CASTNET Quality Assurance Report Third Quarter 2025

Summary of Quarterly Operations (July through September)

Submitted to U.S. Environmental Protection Agency (EPA)
Clean Air and Power Division

EPA Contract No. 68HERH21D0006

Introduction

This quarterly report summarizes results from the Clean Air Status and Trends Network (CASTNET) quality assurance/quality control (QA/QC) program for data collected during third quarter 2025. The various QA/QC criteria and policies are documented in the CASTNET Quality Assurance Project Plan [QAPP; WSP Environment & Infrastructure Inc. (WSP), 2025]. The QAPP is comprehensive and includes standards and policies for all components of project operation from site selection through final data reporting. It is reviewed annually and updated as warranted.

Quarterly Summary

On July 22, 2025, WSP was notified by the American Association for Laboratory Accreditation that WSP’s field and analytical laboratories were approved for renewal of International Organization for Standardization/International Electrotechnical Commission 17025:2017 accreditation for two years, through May 31, 2027.

WSP continued preparations for updating CASTNET ozone (O₃) transfers to the new cross-section. As of the end of third quarter 2025, four CASTNET transfers, which included one Thermo 49i-PS bench standard and three Thermo 49i Level 2 traveling transfers, were updated with the new cross-section and verified following the guidelines in EPA’s 2023 Transfer Standards for Calibration of Air Monitoring Analyzers for Ozone Technical Assistance Document (TAD; EPA, 2023). During September, WSP sent an out-of-certification Thermo 49i Level 2 traveling transfer standard to EPA Region 4 for reverification and updating to the new cross-section by the EPA Region 4 standard reference photometer (SRP). The transfer was damaged during shipping to EPA Region 4, and it was sent back to WSP for repair without being updated. WSP will repair the damaged transfer and will ship it back to EPA Region 4 where it will be reverified and updated to the new cross-section by the EPA Region 4 SRP in early fourth quarter 2025. WSP has an additional two transfers still to be updated to the new cross-section and verified following 2023 O₃ TAD guidelines: a Thermo 49i PS bench standard and a Thermo 49i Level 2 traveling transfer. The certification for one of the two transfers expires in October 2025 and the other expires in November 2025.

WSP continued updating the onsite Level 3 O₃ transfer standards to the new cross-section following EPA’s updated 2023 O₃ TAD guidelines. The new process includes updating the site data logger program and completing updated documentation during the reverification of the site transfer standard. Beginning January 1, 2025, data from O₃ systems not using the new cross-section are required to be flagged “XS” in EPA’s Air Quality System (AQS). WSP is appending “XS” qualifier flags to the 2025 O₃ data for EPA-sponsored CASTNET sites to indicate they were obtained from O₃ systems that were not updated to the new cross-section. WSP will cease application of the “XS” flags following the cross-section update. During third quarter 2025, the sites listed in Table 1 were updated to the new O₃ cross-section as of the date and time listed for each site.

EPA’s Office of Air Quality Planning and Standards (OAQPS) is encouraging O₃ monitoring organizations to transition to scrubber-free O₃ transfer standards. WSP believes the best option for transitioning to scrubber-free transfer standards is through equipment changes over time. Under Task Order 68HERH24F0336, Non-routine Maintenance and Repairs (3006), WSP recommended replacement of the current Thermo Model 49i O₃ system with the new Teledyne API (TAPI) O₃ system (models N400 and N703U). EPA’s Clean Air and Power Division (CAPD) approved WSP’s purchase of the TAPI O₃ systems, and WSP ordered 62 Teledyne Model N400 O₃ analyzers in third quarter 2025. WSP will order N703U during fourth quarter 2025 and begin replacing the Thermo 49i systems at CASTNET sites in early 2026.

The QA Manager completed review of the results of the Level 4 O₃ validation review by Air Resource Specialists, Inc. (ARS). ARS reviewed the O₃ data set for data collected from 2021 through 2023. WSP will make the recommended changes and resubmit the updated data to CAPD and EPA’s AQS.

EPA’s OAQPS finalized the parameters for submission of CASTNET filter pack data to EPA’s AQS. During third quarter 2025, WSP completed uploading validated filter pack data to AQS. Validated filter pack data from EPA-sponsored CASTNET sites from 1990 through 2023 were uploaded, except for data from CHE185, OK, which were uploaded through 2024. Validated filter pack data from National Park Service (NPS)-sponsored CASTNET sites were uploaded for sites active in AQS through 2024. Filter pack data for Bureau of Land Management-sponsored sites active in AQS were uploaded through March 2025. Going forward, validated filter pack data will be updated annually in AQS.

