For a simple molecule, ozone (O 3 ) 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 O 2 molecules in the lower atmosphere, forming O 3 . 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.

To explain these trends in the measurements, you have to account for ozone moving into and out of those areas. Convection in storms can lift ozone high above the surface to travel on the winds before other storm systems pull it back down. Those same storm systems can pull down some stratospheric ozone as well, complicating the local equation.

To work out where the ozone came from, the researchers used a model that simulates atmospheric chemistry and transport based on the available observations. The model showed that both China and the western US were gifted some extra stratospheric ozone—enough to account for half of the increased ozone over China, in fact.

But in a separate simulation where Chinese emissions of ozone precursors were held constant, decreased emissions in the US would have been enough to bring ozone concentrations down a bit despite the addition from the stratosphere. Some of the extra ozone produced by pollution in China was traveling across the Pacific to prop up ozone concentrations in the western US.

The researchers calculated that roughly 40 percent of the ozone reduction that the western US should have gotten from cutting their own pollution was offset by this ozone transported from China. However, this had a greater impact on high-altitude ozone than on the ground-level ozone people breathe.

In an article accompanying the paper in Nature Geoscience, University of Edinburgh researcher Ruth Doherty points out that this is a short time period that may not necessarily represent a longer-term trend. Ozone transport depends on weather that is influenced by things like El Niño/La Niña patterns, which vary from year to year. She also notes that “China itself lies downwind from India and other parts of Asia, and it remains to be established how the free tropospheric ozone trend over China is in turn influenced by emissions upwind.”

The atmosphere, it turns out, encircles the entire planet, and gases don’t recognize political boundaries. And as Doherty writes, “Currently, there are no international agreements that address the intercontinental transport of tropospheric ozone and its precursors.”

Nature Geoscience, 2015. DOI: 10.1038/NGEO2493 (About DOIs).