The U.S. Environmental Protection Agency had outlined a final plan for upgrades at three Arizona coal-fired power plants.
The agency on Friday followed through with a proposal to partially reject Arizona's air quality plan. It came up with one of its own for nitrogen oxide emissions that impair visibility at 18 national parks and wilderness areas.
Instead of sticking with technology such as low nitrogen-oxide burners, the Cholla, Coronado and Apache generating stations likely will have to install selective catalytic reduction technology to further reduce the emissions by 22,700 tons per year.
The state Department of Environmental Quality was quick to denounce the EPA's decision, saying Arizona has a right to control pollutants within its borders. The department says visibility improvements from more than $500 million in upgrades will be imperceptible.