Direct Observation of Hierarchic Molecular Interactions Critical to Biogenic Aerosol Formation

Abstract

Small clusters consisting of sulfuric acid/bisulfate and oxidized organics have been identified in both aerosol field measurements and laboratory experiments, and their formation is suggested to be the rate-limiting step in the formation of new particles. However, the underlying mechanism for cluster formation is still largely unclear. Here we show, through an integrated negative ion photoelectron spectroscopy and quantum chemical study on a series of (HSO₄⁻)(organic molecule) surrogate binary clusters, that the functional groups are more important in determining the extent of the enhanced role of the organics in aerosol formation process than the average carbon oxidation states or O/C ratios. This extent is quantified explicitly for specific functional groups, revealing highly hierarchic intermolecular interactions critical to aerosol formation. Born–Oppenheimer molecular dynamics simulations are employed to probe the water-binding abilities of these clusters under ambient conditions, and their statistical hydrogen-bonding networks.

Studying the early stages of aerosol formation is a challenge in physical and environmental chemistry. Here photoelectron spectroscopy, quantum chemical calculations, and molecular dynamics simulations quantify how specific functional group interactions stabilize clusters of bisulfate anions and organic molecules.