Impact of model resolution on chemical ozone formation in Mexico City; application of the WRF-Chem model

Abstract. The resolution of regional chemical/dynamical models has important effects on the calculation of distributions of air pollutants in large cities. In this study, the sensitivity of air pollutants and photochemical O3 production to different model resolutions is studied by using a regional chemical/dynamical model (version 3 of Weather Research and Forecasting Chemical model – WRF-Chemv3) in Mexico City. The model results with 3, 6, 12, and 24 km resolutions are compared to the surface measurements for CO, NOx, and O3. The study shows that the model resolutions with 3 and 6 km have reasonable simulations of surface CO, NOx, and O3 concentrations and diurnal variations. The model results intend to underestimate the measurements when the resolution is reduced to 12 km. The calculated surface CO, NOx, and O3 concentrations significantly underestimate the measured values at 24 km resolution. This study suggests that 12 km is a threshold resolution for the O3 and O3 precursor calculations for using a regional chemical/dynamical model in Mexico City. There are three major factors related to the effects of model resolution on the calculations of O3 and O3 precursors, including; (1) the calculated meteorological conditions with different model resolutions, (2) the emission spatial distribution of ozone precursors, and (3) the non-linearly O3 photochemical productions with different resolutions. Model studies suggest that model resolution (resulting in different meteorological condition and transport process) have larger impacts than emission inventory resolutions for the calculations of O3 and O3 precursors. The model calculations show that with coarse resolution of emission inventory (24 km) and fine meteorological condition resolution (6 km), the calculated CO and O3 are considerably improved compared to the calculation with coarse resolution for both emission inventory and meteorological condition (24 km), suggesting that the impacts of resolution on meteorological condition and transport process are largest for the calculations of O3 and O3 precursors. The emission resolution has important effects on the calculation, but the effects are smaller than the model resolution. This study also suggests that the changes of O3 precursors at different resolutions lead to important impacts on O3 chemical formation due to the non-linear relationship between O3 formation and O3 precursors. Finally, this study suggests that with the balance between the model performance and required computation time, the 6 km resolution is an optimal resolution for the calculation of O3 and O3 precursors in Mexico City.