CASTNET

2019 ANNUAL REPORT
i

Table of Contents

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

Ozone Concentrations.........................................................................................................................................................................5

Nitrogen Atmospheric Concentrations .........................................................................................................................................8

Improving Characterization of Reduced Nitrogen at IMPROVE and CSN Monitoring Sites................................ 15

Improving Total Nitrogen Deposition Budget ........................................................................................................................ 19

Sulfur Atmospheric Concentrations ............................................................................................................................................ 22

Atmospheric Deposition .................................................................................................................................................................. 26

References ............................................................................................................................................................................................. 34
1

CASTNET Overview

Introduction

The Clean Air Status and Trends Network (CASTNET) operated 98 monitoring stations throughout the 

contiguous United States, Alaska, and Canada in 2019. Ninety-three sites included filter pack systems to 

sample weekly concentrations of acidic gases and particulate matter (PM). Eighty-six sites included 

continuous ozone (O 

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

(NPS), and the Bureau of Land Management-Wyoming State Office (BLM) are the primary sponsors of 

CASTNET. NPS began its participation in 1994 and operated 30 sites during 2019. The BLM operated five 

sites in Wyoming. Site locations can be viewed 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. 

Locations of Monitoring Sites

A map of CASTNET monitoring sites is given in Figure 1. In 2019, 98 monitoring sites were operated at 

96 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 2019. 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.

Measurements Recorded at CASTNET Sites

During 2019, all but five CASTNET sites measured weekly ambient concentrations of acidic pollutants, base 

cations, and chloride (Cl-

) using a 3-stage filter pack with a controlled flow rate (Wood, 2019; 

https://epa.gov/castnet/documents-reports). Gaseous pollutant concentrations include nitric acid (HNO 

3) 

and sulfur dioxide (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 replaced 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.
CASTNET 2019 Annual Report

2 

Figure 1 CASTNET Sites Operational during 2019 

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

3 monitoring system. 

Ozone concentrations were measured at 86 of the 98 sites. The O 

3 inlet and filter pack are located atop a 

10-meter tower. Some CASTNET sites also measure trace-level SO 

2, carbon monoxide, and nitric oxide/total 

reactive oxides of nitrogen (NO/NOy). One site measures NO/total nitrogen oxides (NOx). In 2019, 

meteorological parameters were measured at 5 EPA-, 30 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).

Trends in fourth highest daily maximum 8-hour average (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 2019 

and for 16 western reference sites for 1996 through 2019. The reference sites were selected for their longterm data records and consistent performance. All data used to create the figures and maps in this report 

are available on the CASTNET website: https://epa.gov/castnet/. 
CASTNET 2019 Annual Report

3

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/)

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 2019 QA program elements are documented in the CASTNET 

Quality Assurance Project Plan (QAPP; Wood, 2019). 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. QA program metrics are available in 

quarterly and annual QA reports posted on the EPA CASTNET website: https://epa.gov/castnet/documentsreports.

Network Safety and Natural Disaster Preparedness

The CASTNET 2018 Annual Report (Wood, 2020) discussed network operational challenges associated with 

instrument and equipment wear and extreme weather. Safety is another major element of network 

operation. CASTNET is operated to ensure the safety of project participants. All Wood and subcontractor

personnel must comply with the requirements of the CASTNET Health and Safety Plan (Wood, 2019), which 

addresses the health and safety hazards of each project task. 
CASTNET 2019 Annual Report

4 

Guidance for the operation of CASTNET monitoring sites during natural disasters was developed in 

response to the threat and damage caused by hurricanes during 2018. Natural disasters are meteorological 

or geological events that pose potentially catastrophic risk to CASTNET monitoring. They include

earthquakes and severe weather events associated with hurricanes, tropical storms, tornadoes, floods, and 

wildfires. 

