Researchers have been observing for some time that the dirt cloud containing aerosols, soot, and other pollutants normally found over South Asia disappears during the rainy season. Why and where was so far unclear.
In the summer of 2015, Jos Lelieveld, Max Planck Institute for Chemistry, Mainz, Germany, and colleagues have carried out several measuring flights with the HALO research aircraft into the monsoon clouds. Up to 15 km into the monsoon anticyclone, a huge wind vortex that hangs over Southern Asia during the rainy season. There they analyzed the chemical composition of air samples.
The researchers found that although some of the air pollutants are washed out by rain and chemical reactions in the clouds, the large remainder, however, is transported by the rising air masses into the upper troposphere. That concerns around 10 % of the sulfur dioxide from South Asia as well as sulfur oxides, aerosols, chlorine compounds, and other pollutants.
Once in the upper troposphere, the pollutants accumulate and form a chemically reactive reservoir for weeks to months, from which carbon, nitrogen, sulfur and halogen-containing decomposition products are redistributed globally. About one-third of the air pollutants reach the stratosphere and can also damage the ozone layer there.
In addition, monsoon storms cause increased hydroxyl molecules due to chemical reactions in the air. These are very reactive and oxidize pollutants, which can then be washed out by rain and deposited to Earth’s surface. By this, the monsoon helps to clean the lower layers of air.
The researchers expect that the rapidly increasing emissions of South Asia will also increase the influx of pollutants into the anticyclone in the coming years. If then the positive or negative side of the monsoon gains the upper hand, will show then.
- The South Asian monsoon—Pollution pump and purifier,
J. Lelieveld, E. Bourtsoukidis, C. Brühl, H. Fischer, H. Fuchs, H. Harder, A. Hofzumahaus, F. Holland, D. Marno, M. Neumaier, A. Pozzer, H. Schlager, J. Williams, A. Zahn, H. Ziereis,
Science 2018.
https://doi.org/10.1126/science.aar2501
