Insights into ultrafine particle exposure to bus commuters gained from measurements in Manchester, UK

Prior to this study, little information existed regarding the exposure of bus commuters to UFP. Ad-hoc measurements of UFP number concentrations on buses in south Manchester were made in order to grasp the key variables which control those concentrations, and thus commuter exposure. These limited results indicate that those on-bus UFP concentrations related to an urban background source could be estimated by reference to urban background NO_x concentrations measured at a fixed point, and that this component was significant. Such background measurements, however, do not capture the way that the bus moves through a variable concentration field. We were unable to conclude whether the direction of movement through this varying background was a significant factor. Work is ongoing to investigate how a GIS-based description of the spatial variation in background pollution can inform this work. Local emissions were found to be important in two ways: firstly, the general level of particle emission on the road along which the bus is travelling was found to be important as indicated by the higher UFP:NO ratio found on routes with busy traffic; secondly, intermittently high concentrations could be experienced on the bus when it followed gross polluters, particularly older buses. The effect of brief encounters with such buses was extended in time by the lag introduced by restricted ventilation, therefore higher concentrations in general would be expected on routes with a higher frequency of buses, especially older buses. High frequency routes are inevitably the routes carrying the most passengers, and therefore the most important for exposure. For these reasons a key element of the subsequent study involved gathering data on the nature of these inter-bus encounters, with a focus on the very high frequency route of Oxford Road. Pedestrian measurements made at either end of a bus journey indicate that concentrations can be of a similar magnitude as those experienced during the bus-part of the journey, and should not be discounted when trying to assess exposure to UFP during commuting activities. Subsequent studies will attempt to investigate this element systematically and build it into a GIS-based exposure assessment tool.