CSN (Chemical Speciation Network) is a long-term national monitoring program for the EPA (Environmental Protection Agency). Created in 2000 as a response to support implementation of the 1997 PM2.5 National Ambient Air Quality Standards (NAAQS) as well as the 1999 Regional Haze Rule (regulations for states to establish goals in improving visibility), CSN is a PM2.5 sampling network with sites located principally in densely populated areas.

Visible air pollution contains particles that block visibility, cause health problems for humans and wildlife, and damage environments and infrastructures. In 1985, the EPA created the IMPROVE network along with UC Davis air quality researchers at Crocker Nuclear Lab.