Abstract
Abstract. Mobile emissions represent a significant fraction of the total anthropogenic emissions burden in the Mexico City Metropolitan Area (MCMA) and, therefore, it is crucial to use top-down techniques informed by on-road exhaust measurements to evaluate and improve traditional bottom-up official emissions inventory (EI) for the city. We present the measurements of on-road fleet-average emission factors obtained using the Aerodyne mobile laboratory in the MCMA in March 2006 as part of the MILAGRO/MCMA-2006 field campaign. A comparison of our on-road emission measurements with those obtained in 2003 using essentially the same measurement techniques and analysis methods indicates that, in the three year span, NO emission factors remain within the measured variability ranges whereas emission factors of aldehydes and aromatics species were reduced for all sampled driving conditions. We use a top-down fuel-based approach to evaluate the mobile emissions from the gasoline fleet estimated in the bottom-up official 2006 MCMA mobile sources. Within the range of measurement uncertainties, we found probable slight overpredictions of mean EI estimates on the order of 20–28% for CO and 14–20% for NO. However, we identify a probable EI discrepancy of VOC mobile emissions between 1.4 and 1.9; although estimated benzene and toluene mobile emissions in the inventory seem to be well within the uncertainties of the corresponding emissions estimates. Aldehydes mobile emissions in the inventory, however, seem to be underpredicted by factors of 3 for HCHO and 2 for CH3CHO. Our on-road measurement-based estimate of annual emissions of organic mass from PM1 particles suggests a severe underprediction (larger than a factor of 4) of PM2.5 mobile emissions in the inventory. Analyses of ambient CO, NOx and CO/NOx concentration trends in the MCMA indicate that the early morning ambient CO/NOx ratio has decreased at a rate of about 1.9 ppm/ppm/year over the last two decades due to reductions in CO levels rather than by NOx. These trends, together with the analysis of fuel sales and fleet size, suggest that the relative contribution of diesel vehicles to overall NOx levels has increased over time in the city. Despite the impressive increase in the size of the vehicle fleet between 2000 and 2006, the early morning ambient concentrations of CO and NOx have not increased accordingly, probably due to the reported low removal rates of older vehicles, which do not have emissions control technologies, and partially due to the much lower emissions from newer gasoline vehicles. This indicates that an emission-based air quality improvement strategy targeting large reductions of emissions from mobile sources should be directed towards a significant increase of the removal rate of older, highly-polluting, vehicles.
Highlights
As in many urban areas, mobile sources in the Mexico City metropolitan area (MCMA) are responsible for a significant fraction of fine particulate matter (PM), CO2, CO, NOx, NH3, and volatile organic compounds (VOCs) emitted to the Published by Copernicus Publications on behalf of the European Geosciences Union.M
In this work we present the measurements of on-road fleetaverage emission factors obtained using the Aerodyne mobile laboratory in Mexico City in March 2006
We have presented measured on-road, fleet-average exhaust emission factors obtained with the Aerodyne mobile laboratory in Mexico City in 2006
Summary
As elsewhere, MCMA mobile emissions have shown to include an array of toxic VOCs and carbonaceous PM compounds (Jiang et al, 2005; Rogers et al, 2006; Zavala et al, 2006; Wohrnschimmel et al, 2008), directly impacting human health. This suggests that significant human health, ecosystems and climate benefits can be obtained by controlling mobile emission sources, as long as mobile emissionsbased air quality control policies are based on clear understanding and reliable quantification of these emissions
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