Guidance for CASTNET monitoring sites under threat of natural disasters includes five main categories: 

• Prevention – Preparing for potential disasters such as developing evacuation plans and 

environmental planning and design standards 

• Preparedness – Planning and organizing for emergency activities including evaluating potential 

actions, training, providing appropriate equipment, and taking corrective action 

• Response – Coordinating the response to the disaster requires managing resources and personnel 

• Recovery – Restoring critical services (e.g., power, internet, telephone) and making infrastructure 

repairs are key to resuming normal site operations 

• Mitigation – Assessing the potential for natural disasters and making structural and non-structural 

changes limits the impact of future disasters 

Figure 2 Elevated Monitoring Station – Beaufort, NC (BFT142)

The effectiveness of the natural disaster guidance will vary depending upon the type and severity of the 

event. For example, advance warning for an approaching storm may allow for emergency preparations 

ahead of any danger, but advance warning is not feasible for an earthquake. Action/recovery plans will be 

event-dependent. Figure 2 shows an elevated shelter at BFT142, NC, a mitigation measure taken after 

flooding from Hurricane Florence in 2018.
CASTNET 2019 Annual Report

5 

Ozone Concentrations 

CASTNET O3 data collection is subject to rigorous QA/quality control procedures specified by the CASTNET

QA Program. Hourly average concentrations were measured at 86 CASTNET sites in 2019. One site, Duke 

Forest, NC (DUK008), did not meet siting criteria and was non-regulatory. Data from the other 85 sites were 

delivered routinely to the EPA Air Quality System (AQS) database (epa.gov/aqs). With the exception of data 

from two co-located sites that were designated as National Ambient Air Quality Standards (NAAQS)-

excluded and used for QA purposes, only, data from these sites were used to calculate the three-year

average of the fourth highest DM8A O3 concentrations if three years of Title 40 Code of Federal 

Regulations Part 58-compliant data were available (Figure 3). CASTNET measurements provided data to 

support comparison with the O3 NAAQS and to assess long-term 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. On October 1, 2015, the EPA promulgated a revision to the O3 NAAQS, 

lowering the level of both the primary and secondary standards to 0.070 parts per million (ppm; 

https://www.federalregister.gov/documents/2015/10/26/2015-26594/national-ambient-air-qualitystandards-for-ozone). On December 23, 2020, EPA decided to retain the 2015 O3

 NAAQS without changes 

to the form or level. To attain the 2015 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. For this report, the 2015 standard and related calculations were used. The 

design values for each standard can be downloaded from the EPA Air Quality 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 O3

 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 daily maximum values for this report (https://www.epa.gov/air-qualityanalysis/exceptional-events-rule-and-guidance).

Three-year averages of the fourth highest DM8A O3 concentrations for 2017 through 2019 are presented in

Figure 3. 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 2017–2019 

O3

 design values greater than 70 ppb, the 2015 O3 NAAQS.
CASTNET 2019 Annual Report

6 

Figure 3 Three-year Averages of Fourth Highest DM8A O 

3 Concentrations for 2017–2019

Table 1 2017–2019 O 

3 Design Values Greater than 70 ppb

Site ID State Sponsor 3-year Average

JOT403 California NPS 88

SEK430 California NPS 86 

YOS404 California NPS 77

BEL116 Maryland EPA 72 

WSP144 New Jersey EPA 72

ABT147 Connecticut EPA 71 
CASTNET 2019 Annual Report

7

Figure 4 shows 2019 fourth highest DM8A O 

3 concentrations that met percent completeness criteria. 

Figure 4 Fourth Highest DM8A O 

3 Concentrations for 2019

Joshua Tree National Park – Black Rock, CA (JOT403)
CASTNET 2019 Annual Report

8

Figure 5 provides box plots depicting trends in 1-year mean, median, and annual distributions of fourth 

highest DM8A O 

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

reference sites (bottom).

Figure 5 Trends in Fourth Highest DM8A O 

3 Concentrations

Eastern Reference Sites

Western Reference Sites

Nitrogen Atmospheric Concentrations

Annual mean concentrations of total NO3 (HNO 

3 + NO3), NH+ 4 , and ammonia (NH 3) for 2019 are presented 

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

4 over the 30-year period, 1990 

through 2019, were calculated from the eastern reference sites and for the 24-year period, 1996 through 

2019, from the western reference sites. Trends in NH 

3 were estimated from AMoN data from 2008 through 

2019.

Trace-level gas analyzers measured NOy at six EPA-sponsored and two NPS-sponsored CASTNET sites 

during 2019. These are presented at the end of this section. 
CASTNET 2019 Annual Report

9

Total Nitrate Concentrations

Mean total NO3 concentrations measured in 2019 are presented in the map in Figure 6. To illustrate trends, 

Figure 7 provides box plots of total NO3 levels for the eastern and western reference sites through 2019. 

