The Clean Air Status and Trends Network (CASTNET) was established in 1987 to assess trends in ambient air quality and deposition of acidic pollutants due to emission reduction programs. CASTNET currently operates more than 90 monitoring stations throughout the contiguous US, Alaska, and Canada. While CASTNET is managed by federal agencies (US EPA, National Park Service, and the Bureau of Land Management), there are more than 50 partners that provide invaluable support in the day-to-day operation of the network, including universities, Native American tribes, state agencies, local site operators, and private land owners that provide site operator support, and property.

CASTNET has maintained three long-term tribal partnerships. In eastern Oklahoma, the Cherokee Nation operates a CASTNET filter pack and ozone analyzer at their NCore station. This site has been operating as a CASTNET site since 2002. The Alabama-Coushatta tribe has operated a CASTNET site in eastern Texas since 2004, while the Santee Sioux in northern Nebraska have been operating a CASTNET site since 2006.

In 2012, CASTNET developed a small-footprint, low power monitoring site that does not require a temperature-controlled shelter. The small footprint site consists of a 10-meter tower, a typical CASTNET filter pack, and a 9-meter temperature sensor, with an enclosure located on the tower that includes a pump, mass flow controller, data logger, and a cellular modem. Since the development of the small-footprint site CASTNET has increased the number of monitoring sites by offering this low impact, low-cost setup for measuring weekly sulfur and nitrogen species. Two tribal partners, Kickapoo Nation located in Northeast Kansas and Red Lake Nation located in northern Minnesota joined CASTNET 2014 with the deployment of the small-footprint site. In 2015 the Nez Perce Tribe located in Idaho added an off-the-grid small-footprint CASTNET site. This site will be converted from a solar/wind powered site to a traditional site in 2016 when the tribe installs a CASTNET ozone monitor.

EPA will continue their outreach efforts to existing and new tribal partners to expand tribal monitoring capacity. CASTNET managers develop tools for viewing data, reports on air quality and deposition fluxes in tribal regions, training documents, and Frequently Asked Questions for tribal air monitoring groups. CASTNET data and monitoring information are publically available online at https://www.epa.gov/castnet.