Linking Urban and Global Air Pollution: Control of Urban Ozone in Mexico City, and Control of Global Ozone through Methane Emission Reductions
J. Jason West, Ph.D.
Program in Atmospheric & Oceanic Sciences, and
Woodrow Wilson School of Public & International Affairs
Princeton University
 

Controlling urban and regional ozone has proven difficult, in part due to complex chemistry. Meanwhile, global background concentrations of ozone throughout the troposphere are increasing, exacerbating urban and regional challenges. On an urban scale, emissions of non-methane volatile organic compounds (NMVOCs) are commonly underestimated and this underestimate is important for modeling ozone sensitivity. In Mexico City, ratios of total non-methane hydrocarbons (NMHCs)/NOX and CO/NOX in morning measurements are found to be greater than these ratios in the official emissions inventory, by factors of two to three. When applying the CIT three-dimensional photochemical airshed model to the IMADA measurement campaign of March 1997, the model significantly underestimates measurements of both total NMHCs and of CO when using the official emissions. A best fit to the measurements is found when increasing CO emissions by a factor of two and NMVOC emissions by a factor of three. Using these corrections, the model produces good estimates of ozone and of NOX, with average normalized biases over six days of 3% and 32% respectively. Although these two independent methods agree, confidence in the appropriate correction is low. Modeled ozone peaks that occur early in the day are found to be sensitive to changes in NMVOC emissions, while later peaks are NOX-sensitive.

Global background concentrations of ozone are sensitive to emissions of NOX and of methane, yet methane control is currently considered only for climate purposes. Recent estimates of the sensitivity of ozone to reductions in methane emissions are combined with estimates of methane control costs and the health and agricultural benefits of ozone reductions, to assess the viability of controlling ozone through methane. Identified global cost-saving methane abatement measures will reduce ozone by 0.4-0.7 ppb. Methane controls produce ozone reductions that are widespread globally and are realized gradually (~12 yr). In contrast, controls on NOX and NMVOCs target high-ozone episodes in polluted regions and affect ozone rapidly, but have little climate benefit. A coarse estimate of the monetized global benefits of ozone reductions for agriculture, forestry, and human health (neglecting ozone mortality) justifies reducing ~17% of global anthropogenic methane emissions. These benefits further suggest that climate-motivated methane reductions have air quality-related ancillary benefits comparable to those for CO2. Air quality planning should consider reducing methane emissions alongside NOX and NMVOCs, and because the benefits of methane controls are shared internationally, international methane controls should be considered in both climate change and ozone policies.
 

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