Figure 6 Annual Mean Total NO3 Concentrations for 2019

Figure 7 Trends in Annual Mean Total NO3 Concentrations (1 of 2)

Eastern Reference Sites
CASTNET 2019 Annual Report

10

Figure 7 Trends in Annual Mean Total NO3 Concentrations (2 of 2)

Western Reference Sites

Particulate Ammonium Concentrations

A map of 2019 mean particulate NH+ 

4 concentrations is provided in Figure 8. Figure 9 shows box plots of 

NH+ 

4 concentrations. 

Figure 8 Annual Mean NH+ 

4 Concentrations for 2019
CASTNET 2019 Annual Report

11

Figure 9 Trends in Annual Mean NH+ 

4 Concentrations

Eastern Reference Sites

Western Reference Sites

Blackwater, MD (BWR139)
CASTNET 2019 Annual Report

12

Ammonia Concentrations

Average annual NH 

3 concentrations for 2019 for AMoN sites that met completeness requirements are 

mapped in Figure 10. Trends are shown in Figure 11.

Figure 10 Annual Mean NH 

3 Concentrations at AMoN sites for 2019

Figure 11 Trends in Annual Mean NH 

3 Concentrations at AMoN Sites
CASTNET 2019 Annual Report

13

Continuous Trace-level NOy and Filter Pack Total Nitrate Concentrations

Nitric acid 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

concentrations with filter pack total NO3 levels at BVL130, IL; PNF126, NC; and ROM206, CO for 2019 was 

used to evaluate the 2019 measurements (Figures 12 through 14). 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 eight sites operating continuous NOy analyzers. 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 12 Comparison of BVL130, IL Weekly Mean Continuous Trace-level NOy and Filter Pack Total 

NO3 Concentrations

0

1

2

3

4

0

2

4

6

8

10

12

12/22/18

1/6/19

1/20/19

2/3/19

2/17/19

3/3/19

3/17/193/31/194/14/194/28/195/12/195/26/19

6/9/19

6/23/19

7/7/19

7/21/19

8/4/19

8/18/19

9/1/19

9/15/199/29/19

10/13/1910/27/1911/10/1911/24/19

12/8/19

Total Nitrate Concentration (ppb)

NOy Concentration (ppb)

Mean Trace NOy

Filter Pack Total NO3

Mean Continuous NOy

Filter Pack Total NO3
CASTNET 2019 Annual Report

14

Figure 13 Comparison of PNF126, NC Weekly Mean Continuous Trace-level NOy and Filter Pack 

Total NO3 Concentrations

Figure 14 Comparison of ROM206, CO Weekly Mean Continuous Trace-level NOy and Filter Pack 

Total NO3 Concentrations

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.5

1.0

1.5

2.0

2.5

12/23/18

1/6/19

1/20/19

2/3/19

2/17/19

3/3/19

3/17/193/31/194/14/194/28/195/12/195/26/19

6/9/19

6/23/19

7/7/19

7/21/19

8/4/19

8/18/19

9/1/19

9/15/1910/6/19

10/27/1911/10/1911/24/19

12/8/19

12/22/19

Total Nitrate Concentration (ppb)

NOy Concentration (ppb)

Mean Trace

Filter Pack Total

NOy

NO3

-

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

12/23/18

1/6/19

1/20/19

2/3/19

2/17/19

3/3/19

3/17/193/31/194/14/194/28/195/12/195/26/19

6/9/19

6/23/19

7/7/19

7/21/19

8/4/19

8/18/19

9/1/19

9/15/199/29/19

10/13/1910/27/1911/10/1911/24/19

12/8/19

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 2019 Annual Report

15

Table 2 Summary of Total NO3 and NOy Measurements for 2019

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

DUK008, NC 164* 0.52 2.77 5.56

BVL130, IL 213 1.07 4.32 4.55

MAC426, KY 243 0.60 2.39 4.30

HWF187, NY 497 0.16 1.13 8.64

GRS420, TN 793 0.39 1.45 3.42

PNF126, NC 1216 0.27 1.00 3.93

PND165, WY 2386 0.13 0.62 5.04

ROM206, CO 2742 0.19 1.11 6.19

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

Improving Characterization of Reduced Nitrogen at 

IMPROVE and CSN Monitoring Sites

Introduction 

Over the past two decades, the chemical composition of the atmosphere has changed markedly from one

dominated by oxidized secondary particles including NO3, SO2-

4 , carbonaceous aerosols, and their 

associated precursor gases (NOx, SOx, volatile organic compounds, and semivolatile organic compounds) to 

an atmospheric mixture that includes a significant amount of reduced inorganic nitrogen (NHx = NH+ 

4 + 

NH 

3; Li et al., 2016; Du et al., 2014).

