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Average In-Use Emissions by Year and Model
In this analysis we compared average emissions by vehicle model year and model, using IM240 data from three states. There is a wide range in average in-use emissions by vehicle model, suggesting that vehicle design can be improved to reduce in-use emissions. Average emissions by model are quite consistent across three state IM240 programs. The figure shows average NOx emissions of MY91 car models with at least 100 individual vehicles tested on the IM240, in three states. The vertical axis shows average NOx emissions by model in Arizona, while the vertical axis shows average emissions in Colorado (diamonds) and Wisconsin (open triangles). The figure indicates two models with consistently high NOx emissions in each state, and three models with consistently low emissions in each state. The figure demonstrates that in-use NOx emissions of the dirtiest car models can be as much as 3 times higher as those of the cleanest models of the same age.
Models tested at I/M stations in relatively low-income areas have consistently higher emissions than the same models tested at stations in relatively high-income areas. We believe this result demonstrates that in-use emissions are sensitive to vehicle maintenance, and that some vehicle models are less sensitive to maintenance practices than others. Average emissions by model using IM240 data do not correlate well with average emissions using remote sensing data, in part because remote sensing measures emissions concentrations, while the IM240 measures mass emissions. An adjustment to account for the fuel economy of each vehicle model, or conversion of emissions measured under each test to grams of pollutant per gallon of fuel, is needed to properly compare emissions across measurement techniques. As a member of the Mobile Sources Technical Review Subcommittee, Innovative and Incentive Based Transportation Policies Workgroup, Tom incorporated these findings into the Workgroup's recommendations to EPA ( http://transaq.ce.gatech.edu/epatac/incent/incentprod.htm). He also presented these results at CRC's-On-Road Vehicle Emissions Workshops- in 1997 (163K.pdf), 1998 (293K.pdf) and 1999 (195K.pdf).
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