China’s ozone pollution is wafting over and offsetting half the reductions the US has achieved

Gone with the wind.
Gone with the wind.
Image: Reuters/China Daily
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Like a lot of other chemicals, ozone can be both useful and harmful. In the planet’s stratosphere, it protects us against harmful UV rays. Any lower, however, it can cause respiratory illnesses and hurt the growth of plants.

No wonder then that governments around the world would like to regulate the amount of ozone that is spewed in the atmosphere. The US has been tightening laws to control the creation of ozone pollution over the past decades. Sadly, according to a new study, China’s growth has offset nearly half the reductions the US has achieved.

Winds blowing from the west to east carry ozone and other precursors, such as nitrogen oxides (NOx), from China to the US. The study, published in Nature Geoscience, calculates that these winds offset as much as 43% of the improvements in air quality expected in the western US in response to tighter regulations.

To arrive at this conclusion, Willem Verstraeten of the Royal Netherlands Meteorological Institute used data from NASA’s Aura satellite—a probe launched in 2004 to study the earth’s ozone levels and air quality—over the course of five years. Then he used a chemical-transport model to predict how ozone spread.

Between 2005 to 2010, Verstraeten found, China’s NOx emissions went up 21% while those from the US dropped 21%. The rise in emissions has led to a 7% increase in ozone in the troposphere, the lowest layer of the Earth’s atmosphere, above China. But the US saw no significant drop in ozone levels during the same period. Comparing simulations with and without the rise in emissions over China, Verstraeten found that the reductions that the US should have seen from tighter regulations were offset by the effect of growing emissions over China.

Such long-range pollutant transport happens more frequently in winter and spring, when cyclones are more frequent. Pollution from east Asia is transported via the downwind to western North America. China itself lies downwind from India and other parts of Asia, but it is not yet clear how China’s ozone levels are influenced by them.

Currently there are no international agreements regarding intercontinental transport of tropospheric ozone.