Total reduced inorganic nitrogen measurements are not routinely performed in the United States, and 

currently no regulatory requirements exist to measure NHx. However, scientific and policy interest in the 

increasing trend in NHx concentrations in many regions of the United States has grown. NHx can lead to 

atmospheric PM2.5 formation and visibility impairment and, once deposited, can cause eutrophication, loss 

of species biodiversity, and algal blooms. The Southeastern United States Reduced Nitrogen Study (Rogers 

et al., 2021) was sponsored by EPA and designed to measure reduced nitrogen and to assess measurement 

methods for implementation in long-term monitoring networks, specifically the IMPROVE and Chemical 

Speciation (CSN) networks. IMPROVE provides routine measurements of speciated particulate matter that 

support regional haze rules and PM NAAQS attainment designations. CSN provides data to support 

emission control strategies, health effects research, and model evaluation.

The goal of the study was to determine if phosphorous acid (H3PO3)-coated filters could be deployed at 

existing sites using standard network equipment to measure reduced nitrogen concentrations in hot, 

humid environments. Incorporating an NHx measurement into the CSN and IMPROVE networks would 

leverage the existing networks’ infrastructure, reduce costs, and provide national coverage. The addition of 

NHx also offers the potential of providing NHx and NH+ 

4 estimates at sites co-located with AMoN. Because 

of cost and logistical issues, the use of speciation samplers for 24-hour NHx samples, collected every 

3 days, would be a cost-effective way to measure spatial and temporal variability of NHx and reduce its 
CASTNET 2019 Annual Report

16

uncertainties. When co-located with AMoN, monthly, seasonal, and annual averages of particulate NH+ 

4 can 

be generated. Chen et al. (2014) performed a similar study in the western and midwestern United States 

using only the IMPROVE sampler. In that study, the acid-impregnated filter data compared well with 

measurements from the URG Corp annular denuder filter pack system (ADFPS). 

Ammonia is the precursor emission while particulate NH+ 

4 is a controllable pollutant under the PM NAAQS. 

Measurements of NHx would provide a metric for evaluating emission inventories, evaluating chemical 

transport models, and providing input into nitrogen deposition estimates. The importance of 

understanding trends and the spatial variability of reduced nitrogen is already recognized. Particulate NH+ 

4

concentrations are measured at more than 150 CSN sites and more than 90 CASTNET sites, and NH 

3

concentrations are measured at more than 100 sites as part of AMoN. Data from approximately 60 colocated CASTNET and AMoN sites can be combined to provide measured NHx concentrations, but these 

are integrated over 1 and 2 weeks, respectively. Additionally, the CSN measurements are known to 

underestimate NH+ 

4 concentrations (Yu et al., 2006). In short, CASTNET weekly integrated samples and 

AMoN two-week samples are most suitable for seasonal and annual characterizations in rural areas while 

CSN and some IMPROVE sites could potentially measure NHx concentrations in urban environments.

Locations of Monitoring Sites

Acid-impregnated filters were deployed in both CSN and IMPROVE samplers in North Carolina (Duke 

Forest) and Florida (CASTNET shelter in Gainesville) during the summer of 2017 (Figure 15). In this study, a 

spare module [37 millimeter (mm) filter diameter for IMPROVE and 47 mm filter diameter for CSN] was 

fitted with a H3PO3-impregnated cellulose filter to collect and sustain (no volatile losses) both particulate 

NH+ 

4 and NH 3. A URG ADFPS was used as a reference method. The ADFPS included an NH 3 denuder, 

followed by a nylon filter to collect particles, and a backup denuder to capture any NH 

3 volatilized from the 

nylon filter. The sum of NH 

3 from both denuders and NH+ 4 from the nylon filter constituted a reference NHx

sample. Figure 16 shows the sampling equipment at Duke Forest.
CASTNET 2019 Annual Report

17

Figure 15 Monitoring Locations in the Southeastern U.S.

