CASTNET 2017 Annual Report

Executive Summary

Key Results and Highlights through 2017

The median fourth highest daily maximum 8-hour average (DM8A) ozone (O₃) 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 O₃ 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 O₃ 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 O₃ 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 NO_x control programs have resulted in substantive reductions in emissions. NO_x emissions declined by 89 percent over the 28-year period, 1990 through 2017, at regulated electric generating units (EGUs) in the East. Three-year 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 (SO₂) concentrations measured at the eastern reference sites have declined significantly over the 28-year period, 1990 through 2017. Three-year mean annual SO₂ levels at eastern sites decreased 89 percent reflecting the reduction in regulated eastern EGU SO₂ emissions (96 percent). SO₂ concentrations measured at the western reference sites declined by 45 percent over the 21 years from 1996 through 2017.

Pollutant	Western Reference Sites		Eastern Reference Sites		Percent Changed
	1996-1998	2014-2016	1990-1992	2014-2016	West	East
O3 (ppb)	74	66	85	64	-10	-24
Total NO3- (µg/m  3 )	1.0	0.6	3.0	1.5	-37	-51
SO2 (µg/m  3 )	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 O₃ 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 O₃ to ground-level concentrations in the Intermountain West and the effects of the California wildfires in October and December 2017 on air quality.

Chapter 1: CASTNET Update

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 (O₃) 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).

The U.S. Congress established the Acid Rain Program (ARP) in 1990 to reduce emissions of sulfur dioxide (SO₂) and nitrogen oxides (NO_x) 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 Air Quality 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)
• EPA’s National Core Monitoring (NCore)
• BLM’s Wyoming Air Resources Monitoring System (WARMS)
• Interagency Monitoring of Protected Visual Environments (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 O₃. 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.

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 (HNO₃) and SO₂. Particulate concentrations include nitrate (NO₃^-), ammonium (NH₄⁺), sulfate (SO₄^2-), magnesium (Mg  ²⁺ ), calcium (Ca  ²⁺ ), 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 O₃ monitoring system. O₃ concentrations were measured at 82 sites. The O₃ inlet and filter pack are located atop a 10-m tower. Some CASTNET sites also measure trace-level SO₂, carbon monoxide (CO), and NO/NO_y. 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).

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.

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. 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.

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.

Chapter 2: Ozone Concentrations

CASTNET O₃ 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 O₃ 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 O₃ concentrations for O₃ NAAQS compliance and for presenting information on trends and geographic patterns in regional O₃.

The primary O₃ NAAQS is designed to protect public health. The secondary standard is designed to protect public welfare and the environment. Both the primary and secondary O₃ 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 O₃ 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 O₃ concentrations through national and local monitoring programs. Wood followed EPA procedures (2015a) to estimate O₃ design values and 2017 fourth highest DM8A O₃ 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, 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.

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 O₃ 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 O₃ 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 O₃ design values greater than 70 ppb.

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.

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 O₃ concentrations that met percent completeness criteria. During 2017, three California sites, two eastern sites, and one site in Utah measured fourth highest DM8A O₃ concentrations greater than 70 ppb. These are the same six sites with 2015–2017 design values greater than 70 ppb.

Figure 2-3 provides box plots depicting trends in 1-year mean, median, and annual distributions of fourth highest DM8A O₃ 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 O₃ data show an overall decline of 24 percent in fourth highest DM8A O₃ 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 O₃ data show a reduction of 10 percent in fourth highest DM8A O₃ concentrations from 1996–1998 through 2015–2017. The 2017 median of the fourth highest DM8A O₃ concentrations for the western reference sites was 67 ppb, a slight increase from the 2016 concentration of 65 ppb.

Chapter 3: Stratospheric Ozone Intrusions

A stratospheric O₃ intrusion occurs when O₃-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 O₃. 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 O₃ concentrations at ground level. Satellite data show vertically integrated concentrations of O₃, CO, and humidity. Low concentrations of CO and humidity, in conjunction with elevated O₃ 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 O₃ 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 O₃, CO, relative humidity, and other parameters and their ratios;
• Estimates of tropopause height, isentropic potential vorticity (PV), and potential temperature;
• Presentation of upper air radiosonde data;
• Trajectory analyses, light detection and ranging, O₃ 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 O₃ 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 O₃ 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 O₃ concentrations displayed on the map in Figure 3-1.

