AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Unambiguous Elucidation of the Structure and Formation Mechanism of Dimer Esters in Monoterpene Secondary Organic Aerosol
CHRISTOPHER KENSETH, Yuanlong Huang, Nicholas Hafeman, Nathan Dalleska, Brian Stoltz, John Seinfeld, California Institute of Technology
Abstract Number: 722 Working Group: Aerosol Chemistry
Abstract High-molecular-weight, low-volatility dimeric compounds have been identified as significant components of both ambient and laboratory-derived monoterpene secondary organic aerosol (SOA), and have been implicated as key players in new particle formation and growth, particle viscosity, and cloud condensation nuclei (CCN) activity. In particular, covalent dimer esters have been routinely detected as major products in monoterpene SOA. 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. However, the exact structures of the dimer esters, and thus the mechanisms underlying their production, remain unresolved, due in large part to the chemical complexity of the SOA matrix and, in turn, the speculative nature of structural assignments inferred solely from accurate mass and fragmentation data. Here, informed by detailed chromatographic and mass spectrometric (MS and MS/MS) analysis, coupled with 13C isotopic labeling, H/D exchange, and OH/SCI scavenging, we explicitly determine, for the first time to our knowledge, the molecular structures and abundances of select dimer esters in SOA derived from ozonolysis of α- and β-pinene through synthesis of authentic standards. Constrained by their molecular structures, the reactive intermediates and mechanism of dimer ester formation are unambiguously elucidated from a series of targeted SOA formation experiments. Identification of the chemistry underlying dimer ester production provides a missing link tying the atmospheric degradation of monoterpenes to the observed formation of low-volatility dimeric compounds capable of driving atmospheric particle formation and growth.