Mexico City’s Weekly Pollution Patterns Reveal Chemical Sensitivity of Ozone Formation

A recent study¹ by scientists from ACD and the Méxican government concluded that México City’s ambient ozone (O3) can be reduced by decreasing emissions of volatile organic compounds (VOCs), while decreases in emissions of nitrogen oxides (NOx) may be less effective, and possibly even counterproductive, in reducing local O3 concentrations. Tropospheric O3, a major urban pollutant, is produced in the atmosphere when VOCs and NOx emitted by human activities are exposed to solar ultraviolet radiation. Depending on the prevailing environmental conditions and on the specific mixture of urban pollutants, the chemistry of O3 formation can be sensitive either to VOCs or NOx, or both. Knowledge of this sensitivity is critical to the design of regulatory policies for reducing O3 pollution.

Figure 1: Daily variation of surface concentrations in México City for (a) carbon monoxide, CO, and (b) nitrogen oxides, NOx. Morning rush hour (6-11 am) values are significantly lower on weekends, due to decreased emission from human activities.

The new study analyzed measurements of O3, NOx, and CO (which is highly correlated with VOCs) obtained at 32 monitoring stations within México City over 1986-2007, contrasting workdays (Monday-Friday) to weekends (Saturdays and Sundays). Because human activities are usually less intense during weekends, the workday-weekend differences provide an opportunity to measure how O3 responds to reductions in NOx and VOC emissions. Figure 1 shows that the emissions of CO and NOx during the morning rush hour (7-10am) are indeed lower on Saturdays, and even more so on Sundays. However, the amount of O3 produced by the afternoon (Figure 2) is nearly the same on all days of the week.

Figure 2: Daily variation of surface concentrations of O3 in México City.
Photochemically produced afternoon O3 is nearly the same on weekends as during workdays.

A theoretical analysis of these measurements shows that Mexico City’s high NOx concentrations actually inhibit O3 production by removing photochemically produced radicals (Figure 3). Lower weekend NOx concentrations allow stronger O3 production, although this is offset by lower weekend CO and VOC concentrations so that O3 remains relatively constant.

Figure 3: Fraction of radical loss due to reactions with nitrogen oxides, FNOx. Values of FNOx were derived from the changes in CO, NOx, and O3 between workdays and Saturday (blue curve), and between Saturday and Sunday (red curve). Values of FNOX larger than 0.5 indicate that in México City the formation of O3 is limited by VOCs.

The study is the first to provide observation-based evidence that the production of O3 in Mexico City is limited by, and therefore sensitive to, VOC emissions. While VOC emission reductions would be most effective in reducing local O3 production, NOx emission reduction may be more important for controlling regional oxidants.

¹ Stephens, S., S. Madronich, F. Wu, J. Olson, R. Ramos, A. Retama, and R. Muñoz, Weekly patterns of México City’s surface concentrations of CO, NOx, PM10, and O3 during 1986-2007, Atmos. Chem. Phys. Discuss., 8, 8357-8384, 2008.