Recent laboratory and field experiments indicate that the autoxidation of volatile organics play a significant role in the formation and growth of secondary organic aerosol particles in the atmosphere. This autoxidation process is initiated by an oxidant such as OH or O3, and propagates by hydrogen shift reactions of peroxy radical intermediates followed by subsequent oxygen addition, as illustrated in the figure. We have used an experimentally verified theoretical approach based on Multi-Conformer Transition State Theory (MC-TST), including Eckart tunneling, to calculate temperature dependent rate coeffcients of peroxy radical H-shift reactions. We find these calculated rate coeficients to be in good agreement with the few available experimental values. Based on our theoretical results, we find that substitution at the hydrogen abstraction site, with hydroxy, hydroperoxy, or carbonyl groups leads to increases in the rate coefficient by factors of up to ~1000. In addition, reactions leading to secondary carbon radicals (alkyl substituent) are 100 times faster than those leading to primary carbon radicals. When the ring size in the transition state is 6, 7 or 8 atoms (1,5; 1,6 or 1,7 H-shift), the H-shift reaction rate constants can reach 1 s-1. Furthermore, we find that the chirality of the reactant is crucial to consider as diastereomers can have rate coefficients that differ by up to almost three orders of magnitude. Implementation of calculated peroxy radical H-shift reaction rate coefficients into the most recent GEOS-Chem model for isoprene oxidation shows that at least 30 % of all isoprene molecules emitted to the atmosphere undergo a minimum of one peroxy radical hydrogen shift reaction, highlighting the importance of this reaction class in atmospheric oxidation. Thus H-shift reactions in peroxy radicals are likely much more prevalent in the atmosphere than previously considered.

Atmospheric autoxidation via fast peroxy radical hydrogen shift reactions

2019 Fall National Meeting

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