Methodology for Evaluating Mobile Source Air Toxic Emissions

Abstract

With the final update to its on-road mobile source emission factor model, MOBILE6.2, the U.S. Environmental Protection Agency (EPA) added capabilities for predicting emissions factors for a select group of mobile source air toxics (MSATs), commonly referred to as the six priority MSATs. These are acetaldehyde, acrolein, benzene, 1,3-butadiene, diesel particulate matter, and formaldehyde. This paper describes a methodology for computing and evaluating emissions of MSATs among a group of transportation project alternatives. When the future options for upgrading a transportation corridor are evaluated, the major mitigating factor in reducing MSAT emissions is the implementation of EPA's new motor vehicle emission control standards. Substantial decreases in MSAT emissions will be realized from a current base year through an estimated time of completion for a planned upgrading project and its design year some 25 years in the future. Even when anticipated increases in vehicle miles of travel and varying degrees of efficiency of vehicle operation are accounted for, total MSAT emissions are predicted to decline more than 61% from 2005 to 2030. Although benzene emissions are predicted to decline more than 35%, emissions of diesel particulate matter are predicted to decline considerably more (i.e., >92%). The ability to discern remarkable differences in MSAT emissions among transportation alternatives is difficult, given the uncertainties associated with the forecasting of travel activity and air emissions 25 years or more into the future. In this hypothetical congestion mitigation project, differences in MSAT emissions between the build and no-action alternatives range from more than 6% to less than 16%.