Figure 16 Duke Forest NHx Sampling Equipment

Results

Ambient concentrations of NHx from the Duke Forest site from each of the sampling systems are shown in 

Figure 17. The ADFPS NHx value was reported as the nylon NH+ 

4 concentration plus the NH 3 collected on 

the primary denuder and the volatilized NH+ 

4 collected on the backup acid-coated denuder.

URG denuder/filter pack

• Separates NH 

3 and NH+ 4

• Acid-coated denuder (NH 

3)

• Nylon filter (NH+ 

4 )

• Backup denuder (volatile NH 

3)

• Duplicates

• PM2.5 inlet at 10 liters per minute

CSN

• One module collecting NH+ 

4 on nylon filter

• Second module collecting total NHx on 

acid-impregnated cellulose filter

• PM2.5 inlet at 6.7 liters per minute

IMPROVE

• Acid-impregnated cellulose filter to 

capture total NHx

• PM2.5 inlet at 22.8 liters per minute Duke Forest NHx study site
CASTNET 2019 Annual Report

18

Figure 17 Comparison of NHx Concentrations Measured by IMPROVE, CSN, and ADFPS Methods at 

Duke Forest, NC

The linear relationship between the ADFPS and the CSN SuperSASS sampler and the IMPROVE sampler 

from Duke Forest are shown in Figure 18. There was good agreement between the ADFPS and IMPROVE 

systems with a slope of 0.95 and R2 equal to 0.89. These results were similar to results presented in the 

western United States in the NHx pilot network (Chen et al., 2014). Agreement between the ADFPS and the 

CSN system was fair with a slope of 0.69 and an R2 equal to 0.71. The CSN was biased low as compared to 

the ADFPS. There seemed to be less NHx retained on the acid-impregnated filter in the CSN system at 

higher concentrations. 

View of tower at Duke Forest, NC (DUK008)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

5/25/17 6/25/17 7/25/17 8/25/17 9/25/17 10/25/17

Concentration (μg/m3) D-IMPROVE D-ADFPS NHx (Denuder+Filter Pack) D-ADFPS NHx Duplicate D-CSN
CASTNET 2019 Annual Report

19

Figure 18 Linear Relationship between the ADFPS versus CSN NHx Results and ADFPS versus 

IMPROVE NHx Results at Duke Forest

Equipment issues at the Gainesville site made it difficult to draw conclusions, but the available samples 

seemed to indicate that the high humidity interfered with the acid-impregnated filter’s ability to capture 

NH 

3.

Overall, the acid-impregnated filters performed well to capture NHx in the CSN/Super SASS and IMPROVE 

PM sampler, particularly at the Duke Forest site. The median NHx concentrations from the ADFPS, CSN, and 

IMPROVE from Duke Forest were reasonably close (0.59, 0.57, and 0.68 μg/m3, respectively). The 

performance of the acid-impregnated filters at the Gainesville site was poor with the filters significantly

underestimating the NHx concentrations. Additional testing may be performed to evaluate the filter capture 

efficiency in a controlled environment and to further evaluate the effects of meteorological conditions on 

the results.

Improving Total Nitrogen Deposition Budget

Total Reactive Nitrogen

Accurate and complete deposition budgets of nutrients and acidity are fundamental requirements for the 

development of critical loads and protection of human welfare. While much progress has been made in 

developing deposition budgets for the United States over the past several years, improvement in the 

completeness, accuracy, and spatial representativeness of total reactive nitrogen (Nr) deposition budgets 

remains limited by key data and knowledge gaps.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

NHx CSN or NHx IMPROVE (μg/m3)

NHx ADFPS (μg/m3)

IMPROVE

0.95*NHx_ADFPS + 0.11

R2 = 0.89

CSN

0.69*NHx_ADFPS + 0.15

R2 = 0.71
CASTNET 2019 Annual Report

20

Total nitrogen deposition refers to the transfer of nitrogen pollutants from the atmosphere to the earth’s 

surface via wet and dry processes. Air pollutants containing nitrogen are emitted from a variety of sources. 

In general, those coming from vehicles, power plants, and other industries are oxidized (NOx), while those 

coming from agriculture, including animal production and crop fertilization, are reduced (NHx). The 

deposition of these pollutants in excess of critical loads (ecosystem thresholds) causes changes in terrestrial 

and aquatic biodiversity, soil and lake acidification and eutrophication, drinking water contamination, and 

reduces the resilience of ecosystems to climate variability and other stressors. 

