records may have been impacted by a stratospheric intrusion. The flagging 

request applies to GTH161 from April 22, 2017, 6:00 a.m. through April 23, 2017, 3:00 a.m. and to MEV405 

from April 22, 2017, 9:00 a.m. through April 23, 2017, 7:00 a.m. (Harshfield, 2018). 

Figure 3-3 Time Series of GTH161 Hourly O 

3 Concentrations 

Figure 3-3 displays the abrupt increase and abrupt decrease of the IO-flagged O 

3 data at the time of arrival 

of the suspected stratospheric intrusion (orange). These records are shown with all valid hourly O 

3 data 

(gray) collected during the month of April 2017 from the GTH161 site. This month had a median hourly O 

3

40

45

50

55

60

65

70

75

80

85

90

0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00

Concentration (ppb)

GTH161

MEV405

CNT169

4/21/17 4/22/17 4/23/17 4/24/17

20

30

40

50

60

70

80

90

4/1/17 4/6/17 4/11/17 4/16/17 4/21/17 4/26/17

Concentration (ppb)

IO-flagged hourly ozone data

Hourly ozone data
CASTNET 2017 Annual Report 

14 Chapter 3: Stratospheric Ozone Intrusions 

concentration of 56 ppb and a median daily O 

3 production value of 11 ppb, i.e., the production of O 3 from 

the minimum concentration typically around sunrise to the time of the maximum hourly concentration. This 

diurnal O 

3 profile is consistent with other Intermountain West sites. 

Figure 3-4 Histogram of April 2017 GTH161 Hourly O 

3 Values 

Figure 3-4 shows a histogram of valid hourly O 

3 records measured for the month of April 2017 with the 

April 22–23, 2017 IO-flagged hourly records highlighted in orange. The IO-flagged data were the highest 

values measured. 

Hourly O 

3 and temperature measurements observed at GTH161 and supporting meteorological records 

from the Rocky Mountain Biological Laboratory’s Billy Barr weather station located approximately 1,000 m 

north-northwest of GTH161 are shown in Figure 3-5. The data were collected prior to the start of the 

suspected stratospheric intrusion to 48 hours after the end of the event. A negative correlation between 

solar radiation and relative humidity was recorded throughout the period. The drop in relative humidity on 

April 22, 2017, from 85 percent at 6:00 a.m. to 21 percent at 3:00 p.m., coincided with an increase in O 

3

concentrations from 64 ppb at 6:00 a.m. to 86 ppb at 3:00 p.m. 
CASTNET 2017 Annual Report 

15 Chapter 3: Stratospheric Ozone Intrusions 

Figure 3-5 GTH161 Measured Hourly O 

3, Temperature, Relative Humidity, and Solar Radiation 

Source: https://www.digitalrmbl.org/collections/weather-stations/ 

Additional information that helped identify the stratospheric intrusion was based on upper air sounding 

data from Grand Junction, CO, which is located approximately 85 miles west of GTH161. The data are 

plotted on Skew T log P diagrams (https://www.weather.gov/jetstream/skewt), which are commonly used 

to characterize vertical profiles of the atmosphere. In these plots, the bold black line on the left-hand side 

represents the sounding dew point and the bold black line on the right indicates the sounding 

temperature; the distance between these two lines is inversely proportional to relative humidity. The red 

dotted line that intersects the dew point line and extends to the y-axis identifies the lower bound altitude 

of a large dry air mass. The green dotted line shows the altitude (2915 m) of GTH161. The soundings were 

provided by the University of Wyoming (http://weather.uwyo.edu/upperair/sounding.html). 

The first three upper air sounding plots in Figure 3-6 illustrate the vertical descent of a parcel of dry air, 

which is demarked by a dew point temperature of -35 degrees Celsius (C). The parcel began at 550 mb at 

5:00 p.m. on April 21 (Figure 3-6a), declined to 600 mb at 05:00 a.m. on April 22 (Figure 3-6b), and then to 

650 mb at 5:00 p.m. on April 22 (Figure 3-6c), coinciding with the peak hourly O 

3 . The elevation of GTH161 

is equivalent to a pressure level of about 720 mb. The fourth upper air sounding plot (Figure 3-6d) shows 

the dissipation of the presumed stratospheric air and the removal of the horizontal line that signified the 

dry air. 
CASTNET 2017 Annual Report

16 Chapter 3: Stratospheric Ozone Intrusions

Table 3-2 summarizes the data in Figure 3-6 and illustrates how the lower bound altitude reached 

approximately 200-800 m above GTH161 while the temperature increased and relative humidity dropped 

during the elevated O 

3 event.

Table 3-2 Summary of Atmospheric Sounding Data April 21–22, 2017

Pressure (mb) Altitude (m) O 

3 (ppb) Temp (°C) RH (%)

550 5000-5600 62 -1 82

600 4400-5000 60 -8 86

675 3100-3800 87 4 49

720 2915 68 -4 82

Note: RH = relative humidity.

Figure 3-6 Upper Air Soundings Data for April 21, 2017, from 5:00 p.m. MST to April 23, 2017,

5:00 a.m. MST at Grand Junction, CO (1 of 3)

Figure 3-6a. Upper air soundings data 

for April 21, 2017 at 5:00 p.m. MST (13 

hours prior to the IO-flagged event). 
CASTNET 2017 Annual Report

17 Chapter 3: Stratospheric Ozone Intrusions

Figure 3-6 Upper Air Soundings Data for April 21, 2017, from 5:00 p.m. MST to April 23, 2017,

5:00 a.m. MST at Grand Junction, CO (2 of 3)

Figure 3-6b. Upper air soundings data 

for April 22, 2017 at 5:00 a.m. MST (one 

hour prior to the IO-flagged event).

Figure 3-6c. Upper air soundings data 

for April 22, 2017, 5:00 p.m. MST 

coinciding with the peak hourly ozone.
CASTNET 2017 Annual Report

18 Chapter 3: Stratospheric Ozone Intrusions

Figure 3-6 Upper Air Soundings Data for April 21, 2017, from 5:00 p.m. MST to April 23, 2017,

5:00 a.m. MST at Grand Junction, CO (3 of 3)

Figure 3-6d. Upper air soundings data 

for April 23, 2017 at 5:00 a.m. MST (two 

hours after the last IO-flagged data 

record).

Figure 3-7 shows the North American Mesoscale (NAM) model vertical profiles of isentropic PV (color 

gradient), isobaric temperature (contour lines), ground surface topography (bold black line), and location of 

Gothic (black vertical line) for the period of April 21, 2017, 5:00 p.m. Mountain Standard Time (MST)

through April 22, 2017, 5:00 p.m. MST at 12-hour increments. Potential vorticity was first identified as a 

tracer of stratospheric air in a study examining the exchange of stratospheric O 

3, radioactivity, and PV to 

the troposphere using aircraft and radiosonde measurement data (Danielsen, 1968). The plots in Figure 3-7

illustrate the sporadic nature of tropopause folds as measured by PV where exceedances of ~10-6 square 

meters per second (m2 /sec) times Kelvin/kilogram (K/kg) or one PV unit (PVU)1 is a tracer for stratospheric 

air (Browell et al., 1987; Danielsen 1968; Danielsen et al., 1987; Xing et al., 2016).

The surface meteorological conditions and O 

3 concentrations shown in Figures 3-7a through 3-7c are the 

same as shown in Figure 3-6. A tropospheric fold appeared at approximately 1,000 to 2,000 m above the 

ground surface in Figure 3-7a. In Figure 3-7b the tropopause fold ranged from the ground surface to 

approximately 1,000 m above Gothic. The tropopause fold appears to touch the ground surface 

immediately above Gothic in Figure 3-7c.

