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 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 land owners, and universities.  

Eighty-two CASTNET sites report hourly O₃ concentrations. The data provide accountability for EPA’s regional Nox emission reduction programs (i.e. Nox SIP Call, Cross State Air Pollution Rule, and Cross State Air Pollution Rule Update) and exposure effects on vegetation. The National Park Service uses CASTNET O₃ data to develop strategies for improving visibility and protecting resources within park boundaries.  Additionally, eighty 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). Populations located in areas that exceed the primary NAAQS are more susceptible to adverse health effects. CASTNET provides a unique dataset to rural populations where state-operated O₃ monitors are not required.

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 2014-2016 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 National Park, CA; Yosemite National Park, CA; and Washington Crossing, NJ.

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