Modeling Mobile-Source Emissions (2000) / Chapter Skim
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1 Overview of Mobile-Source Emissions
1 Overview of Mobile-Source Emissions
Pages 15-32
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From page 15... ...
Mobile Source Emissions Factor MOBILE model is the primary tool used by air-quality planners at national, state, and local levels to estimate on-road mobile-source emissions, and hence is key to assessing associated environmental impacts. Because of the model's importance in assessing air-quaTity control programs and because of concerns about weaknesses in the accuracy and reliability of the model, Congress asked the National Academy of Sciences to review and evaluate the MOBILE model.
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The National Ambient Air Quality Standards (NAAQS) sets a primary standard for ambient concentrations of criteria pollutants to protect public health with "an adequate margin of safety", and a secondary standard to protect public welfare against environmental and property damage.
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As more is known about the specific aspects of the toxicity of PM and air tonics, a better understanding of the contribution of motor vehicles to these pollutants becomes important. Mobile-Source Contributions Total mobile source emissions (on-road plus off-road 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,
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An issue unique to mobile sources is the proximity of sources to receptors. Air vents on cars can scoop up exhaust from a vehicle just ahead so
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Because health effects are a function of what is actually inhaled, this variation in exposure is a critical feature of mobile-source pollutants. Environmental Concerns Mobile sources contribute significantly to the risk of detrimental environmental effects of ozone, acidic deposition, PM, and air tonics.
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From page 21... ...
The large number of individual sources, the large variability of emissions characteristics among these sources, and the need for emissions estimation methods to fulfill many applications creates daunting challenges. To aid in identifying, estimating, and reducing risks of motor-vehicle emissions, EPA has developed a series of models referred to as the MOBILE model.
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A single set of emissions standards applies to all passenger cars, regardless of size, passenger occupancy, or use. Emissions are regulated on the basis of grams of pollutant per mile (g/mi)
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This allows NMHCs and CO emissions standards to "float", in that fleet emissions rates depend on the mix of vehicles used to meet the NOx standard. The emissions standards shown for NMHCs and CO are those that would result given the mix assumed in the Notice of Final Rulemaking (EPA 1999a)
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From page 25... ...
mandates the same emissions standards to passenger cars and LDTs.7 As with passenger cars, pollutant emissions limits for LDTs are expressed in grams of pollutant per mile and vehicles are certified on a chassis dynamometer. The MOBILE model retains the historical distinction between passenger cars and LDTs because current emissions standards and control technologies differ among vehicle classes.
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Originally, evaporative emissions were regulated for a combined sum of hot soak and diurnal emissions. In more recent years, a separate limit was placed on running loss evaporative emissions.
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OVERVIEW OF MOBILE-SOURCE EMISSIONS 27 in o ._ cn _` co in — t— ~ O a ·_ co x O ~ ~ O _ o .
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The MOBILE model is now used for purposes for which it was not originally intended, such as providing detailed air-quality modeling inputs, selecting control strategies based on prospective emissions reductions, and demonstrating conformity of transportation projects with the Clean Air Act. The General Accounting Office (GAO)
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The Act also prescribes technical criteria; for example, each plan must contain a current emissions inventory, adequate ambient air-quality data, and an analysis of future air quality based on photochemical grid modeling. Outside of California, the MOBILE model is used to estimate emissions from on-road mobile sources as part of the SIP.
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to quantify the emissions of either a regionally significant project or a regional transportation plan. Regulatory Initiatives EPA has the authority to adopt standards for a broad range of mobile sources to assist states in achieving the NAAQS.
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. The committee will consider the adequacy of the model's input data, assumptions, structure, and results used to characterize mobile source emissions.
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Alternative data sources and analytical techniques currently used for similar purposes by others (e.g. the California Air Resources Board in their EMFAC and other related mobile source emissions models)
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