Implementation and initial application of the near-explicit Master Chemical Mechanism in the 3D Community Multiscale Air Quality (CMAQ) model

A modified Master Chemical Mechanism (MCM) with 4642 species and 13,566 reactions was incorporated into the 3D Community Multiscale Air Quality (CMAQ) model (CMAQ–MCM) and applied to study a three-week high ozone episode in Southeast Texas during the Texas Air Quality Study 2000 (TexAQS 2000). The 3D CMAQ–MCM model successfully reproduced the observed ozone concentrations throughout the Southeast Texas region, with model performance similar to that of the standard SAPRC07 mechanism. The CMAQ–MCM has been used to evaluate the VOC emission inventory by directly simulating the concentrations of a large number of VOCs. The simulated 1-h and 24-h average concentrations of major ozone precursor VOCs show that most of the alkanes and alkenes are universally lower than the observed values by a factor of 2–5. The under-prediction of ethane and propane is more significant, by a factor of approximately 5–10. Major aromatic compounds generally agree better with observation within a factor of 2. At the Clinton Drive site, most of the under-predictions occur in the afternoon when industrial facilities are in the immediate upwind direction. The observed concentrations of a number of the under-predicted species show strong correlations with temperature, suggesting that evaporative emissions are underestimated in the emission inventory.