The QA Manager organized documentation for the 2024 managerial review. He prepared the report and provided it to the CASTNET management team for review. The presentation of the report’s findings will take place in October 2025. The CASTNET management team and WSP corporate QA and management personnel will attend the presentation.

Review of the CASTNET QAPP to determine if updates are needed continued during third quarter. Updates that will be included in the next version of the QAPP include, but are not limited to, adding procedures for installation and operation of the PurpleAir particulate matter (PM) sensors.

The QA Manager began reviewing the WSP corporate Quality Management Plan (QMP) to assess if it addresses updated EPA guidelines. Once he completes his review, he will submit proposed changes to WSP corporate personnel for incorporation into the WSP QMP.

WSP received final results of analyses of samples for proficiency test (PT) 126 for Rain and Soft Waters from the Water Science and Technology Directorate, a branch of Environmental Science and Technology Laboratories with Environment and Climate Change Canada, during third quarter 2025. There were two warning flags: one for an ammonia sample and one for a pH sample. Overall, WSP’s results for PT 126 received a rating of “Good.”

The QA Manager began the process to perform an audit of data quality for per- and polyfluoroalkyl substances (PFAS) data collected from January 2023 to December 2024. The QA Manager received data from Martin Shafer with the National Atmospheric Deposition Program analytical laboratory. The results of the audit will be submitted to EPA in fourth quarter 2025.

During third quarter 2025, National Performance Audit Program (NPAP) and state agency audits were performed at the sites listed in Table 2. Results for all sites were within the established audit criteria.

Table 3 lists the quarters of data that were validated to Level 3 during third quarter 2025 by site calibration group. Table 4 lists the sites in each calibration group along with the calibration schedule. Table 5 presents the measurement criteria for laboratory filter pack measurements. These criteria apply to the QC samples listed in the following section of this report. Table 6 presents the critical criteria for O₃ monitoring. Table 7 presents the critical criteria for trace-level gas monitoring.

Quality Control Analysis Count

The QC sample statistics presented in this report are for reference standards (RF) and continuing calibration verification spikes (CCV) used to assess accuracy and for replicate sample analyses (RP) used to assess “in-run” precision. In addition, laboratory method blanks (MB) containing reagents without a filter; laboratory blanks (LB) containing reagents and a new, unexposed filter; and field blanks (FB) containing reagents and an unexposed filter that had been loaded into a filter pack assembly and shipped to and from the monitoring site while remaining in sealed packaging are also included. Table 8 presents the number of analyses in each category that were performed during third quarter 2025.

Sample Receipt Statistics

Ninety-five percent of field samples from EPA-sponsored sites should be received by the CASTNET laboratory in Gainesville, FL no later than 14 days after removal from the sampling tower. Table 9 presents the relevant sample receipt statistics for third quarter 2025.

Data Quality Indicator (DQI) Results

Figures 1 through 3 present the results of RF, CCV, and RP QC sample analyses for third quarter 2025. All results were within the criteria listed in Table 5.

Table 10 presents summary statistics of critical criteria measurements at O₃ sites collected during third quarter 2025. The statistics presented contain data validated at Level 2 and Level 3. All data associated with QC checks that fail to meet the criteria listed in Table 6 were or will be invalidated unless the cause of failure has no effect on ambient data collection and passing results still meet frequency criteria. Results in shaded cells either exceed documented criteria or are otherwise notable. Table 11 presents observations associated with the shaded cell results in Table 10.

Table 12 presents summary statistics of critical criteria measurements at trace-level gas monitoring sites collected during third quarter 2025. The statistics presented contain data validated at Level 2 and Level 3. All data associated with QC checks that fail to meet the criteria listed in Table 7 were or will be invalidated unless the cause of failure has no effect on ambient data collection and passing results still meet frequency criteria. Results in shaded cells either exceed documented criteria or are otherwise notable. Table 13 presents observations associated with the shaded cell results in Table 12.

