Clean Air Status and Trends

Network

CASTNET

2018 ANNUAL REPORT
i Table of Contents

Table of Contents

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

Network Operation Challenges........................................................................................................................................................4

Ozone Concentrations.........................................................................................................................................................................7

Nitrogen Atmospheric Concentrations ...................................................................................................................................... 10

Sulfur Atmospheric Concentrations ............................................................................................................................................ 15

Atmospheric Deposition .................................................................................................................................................................. 18

Monitor for Aerosols and Gases (MARGA) Update............................................................................................................... 26

References ............................................................................................................................................................................................. 27

The cover photo shows the CASTNET site at Palo Duro Canyon State Park, TX (PAL190). Locally, the 

monitoring station is located on a flat, grassy plain south of the main canyon. On a regional scale, the site 

is surrounded by complex terrain.
1 CASTNET Overview

CASTNET Overview

Introduction

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

contiguous United States, Alaska, and Canada in 2018. Ninety-five sites included filter pack systems to 

sample weekly concentrations of acidic gases and particulate matter. Eighty-three 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 27 sites during 

2018. The Bureau of Land Management-Wyoming State Office (BLM) operated five sites in Wyoming. View 

site locations on the CASTNET website (https://www.epa.gov/castnet/castnet-site-locations).

Many organizations, including Native American tribes, state and federal government agencies, and 

universities, contribute to CASTNET (https://www.epa.gov/castnet/castnet-program-partners). These 

participants sponsor individual CASTNET sites and provide in-kind services that support the overall 

performance of the network. For example, these participants operate and repair site instruments, change 

weekly filter packs, and perform general site maintenance. Many also provide the land for the CASTNET site. 

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 and to characterize rural air quality nationwide. 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/monitoringnetworks-data/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/) 
CASTNET 2018 Annual Report

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Locations of Monitoring Sites

A map of CASTNET monitoring sites is given in Figure 1. In 2018, 96 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 2018. 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.

Figure 1 CASTNET Sites Operational during 2018

Measurements Recorded at CASTNET Sites

During 2018, all but one CASTNET site (CHC432, NM), measured weekly ambient concentrations of acidic 

pollutants, base cations, and chloride (Cl-

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

2018; https://java.epa.gov/castnet/documents.do). 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 all sites in part to enable conversion of concentrations to local conditions. 

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

3 monitoring system. Ozone 

concentrations were measured at 83 sites. The O 

3 inlet and filter pack are located atop a 10-m tower. Some 
CASTNET 2018 Annual Report

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CASTNET sites also measure trace-level SO 

2, carbon monoxide (CO), and nitric oxide/total reactive oxides of 

nitrogen (NO/NOy). One site measures NO/total nitrogen oxides (NOx). In 2018, meteorological parameters 

were measured at 6 EPA-, 27 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 2018 QA program elements are documented in the CASTNET 

Quality Assurance Project Plan (QAPP; Wood, 2018). 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 2018 and 

historical data collected over the period 1990 through 2017 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://java.epa.gov/castnet/documents.do.

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

testing (PT) programs such as the Environment and Climate Change Canada (ECCC) PT Program for 

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

The ECCC PT program conforms to the requirements of the American Association for Laboratory 

Accreditation (A2LA). The program meets the International Organization for Standardization 

(ISO)/International Electrotechnical Commission (IEC) 17043:2010 conformity assessment – general 

requirements for proficiency testing with scope of accreditation 2867.01. The results reported are evaluated 

for systematic bias and precision. Systematic bias is assessed using the Youden non-parametric analysis

(1969; https://archive.org/details/precisionmeasure3001kuha), 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://java.epa.gov/castnet/documents.do.

Results

Trends in fourth highest DM8A O 

3 concentrations and annual sulfur and nitrogen levels are depicted in the 

following sections using box plots showing 1-year mean, median, and annual distributions of 

concentrations aggregated from 34 eastern reference sites for 1990 through 2018 and for 16 western 

reference sites for 1996 through 2018. The reference sites were selected for their long-term data records 
CASTNET 2018 Annual Report

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and consistent performance. All data used to create the figures and maps in this report are available on the 

CASTNET website: https://java.epa.gov/castnet/clearsession.do.

