Modeled and observed ozone sensitivity to mobile-source emissions in Mexico City M. Zavala1,2, W. Lei1,2, M. J. Molina2,*, and L. T. Molina1,2 1Molina Center for Energy and the Environment, La Jolla, California, USA 2Massachusetts Institute of Technology, Cambridge, Massachusetts, USA *now at: Department of Chemistry and Biochemistry, University of California, San Diego, California, USA
Abstract. The emission characteristics of mobile sources in the Mexico City
Metropolitan Area (MCMA) have changed significantly over the past few
decades in response to emission control policies, advancements in vehicle
technologies and improvements in fuel quality, among others. Along with
these changes, concurrent non-linear changes in photochemical levels and
criteria pollutants have been observed, providing a unique opportunity to
understand the effects of perturbations of mobile emission levels on the
photochemistry in the region using observational and modeling approaches.
The observed historical trends of ozone (O3), carbon monoxide (CO) and
nitrogen oxides (NOx) suggest that ozone production in the MCMA has
changed from a low to a high VOC-sensitive regime over a period of 20 years.
Comparison of the historical emission trends of CO, NOx and
hydrocarbons derived from mobile-source emission studies in the MCMA from
1991 to 2006 with the trends of the concentrations of CO, NOx, and the
CO/NOx ratio during peak traffic hours also indicates that fuel-based
fleet average emission factors have significantly decreased for CO and VOCs
during this period whereas NOx emission factors do not show any strong
trend, effectively reducing the ambient VOC/NOx ratio.
This study presents the results of model analyses on the sensitivity of the
observed ozone levels to the estimated historical changes in its precursors.
The model sensitivity analyses used a well-validated base case simulation of
a high pollution episode in the MCMA with the mathematical Decoupled Direct
Method (DDM) and the standard Brute Force Method (BFM) in the 3-D CAMx
chemical transport model. The model reproduces adequately the observed
historical trends and current photochemical levels. Comparison of the BFM
and the DDM sensitivity techniques indicates that the model yields ozone
values that increase linearly with NOx emission reductions and decrease
linearly with VOC emission reductions only up to 30% from the base case.
We further performed emissions perturbations from the gasoline fleet, diesel
fleet, all mobile (gasoline plus diesel) and all emission sources
(anthropogenic plus biogenic). The results suggest that although large ozone
reductions obtained in the past were from changes in emissions from gasoline
vehicles, currently significant benefits could be achieved with additional
emission control policies directed to regulation of VOC emissions from
diesel and area sources that are high emitters of alkenes, aromatics and
Citation: Zavala, M., Lei, W., Molina, M. J., and Molina, L. T.: Modeled and observed ozone sensitivity to mobile-source emissions in Mexico City, Atmos. Chem. Phys., 9, 39-55, doi:10.5194/acp-9-39-2009, 2009.