 

1 PVU units are m2 

/sec times K/kg.
CASTNET 2017 Annual Report

19 Chapter 3: Stratospheric Ozone Intrusions

Figure 3-7 NAM Meteorological Data for April 21, 2017 from 5:00 p.m. MST to April 22, 2017 

5:00 p.m. MST Transect Across Gothic, CO

Figure 3-7a. NAM lateral 

transect for April 21, 2017 at

5:00 p.m. MST (13 hours prior 

to the IO-flagged event) 

Figure 3-7b. NAM vertical 

profile for April 22, 2017 at 

5:00 a.m. MST (one hour prior 

to the IO-flagged event)

Figure 3-7c. NAM vertical 

profile for April 22, 2017 at 

5:00 p.m. MST coinciding with 

the peak hourly O 

3

concentration for the month 

of April 2017 of 87 ppb
CASTNET 2017 Annual Report

20 Chapter 3: Stratospheric Ozone Intrusions

Figure 3-8 provides a GOES-derived satellite image of mid-level moisture measured on April 21, 2017 at

5:00 p.m. MST that shows a scattered band of air with low relative humidity (green) ranging from -30 to 

-40 °C from Idaho across Colorado.

Figure 3-8 GOES-Derived Satellite Images of Band 12 Mid-level Moisture on April 21, 2017 at 

5:00 p.m. MST

Figure 3-8 is approximately 13 hours prior to 

the start of the IO-flagged event.

Figure 3-9 provides a GOES-derived satellite image of total column O 

3 measured on April 21, 2017 at 

5:00 p.m. MST that shows a widely scattered band of O 

3 mixing ratios (green) ranging from 275 to 400 

Dobson Units from Idaho across southwest Wyoming to southwest Colorado.

Figure 3-9 GOES-Derived Satellite Images of Total Column O 

3 on April 21, 2017 at 

5:00 p.m. MST
CASTNET 2017 Annual Report

21 Chapter 3: Stratospheric Ozone Intrusions

Figure 3-10 gives a Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2)

image with a time-averaged O 

3 mass mixing ratio at 400 hectopascal (hPa2

; i.e., 400 mb) from April 21, 

2017 from 5:00 p.m. MST (13 hours before the first IO-flagged data record) to April 22, 2017 at 4:00 p.m. 

MST (one hour before the peak hourly O 

3 measured at Gothic). This figure illustrates a distinct band of high 

O 

3 concentrations from the Idaho/Montana border across Colorado to the Oklahoma Panhandle.

Figure 3-10 MERRA-2 Average Ozone Concentrations at 400 hPa (mb) from April 21, 2017 

5:00 p.m. MST to April 22, 2017 4:00 p.m. MST

Figure 3-11 illustrates RAQMS simulations of the downward transport of O 

3 along a longitudinal crosssection at 40 degrees north (GTH161 is sited at 38.95627, -106.98587), which is located above the 

mountain peak silhouette (254.01 east longitude model location). The model results suggest a pocket of 

elevated O 

3 approached the ground surface near GTH161. 

 

2 1 hectopascal = 1 mb
CASTNET 2017 Annual Report

22 Chapter 3: Stratospheric Ozone Intrusions

Figure 3-11 RAQMS Simulations of the Downward Transport of O 

3 along a Longitudinal Cross 

Section at 40 Degrees North

Source: http://raqms-ops.ssec.wisc.edu/

In summary, the CDPHE requested that CASTNET managers append the IO informational stratospheric 

intrusion flags to the GTH161 and MEV405 hourly O 

3 values in AQS for April 22 and 23, 2017 to signify to 

data users that these records may have been impacted by a stratospheric O 

3 intrusion. Consequently, EPA, 

BLM, Wood scientists, and the SI Workgroup analyzed O 

3 data from GTH161, MEV405, and other sites;

temperature and dew point soundings from Grand Junction; GOES satellite images; and various output 

from NAM and RAQMS model simulations to demonstrate that atypically high, ground-level O 

3

concentrations measured at GTH161 and MEV405 were likely influenced by a stratospheric intrusion.

Additional Examples of Stratospheric Ozone Intrusion – Ute Indian Tribe, 

Utah and Thunder Basin, Wyoming

Technical support for exceptional events documentation (Ute Indian Tribe of the Uintah and Ouray 

Reservation, 2016) was prepared for O 

3 NAAQS exceedances measured in the Uinta Basin, UT on June 8 

and 9, 2015. EPA Region 8 scientists worked with the Tribe to analyze ozonesonde and satellite-based O 

3

data and RAQMS simulations. The Ute Indian Tribe operates O 

3 monitors at four locations in the Uinta 

Basin (Ouray, Redwash, Myton, and Whiterocks). CASTNET operates a monitor at Dinosaur National 

Monument (DIN431) in Utah. The four tribal monitors and DIN431 recorded their highest and second 

highest DM8A O 

3 concentrations in 2015 on June 8 and 9 (Table 3-3). Data presented in the exceptional 

events document show that high O 

3 concentrations measured on those days were unseasonably high, not 

consistent with historical readings and patterns, and coincided with the intrusion of stratospheric air into 

the troposphere contributing O 

3 to the surface measurements.

Gothic, CO
CASTNET 2017 Annual Report

23 Chapter 3: Stratospheric Ozone Intrusions

Table 3-3 Ute Indian Tribe of the Uintah and Ouray Reservation and DIN431 O 

3 Data during 

Period of Likely Stratospheric Intrusion (June 2015)

Monitor Name Monitor Number

8-Jun-15 8-Jun-15 9-Jun-15 9-Jun-15

DM8A O 

3 Annual Rank DM8A O 3 Annual Rank

Ouray 49-047-2003 71 ppb 1st 71 ppb 2nd

Redwash 49-047-2002 74 ppb 1st 72 ppb 2nd

Myton 49-013-7011 71 ppb 2nd 72 ppb 1st

Whiterocks 49-047-7022 73 ppb 1st 73 ppb 2nd

Dinosaur (DIN431) 49-047-1002 74 ppb 1st 72 ppb 2nd

In another example of a stratospheric intrusion event, an exceptional event demonstration package was 

prepared for EPA for Thunder Basin, WY for an O 

3 NAAQS exceedance on June 6, 2012 by the WDEQ/AQD

(2017). The Thunder Basin site is located in the northeast corner of the state. Table 3-4 shows the highest 

and second highest DM8A O 

3 concentrations in 2012 on June 6 and 7. During June 6, 2012, an upper 

atmospheric disturbance associated with a stratospheric intrusion moved over Wyoming likely contributing 

to elevated O 

3 readings resulting in a DM8A value of 88 ppb at the Thunder Basin O 3 monitor. Additionally, 

the monitors at Gillette and Campbell County measured elevated 1-hour O 

3 values of 75 ppb during the 

stratospheric event. Concentrations also exceeded 70 ppb at South Pass and Boulder. 

The WDEQ/AQD demonstration included statistical analyses of ground-level O 

3 measurements and an 

examination of hourly GOES total column O 

3 data, data from NASA’s Alpha Jet Atmospheric eXperiment 

(AJAX) Flight 47, and RAQMS and Rapid Refresh Model (RAP) simulations of O 

3, water vapor, and CO.

Table 3-4 Wyoming O 

3 Data During Period of Likely Stratospheric Intrusion

Monitor Name Monitor Number

6-Jun-12 6-Jun-12 7-Jun-12 7-Jun-12

DM8A O 

3 Annual Rank DM8A O 3 Annual Rank

Northeast Wyoming

Thunder Basin 56-005-0123 88 ppb 1st 52 ppb 138th

Campbell County 56-005-0456 75 ppb 1st 45 ppb 158th

Gillette 56-005-0800 75 ppb 1st 45 ppb 162th

West Wyoming

South Pass 56-013-0099 67 ppb 4th 73 ppb 1st

Boulder 56-035-0099 71 ppb 3rd 70 ppb 4th

Daniel 56-035-0100 67 ppb 4th 70 ppb 2nd

Pinedale (WDEQ/AQD) 56-035-0101 64 ppb 7th 69 ppb 1st

Big Piney 56-035-0700 68 ppb 3rd 69 ppb 2nd

The WDEQ/AQD concluded that a stratospheric intrusion event occurred during June 6, 2012 resulting in 

an exceptional event. Statistical analysis of June 6, 2012 data showed that the exceptional event was 

statistically significantly higher than data recorded during prior months of June from 2001 to 2012 at the

Thunder Basin monitor. The stratospheric intrusion allowed O 

3-rich air to descend to the Thunder Basin 

monitor area and likely contributed to elevated 1-hour average O 

3 values. 
24 Chapter 4: Nitrogen Pollutant Concentrations

Chapter 4

Nitrogen Pollutant Concentrations

During 2017, weekly average concentrations of nitric acid, particulate nitrate, and particulate 

ammonium were measured using 3-stage filter packs at 95 CASTNET monitoring stations. Maps of 2017

annual mean total nitrate (nitric acid plus nitrate) and ammonium concentrations illustrate the 

geographic distribution of the two pollutant indicators. Box plots show trends in annual mean total 

nitrate and ammonium concentrations aggregated over 34 eastern and 16 western reference sites. The 

nitrogen pollutants measured at the 34 eastern reference sites declined over the 28-year period from 

1990 through 2017. At the eastern reference sites, annual mean concentrations of total nitrate were 

reduced by 51 percent, and ammonium concentrations declined by 68 percent. Total nitrate and 

ammonium concentrations measured at the 16 western reference sites were reduced by 37 percent and 

32 percent, respectively, over the 22-year period 1996 through 2017.

