Abstract
Abstract. Mobile sources produce a significant fraction of the total anthropogenic emissions burden in large cities and have harmful effects on air quality at multiple spatial scales. Mobile emissions are intrinsically difficult to estimate due to the large number of parameters affecting the emissions variability within and across vehicles types. The MCMA-2003 Campaign in Mexico City has showed the utility of using a mobile laboratory to sample and characterize specific classes of motor vehicles to better quantify their emissions characteristics as a function of their driving cycles. The technique clearly identifies "high emitter" vehicles via individual exhaust plumes, and also provides fleet average emission rates. We have applied this technique to Mexicali during the Border Ozone Reduction and Air Quality Improvement Program (BORAQIP) for the Mexicali-Imperial Valley in 2005. We analyze the variability of measured emission ratios for emitted NOx, CO, specific VOCs, NH3, and some primary fine particle components and properties by deploying a mobile laboratory in roadside stationary sampling, chase and fleet average operational sampling modes. The measurements reflect various driving modes characteristic of the urban fleets. The observed variability for all measured gases and particle emission ratios is greater for the chase and roadside stationary sampling than for fleet average measurements. The fleet average sampling mode captured the effects of traffic conditions on the measured on-road emission ratios, allowing the use of fuel-based emission ratios to assess the validity of traditional "bottom-up" emissions inventories. Using the measured on-road emission ratios, we estimate CO and NOx mobile emissions of 175±62 and 10.4±1.3 metric tons/day, respectively, for the gasoline vehicle fleet in Mexicali. Comparisons with similar on-road emissions data from Mexico City indicated that fleet average NO emission ratios were around 20% higher in Mexicali than in Mexico City whereas HCHO and NH3 emission ratios were higher by a factor of 2 in Mexico City than in Mexicali. Acetaldehyde emission ratios did not differ significantly whereas selected aromatics VOCs emissions were similar or smaller in Mexicali. Nitrogen oxides emissions for on-road heavy-duty diesel truck (HDDT) were measured near Austin, Texas, as well as in both Mexican cities, with NOy emission ratios in Austin < Mexico City < Mexicali.
Highlights
Emissions from transportation sources, primarily on-road motor vehicles, are generally the largest contributors to criteria air pollutants such as CO, NOx, and selected volatile organic compounds (VOCs) in urban areas; on-road vehicles are major sources of fine primary particle emissions and Published by Copernicus Publications on behalf of the European Geosciences Union.M
Comparisons with similar on-road emissions data from Mexico City indicated that fleet average NO emission ratios were around 20% higher in Mexicali than in Mexico City whereas HCHO and NH3 emission ratios were higher by a factor of 2 in Mexico City than in Mexicali
The mobile laboratory was situated in the prevailing downwind direction from the emitting vehicles as consistently as possible because, in this type of operational sampling mode, a successful measurement of an emission exhaust signature from a passing vehicle is highly dependant on the predominant wind direction and speed at the time of the exhaust event (Herndon et al, 2005)
Summary
Zavala et al.: Variability of mobile emissions sampled using a mobile laboratory specific air toxics (Molina et al, 2004) Despite their importance in determining air quality levels, the estimation of mobile emission sources is challenging because multiple parameters affect the variability of on-road mobile emissions within and across vehicles types (Cadle et al, 2007). Vehicle speed and acceleration under real-world operating conditions are affected by traffic congestion, road infrastructure (e.g. traffic signals and posted speed limits) and even personal driving behaviors. The combination of these operating conditions impacts the engine load and the pollutant emissions rates. A similar dependency of the effects of driving conditions (categorized for example by idling, acceleration and cruising modes) on vehicle emissions has been found in a more limited number of on-road measurement studies (Tong et al, 2000; Shah et al, 2006; Durbin et al, 2008)
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