American Association for Aerosol Research - Abstract Submission

AAAR 38th Annual Conference
October 5 - October 9, 2020

Virtual Conference

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Ozonolysis of Unsaturated Products from Toluene Photo-oxidation Can Form Highly Oxygenated Organic Molecules (HOM)

OLGA GARMASH, Matthieu Riva, Matti P. Rissanen, Pekka Rantala, Otso Peräkylä, Liine Heikkinen, Yanjun Zhang, Mikael Ehn, University of Helsinki

     Abstract Number: 618
     Working Group: Aerosol Chemistry

Abstract
Recent studies have shown that highly oxygenated organic molecules (HOM) contribute to atmospheric particle formation and growth. HOM are formed in the gas phase through a chemical process called autoxidation and involve peroxy radicals generated from the oxidation of volatile organic compounds (VOCs). The highest HOM molar yields have been observed in the ozonolysis of monoterpenes that contain endocyclic double bond, such as α-pinene and limonene. In our initial study, we demonstrated that OH-initiated oxidation of aromatic VOCs yields HOM. More specifically, multiple OH oxidation steps of benzene were responsible for the significant increase in the observed HOM molar yields.

In this study, we explored the oxidation of toluene in presence of OH radicals and ozone (O3) in a 2m3 PTFE chamber for its potential to form HOM through “secondary” ozonolysis. We hypothesize that the initial OH attack forms non-aromatic, unsaturated ring products that can further react with O3 and lead to larger HOM formation. While typically reaction with O3 is orders of magnitude slower than with OH radicals, atmospheric concentration of O3 is significantly larger.
Using state-of-the art instrumentation, we observed an increase of HOM yield (in respect to consumed toluene by OH) when O3 rised from 40 to 80ppb while OH radical concentration was maintained constant. Only C7-monomer and C14-dimeric-HOM were considered. Using simple calculations, we simulated the first and second generation of HOM produced in the chamber and we were able to reveal that observed HOM yield cannot be fully explained by typical first-generation oxidation products. In sum, our results contribute to understanding HOM and secondary organic aerosol formation from the oxidation of aromatic species. Finally, this study highlights the role of O3 in the gas-phase oxidation of aromatics, which has been widely dismissed in the literature.