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 SO2, 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-NOx SOFA burners in 2009–2011, reducing emissions of NOx, a nitrate precursor, by 40% The plant shut down completely in 2019. Emissions from the