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The Two Faces of the South Asian Monsoon

Scientists investigate how monsoon influences global distribution of air pollutants and atmospheric self-purification

Jülich, 15 June 2018 – The summer monsoon, the world’s largest weather phenomenon, which occurs above Asia, transports air pollutants up into the upper atmosphere, where they can spread across the entire globe. At the same time, however, it strengthens the self-purification mechanism of the polluted air. This was verified by an international team of researchers involving Jülich scientists using measurement data from the OMO flight mission (“oxidation mechanism observations”), which was conducted in 2015. During the campaign, the scientists used the German research aircraft HALO to analyse the composition of the air between the Eastern Mediterranean and the Indian Ocean at altitudes of up to 15 km. Instruments from Forschungszentrum Jülich and the Max Planck Institute for Chemistry in Mainz were also on board the aircraft: they measured hydroxyl radicals, which are responsible for the decomposition of air pollutants.

No weather phenomenon affects South Asia as strongly as the monsoon: the gigantic air flow leads to dry spells and drought during winter, and during summer causes heavy rainfalls which can last for months. The monsoon is triggered by air masses above the Indian subcontinent heating up intensely during the summer months. The warm air rises, moist ocean air is sucked in and flows towards the Himalayas across the land, forming huge clouds.

Until recently, it was just an assumption that the rising air masses also transport polluted air high up into the atmosphere, beyond the rain clouds. Air pollution is a growing problem in South Asia. The nitrogen oxide and sulfur dioxide emissions in the region have risen by 50 % in the past decade – mainly due to the combustion of coal and other fossil energy carriers. Every year, a huge haze layer is formed from anthropogenic pollutants above South Asia during the dry winter (December to March). This “atmospheric brown cloud” ranges from the North of the Indian subcontinent to the Indian Ocean. During the rainy season in summer, the pollutant cloud disappears – and the role the monsoon plays in this regard has now been investigated in detail for the first time using direct measurements.

atmospheric brown cloudA huge dust cloud above South Asia: every year, the atmospheric brown cloud forms during the winter months due to the combustion of biomass and fossil fuels.
Copyright: NASA

Pollution pump above the clouds

Using the HALO research aircraft, the scientists ascended to altitudes of up to 15 km and collected data in an area spanning from the Eastern Mediterranean to the Indian Ocean. They also traversed regions above the Middle East, the Mediterranean, and northern Africa in order to investigate the expansion of the phenomena. The data of the flight mission have now been analysed and clearly show that some of the pollutants – driven by the monsoon – reach the upper troposphere at an altitude of 15 km. There, they enter into huge wind circulations (named “anticyclones”) above the clouds, and then spread worldwide. Thus, the researchers were able to show that the amounts of carbon monoxide and sulfur dioxide were significantly higher within the anticyclones. They demonstrated that major proportions of the pollutants originate in South Asia, and not, as often presumed, in China. Almost 10 % of the sulfur dioxide reaches the stratosphere, affecting both our climate and the ozone layer.

In their analysis, the researchers took into consideration measurements of numerous chemical compounds: sulfur and nitrogen oxide, ozone, aerosol particles, chlorinated molecules, and hydrocarbons and their degradation products. Methane measurements served as an indicator of whether or not the analysed air came from anticyclones: during the monsoon, large quantities of this greenhouse gas escape from rice fields, and it also has a long lifetime.

Additional Detergent

“Particular emphasis was put on measuring hydroxyl radicals, OH radicals for short. We used measuring instruments which were developed for use on HALO by researchers from Mainz and Jülich over several years of collaboration,” explains Frank Holland from Jülich’s Institute of Energy and Climate Research. The measurements of the OH radicals, which are viewed as the atmosphere’s detergent, show that the atmospheric self-purification effect is considerably increased during the monsoon: under the influence of updraughts, thunderstorms, and chemical reactions, a significant proportion of the transported air pollutants oxidise. This means that they become more readily soluble in water and can washed out by precipitation.

