How iodine-containing molecules contribute to the formation of atmospheric aerosols, affect climate

"Essentially all uncertainty around climate change and the atmosphere has something to do with particles and cloud droplets," said Neil Donahue, Thomas Lord University Professor of Chemistry and a professor in the departments of Chemical Engineering, and Engineering and Public Policy. The Donahue lab has been a longtime member of the CERN CLOUD experiment, an international collaboration of scientists that use a special chamber at CERN in Switzerland to remedy that uncertainty by studying how cosmic rays affect the formation of particles and clouds in the atmosphere. The chamber allows researchers to precisely mix vaporous compounds and observe how particles form and grow from them.

In a study published today in the journal Science, the CLOUD collaboration looked in particular at how vapors containing iodine affect this nucleation process. For reasons not yet fully understood, Donahue said, concentrations of iodine-containing vapor compounds have been increasing in recent years in the atmosphere.

"This represents a new pathway for particle formation, which in turn governs the properties of clouds in the marine atmosphere," Donahue said.

Building on previous research his lab conducted on discovering a new rapid mechanism for atmospheric particle formation from nitric acid and ammonia vapors, Donahue and his team have now helped the CLOUD collaboration discover that the nucleation rates of iodic acid particles are very fast. This means that increasing concentrations of iodine-containing vapors in the atmosphere can lead to large increases in the number of particles that form clouds.

Specifically, Donahue and his collaborators, including current Ph.D. candidates Mingyi Wang and Victoria Hofbauer, alumna Qing Ye and former postdoctoral scholar Dexian Chen, contributed their use of a state-of-the-art chemical ionization mass spectrometer that can measure the amount and composition of extremely small particles less than 10 nanometers in size just following their formation.

"The CMU measurements showed that the newly formed particles are composed largely of iodic acid, confirming that this critical molecule not only is present as a vapor while particles are forming but definitively drives their growth," Donahue said.

While clouds forming may sound like a relatively benign outcome, clouds play an important role in regulating Earth's temperature because they are highly reflective. Much of the sun's energy is reflected by clouds back into space, keeping Earth from becoming too hot. However, that reflectivity can work both ways, which is a particular problem at Earth's poles. Typically, the white snow and ice surfaces reflect a lot of sunlight back into space, thus keeping the surface there cool. However, increased cloud formation in those regions can mean that the light reflected off the surface can be reflected back onto the ice and snow by the cloud cover.

"The Arctic is an especially vulnerable region, with twice the rate of warming and the huge consequences of both sea ice and ice sheet melting," Donahue said. He and his lab are already planning future research into the complex feedbacks between iodic acid and sulfur compounds and how these affect the polar atmosphere and climate change.

"We have a great deal more to learn in this area, especially regarding the interactions of the iodine compounds and particles, and dimethyl sulfide oxidation and its particle formation," Donahue said.