Development and evaluation of a photooxidation mechanism for isoprene

A detailed mechanism for isoprene photooxidation is developed that includes the recent developments on each of isoprene's atmospherically important reactions: O₃, OH, O(³P), and NO₃ as well as the OH reactions of methacrolein and methyl vinyl ketone. The mechanism also attempts to account for the gaps in the product distributions for a few reactions. The CO yield from isoprene oxidation in the presence of NOₓ, more than about 50 ppt, is calculated as about 3.4. The mechanism is tested against chamber data that include a range of mixtures of the primary oxidants, isoprene concentrations, and hydrocarbon/NOₓ ratios. It performs well under conditions where the OH and O(³P) reactions dominate and predicts peak O₃ concentrations within about 25% under all conditions. The mechanism underpredicts methyl vinyl ketone yields as well as O₃ levels under conditions of low available NOₓ. Sensitivity of the mechanism to assumptions about the NO₃ and O₃ reactions are examined, and possible sources of the discrepancies are discussed. A condensed mechanism is developed and compared to the explicit mechanism.