1University of Helsinki, Department of Physical Sciences, Finland
2Stockholm University, Department of Applied Environmental Science, Sweden
3Finnish Meteorological Institute, Air Quality Research, Finland
4Institute for Atmospheric Science, School of Earth and Environment, University of Leeds, UK
5Helsinki Polytechnic, Department of Technology, Finland
6School of Mathematical Science, Queensland University of Technology, Brisbane, Australia
7Department of Chemistry, University of Copenhagen, Denmark
Abstract. This study presents an evaluation and modeling exercise of the size fractionated aerosol particle number concentrations measured nearby a major road in Helsinki during 23 August–19 September 2003 and 14 January–11 February 2004. The available information also included electronic traffic counts, on-site meteorological measurements, and urban background particle number size distribution measurement. The ultrafine particle (UFP, diameter<100 nm) number concentrations at the roadside site were approximately an order of magnitude higher than those at the urban background site during daytime and downwind conditions. Both the modal structure analysis of the particle number size distributions and the statistical correlation between the traffic density and the UFP number concentrations indicate that the UFP were evidently from traffic related emissions. The modeling exercise included the evolution of the particle number size distribution nearby the road during downwind conditions. The model simulation results revealed that the evaluation of the emission factors of aerosol particles might not be valid for the same site during different time.