In two papers1,2 published today in the journal Nature, new results from the CLOUD3 experiment at CERN4 imply the baseline pristine pre-industrial climate may have been cloudier than presently thought. CLOUD 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 fossil fuels - was essential to initiate aerosol particle formation. CLOUD finds that these so-called biogenic vapours are also key to the growth of the newly-formed particles up to sizes where they can seed clouds.
"These results are the most important so far by the CLOUD experiment at CERN," said CLOUD spokesperson, Jasper Kirkby. "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 clouds and climate."
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 change5. CLOUD is designed to understand how new aerosol particles form and grow in the atmosphere, and their effect on clouds and 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 pre-industrial times than in today's polluted atmosphere.
A paper published simultaneously in Science (Bianchi, F., et al. Science, doi 10.1126/ science.aad5456 (link is external), 2016) describes an observation of pure organic nucleation at the Jungfraujoch observatory by the same mechanism reported by CLOUD. The measurements did not involve CLOUD directly but most of the authors are also members of the CLOUD collaboration.
"The observation of pure organic nucleation at the Jungfraujoch is very satisfying," said Kirkby. "It confirms that the same process discovered by CLOUD in the laboratory also takes place in the atmosphere."
25 May 2016
1. Kirkby, J., et al. Ion-induced nucleation of pure biogenic particles. Nature, doi 10.1038/nature 17953 (link is external) (2016).
2. Tröstl, J., et al. The role of low-volatility organic compounds in initial particle growth in the atmosphere. Nature, doi 10.1038/nature18271 (link is external) (2016).
3. 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 controled 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.
4. CERN, the European Organization for Nuclear Research, is the world's leading laboratory for particle physics. Its headquarters are in Geneva. Its Member States are: Austria, Belgium, Bulgaria, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and United Kingdom. Romania is a Candidate for Accession. Cyprus and Serbia are Associate Member States in the pre-stage to Membership. Pakistan and Turkey are Associate Member States. European Union, India, Japan, JINR, Russian Federation, UNESCO and United States of America have Observer status.
5. Boucher, O. et al. in Climate Change 2013: The Physical Science Basis. Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (eds. Stocker, T.F. et al.) 571-658 (Cambridge Univ. Press, 2013).