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 throughout the United States and Canada. CASTNET is managed collaboratively by the Environmental Protection Agency – Clean Air Markets Division (EPA), the National Park Service – Air Resources Division (NPS), and the Bureau of Land Management – Wyoming State Office (BLM-WSO). In addition to EPA, NPS, and BLM-WSO, numerous other participants provide site operator support and grant land access including North American tribes, other federal agencies, States, private landowners, and universities.

Eighty-eight CASTNET sites report hourly O₃ concentrations used to assess regional trends, evaluate climate impacts on air quality, and model exposure effects on vegetation. Additionally, 87 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). One CASTNET site was established as a research site located above a forest canopy at Duke Forest, NC.

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.

Preliminary 2018-2020 3-year average of the fourth highest daily maximum rolling 8-hour averages calculated using the 2015 ozone NAAQS indicate that four CASTNET sites exceed the 70 ppb O₃ NAAQS including Joshua Tree National Park, CA; Sequoia & Kings Canyon National Parks, CA; Yosemite National Park, CA; and Beltsville, MD. Two additional sites, Rocky Mountain National Park, CO and Washington Crossing, NJ, had a 3-year average of 70 ppb.

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