Elucidation of the Structures and Formation Mechanism of Dimer Esters in α-Pinene and β-Pinene Secondary Organic Aerosol

CHRISTOPHER KENSETH, Nicholas Hafeman, Samir Rezgui, Yuanlong Huang, Nathan Dalleska, Brian Stoltz, John Seinfeld, California Institute of Technology

     Abstract Number: 385
     Working Group: Aerosol Chemistry

Abstract
Multifunctional dimer esters have been identified using advanced mass spectrometric techniques as significant components of secondary organic aerosol (SOA) formed from oxidation of α-pinene and β-pinene, and have been implicated as key players in new particle formation and growth, particle viscosity, and cloud condensation nuclei (CCN) activity. Particle-phase reactions of closed-shell monomers (e.g., esterification and peroxyhemiacetal/diacyl peroxide decomposition) and gas-phase reactions involving early-stage oxidation products and/or reactive intermediates [e.g., stabilized Criegee intermediates (SCIs), carboxylic acids, and organic peroxy radicals (RO₂)] have been advanced as possible dimer ester formation pathways. Due to a lack of authentic standards, however, structures of the dimer esters are inferred from accurate mass/fragmentation data and, therefore, mechanistic understanding of their formation remains unconstrained. Here, informed by detailed structural analyses (MS and MS/MS, ¹³C and ¹⁸O isotopic labeling, and H/D exchange) from current and past work (Kenseth et al., PNAS, 2018), we synthesize the first authentic standards of the major dimer esters identified in SOA from ozonolysis of α-pinene and β-pinene and elucidate their formation mechanism from a series of targeted environmental chamber experiments using chemical ionization mass spectrometry (CIMS) and liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) for respective analysis of gas- and particle-phase molecular composition. Identification of the chemistry underlying dimer ester production provides a missing link tying the atmospheric degradation of α-pinene and β-pinene to the observed formation of low-volatility compounds capable of driving particle formation and growth.