AirNow was established in 1998 in order to gather the data on air quality collected by U.S. local, state and federal agencies and to provide the public with easy access to this information. AirNow gathers data U.S.-wide, but by now it also covers parts of Canada. AirNow has a network of partner organizations from different levels, among others the U.S. Environmental Protection Agency (EPA), the National Oceanic and Atmospheric Administration (NOAA) and the National Park Service (NPS) as well as tribal, state and local agencies. AirNow provides an Air Quality Index (AQI) for more than 300 cities in the U.S. as well as national and regional air quality maps (including parts of Canada). Maps are updated every hour. The email alert service EnviroFlash warns users if air quality becomes a concern. Links to local air quality websites can also be found. Information provided by AirNow includes current AQI and ozone/PM2.5 concentration as well as forecasted AQI. The AQI takes into account ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide. The service targets the general public.

References:
AirNow Website (http://airnow.gov/, accessed on February 10, 2012)
T. Dye et al., EPA (2004): From Raw Air Quality Data to the Nightly News: An Overview of How EPA’s AirNow Program Operates. (http://www.epa.gov/international/air/pdf/AIRNow.pdf, accessed on February 10, 2012)
V. Grasso and A. Singh, UNEP (Dec 2011, forthcoming): Early Warning Systems: State-of-Art Analysis and Future Directions.