Experiment sheds new light on cloud formation

CERN 's CLOUD experiment has shown that biogenic vapours emitted by trees and oxidised in the atmosphere have a significant impact on the formation of clouds, thus helping to cool the planet. These biogenic aerosols are what give forests seen from afar their characteristic blue haze. The CLOUD study shows that the oxidised biogenic vapours bind with sulphuric acid to form embryonic particles which can then grow to become the seeds on which cloud droplets can form. This result follows previous measurements from CLOUD showing that sulphuric acid alone could not form new particles in the atmosphere as had been previously assumed. "This is a very important result," said CLOUD spokesperson Jasper Kirkby, "since it identifies a key ingredient responsible for formation of new aerosol particles over a large part of the atmosphere - and aerosols and their impact on clouds have been identified by the Intergovernmental Panel on Climate Change as the largest source of uncertainty in current climate models." Cloud droplets form on aerosol particles that can either be directly emitted, such as evaporated sea spray, or else form through a process known as nucleation, in which trace atmospheric vapours cluster together to form new particles that may grow to become cloud seeds. Around half of all cloud seeds are thought to originate from nucleated particles, but the process of nucleation is poorly understood. The CLOUD chamber has achieved much lower concentrations of contaminants than previous experiments, allowing nucleation to be measured in the laboratory under precisely controlled atmospheric conditions.