Spatial distribution of ultrafine particles at urban scale: The road-to-ambient stage

The spatial variability of ultrafine particles (UFPs) is believed to be an important issue to assess urban air pollution fate and exposure in connection with traffic motorised emissions. In this work, the high-time resolution total number concentration of UFPs was measured at traffic-oriented and urban background locations in a middle-size city in Italy. The major objective was to study valuable connections with local traffic sources, as well as measurement sites representativeness. On the one hand, it was found that the total concentration at the traffic site can be representative of vehicle exhaust sources in ambient air. On the other hand, it was possible to identify three prevailing contributions for the total UFPs number concentration at urban scale: a very low urban background concentration, a significant contribution due to local traffic sources, and a significant contribution due to secondary transformation processes closely linked to meteorology.