Quantification of Self Pollution from Two Diesel School Buses using Three Independent Methods

Abstract

We monitored two Seattle school buses to quantify the buses' self pollution using the dual tracers (DT), lead vehicle (LV), and chemical mass balance (CMB) methods. Each bus drove along a residential route simulating stops, with windows closed or open. Particulate matter (PM) and its constituents were monitored in the bus and from a LV. We collected source samples from the tailpipe and crankcase emissions using an on-board dilution tunnel. Concentrations of PM(1), ultrafine particle counts, elemental and organic carbon (EC/OC) were higher on the bus than the LV. The DT method estimated that the tailpipe and the crankcase emissions contributed 1.1 and 6.8 mug/m(3) of PM(2.5) inside the bus, respectively, with significantly higher crankcase self pollution (SP) when windows were closed. Approximately two-thirds of in-cabin PM(2.5) originated from background sources. Using the LV approach, SP estimates from the EC and the active personal DataRAM (pDR) measurements correlated well with the DT estimates for tailpipe and crankcase emissions, respectively, although both measurements need further calibration for accurate quantification. CMB results overestimated SP from the DT method but confirmed crankcase emissions as the major SP source. We confirmed buses' SP using three independent methods and quantified crankcase emissions as the dominant contributor.

Figure 1

Schematic of various measurements made in bus cabin, on LV and during tailpipe and crankcase sampling.

Figure 2

Comparison of 2-h self-pollution estimates from the LV method with different measurements vs. the tracers estimates for PM from the tailpipe (PM_tp) and crankcase (PM_ck). Filled and void symbols represent window open and closed runs, respectively. A 1:1 dotted line is also included for reference.

Figure 3

One-hour self-pollution estimates (red symbols) from the LV method vs. the tracer estimates for tailpipe (PM_tp) and crankcase PM_2.5 (PM_ck). The 2-h samples and a 1:1 dotted line are included for reference.

Figure 4

Estimates of in-cabin PM_2.5 contributions from the tailpipe (TP), crankcase (CK), and other sources using the dual tracers (DT) and the chemical mass balance (CMB) with partial least square methods.