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Mobile emissions represent a significant fraction of the total anthropogenicemissions burden in the Mexico City Metropolitan Area (MCMA) and, therefore,it is crucial to use top-down techniques informed by on-road exhaustmeasurements to evaluate and improve traditional bottom-up officialemissions inventory (EI) for the city. We present the measurements ofon-road fleet-average emission factors obtained using the Aerodyne mobilelaboratory in the MCMA in March 2006 as part of the MILAGRO/MCMA-2006 fieldcampaign. A comparison of our on-road emission measurements with thoseobtained in 2003 using essentially the same measurement techniques andanalysis methods indicates that, in the three year span, NO emission factorsremain within the measured variability ranges whereas emission factors ofaldehydes and aromatics species were reduced for all sampled drivingconditions. We use a top-down fuel-based approach to evaluate the mobile emissions fromthe gasoline fleet estimated in the bottom-up official 2006 MCMA mobilesources. Within the range of measurement uncertainties, we found probableslight overpredictions of mean EI estimates on the order of 20–28% for COand 14–20% for NO. However, we identify a probable EI discrepancy of VOCmobile emissions between 1.4 and 1.9; although estimated benzene and toluenemobile emissions in the inventory seem to be well within the uncertaintiesof the corresponding emissions estimates. Aldehydes mobile emissions in theinventory, however, seem to be underpredicted by factors of 3 for HCHO and 2for CH3CHO. Our on-road measurement-based estimate of annual emissionsof 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 theMCMA indicate that the early morning ambient CO/NOx ratio has decreasedat a rate of about 1.9 ppm/ppm/year over the last two decades due toreductions in CO levels rather than by NOx. These trends, together withthe analysis of fuel sales and fleet size, suggest that the relativecontribution of diesel vehicles to overall NOx levels has increasedover time in the city. Despite the impressive increase in the size of thevehicle fleet between 2000 and 2006, the early morning ambientconcentrations of CO and NOx have not increased accordingly, probablydue to the reported low removal rates of older vehicles, which do not haveemissions control technologies, and partially due to the much loweremissions from newer gasoline vehicles. This indicates that anemission-based air quality improvement strategy targeting large reductionsof emissions from mobile sources should be directed towards a significantincrease of the removal rate of older, highly-polluting, vehicles

Evaluation of mobile emissions contributions to Mexico City's emissions inventory using on-road and cross-road emission measurements and ambient data