Modeling Mobile-Source Emissions (2000) / Chapter Skim
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2 Uses of MOBILE in Air-Quality Management
2 Uses of MOBILE in Air-Quality Management
Pages 33-60
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From page 33... ...
(California uses the Motor Vehicle Emissions Inventory modeling suite for the assessment of vehicle emissions and their controls, as discussed in Chapter 5.) MOBILE is used in the development of national, regional, and urban emissions inventories; the simulation of regional air chemistry and microscale dispersion of pollutants; the assessment of the effectiveness of control strategies; the documentation of emissions reductions in State Implementation Plans (SIPs)
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From page 34... ...
Current uses of the model include developing emissions inventories and reductions in SIPs, demonstrating conformity of transportation and air-quality plans, and providing emissions estimates for dispersion and photochemical air-quality modeling. Thus, the original role of MOBILE has been expanded in ways that now require higher standards of accuracy that incorporate a greater degree of complexity.
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From page 35... ...
MOBILE estimates emissions rates based on vehicle type, average speed, ambient temperature, and other factors. The product of the transportation activity and the emissions rates from MOBILE results in emissions estimates for each modeled pollutant (carbon monoxide (CO)
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From page 36... ...
The travel-demand models answer: "Will I travel," "How often and when," "Where," "By which mode," and "By which route"? They provide the MOBILE model with information on average vehicle speeds for each roadway segment that may be aggregated by roadway type or facility (e.g., freeways, arterials, collectors, and freeway ramps)
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From page 37... ...
the emissions type and season of interest. Adjustments for weekend/weekday Travel models typically simulate weekday traffic, with adjustments for weekend emissions inventories often being required for air-quality assessments.
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From page 38... ...
Emissions Modeling A primary use of MOBILE is for developing on-road mobile-source emissions inventories for use in air-quality planning. Emissions rates developed in MOBILE are combined with average vehicle speeds and travel activity estimates to develop these inventories.
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From page 39... ...
MOBILE calculates a typical urban area VMT mix based on national data for several variables, including registration distributions, annual mileage accumulation rates, percentage of diesel sales, and number of vehicles. EPA recommends that users develop locally specific estimates of VMT mix for SIP emissions inventories.
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From page 40... ...
Temperature MOBILE requires locally specific temperature data, in part because no national defaults would be appropriate for temperature. Thus, users must develop average temperature data to develop on-road mobile-source emissions inventories.
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From page 41... ...
However, there are many other factors that contribute to the poor performance of air quality models, including errors in the atmospheric chemical mechanisms and meteorological inputs within the air quality models as well as uncertainties in the travel activity inputs discussed earlier. After a model achieves acceptable performance, it is used to test control strategies, in particular to identify the set of controls that are likely to lead to attainment.
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From page 42... ...
In part because of the difficulties in estimating emissions inventories, guidelines for future ozone air-quality modeling might be used in a more relative sense. Thus, rather than ensuring that all concentrations simulated by the model are at or below 0.12 ppm (or 0.08 ppm for the new 8-fur standard)
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From page 43... ...
Air toxic agents are a societal concern, and automotive emissions contain significant quantities of hazardous air pollutants (HAPs) or air tonics, such as benzene, formaldehyde, and 1,3 butadiene.
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From page 44... ...
Such analyses are used by state environmental agencies, transportation consultants, metropolitan planning organizations, and state departments of transportation to support development of SIPs and transportation conformity analysis (Figure 2-31. Such analyses are also used by EPA to estimate the impacts of national mobile-source regulatory strategies on overall emissions levels.
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From page 45... ...
It is common in travel-demand modeling to compare predicted traffic volumes on a roadway segment or ridership on a transit line with independently observed values. Such standard caTibration and validation procedures and metrics have not been developed for applications of MOBILE.
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From page 46... ...
