Experiment provides unprecedented insight into cloud formation

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."

Atmospheric aerosols play an important role in the climate. Aerosols reflect sunlight and produce cloud droplets. Additional aerosols would therefore brighten clouds and extend their lifetime. By current estimates, about half of all cloud droplets begin with the clustering of molecules that are present in the atmosphere only in minute amounts. Some of these embryonic clusters eventually grow large enough to become the seeds for cloud droplets. Trace sulphuric acid and ammonia vapours are thought to be important, and are used in all atmospheric models, but the mechanism and rate by which they form clusters together with water molecules have remained poorly understood until now.

The CLOUD results show that a few kilometres up in the atmosphere sulphuric acid and water vapour can rapidly form clusters, and that cosmic rays enhance the formation rate by up to ten-fold or more. However, in the lowest layer of the atmosphere, within about a kilometre of Earth's surface, the CLOUD results show that additional vapours such as ammonia are required. Crucially, however, the CLOUD results show that sulphuric acid, water and ammonia alone -- even with the enhancement of cosmic rays -- are not sufficient to explain atmospheric observations of aerosol formation. Additional vapours must therefore be involved, and finding out their identity will be the next step for CLOUD.

"It was a big surprise to find that aerosol formation in the lower atmosphere isn't due to sulphuric acid, water and ammonia alone," said Kirkby. "Now it's vitally important to discover which additional vapours are involved, whether they are largely natural or of human origin, and how they influence clouds. This will be our next job."

The CLOUD experiment consists of a state-of-the-art chamber in which atmospheric conditions can be simulated with high control and precision, including the concentrations of trace vapours that drive aerosol formation. A beam of particles from CERN's Proton Synchrotron accelerator provides an artificial and adjustable source of cosmic radiation.