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.
Kelly Barr, an official with the Salt River Project, which owns and operates the Coronado Generating Station near St. Johns, 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.