(Excerpted from EPA's visibility web site at http://www.epa.gov/oar/visibility/regional.html)

Because the pollutants that lead to regional haze can originate from sources located across broad geographic areas, EPA has encouraged the States and Tribes across the U.S. to address visibility impairment from a regional perspective. EPA currently provides funding to five regional planning organizations to address regional haze and related issues. These organizations will first evaluate technical information to better understand how their States and Tribes impact national park and wilderness areas (class I areas) across the country, and they will then pursue the development of regional strategies to reduce emissions of particulate matter and other pollutants contributing to regional haze.