Network Operation Challenges

CASTNET began operating monitoring sites in 1987 (https://www3.epa.gov/castnet/docs/CASTNETFactsheet-2019.pdf) with about 50 sites in operation during the first five years of the network. Years of use 

and weather take a toll on equipment and equipment shelters, and regular maintenance is needed. The 

non-permanent shelters at the monitoring sites, while sturdy, have a limited life expectancy compared to a 

standard, permanent structure and need additional upkeep. All monitoring shelters are also subject to 

extreme weather and associated storm damage (e.g., hurricanes, flooding, and lightning) depending on 

location. 

Hurricane Damage to Beaufort, NC (BFT142) and Sumatra, FL (SUM156)

During 2018, Hurricane Florence made landfall as a Category 1 hurricane on the morning of September 14 

over Wrightsville Beach, NC, a few miles east of Wilmington and not far from the CASTNET site at BFT142. 

The hurricane came ashore with 90 mph winds and significant storm surge. In anticipation of the storm, 

BFT142 was shut down on September 11. Hurricane Florence severely flooded the site and surrounding area. 

Site shutdown preparations included checking the guy wires to the sampling tower and removing critical 

equipment. The tower itself was left in place. Site shutdown preparations also included removing the 

deposition filter pack and returning it to the CASTNET laboratory in Gainesville, FL. The following 

instruments and equipment were removed.

• ozone analyzer

• ozone transfer standard

• air compressor

• flow pump

• mass flow controller

• data logger

• weighing precipitation gauge

• site records including the logbook and report forms

After the hurricane, the BFT142 site required repairs to both power and infrastructure. The shelter was 

knocked off its foundation, and the sampling tower was damaged. The soft, super-saturated ground 

prevented vehicles from accessing the monitoring site for repairs until March 2019. During repairs, the 

shelter was raised as a preventive measure. The site was eventually reopened on March 18, 2019, six 

months after the hurricane.

Hurricane Michael was the first Category 5 hurricane to strike the contiguous United States since Hurricane 

Andrew in 1992. It made landfall in Florida on October 10, 2018, between Mexico Beach and Tyndall Air 

Force Base near Panama City and approximately 25 miles west of SUM156. Communication with the 
CASTNET 2018 Annual Report

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SUM156 site was lost on October 11. Ultimately, the SUM156 infrastructure was not damaged except for 

the power pole, which was replaced. Routine monitoring was re-established on December 28, 2018. 

Aging Infrastructure and the Efforts to Counteract the Effects of Aging

In general, CASTNET monitoring sites are affected by weather ranging from extreme cold with frost heaving 

to extreme heat and humidity. Damage from insects and animals also occurs. Ants, bees, roaches and other 

insects are attracted to the shelters. Sites are routinely checked for the following:

• damage to flow towers (e.g., cracks in the tower, loose or broken guy wires);

• roof leaks that cause wall, floor, and counter damage along with mold and mildew;

• aging weather stripping and insulation that can impact shelter and bench temperatures;

• frost heaving that can damage shelter foundations and tower bases;

• site flooding;

• extreme heat and extreme cold affecting air conditioning and heating, data logger batteries, and 

other instruments;

• storm damage from lightning, heavy rain, hail, and high winds;

• extreme weather damage from hurricanes, tornadoes, and blizzards;

• insect and animal damage to wiring connections and structures; and

• damage from other sources (e.g., vehicles, snowplows, etc.).

The following list describes some recent examples of the routine infrastructure repairs that are needed.

• Coweeta, NC (COW137)

On July 20, 2017, the flow tower cracked. A new tower was installed on September 20.

• Perkinsville, WI (PRK134)

The flow tower was damaged. The flow tower was replaced on September 19, 2017.

• Cranberry, NC (PNF126)

The leaking roof resulted in floor damage. On November 6, 2017, the floor and subfloor insulation 

were replaced. All exterior seams and rivets were sealed. The improvements to the PNF126 site are 

detailed in the following photographs. 
CASTNET 2018 Annual Report

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• Abington, CT (ABT147) 

The shelter shifted due to frost heave and gravity and slid off the mounting posts. The shelter was 

remounted and stabilized on May 2, 2018 (see the following photographs).

