Imports from China are blocking US ozone improvements

For a simple molecule, ozone (O₃) wears many hats. Up in the stratosphere, the ozone layer provides planetary sunscreen, absorbing UV radiation before it can reach the Earth. Between the surface and the stratosphere, ozone’s significance comes from absorbing infrared radiation from the Earth—it’s a greenhouse gas. And down at ground level, ozone's reactivity makes it a harmful enough lung irritant that it’s a part of the daily weather report in some cities.

Ozone is produced in the atmosphere naturally, but it’s also created as a result of air pollution. In particular, nitrogen oxides react with sunlight to let rogue oxygen atoms loose, which can get together with friendly O₂ molecules in the lower atmosphere, forming O₃. By cutting the emissions of those “ozone precursors,” we can dial back the amount of ozone down here where we breathe. In the western US, emissions of nitrogen oxides were reduced by 21 percent between 2005 and 2010, yet ozone stayed about constant. Part of the answer to that conundrum lies across the Pacific, according to a new study led by Wageningen University’s Willem Verstraeten.

The suggestion that ozone from China is ending up in the US isn’t new, but the researchers turned to data from NASA’s Aura satellite (launched in 2004) to improve past estimates. The satellite data confirmed that nitrogen oxides had declined across the American West, while average ozone hadn’t changed much (some areas saw a slight decrease and some a slight increase). In China, on the other hand, emissions of nitrogen oxides rose about 21 percent between 2005 and 2010, and ozone increased by around seven percent.