Bus commuter exposure and the impact of switching from diesel to biodiesel for routes of complex urban geometry.

We report on commuters' exposure to black carbon (BC), PM2.5 and particle number (PN, with aerodynamic diameter, da, in the range 0.01 <da< 1.0 μm) collected on-board diesel- and biodiesel-fuelled buses of the Bus Rapid Transit (BRT) system of the city of Curitiba, Brazil. Particulate concentrations measured at high sampling rates allowed the capture of fine gradients along the route and the comparison of in-cabin air pollution on buses of different technologies. Of all metrics, BC showed the largest discrepancies, with mean concentrations of 20.1 ± 20.0 μg m-3 and 3.9 ± 26.0 μg m-3 on diesel- and biodiesel-fuelled buses, respectively. Mean PM2.5 concentrations were similar (31.6 ± 28.5 μg m-3 and 29.0 ± 17.8 μg m-3), whilst mean PN concentrations were larger on the biodiesel buses (56,697 ± 26,800 # cm-3vs. 43,322 ± 32,243 # cm-3). The results are in line with studies on biodiesel emission factors that reported lower BC mass but more particles with smaller diameters. Our hypothesis is that different emission factors of diesel and biodiesel engines reflected in differences of in-cabin particulate concentrations. We found that the passenger exposure during the bus commutes was affected not only by the fuel used but also by the street geometry along the route, with segments with canyon configurations resulting in peak exposure to particulates. The results suggest that i) switching from diesel to biodiesel may help abate commuters' exposure to BC particles on-board buses of the BRT system, whilst it would need to be complemented with after-treatment technologies to reduce emissions; ii) further reductions in exposure (to peaks in particular) could be achieved by changing bus routes to ones that avoid passing through narrow urban street canyons.