• Blackwater National Wildlife Refuge, MD (BWR139)

The bottom of a shelter wall rotted due to a leak in the roof. Repairs, including a new roof, inside 

wall, and floor replacement, were completed November 7, 2018.

Infrastructure issues are identified by the local site operator, Wood, subcontractor calibrators, and external 

auditors. The biggest hurdle to fixing these issues is finding a contractor to perform the work at the remote 

site locations. Remedial actions are prioritized by potential safety issues or data loss and then completed as 

soon as possible.

Infrastructure problems can affect data completeness of O 

3 measurements and filter pack concentrations.

Data collection must meet certain criteria specific to the type of data collected for the data to be valid (see 

the CASTNET QAPP; Wood, 2018). Examples of problems that affect data collection and completeness 

include loss of power to the instruments, air conditioner and heater failures that affect shelter and bench 

temperature, and water damage from a leaking roof. Completeness statistics are discussed in quarterly 

data and QA reports (https://java.epa.gov/castnet/documents.do). Completeness requirements are 

different for O 

3 and filter pack concentrations. Ozone design value maps require 90 percent completeness 

for all three years. Annual O 

3 concentration maps require 75 percent of daily maximum values to be 

available during the O 

3 season to meet completeness. Filter pack concentration maps require 9 out of 13

valid filter packs within a quarter for a quarterly value and 3 valid quarters for an annual value.

In summary, most of the infrastructure improvements have involved the shelter and sampling towers, including: 

• sealing exterior shelter seams and rivets,

• adding shelter roofs where needed,

• restoring the inside of shelters due to roof leak damage,

• replacing damaged towers as needed, and 

• providing equipment upgrades as needed.
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Ozone Concentrations

CASTNET O 

3 data are based on rigorous QA/quality control procedures specified by the CASTNET QA 

Program. Hourly average concentrations were measured at 83 CASTNET sites in 2018. 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 O 

3 design values using fourth highest daily 

maximum 8-hour average (DM8A) O 

3 concentrations when three years of Title 40 Code of Federal 

Regulations (CFR) Part 58-compliant data become available. CASTNET measurements provide data to 

support Ozone National Ambient Air Quality Standards (NAAQS) and to assess long-term 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. On October 1, 2015, the EPA promulgated a revision to the ozone 

NAAQS (2015 O 

3 NAAQS), lowering the level of both the primary and secondary standards to 0.070 parts 

per million (ppm; https://www.epa.gov/ground-level-ozone-pollution/2015-national-ambient-air-qualitystandards-naaqs-ozone). To attain the 2015 O 

3 NAAQS, 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 2008 O 

3 NAAQS remains in effect at 0.075 ppm. For this report the 2015 

standard and related calculations were used. The design values for each standard can be downloaded from 

the EPA Design Values website (https://www.epa.gov/air-trends/air-quality-design-values).

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

3 concentrations through 

national and local monitoring programs. 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 the fourth highest DM8A values for this report (https://www.epa.gov/air-qualityanalysis/exceptional-events-rule-and-guidance).

Three-year averages of the fourth highest DM8A O 

3 concentrations for 2016 through 2018 are presented in 

Figure 2 for the 2015 NAAQS. 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. Table 1 lists 

sites with 2016–2018 O 

3 design values greater than 70 ppb.
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Figure 2 Three-year Averages of Fourth Highest DM8A O 

3 Concentrations for 2016–2018

Table 1 2016–2018 O 

3 Design Values Greater than 70 ppb

Site ID State Sponsor

3-year Average

(ppb)

JOT403 California NPS 89

SEK430 California NPS 89

YOS404 California NPS 79

WSP144 New Jersey EPA 74

ABT147 Connecticut EPA 73

DIN431 Utah NPS 72

ANA115 Michigan EPA 71

BEL116 Maryland EPA 71

Figure 3 shows 2018 fourth highest DM8A O 

3 concentrations that met percent completeness criteria. 
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Figure 3 Fourth Highest DM8A O 

3 Concentrations for 2018

Figure 4 provides box plots depicting trends in 1-year mean, median, and annual distributions of fourth 

highest DM8A O 

3 concentrations from the 34 eastern reference sites (top) and for the 16 western reference 

sites (bottom).

