Articles | Volume 16, issue 8
Atmos. Chem. Phys., 16, 4817–4835, 2016
Atmos. Chem. Phys., 16, 4817–4835, 2016

Research article 19 Apr 2016

Research article | 19 Apr 2016

Modeling and measurements of urban aerosol processes on the neighborhood scale in Rotterdam, Oslo and Helsinki

Matthias Karl1, Jaakko Kukkonen2, Menno P. Keuken3, Susanne Lützenkirchen4, Liisa Pirjola5,6, and Tareq Hussein6,7 Matthias Karl et al.
  • 1Helmholtz-Zentrum Geesthacht, Institute of Coastal Research, Geesthacht, Germany
  • 2Atmospheric Composition, Finnish Meteorological Institute, Helsinki, Finland
  • 3TNO, Netherlands Organization for Applied Research, Utrecht, the Netherlands
  • 4City of Oslo – Agency for Urban Environment, Oslo, Norway
  • 5Department of Technology, Metropolia University of Applied Sciences, Helsinki, Finland
  • 6University of Helsinki, Department of Physics, P.O. Box 64, 00014 UHEL, Helsinki, Finland
  • 7The University of Jordan, Department of Physics, Amman 11942, Jordan

Abstract. This study evaluates the influence of aerosol processes on the particle number (PN) concentrations in three major European cities on the temporal scale of 1 h, i.e., on the neighborhood and city scales. We have used selected measured data of particle size distributions from previous campaigns in the cities of Helsinki, Oslo and Rotterdam. The aerosol transformation processes were evaluated using the aerosol dynamics model MAFOR, combined with a simplified treatment of roadside and urban atmospheric dispersion. We have compared the model predictions of particle number size distributions with the measured data, and conducted sensitivity analyses regarding the influence of various model input variables. We also present a simplified parameterization for aerosol processes, which is based on the more complex aerosol process computations; this simple model can easily be implemented to both Gaussian and Eulerian urban dispersion models. Aerosol processes considered in this study were (i) the coagulation of particles, (ii) the condensation and evaporation of two organic vapors, and (iii) dry deposition. The chemical transformation of gas-phase compounds was not taken into account. By choosing concentrations and particle size distributions at roadside as starting point of the computations, nucleation of gas-phase vapors from the exhaust has been regarded as post tail-pipe emission, avoiding the need to include nucleation in the process analysis. Dry deposition and coagulation of particles were identified to be the most important aerosol dynamic processes that control the evolution and removal of particles. The error of the contribution from dry deposition to PN losses due to the uncertainty of measured deposition velocities ranges from −76 to +64 %. The removal of nanoparticles by coagulation enhanced considerably when considering the fractal nature of soot aggregates and the combined effect of van der Waals and viscous interactions. The effect of condensation and evaporation of organic vapors emitted by vehicles on particle numbers and on particle size distributions was examined. Under inefficient dispersion conditions, the model predicts that condensational growth contributes to the evolution of PN from roadside to the neighborhood scale. The simplified parameterization of aerosol processes predicts the change in particle number concentrations between roadside and urban background within 10 % of that predicted by the fully size-resolved MAFOR model.

Short summary
Particles emitted from road traffic are subject to complex dilution processes as well as microphysical transformation processes. Particle measurements at major roads in Rotterdam, Oslo and Helsinki were used to analyze the relevance of microphysical transformation processes. Transformation processes caused changes of the particle number concentration of up to 20–30 % on the neighborhood scale. A simple parameterization to predict particle number concentrations in urban areas is presented.
Final-revised paper