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 United States may hamper the ability of western states to meet federal air quality rules, the study observes.
Asian Emissions Rising
A team of 20 scientists analyzed 100,000 air quality measurements covering a 25-year span ending in 2006, focusing on springtime measurements when trans-Pacific winds from Asia to the Western U.S. are the strongest.
The study reported South and East Asian emissions of nitrogen oxides, a key ozone precursor, increased 44 percent between 2001 and 2006, with China’s emissions increasing 55 percent. During this same period, U.S. ozone precursor emissions decreased by more than one third.
“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.
West Faces Uphill Battle
“We show a strong increase in springtime ozone mixing ratios during 1995-2008 [in the western United States]…. 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 added.
“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.
The study examined ozone levels roughly two to five miles above the earth’s surface, and did not calculate how much of this ozone actually makes it to the surface, where the ozone air quality standard applies.
“We know from previous studies that air in the 2 to 5 mile layer can reach the surface, but more work is required to quantify the extent to which the increase in upper level ozone impacts the surface,” said study co-author Owen Cooper, a research scientist at the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colorado.
“Because of this need, NOAA is making ozone measurements from the surface to the stratosphere this spring/summer along the coast of California to get a better idea of how much ozone is flowing into the western U.S. close to the surface,” Cooper noted.
“There has been a strong and significant increase in ozone in the mid-troposphere in the West, and it doesn’t seem the United States is contributing to the increase,” Cooper explained.
Alyssa Carducci ([email protected]) writes from Tampa, Florida.