A research group from the CERN Cloud experiment, including scientists from Carnegie Mellon University's College of Engineering and Mellon College of Science, have uncovered the processes behind the formation and evolution of small atmospheric particles free from the influence of pollution. Their findings are key to creating accurate models to understand and predict global climate change. The findings are published in the May 26 issue of Nature.

Clouds and aerosols-small airborne particles that can become the seeds upon which clouds form-are essential to climate predictions because they reflect sunlight back into space. Reflecting light away from Earth can have a cooling effect, masking some of the warming caused by greenhouse gases.

"The best estimate is that about one-third of the warming by greenhouse gas emissions is masked by this aerosol cooling, but the fraction could be as large as half and as little as almost nothing," says Neil Donahue, professor of chemical engineering, engineering and public policy, and chemistry at Carnegie Mellon.

In order to have complete climate prediction models, scientists need to incorporate clouds and aerosols into their calculations, but understanding how new aerosol particles form and grow in the atmosphere, and how they affect clouds and climate, has been challenging.

Scientists involved with CERN's CLOUD experiment study use a large chamber to simulate the atmosphere and track the formation and growth of aerosol particles and the clouds they seed. The latest research shows that new particles can form exclusively from the oxidation of molecules emitted by trees without the presence of sulfuric acid. Sulfuric acid largely arises from fossil fuels, so the new findings provide a mechanism by which nature produces particles without pollution.

"This softens the idea that there may be many more particles in the atmosphere today due to pollution than there were in 1750, and suggests that the pristine pre-industrial climate may have had whiter clouds than presently thought," says Donahue.

The team's research has lasting climate implications.

"Earth is already more than 0.8C than it was in the pre-industrial epoch, and this is with some masking by aerosol particles. As the pollution subsides, up to another 0.8C of hidden warming could emerge," says Donahue.