CASTNET is a long-term environmental monitoring network with 96 sites located throughout the United States and Canada. CASTNET is managed and operated by the U.S. Environmental Protection Agency (EPA) in cooperation with the National Park Service (NPS); Bureau of Land Management, Wyoming State Office (BLM); and other federal, state, and local partners (www.epa.gov/castnet) including six Native American tribes that operate CASTNET sites on tribal lands. The network was established under the 1991 Clean Air Act Amendments to assess the trends in acidic deposition due to emission reduction programs. CASTNET measures ambient concentrations of sulfur and nitrogen species as well as rural ozone concentrations. Results are used to report on geographic patterns and temporal trends in acidic pollutants and deposition. CASTNET is the only network in the United States that provides a consistent, long-term data record of acidic dry deposition fluxes.

CASTNET complements the National Atmospheric Deposition Program’s (NADP’s) National Trends Network (NTN). The NTN is considered the nation’s primary source of wet deposition data. Nearly all CASTNET sites are co-located with or near an NTN site. Together, the CASTNET and NADP/NTN programs provide data necessary to estimate long-term temporal and spatial trends in total deposition (dry and wet) as well as ecosystem health. In recent years, the NADP Total Deposition Science Committee (TDEP) developed a hybrid method which combines the NADP and CASTNET measurements with model output from the Community Multi-Scale Air Quality (CMAQ) model to provide continuous spatial and temporal estimates of total deposition. CMAQ output includes estimates of non-measured nitrogen pollutants and improves understanding of the complicated processes that factor into dry deposition. The maps produced by this hybrid method are being used to identify areas where more monitoring data are needed to help evaluate and refine model outputs.

Many CASTNET sites are also co-located with the NADP’s AIRMoN, AMoN, MDN, and AMNet networks. CASTNET also collaborates with the EPA’s National Core monitoring network (Ncore) and the Interagency Monitoring of Protected Visual Environments (IMPROVE). EPA, NPS, and BLM have deployed more than 65 NADP Ammonia Monitoring Network (AMoN) sites at CASTNET sites. AMoN sites measure biweekly concentrations of ambient ammonia (NH₃). Results from CMAQ estimated that NH₃ accounts for an additional 10 to 40% of the total nitrogen deposition budget.

For increased flexibility in siting and lower operating costs, CASTNET developed a small-footprint monitoring station that does not require a temperature-controlled shelter. These sites can be operated using alternative power sources (wind/solar). During 2018, 12 CASTNET small-footprint sites, including 3 off-grid, alternate power sites, were operated. Three of the CASTNET sites operated on tribal lands utilize the small-footprint station. The availability of the small footprint design makes it more affordable for monitoring agencies, universities, and others to join the CASTNET network in areas of the country where CMAQ has identified the need for additional monitoring.

With more than 90 monitoring locations available, CASTNET also offers the infrastructure and platform for further collaboration with other networks, organizations, and research partners. CASTNET sites and existing data products present the opportunity for ground truthing of satellite observations, co-location with short-term high resolution monitoring technologies, and additional long-term flux measurements.