FLAGSTAFF - The U.S. Environmental Protection Agency has prescribed new pollution limits on three coal-fired Arizona power plants, aiming to protect the environment and air quality for wilderness areas and landmarks such as the Grand Canyon.

The EPA set limits for the Apache, Cholla and Coronado generating stations that will require technology upgrades to keep 22,700 tons of nitrogen oxide out of the air each year.

The Arizona Electric Power Cooperative (AEPCO) owns and operates the 605-megawatt Apache plant east of Benson for a consortium of electric cooperatives, including Trico Electric Cooperative in Marana.

Coronado is owned and operated by the Salt River Project, and Cholla is operated and mainly owned by Arizona Public Service Co.

State officials quickly denounced the decision Friday, saying it needlessly was harsh, would require a $500 million investment by utilities and will make electricity more expensive for consumers.

"We are disappointed that EPA would choose to unilaterally decide what's best for Arizona rather than" partner with the state Department of Environmental Quality on a proposal, said department director Henry Darwin.

He added that any air quality improvements, specifically a reduction in haze that obscures mountains and sunsets and can make breathing difficult for people with respiratory problems, would be imperceptible.

States have the authority to come up their own plans to reduce haze, but they are subject to EPA review. The agency approved Arizona's plan for sulfur dioxide and particulate matter, but said the state didn't do enough to limit nitrogen oxide emissions.

Colleen McKaughan, associate director in the EPA's air division, agreed with the state's estimate on the cost of upgrades, but said the power plants should be able to afford the improvements.

The limits set by the EPA under the new rule finalized late Thursday are less strict than a July proposal and give the power plants more flexibility, McKaughan said.

"We're not picking on power plants, we're not picking on the state of Arizona," she said. "This is a nationwide effort looking at all sources of visibility impairing pollutants."

Environmentalists touted the decision as a victory for public health. The regional haze program under the federal Clean Air Act is meant to address visibility in Class I areas like the Grand Canyon, Mesa Verde and the Petrified Forest, but environmentalists say the same pollutants are linked to respiratory illnesses and death.

The plants' operators now have five years to comply with new limits for nitrogen oxide at most of the units. The rule is subject to challenge through a petition for reconsideration or a lawsuit.

The operators haven't decided what route they'll take but said Friday they were disappointed with the EPA.

AEPCO said in a news release the ruling requires further analysis, but the necessary upgrades would cost $171 million to install at Apache, compared with an estimated $21 million for upgrades the company has proposed.

Any technology the company uses to meet the EPA standard will have a "hugely negative financial impact" on plant operations and customers, with negligible environmental benefits, AEPCO spokesman Geoff Oldfather said.

Kelly Bar, an official with the Salt River Project, which owns and operates the Cholla Power Plant near Holbrook, said she's unsure if the company can comply with the new limits.

"We don't believe there's going to be any perceptible improvement in visibility," she said. "The rule puts us in a difficult position."

The EPA will address the remaining portions of Arizona's air quality plan by Dec. 8.

Arizona Daily Star Assistant Business Editor David Wichner contributed to this report.