Atmospheric Chemistry and Physics www.atmos-chem-phys.net 1680-7316 1680-7324 9 17 2009 10.5194/acp-9-6305-2009 http://www.atmos-chem-phys.net/9/6305/2009/ http://www.atmos-chem-phys.net/9/6305/2009/acp-9-6305-2009.html http://www.atmos-chem-phys.net/9/6305/2009/acp-9-6305-2009.pdf 6305 6317 2009-09-02 Evaluation of mobile emissions contributions to Mexico City's emissions inventory using on-road and cross-road emission measurements and ambient data M. Zavala miguelz@mce2.org S. C. Herndon E. C. Wood T. B. Onasch W. B. Knighton L. C. Marr C. E. Kolb L. T. Molina Molina Center for Energy and the Environment, La Jolla, California and Massachusetts Institute of Technology, Cambridge, Massachusetts, USA Center for Atmospheric and Environmental Chemistry, Center for Cloud and Aerosol Chemistry, Aerodyne Research Inc., Billerica, Massachusetts, USA Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana, USA Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, Virginia, USA 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. <br><br> 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 PM_2.5 mobile emissions in the inventory. <br><br> Analyses of ambient CO, NO_x and CO/NO_x concentration trends in the MCMA indicate that the early morning ambient CO/NO_x 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 NO_x. These trends, together with the analysis of fuel sales and fleet size, suggest that the relative contribution of diesel vehicles to overall NO_x levels has increased over time in the city. 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