Modeling Mobile-Source Emissions (2000) / Chapter Skim
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Executive Summary
Executive Summary
Pages 1-14
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Transportation infrastructure decisions are also affected by emissions estimates from MOBILE. iThe MOBILE model estimates emissions factors in grams per mile from information on the emissions characteristics of the vehicle fleet.
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For example, estimating the effect of a regional emissions inspection and maintenance program would not require emissions estimates to be resolved at the level of detail needed for estimating the effect of an effort to coordinate traffic signals along a major corridor. Since its release more than 20 years ago, MOBILE has been used increasingly in regulatory applications.
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MOBILE-SOURCE EMISSIONS MODELING RECOMMENDATIONS Development of a Toolkit of Models Finding Since its development in 1978, MOBILE has evolved from a tool for estimating regional emissions inventories to such uses as determining the conformity of transportation projects with requirements of State Implementation Plans and assessing the emissions impacts of transportationcontroT measures. The further the model's application deviates from its original purpose of estimating aggregate regional emissions, the more difficult it becomes to verify the accuracy of its predictions and, as a result, the less appropriate it becomes for air-quaTity management.
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emissions through the coupling of vehicle operating conditions with appropriate emissions factors; and a microscale ir~stantan~eous emissions modeling component that uses instantaneous operating conditions of individual vehicles to estimate continuous vehicle emissions and that can be used for a variety of applications, including generating emissions factors for microscaTe traffic-simulation models, mesoscale emissions models, traffic data sets, and dispersion models. To allow for a nested progression of models, the models in the toolkit should be designed and maintained to be consistent, despite differences in the level of detail in inputs (e.g., variations in roadway network detail and meteorological variables)
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Recommendation Enhanced model evaluation studies shoulc! begin immediately anal coatinue throughout the lor~g-term evolution and development of mobile-source emissions models.
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Specifically, the analyses should: include a rigorous study of the model's sensitivity to all the input data to provide users with information on the most critical factors affecting model results; include a rigorous study of uncertainties and bias in all model components and in the data used to develop model parameters and relationships; explicitly define the levels of accuracy needed to fulfill EPA's regulatory responsibilities; and be used to design future versions of MOBILE and other models in the toolkit. Long-Term Planning Finding EPA has not engaged adequately in long-term planning to coordinate future model-application needs with model developments.
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Department of Transportation, and others to complete a tong-rar~ge plan that addresses improvements of or resew approaches to mobile-source emissions models. EPA should also arrange for a short policy study by an independent body to identify possible institutional barriers anal ways to enhance institutional coordination in model development.
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A critical parameter in estimating the effectiveness of future inspection and maintenance programs is the fraction of motorists that get repairs in response to malfunction indicator lights of on-board diagnostic systems. The response rates must be established for areas with and without vehicle-emissions inspection and maintenance programs.
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The proposed MOBILES emissions factors use engine certification data (in grams per brake horsepower hour) and conversion factors to estimate gram per mile emissions factors.
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Random roadside pullover testing of exhaust emissions, such as those currently performed in California, appears to be one of the most promising means of identifying vehicles with high exhaust emissions. Remote sensing of exhaust emissions has shown some promise as well.
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Information that invalidated assumptions in MOBILES about the deterioration of light-duty vehicles and the effectiveness of vehicle emissions inspection and maintenance programs, oxygenated fuel, and other control programs has been available for several years and has not been incorporated into the model. Thus, emissions inventories and control strategies being developed are based on out-of-date assumptions and inaccurate predictions, perhaps resulting in the selection and propagation of inefficient or ineffective controls.
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EPA should assess weaknesses in the mobile-source taxies databases, design and run test programs to fill data gaps, and incorporate new test data into a timely update of MOBILE6. OTHER RECOMMENDATIONS Off-Road Emissions Finding As emissions from on-road vehicles decrease due to tighter emissions standards, fuel-sulfur controls, and less deterioration of emissions-control devices, the emissions from off-road mobile sources will continue to increase in importance.
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Recent reorganization of the EPA Office of Mobile Sources, the EPA office responsible for MOBILE, into the Office of Transportation and Air Quality might have an impact on MOBILE's continued development. The reorganization should not impede rigorous development of MOBILE, which must be seen as an accurate reflection of mobile-source emissions.
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Human Health Concerns Total emissions from mobile sources contribute significantly to the detrimental health effects resulting from exposure to ambient ozone, CO, PM, and air toxics. Urban ozone has been one of the most persistent health concerns.
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