Annual mean concentrations of total NO3 (HNO 

3 + NO3) and NH+ 4 for 2017 are presented in two maps in 

this chapter. Trends in annual mean concentrations over the 28-year period, 1990 through 2017, were 

calculated from measurements from the 34 CASTNET eastern reference sites and for the 22-year period, 

1996 through 2017, from data measured at the 16 CASTNET western reference sites. See Appendix A for 

the designated reference sites.

Trace-level gas analyzers measured NOy at six EPA and two NPS CASTNET sites during 2017. A new system 

for measuring all species of total reactive nitrogen, reduced plus oxidized, was added at the DUK008, NC 

site in May. Table 4-1 lists the site locations, start dates, and the trace-level gas parameters measured at 

the eight sites.

Table 4-1 Continuous, Trace-level Gas Monitoring Stations Operated at CASTNET Sites 

during 2017

Site Location Start Dates Measurements

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, SO 

2, and CO

Duke Forest, NC (DUK008) May 2017 NO/NOy, SO 

2, and NHx

Note: *Operated by NPS.
CASTNET 2017 Annual Report

25 Chapter 4: Nitrogen Pollutant Concentrations

Total Nitrate Concentrations

Mean total NO3 concentrations measured in 2017 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 2017. Each box presents the mean and median concentrations and the 10th, 25th, 75th, and 90th 

percentiles for that year. The data shown in the upper half of the figure were aggregated from the 34 

eastern reference sites. Over the history of the network, 3-year mean levels declined from 3.0 micrograms 

per cubic meter (μg/m3 ) of air for 1990–1992 to 1.5 μg/m3 for 2015–2017, producing a 51 percent 

reduction in total NO3. 

The lower half of Figure 4-2 shows data aggregated from the 16 western sites. The 3-year mean total NO3

concentration for 2015–2017 was 37 percent lower than the corresponding 1996–1998 level. The 3-year 

mean concentration was 1.0 μg/m3 for 1996–1998 and 0.6 μg/m3 for 2015–2017.

Figure 4-1 Annual Mean Total NO3 Concentrations for 2017
CASTNET 2017 Annual Report

26 Chapter 4: Nitrogen Pollutant Concentrations

Figure 4-2 Trends in Annual Mean Total NO3 Concentrations

Eastern Reference Sites

Western Reference Sites

Particulate Ammonium Concentrations

A map of 2017 mean particulate NH+ 

4 concentrations is provided in Figure 4-3. Figure 4-4 shows box plots 

of NH+ 

4 concentrations. The trend diagram for the eastern sites (top) shows a reduction in mean NH+ 4 levels 

from 1990–1992 to 2015–2017. The 1990–1992 mean concentration was 1.8 μg/m3 , and the 2015–2017 

value was 0.6 μg/m3 , a 68 percent decline. The western reference sites show a decline from 0.3 μg/m3 in 

1996–1998 to 0.2 μg/m3 in 2015–2017, a 32 percent reduction.

The 2017 average total nitrate concentration for the eastern reference sites was 1.4 μg/m3 . The eastern total 

nitrate data show a substantive decline of 48 percent since 2000 with a reduction in 3-year mean total nitrate 

concentrations over the period 1990 through 2017 of 51 percent. This decline is less than the reduction (89 

percent) in NOx emissions from EGUs operating in the eastern United States.
CASTNET 2017 Annual Report

27 Chapter 4: Nitrogen Pollutant Concentrations

QA PROGRAM SUMMARY

Figure 4-b Historical* and 2017 Percent Completeness of 

Measurements

65 70 75 80 85 90 95 100

Atmospheric Concentrations

Meteorological Parameters

Filter Pack Flow

CO

NOYDIF

NO

NOY

SO2

Ozone

Percent Completeness

Historical (1990-2016)

2017

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 2016) 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 2017 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 2017 measurements met the criterion for each analyte.

The 2017 analytical precision results for 10 analytes 

are also 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

CASTNET measurements and supporting activities 

are assessed using DQI such as precision, accuracy, 

and completeness. Completeness is defined as the 

percentage of valid data points obtained from a 

measurement system relative to total possible data 

points. The CASTNET DQI measurement criterion 

for completeness requires a minimum 

completeness of 90 percent for every parameter for 

each quarter. 

The historical results and the results for 

2017 are given in Figure 4-b. Historical 

results for trace-level gas measurements 

represent data from 2013–2016. 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 met the completeness 

requirements of 40 CFR Part 50 (EPA, 

2015b) except for CO, which had a 

completeness less than 75 percent. 

Figure 4-a Historical and 2017 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 2016

Mackville, KY - 2017

Rocky Mtn, CO - 2017

Laboratory Replicates - 2017

Note: *Black bars are 1990–2016 for long-term data and 2013–2016 for trace-level gas data
CASTNET 2017 Annual Report

28 Chapter 4: Nitrogen Pollutant Concentrations

Figure 4-3 Annual Mean NH+ 

4 Concentrations for 2017
CASTNET 2017 Annual Report

29 Chapter 4: Nitrogen Pollutant Concentrations

Figure 4-4 Trends in Annual Mean NH+ 

4 Concentrations

Eastern Reference Sites

Western Reference Sites

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. 

NOy is defined as NOx (NO and nitrogen dioxide) plus NOz (HNO 

3, nitrous acid, peroxyacetyl nitrate, 

peroxyproyl nitrate, other organic nitrates, and nitrite). It consists of reactive gases that are considered 

precursors of O 

3 and PM2.5. 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 2017 was used to evaluate the measurements (Figures 

4-5 through 4-7). The scales for NOy and total NO3 levels are different in the three figures. 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 for 

all eight sites are listed in Table 4-2. 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 3.66 at PNF126 to 5.88 at ROM206. 
CASTNET 2017 Annual Report

30 Chapter 4: Nitrogen Pollutant Concentrations

Figure 4-5 Comparison of BVL130, IL Weekly Mean Continuous Trace-level NOy and Filter Pack 

Total NO3 Concentrations

Figure 4-6 Comparison of PNF126, NC Weekly Mean Continuous Trace-level NOy and Filter 

Pack Total NO3 Concentrations

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0

1

2

3

4

5

6

7

8

9

12/25/16

1/8/17

1/22/17

2/5/17

2/19/17

3/5/17

3/19/17

4/2/17

4/16/174/30/175/14/175/28/176/11/176/25/17

7/9/17

7/23/17

8/7/17

8/20/17

9/3/17

9/17/1710/1/17

10/15/1710/29/1711/12/1711/26/1712/10/17

Total Nitrate Concentration (ppb)

NOy Concentration (ppb)

Mean Trace NOy

Filter Pack Total NO3

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0

0.5

1.0

1.5

2.0

2.5

3.0

12/25/16

1/8/17

1/22/17

2/5/17

2/19/17

3/5/17

3/19/17

4/2/17

4/16/174/30/175/14/175/28/176/11/176/25/17

7/9/17

7/23/178/13/178/27/179/10/179/24/1710/8/17

10/22/17

11/5/17

11/19/17

12/3/17

12/17/17

Total Nitrate Concentration (ppb)

