Asian Pollution Increasing U.S. Ozone Levels

Pollution from Asia is crossing the Pacific Ocean and causing increases in U.S. ozone levels even though U.S. ozone precursor emissions are declining, reports a new study in the journal Nature. The transport of Asian pollution to the U.S. may hamper the ability of western states to meet federal air quality rules, the study observes.

A team of 20 scientists analyzed 100,000 measurements over 25 years and found ozone levels in the air over the western U.S. have been increasing, despite the decline in U.S. emissions, in a manner that coincides with Asian emissions and trans-Pacific wind currents.

“At present, east Asia has the fastest-growing ozone precursor emissions. Much of the springtime east Asian pollution is exported eastwards towards western North America,” the study observed.

“The changes we have seen over the past 25 years coincide with when China was transforming itself into an economic powerhouse,” said study co-author Dan Jaffe, professor of environmental chemistry at the University of Washington-Bothell.

According to the study, south and east Asian emissions of nitrogen oxides, a key ozone precursor, increased 44 percent from 2001 to 2006, with an increase of 55 percent within China. This continues a longstanding pattern of rising Asian emissions, with Chinese emissions alone rising up to 29 percent annually since 1996. Meanwhile, U.S. ozone precursor emissions have declined by more than a third since 1985.

Despite the decline in U.S. ozone precursor emissions, “there has been a strong and significant increase in ozone in the mid-troposphere in the West and it doesn’t seem the U.S. is contributing to the increase,” said study co-author Owen Cooper, a research scientist at the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colorado.

“We show a strong increase in springtime ozone mixing ratios during 1995-2008 [in the western U.S.] …. We find that the rate of increase in ozone mixing ratio is greatest when measurements are more heavily influenced by direct transport from Asia,” the study reports.

“With western North America being particularly sensitive to rising Asian emissions … [w]e suggest that the observed increase in springtime background ozone mixing ratio may hinder the USA’s compliance with its ozone air quality standard,” concluded the study.