Environment & Science

Smog from Asia causing ozone emissions to rise in US national parks

The sun shines on Joshua Tree National Park.
The sun shines on Joshua Tree National Park.
Nillie De Grakovac

Smog blowing over from Asia is undermining U.S. efforts to reduce air pollution in some of the most pristine places: national parks in the American West.

A new study in the journal, Atmospheric Chemistry and Physics, found that smog — also called ozone — from China, India and other Asian countries has caused smog levels in parks like Yosemite, Joshua Tree and Yellowstone National Park to rise over the past 25 years, particularly during the spring. The increase has more than offset the gains made by U.S. pollution reduction programs.

“Air pollution is a global phenomenon,” said lead author Meiyun Lin, a researcher with Princeton University and the National Oceanic and Atmospheric Administration. “It’s important to know how pollution in Asia influences our efforts to control air quality in the U.S.”

Since 2000, smog-forming emissions have decreased by approximately 50 percent in the U.S. due to Clean Air Act enforcement, pollution controls on large emitters and regulations on cars and trucks.

Yet in the rural West, where most national parks are located, ozone levels haven’t dropped. That’s because these areas are usually at higher elevations and closer to the layers in the atmosphere where smog travels over from Asia.

Take Joshua Tree National Park, whose desert terrain ranges from 934 to over 5,800 feet. There, rising emissions from Asia have actually offset Southern California’s efforts to combat smog. Emissions of nitrogen oxide, a precursor to ozone, have fallen 30 percent in greater Los Angeles since 2000, yet according to Lin’s study, springtime ozone levels at Joshua Tree held steady from 1990 to 2010.

Some high-elevation western cities are also more susceptible. Take, for instance, Denver, where ozone levels have remained stubbornly high despite significant local reductions in smog-forming emissions.

Lin says by virtue of being at sea level, cities like Los Angeles are less impacted by Asian smog, although they have their own local smog problems.

Ian Faloona, an atmospheric scientist at UC Davis who studies the sources of ozone pollution in California, said Lin’s study stands out for its long-term timespan: 1980 to 2014.

Faloona's own work shows lower-elevation cities in California may be more impacted by Asian smog than Lin’s model, however. In work funded by the San Joaquin Valley Air Pollution Control District, Faloona determined an average of five percent of the ozone pollution in Bakersfield and Fresno has blown in from Asia.

While it doesn’t sound like much, Faloona said an additional five percent can be enough to push an already highly polluted region over the threshold for violating federal air standards.

“Regional air basins are penalized for not meeting the standards,” he said. “The important question is, how much is in our control?”