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Number concentrations and size distributions of traffic related aerosolparticles were measured at a roadside in Helsinki during two wintercampaigns (10–26 February 2003, 28 January–12 February 2004) and two summercampaigns (12–27 August 2003, 6–20 August 2004). The measurements wereperformed simultaneously at distances of 9 m and 65 m from the highway.Total number concentrations were measured by a condensation particle counter(CPC) and particle size distributions by a scanning mobility particle sizer(SMPS) and an electrical low pressure impactor (ELPI). This studyconcentrates on data that were measured when the wind direction was from theroad to the measurement site. The total concentrations in the wintertimewere 2–3 times higher than in the summertime and the concentrations weredominated by nucleation mode particles. The particles smaller than 63 nm(aerodynamic diameter) constituted ~90% of all particles in thewintertime and ~80% of particles in the summer time. The particletotal concentration increased with increasing traffic rate. The effect oftraffic rate on particles smaller than 63 nm was stronger than on the largerparticles. The particle distributions at the roadside consisted of twodistinguishable modes. The geometric mean diameter (GMD) of nucleation mode(Mode 1) was 20.3 nm in summer and 18.9 nm in winter. The GMD of the largermode consisting mostly of traffic related soot particles (Mode 2) was 72.0 nmin summer and 75.1 nm in winter. The GMD values of the modes did notdepend on the traffic rate. The average particle density for each mode wasdetermined by a parallel density fitting method based on the sizedistribution measurement made by ELPI and SMPS. The average density valuefor Mode 1 particles was 1.0±0.13 g/cm3 and 1.0±0.07 g/cm3both in summer and winter respectively, while the average densityvalue for Mode 2 was 1.5±0.1 g/cm3 and 1.8±0.3 g/cm3for summer and winter, respectively

Winter and summer time size distributions and densities of traffic-related aerosol particles at a busy highway in Helsinki