The deposition of Nr is monitored by several different networks, including NADP/NTN, which measures 

NO3 and NH+ 

4 in wet deposition; AMoN, which measures ambient NH 3; and standard CASTNET 

measurements. The NADP Total Deposition (TDep) Science Committee uses these measurements, along 

with regional model simulations, to estimate total Nr deposition. (See the subsequent section, 

“Atmospheric Deposition,” for TDep deposition estimates.) These estimates are then compared to critical 

loads to inform resource management and policy decisions and track trends, determine spatial patterns, 

assess ecosystem health, and determine sources of air pollution.

The mission of the NADP’s TDep Science Committee is to improve estimates of atmospheric deposition by 

advancing the science of measuring and modeling atmospheric wet, dry, and total deposition of species,

such as sulfur, nitrogen, and mercury, by providing a forum for the exchange of information on current and 

emerging issues within a broad multi-organization context including atmospheric scientists, ecosystem 

scientists, resource managers, and policy makers. CASTNET is critical for providing measurement data to 

support the continuing development of TDep maps and to support the ongoing research to reduce 

uncertainties in estimates of atmospheric deposition.

Addressing Key Knowledge Gaps and Research Needs

The white paper, “Science Needs for Continued Development of Total Nitrogen Deposition Budgets in the 

United States,” summarized the work of the TDEP Science Committee and identified knowledge gaps in 

atmospheric nitrogen deposition. The white paper resulted in nine peer-reviewed journal articles and four 

magazine articles. All are available at http://nadp.slh.wisc.edu/committees/tdep/reports/.

Key knowledge gaps in Nr deposition are of interest to a broad group of stakeholders, including many 

federal (EPA, USGS, NPS, USFS, FWS, BLM, NOAA, NSF, and NASA), tribal, and state agencies; academia; 

industry; and non-profit groups. These stakeholders share a wide variety of interconnected scientific and 

policy interests and have prioritized the three following research needs:

Total Nr deposition: 55% is dry across the United States (Figure 26)

90% is dry over broad areas of the Southwest and 

other arid regions (Figure 26)

50% is reduced N (NHx; Figure 28)
CASTNET 2019 Annual Report

21

• Better characterization of patterns and trends in Nr deposition: a complete Nr budget with good 

geographical coverage and over long time periods is important so that critical loads can be 

accurately assessed and changes in atmospheric chemistry can be identified, such as the shift from 

a predominance of oxidized forms of Nr (NOy) to reduced forms (NH 

3 + NH+ 4 ). This can be 

accomplished by filling in key geographical gaps with satellite air pollutant measurements, 

additional monitoring sites (e.g., NTN and CASTNET in urban areas and AMoN in agricultural 

regions), expanding the capability to measure dry deposition, and measuring additional 

compounds not currently captured in the national networks that contribute significantly to total Nr 

deposition. Adaptation of NTN methods to include bulk water soluble organic Nr (ON) in wet 

deposition would be an important step as well as expanding nitrogen dioxide measurements, which 

are currently mostly located in urban areas.

• Quantification and reduction of uncertainty in deposition estimates used for critical loads 

applications: deposition estimates are made with measurements, models, or a combination of both. 

Lack of dry deposition measurements imposes a reliance on models, resulting in a much higher 

degree of uncertainty relative to wet deposition that is routinely measured. The uncertainty 

inherent in using complex models to estimate total deposition could be quantified by a 

comprehensive comparison of the deposition processes within them. Developing more modular 

codes and software architectures that allow for easier implementation and comparison of different 

parameterizations is needed. Such efforts could draw on modeling expertise within EPA, NPS, 

Environment and Climate Change Canada, and NOAA. The incorporation of NOAA and NASA 

satellite data within the hybrid TDep methodology could also help reduce uncertainty. However, 

the satellite data have to be coupled with modeled deposition velocity data to estimate deposition.

• Improvement of the understanding of the linkages between agricultural emissions and Nr 

deposition: with the shift in atmospheric chemistry from oxidized to more reduced forms of Nr, 

agriculture has become a more important source of N emissions. Better estimates of NH 

3 emissions 

from agricultural sources (animal production and crop fertilization) are needed for source 

apportionment and modeling of air quality, visibility, and Nr deposition. It is also important to 

understand the contribution of NH 

3 to atmospheric PM and the indirect effects of agriculture on 

land use change (i.e., biodiversity impacts of NHx deposition).
CASTNET 2019 Annual Report

22

Sulfur Atmospheric Concentrations

Annual mean concentrations of SO 

2 and SO2-4 for 2019 are presented in this section. Trends in annual 

mean concentrations are also shown.

