Atmospheric autoxidation via fast peroxy radical hydrogen shift reactions

Abstract

Recent lab. and field expts. indicate that the autoxidn. of volatile orgs. play a significant role in the formation and growth of secondary org. aerosol particles in the atm. This autoxidn. 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 addn., as illustrated in the figure. We have used an exptl. verified theor. approach based on Multi-Conformer Transition State Theory (MC-TST), including Eckart tunneling, to calc. temp. dependent rate coeffcients of peroxy radical H-shift reactions. We find these calcd. rate coeficients to be in good agreement with the few available exptl. values. Based on our theor. results, we find that substitution at the hydrogen abstraction site, with hydroxy, hydroperoxy, or carbonyl groups leads to increases in the rate coeff. by factors of up to ~1000. In addn., 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 consts. can reach 1 s⁻¹. Furthermore, we find that the chirality of the reactant is crucial to consider as diastereomers can have rate coeffs. that differ by up to almost three orders of magnitude. Implementation of calcd. peroxy radical H-shift reaction rate coeffs. into the most recent GEOS-Chem model for isoprene oxidn. shows that at least 30 % of all isoprene mols. emitted to the atm. undergo a min. of one peroxy radical hydrogen shift reaction, highlighting the importance of this reaction class in atm. oxidn. Thus H-shift reactions in peroxy radicals are likely much more prevalent in the atm. than previously considered.

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