Over the past few decades, the U.S. has put in new laws to control the creation of low-level ozone. This is because unlike the protective ozone layer in our planet's stratosphere, ozone down in the troposphere – the atmosphere's lowest layer – is a greenhouse gas and a pollutant that harms human health.

Despite these efforts, the levels seen in North America haven’t dropped dramatically, and many have speculated that the reason is that ozone created by emissions in East Asia is making its way across the Pacific Ocean. To quantify this effect, a team led by Willem Verstraeten of Wageningen University turned to NASA satellite data. According to their analyses, 43% of the effect that the Western U.S. should have seen from tighter regulations was offset by growing emissions over China. These findings are published in Nature Geoscience this week. 

Tropospheric ozone is what's called a secondary pollutant. It's formed by reactions in the atmosphere involving precursor chemicals like nitrogen oxides (NOx), and can last several weeks, long enough to be blown around between continents. Many researchers suspected that this atmospheric transport from Asia could have a major impact on ozone levels in the Western U.S., but there’s been little hard data to prove the effect.

The team looked at observations taken by NASA's Aura satellite – a probe launched in 2004 to study the planet's ozone levels and air quality -- to pin down ozone levels in China and the U.S. between 2005 and 2010. Then they turned to a commonly accepted model for the transportation of chemicals around the world's atmosphere to predict how ozone spreads.

In China, where emissions rose by about 21% between 2005 and 2010, ozone levels rose by 7%. Meanwhile, the movement of ozone and its precursor chemicals produced in China offset nearly half of the gains that the U.S. expected from tighter regulations on vehicles and power plants. Despite a 21% drop in NOx emissions over the course of the five years, the U.S. saw no significant drop in tropospheric ozone during that period.

The upshot is clear, according to an accompanying News and Views article: It's not enough for a town or a state or a nation to simply regulate its own emissions and expect tropospheric ozone to drop. China is downwind from India, North America is downwind from China, and everyone is affected by emissions from other parts of the world. Yet today, despite global agreements to protect the stratospheric ozone layer, there’s no international deal to regulate the movements of ozone or the chemicals that create it.