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

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 (O3), carbon monoxide (CO) and nitrogen oxides (NOx), and the maximum and minimum O3 concentrations are generally consistent with surface measurements. Our analysis shows that the strong diurnal cycle in O3 is mainly attributable to photochemical variations, while diurnal cycles of CO and NOx 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 O3 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 O3 concentrations to NOx and HC emissions was assessed. Our results show that daytime O3 production is HC-limited in the Mexico City metropolitan area, so that increases in HC emissions increase O3 chemical production, while increases in NOx emissions decrease O3 concentrations. However, increases in both NOx and HC emissions yield even greater O3 increases than increases in HCs alone. Uncertainties in HC emissions estimates give large uncertainties in calculated daytime O3, while NOx emissions uncertainties are less influential. However, NOx emissions are important in controlling O3 at night.