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 underprediction 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 under predicted 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 and that the decrease has been driven by reductions in CO levels rather than by NOx concentration changes, suggesting that the relative contribution of diesel vehicles to overall NOx levels has increased over time in the city. Despite the impressive increases 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 control 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.

Physical and chemical properties of the regional mixed layer of Mexico's Megapolis

“…The O 3 at Altzomoni has to be the result of photochemical production as the precursor gases are transported from their sources (Madronich, 2006). As was observed in comparisons between Altzomoni and Mexico City, the CO at the research site was a factor of ten less due to dilution; however, the average, maximum concentration of O 3 at Altzomoni was on the order of 0.1 ppm, similar to maximum values measured in Mexico City during March, 2006 (Zavala et al, 2009). …”

Ozone response to emission changes: a modeling study during the MCMA-2006/MILAGRO Campaign

“…The total VOC adjustment in the MCMA is smaller than the value used by Lei et al (2007Lei et al ( , 2008 (1.26 vs. 1.65), probably due to the VOC emissions changes over the years in both the emission inventories and in actual emissions. Zavala et al (2009b) (Lei et al, 2007(Lei et al, , 2008, the NO x emissions increased slightly (6%), but the VOCs emissions decreased by about 20%, leading to changes in the NO x /VOC ratio from 4.9 in 2003 to 3.7 (mass -based) in 2006. In particular, the total emissions of highly reactive VOCs (alkenes and aromatics) decrease significantly (16% and 48%, respectively), resulting in the reduction of VOC reactivity in 2006 compared to 2003.…”

Application of positive matrix factorization to on-road measurements for source apportionment of diesel- and gasoline-powered vehicle emissions in Mexico City

“…The benefits of stringent emission control standards and technological advancements in fuels and vehicles are undeniable (Kirchstetter et al, 1999b;Sawyer et al, 2000). Such improvements have been responsible for reducing mobile source emissions, or at least preventing them from growing in the face of an increasing number of vehicles on the road and distances driven (Zavala et al, 2009b). Despite its efforts, the MCMA still experiences serious air pollution problems with many pollutants regularly exceeding Mexican and US air quality standards.…”