quality:correction
Air quality

As of 1999 the Grand Canyon area had some of the cleanest air in the United States. However, air quality in the area can be affected by air pollution from coal-fired power plants, mining, oil and gas, vehicles, and urban and industrial pollution from nearby states, California and Mexico. Events such as forest fires and dust storms in the Southwest can have a considerable impact.

Differences in visibility tend to be seasonal: best during the winter and poorest during the summer. Winter cold fronts tend to carry clean, crisp air from the northwest, a less populated area. However they can also carry pollution from nearby mines and power plants. The average visibility during the winter is 160 miles, with the potential to reach 210 miles under ideal conditions. In the summer, prevailing winds from the southwest carry pollution from major urban and industrial centers, and visibility on average is only 100 miles. Within the canyon itself, smog can be trapped.

Air quality and visibility in the canyon are affected mainly by sulfates, soils, and organics. The sulfates largely result from urban emissions in southern California, borne on the prevailing westerly winds throughout much of the year, and emissions from Arizona's copper smelter region, borne on southerly or southeasterly winds during the monsoon. Airborne soils originate with windy conditions and road dust. Organic particles result from vehicle emissions, long-range transport from urban areas, and forest fires, as well as from VOCs emitted by vegetation in the surrounding forests. Nitrates, carried in from urban areas, stationary sources, and vehicle emissions; as well as black carbon from forest fires and vehicle emissions, also contribute to a lesser extent.

A number of actions have been taken to preserve and further improve air quality and visibility at the canyon.In 1990, amendments to the Clean Air Act established the Grand Canyon Visibility Transport Commission (GCVTC) to advise the US EPA on strategies for protecting visual air quality on the Colorado Plateau. The GCVTC released its final report in 1996 and initiated the Western Regional Air Partnership (WRAP), a partnership of state, tribal and federal agencies to help coordinate the implementation of the commission's recommendations.

In 1999, the Regional Haze Rule established a goal of restoring visibility in national parks and wilderness areas (Class 1 areas), such as the Grand Canyon, to natural background levels by 2064. Subsequent revisions to the rule provide specific requirements for making reasonable progress toward that goal.

In the early 1990s, studies indicated that emissions of SO, a sulfate precursor, from the Navajo Generating Station affected visibility in the canyon, mainly in the winter. As a result, scrubbers were added to the plant's three units in 1997 through 1999 to reduce SO2 emissions by 90% on an annual average. Before and after observations showed a wintertime decrease of 33% for particulate sulfur (Sp), improving visibility. The plant also installed low-NO SOFA burners in 2009–2011, reducing emissions of NO, a nitrate precursor, by 40% The plant shut down completely in 2019. Emissions from the Mohave Generating Station to the west were similarly found to affect visibility in the canyon. The plant was required to have installed SO scrubbers, but was instead shut down in 2005, completely eliminating its emissions.

Prescribed fires are typically conducted in the spring and fall in the forests adjacent to the canyon to reduce the potential for severe forest fires and resulting smoke conditions. Although prescribed fires also affect air quality, the controlled conditions allow the use of management techniques to minimize their impact.