The Cosmics Leaving OUtdoor Droplets (CLOUD) experiment at CERN was conceived to examine the influence of ionisation from galactic cosmic rays (GCRs) on aerosols and clouds as a potential mechanism to explain solar-climate variability. The essential concept is to reproduce selected and well-controlled atmospheric conditions inside a large chamber and to simulate cosmic rays with an ionising particle beam from CERN's Proton Synchrotron. The aerosol processes studied in CLOUD are poorly known yet climatically important because they create the seeds for more than 50% of global cloud droplets. Anthropogenic increases of aerosols and clouds are considered by the Intergovernmental Panel on Climate Change (IPCC) to be the largest source of uncertainty limiting the ability of climate models to make precise climate projections for the 21st century. Aerosol particle formation is also responsible for urban smog, which ranks fifth in risk factors for mortality worldwide. Consequently, since starting to take data in 2009, CLOUD has broadened its scientific objectives to include improved understandings of anthropogenic aerosol radiative forcing and of urban smog. Following a series of high-impact publications, CLOUD is now widely recognised as the world’s leading laboratory experiment for atmospheric aerosol formation. Expertise from the EN and EP Departments and elsewhere at CERN has been - and continues to be - crucial for the unprecedented technical performance of CLOUD. This talk will present an overview of the technical challenges of CLOUD and some of the scientific advances from the experiment so far, including the recent discovery of a key mechanism that can explain winter urban smog formation.
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