NOy Concentration (ppb)

Mean Trace

Filter Pack Total

NOy

NO3

-

Mean Continuous NOy

Mean Continuous NOy

Filter Pack Total NO3

Filter Pack Total NO3
CASTNET 2017 Annual Report

31 Chapter 4: Nitrogen Pollutant Concentrations

Figure 4-7 Comparison of PND165, WY Weekly Mean Continuous Trace-level NOy and Filter 

Pack Total NO3 Concentrations

Table 4-2 Summary of Total NO3 and NOy Measurements for 2017

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

DUK008, NC 164 0.54 2.86 5.36

BVL130, IL 213 0.92 4.28 4.99

MAC426, KY 243 0.61 2.46 4.18

HWF187, NY 497 0.18 0.84 4.81

GRS420, TN 793 0.43 1.50 3.73

PNF126, NC 1216 0.33 1.16 3.66

PND165, WY 2386 0.14 0.60 4.54

ROM206, CO 2742 0.18 0.98 5.88

0.0

0.1

0.2

0.3

0.4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

12/25/16

1/8/17

1/22/17

2/5/17

2/19/17

3/5/17

3/19/17

4/2/17

4/16/174/30/175/14/175/28/176/11/176/25/17

7/9/17

7/23/17

8/7/17

8/20/17

9/3/17

9/17/1710/1/17

10/15/1710/29/1711/12/1711/26/17

Total Nitrate Concentration (ppb)

NOy Concentration (ppb)

Mean Trace NOy

Filter Pack Total NO3

Mean Continuous NOy

Filter Pack Total NO3
32 Chapter 5: Update on the Ammonia Monitoring Network

Chapter 5

Update on the Ammonia Monitoring Network

The Ammonia Monitoring Network (AMoN) operates passive ammonia samplers at 92 locations, 65 of 

which are located at CASTNET sites. AMoN is managed by the National Atmospheric Deposition 

Program. The network has been in operation since 2007 and provides information on 2-week integrated 

ammonia concentrations. The goal of AMoN is to operate a long-term (i.e., for several decades), spatially 

diverse network with consistent measurements across the continental United States.

Reduced nitrogen [ammonia (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. Ammonia 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, 2014). Although 

NH 

3 is beneficial when used as NH 3-based fertilizer, it can have a negative effect on the environment when 

it reacts with acidic ions such as SO2-

4 and NO3 to form fine particulate matter (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.

Average annual NH 

3 concentrations for 2017 for the sites that met completeness requirements are mapped 

in Figure 5-1 and show a wide range of concentrations from a low of 0.3 μg/m3 at three sites in upstate 

New York (NY98), New Hampshire (NH02), and Nova Scotia (NS01) to a high of 16.9 μg/m3 in northern 

Utah (UT01). 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 frequently stagnant weather. The next highest concentrations ranged from 4.2 to 4.6

μg/m3 at three sites west of the Mississippi River. The highest concentration east of the Mississippi was 

observed at Cranberry, NC (NC02) with a measured concentration of 5.2 μg/m3 . High NH 

3 concentrations 

at this site can be attributed to local emissions from hog and cattle feeding and waste and crop fertilization 

and production.

Figure 5-2 provides box plots showing trends in NH 

3 concentrations from 2008 through 2017. The box 

plots were constructed with data from the 22 sites shown in Table 5-1. These sites were selected based on 

data completeness over the 10 years. The figure shows an increase in annual mean NH 

3 concentrations. In 

particular, 3-year mean concentrations increased from 1.32 μg/m3 to 1.71 μg/m3 (30 percent) from 2008-
CASTNET 2017 Annual Report

33 Chapter 5: Update on the Ammonia Monitoring Network

2010 through 2015-2017 and 3-year median concentrations increased by 16 percent from 1.11 μg/m3 to 

1.29 μg/m3 . Two sites (TX43 and CO13) with relatively high concentrations reported the largest increases.

Figure 5-1 Annual Mean NH 

3 Concentrations at AMoN sites for 2017

Figure 5-2 Trends in Annual Mean NH 

3 Concentrations
CASTNET 2017 Annual Report

34 Chapter 5: Update on the Ammonia Monitoring Network

Table 5-1 AMoN Sites Included in Trends Analysis

AMoN Site ID Site Location/Name State CASTNET Site ID

CO13 Fort Collins CO

ID03 Craters of the Moon National 

Monument

ID

IL11 Bondville IL BVL130

IN99 Indianapolis IN

MD08 Piney Reservoir MD

MD99 Beltsville MD

MI96 Detroit MI

MN18 Fernberg MN

NC06 Beaufort NC BFT142

NC30 Duke Forest NC DUK008

NC35 Clinton Crops Research Station NC

NM98 Navajo Lake NM

NM99 Farmington NM

NY16 Cary Institute NY

NY67 Ithaca NY CTH110

OH02 Athens Super Site OH

OH27 Cincinnati OH

OK99 Stilwell OK CHE185

PA00 Arendtsville PA ARE128

SC05 Cape Romain National Wildlife Refuge SC

TX43 Cañónceta TX PAL190

WI07 Horicon Marsh WI
35 Chapter 6: Sulfur Pollutant Concentrations

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 2017. Maps of 2017 annual mean concentrations provide 

the geographic distribution of the two pollutants across the United States. Annual sulfur dioxide and 

sulfate concentrations were aggregated over 34 eastern and 16 western reference sites to show trends 

using box plots. The sulfur dioxide pollutants measured at the eastern reference sites declined by 89

percent over the 28-year period from 1990 through 2017. Particulate sulfate concentrations at the eastern 

reference sites were reduced by 75 percent. Sulfur dioxide and sulfate concentrations measured at the 16 

western reference sites have decreased by 45 and 35 percent, respectively, over the 22-year period, 1996 

through 2017.

Annual mean concentrations of SO 

2 and SO2-4 for 2017 are presented in this chapter. Trends in annual mean 

concentrations over the 28-year period, 1990 through 2017, were derived from measurements from the 34 

CASTNET eastern reference sites and for the 22-year period, 1996 through 2017, from data measured at 

the 16 CASTNET western reference sites. See Appendix A for descriptions of the designated reference sites.

Sulfur Dioxide Concentrations

Annual mean SO 

2 concentrations for 2017 are shown in Figure 6-1. Annual mean concentrations were 

highest in the Midwest and East near and downwind of the Ohio River. The highest annual mean SO 

2

concentration was measured in the West at Basin, WY (BAS601), which measured unexpectedly high SO 

2

concentrations during fourth quarter 2017. See the additional discussion of BAS601 SO 

2 concentrations at 

the end of this chapter.

Box plots of annual mean SO 

2 concentrations aggregated over the 34 eastern reference sites from 1990 

through 2017 (top) and the 16 western reference sites from 1996 through 2017 (bottom) are depicted in 

Figure 6-2. The y-axes on the western and eastern plots have different scales because concentrations 

measured at the western CASTNET sites were much lower than those measured at the eastern sites. Threeyear mean SO 

2 concentrations for the CASTNET eastern reference sites for 1990–1992 and 2015–2017 were 

8.8 μg/m3 and 0.9 μg/m3 , respectively. This change constitutes an 89 percent reduction in 3-year mean SO 

2

concentrations between the two periods. 

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 2015–2017 were 

0.6 μg/m3 and 0.3 μg/m3 , respectively. This change constitutes a 45 percent reduction in 3-year mean SO 

2

concentrations at the CASTNET western reference sites over the 22 years.
CASTNET 2017 Annual Report

36 Chapter 6: Sulfur Pollutant Concentrations

The 2017 average sulfur dioxide concentration for the eastern reference sites was 0.7 μg/m3 . The eastern sulfur 

dioxide data show a substantive decline since 1997. The reduction (89 percent) in sulfur dioxide concentrations 

over the period 1990 through 2017 is consistent with the reduction (96 percent) in sulfur dioxide emissions from 

EGUs operating in the eastern United States.

