Evaporation and condensation kinetics of ice clarify confusing observations.

The tragic events at the World Trade Center, New York, US, on 11 September 2001 caused all air traffic to be grounded for three days. This time gave atmospheric chemists a unique opportunity to find out whether contrails from aeroplanes had a real impact on climate. Following on from this research, Swiss chemists have done an in-depth study looking at ice formation in clouds and in aeroplane contrails.

Michel Rossi and Christophe Delval at the Swiss Federal Institute of Technology, Lausanne, were puzzled by observations that in cirrus clouds and aviation contrails, ice particles lived longer than predicted. By taking a close look at the kinetics that govern ice formation in clouds, Rossi and Delval tried to solve the puzzle. They realised they needed to look at both evaporation and condensation under the same experimental conditions, and used well established techniques to do just that. The quartz crystal microbalance technique they used to measure mass accurately hasn't been used for ice before, but coupled with other techniques, provides real kinetic data to show that the evaporation of ice is faster than previously thought, hence explaining the observations from field measurements.

Rossi explains that 'water is the most important greenhouse gas in the terrestrial atmosphere.our contribution addresses the kinetics of condensational growth of ice particles at atmospheric temperatures'. He adds that 'of equal importance, the work obtains the evaporation lifetimes of ice particles that is found to be longer than expected by a factor of five to 10'. The results will be important for future models of global atmospheric changes and the chemistry going on in ice particles in the atmosphere.

Katharine Sanderson