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Research
A force of nature with two faces
It can even be seen from space: the gigantic “Asian brown cloud”. In winter, the smog cloud in the lower atmosphere covers the South Asian region for months. The onset of the summer monsoon puts an end to the billowing spectre. The monsoon purifies the air, but also transports pollutants to high altitudes where they are distributed worldwide, as researchers have found out.
“It feels like having sandpaper in your throat,” says Dr. Fred Stroh from the Jülich Institute of Energy and Climate Research (IEK-7). Stroh has already taken part in several measurement campaigns in South Asia and inhaled the thick air – a composition of dust, sooty particles, sulphur dioxide and nitrogen oxides from factory chimneys, exhaust pipes and burning stubble fields. In broad daylight, the sun disappears behind a grey-brown veil, the smog gnaws at historical buildings, and airplanes cannot land because of low visibility. In November 2017, fine dust concentrations of over 1,000 micrograms per cubic meter of air were measured repeatedly in the megacity New Delhi. The World Health Organization WHO considers 25 micrograms per cubic meter over a period of 24 hours to be just barely acceptable. The pollutants are suspected of causing cancer and heart attacks – those who can move away do so.
T Low pressure area H High pressure area How summer monsoon develops
The air over large land masses such as India heats up very strongly in summer and rises rapidly. This takes moist ocean air in. Huge clouds then form over land, from which it can rain for months.
Every year, the summer monsoon brings relief for the people living there, purifying the air locally and washing out the pollutants – the smog disappears. At the same time, the monsoon increases the self-cleaning properties of the atmosphere – by increasing the formation of the atmospheric “detergent”, the OH radicals. These transform the pollutants through chemical reactions into water-soluble compounds that can be washed out with the rain. “During our measurement flights at altitudes of up to 15 kilometres, we were able to prove that the OH radicals consumed by pollutants are effectively reproduced through the lightning activity, which is abundant during the monsoon storms,” says Dr. Andreas Hofzumahaus of the Jülich Institute of Energy and Climate Research (IEK-8). The reason is the nitrogen oxides in the thunderclouds. “Nitrogen oxides are not only generated by combustion processes, but also by lightning in the atmosphere, meaning it occurs particularly frequently in the monsoon thunderclouds.” They recycle the OH radicals, which can thus clean repeatedly.
But the monsoon has another face as well, as the researchers were able to prove for the first time: rapidly rising air transports a portion of unwashed pollutants, including sulphur-containing compounds to high altitudes of over 15 kilometres. There they are captured by powerful air vortices – the anticyclones. Some of them can reach the stratosphere at altitudes of up to 50 kilometres and be distributed globally in the process.
What happens to pollutants in the monsoon?
Lightning in the thunderclouds intensifies the self-cleaning capacity of the atmosphere: pollutants become more water-soluble. The thunderclouds can reach up to 14 kilometres high.
According to the researchers, the pollutants thus influence the global climate. Sulphate aerosols, for example, which reach the stratosphere or are formed there from sulphur-containing pollutants, have a cooling effect because they reflect sunlight and can serve as cloud germs in deeper layers of the atmosphere. “Aerosols are also suspected of amplifying reactions of chlorine compounds in the atmosphere, which in turn destroy the protective ozone layer,” reports Hofzumahaus. “Therefore, effects of aerosols are considered one of the greatest uncertainties in predicting climate change.”
All these data and findings now need to be integrated into existing models. A central question is whether the cleaning and transport mechanisms will persist as air pollution increases. Or will one of the processes dominate?
To investigate the climatic effects of air pollution in higher atmospheric layers, the researchers flew even higher in another major measurement campaign called StratoClim in the summer of 2017. Fred Stroh reports: “With the help of the Russian high-altitude aircraft M55-Geophysika, we were able to examine the air composition at 20 kilometres above Nepal, India and Bangladesh in detail for the very first time.” The evaluation of these data is still ongoing, but the scientists are optimistic to learn more about the two faces of the monsoon with the material.
Brigitte Stahl-Busse
Halfway around the world
In the extensive “Oxidation Mechanism Observations” (OMO) measuring campaign, scientists explored exactly how the monsoon purifies the air. Equipped with state-of-the-art measurement technology, the German research aircraft HALO (photo) covered more than 100,000 flight kilometres in over 120 flying hours at altitudes of up to 15 kilometres between July and August 2015. The route went from Oberpfaffenhofen to Cyprus to the Maldives and back. The scientists evaluated the extensive data collection for three years. Around 60 researchers from the Max Planck Institute for Chemistry, Forschungszentrum Jülich, the German Aerospace Centre, the Karlsruhe Institute of Technology and the universities of Bremen, Heidelberg, Leipzig and Wuppertal were involved.
With StratoClim into the monsoon
A blog of the Jülich climate researcher Corinna Kloss
Images: Perfect Lazybones/Shutterstock.com, Forschungszentrum Jülich, Grafiken: SeitenPlan