In an open access paper published in the journal Science today, 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, 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.Jasper Kirkby, CLOUD experiment

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. The experiment has several unique aspects, including the ability to control the “cosmic ray” beam intensity from the CERN PS, the capability to suppress ions completely by means of a strong electric clearing-field, precise adjustment of "sunlight" from a UV fibre-optic system, and highly-stable operation at any temperature in the atmosphere.

Sulphuric acid is thought to play a key role, but previous CLOUD experiments have shown that, on its own, sulphuric acid has a much smaller effect than had been assumed. Sulphuric acid in the atmosphere originates from sulphur dioxide, for which fossil fuels are the predominant source. The new result shows that oxidised biogenic vapours derived from alpha-pinene emitted by trees rapidly form new particles with sulphuric acid. Ions produced in the atmosphere by galactic cosmic rays are found to enhance the formation rate of these particles significantly, but only when the concentrations of sulphuric acid and oxidised organic vapours are relatively low. The CLOUD paper includes global modelling studies which show how this new process can account for the observed seasonal variations in atmospheric aerosol particles, which result from higher global tree emissions in the northern hemisphere summer.

"The reason why it has taken so long to understand the vapours responsible for new particle formation in the atmosphere is that they are present in minute amounts near one molecule per trillion air molecules", explains Jasper Kirkby. "Reaching this level of cleanliness and control in a laboratory experiment is at the limit of current technology, and CERN know-how has been crucial for CLOUD being the first experiment to achieve this performance."

Biogenic vapours join another class of trace vapours, known as amines, that have previously been shown by CLOUD to cluster with sulphuric acid to produce new aerosol particles in the atmosphere. Amines, however, are only found close to their primary sources such as animal husbandry, whereas alpha-pinene is ubiquitous over landmasses. This latest result from CLOUD could therefore explain a large fraction of the birth of cloud seeds in the lower atmosphere around the world. It shows that sulphuric acid aerosols do indeed have a significant influence on the formation of clouds, but they need the help of trees.