Figure 6-1 Annual Mean SO 

2 Concentrations for 2017
CASTNET 2017 Annual Report

37 Chapter 6: Sulfur Pollutant Concentrations

Figure 6-2 Trends in Annual Mean SO 

2 Concentrations

Eastern Reference Sites

Western Reference Sites

The trend in SO 

2 emissions from regulated EGUs from 1990 through 2017, aggregated over the eastern 

United States, was 96 percent, which is consistent with the 89 percent reduction (Figure 6-2) in annual 

mean SO 

2 concentrations aggregated over the CASTNET eastern reference sites.

Particulate Sulfate Concentrations

Figure 6-3 shows a map of 2017 annual mean particulate SO2-

4 concentrations. Figure 6-4 provides box 

plots of annual mean SO2-

4 concentrations from the 34 eastern reference sites and 16 western reference 

sites. The figure shows a substantial decline in SO2-

4 over the 28 years for the eastern CASTNET reference 

sites. The difference between 3-year means from 1990–1992 to 2015–2017 depicts a 75 percent reduction 

in SO2-

4 from 5.4 μg/m3 to 1.3 μg/m3 . The 2017 mean SO2-4 level of 1.1 μg/m3 for the eastern reference sites 

was the lowest in the history of the network. The box plots for the western reference sites are provided on 

the lower half of Figure 6-4. The data show a 35 percent reduction in annual mean SO2-

4 concentrations 

aggregated over the 16 sites with 1996–1998 and 2015–2017 concentrations of 0.8 μg/m3 and 0.5 μg/m3 

, 

respectively.
CASTNET 2017 Annual Report

38 Chapter 6: Sulfur Pollutant Concentrations

The 2017 average sulfate concentration for the eastern reference sites was 1.1 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.

Figure 6-3 Annual Mean SO2-

4 Concentrations for 2017
CASTNET 2017 Annual Report

39 Chapter 6: Sulfur Pollutant Concentrations

Figure 6-4 Trends in Annual Mean SO2-

4 Concentrations

Eastern Reference Sites

Western Reference Sites

Sulfur Dioxide Concentrations at Basin, WY

The BLM-sponsored site at Basin, WY (BAS601) is one of eight CASTNET monitoring sites in Wyoming 

(Figure 1-1). BAS601 measured unexpectedly high SO 

2 concentrations during fourth quarter 2017 

causing concern about increased pollution. The data show the SO 

2 concentrations at BAS601 were 

consistently higher than levels measured at other Wyoming CASTNET sites (Figure 6-5). Although 

hundreds of oil and gas, mining, and other industrial facilities operate in the Bighorn Basin of 

Wyoming, no one particular activity can explain the high SO 

2 levels measured at BAS601. High 

concentrations persisted through first quarter 2018 and then returned to levels more consistent with 

the other Wyoming CASTNET sites.
CASTNET 2017 Annual Report

40 Chapter 6: Sulfur Pollutant Concentrations

Figure 6-5 Quarterly Mean SO 

2 Concentrations for Wyoming CASTNET Sites (Fourth Quarter 

2012 through Second Quarter 2018)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

Q4 2012Q1 2013Q2 2013Q3 2013Q4 2013Q1 2014Q2 2014Q3 2014Q4 2014Q1 2015Q2 2015Q3 2015Q4 2015Q1 2016Q2 2016Q3 2016Q4 2016Q1 2017Q2 2017Q3 2017Q4 2017Q1 2018Q2 2018

Concentration (μg/m3)

BAS601

BUF603

FOR605

SHE604

YEL408
41 Chapter 7: Atmospheric Deposition of Nitrogen

Chapter 7

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 2017, EPA used NADP’s Total 

Deposition Science Committee Hybrid Method. The hybrid method combines measured pollutant 

concentrations with output from the CMAQ modeling system. 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 primary goal of CASTNET is to estimate the dry deposition of pollutants from the 

atmosphere to sensitive ecosystems. The NADP Total Deposition (TDep) Hybrid Method (EPA, 2015c; 

Schwede and Lear, 2014) combines CASTNET monitoring data with output from the CMAQ modeling 

system (Byun and Schere, 2006) to estimate dry deposition. The TDep method preferentially weights

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 web page 

(http://nadp.slh.wisc.edu/committees/tdep/). See also the NADP Fact Sheet, “Hybrid Approach to Mapping 

Total Deposition,” (http://nadp.slh.wisc.edu/lib/brochures/tdepsheet.pdf).

For wet deposition, the TDep method combines precipitation amounts measured at NADP/NTN sites with a 

PRISM-modeled grid of precipitation data. PRISM uses terrain elevation, slope, and aspect and climatic 

measurements to estimate precipitation. Pollutant concentrations in precipitation are measured by 

NADP/NTN, extrapolated into a grid using an inverse distance weighting algorithm, and then multiplied by 

the combined precipitation grid to generate a wet deposition surface. 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 ).

A map of the dry deposition fluxes of nitrogen (N) for 2017 generated by the TDep method is shown in 

Figure 7-1. The magnitude of the deposition fluxes is illustrated by the shading in the figure legend. A map 

of total deposition of N for 2017 is given in Figure 7-2. The percentage of total deposition of N due to dry 

deposition is shown in Figure 7-3. A map of TDep method estimates of total deposition of reduced N 

species for 2017 is given in Figure 7-4. Figure 7-5 gives the percentage of total deposition of reduced N 

species from dry deposition. Dry deposition of N species that are not routinely measured but are modeled 

by CMAQ and used in the TDep method is shown in the map in Figure 7-6, and its percentage of total 

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

3 for 2017 is shown in Figure 7-8, and 

the net dry deposition of NH 

3 is given in Figure 7-9. The net dry deposition figure shows the direction of 

the flux (i.e., positive values for deposition and negative values for emissions).
CASTNET 2017 Annual Report

42 Chapter 7: Atmospheric Deposition of Nitrogen

Figure 7-1 TDep Dry Deposition Estimates of N (kg ha-1

 yr-1 ) for 2017

Source: CASTNET|CMAQ|NADP

Figure 7-2 TDep Total Deposition Estimates of N (kg ha-1

 yr-1 ) for 2017

Source: CASTNET|CMAQ|NADP
CASTNET 2017 Annual Report

43 Chapter 7: Atmospheric Deposition of Nitrogen

Figure 7-3 TDep Percent of Total Deposition of N from Dry Deposition for 2017

Source: CASTNET|CMAQ|NADP

Figure 7-4 TDep Total Deposition Estimates of Reduced N Species (kg ha-1

 yr-1 ) for 2017

Source: CASTNET|CMAQ|NADP
CASTNET 2017 Annual Report

44 Chapter 7: Atmospheric Deposition of Nitrogen

Figure 7-5 TDep Percent of Total Deposition of Reduced N Species from Dry Deposition 

for 2017

Source: CASTNET|CMAQ|NADP

Figure 7-6 TDep Dry Deposition Estimates of Unmonitored N Species (kg ha-1

 yr-1 ) for 2017

Source: CASTNET|CMAQ|NADP
CASTNET 2017 Annual Report

45 Chapter 7: Atmospheric Deposition of Nitrogen

Figure 7-7 TDep Percent of Total Deposition of N from Dry Deposition of Unmonitored Species 

for 2017

Source: CASTNET|CMAQ|NADP

Figure 7-8 TDep Gross Dry Deposition of NH 

3 as N (kg ha-1 yr-1 ) for 2017

Source: CASTNET|CMAQ|NADP
CASTNET 2017 Annual Report

46 Chapter 7: Atmospheric Deposition of Nitrogen

Figure 7-9 TDep Net Dry Deposition of NH 

3 as N (kg ha-1 yr-1 ) for 2017

Source: CASTNET|CMAQ|NADP
47 Chapter 8: Effects of California Wildfires on Air Quality

Chapter 8

Effects of California Wildfires on Air Quality

Wildfires and prescribed agricultural and forest management burns produce large amounts of aerosols 

and trace-level gases from burning vegetation, buildings, and other materials. Smoke from wildfires is a 

significant source of air pollution and can cause risks to health, visibility, and safety. The 2017 wildfire 

season was extensive with major fires in California in October and December. The October 2017 wildfires

in Northern California were part of 250 wildfires that started burning across the state beginning in early 

October. These wildfires affected Napa, Sonoma, Lake, Mendocino, Butte, and Solano counties. Twentynine wildfires ignited across Southern California in December 2017. The Thomas Fire, which started on 

December 4, affected Ventura and Santa Barbara counties. CASTNET ozone measurements and air 

quality data from local air pollution control districts showed elevated levels of ozone and fine particulate 

matter during the California wildfire events.

