Concentrations of polluting ozone in the low troposphere have increased in China between 2005 and 2010, in response to both rising emissions and increased downward transport from higher atmospheric layers, concludes a study published online in Nature Geoscience. The rise in ozone and emissions of its precursor substances from China has offset 43% of the improvements in air quality expected in the western United States in response to tighter regulations.

Ozone acts as a greenhouse gas in the lowest layer of the atmosphere, the troposphere, and high ozone concentrations near the surface have adverse effects on human and environmental health. Ozone levels are largely determined by local emissions of precursor substances such as nitrogen oxides and hydrocarbons, as well as long-range transport by the winds.

Willem Verstraeten and colleagues assess changes in tropospheric ozone concentrations over China and the western United States between 2005 and 2010, using satellite observations along with a chemical transport computer model. They find a significant increase in ozone concentrations over China, and no significant change over the western US - despite tightening air pollution regulations. Comparing simulations with and without the rise in emissions over China, they quantify the offset from emissions over China at 43%.