Evidence for trans-β-IEPOX as the Main Surface-Active Component of Isoprene-Derived SOA Particles

Secondary organic aerosol (SOA) particle formation ranks among the least understood chemical processes in the atmosphere, due in part to a lack of information regarding chemical composition at the gas/aerosol interface. Reactive uptake of surface-active organic oxidation products of biogenic volatile organic compounds (BVOCs) at the gas/aerosol interface can potentially decrease the aerosol surface tension and therefore influence cloud condensation nuclei (CCN) formation. The combination of surface-selective nonlinear vibrational spectroscopy with organic synthesis allows us to gain molecular-level insight into the structure and orientation of terpenes and their oxidation products, enhancing our understanding of SOA formation and surface composition. We utilize a spectroscopic technique called sum frequency generation (SFG) that uses infrared and visible laser light fields, overlapped spatially and temporally at a surface, to probe vibrational transitions within molecules. Use of this technique allows us to analyze the chemical identity of species at the aerosol surface, where interactions with the gas phase can occur. Specifically, we have focused on the synthesis and spectroscopic analysis of the epoxide (IEPOX) and 2-methyltetraol products derived from isoprene oxidation by hydroxyl radicals. Furthermore, dynamic surface tension measurements of synthesized isoprene-derived SOA particle constituents using pendant drop tensiometry demonstrate that these oxidation products depress the surface tension of aqueous droplets, indicating that these compounds may influence the surface chemistry of aerosol particles and have the potential to influence CCN activity.