API PUBL 4603
Investigation of MOBILE5a Emission Factors: Assessment of Exhaust and Nonexhaust Emission Factor Methodologies and Oxygenate Effects
|Publication Date:||1 January 1994|
This report presents the results of an evaluation of the EPA's onroad vehicle emissions model, MOBILE5a. MOBILE5a is a computer model developed by EPA to predict onroad vehicle emission rates and to evaluate alternative vehicle emissions control strategies. It is also the model that states must use to prepare regional emission estimates for various requirements of the Clean Air Act Amendments of 1990, including State Implementation Plan inventories. MOBILE5a was released in March 1993 and replaces the previous version of the model, MOBILE4.1. The model is continually being updated and revised with new data and methodologies. The MOBILE model has frequently been criticized for underpredicting emissions of hydrocarbons and carbon monoxide. These criticisms are, in part, motivated by findings from tunnel studies and remote sensing of vehicle emissions, which have indicated discrepancies between in-use vehicle emission rates and model predictions. MOBILE has also been criticized for its reliance upon the federal test procedure (FTP) driving cycle to develop exhaust emission rates. It has been shown recently by EPA and other researchers that this driving cycle does not include some in-use driving patterns that typically result in high emission events, such as hard accelerations.
This study was undertaken because only limited documentation is available from EPA on MOBILE5a and updates made since MOBILE4.1. Also, this version of the model has not previously undergone extensive review outside the agency. Not all features of the model were reviewed. For example, corrections applied to account for nonstandard speed and temperature conditions were not evaluated. Instead, the focus was on the basic mathematical relations used to predict exhaust and nonexhaust emissions. Fuel oxygenate effects upon emissions were also reviewed.
Descriptions of the current MOBILE5a model algorithms and underlying databases were based on available model documentation and conversations with EPA. When possible, copies of the model databases were obtained. Lists and criticisms of assumptions and extrapolations within the model were prepared. A critical evaluation of the methodologies was developed. Suggestions were made for sensitivity analyses that should be performed to further quantify the potential effects of alternative methods or assumptions that could be used within the model, or the effects of updates made since MOBILE4.1 was released. These suggested analyses were subsequently prioritized according to the resources required to perform them and the importance of the effect to be analyzed. A subset of the proposed analyses was performed, and results are included in this report.
Findings presented in this report are grouped according to the component of MOBILE5a analyzed. Separate sections present the discussion and results for exhaust rates, nonexhaust rates, and fuel oxygenate effects. Each section begins with a description and critical evaluation of the methodologies and databases used, grouped by the subcategories of that model component. For example, the Section 2 discussion of nonexhaust rate methodologies provides a separate discussion for each nonexhaust mode (e. g., hot soak, diurnal). These sections are followed by a list and critical evaluations of the extrapolations and analogies used for that component of the model. This is provided in tabular format. Each section concludes with a discussion of the sensitivity analyses performed for that component of the model. In some cases this includes extensive tabular or graphical summations of analyses results, which are grouped at the end of the relevant section or as appendices to the report.