First author

Humans, plants and animals are vulnerable to the harmful effects of ozone when exposed to high levels of the air pollutant. Since the 1970s, scientists have been monitoring ozone levels in the atmospheric boundary and free troposphere over western North America. Previous studies had shown that surface ozone is on the rise, but could not identify the sources responsible for this, whereas studies in the free troposphere, 2–12 kilometres above Earth's surface, showed no increase. Owen Cooper at the University of Colorado in Boulder and his colleagues gathered more data and revisited the problem (see page 344). Cooper tells Nature more.

Why did you conduct this study?

There were conflicting results on whether trophospheric ozone was increasing, and whether pollution from east Asia, the fastest-growing producer of ozone-causing emissions, was playing a part. Ozone levels in the atmosphere vary a great deal — in part because ozone is a highly reactive molecule that doesn't last long before breaking down — and I suspected that the free tropospheric studies didn't have enough data. So we decided to conduct an expanded study to determine whether ozone levels in the troposphere had been rising.

What did you find?

We compiled springtime ozone data from various platforms, including weather balloons [ozonesondes] and research and commercial aircraft. We found that from 1995 to 2008 there was a 14% increase in ozone concentrations above western North America, and from 1984 to 2008 there was a 29% increase.

Where is the ozone coming from?

I can't pin down any one nation as the source. However, we know that the level of ozone is rising in the air coming into North America from abroad, and that the rate of increase is highest when the transport is coming from south and east Asia. Global shipping is another source that needs to be considered because it's estimated to produce some 10–15% of the world's emissions of nitrogen oxides, which are ozone precursors.

Has your study affected environmental research?

This summer, the US National Oceanic and Atmospheric Administration plans to launch ozonesondes in six coastal locations in California to measure ozone levels and air quality. That information will be used to calculate an ozone budget for the state — if ozone levels increase, policy-makers may have to decide whether local emissions need to be reduced as a result.