Table 3-1 2017 DM8A O₃ Concentrations

Date	O3 (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

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.

Figure 3-2 shows time series of hourly O₃ concentrations measured at three CASTNET sites (GTH161, CNT169, and MEV405) over the period April 21–23, 2017. The time series show hourly O₃ values at GTH161 exceeded 85 ppb on April 22, 2017.

The Colorado Department of Public Health and the Environment (CDPHE) requested that CASTNET managers append informational stratospheric intrusion flags (“IO”) to the hourly O₃ 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 displays the abrupt increase and abrupt decrease of the IO-flagged O₃ data at the time of arrival of the suspected stratospheric intrusion (orange). These records are shown with all valid hourly O₃ data (gray) collected during the month of April 2017 from the GTH161 site. This month had a median hourly O₃ concentration of 56 ppb and a median daily O₃ production value of 11 ppb, i.e., the production of O₃ from the minimum concentration typically around sunrise to the time of the maximum hourly concentration. This diurnal O₃ profile is consistent with other Intermountain West sites.

Figure 3-4 shows a histogram of valid hourly O₃ 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 O₃ 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 O₃ concentrations from 64 ppb at 6:00 a.m. to 86 ppb at 3:00 p.m.

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.

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 O₃ . 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.

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 O₃ event.

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

Pressure (mb)	Altitude (m)	O3 (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 3-7 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 O₃, 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 (m  ²/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 O₃ 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 m  ²/sec times K/kg.

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 3-9 provides a GOES-derived satellite image of total column O₃ measured on April 21, 2017 at 5:00 p.m. MST that shows a widely scattered band of O₃ mixing ratios (green) ranging from 275 to 400 Dobson Units from Idaho across southwest Wyoming to southwest Colorado.

Figure 3-10 gives a Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) image with a time-averaged O₃ mass mixing ratio at 400 hectopascal (hPa  ²; 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 O₃ measured at Gothic). This figure illustrates a distinct band of high O₃ concentrations from the Idaho/Montana border across Colorado to the Oklahoma Panhandle.

Figure 3-11 illustrates RAQMS simulations of the downward transport of O₃ 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 O₃ approached the ground surface near GTH161.

In summary, the CDPHE requested that CASTNET managers append the IO informational stratospheric intrusion flags to the GTH161 and MEV405 hourly O₃ values in AQS for April 22 and 23, 2017 to signify to data users that these records may have been impacted by a stratospheric O₃ intrusion. Consequently, EPA, BLM, Wood scientists, and the SI Workgroup analyzed O₃ 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 O₃ 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 O₃ 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 O₃ data and RAQMS simulations. The Ute Indian Tribe operates O₃ 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 O₃ concentrations in 2015 on June 8 and 9 (Table 3-3). Data presented in the exceptional events document show that high O₃ 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 O₃ to the surface measurements.

Table 3-3 Ute Indian Tribe of the Uintah and Ouray Reservation and DIN431 O₃ 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 O3	Annual Rank	DM8A O3	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 O₃ 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 O₃ 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 O₃ readings resulting in a DM8A value of 88 ppb at the Thunder Basin O₃ monitor. Additionally, the monitors at Gillette and Campbell County measured elevated 1-hour O₃ 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 O₃ measurements and an examination of hourly GOES total column O₃ data, data from NASA’s Alpha Jet Atmospheric eXperiment (AJAX) Flight 47, and RAQMS and Rapid Refresh Model (RAP) simulations of O₃, water vapor, and CO.

Table 3-4 Wyoming O₃ Data During Period of Likely Stratospheric Intrusion

Monitor Name	Monitor Number	6-Jun-12	6-Jun-12	7-Jun-12	7-Jun-12
		DM8A O3	Annual Rank	DM8A O3	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	162nd
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 O₃-rich air to descend to the Thunder Basin monitor area and likely contributed to elevated 1-hour average O₃ values.

