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Impact of model resolution on chemical ozone formation in Mexico City: Application of the WRF-Chem model

by X. Tie, G. Brasseur, Z. Ying
Atmospheric Chemistry and Physics ()
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The resolution of regional chemical/dynamical models has important effects on the calculation of the distri-butions of air pollutants in urban areas. In this study, the sen-sitivity of air pollutants and photochemical ozone production to different model resolutions is assessed by applying a re-gional chemical/dynamical model (version 3 of Weather Re-search and Forecasting Chemical model – WRF-Chemv3) to the case of Mexico City. The model results with 3, 6, 12, and 24 km resolutions are compared to local surface measure-ments of CO, NO x , and O 3 . The study shows that the model resolutions of 3 and 6 km provide reasonable simulations of surface CO, NO x , and O 3 concentrations and of diurnal vari-ations. The model tends to underestimate the measurements when the resolution is reduced to 12 km or less. The cal-culated surface CO, NO x , and O 3 concentrations at 24 km resolution are significantly lower than measured values. This study suggests that the ratio of the city size to the thresh-old resolution is 6 to 1, and that this ratio can be considered as a test value in other urban areas for model resolution set-ting. There are three major factors related to the effects of model resolution on the calculations of O 3 and O 3 precur-sors, including; (1) the calculated meteorological conditions, (2) the spatial distribution for the emissions of ozone precur-sors, and (3) the non-linearity in the photochemical ozone production. Model studies suggest that, for the calculations of O 3 and O 3 precursors, spatial resolutions (resulting from different meteorological condition and transport processes) have larger impacts than the effect of the resolution associ-ated with emission inventories. The model shows that, with coarse resolution of emission inventory (24 km) and high res-Correspondence to: X. Tie ( olution for meteorological conditions (6 km), the calculated CO and O 3 are considerably improved compared to the re-sults obtained with coarse resolution for both emission in-ventory and meteorological conditions (24 km). The resolu-tion of the surface emissions has important effects on the cal-culated concentration fields, but the effects are smaller than those resulting from the model resolution. This study also suggests that the effect of model resolution on O 3 precursors leads to important impacts on the photochemical formation of ozone. This results directly from the non-linear relation-ship between O 3 formation and O 3 precursor concentrations. Finally, this study suggests that, considering the balance be-tween model performance and required computation time on current computers, the 6 km resolution is an optimal resolu-tion for the calculation of ozone and its precursors in urban areas like Mexico City.

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