Wildfire smoke is a mixture of CO, volatile organic compounds, PM2.5, and smaller particles that include 

alkaline ash, black and organic carbon, and polyaromatic hydrocarbons. Wildfire smoke is also a precursor 

to downwind O 

3 concentrations. The 2017 wildfire season was extensive with major fires in California in 

October and December. Wildfires were not confined to the West. In late November, major fires were 

reported in Arkansas, Kansas, Kentucky, Missouri, Oklahoma, and Pennsylvania (Peltier, 2017).

Measurements from CASTNET and other federal, state, and county networks provided information to 

characterize the impacts of wildfires on air quality and visibility near critical resources. Measurements from 

local air pollution control districts (APCDs) and AirNow (https://www.airnow.gov/) provided additional 

information.

Northern California Wildfires

The October 2017 Northern California wildfires were part of 250 wildfires that started burning across the 

state beginning in early October. Twenty-one became major fires that burned at least 245,000 acres. Fires 

started on the evening of Sunday, October 8, with the largest fires occurring in Napa and Sonoma counties. 

Weather and dry vegetation at the time were favorable for rapid fire growth. The wildfires spread 

throughout Napa, Sonoma, Lake, Mendocino, Butte, and Solano counties during severe fire weather 

conditions (CAL FIRE, October 30, 2017).

Figure 8-1 shows satellite images (Alrick, 2018) of the smoke plumes from the fires in Northern California. A 

daily weather map for October 9, 2017 [National Oceanic and Atmospheric Administration (NOAA), 2017] is 

given in Figure 8-2. A strong pressure gradient between the Bay Area and a surface high over Idaho 

resulted in brisk offshore winds that spread the fires. 
CASTNET 2017 Annual Report

48 Chapter 8: Effects of California Wildfires on Air Quality

An air quality index (AQI) is a form of measurement used by EPA and other governmental agencies to 

inform the public about levels of air pollution in their communities. Figure 8-3 provides a map of daily 

average PM2.5 concentrations color-coded by the AQI category for each area on October 9. 

Figure 8-1 Satellite Image of October 2017 Northern California Wildfires

Source: Alrick, 2018

Figure 8-2 Daily Weather Map for October 9, 2017

Source: NOAA, 2017 (https://www.wpc.ncep.noaa.gov/dailywxmap/index_20171009.html)
CASTNET 2017 Annual Report

49 Chapter 8: Effects of California Wildfires on Air Quality

Figure 8-3 Map of Daily Average PM2.5 Concentrations (μg/m3 ) Color-Coded by AQI Category 

for October 9, 2017

Source: Alrick, 2018

The winds switched to more northerly by October 12 and varied throughout the period October 8 through 

October 19. By October 12 (Figure 8-4), smoke from the wildfires had spread nearly 100 miles, registering 

"unhealthy" AQI in the cities of Oakland, San Francisco, and San Rafael. Downwind from the fire, the air 

quality in the city of Napa was ranked the poorest in the nation due to high levels of PM2.5 and O 

3 (Ho and 

Lyons, 2017). 
CASTNET 2017 Annual Report

50 Chapter 8: Effects of California Wildfires on Air Quality

Figure 8-4 Map of Daily Average PM2.5 Concentrations (μg/m3 ) Color-Coded by AQI Category 

for October 12, 2017

Source: Alrick, 2018

Southern California Wildfires

Twenty-nine wildfires ignited across Southern California in December 2017. Six of the fires became 

significant wildfires that led to widespread evacuations and property losses. The wildfires burned over 

307,900 acres and caused high air pollutant concentrations, traffic disruptions, school closures, and power 

outages. The largest of the wildfires was the Thomas Fire, which grew to 281,893 acres. These wildfires were 

exacerbated by unusually powerful and long-lasting Santa Ana winds and large amounts of dry vegetation, 

due to an extensive dry period following the 2016–2017 rainy season. The Santa Ana winds also occurred at 

the end of an active and destructive wildfire season in 2016 (Byrne, 2018).

The Thomas Fire started on December 4 near Thomas Aquinas College just north of Santa Paula in Ventura 

County from which it received its name. Fast-moving, it quickly reached the City of Ventura. Figure 8-5 

shows a photograph of the smoke plume from a location in north Oxnard looking toward the City of 

Ventura. The fire began burning through the rugged Santa Ynez Mountains as it threatened several small 

communities along the Rincon Coast north of Ventura, expanded into the Los Padres National Forest, and 

reached Santa Barbara County.

The unusually strong and persistent Santa Ana winds were the largest factor in the spread of the fire. The 

region experienced an on-and-off Santa Ana wind event for a little over two weeks, which contributed to 
CASTNET 2017 Annual Report

51 Chapter 8: Effects of California Wildfires on Air Quality

the Thomas Fire's persistent growth. The winds also dried out the air, resulting in extremely low humidity. 

Figure 8-6 presents satellite views of the Thomas Fire in Ventura and Santa Barbara Counties.

Figure 8-7 shows Santa Barbara County air quality monitoring locations. Tables 8-1 through 8-3 present 

daily average O 

3, PM10, and PM2.5 concentrations color-coded by AQI Category for December 9, 14, and 18,

2017. Fine particulate matter concentrations measured at the Santa Barbara monitor indicated unhealthy 

conditions on December 9 and 14. Conditions returned to moderate or better levels at all monitors by 

December 18, 2017. Surprisingly, O 

3 concentrations remained mostly in the good category throughout the 

period.

Figure 8-8 shows monitoring locations in Ventura County. Figures 8-9 and 8-10 depict PM2.5 and O 

3

concentrations, respectively, for five Ventura County monitors for December 2017. Figure 8-10 also shows 

O 

3 concentrations from the CASTNET site at Joshua Tree National Park (JOT403). See Figure 1-1 for the 

specific location. Concentrations of PM2.5 reached over 500 μg/m3 on December 6 and 8 at Ojai at the 

height of the Thomas Fire. Ozone concentrations remained low during the fire. It is interesting that the 

lowest O 

3 concentration (17 ppb) at Ojai occurred during the period with the highest PM2.5 concentration 

(557 μg/m3 ).

Figure 8-5 Thomas Fire Viewed from North Oxnard

Source: J. J. Lavery 
CASTNET 2017 Annual Report

52 Chapter 8: Effects of California Wildfires on Air Quality

Figure 8-6 Satellite Views of the Thomas Fire in Ventura and Santa Barbara County

December 5, 2017

Source: National Aeronautics and Space Administration Terra satellite photo of the smoke plumes on December 5, 2017.

https://earthobservatory.nasa.gov/images/91379/smoke-and-fire-in-southern-california

Figure 8-7 Santa Barbara County Air Quality Monitoring Locations

Source: Santa Barbara County Air Pollution Control District (https://www.ourair.org/location-of-santa-barbara-county-monitoring-stations/)
CASTNET 2017 Annual Report

53 Chapter 8: Effects of California Wildfires on Air Quality

Table 8-1 Daily Average O 

3, PM10, and PM2.5 Concentrations by Color-Coded AQI Category for 

December 9, 2017

Stations Monitoring Ozone and Particulates

Location Ozone (ppb) PM10 (μg/m3 ) PM2.5 (μg/m3 )

Santa Barbara Good (10) Unhealthy for Sensitive Groups (109) Unhealthy (172)

Santa Maria Good (27) Good (28) Good (48)

Lompoc H St Good (18) Moderate (53) Moderate (96)

Goleta Good (6) Moderate (77) Unhealthy (153)

El Capitan Good (34) Moderate (68)

Las Flores Canyon Good (43) Moderate (59)

VAFB Good (44) Moderate (52)

Carpinteria Good (27)

Lompoc HS and P Good (44)

Nojoqui Good (34)

