Evaluation of the Community Multiscale Air Quality (CMAQ) model version 4.5: Sensitivities impacting model performance Part I - Ozone

This study examines ozone (O-3) predictions from the Community Multiscale Air Quality (CMAQ) model version 4.5 and discusses potential factors influencing the model results. Daily maximum 8-h average O-3 levels are largely underpredicted when observed O-3 levels are above 85ppb and overpredicted when they are below 35ppb. Using a clustering approach, model performance was examined separately for several different synoptic regimes. Under the most common synoptic conditions of a typical summertime Bermuda High setup, the model showed good overall performance for O-3, while associations have been identified here between other, less frequent, synoptic regimes and the O-3 overprediction and underprediction biases. A sensitivity test between the CB-1V and CB05 chemical mechanisms showed that predictions of daily maximum 8-h average O-3 using CB05 were on average 7.3% higher than those using CB-IV. Boundary condition (BC) sensitivity tests show that the overprediction biases at low O-3 levels are more sensitive to the BC 03 levels near the surface than BC concentrations aloft. These sensitivity tests also show the model performance for O-3 improved when using the global GEOS-CHEM BCs instead of default profiles. Simulations using the newest version of the CMAQ model (v4.6) showed a small improvement in O-3 predictions, particularly when vertical layers were not collapsed. Collectively, the results suggest that key synoptic weather patterns play a leading role in the prediction biases, and more detailed study of these episodes are needed to identify further modeling improvements. Published by Elsevier Ltd.