10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Constraining the Abundance of Oligomers in Monoterpene Secondary Organic Aerosol
CHRISTOPHER KENSETH, Yuanlong Huang, Ran Zhao, Nathan Dalleska, Caleb Hethcox, Brian Stoltz, John Seinfeld, California Institute of Technology
Abstract Number: 1627 Working Group: Aerosol Chemistry
Abstract High molecular weight oligomeric 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. Electrospray ionization mass spectrometry (ESI-MS), typically coupled with liquid chromatographic (LC) separation, is among the most widely used analytical techniques for characterizing the identity and abundance of SOA molecular constituents. Due to a lack of authentic standards, oligomers in monoterpene SOA are typically quantified using commercially available terpenoic acids as surrogates (e.g., cis-pinonic acid and cis-pinic acid). However, due to fundamental differences in the ionizing behavior of monomers and oligomers, namely that oligomers produce ions with more delocalized charge, we have found through a combination of experimental and computational approaches that the sensitivity of ESI-MS toward oligomers is approximately two to ten times higher than that toward monomers, suggesting that the mass contribution of oligomers to monoterpene SOA has been significantly overestimated in past studies. In this work, we synthesize a series of monoterpene-derived oligomeric compounds and characterize their ESI efficiencies both empirically and theoretically. Using these oligomers as more representative surrogates, we constrain the abundance of oligomeric products in SOA derived from the O3-initiated oxidation of α- and β-pinene, the most abundant monoterpenes in the atmosphere (total global emissions estimated at 85 Tg yr-1). This critical analytical aspect of SOA formation experiments constitutes a potentially significant source of systematic bias in chamber-derived parameterizations of SOA production from monoterpene oxidation.