Onboard,Real-World Second-by-Second Particle Number Emissions from 2010 Hybrid and Comparable Conventional Vehicles

Despite hybrid-electric vehicles becoming more prominent in the on-road light-duty vehicle fleet, few studies have evaluated their real-world particle emissions. Hot-stabilized tailpipe particle number concentrations, emission rates, and number distributions emitted from two 2010 Toyota Camry vehicles—one hybrid and one conventional—were quantified during city driving in Burlington, Vermont, at ambient temperatures between −5°C and 9°C with the University of Vermont total onboard tailpipe emissions measurement system. Across replicate real-world runs, the average total cumulative particle number (3- to 3,000-nm particle diameter) emission rates measured for the hybrid vehicle were two times higher than that of the conventional vehicle, despite the hybrid vehicle's internal combustion engine being off 16% to 57% of the run duration. Average second-by-second number distributions for the conventional vehicle had three particle modes (10, 50, and 400 nm), whereas the broader hybrid vehicle distributions had two distinct modes (50 and 400 nm) with shoulders at 10 and 20 nm. These distributions changed little over the route for a given vehicle type. The unexpected higher particle number emission rates from the hybrid, and possibly the differences in number distributions, may be explained by an observed pattern of internal combustion engine restarts under stop-and-go driving that resulted in extreme particle emission rates up to 3.5 × 10¹⁰ (number per second) at every engine restart. The hybrid restart behavior resulted in high particle number emissions localized primarily at intersections. More detailed study is needed on different hybrid vehicle designs to determine the broader significance of the observed particle emission patterns.