The Importance of Peroxy Radical Hydrogen-Shift Reactions in Atmospheric Isoprene Oxidation

With an annual emission of about 500 Tg, isoprene is an important molecule in the atmosphere. While much of its chemistry is well constrained by either experiment or theory, the rates of many of the unimolecular peroxy radical hydrogen-shift (H-shift) reactions remain speculative. Using a high-level multiconformer transition state theory (MC-TST) approach, we determine recommended temperature dependent reaction rate coefficients for a number of the H-shift reactions in the isoprene oxidation mechanism. We find that most of the (1,4, 1,5, and 1,6) aldehydic and (1,5 and 1,6) α-hydroxy H-shifts have rate constants at 298.15 K in the range 10^-2 to 1 s^-1, which make them competitive with bimolecular reactions in the atmosphere under typical atmospheric conditions. In addition, we find that the rate coefficients of different diastereomers can differ by up to 3 orders of magnitude, illustrating the importance of chirality. Implementation of our calculated 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 during their complete oxidation to CO₂ and deposited species. This highlights the importance of peroxy radical H-shifts reactions in atmospheric oxidation.