Laboratory Control Sample Analysis

The laboratory control sample (LCS) is a reagent blank spiked with the target analytes from the established analytical methods and carried through the same extraction process that field samples must undergo. LCS analyses are performed by the laboratory to monitor for potential sample handling artifacts and provide a means to identify possible analyte loss from the extraction process. Figure 4 presents LCS analysis results for third quarter 2025. All recovery values were between 91 percent and 110 percent.

Blank Results

Figures 5 through 7 present the results of MB, LB, and FB QC sample analyses for third quarter 2025. All third quarter results were within criteria (two times the reporting limit listed in Table 5).

Suspect/Invalid Filter Pack Samples

Filter pack samples that were flagged as suspect or invalid during third quarter 2025 are listed in Table 14. This table also includes associated site identification and a brief description of the reason the sample was flagged. During third quarter, nine filter pack samples were invalidated.

Field Problem Count

Table 15 presents counts of field problems affecting continuous data collection for more than one day for third quarter 2025. The problem counts are sorted by a 30-, 60-, or 90-day time period to resolution. A category for unresolved problems is also included.

References

American Society for Testing and Materials (ASTM). 2022. ASTM E29-22, Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications. ASTM International, West Conshohocken, PA, DOI:10.1520/E0029-22. www.astm.org.

U.S. Environmental Protection Agency (EPA). 2024. Title 40 Code of Federal Regulations Part 58, Appendix A to Part 58 – Quality Assurance Requirements for Monitors used in Evaluations of National Ambient Air Quality Standards. https://www.epa.gov/amtic/ambient-air-monitoring-quality-assurance.

U.S. Environmental Protection Agency (EPA). 2023. Transfer Standards for Calibration of Air Monitoring Analyzers for Ozone Technical Assistance Document. Publication No. EPA-454/B-22-003, January 2023: https://www.epa.gov/system/files/documents/2023-11/o3_tad_508_20230906_final.pdf.

WSP USA Environment & Infrastructure Inc. (WSP). 2025. Clean Air Status and Trends Network (CASTNET) Quality Assurance Project Plan (QAPP) Revision 10.2. Prepared for U.S. Environmental Protection Agency (EPA), Office of Air and Radiation, Clean Air and Power Division, Washington, DC. Contract No. 68HERH21D0006. Gainesville, FL. https://www.epa.gov/castnet/documents-reports.

Notes:

• * The sites contained in each calibration group are listed in Table 4.
• † Contains ROM206 of the ROM406/ROM206 co-located pair
• ‡ Contains MCK131/231 co-located pair

Notes:

• ¹ Trace-level gas calibrations are performed quarterly in January, April, July, and October.
• ² Trace-level gas calibrations are performed quarterly in March, June, September, and December.

Notes:

• ¹ This column lists precision goals for both network precision calculated from co-located filter samples and laboratory precision based on replicate samples for samples > five times the reporting limit. The criterion is ± the reporting limit if the sample is ≤ five times the reporting limit.
• ² This column lists laboratory accuracy goals based on reference standards and continuing calibration verification spikes. The criterion is 90–110 percent for ICP-OES reference standards.
• ³ The reporting limit for sulfate on cellulose filters (reported as SO₄^2- with correction factor applied) is 0.080 mg/L (2.0 µg/filter) because there are two cellulose filters in each filter pack.

AC

automated colorimetry

IC

ion chromatography

ICP-OES

inductively coupled plasma-optical emission spectrometry

MARPD

mean absolute relative percent difference

mg/L

milligrams per liter

µg/Filter

micrograms per filter

*

as nitrogen

Values are rounded according to American Society for Testing and Materials (ASTM) E29-22, “Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications” (ASTM, 2022).

For more information on analytical methods and associated precision and accuracy criteria, see the CASTNET QAPP, (WSP, 2025).

Notes:

• * Applies to CASTNET sites that are configured and operated in accordance with Part 58 of Title 40 of the Code of Federal Regulations (EPA, 2024). The minimum frequency for these checks is once every two weeks.
• Values are rounded according to ASTM E29-22, “Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications” (ASTM, 2022).

Notes:

• * Applies to CASTNET sites that are configured and operated in accordance with Part 58 of Title 40 of the Code of Federal Regulations (EPA, 2024). The minimum frequency for these checks is once every two weeks.
• Values are rounded according to ASTM E29-22, “Standard Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications” (ASTM, 2022).

SO₂

sulfur dioxide

NO_y

total reactive oxides of nitrogen

CO

carbon monoxide

ppb

parts per billion

Note: Sample shipments for the Egbert, Ontario site (EGB181) are sent in groups of four. Samples associated with EGB181 are excluded from this statistic.

