Ultrafine Particle Number Concentrations from Hybrid Urban Transit Buses

Abstract

Recent studies have focused on mass-based quantification of gas and particulate matter (PM) transit bus exhaust emissions under laboratory dynamometer testing conditions because transit buses frequent heavily populated areas and are major contributors to the ambient fine particles in urban regions. This study examines the ultrafine particle (particle mobility diameters of <100 nm) number concentrations and size distributions for conventional diesel and hybrid electric diesel transit buses by the use of onboard exhaust measurements with a scanning mobility particle sizer from TSI, Inc., operated in single-diameter mode. The buses were run on three bus routes (freeway commuter, local, and high-grade arterial routes), with different fuels (No. 1 diesel and ultra-low-sulfur diesel), and with and without diesel particulate filter (DPF) aftertreatment at different points throughout the study. This is the first urban bus emissions study conducted with buses traveling on actual bus routes. Particle number distributions varied by route but not by bus type or fuel sulfur content. Particle number concentrations were higher, on average, for high-load routes (freeway 65-mph commuter and steep-grade routes). There were no significant differences in particle number distributions between the 2003 parallel hybrid electric and the 2002 conventional diesel bus types, likely because of the similar diesel engine specifications and a hybrid control strategy that was not optimized for particulate emissions benefits. For both bus types, use of a DPF resulted in 95% to 99% number concentration reductions for particles of all diameters sampled (10 to 130 nm) on all routes. The study results point to DPFs as a cost-effective solution for achieving particulate emissions control from diesel transit buses.