Chapter 4: Nitrogen Pollutant Concentrations

Annual mean concentrations of total NO₃^- (HNO₃ + NO₃^-) and NH₄⁺ 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 NO_y 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, SO2, and CO
Bondville, IL (BVL130)	July 2012	NO/NOy, SO2, 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, SO2, and CO
Duke Forest, NC (DUK008)	May 2017	NO/NOy, SO2, and NHx

Note: *Operated by NPS.

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/m  ³) of air for 1990–1992 to 1.5 µg/m  ³ 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/m  ³ for 1996–1998 and 0.6 µg/m  ³ for 2015–2017.

The 2017 average total nitrate concentration for the eastern reference sites was 1.4 µg/m  ³ . 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 NO_x emissions from EGUs operating in the eastern United States.

Particulate Ammonium Concentrations

A map of 2017 mean particulate NH₄⁺ concentrations is provided in Figure 4-3. Figure 4-4 shows box plots of NH₄⁺ concentrations. The trend diagram for the eastern sites (top) shows a reduction in mean NH₄⁺ levels from 1990–1992 to 2015–2017. The 1990–1992 mean concentration was 1.8 µg/m  ³ , and the 2015–2017 value was 0.6 µg/m  ³ , a 68 percent decline. The western reference sites show a decline from 0.3 µg/m  ³ in 1996–1998 to 0.2 µg/m  ³ in 2015–2017, a 32 percent reduction.

Continuous Trace-level NO_y and Filter Pack Total Nitrate Concentrations

HNO₃ and particulate NO₃^- are measured on CASTNET filter packs, and the sum is reported as total NO₃^-. NO_y is defined as NO_x (NO and nitrogen dioxide) plus NOz (HNO₃, nitrous acid, peroxyacetyl nitrate, peroxyproyl nitrate, other organic nitrates, and nitrite). It consists of reactive gases that are considered precursors of O₃ and PM_2.5. Because HNO₃ and particulate NO₃^- are measured as components of NO_y, NO_y concentrations should always be higher than total NO₃^- levels (i.e., the ratio of NO_y to total NO₃^- should always be greater than 1.0). A comparison of weekly mean continuous NO_y 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 NO_y and total NO₃^- levels are different in the three figures. The NO_y 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 NO_y 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 NO_y values matching the run time of the weekly filter packs, and the average of the ratios calculated for each week. Weekly NO_y levels were higher than the weekly total NO₃^- concentrations with ratios of NO_y to total NO₃^- varying from 3.66 at PNF126 to 5.88 at ROM206.

Table 4-2 Summary of Total NO and NO_y Measurements for 2017

Site Location	Elevation (m)	Total NO (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	1.5	3.73
PNF126, NC	1216	0.33	1.16	3.66
PND165, WY	2386	0.14	0.6	4.54
ROM206, CO	2742	0.18	0.98	5.88

Chapter 5: Update on the Ammonia Monitoring Network

Reduced nitrogen [ammonia (NH₃) + NH₄⁺] is an important component to total nitrogen deposition. Wet deposition of NH₄⁺ measured by NTN has been increasing in many areas of the United States over the past 10 years; however, until 2007, gaseous NH₃ 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 NH₃ emissions (EPA National Emissions Inventory, 2014). Although NH₃ is beneficial when used as NH₃-based fertilizer, it can have a negative effect on the environment when it reacts with acidic ions such as SO₄^2- and NO₃^- to form fine particulate matter (PM_2.5), 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 NH₃ 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/m  ³ at three sites in upstate New York (NY98), New Hampshire (NH02), and Nova Scotia (NS01) to a high of 16.9 µg/m  ³ 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/m  ³ 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/m  ³ . High NH₃ 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 NH₃ 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 NH₃ concentrations. In particular, 3-year mean concentrations increased from 1.32 µg/m  ³ to 1.71 µg/m  ³ (30 percent) from 2008-2010 through 2015-2017 and 3-year median concentrations increased by 16 percent from 1.11 µg/m  ³ to 1.29 µg/m  ³ . Two sites (TX43 and CO13) with relatively high concentrations reported the largest increases.

