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Utah's Air Quality Board approved a revised plan on Wednesday for clearing "regional haze" that obscures vistas at the state's five national parks, despite objections from National Park Service officials who insist only steep emission cuts will clear the air.

To the dismay of environmentalists, the plan imposes no new pollution controls on coal-fired power plants run by Utah's largest electrical utility.

"It appears that the state has spent the past seven years honing its justification for why the plan does not require the installation of selective catalytic reduction [SCR] — industry-standard pollution control in place at over 200 similar plants across the country," Cory MacNulty of the National Parks Conservation Association told the board.

"We see the vote today as a missed opportunity to cut an additional 14,000 tons per year of nitrogen oxide emissions — a 75-percent reduction in pollution that obscures up to 30 miles of the landscape."

NPCA is leading green groups urging the state Division of Air Quality (DAQ) to get tougher on Rocky Mountain Power.

The utility says it would cost about $170 million per unit to retrofit with SCR, which it has been required to do at power stations in neighboring states.

State air-quality regulators claim the benefits of the best pollution controls are too tenuous to justify such a huge cost.

Federal officials who oversee Bryce, Zion, Arches, Canyonlands and Capitol Reef national parks say Utah should do more to protect the air over treasured landscapes that draw millions to Utah from around the world.

But DAQ environmental scientist Colleen Delaney argued Wednesday that strides have already been made in cutting sulfur dioxide (SO2) emissions at the company's Hunter and Huntington coal-fired power plants, in addition to the recent closure of the nearby Carbon Power Plant, which was retired early in April because of new mercury rules.

"The Regional Haze Rule has two pathways you can take. Each one is equally valid," Delaney said. "We are focused on the results, which is substantial emissions reductions. We see a big success."

She said cuts in SO2 emissions have "the biggest impact on visibility. Its impact is year-round."

The state is revisiting the 2008 plan because the U.S. Environmental Protection Agency rejected the portions pertaining to nitrogen oxide (NOx), asking Utah to re-evaluate "best available retrofit technology," or BART, for reducing the pollutant.

Developed as a component of the Clean Air Act, the Regional Haze Rule is meant to protect 156 national parks and wilderness areas from visibility-impairing pollution by mandating upgrades at the nation's oldest and dirtiest power plants.

Utah is the last state to comply with the rule and is now at risk of having an emissions-reduction plan imposed by the EPA, critics say.

The National Park Service noted much is at stake given the importance of the parks to Utah's economy and quality of life.

The state's parks logged 10 million visits last year, adding $730 million to Utah's economy and supporting 12,000 jobs.

Parks managers argue that DAQ overestimated the costs of SCR and underestimated its benefits, and provided the technical data to back that up that criticism. In its revised plan, DAQ proposed an alternative to BART that gives credit for emission cuts at plants not subject to the Regional Haze Rule, as well as subsequent investments at Hunter and Huntington.

"It's taking advantage of things going on for other reasons," Delaney said. "You get co-benefits from meeting these other standards."

Rocky Mountain Power has reduced its NOx emissions, which form view-obscuring ammonium nitrate in the atmosphere. However, "the wintertime levels of ammonium nitrate are not improving," she said. "We think there is more going on there."

While the poorest visibility days remain bad, average visibility is improving and so are the cleanest days. A key culprit could be smoke from fires, dust storms and other sources DAQ has no control over, she said.