Ozone and the “weekend effect”

Research from MIRAGE (Megacities Impacts on Regional and Global Environments), a field campaign held in Mexico City in 2006, is coming to fruition as scientists begin to publish their findings. “Weekly patterns of Mexico City’s surface concentrations of CO, NOx, PM10 and 03 during 1986–2007,” published online in Atmospheric Chemistry and Physics, details the “weekend effect” in Mexico City and its implications for local air pollution.

“Surface pollutant concentrations in Mexico City show a distinct pattern of weekly variations,” write NCAR scientists Sherry Stephens, Sasha Madronich, and Fei Wu in conjunction with colleagues in Mexico. The study found that carbon monoxide, nitrogen oxides, and small particulates are less prevalent on Saturdays and especially Sundays, due to less driving and other pollution-generating activity on weekends than during the week. This effect has been observed in other large metropolitan areas as well.

However, ozone in Mexico City doesn’t follow this weekend pattern. As expected, ozone increased though the morning and noon hours each day as volatile organic compounds (VOCs) interacted with sunlight. Each night, ozone levels dropped back. The strength of the afternoon ozone peaks changed little from day to day, though, despite the enhanced VOCs and nitrogen oxides present during the week as opposed to weekends. The ACD team showed why: on weekdays, the enhanced nitrogen oxides apparently removed sunlight-generated radicals that would have otherwise helped produce ozone. On weekends, the presence of more radicals was counterbalanced by fewer VOCs, so ozone production stayed relatively constant.

The study, which was supported by NCAR and the Mexican government, concludes that efforts to reduce ozone in Mexico City should focus on cutting back VOCs rather than nitrogen oxides.


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