Top on the list of tools needed to reliably forecast any potential adverse health outcomes from proposed highway projects is, predictably, mobile source emission factor models. This paper provides a comparison of the two current regulatory models Emfac2007, developed by the California Air Resources Board for applications in California and MOBILE6.2, developed by the U.S. Environmental Protection Agency for regulatory applications elsewhere in the nation. Mobile source air toxic (MSAT) emission factors computed by the models are evaluated. While the two models share the same fundamental principal, i.e., emissions from motor vehicles are based on testing with correction factors utilized to account for on-road vehicle use; the models produce strikingly divergent results for some MSAT compounds. And unexpectedly enough, Emfac2007 predicts higher MSAT emissions for a supposedly cleaner vehicle fleet. The reasons for these calculation differences are discussed. The implications pertaining to the evaluation of highway project alternatives are also discussed.
Samenvatting