Smog in the Western U.S.: Blame China?
The study, published in the journal Nature, probes a phenomenon that has puzzled scientists in the last decade: Ground-level ozone has dropped in cities thanks to tighter pollution controls, but it has risen in rural areas in the Western U.S., where there is little industry or automobile traffic.The study, led by Owen R. Cooper, an atmospheric scientist at the University of Colorado, examined nearly 100,000 observations in the free troposphere — the region two to five miles above ground — gathered from aircraft, balloons and ground-based lasers.
It found that baseline ozone — the amount of gas not produced by local vehicles and industries — has increased in springtime months by 29% since 1984. The study has important implications both for the curbing of conventional pollution that damages human health and for controls on greenhouse gases that are changing the planet’s climate, experts said.
It shows the need, said Mary Nichols, chairwoman of the California Air Resources Board, for a transformation of global energy and transportation systems. “Atmospheric scientists keep finding more evidence that pollutants travel around the globe and move up and down as they travel,” she said. “There is not a bright line separating greenhouse gases from regular air pollution.”The study, co-authored by researchers from nine institutes in the U.S. and abroad, is only a first step in understanding the complexities of cross-border pollution, Cooper said. More research will be needed to probe the sources of ozone at ground level and at other times of the year. The researchers began with the free troposphere because it is easier to eliminate local sources from baseline ozone calculations. They chose the months of April and May because that is when winds from Asia are strongest.
“Ozone is a difficult gas to pin down,” said Cooper, who works at the Earth System Research Laboratory of the National Oceanic and Atmospheric Administration in Boulder, Colo. “The study of intercontinental air pollution has been going on for a decade, but whether it was increasing overall was uncertain. And in places where it had spiked, along coasts and in national parks, we didn’t know how much was from local sources and how much was from Asia.”
Cooper said they have not yet determined exactly how much of the ozone increase comes from Asia, but they found that the increase was about twice as much when prevailing winds came from South and East Asia.
Still, the new study offers “the most conclusive evidence so far of increasing ozone levels in the free troposphere over North America,” wrote atmospheric chemist Kathy Law, an expert in long-range pollution transport, in a Nature commentary on the paper.Moreover, she added, the increases “certainly have implications for climate change, causing warming
either at the mid-latitudes where ozone forms, or in sensitive regions such as the Arctic to which ozone
might be transported.”
According to the U.S. Environmental Protection Agency, ground-level ozone is linked to serious health problems, ranging from aggravation of asthma to increased risk of premature death in people with heart or lung disease. In Southern California, which has some of the highest pollution levels in the U.S, smog levels exceed health standards more than 80 days a year. Based on new studies, the EPA announced this month that it might tighten federal ozone rules.
Ozone is not the only substance that crosses borders. Last year, a study by the National Research Council found that “significant concentrations” of three other types of air pollutants are also transported across the Northern Hemisphere: particulate matter such as dust, sulfates or soot; mercury; and persistent organic pollutants such as DDT. Emissions of nitrogen oxides, a key ingredient for ozone formation, have increased by more than 50% in China over the last decade while decreasing in the U.S. and Europe.The issue is also of concern to U.S. industry, which is chafing under domestic controls. A 2006 petition
from the U.S. Chamber of Commerce on fine particulate pollution asked the EPA “to develop national
regulations to clarify how states and localities should address the impact of international transport in air quality planning.”
-- Margot Roosevelt
Photo: Beijing traffic. Credit: AFP/Getty Images