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

Chapter 6: Sulfur Pollutant Concentrations

Annual mean concentrations of SO₂ and SO₄^2- 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 SO₂ 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 SO₂ concentration was measured in the West at Basin, WY (BAS601), which measured unexpectedly high SO₂ concentrations during fourth quarter 2017. See the additional discussion of BAS601 SO₂ concentrations at the end of this chapter.

Box plots of annual mean SO₂ 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 SO₂ concentrations for the CASTNET eastern reference sites for 1990–1992 and 2015–2017 were 8.8 µg/m  ³ and 0.9 µg/m  ³ , respectively. This change constitutes an 89 percent reduction in 3-year mean SO₂ concentrations between the two periods.

The box plots for the western reference sites indicate a decline in annual mean SO₂ concentrations aggregated over the 16 sites. Three-year mean SO₂ concentrations for 1996–1998 and 2015–2017 were 0.6 µg/m  ³ and 0.3 µg/m  ³ , respectively. This change constitutes a 45 percent reduction in 3-year mean SO₂ concentrations at the CASTNET western reference sites over the 22 years.

The 2017 average sulfur dioxide concentration for the eastern reference sites was 0.7 µg/m  ³ . 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.

The trend in SO₂ 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 SO₂ concentrations aggregated over the CASTNET eastern reference sites.

Particulate Sulfate Concentrations

Figure 6-3 shows a map of 2017 annual mean particulate SO₄^2- concentrations. Figure 6-4 provides box plots of annual mean SO₄^2- concentrations from the 34 eastern reference sites and 16 western reference sites. The figure shows a substantial decline in SO₄^2- 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 SO₄^2- from 5.4 µg/m  ³ to 1.3 µg/m  ³ . The 2017 mean SO₄^2- level of 1.1 µg/m  ³ 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 SO₄^2- concentrations aggregated over the 16 sites with 1996–1998 and 2015–2017 concentrations of 0.8 µg/m  ³ and 0.5 µg/m  ³ , respectively.

The 2017 average sulfate concentration for the eastern reference sites was 1.1 µg/m  ³ , 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.

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 SO₂ concentrations during fourth quarter 2017 causing concern about increased pollution. The data show the SO₂ 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 SO₂ levels measured at BAS601. High concentrations persisted through first quarter 2018 and then returned to levels more consistent with the other Wyoming CASTNET sites.

Chapter 7: Atmospheric Deposition of Nitrogen

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. See also the NADP Fact Sheet, "Hybrid Approach to Mapping Total Deposition."

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 NH₃ for 2017 is shown in Figure 7-8, and the net dry deposition of NH₃ 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).

Chapter 8: Effects of California Wildfires on Air Quality

Wildfire smoke is a mixture of CO, volatile organic compounds, PM_2.5, and smaller particles that include alkaline ash, black and organic carbon, and polyaromatic hydrocarbons. Wildfire smoke is also a precursor to downwind O₃ 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.

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 PM_2.5 concentrations color-coded by the AQI category for each area on October 9.

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 PM_2.5 and O₃ (Ho and Lyons, 2017).

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 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 O₃, PM₁₀, and PM_2.5 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, O₃ 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 PM_2.5 and O₃ concentrations, respectively, for five Ventura County monitors for December 2017. Figure 8-10 also shows O₃ concentrations from the CASTNET site at Joshua Tree National Park (JOT403). See Figure 1-1 for the specific location. Concentrations of PM_2.5 reached over 500 µg/m  ³ 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 O₃ concentration (17 ppb) at Ojai occurred during the period with the highest PM_2.5 concentration (557 µg/m  ³ ).

