A feasibility study of mapping light-absorbing carbon using a taxi fleet as a mobile platform

Carbon-containing particles are associated with adverse health effects, and their light-absorbing fractions were recently estimated to be the second largest contributor to global warming after carbon dioxide. Knowledge on the spatiotemporal variability of light-absorbing carbon (LAC) particles in urban areas is relevant for air quality management and to better diagnose the population exposure to these particles. This work reports on the first mobile LAC mass concentrations (MLAC) measured on-board four taxis in the Stockholm metropolitan area in November 2011. On average, concentrations were higher and more variable during daytime (median of 1.9 µg m−3 and median absolute deviation of 2.3 µg m−3). Night-time (21:00–05:00) measurements were very similar for all road types and also compared to levels monitored at an urban background fixed site (median of 0.9 µg m−3). We observed a large intra-urban variability in concentrations, with maxima levels inside road tunnels (median and 95th percentile of 7.5 and 40.1 µg m−3, respectively). Highways presented the second ranked concentrations (median and 95th percentile of 3.2 and 9.7 µg m−3, respectively) associated with highest vehicle speed (median of 65 km h−1), traffic rates (median of 62 000 vehicles day−1 and 1500 vehicles h−1) and diesel vehicles share (7–10%) when compared to main roads, canyon streets, and local roads. Multiple regression modelling identified hourly traffic rate and MLAC concentration measured at an urban background site as the best predictors of on-road concentrations, but explained only 25% of the observed variability. This feasibility study proved to be a time- and cost-effective approach to map out ambient MLAC concentrations in Stockholm and more research is required to represent the distribution in other periods of the year. Simultaneous monitoring of other pollutants, closely correlated to MLAC levels in traffic-polluted environments, and including video recording of road and traffic changes would be an asset.