Notes:

• ¹ Percentage of comparisons that pass the criteria listed in Table 6. Values falling below 90 percent are addressed in Table 11.
• ² Absolute value of the average percent differences between the on-site transfer standard and the site monitor. Values exceeding the criteria listed in Table 6 are addressed in Table 11.
• %D = percent difference
• ppb = parts per billion

Note: %D = percent difference

Notes:

• ¹ Percentage of comparisons that pass the criteria listed in Table 7. Values falling below 90 percent are addressed in Table 13.
• ² Absolute value of the average percent differences between the supplied and observed concentrations. Values exceeding the criteria listed in Table 7 are addressed in Table 13.
• %D = percent difference
• ppb = parts per billion

Figure 1 Reference Standard Results for Third Quarter 2025 (percent recovery)

Four scatter charts plot the percent recovery of individual reference standard (RF) analyses by analysis date from June 27, 2025 through October 1, 2025. Each chart's vertical axis runs from 90 to 110 percent recovery. No numeric data labels are printed; all results were within the criteria listed in Table 5.

• RF - Teflon ICP-OES (series Ca, Mg, Na, K): recoveries scatter between roughly 92 and 105 percent with no trend across the quarter. Mg results tend to run at or above 100 percent, K and Ca results cluster near 98 to 104 percent, and Na results tend to run below 100 percent, mostly between roughly 92 and 98 percent.
• RF - Teflon IC & AC (series TSO4, TNO3, TNH4, Cl): TSO4, TNO3, and Cl results cluster tightly between roughly 99 and 103 percent throughout the quarter. TNH4 results run lower, mostly between roughly 91 and 96 percent.
• RF - Nylon IC (series NSO4, NHNO3): NSO4 results fall mostly between roughly 100 and 105 percent; NHNO3 results fall mostly between roughly 98 and 102 percent.
• RF - Cellulose IC (series WSO2): results fall between roughly 99 and 105 percent.

Figure 2 Continuing Calibration Spike Results for Third Quarter 2025 (percent recovery)

Four scatter charts plot the percent recovery of individual continuing calibration verification spike (CCV) analyses by analysis date from June 27, 2025 through October 1, 2025, on vertical axes running from 90 to 110 percent recovery. Analyses occur in vertical clusters on each run date. No numeric data labels are printed; all results were within the criteria listed in Table 5.

• CCV - Teflon ICP-OES (series Ca, Mg, Na, K): recoveries cluster around 100 percent, mostly between roughly 97 and 104 percent, with occasional single results near 95 to 96 percent.
• CCV - Teflon IC & AC (series TSO4, TNO3, TNH4, Cl): most results fall between roughly 98 and 103 percent, with one Cl result near 104 percent in late June. TNH4 results extend lower than the other series, with clusters reaching roughly 93 to 97 percent, including a group near 93 to 96 percent in late July and results near 93 to 94 percent in mid September.
• CCV - Nylon IC (series NSO4, NHNO3): both series fall mostly between roughly 97 and 102 percent.
• CCV - Cellulose IC (series WSO2): results fall between roughly 96 and 105 percent.

Figure 3 Replicate Sample Analysis Results for Third Quarter 2025 (percent difference)

Four scatter charts plot the percent difference of individual replicate sample (RP) analyses by analysis date from June 27, 2025 through October 1, 2025, on vertical axes running from 0 to 20 percent difference. No numeric data labels are printed; all results were within the criteria listed in Table 5.

• RP - Teflon ICP-OES (series Ca, Mg, Na, K): most differences are below 5 percent, with scattered results between 5 and 10 percent, one Ca result near 14 percent in mid August, and one Mg result near 16 percent in mid September.
• RP - Teflon IC & AC (series TSO4, TNO3, TNH4, Cl): most differences are below 5 percent; TNO3 results occasionally reach roughly 4 to 6.5 percent, and two TNH4 results are near 10 percent (one in late June and one in mid September).
• RP - Nylon IC (series NSO4, NHNO3): NSO4 differences are more variable, with many results between 5 and 12 percent and maximums near 17 to 17.5 percent; NHNO3 differences are mostly below 8 percent.
• RP - Cellulose IC (series WSO2): few replicates are plotted; all differences are below roughly 6 percent, with most below 2.5 percent.

