Air Pollution:

Limitations of EPA's Motor Vehicle Emissions Model and Plans to Address Them

RCED-97-210: Published: Sep 15, 1997. Publicly Released: Sep 30, 1997.

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Peter F. Guerrero
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Pursuant to a congressional request, GAO reviewed the Environmental Protection Agency's (EPA) MOBILE series of complex computer models to estimate motor vehicle emissions, focusing on the model's major limitations and EPA's process for improving the current and future versions of the model.

GAO noted that: (1) EPA and a group of stakeholders have identified 14 major limitations in the current MOBILE model; (2) some vehicle emissions-producing activities are not accounted for in the current model, and other emissions-producing activities may not be adequately represented on the basis of the most recent information; (3) according to EPA, much of this information has become available since MOBILE5a was released; (4) these limitations cause the model to underestimate vehicle emissions in some cases and overestimate them in others; (5) other studies indicate that some activities are inadequately represented in the model; (6) another study indicates that carbon monoxide and hydrocarbon emissions from higher mileage vehicles may be significantly less than the model's estimates; (7) EPA plans to address most of these limitations in its next revision to the MOBILE model, however, according to agency officials, three of the limitations will probably not be addressed until later because of a combination of factors; (8) according to agency officials, these include the negligible impact on emissions inventory predictions, a relatively low priority ascribed by EPA and stakeholders, the cost and length of time required for these studies relative to the schedule for release of MOBILE6, and the emergence of new technologies that will make the improvements more feasible or cost effective in a few years; (9) EPA officials pointed out that they have updated the estimating capabilities of the MOBILE model 10 times since it was first introduced in 1978; (10) irrespective of these limitations, there are specific actions, most of which were recommended by the Science Advisory Board in its 1989 resolution, that, when followed, can enhance a model's estimating capabilities; (11) among other things, these actions involve documenting the implicit and explicit assumptions that are the basis of the formulas contained in the model, obtaining external stakeholders' input during the model's development, and having the model peer reviewed before it is used; (12) EPA officials acknowledged that, primarily because of resource limitations, until recently such actions have been delayed or forgone; (13) however, EPA is developing the next model, MOBILE6, with significantly increased openness and input from other stakeholders; and (14) EPA also plans to carry out the actions recommended by the Science Advisory Board, such as peer review, as part of its program for developing MOBILE6, due to be issued in late 1998.

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