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CLOUD1 experiment at CERN2 has reported its first results. The CLOUD experiment has been designed to study the effect of cosmic rays on the formation of atmospheric aerosols - tiny liquid or solid particles suspended in the atmosphere - under controlled laboratory conditions. Atmospheric aerosols are thought to be responsible for a large fraction of the seeds that form cloud droplets. Understanding the process of aerosol formation is therefore important for understanding the climate.

The CLOUD results show that trace vapours assumed until now to account for aerosol formation in the lower atmosphere can explain only a tiny fraction of the observed atmospheric aerosol production. The results also show that ionisation from cosmic rays significantly enhances aerosol formation. Precise measurements such as these are important in achieving a quantitative understanding of cloud formation, and will contribute to a better assessment of the effects of clouds in climate models.

“These new results from CLOUD are important because we’ve made a number of first observations of some very important atmospheric processes,” said the experiment’s spokesperson, Jasper Kirkby. “We’ve found that cosmic rays significantly enhance the formation of aerosol particles in the mid troposphere and above. These aerosols can eventually grow into the seeds for clouds. However, we’ve found that the vapours previously thought to account for all aerosol formation in the lower atmosphere can only account for a small fraction of the observations - even with the enhancement of cosmic rays.”