“After they are used up by the reaction with air pollutants, OH radicals are effectively regenerated in the atmosphere through chemical reactions with nitrogen oxides,” explains Jülich atmospheric chemist Andreas Hofzumahaus. “Flashes of lightning are an important source of nitrogen oxide – and lighting occurs particularly frequently in monsoon thunderclouds.” This explanation is confirmed by atmospheric researcher Jos Lelieveld from the Max Planck Institute for Chemistry: using a numerical modelling system, he conducted calculations which image the global chemical processes and concentrations of individual chemical compounds in the atmosphere. When the nitrogen oxides formed by lightning were removed from the model, the OH concentration in the model also decreased by a factor of two to three.

Modellergebnisse verdeutlichen die Luftverschmutzung über Südasien.Model results illustrate the air pollution above South Asia: the image on the left shows carbon monoxide emissions (CO) at altitudes of 12–17 km; the image on the right shows the same scene without emissions from South Asia. The left-hand image also shows the winds in the region, revealing the anticyclone formed by the monsoon.
Copyright: MPI für Chemie

The oxidation of trace gases with OH radicals, however, can cause chemical products to develop, which form fine particles. Since anticyclones extend over large areas and spread these aerosol particles, this effect can also influence the global climate.

Uncertain future

It must be assumed that pollutant emissions will continue to rise in the coming years – and this is why atmospheric researchers are interested in how the face of the South Asian monsoon will develop: will the self-purification and transport mechanisms remain in place simultaneously, or will they tip one way or the other?

About OMO

In 2015, the OMO mission (“oxidation mechanism observations”) was conducted, headed by the Max Planck Institute for Chemistry (MPI) in Mainz together with colleagues from Forschungszentrum Jülich, the Karlsruhe Institute of Technology, the German Aerospace Center, and the universities of Bremen, Wuppertal, and Heidelberg. The aim was to investigate the atmospheric self-purification ability and the chemistry of atmospheric air pollutants in the free troposphere under the influence of emissions from South Asia during the summer monsoon. One focus of the investigations was on the measurement of free radicals (OH, HO2) by Jülich and Mainz scientists. The measurement flights were conducted with the HALO research aircraft.

About AirLIF

AirLIF is Forschungszentrum Jülich’s instrument designed to measure OH radicals. Just like the Max Planck Institute’s instrument, it detects highly reactive molecules by means of laser-induced fluorescence with great sensitivity. Important components necessary for the instruments on-board HALO were developed in cooperation between the two atmospheric research institutes. The special air inlet, which decelerates and collects incoming air free of contamination was developed at Jülich. The inlet was developed together with Jülich’s Central Institute of Engineering and Technology and also manufactured there.

Lufteinlass des Jülicher OH-Radikal-Messinstruments (AirLIF) für den Einsatz auf dem Höhenforschungsflugzeug HALOAir inlet of the Jülich OH-radical measuring instrument (AirLIF) for use on board the high-altitude research aircraft HALO
Copyright: Forschungszentrum Jülich / Michael Schmitt

About HALO

The HALO research aircraft is a joint initiative of German environmental and climate research institutions. HALO was procured using funds from the Federal Ministry of Education and Research, the Helmholtz Association, and the Max Planck Society. The operation of HALO is funded by the German Research Foundation (DFG), the Max Planck Society, Forschungszentrum Jülich, the Karlsruhe Institute of Technology, the German Research Centre for Geosciences in Potsdam (GFZ), and the Leibniz Institute for Tropospheric Research e.V. in Leipzig (TROPOS). The German Aerospace Centre (DLR) owns and operates the aircraft.

Further information:

Website HALO

Website OMO

Institute for Energy und Climate Research, Troposphere (IEK-8)

OMO at the IEK-8

Original publication: 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, 14 June 2018, DOI: 10.1126/science.aar2501


PD Dr. Andreas Hofzumahaus
Institute for Enryg and Climate Research - Troposphere (IEK-8)
Tel.: +49 2461 61-3239

Dr. Frank Holland
Institute for Enryg and Climate Research - Troposphere (IEK-8)
Tel.: +49 2461 61-6078


Dr. Regine Panknin
Press officer
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