Figure 4 Trends in Fourth Highest DM8A O 

3 Concentrations

Eastern Reference Sites

Western Reference Sites
CASTNET 2018 Annual Report

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Nitrogen Atmospheric Concentrations

Annual mean concentrations of total NO3 (HNO 

3 + NO3), NH+ 4 , and NH 3 for 2018 are presented in three 

maps. Trends in annual mean concentrations of total NO3 and NH+ 

4 over the 29-year period, 1990 through 

2018, were calculated from the eastern reference sites and for the 23-year period, 1996 through 2018, from 

the western reference sites. Trends in NH 

3 were estimated from AMoN data from 2008 through 2018.

Trace-level gas analyzers measured total reactive oxides of nitrogen (NOy) at six EPA and two NPS 

CASTNET sites during 2018. These are presented at the end of this section. 

Total Nitrate Concentrations

Mean total NO3 concentrations measured in 2018 are presented in the map in Figure 5. To illustrate trends, 

Figure 6 provides box plots of total NO3 levels for the eastern and western reference sites through 2018. 

Figure 5 Annual Mean Total NO3 Concentrations for 2018
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Figure 6 Trends in Annual Mean Total NO3 Concentrations

Eastern Reference Sites

Western Reference Sites

Particulate Ammonium Concentrations

A map of 2018 mean particulate NH+ 

4 concentrations is provided in Figure 7. Figure 8 shows box plots of 

NH+ 

4 concentrations. 

Coweeta (Hydrologic Laboratory), NC (COW137)
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Figure 7 Annual Mean NH+ 

4 Concentrations for 2018

Figure 8 Trends in Annual Mean NH+ 

4 Concentrations

Eastern Reference Sites

Western Reference Sites
CASTNET 2018 Annual Report

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Ammonia Concentrations

Average annual NH 

3 concentrations for 2018 for AMoN sites that met completeness requirements are 

mapped in Figure 9. Trends are shown in Figure 10.

Figure 9 Annual Mean NH 

3 Concentrations at AMoN sites for 2018

Figure 10 Trends in Annual Mean NH 

3 Concentrations

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 + 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 fine particulate matter (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
CASTNET 2018 Annual Report

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concentrations with filter pack total NO3 levels at BVL130, IL; MAC426, KY; and ROM206, CO for 2018 was 

used to evaluate the 2018 measurements (Figures 11 through 13). 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 are listed in Table 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. 

Figure 11 Comparison of BVL130, IL Weekly Mean Continuous Trace-level NOy and Filter Pack Total 

NO3 Concentrations

Figure 12 Comparison of MAC426, KY Weekly Mean Continuous Trace-level NOy and Filter Pack 

Total NO3 Concentrations

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NOy Concentration (ppb)

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Total Nitrate Concentration (ppb)

NOy Concentration (ppb)

Mean Trace NOy

Filter Pack Total NO3

Mean Continuous NOy

Filter Pack Total NO3

Mean Continuous NOy

Filter Pack Total NO3
CASTNET 2018 Annual Report

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Figure 13 Comparison of ROM206, CO Weekly Mean Continuous Trace-level NOy and Filter Pack 

Total NO3 Concentrations

Table 2 Summary of Total NO3 and NOy Measurements for 2018

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

DUK008, NC* 164 0.51 2.92 5.74

BVL130, IL 213 1.06 4.49 4.66

MAC426, KY 243 0.69 2.61 4.03

HWF187, NY 497 0.19 0.80 4.74

GRS420, TN 793 0.40 1.50 3.61

PNF126, NC 1216 0.30 1.11 4.14

PND165, WY 2386 0.15 0.57 4.15

ROM206, CO 2742 0.21 1.02 5.35

Note: *Enhanced NOy monitor located at the top of the 30-m tower

Sulfur Atmospheric Concentrations

Annual mean concentrations of SO 

2 and SO2-4 for 2018 are presented in Figures 14 and 16, respectively. 

