The Clean Air Status and Trends Network (CASTNET) is a national air quality monitoring network designed to provide data to assess trends in air quality, atmospheric deposition, and ecological effects due to changes in air pollutant emissions. CASTNET began collecting measurements in 1991 with the incorporation of 50 sites from the National Dry Deposition Network, which had been in operation since 1987. The National Park Service (NPS) operates more than 20 CASTNET sites within national parks and Class 1 areas. The Bureau of Land Management, Wyoming State Office joined CASTNET in 2012 when they converted five Wyoming Air Resource Monitoring System (WARMS) sites to CASTNET protocol sites. CASTNET provides long-term monitoring of air quality in rural areas to determine trends in regional atmospheric nitrogen, sulfur, and ozone concentrations and deposition fluxes of sulfur and nitrogen pollutants in order to evaluate the effectiveness of national and regional air pollution control programs. CASTNET operates more than 85 regional sites throughout the contiguous United States, Alaska, and Canada. Sites are located in areas where urban influences are minimal.

CASTNET measurements are used to assess long-term, regional trends in sulfur and nitrogen pollutants. Significant reductions in SO₂, SO₄, HNO₃, total NO₃, and NH₄ have been realized in the Eastern US. For example, between 1990-1992 and 2010-2012, SO₂ concentrations were reduced from 8.9 μg/m³ to 2.1 μg/m³, a 76% reduction. Reductions in regional ozone concentrations have been most dramatic in the East. Additional information about CASTNET, including results, data quality, and descriptions of on-going reearch studies can be found in the most recent Annual Report.

In 2011, all CASTNET ozone monitors became compliant with the requirements in the Code of Federal Regulations (40 CFR). All CASTNET sites that have meet the QA and completeness criteria will be used for comparison to the ozone NAAQS beginning with the 2011-2013 time period. Learn more about the CASTNET ozone monitoring program here.