Comparison of surface ozone simulation among selected regional models in MICS-Asia III - Effects of chemistry and vertical transport for the causes of difference

Abstract

In order to clarify the causes of variability among the model outputs for surface ozone in the Model Intercomparison Study Asia Phase III (MICS-Asia III), three regional models, CMAQ v.5.0.2, CMAQ v.4.7.1, and NAQPMS (abbreviated as NAQM in this paper), have been selected. Detailed analyses of monthly averaged diurnal variation have been performed for selected grids covering the metropolitan areas of Beijing and Tokyo and at a remote oceanic site, Oki. The chemical reaction mechanism, SAPRC99, used in the CMAQ models tended to give a higher net chemical ozone production than CBM-Z used in NAQM, agreeing with previous studies. Inclusion of the heterogeneous "renoxification" reaction of HNO 3 (on soot surface)→NO+NO 2 only in NAQM would give a higher NO concentration resulting in a better agreement with observational data for NO and nighttime O 3 mixing ratios. In addition to chemical processes, the difference in the vertical transport of O 3 was found to affect the simulated results significantly. Particularly, the increase in downward O 3 flux from the upper layer to the surface after dawn was found to be substantially different among the models. Larger early morning vertical transport of O 3 simulated by CMAQ 5.0.2 is thought to be the reason for higher daytime O 3 in July in this model. All three models overestimated the daytime ozone by ca. 20 ppbv at the remote site Oki in July, where in situ photochemical activity is minimal.