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“The same phenomenon recurs every year. During the dry season, in winter, burning fossil fuels and biomass in South Asia creates a huge pollution haze: the Atmospheric Brown Cloud. How and why it disappears as soon as the rainy season starts in spring has now been clarified by an international team of scientists led by the Max Planck Institute for Chemistry. The result is that thunderstorm updrafts, lightning and chemical reactions enhance the self-cleansing power of the atmosphere, allowing atmospheric pollutants to be efficiently washed out of the air. However, the pollutants that are not eliminated are transported into the upper troposphere by the monsoon and then spread worldwide.

No weather phenomenon defines South Asia as much as the monsoon: this enormous circulation system leads to dryness in winter but brings intense precipitation in summer. The summer monsoon is created by the heating of air masses over the Indian subcontinent and the warm air rising. As a result, humid ocean air is drawn in and flows over land towards the Himalayas. Deep thunderstorm clouds produce rain over the region for months, guaranteeing water supplies and safeguarding harvests.

Atmospheric researchers have long suspected that the rising air masses also transport pollution high into the atmosphere, even above the rain clouds. “We anticipated that gaseous and particulate pollutants are transferred into an anticyclone, a huge clockwise circulation of winds, which forms above the clouds over South Asia as a result of the thunderstorm convection,” says Jos Lelieveld, Director at the Max Planck Institute for Chemistry. Geographically, the countries of Bhutan, Nepal, Myanmar, Bangladesh, Tibet, India, Sri Lanka, Pakistan and Afghanistan are part of South Asia. In this region, nitrogen oxide and  emissions from burning coal and other fossil fuels have increased by fifty percent over the past decade. However, the pollution cloud is also fuelled by other sources, in particular the combustion of biomass by the region’s large population…”

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