Characterizations of chemical oxidants in Mexico City: A regional chemical dynamical model (WRF-Chem) study

Abstract The formation of chemical oxidants, particularly ozone, in Mexico City were studied using a newly developed regional chemical/dynamical model (WRF-Chem). The magnitude and timing of simulated diurnal cycles of ozone (O 3 ), carbon monoxide (CO) and nitrogen oxides (NO x ) , and the maximum and minimum O 3 concentrations are generally consistent with surface measurements. Our analysis shows that the strong diurnal cycle in O 3 is mainly attributable to photochemical variations, while diurnal cycles of CO and NO x mainly result from variations of emissions and boundary layer height. In a sensitivity study, oxidation reactions of aromatic hydrocarbons (HCs) and alkenes yield highest peak O 3 production rates (20 and 18 ppbv h −1 , respectively). Alkene oxidations, which are generally faster, dominate in early morning. By late morning, alkene concentrations drop, and oxidations of aromatics dominate, with lesser contributions from alkanes and CO. The sensitivity of O 3 concentrations to NO x and HC emissions was assessed. Our results show that daytime O 3 production is HC-limited in the Mexico City metropolitan area, so that increases in HC emissions increase O 3 chemical production, while increases in NO x emissions decrease O 3 concentrations. However, increases in both NO x and HC emissions yield even greater O 3 increases than increases in HCs alone. Uncertainties in HC emissions estimates give large uncertainties in calculated daytime O 3 , while NO x emissions uncertainties are less influential. However, NO x emissions are important in controlling O 3 at night.