Crystal-clear air is essential to enjoy the stunning colors and forms of the Grand Canyon, making visibility a critical air resource.  On the best days, the park can still have some of the clearest air left in the continental U.S.  The clearest generally follow major winter storms.  Visibility is a fragile air resource.  Very low concentrations of fine particles (PM_2.5) produce a noticeable veil of haze.

Most of the fine particles affecting the park travel long distances from urban and industrial areas, mixing en route to form a uniform “regional haze.”  Regional haze is usually thickest in summer, when air from the south and west carries pollutants into the park.  At high concentrations, fine particles not only cause haze, they are also unhealthy.  Consequently, the EPA established a fine particle NAAQS.  Local sources make a very minor contribution to fine particle levels, with one exception – fire.

Forest fires, both beneficial and unwanted, create thick smoke that can obscure the canyon. Fortunately, episodes are generally brief. To address the urban and industrial sources of haze that contribute to regional haze, the National Park Service works with various western states and tribes. Some of this work addresses particular pollution sources. Through the Western Regional Air Partnership (http://www.wrapair.org) state, tribal and federal governments are working on region-wide solutions to haze.

Visibility in Grand Canyon varies from year to year, responding to both air pollution and weather.  Trends over the last 10 years show the best days at Grand Canyon getting clearer.  Unfortunately, the haziest days do not show a trend; they’re not getting worse, but they’re not getting better.  This photo reconstructs the best and worst 24-hour average visibility based on particle concentrations sampled at Indian Garden.