North America’s attempts to control ozone levels are being hampered by pollution from Asia, according to a new study in Nature.

Owen Cooper, of the University of Colorado at Boulder, and his colleagues looked at nearly 100,000 ozone measurements collected by commercial and government aircraft in the troposphere – the lowest level of the atmosphere. They found that ozone concentrations over the Western part of the continent increased significantly between 1995 and 2008. In addition, increases were highest when air was brought over from south and east Asia, although they did not quantify exactly how much of the increase in ozone was down to Asian emissions.

“In springtime, pollution from across the hemisphere, not nearby sources, contributes to the ozone increases above western North America,” says Cooper (press release).

It is not ozone itself which is being emitted in Asia and carried over the Pacific, but pollutants such as nitrogen oxides and carbon monoxide. Chemical reactions then turn these into ozone in the troposphere.

In the troposphere ozone is a potent greenhouse gas. At ground level it can harm human health.

“We still don’t know how much [ozone] is coming down to the surface,” says Cooper (Washington Post). “If the surface ozone is increasing along with the free tropospheric ozone, that could make it more difficult for the U.S. to meet its ozone air quality standard.”

David Tarasick, paper co-author and atmospheric scientist with Environment Canada, says similar things about Canada meeting its own pollution standards (Canwest News).

In a News and Views article published alongside the paper in nature, Kathy Law of UPMC Universite de Paris 06 notes that the new work provides “the most conclusive evidence so far of increasing ozone levels in the free troposphere over western North America”. Previous studies have suggested either small or no increases.

Law argues that the paper is more evidence of the need for strong international agreements on air-pollution.

This story is also discussed on this week’s Nature Podcast.

Image: aircraft such as these were involved in collecting data for this study / NOAA