The Clean Air Status and Trends Network (CASTNET) is a long-term monitoring network designed to measure acidic pollutants and ambient ozone (O₃) concentrations in rural areas in the United States and Canada. CASTNET is managed collaboratively by the Environmental Protection Agency - Clean Air and Power Division (EPA), the National Park Service - Air Resources Division (NPS), the Bureau of Land Management - Wyoming State Office (BLM-WSO), and Alberta's Ministry of Environment and Protected Areas. Numerous other participants provide site operator support and grant land access including Native American tribes, other federal agencies, States, private landowners, and universities.

Each CASTNET monitor measures ambient O₃ concentrations for the entire year. CASTNET O₃ data are submitted to the AIRNow Tech website for near-real time reporting (www.airnowtech.org) and to EPA's Air Quality System (AQS) database (https://aqs.epa.gov/aqs). Annual performance evaluations (PE) and results from the National Performance Audit Program (NPAP) are also submitted to AQS routinely.

Eighty CASTNET sites currently report hourly O₃ concentrations. Seventy-seven of the O₃ monitors at CASTNET sites meet the requirements of Title 40 of the Code of Federal Regulations (CFR) Part 58 and are used to determine compliance with the O₃ National Ambient Air Quality Standard (NAAQS). Two monitors are co-located sites and are submitted to AQS as QA/non-regulatory data, and one monitor is at the Duke Forest, NC research site and measures concentrations above the forest canopy. CASTNET provides a unique dataset to serve rural populations where state or local operated O₃ monitors are not required. During the recent Science Advisory Board (SAB) review panel, CASTNET ozone data were highlighted for their utility in evaluating atmospheric chemical transport models and in ground truthing satellite estimates. Preliminary analysis of the fourth highest daily maximum rolling 8-hour averages for 2023 indicate that 18 CASTNET sites had values greater than 70 ppb.

CASTNET is currently starting to evaluate options for the replacement of its aging ozone analyzers and site transfer standards. In conjunction with NPS and Environment and Climate Change Canada (ECCC), EPA will review various commercially available Federal Reference Method (FRM) or Federal Equivalent Method (FEM) equipment during 2024 to determine how to proceed.

Ozone data and additional information about the CASTNET monitoring program can be found on the CASTNET webpage at https://www.epa.gov/castnet.

¹ WSP USA Environment & Infrastructure, Jacksonville, FL

² U.S. Environmental Protection Agency, Clean Air and Power Division, Washington, DC

³ WSP USA Environment & Infrastructure, Gainesville, FL

*Corresponding Author: selma.isil@wsp.com