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How ice clouds develop – Asian monsoon influences large parts of the Northern Hemisphere

Air pollutants form the condensation nuclei for ice clouds or cirrus clouds (here: Cirrus spissatus). When ammonia, nitric acid and sulfuric acid are present together, they form such condensation nuclei particularly effectively. Credit: Joachim Curtius, Goethe-University Frankfurt

Atmospheric researchers from the international CLOUD consortium have discovered a mechanism that allows nuclei for ice clouds to form and rapidly grow in the upper troposphere. The discovery is based on cloud chamber experiments to which a team from Goethe University contributed highly specialised measurements. Although the conditions for nucleus formation are only fulfilled in the Asian monsoon region, the mechanism is expected to have an impact on ice cloud formationacross large parts of the Northern Hemisphere.

The Asian monsoon transports enormous amounts of air from atmospheric layers close to Earth’s surface to a height of around 15 kilometres. Like in a gigantic elevator, human-induced pollutants also end up in the upper troposphere in this way. A research team from the CLOUD consortium (Cosmics Leaving Outdoor Droplets), including atmospheric researchers from Goethe University in Frankfurt, have reproduced the conditions prevailing there, among them cosmic radiation, in their experimental chamber at the CERN particle accelerator centre in Geneva.

In the process, they identified that up to 100 times more aerosol particles form from ammonia, nitric acid and sulphuric acid than when only two of these substances are present. These particles are then available on the one hand as condensation nuclei for liquid water droplets in clouds and on the other hand as solid seeds for pure ice clouds, so-called cirrus clouds. The research team also observed that ice clouds with the three-component particles already form at lower water vapour supersaturation than anticipated. This means that the ice clouds already develop under conditions that atmospheric researchers worldwide had so far assumed did not lead to the formation of cirrus clouds. With model calculations from around the globe, the CLOUD research team was also able to show that the cloud nuclei can spread across large parts of the Northern Hemisphere within just a few days.

“The experiment in the cloud chamber was a reaction to the results of field experimentsover Asia. These measurements showed that ammonia is present there in the upper troposphere during the monsoon,” explains Professor Joachim Curtius from Goethe University. “Previously, we had always assumed that ammonia, due to its water solubility, was rinsed out of the rising air masses before it reached the upper troposphere.” As the CLOUD researchers’ experiment now corroborates, ammonia is an essential ingredient for more cloud formation. Ammonia emissions in Asia come predominantly from agriculture.

The international CLOUD research collaboration (Cosmics Leaving Outdoor Droplets) is made up of teams from 21 research institutions. In the experiment of which the research team is now presenting the results in the current issue of “Nature”, the researchers led by Curtius were responsible for the mass spectrometric measurement of the sulphuric acid concentration. This concentration changed over the course of the experiment, but was still always very low, like in the upper troposphere: for a single sulphuric acid molecule there are over a trillion other gas molecules. “Apart from the very best measuring equipment, such measurements require highly specialised expertise. That is why you need teams with complementary skills to conduct such an experiment,” explains Curtius, who is a member of the CLOUD steering committee and was coordinator of the EU project CLOUD-MOTION successfully completed just recently. Like in the atmosphere, sulphuric acid forms in the CLOUD chamber from sulphur dioxide and hydroxyl radicals.

Clouds are an important and at the same time still insufficiently understood element of global climate. Depending on whether they float high up or low down, their water or ice content, how thick they are or over which region of the globe they form, it gets warmer or colder beneath them. To improve the precision of climate models, researchers worldwide require exact knowledge of all the processes surrounding clouds as a climate factor. The CLOUD research team’s findings are helping them a long way towards increasingly reliable climate predictions.

Publication: Mingyi Wang et al., Synergistic HNO3 H2SO4 NH3 upper tropospheric particle formation. Nature www.nature.com/articles/s41586-022-04605-4, DOI 10.1038/s41586-022-04605-4

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