This means residents, visitors, and vegetation in the park itself are being exposed to elevated levels of the gas, especially during those warm summer days that favour its production.

The USGS Forest and Rangeland Ecosystem Science Center study shows that the Mt. Rainier ozone blows in from Seattle and other urban centres in western Washington and adds to the growing body of evidence that protected areas such as national parks are vulnerable to pollution from outside sources.

“It’s well documented that both periodic episodes of high ozone exposure and chronic moderate ozone exposure can be harmful to plants,” said the author of the study, Dr. David Peterson. “We know this from other regions of North America including national parks such as Sequoia and Great Smoky Mountains.”

Sources of ozone-producing compounds, including automobile exhaust and fuel-burning industries, tend to be concentrated in urban areas. But the common assumption that ozone pollution is strictly an urban problem is proving to be false, Peterson said.

Peterson and his students monitored ozone at Mount Rainier National Park from 1993 to 1997, and quantified the spatial distribution of ozone throughout western Washington in 1996. The park consistently had the highest average weekly levels of tropospheric ozone measured anywhere in the state. Ozone concentrations tend to increase at higher altitudes, partly because of passive dispersion from the stratosphere, but largely due to the transport of pollutants by prevailing winds inland from urban sources, such as the Seattle metropolitan area.

A common visitor destination in the park, known as Paradise, is on the southern slope of Mount Rainier at an elevation of 5,400 feet and a distance of about 60 miles from Seattle. Peterson said that Paradise had almost twice the monthly mean ozone concentration as Lake Sammamish, which is near sea level and less than ten miles east of Seattle.

A number of other North American wildlands have concentrations of tropospheric ozone sufficiently high to harm vegetation. They, too, are located downwind of metropolitan areas. These areas include El Desierto de Los Leones near Mexico City, San Bernardino National Forest to the east of Los Angeles, Sequoia National Park and Sequoia National Forest to the east of several metropolitan areas in California, and Great Smoky Mountains National Park in the eastern US.

Peterson noted that in the rapidly urbanizing Puget Sound region, with its higher population and additional vehicles will produce more local pollution sources in the future. “It is time to consider the potential for damage to park ecosystems and even potential health hazards to park visitors,” Peterson said. “We’ve long known that parks cannot be managed as islands, separate from their surroundings. These findings about tropospheric ozone concentrations lend additional support to that management position.”

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