Trends in annual mean concentrations of SO 

2 and SO2-4 for 2018 are also shown in Figures 15 and 17, 

respectively. The Basin, WY (BAS601) site (Figure 1) measured unexpectedly high SO 

2 concentrations during 

fourth quarter 2017. High concentrations persisted through first quarter 2018 and then returned to levels 

more consistent with the other Wyoming CASTNET sites. The high concentration measured during the first 

quarter affected the annual mean concentration for BAS601 shown in Figure 14.

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Total Nitrate Concentration (ppb)

NOy Concentration (ppb)

Mean Trace NOy

Filter Pack Total NO3

Mean Continuous NOy

Filter Pack Total NO3
CASTNET 2018 Annual Report

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Figure 14 Annual Mean SO 

2 Concentrations for 2018

Figure 15 Trends in Annual Mean SO 

2 Concentrations

Eastern Reference Sites

Western Reference Sites
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Figure 16 Annual Mean SO2-

4 Concentrations for 2018

Figure 17 Trends in Annual Mean SO2-

4 Concentrations

Eastern Reference Sites

Western Reference Sites
CASTNET 2018 Annual Report

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Figure 18 provides a diagram that compares weekly filter pack SO 

2 concentrations with continuous tracelevel gas data measured at Great Smoky Mountains National Park, TN (GRS420). The continuously 

measured trace-level concentrations were comparable to the filter pack concentrations.

Figure 18 Comparison of GRS420 Weekly Mean SO 

2 Concentrations

Atmospheric Deposition

Gaseous and particulate nitrogen, sulfur, and other trace 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) 

measurement-model fusion technique (Schwede and Lear, 2014) combines CASTNET monitoring data with 

output from the Community Multiscale Air Quality (CMAQ) modeling system (Byun and Schere, 2006) to 

estimate dry deposition. The TDep method uses measurement data from air quality monitoring sites when 

available and CMAQ output in areas where monitoring data are not available. CMAQ also provides 

modeled data for species that are not routinely measured. TDep 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 

Parameter-elevation Regressions on Independent Slopes Model (PRISM) modeled grid of precipitation data 

to develop a continuous 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 estimates of wet deposition. For 2018, dry and wet deposition 

rates were added to obtain estimates of total deposition of nitrogen, sulfur, base cations, and Cl-

, which are 

presented on the maps in this section as kilograms per hectare per year (kg ha-1

 yr-1 ). 

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Concentration (ppb)

Mean Trace

Filter Pack

SO2

SO2

Mean Continuous SO 

2

Filter Pack SO 

2
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Nitrogen Deposition

A map of the dry deposition fluxes of nitrogen (N) estimated for 2018 by the TDep method is shown in 

Figure 19. The magnitude of the deposition fluxes is illustrated by the shading in the figure legend. A map 

of total deposition of N estimated for 2018 is given in Figure 20. The percentage of total deposition of N 

due to dry deposition is shown in Figure 21. A map of TDep estimates of total deposition of reduced N 

species for 2018 is given in Figure 22. Figure 23 gives the percentage of total deposition of reduced N. Dry 

deposition of N species that are not routinely measured but are modeled by CMAQ and used by TDep is 

shown in the map in Figure 24, and its percentage of total deposition is shown in Figure 25. Estimates of 

annual gross dry deposition of NH 

3 as N for 2018 is shown in Figure 26, and the net dry deposition of NH 3

as N is shown in Figure 27. The net dry deposition figure shows the direction of the flux (i.e., positive values 

for deposition and negative values for emissions).

Sulfur Deposition

Figure 28 shows a map of TDep estimates of total deposition rates of sulfur (S) for 2018. The magnitude of 

the deposition fluxes is illustrated by the shading in the figure legend. The percentage of total deposition 

of S due to dry deposition is shown in Figure 29.

Deposition of Base Cations and Chloride

A map of 2018 estimated total deposition of base cations (Ca2+

 , K+ , Mg2+ , and Na+ ) is given in Figure 30. 

Figure 31 provides 2018 estimated rates of deposition of total Cl-.

