Jeremy Nichols, of the Colorado-based WildEarth Guardians, said the results illustrate how plants in the West lag on these emissions controls. He noted that the problem can be corrected with tried-and-true retrofits.
"We're running up against the reality here that our fleet and Utah is an example that we need to do a lot more to reduce NOX emissions to reduce haze," he said.
The EPA last month told Utah regulators that more needs to be done to reduce NOX in the state's regional haze plan.
And Rocky Mountain Power has spent $1.4 billion on pollution controls to comply with Clean Air Act limits, with about $1 billion more in upgrades planned in the coming decade.
By contrast, the Intermountain Power Plant near Delta is a big plant that has not been required so far to cut its NOX emissions under recent Clean Air Act changes, noted Bryce Bird, director of Utah's Division of Air Quality.
"Without a [regulatory] trigger," Bird said, "we don't have a mechanism to require a retrofit."
Meanwhile, Bird notes that Utah plants have made solid strides in some areas. He pointed to sulfur dioxide, another pollutant counted in the GAO report, that is low in Utah and the rest of the West thanks to the Regional Haze Rule.
Utah plants produce 1.10 pounds of sulfur dioxide per megawatt hour of electricity, according to GAO. That's less than one-eighth the sulfur pollution from plants in New Hampshire and Ohio at the top of the nationwide list.
The GAO did its inventory at the request of Sen. Sheldon Whitehouse, D-R.I., and chairman of a Senate Environment and Public Works subcommittee.
In a letter describing its findings, the agency drew the link between aging facilities and pollution.
"Older electricity-generating units those that began operating in or before 1978 provided 45 percent of electricity from fossil fuel units in 2010 but produced a disproportionate share of emissions, both in aggregate and per unit of electricity generated," the GAO reported, noting that 93 percent of the electricity produced by older fossil-fuel units in 2010 was generated by coal-fired plants.
"For each unit of electricity generated, older units collectively emitted about 3.6 times as much sulfur dioxide, 2.1 times as much nitrogen oxides, and 1.3 times as much carbon dioxide as newer units."
An EPA spokeman said the agency is still reviewing the GAO report.
"It is important to note that the New Source Review program [under the nation's clean-air law] was not designed to prompt retrofits of older plants," EPA commented in an email. "It is a pre-construction permitting program designed to ensure that newly built facilities do not worsen air quality or slow progress toward clean air."
Tim Wagner, of the Utah Sierra Club, had a different take.
"The fact that IPP can continue to emit these high levels of nitrogen oxides but no regulatory measure exists that allows us to do anything about it simply indicates that we not only have a pollution problem," said Wagner, "we have a political problem."
Power production and pollution
The Government Accountability Office looked at 3,443 U.S. electric plants that burned fossil fuels, such as coal and natural gas, in 2010. Combined, the plants produced 2.8 billion megawatts of electricity 39.2 million in Utah.