Modeled and observed ozone sensitivity to mobile-source emissions in Mexico City

摘要

The emission characteristics of mobile sources in the Mexico CityMetropolitan Area (MCMA) have changed significantly over the past fewdecades in response to emission control policies, advancements in vehicletechnologies and improvements in fuel quality, among others. Along withthese changes, concurrent non-linear changes in photochemical levels andcriteria pollutants have been observed, providing a unique opportunity tounderstand the effects of perturbations of mobile emission levels on thephotochemistry in the region using observational and modeling approaches.The observed historical trends of ozone (O₃), carbon monoxide (CO) andnitrogen oxides (NO_x) suggest that ozone production in the MCMA haschanged from a low to a high VOC-sensitive regime over a period of 20 years.Comparison of the historical emission trends of CO, NO_x andhydrocarbons derived from mobile-source emission studies in the MCMA from1991 to 2006 with the trends of the concentrations of CO, NO_x, and theCO/NO_x ratio during peak traffic hours also indicates that fuel-basedfleet average emission factors have significantly decreased for CO and VOCsduring this period whereas NO_x emission factors do not show any strongtrend, effectively reducing the ambient VOC/NO_x ratio.This study presents the results of model analyses on the sensitivity of theobserved ozone levels to the estimated historical changes in its precursors.The model sensitivity analyses used a well-validated base case simulation ofa high pollution episode in the MCMA with the mathematical Decoupled DirectMethod (DDM) and the standard Brute Force Method (BFM) in the 3-D CAMxchemical transport model. The model reproduces adequately the observedhistorical trends and current photochemical levels. Comparison of the BFMand the DDM sensitivity techniques indicates that the model yields ozonevalues that increase linearly with NO_x emission reductions and decreaselinearly with VOC emission reductions only up to 30% from the base case.We further performed emissions perturbations from the gasoline fleet, dieselfleet, all mobile (gasoline plus diesel) and all emission sources(anthropogenic plus biogenic). The results suggest that although large ozonereductions obtained in the past were from changes in emissions from gasolinevehicles, currently significant benefits could be achieved with additionalemission control policies directed to regulation of VOC emissions fromdiesel and area sources that are high emitters of alkenes, aromatics andaldehydes.