Ozone-forming chemicals produced in eastern China have been drifting across the Pacific to the western United States, according to a new study. The pollution has combined with ozone released by natural processes to cancel out many of the gains from stricter pollution controls recently enacted in the United States, according to a new study.

Researchers from the NASA Jet Propulsion Laboratory in California and Wageningen University in the Netherlands used satellites to measure levels of ozone in a layer of atmosphere called the mid-tropospheric region, which is between 10,000 and 30,000 feet above the Earth's surface. They then used the measurements in computer models that traced the origin of ozone seen in the troposphere above the western United States.

The authors found that overall, ozone levels in the western United States not drop in some areas over that time period, contrary to expectations. Some of areas where nitrogen oxide levels dropped the most did see slight decreases in ozone, but the region as a whole saw no change.



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"We tend to think of pollution as a local issue that can be addressed at the local, state or national level," said Jessica Neu, a researcher at the jet propulsion laboratory and one of the paper's co-authors. "This paper is really showing that long-range transport of pollution is an international issue, in the same way that climate change has become an international issue."

Natural processes were responsible for a little more than half of the ozone increase. A particularly strong El Niño in 2009 and 2010 drew a large amount of ozone out of the stratosphere—the one of the outermost layers of the atmosphere. Ozone naturally occurs there—it's produced by solar energy reacting with oxygen molecules and forms the "ozone layer" which protects the Earth from solar radiation.