The influence of isoprene peroxy radical isomerization mechanisms on ozone simulation with the presence of NOx

Isoprene peroxy radical isomerizations (1,5- and 1,6-H shifts) have recently been proposed as important pathways regenerating and recycling HO_x (OH?+?HO₂) in the atmosphere under low-NO_x conditions (Peeters et al. Phys. Chem. Chem. Phys. 28: 5935?C5939 2009; da Silva et al. Environ. Sci. Technol. 44:250?C256 2010). Evaluation and comparison of the isoprene peroxy radical isomerization mechanisms from recent studies have been performed against isoprene-NO_x experiments conducted in the UNC dual outdoor smog chambers. Five different kinetic mechanisms were tested in this study, including the original Master Chemical Mechanism (MCM) v3.1; two modified MCM mechanisms both implementing isoprene peroxy radical isomerization reactions but with different rate coefficients; the Carbon Bond 6 (CB6) mechanism; and the ISO-UNC mechanism. Sensitivity analyses of the unsaturated hydroxyperoxy aldehydes (HPALDs) reaction mechanisms under fast isomerization have also been performed. The results indicate that the fast isomerization mechanism and the mechanisms with high OH yields from HPALDs photolysis both significantly enhance HO_x estimates with increasing isoprene/NO_x ratios. However, O₃ predictions, as well the isoprene decay rates are substantially overestimated. Our results suggest that given the current state of our knowledge, it is difficult to improve both HO_x levels and maintain reasonable O₃ simulations using the Peeters et al. (Peeters et al. Phys. Chem. Chem. Phys. 28: 5935?C5939 2009) mechanism.