Measurement report: Wintertime aerosol characterization at an urban traffic site in Helsinki, Finland

Abstract. Physical and chemical properties of particulate matter and concentrations of trace gases were measured at an urban site in Helsinki, Finland, for 5 weeks to investigate the effect of wintertime conditions on pollutants. The measurement took place in a street canyon (traffic supersite) in January–February 2022. In addition, measurements were conducted in an urban background station (UB supersite, SMEAR III, located approx. 0.9 km from the traffic supersite). Measurements were also made using the mobile laboratory. The measurements were made driving the adjacent side streets and the street along the traffic supersite. Source apportionment was performed for the soot particle aerosol mass spectrometer measurements to identify organic factors connected to different particulate sources. Particle number concentration time series and the pollution detection algorithm were used to compare local pollution level differences between the sites. During the campaign three different pollution events were observed with increased pollution concentrations. The increased concentrations during these episodes were due to both trapping of local pollutants near the boundary layer and the long-range and regional transport of pollutants to the Helsinki metropolitan area. Local road vehicle emissions increased the particle number concentrations, especially sub-10 nm particles, and long-range-transported and regionally transported aged particles increased the PM mass and particle size.