Project title: Comparison of Methods to Estimate On-bus Pollution from Diesel School Buses - A Pilot Study
Completed in: 2006
Nationwide, over 24 million children breathe diesel exhaust (DE) on any weekday while riding diesel school buses, prompting a regulatory focus on retrofitting diesel engines. To determine the effectiveness of retrofit technologies, it is important to differentiate the on-bus DE contribution of bus self-pollution from on-road vehicle exhaust and background pollution. We conducted an intensive sampling campaign on two representative Seattle school buses to determine self-pollution using three approaches, including dual-tracers, lead-vehicle, and chemical mass balance (CMB) methods. The dual-tracers method involved spiking diesel fuel with iridium and lubricating oil with N-hexatriacontane-D74. The lead vehicle, a 1996 Chrysler Minivan, operated ahead of the bus to measure on-road traffic exhaust. PM2.5, PM1, ultrafine PM, PAHs, EC/OC, BC, CO, and NO2 were monitored with both continuous and integrated instruments for four 2-h runs per bus. With windows alternating between open and closed, the buses drove along a residential route, simulation typical pick-ups and drop-offs. Source samples collected using an on-board dilution tunnel connected o the tailpipe and crankcase were analyzed for the tracers, selected elements, and organics. Chemical analysis results indicate that OC, hopanes, and steranes dominated crankcase emissions, while EC dominated tailpipe emissions. On-bus measurements for PM1 and most PM components were higher than those measured in the lead vehicle, indicated significant bus self-pollution. The self-pollution levels ranged between -1.1 and 9.8μg/m3 for PM1 and between 0.7 and 2.79 μg/m3 for BC using the aethalometer. The dual-tracers method indicates that PM2.5 from the crankcase (range: 0.5-20.2 μg/m3 ) and tailpipe (range: 0.2-2.3μg/m3) accounted for an average of 80 +/- 18% and 20 +/- 18% of self-pollution, respectively. CMB results also quantified crankcase emissions as the major source of self-pollution. In general, when windows were open, self-pollution levels diminished. PM1 results from the lead-vehicle approach correlated well with those from dual-tracers method for the crankcase (R2 = 0.88) and tailpipe (R2 = 0.82), denoting the lead-vehicle method as a cost-effective means for determining self-pollution. This is the first study that provides a definite and crosschecked determination of bus self-pollution, which is dominated by the crankcase emissions.