› Larger image A new study finds that the western United States reduced its production of ozone-forming pollutants by a whopping 21 percent between 2005 and 2010, but ozone in the atmosphere above the region did not drop as expected in response. The reason: a combination of naturally occurring atmospheric processes and pollutants crossing the Pacific Ocean from China.
Scientists from the Netherlands and from NASA's Jet Propulsion Laboratory, Pasadena, California, looked at ozone in the mid-troposphere, about 10,000 to 30,000 feet (3 to 9 kilometers) above ground level. Ozone is formed throughout the atmosphere by chemical reactions, and it travels through the atmosphere upward, downward and sideways -- from ground level to many miles up into the stratosphere. In the mid-troposphere, ozone has a measurable greenhouse effect.
The researchers focused on ozone above eastern China and the western United States, using measurements of ozone and key ozone-forming pollutants from instruments on NASA's Aura satellite, and a computer model of global atmospheric chemistry and weather. Their study covered 2005 through 2010.
Over China, ozone increased about seven percent in the mid-troposphere. The researchers found two causes. First, Chinese emissions of ozone-forming pollutants increased 21 percent during these years. Second, an unusually large amount of ozone drifted down from the stratosphere as the result of several periodic, natural cycles, including an El Niño event in 2009-10.