Comparison of Particle Number and Mass Emissions from Diesel Transit Buses across Temporal and Spatial Scales

Two common metrics of particle pollution measure the total number of particles (particle number) and the total mass of the particles (particle mass). This work analyzes particle number and mass emission rates measured from the exhaust of a 2002 diesel transit bus in real-driving conditions using on on-board mini-dilution system. The number concentrations were measured using the Electrical Low Pressure Impactor (ELPI) across a particle range of 7 to 10,060 nm, with 93% of the total particle number concentration measured below 95 nm. Mass emission rates were derived from the number counts using the bottom stages of the ELPI (7 to 387 nm) and were verified to be consistent with concurrent gravimetric filter measurements made on-board the bus. The behavior of the number and mass emission rates are examined at resolved temporal and spatial scales across three facility types: an urban arterial, a rural arterial and a divided freeway. The time-based particle emission rates are highest on the freeway, but at select 50meter segments the distance-based particle emission rates (i.e., “hot-spots” for exposure assessment) occur at intersections when the bus accelerates from a stop. Generally, the number and mass emissions are highly correlated both temporally and spatially. Some deviations do occur because particle mass emissions are highly elevated during sustained fueling events, such as traveling on high grades and sustained accelerations, while particle number emissions are more sensitive to fuel and engine speed fluctuations. The observations are validated using statistical models across two days of testing. The results should be used with qualification, as the sampling system did not fully measure the nucleation mode concentrations which contain the majority of the particle numbers in diesel exhaust. The size distribution data are consistent with heavy-duty vehicle emission sampled from artificial dilution tunnels. However, much higher nucleation mode concentrations were detected from studies that 1. Sampled particles directly from the exhaust plume, and 2. Accurately measured particles with diameters smaller than 7 nm. Introduction Single metrics are used to represent the entire distribution of particulate matter (PM) emitted from vehicle exhaust. Two common metrics evaluate the total number of particles (particle number) and the total mass of the particles (particle mass). For diesel exhaust from conventional heavy-duty vehicles, ultrafine particles (<100 nm) dominate the total number emissions, but contribute little to the total mass of the diesel particles (Kittelson et al., 2004). Fine particles (< 2.5 um) from heavy-duty diesel vehicles contain the majority of mass, with the peak in particle mass distribution occurring between 100 and 180 nm for heavy-duty diesel vehicles (Robert et al., 2007). Both metrics are useful from a health perspective, however the relative importance of each are not readily understood. Currently all U.S. ambient regulations, tail-pipe emission standards, and regulatory emission models quantify PM according to mass-based metrics (McCarthy et al., 2006). The number of diesel particles has been proposed as a more effective health measurement of PM emissions because ultrafine particles have the ability to diffuse deep within the lungs and absorb into the bloodstream (Brunekreef and Holgate, 2002) and can contain higher air toxics per unit mass than fine particles (Sioutas et al. 2005). The spatial distribution of ambient particle concentrations differs significantly depending on the particle size. Near major roadways, particle number concentrations can be more than ~25