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

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

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.

Mingyi Wang et al., Synergistic HNO3 H2SO4 NH3
upper tropospheric particle formation. Nature, DOI 10.1038/s41586-022-04605-4


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


Professor Joachim Curtius

Institute for Atmospheric and Environmental Sciences

Goethe University, Frankfurt, Germany

Phone +49 (0)69 798-40258

[email protected]

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