Chennai: Every year during the dry season between December and March, a dirty haze appears over South Asia and China. It is a potent mixture of pollutants borne out of wood burning and crop burning, vehicular emissions and industrial combustion, christened the Asian Brown Cloud.
It is a phenomenon that is too familiar to North India in the winter, thanks to the accompanying respiratory ailments, headaches and allergic reactions. Yet, come monsoon, the brown cloud disappears. What happens to the pollutants in the rainy season?
A question that puzzled researchers for two decades has now been answered. Scientists from Germany and Cyprus have discovered that the Southwest Monsoon (also called the Indian Monsoon) cleanses a large chunk of pollutants that collect in the atmosphere. But it is not all benign — the monsoon also spreads pollution from South Asia, to other parts of the world.
Jos Lelieveld, the lead researcher and the head of the Max Planck Institute for Chemistry, Germany called the Indian monsoon, two-faced, likening it to Janus, the Roman god of duality, in his paper published in the journal Science.
Every year, at the end of the summer, dark clouds gather moisture from the Arabian Sea and the Bay of Bengal and move towards land. This is the approaching monsoon. But above this stormy layer, is an accompanying layer of cloud-free, clear atmosphere. This is called the anti-cyclone. Every monsoon comes with a larger layer of anti-cyclone.
While the monsoon air currents draw moisture inwards, building a dense layer of rain clouds, air currents in the anti-cyclone circulates outwards and spreads clouds and moisture over a large area.
So, while the monsoon covers most of the sub-continent, its accompanying anticyclone covers a much larger area, extending from the Mediterranean Sea to the Pacific Ocean.
This is important, because the extent of coverage indicates how far anything caught up in these weather systems can spread.
Along with moisture, the Indian monsoon collects pollutants that have been sitting in the atmosphere throughout the dry season and pushes much of it higher up the atmosphere, into the anticyclone.
Lelieveld and his colleagues set out to find out what happened to these pollutants once they reached the anticyclone. The researchers travelled along the western part of the anticyclone, which is the sky between Cyprus and the Maldives, at an altitude between 9,000-15,000 metres above land. (IANS)
