Dr. Urs Baltensperger,
Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen, Switzerland

Physics and chemistry of new particle formation in the atmosphere

"Globally, a significant source of cloud condensation nuclei for cloud formation is thought to originate from new particle formation (aerosol nucleation). Despite extensive research, many questions remain about the dominant nucleation mechanisms. Specifically, a quantitative understanding of the dependence of the nucleation rate on the concentration of the nucleating substances such as gaseous sulfuric acid, ammonia, water vapor and others has not been reached. This is of relevance for climate as the atmospheric concentrations of sulfuric acid, ammonia and other nucleating agents are strongly influenced by anthropogenic emissions. Ions, produced e.g. by galactic cosmic rays, are also known to influence nucleation rates, however their importance is still debated. By providing extremely well controlled and essentially contaminant free conditions in the CLOUD chamber, we were able to show that indeed sulfuric acid is often an important component for such new particle formation, however, for the typical temperatures encountered in the planetary boundary layer the concentrations of sulfuric acid are not high enough to explain the atmospheric observations [1]. Under these conditions, ammonia [1], amines [2] or oxidized organic molecules [3] are needed for nucleation with sulfuric acid to occur. The relevant organic molecules are highly oxygenated molecules (HOMs). HOMs produced by the oxidation of biogenic precursors are able to trigger new particle formation on their own, even in the absence of sulfuric acid [4,5]. We confirmed that this mechanism does occur in today’s lower free troposphere [6]. Moreover, the latest CLOUD results from new particle formation in urban environments, a topic of extensive current research, will be presented."

References: [1] Kirkby, J. et al., Nature, 476, 42-433, 2011 [2] Almeida, J. et al., Nature, 502, 359-363, 2013. [3] Riccobono F. et al., Science, 344, 717-721, 2014. [4] Kirkby, J. et al., Nature, 533, 521-526, 2016. [5] Tröstl, J. et al., Nature, 533, 527-531, 2016. [6] Bianchi, F. et al., Science, 352, 1109-1112, 2016.