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

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= 298K were calculated to be k(3a)(infinity)= 1.5x 10(-3)s(-1) (1,5-H-shift of the 1,2-isomer) and k(4a)(infinity)= 6.5s(-1) (1,6-H-shift of the (Z)-1,4-isomer). The calculated value of k(4a)(infinity) 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.