Consecutive Chemical Activation Steps in the OH‐Initiated Atmospheric Degradation of Isoprene: An Analysis with Coupled Master Equations

Abstract

ABSTRACT The influence of a two‐step chemical activation on 1,5‐H and 1,6‐H shift reactions of hydroxyl‐peroxy radicals formed in the atmospheric photooxidation of isoprene was investigated by means of a master equation analysis. To account for multiple chemical activation processes, three master equations were coupled. The general approach of this coupling is described, and consequences for steady‐state regimes are examined. The specific calculations show that chemical activation has no substantial influence on the rate coefficients of the above‐mentioned reactions under tropospheric conditions. However, it is demonstrated that high‐pressure limits of the thermal rate coefficients instead of the falloff‐corrected values have to be used for kinetic modeling. This is a consequence of the continuous population of the high‐energy part of the isoprene‐OH‐O 2 adduct distribution by the forming reactions under steady‐state conditions. The rate coefficients of the isomerization reactions at T = 298 K were calculated to be k 3a ∞ = 1.5 × 10 −3 s −1 (1,5‐H‐shift of the 1,2‐isomer) and k 4a ∞ = 6.5 s −1 (1,6‐H‐shift of the ( Z )‐1,4‐isomer). The calculated value of k 4a ∞ is three orders of magnitude larger than a recently reported experimentally observed rate coefficient for the hydrogen shift reactions of the hydroxyl‐peroxy intermediates. It is shown that this discrepancy is in part due to the fact that the experiment does not distinguish between different structural isomers. A comparison of the experimentally determined isotope effect with the calculated value shows a reasonable agreement.