Paris: Progress slashing unhealthy ozone in the western United States has been largely undone by pollution wafting across the Pacific from China, according to a study published today.
Scientists have long suspected this might explain why ozone levels along the US west coast remained constant despite a significant local reduction in ozone-forming chemicals.
The study, published in Nature Geoscience, is the first to make the case using satellite observations coupled with computer models of how air-borne molecules travel in the lower atmosphere, the authors said.
"The dominant westerly winds blew this air pollution straight across to the United States," explained lead research Willem Verstraeten of the Royal Netherlands Meteorological Institute.
"In a manner of speaking, China is exporting its air pollution to the West Coast of America," he said in a statement.
Nitrous Oxide emissions from vehicle traffic and industry, mixed with sunlight, create dirty-yellow blankets of ozone smog that sting the eye and scatch the throat.
Close to the ground, this pollution causes respiratory problems, damages crops, and is an important source of greenhouse gases.
By imposing stringent standards for motor vehicles and industry, state and national government in the US succeeded in cutting ozone-producing nitrous oxide emissions by 20 percent from 2005 to 2010.
Those efforts, however, were undermined by China's galloping growth, which pushed its own ozone levels up over the same period by about seven percent.