Progress slashing unhealthy ozone in the western United States has largely been undone by pollution wafting across the Pacific from China, according to a study published Monday. Scientists have long suspected this might explain why ozone levels along the U.S. 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...