Structures and Formation Mechanisms of Molecular Products in Pinene Secondary Organic Aerosol

CHRISTOPHER KENSETH, Samir Rezgui, Jing Chen, Nathan Dalleska, Henrik Kjaergaard, Brian Stoltz, Joel A. Thornton, Paul Wennberg, John Seinfeld, California Institute of Technology

     Abstract Number: 486
     Working Group: Aerosol Chemistry

Abstract
For decades, advanced mass spectrometric techniques have been employed to characterize the molecular composition of secondary organic aerosol (SOA) formed from the oxidation of α-pinene and β-pinene. Due to a lack of authentic standards, however, the structures of identified molecular products are typically not known but only inferred from accurate mass/fragmentation data. As a result, mechanistic understanding of their formation and evolution remains unconstrained. Here, we unambiguously determine the structures of the most abundant monomeric and dimeric products identified using liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) in SOA from ozonolysis of α-pinene and β-pinene through independent synthesis of authentic standards. Based on targeted environmental chamber experiments featuring ²H, ¹³C, and ¹⁸O isotopic labeling together with chemical ionization mass spectrometry (CIMS) and LC/ESI-MS for analysis of gas/particle-phase molecular composition, we elucidate their gas- and/or particle-phase formation mechanisms. Determination of the structures and formation mechanisms of these key pinene SOA constituents resolves a longstanding puzzle in atmospheric aerosol chemistry and rationalizes a number of notable experimental and ambient observations from past studies.