PNNL

Abstract

Isotopologues of a-pinene were synthesized to probe the importance of the vinyl, bridgehead, methylene bridge, and the majority of the allylic methylene positions in the surface chemical composition of secondary organic aerosol material (SOM) derived from oxidation of a-pinene by ozone. Vibrational sum frequency generation (SFG) spectra of the ring-substituted isotopologues revealed up to 80% suppression of the SFG signal intensity positioned around 2930 cm-1 in the broad band that is emblematic of unlabeled ?-pinene. SFG spectra of SOM prepared from the ozonolysis of two isotopologues featuring a deuterated methylene bridge group show up to 40% less signal intensity than SOM prepared from methyl-substituted isotopologues, suggesting the presence of the dimethylcyclobutane ring at the SOM surface. The synthesis of isotopically labelled SOM and analysis of their surface-active constituents opens the door to studying the dynamics of gas-phase volatile organic compound (VOC) adsorption and reactivity at varying relative humidity, temperature, and consequently particle viscosity in a way that distinguishes the unlabeled gas-phase molecules from those in the particle phase. More broadly, expanding the range of isotopologues of terpene hydrocarbons that are not commercially available offers the atmospheric community new capabilities to elucidate mechanistic questions in aerosol formation and growth.