Figure 19 TDep Dry Deposition Estimates of N (kg ha-1

 yr-1 ) for 2018

Source: CASTNET/CMAQ/NADP

Dry N
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Figure 20 TDep Total Deposition Estimates of N (kg ha-1

 yr-1 ) for 2018

Source: CASTNET/CMAQ/NADP

Figure 21 TDep Percent of Total Deposition of N from Dry Deposition for 2018

Source: CASTNET/CMAQ/NADP

Total N

Percent
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Figure 22 TDep Total Deposition Estimates of Reduced N Species (kg ha-1

 yr-1 ) for 2018

Source: CASTNET/CMAQ/NADP

Figure 23 TDep Percent of Total Deposition of Reduced N Species for 2018

Source: CASTNET/CMAQ/NADP

Total N

Percent
CASTNET 2018 Annual Report

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Figure 24 TDep Dry Deposition Estimates of Unmonitored N Species (kg ha-1

 yr-1 ) for 2018

Source: CASTNET/CMAQ/NADP

Figure 25 TDep Percent of Total Deposition of N from Dry Deposition of Unmonitored Species 

for 2018

Source: CASTNET/CMAQ/NADP

Dry N

Percent
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Figure 26 TDep Gross Dry Deposition of NH 

3 as N (kg ha-1 yr-1 ) for 2018

Source: CASTNET/CMAQ/NADP

Figure 27 TDep Net Dry Deposition of NH 

3 as N (kg ha-1 yr-1 ) for 2018

Source: CASTNET/CMAQ/NADP

Dry N

Dry N
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Figure 28 TDep Total Deposition Estimates of S (kg ha-1

 yr-1 ) for 2018

Source: CASTNET/CMAQ/NADP

Figure 29 TDep Percent of Total Deposition of S from Dry Deposition for 2018

Source: CASTNET/CMAQ/NADP

Total S

Percent
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Figure 30 TDep Total Deposition Estimates of the Base Cations Ca2+, K+ , Mg2+, and Na+ 

(kiloequivalents ha-1

 yr-1 ) for 2018

Source: CASTNET/CMAQ/NADP

Figure 31 TDep Total Deposition Estimates of Total Cl- (kg ha-1

 yr-1 ) for 2018

Source: CASTNET/CMAQ/NADP

Total ClTotal BC
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Monitor for Aerosols and Gases (MARGA) Update

Several Monitor for AeRosol GAses (MARGA) field measurement campaigns have been completed since 

2011. These measurement campaigns produced hourly concentrations of the pollutant gases and aerosols 

historically collected by CASTNET plus NH 

3 to characterize the integrated measurements from the CASTNET 

filter pack and provide more information on atmospheric chemistry and processes that may affect the filter 

pack data. In several of the campaigns, MARGA systems were deployed in a gradient sampling mode and 

paired with sonic anemometers to directly measure dry deposition fluxes. These gradient sampling studies 

were conducted across different ecosystems to improve deposition model parameterizations. These studies 

were supported by EPA’s Clean Air Markets Division through CASTNET and Office of Research and 

Development (ORD). Other sponsors/operators included NADP, the Illinois State Water Survey (ISWS), 

Colorado State University, College of Charleston, and ECCC. In 2020, the United States Department of 

Agriculture’s Agricultural Research Service plans to operate a MARGA in Denton, MD. Table 3 summarizes 

the MARGA field measurements and resulting databases.

The datasets have already been used in published studies (e.g., Rumsey et al., 2014; Rumsey and Walker, 

2016; Evanoski-Cole et al., 2017; Chen et al., 2017), doctoral dissertations (Evanoski-Cole, 2017), 

environmental reports (EPA, 2011), and many presentations. The data are currently being used in several 

EPA studies (publications in preparation). An additional goal of the MARGA sampling program is to make 

the concentration and flux datasets publicly available on the CASTNET website. The datasets that will be 

published are listed in Table 3. The databases will include both EPA and collaborator-collected data. The 

studies are at various QA stages, and the datasets must be organized, made consistent, and documented. 

