Measurement report: The influence of traffic and new particle formation on the size distribution of 1-800nm particles in Helsinki-a street canyon and an urban background station comparison

Most of the anthropogenic air pollution sources are located in urban environments. The contribution of these sources to the population of atmospheric particles in the urban environment is poorly known. In this study, we investigated the aerosol particle number concentrations in a diameter range from 1 to 800nm at a street canyon site and at a background station within 1km from each other in Helsinki, Finland. We use these number size distribution data together with complementary trace gas data and develop a method to estimate the relative contributions of traffic and atmospheric new particle formation (NPF) to the concentrations of sub-3nm particles. During the daytime, the particle concentrations were higher at the street canyon site than at the background station in all analyzed modes: sub-3nm particles, nucleation mode (3-25nm), Aitken mode (25-100nm), and accumulation mode (100-800nm). The population of sub-3nm and nucleation mode particles was linked to local sources such as traffic, while the accumulation mode particles were more related to non-local sources. Aitken mode particles were dominated by local sources at the street canyon site, while at the background station they were mainly influenced by non-local sources. The results of this study support earlier research showing direct emissions of the sub-3nm particles from traffic. However, by using our new method, we show that, during NPF events, traffic contribution to the total sub-3nm particle concentration can be small and during daytime (6:00-20:00) in spring it does not dominate the sub-3nm particle population at either of the researched sites. In the future, the contribution of traffic to particle number concentrations in different urban environments can be estimated with a similar approach, but determining the relationships between the gas and particle concentrations from observations needs to be conducted with longer data sets from different urban environments.