Clouds reflect sunlight back to space and effectively cool the planet (an important component of modern climate change). Cloud droplets begin as an aerosol particle and grow by the addition of atmospheric water vapour. Particles that grow into clouds are called cloud condensation nuclei, CCN. A particle may ‘activate’ and become a cloud droplet depending on the chemical composition of the CCN.  Natural organic films just one molecule thick at the air-water interface of the CCN will lower the condition for cloud formation (critical supersaturation) favouring its occurrence. Using neutron reflection we can accurately measure the amount of organic material at this interface. We have used stearic acid to represent the organic film, and can replicate the atmospheric oxidation removal process of this film by OH radicals in the cloud water droplet. Unexpectedly the oxidation reaction removes the stearic acid to replace it with new, thinner organic films at the interface. Several oxidation steps are required to completely remove the film and increase the condition for cloud formation. The resistance of the film to chemical oxidation was unexpected and may resolve the difference between field and laboratory studies.

Dr Martin King

CO Lucas (Royal Holloway University of London), AR Rennie (Uppsala University, Sweden), A Hughes (ISIS)

Research date: December 2008