Cloud in a chamber

August 25, 2008

Everybody knows that clouds markedly influence weather and climate, but is this influence changing with time? Over the last several years some evidence was found that past changes in cosmic ray intensity have correlated with fluctuations of temperature and precipitation on Earth. There could be a causal link of these quantities via the formation of clouds, because the cosmic rays create ions in the atmosphere which may grow into aerosol particles that act as the initial point for cloud formation.

How these processes take place in detail and in how far they could potentially influence climate will be investigated within the framework of a PhD network funded by the European Union. The project is coordinated by Goethe-University of Frankfurt and the main experiment will be located at the European Centre for Nuclear Research, CERN. Total funding over the next 4 years will be 2.3 Mio Euro.

Highly energetic galactic cosmic rays consist mainly of protons and alpha-particles, released from supernova bursts into space. When cosmic rays traverse the Earth's atmosphere, electrons are pulled out of surrounding gas molecules, and a trail of charged molecules is left behind. These so called ions are potentially ideal condensation nuclei for the formation of new aerosol particles in the atmosphere, which may then grow in size. Without these aerosol particles as condensation nuclei, the water droplets that make up a cloud could not form. The researchers plan to simulate the processes in the laboratory with the "CLOUD" experiment to gain understanding of the details of the aerosol and cloud formation mechanisms as a function of altitude and composition of the atmosphere.

The core of the experiment is an aerosol chamber, a cylinder of 3 meter diameter and 3.7 meter height that is filled with air, water vapour and variable trace amounts of gaseous gases such as sulphuric acid. The contents of the chamber are continuously analysed with an array of sensitive instruments attached to sampling probes. "With the sulphuric acid we include on the one hand side the effects of anthropogenic air pollution by sulphur dioxide" explains Prof. Joachim Curtius from the Institute for Atmospheric and Environmental Sciences at the Goethe-University. But part of the sulphur in the atmosphere also comes from natural sources such as volcanoes and the oceans. The galactic cosmic rays are simulated by a beamline from the Proton Synchrotron accelerator at CERN in Geneva. "With the beam we can simulate the cosmic ray intensity from ground level to 15 km altitude".

Besides CERN, the project participants are the Paul Scherrer Institute near Zurich, the Universities of Helsinki, Leeds, Reading and Vienna, the Institute for Tropospheric Research in Leipzig and the company Ionicon Analytik in Innsbruck. In total 8 PhD students and two post-doctoral researchers are funded within CLOUD-ITN. The contribution of University of Frankfurt will be the measurement of gaseous sulphuric acid and the development of a method for identification of droplets and other aerosol particles with diameters smaller than 3 nanometers. The critical size for aerosol formation in the atmosphere is expected to occur at particle sizes of 1-2 nanometers. This range is currently hardly accessible for direct experimental investigation.

The results of CLOUD-ITN will be important for future climate models because cloud formation is one of the largest factors of uncertainty in the prediction of climate change.

Goethe University Frankfurt

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