Clean Air Status and Trends Network (CASNET)

The Clean Air Status and Trends Network (CASTNET) is an atmospheric monitoring network that was established in response to the 1990 Clean Air Act Amendments. Air sampling sites are located throughout the U.S. and Canada to measure trends in pollutant concentrations, atmospheric deposition, and ecological effects due to changes in air pollutant emissions. The sites measure weekly ambient concentrations of pollutants that are used to monitor and assess emission reduction programs. Data from CASTNET also support assessment of the primary and secondary National Ambient Air Quality Standards for ozone, nitrogen dioxide, sulfur dioxide, and PM2.5. Raw data as well as prepackaged data sets are publicly available for download. Over the past 30 years, CASNET data have helped inform environmental policy decisions resulting in a substantive improvement in air quality.