AMERICA'S national parks, symbols of unspoiled natural splendor, are being threatened by a man-made problem: air pollution. From coast to coast, some of the nation's most scenic natural wonders are being sullied by emissions from cars, factories, and power plants often hundreds of miles away.
The result is a silk-stocking haze that periodically pervades the parks, obscuring views, threatening vegetation, and even prompting health concerns.
This summer parks may begin posting air-quality advisories on especially dingy days. A few have already put up signs explaining reasons for poor visibility. Here at Sequoia National Park, discussions of ozone and sulfur dioxide are included in nature talks about mule deer and the derivation of the park's giant trees.
On a recent sunny day, Tom Pittenger stood on the 6,700-foot crown of Moro Rock, a granite dome that provides one of the park's most scenic views. Years ago a visitor could stand here and look west over the rumpled foothills of the Sierra Nevadas, down to the verdant farmlands of the San Joaquin Valley, and on to the coastal mountains, 70 miles away.
But today, as on most days now, the coastal range can't be seen, nor can the orange and almond groves of the Central Valley. Instead, the view extends out only about 12 miles over the park's ancient sequoia forests and mountains - and even these are diffused in a haze that gives the landscape the appearance of a silkscreen rather than a snapshot.
``It is like looking down through frosted glass,'' says Mr. Pittenger, an air-quality specialist with the park. ``You don't see anything clearly.''
On the eve of the summer travel season, the concern about the ``graying of the national parks'' extends well beyond here:
At Shenandoah National Park in Virginia's Blue Ridge Mountains, it used to be possible to see the Washington Monument and the Capitol, 70 miles away. Typical visibility now is 10 miles.
At Great Smoky Mountains National Park in Tennessee, the natural blue haze for which the area is known turns a smoggy gray-white in the summer.
The haze is so thick over Arizona's Grand Canyon on some days that visitors can't see from one rim to another.
``Air pollution is one of the major threats facing the park system,'' says Molly Ross, assistant chief of the National Park Service's air-quality division. ``Our parks aren't islands anymore.''
A 1988 Park Service study found that man-made pollutants affect vistas in all of the parks more than 90 percent of the time. Studies show the heaviest concentrations in the East, where visibility has decreased by more than 50 percent since 1948.
``A lot of our Eastern parks are as polluted as some of our cities,'' says Christine Shaver of the Park Service's air-quality division in Denver.
Though the skies are less dirty in the West, the impact of haze can be more dramatic. Because the air is so dry and clean, even small amounts of particulates dramatically despoil views - this in a region where scenic vistas are the essence of the parks.
The grime arrives from both near and far, as the problem at Sequoia illustrates. Situated on the edge of the fast-growing Central Valley, the park is affected by all the local activities: farm dust, oil industry emissions, car exhaust, truck plumes.
But it also draws villainous elements from farther away. In the summer, offshore winds push through the coastal mountains at San Francisco and then head south into the Central Valley, whisk-brooming pollutants along with it. Once in the valley, the air is trapped on three sides by mountains, producing a noxious eddy. Some of the air finds its way up to Sequoia.
Besides impairing views, the pollution raises concern about park biology. Researchers have documented ozone and acid rain damage to white pine and milkweed in Eastern parks. In the West, yellow pine and black oak have been injured.
``People come up here from Los Angeles and say, `The air is great,' '' says Diane Ewell, an air-quality specialist at Sequoia. ``But we're seeing 40 percent of our yellow pine with visible ozone injury.''
The impact on the cinnamon-barked sequoias is uncertain. Researchers have fumigated sequoia seedlings with ozone-laden air and found damage. Tests on larger trees will be done this summer.
As concern over the pollution intensifies, so does the politics of what to do about it. In 1977, Congress instructed the Environmental Protection Agency (EPA) to clear up visibility at the parks. Since then, however, little enforcement has been carried out, in part because of the difficulty of pinning the problem on just a few sources of pollution. Environmentalists say industry resistance and federal indifference have also played a part.
Recently, though, after years of prodding by environmentalists, the EPA issued a draft order that would require owners of a coal-fired plant near the Grand Canyon to reduce emissions by 90 percent - the first such action taken by the agency. The EPA acted after a study showed that as much as 70 percent of the haze at the park on certain winter days is caused by the plant, 80 miles to the northeast.
The utility's owners, including the US Bureau of Reclamation, dispute the findings. The order is being reviewed by the White House Office of Management and Budget.
In the US House of Representatives, additions were expected to be made to the Clean Air Act this week that would strengthen federal regulations dealing with regional haze in the West.
Such a move would certainly please Jerry Acosta, who came up to Sequoia from Orange County for a day of hiking. After a trek up Moro Rock, he paused at an exhibit explaining the causes of pollution in the park.
``This is crazy,'' he says, binoculars slung around his neck. ``I knew it was going to be bad, but I didn't think it would be this bad.''