Figure 19 Annual Mean SO 

2 Concentrations for 2019

Washington Crossing State Park, NJ (WSP144)
CASTNET 2019 Annual Report

23

Figure 20 Trends in Annual Mean SO 

2 Concentrations

Eastern Reference Sites

Western Reference Sites

Abington, CT (ABT147)
CASTNET 2019 Annual Report

24

Figure 21 Annual Mean SO2-

4 Concentrations for 2019

Cranberry, NC (PNF126)
CASTNET 2019 Annual Report

25

Figure 22 Trends in Annual Mean SO2-

4 Concentrations

Eastern Reference Sites

Western Reference Sites

Figure 23 provides a diagram that compares weekly filter pack SO 

2 concentrations with continuous tracelevel gas data measured at Mammoth Cave National Park, KY (MAC426). The continuous analyzer provides 

a gross check of the filter pack measurements despite its occasional uncorrected baseline drift. The 

continuous analyzer data can also be used to produce diurnal profiles of hourly SO 

2 concentrations and 

detect the presence of SO 

2 plumes.

Yosemite National Park – Turtleback Dome, CA (YOS404)
CASTNET 2019 Annual Report

26

Figure 23 Comparison of MAC426 Weekly Mean Continuous and Filter Pack 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 TDep measurement-model fusion 

technique (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 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) surface of precipitation data to 

develop a continuous grid of precipitation data. PRISM uses terrain elevation, slope, and aspect and 

climatic measurements to estimate precipitation amounts. 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 2019, 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 chapter as kilograms per hectare per year (kg ha-1

 yr-1 ). 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

12/24/18

1/27/192/10/192/24/193/10/193/24/19

4/7/19

4/21/19

5/5/19

5/19/19

6/2/19

6/16/196/30/19

7/14/19

7/28/198/11/198/25/19

9/8/19

9/22/1910/6/19

10/20/19

11/3/19

11/17/19

12/1/19

12/15/19

Concentration (ppb)

Filter Pack

Mean Trace

SO2

SO2

Mean Continuous SO2

Filter Pack SO2
CASTNET 2019 Annual Report

27

Nitrogen Deposition

A map of the dry deposition fluxes of nitrogen (N) estimated for 2019 by the TDep method is shown in 

Figure 24. The magnitude of the deposition fluxes is illustrated by the shading in the figure legend. A map 

of total deposition of N estimated for 2019 is given in Figure 25. The percentage of total deposition of N 

due to dry deposition is shown in Figure 26. A map of TDep estimates of total deposition of reduced N 

species for 2019 is given in Figure 27. Figure 28 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 29, and its percentage of total deposition is shown in Figure 30. Estimates of 

annual gross dry deposition of NH 

3 as N for 2019 is shown in Figure 31, and the net dry deposition of NH 3

as N is shown in Figure 32. 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 33 shows a map of TDep estimates of total deposition rates of sulfur (S) for 2019. 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 34.

Deposition of Base Cations and Chloride

A map of 2019 estimated total deposition of base cations (Ca2+

 , K+ , Mg2+ , and Na+ ) is given in Figure 35. 

Figure 36 provides 2019 estimated rates of deposition of total Cl-.

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

 yr-1 ) for 2019

Source: CASTNET/CMAQ/NADP

Dry N
CASTNET 2019 Annual Report

28

Figure 25 TDep Total Deposition Estimates of N (kg ha-1

 yr-1 ) for 2019

Source: CASTNET/CMAQ/NADP

Figure 26 TDep Percent of Total Deposition of N from Dry Deposition for 2019

Source: CASTNET/CMAQ/NADP

Total N

Percent
CASTNET 2019 Annual Report

29

Figure 27 TDep Total Deposition Estimates of Reduced N Species (kg ha-1

 yr-1 ) for 2019

Source: CASTNET/CMAQ/NADP

Figure 28 TDep Percent of Total Deposition of Reduced N Species for 2019

Source: CASTNET/CMAQ/NADP

Total N

Percent
CASTNET 2019 Annual Report

30

Figure 29 TDep Dry Deposition Estimates of Unmonitored N Species (kg ha-1

 yr-1 ) for 2019

Source: CASTNET/CMAQ/NADP

Figure 30 TDep Percent of Total Deposition of N from Dry Deposition of Unmonitored Species 

