Millions of families this summer will flee smog-bound cities for the majestic surroundings of America's national parks, only to find that air pollution has beat them there and clouded the post card vistas.

Unfortunately, air pollution in the parks is neither isolated nor new. According to National Park Service statistics, man-made pollution affects the scenic park views more than 90% of the time.

"You don't know how important a clear view is until you travel a long way to see the parks and then find bad air," said Mark Scruggs, chief of research for the National Park Service Air Quality Division in Denver.

According to Scruggs, pollution problems in certain parts of the country have improved in recent years, but the atmosphere at other parks has become much worse.

Because of the extreme variability in weather conditions, the Park Service has not been able to identify a national trend concerning the air quality in parks. But there are notable changes in specific regions. The air quality at western parks (based on selected sites) has improved in recent years, according to data from the Park Service and other sources, while it has continued to deteriorate in eastern parks.

In general, the parks with the best visibility are in eastern Nevada, western Utah and southern Idaho. The next-best air is in the Colorado plateau region, which includes the Grand Canyon, Bryce Canyon and Zion National parks, followed by northern Arizona, most of New Mexico and the Colorado Rockies.

The worst visibility is found in the Shenandoah (Virginia) and Great Smoky Mountains (Tennessee/North Carolina) parks.

In the West, the poorest visibility is along the coasts of California and Washington.

Although a portion of the visibility problem--which is seen in various forms of haze conditions--can be attributed to causes such as traffic dust, it is primarily caused by sulfate particles produced by utilities and heavy industry.

These particles, emitted by smelters, power plants, refineries and oil and gas fields, can travel long distances in the atmosphere. In the Colorado Plateau region, sulfate particles cause 40% to 60% of the pollution, and in Eastern parks these particles are an even greater contributor to haze and smog.

But pollutants from faraway urban areas can also be significant causes of park air pollution. For example, Southern California industry makes a substantial contribution to the pollution of the Rocky Mountain and Grand Canyon national parks, while much of the Shenandoah park pollution can be traced to the industrial centers of Detroit and Gary, Ind.