Paradise Rd Good (31)

Santa Ynez Good (23) NA

Source: Hoffman, 2018

Table 8-2 Daily Average O 

3, PM10, and PM2.5 Concentrations by Color-Coded AQI Category for 

December 14, 2017

Stations Monitoring Ozone and Particulates

Location Ozone (ppb) PM10 (μg/m3 ) PM2.5 (μg/m3 )

Santa Barbara Good (12) Moderate (96) Unhealthy (155)

Santa Maria Good (26) Good (44) Good (43)

Lompoc H St Good (24) Good (44) Moderate (59)

Goleta Good (18) Moderate (62) Unhealthy for Sensitive Groups (113)

El Capitan Good (39) Moderate (52)

Las Flores Canyon Good (47) NA

VAFB Good (48) Moderate (51)

Carpinteria Good (19)

Lompoc HS and P Good (49)

Nojoqui Good (41)

Paradise Rd Moderate (58)

Santa Ynez Good (44) Unhealthy for Sensitive Groups (112)

Source: Hoffman, 2018
CASTNET 2017 Annual Report

54 Chapter 8: Effects of California Wildfires on Air Quality

Table 8-3 Daily Average O 

3, PM10, and PM2.5 Concentrations by Color-Coded AQI Category for 

December 18, 2017

Stations Monitoring Ozone and Particulates

Location Ozone (ppb) PM10 (μg/m3 ) PM2.5 (μg/m3 )

Santa Barbara Good (36) Moderate (61) Moderate (66)

Santa Maria Good (31) Good (45) Good (46)

Lompoc H St Good (28) Good (49) Moderate (53)

Goleta Good (34) Good (45) Moderate (55)

El Capitan Good (39) Good (41)

Las Flores Canyon Good (44) Good (47)

VAFB Good (46) Good (49)

Carpinteria Good (34)

Lompoc HS and P Good (44)

Nojoqui Good (34)

Paradise Rd Good (38)

Santa Ynez Good (39) Moderate (53)

Source: Hoffman, 2018

Figure 8-8 Monitoring Locations in Ventura County

Source: Ventura County Air Pollution Control District (http://www.vcapcd.org/MonitoringStations.htm)
CASTNET 2017 Annual Report

55 Chapter 8: Effects of California Wildfires on Air Quality

Figure 8-9 Time series of 24-hr PM2.5 Concentrations from Ventura County APCD Sites

Figure 8-10 Time series of DM8A O 

3 Concentrations from Ventura County APCD Sites and the 

Joshua Tree National Park CASTNET Site (JOT403)

0

100

200

300

400

500

600

12/1/1712/3/1712/5/1712/7/1712/9/17

12/11/1712/13/1712/15/1712/17/1712/19/1712/21/1712/23/1712/25/1712/27/1712/29/1712/31/17

Concentration (μg/m3)

Thousand Oaks - Moorpark Road

Ojai - Ojai Ave.

Simi Valley - Cochran Street

El Rio - Rio Mesa School #2

Piru - Pacific

0

10

20

30

40

50

60

70

12/1/1712/3/1712/5/1712/7/1712/9/17

12/11/1712/13/1712/15/1712/17/1712/19/1712/21/1712/23/1712/25/1712/27/1712/29/1712/31/17

Concentration (ppb)

Thousand Oaks - Moorpark Road

Ojai - Ojai Ave.

Simi Valley - Cochran Street

El Rio - Rio Mesa School #2

Piru - Pacific

JOT403
56 Chapter 9: Atmospheric Deposition of Sulfur,

Base Cations, and Chloride

Chapter 9

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 TDep method to assess the status of 

dry and wet deposition for 2017. Total deposition was calculated as the sum of estimated dry and wet 

deposition.

Gaseous and particulate sulfur pollutants, base cations, and Clare 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 method (EPA, 2015c; 

Schwede and Lear, 2014) was used to estimate dry and wet deposition. Dry and wet deposition rates were 

summed to obtain estimates of total deposition across the United States.

Sulfur Deposition

Figure 9-1 shows a map of TDep method estimates of dry deposition rates of sulfur (S) for 2017. Estimated 

rates of total deposition of S for 2017 are given in Figure 9-2. 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 9-3.

Deposition of Base Cations and Chloride

A map of 2017 estimated total deposition of base cations (Ca2+

 , K+ , Mg2+ , and Na+ ) is given in Figure 9-4. 

Figure 9-5 provides estimated rates for 2017 for total deposition of Cl-

.

Georgia Station, GA (GAS153)
CASTNET 2017 Annual Report

57 Chapter 9: Atmospheric Deposition of Sulfur,

Base Cations, and Chloride

Figure 9-1 TDep Dry Deposition Estimates of S (kg ha-1

 yr-1 ) for 2017

Source: CASTNET|CMAQ|NADP

Figure 9-2 TDep Total Deposition Estimates of S (kg ha-1

 yr-1 ) for 2017

Source: CASTNET|CMAQ|NADP
CASTNET 2017 Annual Report

58 Chapter 9: Atmospheric Deposition of Sulfur,

Base Cations, and Chloride

Figure 9-3 TDep Percent of Total Deposition of S from Dry Deposition for 2017

Source: CASTNET|CMAQ|NADP

Figure 9-4 TDep Total Deposition Estimates of the Base Cations Ca2+

 , K+ , Mg2+ , Na+ 

(kiloequivalents per hectare per year) for 2017

Source: CASTNET|CMAQ|NADP
CASTNET 2017 Annual Report

59 Chapter 9: Atmospheric Deposition of Sulfur,

Base Cations, and Chloride

Figure 9-5 TDep Total Deposition Estimates of Total Cl-(kg ha-1

 yr-1 ) for 2017

Source: CASTNET|CMAQ|NADP
60 References

References

Alrick, D. 2018. October 2017 Northern California Wildfires. National Air Quality Conference, Austin, 

January 25, 2018.

Browell, E. V., Danielsen, E. F., Ismail, S., Gregory, G. L., and Beck, S. M. (1987), Tropopause Fold Structure 

Determined from Airborne LIDAR and in situ Measurements, J. Geophys. Res., 92(D2), 2112–2120, 

doi:10.1029/JD092iD02p02112.

Byrne, K. 2018. California's 2017 Wildfire Season to Rank among Most Destructive, Costly on Record. 

https://www.accuweather.com/en/weather-news/californias-2017-wildfire-season-to-rank-amongmost-destructive-costly-on-record/70003704. Accessed August 2018.

Byun, D. and Schere, K. L. 2006. Review of the Governing Equations, Computational Algorithms, and Other 

Components of the Models-3 Community Multiscale Air Quality (CMAQ) Modeling System. Applied 

Mechanics Reviews, 59: 51-77. doi:10.1115/1.2128636.

CAL FIRE. 2017. Statewide Fire Summary for Monday Morning, October 30, 2017. 

https://goldrushcam.com/sierrasuntimes/index.php/news/local-news/11736-cal-fire-californiastatewide-fire-summary-for-monday-morning-october-30-2017-5-active-wildfires-with-around-1-

000-firefighters. Accessed August 2018

Danielsen, E. 1968. Stratospheric-Tropospheric Exchange Based on Radioactivity, Ozone, and Potential 

Vorticity. J. Atmos Sciences. Vol 25. Pages 502-518. 

Danielsen, E. F., Hipskind, R. S., Gaines, S. E., Sachse, G. W., Gregory, G. L., and Hill, G. F. 1987. ThreeDimensional Analysis of Potential Vorticity Associated with Tropopause Folds and Observed 

Variations of Ozone and Carbon Monoxide, J. Geophys. Res., 92( D2), 2103– 2111, 

doi:10.1029/JD092iD02p02103.

Harshfield, G. 2018. Letter to Ozone Data Managers. Colorado Department of Public Health and 

Environment (CDPHE), Air Pollution Control Division (APCD). January 12.

Ho, V. and Lyons, J. 2017. Live Updates: Northern California Wildfires Cause Estimated $3 Billion Damage. 

