Particles in the atmosphere are the seeds upon which clouds form. Variations in the quantity of these particles affect climate and precipitation patterns. How these aerosol particles originate has not been fully understood because of insufficient data upon which to build mathematical models. Using laboratory measurements and model simulations, an international team has now established the main mechanisms causing the formation of particles that eventually become cloud droplets. The results were published on Oct. 27, 2016 in Science.  

Among the members of the international team is Athanasios Nenes, a professor in the Schools of Earth and Atmospheric Sciences and of Chemical and Biomolecular Engineering at Georgia Tech. “The formation of new particles is important for climate,” Nenes says. “By current estimates, about half of droplets worldwide are formed on aerosol particles that were created in this way.”

The roles of specific molecules in the early stage of cloud formation—called nucleation—is one of the key findings of the work, Nenes says. Cloud-originating particles are known to form from the clustering of molecules of sulfuric acid, organic compounds, ammonia, and water. But it was unclear which molecules are most important. For many years, sulfuric acid was recognized as a key contributor to nucleation.

“The new study confirms the importance of sulfuric acid,” Nenes says. “But sulfuric acid alone cannot lead to cloud formation. The other molecules have to be present in enough concentrations to get cloud formation going.”

Nenes’s research at Georgia Tech aims to understand the impacts of particles on clouds and climate through a combination of instrument development, field and laboratory observations, and modeling. The Nenes Lab provided the international team with computational tools to link simulations of new particle formation with clouds and climate—an area for which the lab is well-known. This linkage would enable simulation of the climate response to new particles formed throughout the atmosphere.

“Our involvement helps quantify the climate relevance of the most uncertain and elusive processes that contribute to the concentrations of atmospheric particulates,” Nenes says. “In a future where emissions are expected to considerably change in magnitude and nature, there is even greater need to understand new particle formation and how it would change over time and affect clouds and climate.”

The new findings open new areas of inquiry. Nenes says he is interested in studying the impacts of ammonia and organic compounds on future aerosol levels. “Intensification of agriculture to meet global food supply demands, together with reductions in emissions of sulfur, can have a profound impact on rates of particle formation,” he says. “What these changes mean for future levels of atmospheric aerosol—and the susceptibility of clouds and climate to them—is a problem that is more easily addressed because of the results of this new study.”