Table 8-1 Daily Average O₃, PM₁₀, and PM_2.5 Concentrations by Color-Coded AQI Category for December 14, 2017

Stations Monitoring Ozone and Particulates
Location	Ozone (ppb)	PM10 (µg/m)	PM2.5 (µg/m)
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 O₃, PM₁₀, and PM_2.5 Concentrations by Color-Coded AQI Category for December 14, 2017

Stations Monitoring Ozone and Particulates
Location	Ozone (ppb)	PM10 (µg/m)	PM2.5 (µg/m)
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

Table 8-3 Daily Average O₃, PM₁₀, and PM_2.5 Concentrations by Color-Coded AQI Category for December 18, 2017

Stations Monitoring Ozone and Particulates
Location	Ozone (ppb)	PM10 (µg/m)	PM2.5 (µg/m)
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

Chapter 9: Atmospheric Deposition of Sulfur, Base Cations, and Chloride

Gaseous and particulate sulfur pollutants, base cations, and Cl  ^- are deposited to the environment through dry and wet atmospheric processes. A principal goal of CASTNET is to estimate the rate of dry deposition of these compounds from the atmosphere to sensitive ecosystems. The NADP TDep 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 (Ca  ²⁺ , K  ⁺ , Mg  ²⁺ , and Na  ⁺ ) is given in Figure 9-4. Figure 9-5 provides estimated rates for 2017 for total deposition of Cl  ^-.

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Ute Indian Tribe of the Unitah and Ouray Reservation. 2016. Ozone NAAQS Exceedances Occurring June 8 and 9, 2015, Uinta Basin of Utah. Submitted to U.S. Environmental Protection Agency (EPA), Region 8, August 30,.

Wood Environment & Infrastructure Solutions, Inc. (Wood)*. 2017. Clean Air Status and Trends Network (CASTNET) Quality Assurance Project Plan (QAPP) Revision 9.0. Prepared for U.S. Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets Division, Washington, DC. Contract No. EP-W-09-028. Gainesville, FL. http://java.epa.gov/castnet/documents.do.

Xing, J., Mathur, R., Pleim, J., Hogrefe, C., Wang, J., Gan, C.-M., Sarwar, G., Wong, D. C., and McKeen, S. 2016. Representing the Effects of Stratosphere–troposphere Exchange on 3-D O 3 Distributions in Chemistry Transport Models using a Potential Vorticity-based Parameterization, Atmos. Chem. Phys., 16, 10865-10877, https://doi.org/10.5194/acp-16-10865-2016.

Youden, W.J. (Ku, H.H., ed). 1969. Precision Measurement and Calibration. NBS Special Publication 300-Volume 1. U.S. Government Printing Office, Washington, DC.

 

* Formerly known as Amec Foster Wheeler Environment & Infrastructure, Inc.

Appendix A: Locational and Operational Characteristics of CASTNET Sites

   

Note:

NM = National Monument

NP = National Park

NWR = National Wildlife Reserve

SP = State Park

E = eastern reference site

W = western reference site

Appendix B: Acronyms and Abbreviations

A2LA	American Association for Laboratory Accreditation
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
Ca	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	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  -1	kilograms per hectare per year
m	meter
m  2/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
Mg  2+	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
NH3	ammonia
NH4+	particulate ammonium
NO	nitrogen dioxide
NO3-	particulate nitrate
NOx	nitrogen oxides [nitrogen oxide (NO) + nitrogen dioxide (NO2)]
NOy	total reactive oxides of nitrogen
NOAA	National Oceanic and Atmospheric Administration
NPS	National Park Service
NTN	National Trends Network
O3	ozone
PM2.5	fine particulate matter
PM10	particulate matter with a mean aerodynamic diameter less than 10 micrometers
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 (m  2/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
SO2	sulfur dioxide
SO42-	particulate sulfate
TDEP	NADP Total Deposition Hybrid Method
total NO3-	gaseous nitric acid (HNO) + particulate nitrate (NO)
µg/m  3	micrograms per cubic meter
WARMS	Wyoming Air Resources Monitoring System
WDEQ/AQD	State of Wyoming Department of Environmental Quality/Air Quality Division