San Francisco Chronicle (October 15, 2017).
CASTNET 2017 Annual Report

61 References

References (continued)

Hoffman, L. 2018. Southern California Fires – December 2017. Presentation at California Air Response 

Planning Alliance (CARPA) 2017 Wildfire Smoke Air Response and After-Action Review. 

International Organization for Standardization (ISO). 2005. Statistical Methods for the Use in Proficiency 

Testing by Interlaboratory Comparisons, Annex C, Robust Analysis, Section C.1: Algorithm A. 

Standard 13528. ISO 13528:2005(E).

Kaldunski, B., Pierce, B., and Holloway, T. 2017. When Stratospheric Ozone Hits Ground-level Regulation, 

Exceptional Events in Wyoming, doi:10.1175/BAMS-D-14-00133.1. Accessed August 2018.

Martin, R. S. and Baasandorj, M. 2016. A Comparison of AMoN Measurements with Localized, Arrayed Passive 

NH 

3 Samplers in Northern Utah. National Atmospheric Deposition Program (NADP), 2016 Fall 

Meeting, Santa Fe, NM, Oct. 31–Nov. 4.

National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Prediction 

(NCEP), Weather Prediction Center. 2017. Daily Weather Map, October 9, 2017. 

https://www.wpc.ncep.noaa.gov/dailywxmap/index_20171009.html. Accessed August 2018.

National Oceanic and Atmospheric Administration (NOAA) Earth System Research Laboratory. 2018. The 

High Resolution Rapid Refresh (HRRR). https://rapidrefresh.noaa.gov/hrrr/. Accessed August 2018.

Peltier, R. E. 2017. The Hazards of Breathing Wildfire Smoke. https://www.psmag.com/environment/healthhazards-of-breathing-wildfire. Accessed September 2018.

Schwede, D. B. and Lear, G. G. 2014. A Novel Hybrid Approach for Estimating Total Deposition in the United 

States. Atmos. Environ., 92:207-220. doi:10.1016/j.atmosenv.2014.04.008. 

U.S. Environmental Protection Agency (EPA). 2018a. Clean Air Status and Trends Network Factsheet-2018. 

https://www3.epa.gov/castnet/docs/CASTNET-Factsheet-2018.pdf. Accessed September 2018.

U.S. Environmental Protection Agency (EPA). 2018b. Guidance on the Preparation of Exceptional Events 

Demonstrations for Stratospheric Ozone Intrusions. Publication No. EPA-457/B-18-001. 

https://nepis.epa.gov/Exe/ZyPDF.cgi/P100WS4X.PDF?Dockey=P100WS4X.PDF Accessed June 2019.

U.S. Environmental Protection Agency (EPA). 2016. Treatment of Air Quality Data Influenced by Exceptional 

Events. https://www.epa.gov/air-quality-analysis/treatment-air-quality-data-influenced-exceptionalevents. June 2018.
CASTNET 2017 Annual Report

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References (continued)

U.S. Environmental Protection Agency (EPA). 2015a. Interpretation of the Primary and Secondary National 

Ambient Air Quality Standards for Ozone. Title 40 Code of Federal Regulations (CFR) Appendix U to 

Part 50. https://www.ecfr.gov/cgi-bin/textidx?SID=9b5c394e6ac7be772c8f605a6554e08b&mc=true&node=pt40.2.50&rgn=div5#ap40.2.50_1

19.u. Accessed August 2018.

U.S. Environmental Protection Agency (EPA). 2015b. Title 40 Code of Federal Regulations (CFR) Part 50, 

Primary and Secondary National Ambient Air Quality Standards. https://www.ecfr.gov/cgi-bin/textidx?SID=b67492f51b28e27bf9b4226033416c93&mc=true&node=pt40.2.50&rgn=div5. Accessed 

August 2018.

U.S. Environmental Protection Agency (EPA). 2015c. Total Deposition Estimates Using a Hybrid Approach. 

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2018.

U.S. Environmental Protection Agency (EPA). 2014. National Emissions Inventory (NEI) Data. 

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Accessed August 2018.

Ute Indian Tribe of the Unitah and Ouray Reservation. 2016. Ozone NAAQS Exceedances Occurring June 8 

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. 2017. Clean Air Status and Trends Network 

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* Formerly known as Amec Foster Wheeler Environment & Infrastructure, Inc.
CASTNET 2017 Annual Report

Appendix A

Appendix A

Locational and Operational Characteristics of CASTNET Sites
CASTNET 2017 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

CHC432, NM Chaco Culture 

NHP

3/1/2017 36.035 -107.9042 1965 CO99 Desert Complex NPS

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

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

DUK008, NC Duke Forest 4/3/2017 35.9745 -79.099 164 NC41 Forest Rolling EPA

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 2017 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 2017 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: NHP = National Historical Park

NM = National Monument

NP = National Park

NWR = National Wildlife Reserve

SP = State Park

E = eastern reference site

W = western reference site
CASTNET 2017 Annual Report

Appendix B

Appendix B

Acronyms and Abbreviations
CASTNET 2017 Annual Report

B-1 Appendix B

List of Acronyms and Abbreviations

AJAX Alpha Jet Atmospheric eXperiment

AMNet Atmospheric Mercury Network

AMoN Ammonia Monitoring Network 

APCD air pollution control district

AQI air quality index

AQS EPA’s Air Quality System

ARP Acid Rain Program

BLM Bureau of Land Management

°C degrees Celsius

Ca2+ particulate calcium 

CAPMoN Canadian Air and Precipitation Monitoring Network

CASTNET Clean Air Status and Trends Network

CDPHE Colorado Department of Public Health and the Environment

CFR Code of Federal Regulations

Cl- particulate chloride 

CMAQ Community Multiscale Air Quality Modeling System

CO carbon monoxide

DM8A daily maximum 8-hour average

DQI data quality indicator

ECAN Environment Canada

EGUs electric generating units

EPA U. S. Environmental Protection Agency

GOES Geostationary Operational Environmental Satellite system

hPa hectopascal

HNO 

3 nitric acid

IMPROVE Interagency Monitoring of Protected Visual Environments

IO informational stratospheric intrusion data flags

ISO International Organization for Standardization

K Kelvin

K+ particulate potassium

kg kilogram

kg ha-1

yr-1kilograms per hectare per year

m meter

m2 /sec square meter per second

MARPD mean absolute relative percent difference

mb millibar

MDN Mercury Deposition Network

MERRA-2 Modern-Era Retrospective Analysis for Research and Applications, Version 2
CASTNET 2017 Annual Report 

 B-2 Appendix B 

List of Acronyms and Abbreviations (continued) 

Mg2+

 particulate magnesium 

MST Mountain Standard Time 

N nitrogen 

Na+ particulate sodium 

NAAQS National Ambient Air Quality Standards 

NADP National Atmospheric Deposition Program 

NAM North American Mesoscale model 

NCore EPA’s National Core Monitoring 

NH 

3 ammonia 

NH +

4 particulate ammonium 

NO nitrogen oxide 

NO -

3 particulate nitrate 

NOx nitrogen oxides (NO + NO 

2) 

NOy total reactive oxides of nitrogen 

NOAA National Oceanic and Atmospheric Administration 

NPS National Park Service 

NTN National Trends Network 

O 

3 ozone 

PM10 particulate matter with a mean aerodynamic diameter less than 10 micrometers 

PM2.5 fine particulate matter 

ppb parts per billion 

ppm parts per million 

PRISM Parameter-elevation Regressions on Independent Slopes Model 

PT proficiency testing 

PV potential vorticity 

PVU potential vorticity unit (m2 /sec times Kelvin/kilogram) 

QA quality assurance 

QAPP Quality Assurance Project Plan 

RAP Rapid Refresh Model 

RAQMS Realtime Air Quality Modeling System 

S sulfur 

SI Workgroup EPA Ozone Stratospheric Intrusion Workgroup 

SO 

2 sulfur dioxide 

SO2-

4 particulate sulfate 

TDep NADP Total Deposition Hybrid Method 

total NO3 gaseous HNO 

3 + particulate NO3

μg/m3 micrograms per cubic meter 

WARMS Wyoming Air Resources Monitoring System 

WDEQ/AQD State of Wyoming Department of Environmental Quality/Air Quality Division