Mobile sources produce a significant fraction of\r\nthe total anthropogenic emissions burden in large cities and\r\nhave harmful effects on air quality at multiple spatial scales.\r\nMobile emissions are intrinsically difficult to estimate due to\r\nthe large number of parameters affecting the emissions variability\r\nwithin and across vehicles types. The MCMA-2003\r\nCampaign in Mexico City has showed the utility of using a\r\nmobile laboratory to sample and characterize specific classes\r\nof motor vehicles to better quantify their emissions characteristics\r\nas a function of their driving cycles. The technique\r\nclearly identifies ââ?¬Å?high emitterââ?¬Â vehicles via individual exhaust\r\nplumes, and also provides fleet average emission rates.\r\nWe have applied this technique to Mexicali during the Border\r\nOzone Reduction and Air Quality Improvement Program\r\n(BORAQIP) for the Mexicali-Imperial Valley in 2005. We\r\nanalyze the variability of measured emission ratios for emitted\r\nNOx, CO, specific VOCs, NH3, and some primary fine\r\nparticle components and properties by deploying a mobile\r\nlaboratory in roadside stationary sampling, chase and fleet\r\naverage operational sampling modes. The measurements reflect\r\nvarious driving modes characteristic of the urban fleets.\r\nThe observed variability for all measured gases and particle\r\nemission ratios is greater for the chase and roadside stationary\r\nsampling than for fleet average measurements. The fleet average sampling mode captured the effects of traffic conditions\r\non the measured on-road emission ratios, allowing\r\nthe use of fuel-based emission ratios to assess the validity\r\nof traditional ââ?¬Å?bottom-upââ?¬Â emissions inventories. Using the\r\nmeasured on-road emission ratios, we estimate CO and NOx\r\nmobile emissions of 175Ã?±62 and 10.4Ã?±1.3 metric tons/day,\r\nrespectively, for the gasoline vehicle fleet in Mexicali. Comparisons\r\nwith similar on-road emissions data from Mexico\r\nCity indicated that fleet average NO emission ratios were\r\naround 20% higher in Mexicali than in Mexico City whereas\r\nHCHO and NH3 emission ratios were higher by a factor of\r\n2 in Mexico City than in Mexicali. Acetaldehyde emission\r\nratios did not differ significantly whereas selected aromatics\r\nVOCs emissions were similar or smaller in Mexicali. Nitrogen\r\noxides emissions for on-road heavy-duty diesel truck\r\n(HDDT) were measured near Austin, Texas, as well as in\r\nboth Mexican cities, with NOy emission ratios in Austin <\r\nMexico City < Mexicali.
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