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When aerial photographer Rita Donham flew around Northwest Colorado in the 1990s, she often saw a low, yellow-orange cloud. The cloud usually appeared in the springtime.

"You could just see the plume coming from the power plant, heading eastbound toward the Steamboat Springs area and the Mount Zirkel Wilderness," Donham said.

Now, that cloud doesn't seem to appear.

Donham no longer lives in the area, but she still has property and business clients in Steamboat Springs.

"It's just not there as much," she said about the cloud. Some--times she sees steam clouds, but "not that big, yellow-orange-brown thing."

Donham and others have seen benefits first-hand of recent retrofits of the coal-fired power plants at Hayden and Craig. The installations of pollution-control devices at the stations were direct results of lawsuits filed in federal court by the Sierra Club against the owners of the two power plants during the 1990s.

Now, scientists have numbers that substantiate some of the improvements people are seeing.

A recent analysis conducted by state and federal agencies indicates those pollution-control devices seem to be reducing haze and acidic snow and rain caused by power-plant emissions.

The analysis looks at air and water quality from 1995 to 2003. The Hayden Station retrofits were completed in 1999, so the study tracks air and water quality before and after the retrofits.

The study shows that there have been some notable improvements in visibility, and rain and snow are less acidic.

That doesn't mean environmental conditions now are perfect. For instance, although the Mount Zirkel Wilderness Area no longer leads the West for having the most acidic snow, it still ranks among the top five monitored sites.

But for the most part, "it was what we hoped to see," said Dan Ely, an air pollution researcher with the Colorado Department of Public Health and Environment.

Scientists involved agree that more years of data collection will be helpful.

Chemistry of the air

For the past decade, scientists have been digging up snow samples, looking at chemicals in rain, and tracking what's in the air around Steamboat and beyond.

The state health department and the U.S. Geological Survey, with the help of the Environmental Protection Ag----ency and U.S. Forest Service, have measured air quality, rain and snow chemistry and visibility before and after the Hayden Station upgrades.

The Craig Station emission control upgrades are not reflected in the measurements because those upgrades were not complete until 2004, Ely said.

Two chemicals emitted from the power stations are of concern: sulfur dioxide and nitrogen oxides. Both chemicals can create acid rain and snow, and can turn into particles that impair visibility.

Emissions of both were dec--reased by the Hayden upgrades.

Scientists focused their study on sulfur dioxide, however, as decreased emissions of nitrogen oxides were likely too small to be noticed.

Ely's focus was on air chemistry.

One air chemistry monitor at Buffalo Pass Road showed a 40 percent decrease in sulfur dioxide, a significant decrease that was expected. That decrease can be linked to pollution control at the Hayden Station, Ely said.

The biggest sign of an improvement in visibility came from a monitor that tracks the amount of sulfate particles. There was a 20 percent overall decrease in sulfate, but Ely said that trend also was seen regionally.

Unique to the area was a decrease in the number of days in which sulfate significantly impaired visibility. Such murky days were more frequent before the emissions controls were installed and would happen during the right weather conditions.

After the emissions controls, when weather conditions are just right to bring the sulfates toward Steamboat and Buffalo Pass and let them sit, there isn't as much sulfate from the Hayden Station to bring, Ely said. That means there aren't nearly as many days when visibility is significantly impaired.

Chemistry of the water

The USGS looked at water quality by measuring chemicals in snow, rain and lake water.

The strongest sign of improvement was seen at sites that collect snow and rain year-round, said Alisa Mast, a hydrologist with the USGS. In two sites downwind of the Hayden Station, Dry Lake and Buffalo Pass, a 20 percent decrease in sulfate was observed.

Such a decrease was not seen over the larger region, making officials confident the changes were a result of the Hayden Station pollution-control devices.

Across a larger region, there were some places where sulfate concentrations increased, making the local decrease even more noteworthy.

USGS workers also took samples of snowpack at four sites in the Routt National Forest. Those samples include snow that has fallen this winter.

Much smaller declines in sulfates were seen in snowpack samples, Mast said.

She said the Mount Zirkel Wilderness Area ranks among the West's top five locations having the most acidic snow. That is a reminder that there are contributors to pollution besides just the local power plants.

But the wilderness area no longer ranks as No. 1 or No. 2, as it has been in the past, Mast said.

Acidic snow is harmful because when the snow melts in the spring, nearby creeks and ponds are drenched in an acidic pulse. That can be dangerous to salamanders, fish and small aquatic species, which are breeding in the spring.

The lake chemistry did not show much change, which was expected. Because lakes store so much water, sulfates can build up and have to be flushed out before benefits are seen, Mast said.

In all the studies, there are some major complicating factors. For instance, the three years measured before the emission controls were wetter than average, and the three years measured after the emission controls were very dry.

During drought, there is more haze because of wildfires and dust, and water samples are concentrated.

Another complication is that several large regional power plants made big improvements to their emissions at the same time.

To some extent, scientists can account for those issues, but it's difficult, Mast said.

Still, noticeable decreases in sulfates are seen despite the complicating factors, which Mast said is a sign that the Hayden emission controls are having positive effects.

"I think we saw improvements where we really expected it," Mast said. The emission controls do "reduce the acidification potential in the wilderness because you're reducing the sulfate input to the system."

Continued monitoring

Scientists will keep tracking air and water chemistry in the area.

Now that the Craig Station improvements are complete, data collected during the next few years will reflect the results of emission controls at both stations.

Those changes likely will not be as dramatic, Ely said. The emission controls the Hayden Station installed took the plant from no control on sulfur dioxide emissions to stopping the release of 85 percent of those emissions. The Craig Station, however, only went from stopping 70 percent of sulfur dioxide emissions from two units to stopping more than 90 percent.

Representatives plan to come to Steamboat in the next few months to present the new research.

"This is a success story," Ely said. "There were people that were very worried about visibility impairment ... and acid deposition in the wilderness. The news is that the emissions reductions have achieved what we hoped."

Jeff Sorkin, assistant regional air program manager for the U.S. Forest Service, said he thinks it's too early to tell how much the emission controls have improved air quality in the Mount Zirkel Wilderness Area. He said that continued monitoring during the next decade is important.

Mark Stutz, a spokesman for Xcel Energy, which owns the Hayden Station, said he has no reaction to the study at this point, as he has not seen the full report.

Stutz said Xcel Energy is pleased with any reductions seen since their emission controls were installed, but continues to stress that regional air quality is a multilayered issue. Various sources, such as vehicles, contribute to air pollution.

Jim Van Someren is the spokesman for Tri-State Gen--eration and Transmission Assoc--iation, which is the operator and part owner of Craig Station.

"It was a big, big project, obviously very expensive," Someren said about the power plant's emissions controls up----grades. "But we think the investment in it will pay off, not only toward improving the environment," but also to help the facility remain a key resource for years to come, he said.

Reed Zars, the lead Sierra Club attorney on the lawsuits, said he thinks the emissions controls have been successful. The chemicals coming out of the power stations are reduced greatly, and the hazy cloud that used to stretch through the Yampa Valley is gone.

"The visible plume and the layered haze that you would regularly see has been eliminated in the Yampa valley," Zars said.

If what is released still negatively affects wilderness areas and the Routt National Forest, further mitigation might be necessary, he said.

But for the most part, the air in the Yampa Valley seems cleaner.

"Overall, I think it's been a fantastic result," Zars said. "We should all be proud of the thousands and thousands of tons of pollution that have been taken out of the air in the Yampa Valley."