Meteorological and urban determinants of in-bus ultrafine particle exposure in Auckland, New Zealand

Abstract

For many people living in urban areas, exposure to air pollution while commuting represents a significant proportion of their total daily exposure. Most commuter exposure studies carried out to date have focused on commuting by car or compared different modes of commuting for similar journeys. Little attention has been given to buses, particularly in regard to the determinants influencing in-bus commuter exposures. The aim of this study was to identify the processes which affect the exposure of people on board buses to ultrafine particles (UFP). For this study, UFP exposure data were collected simultaneously on near-identical buses travelling along three bus routes: one a short inner city loop, one loop travelling through inner suburbs and one along a longer route in the outer suburbs of Auckland, New Zealand. Data were collected during periods of morning rush hour in October 2013 and again in March 2014. The factors influencing in-bus UFP exposure were assessed both qualitatively and quantitatively using a combination of spatial, statistical and GIS analysis techniques. The results of the study suggest that in-bus UFP exposures on the inner city route were higher compared to those of commuters on the suburban routes. A prominent feature during bus trips was the occurrence of rapid increases in UFP exposures (spikes). Trips which had at least one spike were found to have significantly higher exposures compared to trips which did not. Urban features such as bus speed (congestion), being stopped at bus stops, being at traffic light intersections, and passing tightly packed buildings with awnings were associated with a higher likelihood of occurrence of a spike and thus high exposures. In addition street canyon vortices were observed to play an important part in the occurrence of spikes, as there was an increased likelihood of pollutants infiltrating the bus when the bus stop was located on the leeward side of the road. The results of this study suggest that reducing the likelihood of occurrence of such spikes is expected to be an effective management tool for minimising air pollution exposure for bus commuters. This could be achieved by placing bus stops away from traffic lights, in areas where there is minimal congestion, and areas where buildings are sparsely spaced.