Weather phenomena – the monsoon is both a pollutant remover and a carrier

Every year, a large cloud of pollutants caused by human activities forms over South Asia during the winter dry season. This 'atmospheric brown cloud' extends across the Indian subcontinent and into the Indian Ocean; it is the result of the combustion of fossil fuels and biomass. From December until March, this cloud is so large that, until now, it has been unclear how it disappears again during the rainy monsoon season in the summer. An international team of researchers led by the Max Planck Institute for Chemistry, and involving the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) has now established that updrafts, thunderstorms and chemical reactions provide the atmosphere with the capacity for self-cleaning, as more hydroxyl radicals form due to monsoon storms. These molecules serve as a kind of detergent, as they oxidise air pollutants and particle precursor gases. As a result, some of the pollutants become more easily soluble in water and can rain down on Earth as precipitation. However, the pollutants that are not washed out rise into the upper troposphere, driven by the monsoon, and from there spread out all over the world and up into the stratosphere.

No other weather phenomenon has such a marked effect on South Asia as the monsoon. In winter, the massive flow of air leads to dryness and drought, but it brings large amounts of precipitation in summer. The monsoon occurs when masses of air over the Indian subcontinent heat up very strongly over the summer months, and the warm air rises. As a result, damp ocean air is drawn in and flows across the land towards the Himalayas. Enormous clouds form over the region, which can cause rain for months, ensuring a supply of water and crops. "We were able to study this giant weather machine at altitudes of up to 15 kilometres with the HALO research aircraft," says Hans Schlager of the DLR Institute of Atmospheric Physics.

It has been apparent, albeit hitherto not directly proven, that the rising masses of air also carry pollutants high into the atmosphere and beyond the rain clouds. "We believe that convection causes pollutants and particulates to enter an anticyclone, a huge wind vortex that forms above the cloud layer over South Asia," says Jos Lelieveld, Director of the Max Planck Institute for Chemistry. "Our research flights also showed that pollutants are effectively removed by the monsoon."

Geographically, South Asia is made up of the countries of Bhutan, Nepal, Myanmar, Bangladesh, Tibet, India, Sri Lanka, Pakistan and Afghanistan. Nitrogen oxide and sulphur dioxide emissions have increased by 50 percent in this region over the last decade. This has been caused mainly by the combustion of coal and other fossil fuels, although other causes, in particular the burning of biomass by the many people who live in the region, also add to the cloud of pollution.

Scientists used the HALO research aircraft to study air composition

Evidence that the South Asian monsoon actually carries pollutants to great heights above the cloud layer was established during an extensive expedition with the DLR-operated HALO (High Altitude LOng range) research aircraft. In 2015, the Max Planck Institute for Chemistry began working with the Forschungszentrum Jülich, the Karlsruhe Institute of Technology and DLR on the 'Oxidation Mechanism Observations' (OMO) mission. In July and August, the scientists used the high-flying aircraft to ascend into the tails of anticyclones between the eastern Mediterranean and the Indian Ocean, and studied the composition of the air. In doing so, they also crossed areas of the Middle East, the Mediterranean and North Africa to examine the extent of the phenomenon.

Chemical compounds provided information about the sources of air pollution and degradation processes

In the course of gathering information about the sources of air pollution and the chemical processes that take place in the atmosphere, they identified numerous chemical compounds: sulphur and nitrogen oxides, ozone, aerosol particles, chlorinated molecules, hydrocarbons and their degradation products.

The researchers used methane measurements as an indication of whether or not they were analysing air masses belonging to the anticyclone, an enormous area of high pressure at high altitude. This greenhouse gas escapes from rice fields in large quantities during the monsoon and has a long lifespan.

Pollutants at a height of 15 kilometres

The measurement flights also showed, for instance, that the quantities of carbon monoxide and sulphur dioxide within the anticyclone were significantly higher than those outside it. "The high sulphur dioxide levels are due to combustion activities carried out by humans and are much higher than the natural background concentration," says Schlager. In turn, this means that a significant quantity of air pollution is transported to altitudes of up to 15 kilometres. In addition, the researchers were able to demonstrate that India is a significant source of pollution. Previously, it was assumed that a large proportion of the emissions could also have come from China, as the monsoon’s area of influence extends all the way to East Asia.

The researchers also analysed the concentration of hydroxyl (OH), which was found to be much higher within the anticyclone than outside it. This molecule is well known for its atmosphere-purifying effects, as it is a highly reactive radical and oxidises pollutants effectively. This has two chemical effects. Firstly, their solubility and ability to deposit themselves on airborne particles changes, making it easier for them to be washed out of the air in precipitation and rain down on the ground. Secondly, the oxidised molecules can accumulate to form new aerosol particles. Because the anticyclone expands widely, spreading these particles, this process can affect the global climate.

An 'atmospheric detergent' caused by monsoon storms

The decomposition of ozone and water by sunlight is the primary cause of this 'atmospheric detergent'. Once the radical has reacted with pollutants, it is normally eliminated. However, if nitrogen oxides are present, the radical is recycled and is able to perform its 'cleaning duties' repeatedly. Nitrogen oxides are formed not only by the combustion of diesel fuel, but also by lightning in the atmosphere. As this occurs frequently during monsoon storms, this means that the self-cleaning capacity is maintained by the thunderstorms despite the air pollution at a height of 15 kilometres. According to the scientists, even more hydroxyl is recycled than is produced in the first place, as over 80 percent of hydroxyl in the anticyclone is produced by lightning, as opposed to just 40 percent outside. This means that the monsoon convection not only pumps pollutants high into the atmosphere, but at the same time provides a cleaning mechanism to remove some of them.

This explanation was confirmed by the results of an established numerical model system that maps chemical processes within the atmosphere globally. Among other things, this model allows the levels of individual chemical compounds such as carbon monoxide, sulphur dioxide, hydrocarbons and nitrogen oxides to be determined, as well as the hydroxyl concentration. The latter decreases by a factor of two to three if the scientists do not take nitrogen oxides caused by lightning into account in their model.

Distribution of pollutants around the globe

As effective as the monsoon updraft might be at self-cleansing, the results of the research team's measurements and model also showed the monsoon’s down-side – a large proportion of South Asia's pollutants are carried beyond the monsoon precipitation and do not form part of the anticyclone. Instead, they accumulate and are distributed around the world. For instance, almost 10 per cent of the sulphur dioxide from South Asia reaches the stratosphere, with consequences for the climate and the ozone layer. The monsoon is not only an efficient washing machine for pollutants, but also contributes towards global air pollution.

Since it is assumed that pollutant emissions will continue to increase over the coming years, atmospheric researchers are interested to find out how this Janus-faced dimension to the South Asian monsoon will develop in future. Will its purification and transport mechanisms remain in proportion to one another, or will they start to tilt more in a particular direction?

About HALO

The HALO research aircraft is a joint initiative by German environmental and climate research institutions. HALO was purchased by the German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung; BMBF), the Hermann von Helmholtz Association of German Research Centres and the Max Planck Society (MPG). The operation of HALO is supported by the German Research Foundation (DFG), MPG, DLR, Forschungszentrum Jülich (FZJ), the Karlsruhe Institute of Technology (KIT) and the Leibniz Institute for Tropospheric Research in Leipzig (TROPOS). DLR is also the owner and operator of the aircraft.