for 2019

Source: CASTNET/CMAQ/NADP

Dry N

Percent
CASTNET 2019 Annual Report

31

Figure 31 TDep Gross Dry Deposition of NH 

3 as N (kg ha-1 yr-1 ) for 2019

Source: CASTNET/CMAQ/NADP

Figure 32 TDep Net Dry Deposition of NH 

3 as N (kg ha-1 yr-1 ) for 2019

Source: CASTNET/CMAQ/NADP

Dry N

Dry N
CASTNET 2019 Annual Report

32

Figure 33 TDep Total Deposition Estimates of S (kg ha-1

 yr-1 ) for 2019

Source: CASTNET/CMAQ/NADP

Figure 34 TDep Percent of Total Deposition of S from Dry Deposition for 2019

Source: CASTNET/CMAQ/NADP

Total S

Percent
CASTNET 2019 Annual Report

33

Figure 35 TDep Total Deposition Estimates of the Base Cations Ca2+, K+ , Mg2+, and Na+ 

(kiloequivalents per hectare per year) for 2019

Source: CASTNET/CMAQ/NADP

Figure 36 TDep Total Deposition Estimates of Total Cl- (kg ha-1

 yr-1 ) for 2019

Source: CASTNET/CMAQ/NADP

Total BC

Total Cl-
CASTNET 2019 Annual Report

34

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., Day, D., Schichtel, B., Malm, W., Matzoll, A. K., Mojica, J., McDade, C. S., Hardison, E. D., Hardison,

D. L., Walters, S., Van De Water, M., Collett, J. L. Jr. 2014. Seasonal Ambient Ammonia and 

Ammonium Concentrations in a Pilot IMPROVE NHx Monitoring Network in the Western United 

States. Atmospheric Environment 91, 118-126. 

Du, E., de Vries, W., Galloway, J. N., Hu, X., Fang, J. 2014. Changes in Wet Nitrogen Deposition in the United 

States between 1985 and 2012. Environmental Research Letters 9, 095004. 

Li, Y., Schichtel, B., Walker, J. T., Schwede, D. B., Chen, X. I., Lehmann, C., Puchalski, M., Gay, D., Collett, J. L. Jr.

2016. The Increasing Importance of Deposition of Reduced Nitrogen in the United States. 

Proceedings of the National Academy of Sciences, 113, 5874–5879. 

Rogers, C., Mishoe, K., Stewart, M., Barry, K., Rice, J., Chen, X., Walker, J. T., Puchalski, M., Baumgardner, R., 

and Schichtel, B. 2021. Improving Characterization of Reduced Nitrogen at IMPROVE and CSN 

Monitoring Sites. U.S. Environmental Protection Agency, Washington, DC, 2021.

https://cfpub.epa.gov/si/si_public_record_Report.cfm?dirEntryId=350728&Lab=CEMM. Accessed 

March 2021.

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

States. Atmospheric Environment, 92:207-220. doi:10.1016/j.atmosenv.2014.04.008. 

Wood Environment & Infrastructure Solutions, Inc. (Wood). 2020. Clean Air Status and Trends Network 

(CASTNET) 2018 Annual Report. Prepared for U.S. Environmental Protection Agency (EPA), Office of 

Air and Radiation, Clean Air Markets Division, Washington, DC. Contract No. EP-W-16-015. 

Gainesville, FL. http://java.epa.gov/castnet/documents.do.

Wood Environment & Infrastructure Solutions, Inc. (Wood). 2019. Clean Air Status and Trends Network 

(CASTNET) Quality Assurance Project Plan (QAPP) Revision 9.2. Prepared for U.S. Environmental 

Protection Agency (EPA), Office of Air and Radiation, Clean Air Markets Division, Washington, DC. 

Contract No. EP-W-16-015. Gainesville, FL. http://java.epa.gov/castnet/documents.do.

Yu, X., Lee, T., Ayresa, B., Kreidenweis, S., Malm, W., Collett, J. 2006. Loss of Fine Particle Ammonium from 

Denuded Nylon Filters. Atmospheric Environment, 40 4797–4807.