Figure 4 Laboratory Control Sample Results for Third Quarter 2025 (percent recovery)

Four scatter charts plot the percent recovery of individual laboratory control sample (LCS) analyses by analysis date from June 26, 2025 through September 30, 2025, on vertical axes running from 80 to 120 percent recovery. No numeric data labels are printed; per the report text, all recovery values were between 91 percent and 110 percent.

• LCS - Teflon ICP-OES (series Ca, Mg, Na, K): recoveries fall mostly between roughly 96 and 103 percent, with one Ca result near 110 percent in early August.
• LCS - Teflon IC & AC (series TSO4, TNO3, TNH4, Cl): recoveries fall mostly between roughly 95 and 104 percent, with occasional TNH4 results near 92 to 94 percent.
• LCS - Nylon IC (series NSO4, NHNO3): NSO4 recoveries fall mostly between roughly 95 and 102 percent; NHNO3 recoveries run slightly lower, mostly between roughly 91 and 97 percent.
• LCS - Cellulose IC (series WSO2): recoveries fall between roughly 97 and 101 percent.

Figure 5 Method Blank Analysis Results for Third Quarter 2025 (total micrograms)

Four scatter charts plot method blank (MB) concentrations in total micrograms by analysis date from June 26, 2025 through September 30, 2025. In each chart every result plots at a constant level for every analysis date, corresponding to the reporting limits in Table 5. All results were within criteria (two times the reporting limit listed in Table 5).

• MB - Teflon ICP-OES (series Ca, Mg, Na, K): Ca and K results plot at about 0.15, Na results at about 0.125, and Mg results at about 0.075 micrograms on every analysis date.
• MB - Teflon IC & AC (series TSO4, TNO3, TNH4, Cl): TSO4 results plot at about 1.0, TNH4 and Cl results at about 0.5, and TNO3 results at about 0.2 micrograms on every analysis date.
• MB - Nylon IC (series NSO4, NHNO3): NSO4 results plot at about 1.0 and NHNO3 results at about 0.2 micrograms on every analysis date.
• MB - Cellulose IC (series WSO2): results plot at about 2.0 micrograms on every analysis date.

Figure 6 Laboratory Blank Analysis Results for Third Quarter 2025 (total micrograms)

Four scatter charts plot laboratory blank (LB) concentrations in total micrograms by analysis date from late June through mid September 2025. As in Figure 5, nearly all results plot at constant levels corresponding to the reporting limits in Table 5, and all results were within criteria (two times the reporting limit).

• LB - Teflon ICP-OES (series Ca, Mg, Na, K): Ca and K results plot at about 0.15, Na results at about 0.125, and Mg results at about 0.075 micrograms on every analysis date.
• LB - Teflon IC & AC (series TSO4, TNO3, TNH4, Cl): TSO4 results plot at about 1.0, TNH4 and Cl results at about 0.5, and TNO3 results at about 0.2 micrograms on every analysis date.
• LB - Nylon IC (series NSO4, NHNO3): NSO4 results plot at about 1.0 and NHNO3 results at about 0.2 micrograms on every analysis date.
• LB - Cellulose IC (series WSO2): most results plot at 2.0 micrograms; two early-July results are higher, near 2.07 and 2.21 micrograms.

Figure 7 Field Blank Analysis Results for Third Quarter 2025 (total micrograms)

Four scatter charts plot field blank (FB) concentrations in total micrograms by analysis date from July 7, 2025 through August 30, 2025. Field blank analyses occur on fewer dates than the other blank types. Nearly all results plot at constant levels corresponding to the reporting limits in Table 5, and all results were within criteria (two times the reporting limit).

• FB - Teflon ICP-OES (series Ca, Mg, Na, K): Ca and K results plot at about 0.15, Na results at about 0.125, and Mg results at about 0.075 micrograms on each analysis date.
• FB - Teflon IC & AC (series TSO4, TNO3, TNH4, Cl): TSO4 results plot at about 1.0, TNH4 and Cl results at about 0.5, and TNO3 results at about 0.2 micrograms on each analysis date.
• FB - Nylon IC (series NSO4, NHNO3): NSO4 results plot at about 1.0 and NHNO3 results at about 0.2 micrograms on each analysis date.
• FB - Cellulose IC (series WSO2): most results plot at about 2.0 micrograms, with one higher result near 2.8 micrograms in early August.