Source apportionment of particle number size distribution at the street canyon and urban background sites

Abstract. Particle size is one of the key parameters of aerosol particles affecting their climate and health effects. Therefore, a better understanding of the particle size distributions from different sources is crucial. In urban environments, aerosols are produced in a large number of varying processes, and conditions. This study aims to increase the knowledge of urban aerosol sources using a novel approach to positive matrix factorization (PMF). The particle source profiles are detected in particle number size distribution data measured simultaneously at nearby a street canyon and an urban background station between February 2015 and June 2019 in Helsinki southern Finland. The data is combined into one file so that the data from both stations has the same timestamps. Then PMF finds profiles for the unified data. A total of five different aerosol sources were found. Four of them were detected at both stations: slightly aged traffic (TRA2), secondary combustion aerosol (SCA), secondary aerosol (SecA), and long-range transported aerosol (LRT). One of the sources, fresh traffic was only detected at a street canyon. The factors were identified based on available auxiliary data. This work implies that traffic-related aerosols remain important in urban environments and that aerosol sources can be detected by using only particle number size distribution data as input in the positive matrix factorization method.