Reducing Particulate Matter and Oxides of Nitrogen Emissions from Heavy-Duty Vehicles: The Urban Bus Case

The 1990 Clean Air Act Amendments mandated stricter emissions standards for heavy-duty vehicles. One category of heavy-duty vehicles, urban transit buses, constitutes a highly visible source of pollutant emissions and must meet even more stringent standards. In response, engine manufacturers have produced diesel engines that produce virtually no black smoke and emit several times less particulate matter (PM) than older engines. The tighter PM emissions standards that apply to urban transit buses only were found to be cost-effective ($4,600 to $6,300 per Mg of PM reduced) compared with other control strategies. The Urban Bus Retrofit/Rebuild Program was found to be somewhat less cost-effective ($6,900 to $42,000 per Mg). Both of these programs could be extended to other heavy-duty vehicles. However, the use of compressed natural gas (CNG) in transit buses was found to have a costeffectiveness of $0.9 million to $1.8 million per Mg of PM. Replacing older diesel engines with low-emission diesels is the most cost-effective way to reduce transit bus emissions. By purchasing more clean diesels instead of fewer cleaner CNG buses, transit agencies can produce greater reductions in fleetwide emissions. In fact, the transit agency in this study with the most alternative-fuel buses has the highest fleetwide PM emissions rate. It is premature to expand the alternative-fuel transit bus fleet beyond the experimental level. Regulatory policy should be reassessed as the understanding of the link between diesel emissions and health effects improves.