ASIAN pollution is a global problem. Millions of tonnes of soot, sulphur dioxide and other pollutants are fast-tracked into the stratosphere each year by the summer monsoon.
鈥淭he monsoon is one of the most powerful atmospheric circulation systems on the planet, and it happens to form right over a heavily polluted region,鈥� says William Randel of the US National Center for Atmospheric Research in Boulder, Colorado.
鈥淭he monsoon is extremely powerful, and it happens to form right over a heavily polluted region鈥�
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The stratosphere begins about 12 kilometres up, above the troposphere where weather systems like the monsoon develop. Most pollution stays below the boundary between the two. However, by using satellite instruments to track hydrogen cyanide, a minor but telltale ingredient of the pollution, Randel and his colleagues found 鈥減ipes鈥� of polluted air moving through the boundary.
They think that the exceptional updraughts of air inside the monsoon鈥檚 giant clouds can bust through and send pollution deep into the stratosphere (Science, ). This is where the planet鈥檚 ozone layer sits, filtering out ultraviolet radiation from the sun.
The findings will trigger a radical rethink about the state of the stratosphere. 鈥淩eceived wisdom has been that gases like sulphur dioxide and nitrogen oxides don鈥檛 make it into the stratosphere,鈥� says Peter Bernath of the University of York, UK, a member of the research team. 鈥淣itrogen oxides in particular are of concern,鈥� he says, because they can destroy ozone. Sulphur dioxide can shroud the planet in a cooling haze.
John Pyle, a specialist on the ozone layer at the University of Cambridge, agrees that the research raises key questions. 鈥淗ow much will the transport of pollution change in the future, as emissions increase or the monsoon changes?鈥� he says. It鈥檚 unclear whether climate change will weaken or intensify monsoons.
In the lower atmosphere, pollutants like sulphur dioxide 鈥渞ain out鈥� of the air within days. But in the stratosphere they can stay aloft for years, spread by fast winds known as jets, meaning the threat is global. The effects may have already been unwittingly detected: researchers recently noted an increase in sulphate particles in the stratosphere around the globe, which could be linked to China鈥檚 rapid industrialisation over the past decade.
