CERN clears the sky on CLOUD formation

CERN : How do clouds form? The answer has implications for our understanding of climate change, because clouds can reflect the Sun’s radiation back toward space, thus reducing the amount of heat that reaches Earth.Providing unprecedented insight into cloud formation is the Cosmics Leaving OUtdoor Droplets (CLOUD) experiment at the CERN research center in Switzerland. Researchers there have been studying the effects of cosmic rays on the formation of atmospheric aerosols—tiny particles suspended in the atmosphere—thought to provide the seeds that form cloud droplets.By current estimates, about half of all cloud droplets begin with aerosols. Trace sulfuric acid and ammonia vapors are used in all of CLOUD’s atmospheric models as the genesis for droplet production, but the mechanism and rate by which they form clusters together with water molecules have remained poorly understood.The CLOUD experiment simulates atmospheric conditions inside a chamber and uses CERN’s Proton Synchrotron accelerator to provide an artificial and adjustable source of cosmic radiation. Results confirm that a few kilometers up in the atmosphere sulfuric acid and water vapor can rapidly form clusters, and that cosmic rays enhance the formation rate by up to 10-fold or more.However, in the lowest layer of the atmosphere, within about a kilometer of Earth’s surface, the CLOUD results show that additional vapors such as ammonia are required to generate the aerosols.According to Jasper Kirkby, the lead CLOUD spokesperson, the group’s results, which were published in Nature , show that these trace vapors, which were assumed to account for aerosol formation, can explain only a small fraction of the observed atmospheric aerosol production."It was a big surprise to find that aerosol formation in the lower atmosphere isn’t due to sulfuric acid, water, and ammonia alone,” said Kirkby. “Now it’s vitally important to discover which additional vapors are involved, whether they are largely natural or of human origin, and how they influence clouds. This will be our next job.”