In two new papers published in Nature, researchers from ACES along with colleagues from Europe and the USA, imply that the baseline pristine pre-industrial climate may have been cloudier than presently thought. New results from the CLOUD experiment at CERN, Switzerland, shows that organic vapours emitted by trees produce abundant aerosol particles in the atmosphere in the absence of sulphuric acid.

”Previously it was thought that sulphuric acid – which largely arises from anthropogenic activities – was essential to initiate aerosol particle formation. CLOUD finds that vapours emitted by forests as part of their natural activity are also forming particles that can seed clouds, even in the absence of sulphuric acid” says Ilona Riipinen, Associate Professor at ACES.

The Intergovernmental Panel on Climate Change (IPCC) considers that the increase in aerosols and clouds since pre-industrial times represents one of the largest sources of uncertainty in climate change. CLOUD is designed to understand how new aerosol particles form and grow in the atmosphere, and their effect on clouds and climate.

“These results are the most important so far by the CLOUD experiment at CERN,” says CLOUD spokesperson, Jasper Kirkby, an experimental particle physicist at CERN, Switzerland. “When the nucleation and growth of pure biogenic aerosol particles is included in climate models, it should sharpen our understanding of the impact of human activities on climate.”

CLOUD also finds that ions from galactic cosmic rays strongly enhance the production rate of pure biogenic particles – by a factor 10-100 compared with particles without ions. This suggests that cosmic rays may have played a more important role in aerosol and cloud formation in preindustrial times than in today’s polluted atmosphere. The role of ions in systems not containing these biogenic organic molecules was studied in a separate paper by the CLOUD team that was published in Nature Communications the previous week.

”Besides the often hazardous air pollutants emitted from anthropogenic activities, different natural processes are also an important source of atmospheric particulate matter. Knowing the natural background levels of atmospheric aerosol particles is a prerequisite for understanding how human activities have influenced climate since the industrial revolution. The molecular picture we put together based on the experiments in CLOUD will eventually help us design win-win solutions from both air quality and climate perspectives,” says Ilona Riipinen.


The CLOUD experiment

The CLOUD experiment consists of a large instrumented chamber in which the atmosphere can be precisely simulated, and the formation and growth of aerosol particles and the clouds they seed can be studied under precisely controlled atmospheric conditions. Unwanted contaminants can be suppressed well below the part-per-trillion level. The CLOUD experiment uses a beam from CERN’s Proton Synchrotron to simulate cosmic rays – particles bombarding the atmosphere from space. The experimental collaboration comprises 21 institutes: Aerodyne Research, California Institute of Technology, Carnegie Mellon University, CERN, Finnish Meteorological Institute, Goethe University Frankfurt, Helsinki Institute of Physics, Karlsruhe Institute of Technology, Lebedev Physical Institute, Leibniz Institute for Tropospheric Research, Paul Scherrer Institute, Stockholm University, Tofwerk, University of Beira Interior, University of Eastern Finland, University of Helsinki, University of Innsbruck, University of Leeds, University of Lisbon, University of Manchester, and University of Vienna.


Ilona Riipinen
Head of unit
Phone +46 8 674 7284