1Tampere University of Technology, Institute of Physics, Aerosol Physics, P.O. Box 692, 33 101 Tampere, Finland
2Finnish Meteorological Institute, Air Quality Research, Sahaajankatu 20 E, 00 810 Helsinki, Finland
3Helsinki Polytechnics, Department of Technology, P.O. Box 4020, 00 099 Helsinki, Finland
4Finnish Institute of Occupational Health, Department of Physics, Topeliuksenkatu 41, 00 250 Helsinki, Finland
5University of Helsinki, Department of Physical Sciences, P.O. Box 64, 00 014 University of Helsinki, Finland
Abstract. The number concentration and size distribution of traffic related particles were measured at road-side in Helsinki. Two winter campaigns took place in 10–26 February 2003 and 28 January–12 February 2004 and two summer campaigns in 12–27 August 2003 and 6–20 August 2004. The measurements were performed simultaneously at distances of 9 m and 65 m from the highway. This study concentrates on data that were measured when the wind direction was from the road to the measurement site. The total concentration in winter time was 2–3 times higher than in summer time and it was dominated by nucleation mode particles. The particles smaller than 63 nm (at aerodynamic size) constitute ~90% of all particles in winter time and ~80% of particles in summer time. The particle total concentration increases with increasing traffic rate. The dependence of particles smaller than 63 nm on traffic rate is stronger than for particles larger than 63 nm both during summer and winter. The particle distribution at the roadside consists of two distinguishable modes. The GMD of nucleation mode (Mode 1) was 20.3 nm at summer and 18.9 nm at winter. The GMD of the larger mode (Mode 2) was 72.0 nm at summer and 75.1 nm at winter. The GMD values of the modes do not depend on traffic rate. The average density value for Mode 1 particles was 1.0 g/cm3 both in summer and winter time, while the average density value for Mode 2 was 1.5±0.1 g/cm3 and 1.8±0.3 g/cm3 for summer and winter time, respectively.