Some of the MARGA project tasks in progress or under consideration include the following:

• Developing a publicly available MARGA concentration database including

o Organization of all EPA-related concentration datasets for consistency and compatibility prior to 

merging into the current EPA concentration database,

o Addition of collaborator-collected concentration datasets to EPA database, and

o Documentation of the experimentation and data analysis notes and metadata from each study to 

publish alongside the related database;

• Developing a publicly available MARGA flux database for flux studies (will include similar subsets as the 

MARGA concentration database);

• Publishing additional refereed papers and joint research with collaborators including vetting and 

characterization of integrated measurements from CASTNET filter packs, enhanced NOy monitoring 

systems, and AMoN (e.g., preparation of average diurnal profiles of NH 

3 for different seasons and 

ecosystems);

• Using MARGA data to improve nitrogen and sulfur budgets locally and nationally;

• Using the higher temporal resolution MARGA data to investigate specific acute pollution events and 

combine with source apportionment studies;

• Developing better parameterizations of fluxes and deposition velocities, which will be used to improve 

future versions of models such as CMAQ and the Comprehensive Air Quality Model with Extensions 

(CAMx);
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• Using MARGA data for model algorithm validation (e.g., gas-to-particle speciation and aerosol acidity); 

and

• Expanding the MARGA measurements and research to different types of ecosystems by partnering with 

new organizations.

Table 3 MARGA Datasets

Location Year

Instruments/Co-located 

Measurements

Type of 

Measurement Operating Group

Beltsville, MD 2013-2015 AMoN, MARGA-EPA1 (2013-2015)

MARGA-EPA2 (2013)

MARGA-EPA3 (2013-2016)

CASTNET Filter Pack

Met, NTN, AMoN

Enhanced NO/NOy (2016)

AiRRmonia* (2016)

Concentration CASTNET

2013 MARGA-EPA1 (2013), 

MARGA-EPA2, CASTNET Filter 

Pack, Met, NTN, AMoN

Concentration CASTNET

2013-2016 MARGA-EPA3, CASTNET Filter 

Pack, Met, NTN, AMoN, Enhanced 

NO/NOy, 

AiRRmonia* (2016)

Concentration CASTNET

Charleston, SC 2017 MARGA-GRAD2, Met, Soil Concentration, Flux CASTNET

Bondville, IL 2017 MARGA-EPA3, CASTNET Filter 

Pack, AMoN, Met

Concentration NADP/ISWS

Theodore Roosevelt 

National Park

2013-2014 MARGA-EPA2, Bakken Air Quality 

Study II (BAQS-II)

Concentration Colorado State 

University

Rocky Mountain 

National Park

2013-2014 MARGA-EPA2, Front Range Air 

Pollution and Photochemistry 

Experiment (FRAPPE) 

Concentration Colorado State 

University

Duke Forest, NC 2012 MARGA-GRAD1 Concentration, Flux ORD

2014 MARGA-GRAD1 Concentration ORD

2017 MARGA-GRAD3 Concentration, Flux ORD

Coweeta, NC 2016 MARGA-GRAD3 Concentration, Flux ORD

Egbert, ON (Canada) 2014-2015 MARGA-ECCC, CAPMoN, AMoN, 

CASTNET Filter Pack

Concentration ECCC

Notes: Met = meteorology measurements

* - collaboration with the University of Maryland Baltimore County (UMBC) C. J. Hennigan group

References

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

Chen X., Walker J., Geron C. 2017. Chromatography Related Performance of the Monitor for AeRosols and 

GAses in Ambient Air (MARGA): Laboratory and Field-based Evaluation. Atmos. Meas. Tech. 10: 

3893-3908. doi:10.5194/amt-10-3893-2017
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Evanoski-Cole, A. R. 2017. Impacts of Unconventional Oil and Gas Development on Atmospheric Aerosol 

Particles. Doctoral Dissertation. Colorado State University, Fort Collins, CO.

Evanoski-Cole, A. R., Gebhart, K. A., Sive, B. C., Zhou, Y., Capps, S. L., Day, D. E., Prenni, A. .J., Shurman, M. I., 

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for AeRosols and GAses in Ambient Air (MARGA): a Semi-continuous Method for Soluble 

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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 

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(CASTNET) Quality Assurance Project Plan (QAPP) Revision 9.1. Prepared for U.S. Environmental 

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