Arizona Department of Environmental Quality director Henry Darwin told the Associated Press the limits for the Cholla, Coronado and Apache plants would require about $US500 million in investment to upgrade the technology that would remove 22,700 tons of nitrogen oxide from the air annually.

The decision would also increase customers’ electricity costs, he said.

“We are disappointed that EPA would choose to unilaterally decide what's best for Arizona,” Darwin said, adding the agency could also work with the DEQ on a proposal.

While US states can individually determine haze reduction plans, the EPA must review them.

Arizona’s plan was given approval for sulfur dioxide and particulate matter. However, the EPA decided the state did not go far enough to limit nitrogen oxide emissions.

EPA air division associate director Colleen McKaughan told the news service she concurred with the state’s position on upgrade costs but argued that the facilities should be able to afford it.

“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.”

She also noted an EPA rule finalized late last week was more lenient than its initial proposal set forth in July and would allow plants to have some flexibility.

The operators have five years to comply with the limits, though the rule is subject to challenge through petition for reconsideration or in the case of a lawsuit.

Salt River Project official Kelly Barr told the AP she was not sure if the company, the owner and operator of the Coronado generating station, would be able to comply.

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

The balance of Arizona’s air quality plans will be evaluated by December 8.