SHENANDOAH NATIONAL PARK, Va. — On a clear, cool morning recently atop Hawksbill Summit, the highest peak in this 80-year-old national park, Interior Secretary Sally Jewell gazed at the scenery unfolding to the west.

White clouds covered the valley thousands of feet below. A mountain range rose in the distance. Trees covering every slope had begun to offer the first orange and yellow hints of autumn. The blue sky stretched endlessly.

Not that long ago, Jewell said, “the air quality here was not like this.”

Decades ago, in fact, the average visibility in the park was about 35 miles, and on many days only a fraction of that. These days, the average view in Shenandoah stretches 60 miles, and on the clearest days it can reach twice that far.

Part of that has to do with national efforts over the years to reduce air pollution from cars and trucks and to require power plants and other facilities to install technology to curb emissions of sulfur dioxide and nitrogen oxide, which contribute to the haze that has long clouded some of the nation's most spectacular vistas.

Despite that improvement, the federal government has never fully addressed the haze that obscures views at many of the country's national parks — a problem that was first singled out for attention under the 1977 Clean Air Act Amendments.

Some administrations have tried harder than others. But the Obama administration has been especially aggressive in trying to combat the contributors of haze in every state, and regulators are hoping before a new president takes office to finalize regulations they say will result in cleaner air and increased visibility at parks and wilderness areas over coming decades.

“We've made over the last eight years unbelievable progress in having a regional haze program. It's not an aspirational goal any longer; it actually exists,” said Environmental Protection Agency Administrator Gina McCarthy, who hiked to Hawksbill Summit on Thursday in a group that included Jewell and members of the National Park Service and U.S. Geological Survey.

She said the impetus for trying to tackle the problem extends beyond views alone.

“Haze is nothing but pollution that's in the air and meets sunshine,” McCarthy said, “which means we're trying to reduce overall pollution that really has significant health hazards.”

Under the EPA's “regional haze program,” states periodically submit plans to improve visibility in national parks and wilderness areas, largely by looking for ways to curb pollution from nearby industries. Those plans can vary widely and are subject to approval by the EPA. Some states have taken far longer than others in submitting plans.

In the eastern United States, coal-fired power plants that produce sulfur dioxide and nitrogen oxide traditionally have contributed the most to visual impairment. Out West, particulates from wildfires and carbon monoxide from sources such as cars and trucks also are key culprits.

The administration's efforts haven't always been met with open arms. Some power companies and utilities have complained that the new requirements will be costly and unfairly burdensome. Multiple states have sued the EPA after having parts of their plans for reducing haze rejected by the agency, and others have said they are considering legal action.

“The Obama Administration is misinterpreting and misusing federal agencies to force through a radical agenda based more on the beliefs of his environmentalist base than on common sense,” Texas Attorney General Ken Paxton, a Republican, said in a statement this year after he filed a lawsuit against the EPA. “The steps Washington is demanding we take are extraordinarily expensive, will result in a less-reliable electric grid and ultimately have no significant effect on visibility in Texas.”

On the flip side, environmental groups also sued the EPA in an effort to compel the agency to put new rules in place faster, and some of those same groups have urged it not to weaken the final regulations by delaying deadlines for states or lessening federal oversight.

“It's wonderful that this has begun to get implemented, but it's undergone decades of back-burnering,” said Stephanie Kodish, head of the Clean Air Program for the National Parks Conservation Association.

She said the group hasn't agreed with the EPA on each state plan and has challenged some as too lenient, but overall she praised the administration's work.

Back on Hawksbill Summit, Claire Comer, a National Park Service employee who has worked at Shenandoah since the 1980s, said she welcomes any effort to keep the park's stunning vistas intact and unimpeded by haze.

“People come here for the views, and they spend money in the surrounding counties,” Comer said. “If the visibility goes away, that affects our tourism, our economy.”