In the past industry experts had developed a certain arrogance and sloppy thinking. Frequently they tended to assume that their values were representative of everybody else's. Visibility which was good enough for a polluted city or to land planes in was good enough for a National Park. One famous expert calculated the maximum 24 hour average concentration of sulfur-dioxide from a facility and the maximum 3 hour concentration. It also shows the pitfalls of listening only to like-minded peers.
Many comfortable assumptions are untenable when faced with a significant challenge.
When new restrictions were placed on sulfur emissions from coal-fired powerplants in the 1990 Clean Air Act Amendments, most of the Southwestern powerplants were already controlling their emissions at stricter levels than those specified in the law. The Navajo plant will become the first major facility to be retrofitted with sulfur controls to reduce visibility impairment. This result was obtained primarily because of the data and studies developed by the National Park Service, but we participated in the hearings that supported EPA's decision to require controls. The Navajo Generating Station is located near the northeastern end of the Grand Canyon. The Mohave plant, near the southwestern end of the Grand Canyon, is the subject of further studies.