. Speciation Model I I Biogenics L | Non-Road Mobile |_ | Area Source 1_ | Stationary Source Meteorology Input Data Dispersion Model Air-Quality Model Emissions by Grid and Time of Day = Volatile Organic Compounds, Carbon Monoxide, Nitrogen Oxides, and Primary Particulate Matter Carbon Monoxide Concentrations Ozone and Particulate Concentration Levels by Time and Location Uses: SIP Demonstration of Attainment National Environmental Policy AcVEvaluation of Major Capital Investments CO, 03, and Particulate-Concentration Evaluation FIGURE 2-4 Use of MOBILE in the estimation of ambient pollutant concentrations.
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From page 47... ...
_ Meteorology Input Data _ Dispersion Model Air-Quality Model Emissions by Grid and Time of Day Exposure/Response Model Distribution of Sensitivity to Exposure | Uses: Assessment of health impacts | L Carbon Monoxide Concentrations Ozone and ParticulateConcentration Levels by Time and Location ~ ' Exposure Model Population Characteristics by Exposure and Time Health Impacts .
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From page 48... ...
Laboratory estimates of precursor sensitivity to ozone formation source (traffic counts and transit ridership) by segment (88% correlation, 5% error)
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From page 50... ...
Evaluation of Control Strategies, Emissions Inventory, and Rate of Progress Primary Users ant! Purpose State and local governments, MPOs, consultants, research institutions, and others apply MOBILE to develop regional emissions inventories, evaluate alternative mobile-source emissions-control strategies, and track trends in control-strategy implementation.
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From page 51... ...
has been questioned. The development of SIPs requires accuracy in emissions inventories and crediting of emissions reductions from controls, both of which are particularly sensitive to errors.
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From page 52... ...
Users will also expect that some form of instantaneous modeling will be provided so that both individual projects and larger-area transportation systems can be assessed in a more accurate manner. SIP Demonstration of Attainment Primary Users and Purpose States and, in some cases, metropolitan planning organizations (MPOs)
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From page 53... ...
for critical intersections and sites with violations or possible violations of the NAAQS. Similar to emissions inventories developed for a SIP, the transportation conformity analysis consists of determining emissions estimates as a function of vehicle activity and region-specific emissions factors.
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From page 54... ...
in the conformity analysis is not adequately supported by the aggregate regional and national data and assumptions used in MOBILE. The conformity analysis also must show consistency among the SIP, transportation plan, and TIP.
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From page 55... ...
Since this time, EPA has developed even more dramatic improvements through Tier 2 emissions standards and fuel sulfur reductions. Policy Implication and Future Direction Conformity analysis is often conducted annually for the TIP development and every three years for the transportation plan.
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From page 56... ...
Due to the timeline inconsistency between SIP and transportation plans, this is most relevant for a transportation conformity analysis, because it is these new technologies and initiatives that have the largest impact on out-year emissions. Transportation Control Measure Effectiveness and CMAQ Eligibility Primary Users and Purpose States and MPOs use both travel-demand models and the MOBILE model to aid in the selection of transportation-control measures (T CM)
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From page 57... ...
As with the conformity analysis, this affects the accuracy of the emissions-reduction estimates from MOBILE because MOBILE is used to estimate the effects of a small change in travel parameters on a subset of the overall vehicle fleet. Because no formal blueprint outlines appropriate methodologies at the national level, each region has developed its own approach to evaluating TCM and CMAQ priorities, and these variations may cause important nationwide inconsistencies.
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From page 58... ...
The primary users of the MOBILE model in evaluating major capital transportation investments and developing environmental impact studies are state departments of transportation, federal resource agencies, state resource agencies, MPOs, local governments, consultants (generally working for these government units) , and universities.
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From page 59... ...
SUMMARY OF POLICY IMPLICATIONS AND RECOMMENDATIONS Modeling the air-quality impacts of on-road vehicles is an interdisciplinary effort that encompasses the modeling of travel demand, emissions, and air quality. These individual components must be systematically integrated to develop analyses with adequate consistency, fidelity, accuracy, and precision.
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From page 60... ...
Future focus on new emissions standards, the growing concern about air toxic emissions, and the growing cost of control strategies Will increase the demands for accuracy and detail from MOBILE. A strategic and comprehensive long-range plan is needed to better identify emerging needs for the modeling of automotive emissions, define the levels of detail and accuracy needed to meet those needs, and set priorities to improve MOBILE.
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