Influencing factors for In-cabin Ultrafine Particles Attributed to Tailpipe Exhaust from School Buses

QUNFANG ZHANG, Shusen Liu, Bin Xu, Yifang Zhu

University of California Los Angeles

     Abstract Number: 92
     Last modified: March 30, 2011

Abstract
Self pollution of a school bus is known as the intrusion of tailpipe exhaust into the cabin, and thus increases health risk of school children when commuting in a school bus. But the factors determining the magnitude of in-cabin ultrafine particles (UFPs, diameter < 100 nm) attributed to tailpipe exhaust remain unclear. In this study, data were collected for 11 buses in an open space with little surrounding traffic under stable meteorology condition. Engine operation (on/off), wind direction (perpendicular/parallel), window (closed/open) and bus position (upwind/downwind) were studied simulating the worst exposure scenario when buses were idling to pick up/drop off children. UFP levels were monitored simultaneously inside the cabins of an upwind school bus and a downwind school bus, near the bus tailpipes, and upwind of the two buses. The contribution to in-cabin UFPs from tailpipe emissions strongly depended on wind direction and window position. Parallel wind pushed the tailpipe exhaust into the bus cabin and introduced more tailpipe UFPs into school bus cabins than perpendicular wind. School buses with open windows were usually found to have higher UFP concentrations than those with closed windows if the other parameters were fixed. The average contribution from self-pollution under parallel wind was 4.9 ×10$^3 #/cm$^3 with closed windows and 16.3 ×10$^3 #/cm$^3 with open windows, both of which were statistically significant. The contribution from the tailpipe emissions of other buses parked close to the studied buses was only significant for the downwind buses, with an average increase of 18.2 ×10$^3 #/cm$^3 when the windows were open. A conservative estimate of the proportion of cabin air that came from tailpipe emissions was approximately 0.001%~0.069%, which was comparable to the results of a previous study using SF6 as tracer gas.