A comprehensive performance evaluation of MM5-CMAQ for the summer 1999 southern oxidants study episode, Part III: Diagnostic and mechanistic evaluations

As Part III of a comprehensive evaluation of CMAQ for the summer 1999 Southern Oxidants Study episode, the observed number ( N), volume ( V), surface area ( S), and size distributions of accumulation-mode particles during the Aerosol Research Inhalation Epidemiological Study (ARIES) are used to evaluate CMAQ's capability in reproducing PM size distributions. CMAQ underpredicts V, S, and geometric number mean diameter (by a factor of 1.24–1.54), and overpredicts N, geometric standard deviation, and geometric volume mean diameter (by a factor of 1.46–2.2) on most days. In addition to inaccurate meteorology and emissions, insufficient condensational growth of PM and uncertainty in the initial size distribution may contribute to the underpredictions in V and S. An overestimation of the PM number emission rates (by a factor of 3–5.3) and several other model assumptions/treatments may contribute to the PM number overpredictions. Among the factors that we studied, the floor value of K zz , the boundary conditions (BCONs) of O 3, the emissions of gaseous precursors such as NO x and NH 3 and primary PM species such as POM, and the assumed initial PM size distribution and emission fractions have been identified to be the most influential factors that affect the overall model performance. Sensitivity simulations with a floor value of K zz of 0.1 cm 2 s −1, adjusted emissions of NO x , NH 3, and POM, and adjusted initial PM size distribution and emission fractions provide moderate-to-significant improvements. Further investigation into the uncertainties/deficiencies in model treatments for PM such as gas-to-particle mass transfer will identify additional causes for discrepancies between observations and predictions.