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

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 unCorrespondence to: M. Zavala (miguelz@mce2.org) derpredicted by factors of 3 for HCHO and 2 for CH 3 CHO. Our on-road measurement-based estimate of annual emissions of organic mass from PM 1 particles suggests a severe underprediction (larger than a factor of 4) of PM 2.5 mobile emissions in the inventory.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. Despite the impressive increase in the size of the vehicle fleet between 2000 and 2006, the early morning ambient concentrations of CO and NO x 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.

Experience from Integrated Air Quality Management in the Mexico City Metropolitan Area and Singapore

“…On-road vehicle fleet emission rates in fleet-average mode for various vehicle classes and driving speeds were obtained using a mobile laboratory and identified several discrepancies between the observations and the emission estimates in the 2004 emissions inventory. These included slight over predictions of CO and NO, a probable under prediction of VOC by a factor of 1.4-1.9, and a severely underestimation of the PM emission estimates [71,72]. Recent studies have shown that some chemical species emitted by vehicular traffic in the MCMA have decreased since then [46,73]; however, motor vehicles still play a major role in supplying the NO x and VOC precursors that fuel MCMA's extremely active photochemistry [24], as well as producing abundant amounts of primary PM, elemental carbon, particle-bound polycyclic aromatic hydrocarbons, and a wide range of air toxics.…”

Investigation of the sources and processing of organic aerosol over the Central Mexican Plateau from aircraft measurements during MILAGRO

“…Yokelson et al (2007) compared estimated fire PM amounts from aircraft-based emission measurements to the emissions inventory from the Mexico City metropolitan area and estimated that ∼79-92% of the primary particle mass generated in the Mexico City area is from forest fires near Mexico City, and ∼50%±30% of the aged PM 2.5 (not just of the OA) is from fires. However these authors estimated the urban primary contribution from the MCMA emissions inventory, which has been shown to substantially underestimate primary fine particles (Aiken et al, 2009b;Zavala et al, 2009), and also assumed a doubling of the POA/ CO ratio for BB plumes due to SOA formation, which is likely too large as discussed below, and thus the actual percentage BB contribution to PM 2.5 is likely lower. Querol et al (2008) estimate that BB contributed about 10% of the PM 2.5 during MILAGRO at the T0 urban supersite, which would correspond to about 18% of the OA.…”

Modeling organic aerosols in a megacity: potential contribution of semi-volatile and intermediate volatility primary organic compounds to secondary organic aerosol formation

“…Several aspects of OA formation in Mexico City have already been addressed in recent studies which have shown that the emissions of POA from both anthropogenic activities and biomass burning are reasonably represented in the emission inventories and models, with perhaps some underprediction of the urban POA, and mixed results for the BB POA with overprediction downwind of some large fires but underprediction in the urban area in the early morning (e.g. Hodzic et al, 2009;Zavala et al, 2009). A better prediction of the temporal trend of BBOA was achieved when the emissions profile took into account the emissions during the late evening into the shallow nighttime boundary layer (Aiken et al, 2009b).…”