Changes to manuscript ms-nr acp-2015-903 Modelling and Measurements of Urban Aerosol Processes on the Neighbourhood Scale in Rotterdam, Oslo and Helsinki

This study evaluates the influence of aerosol processes on the particle number (PN) concentrations in three major European cities on the temporal scale of one hour, i.e. on the neighborhood and city scales. We have used selected measured data of particle size distributions from pre5 vious campaigns in the cities of Helsinki, Oslo and Rotterdam. The aerosol transformation processes were evaluated using an aerosol dynamics model MAFOR, combined with a simplified treatment of roadside and urban atmospheric dispersion. We have compared the model predictions of particle 10 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 15 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 com20 pounds 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 pro25 cess 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 veloci30 ties 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 35 by vehicles on particle numbers and on particle size distributions was examined. Under inefficient dispersion conditions, condensational growth might contribute ::: the ::::: model ::::::: predicts ::: that :::::::::::: condensational :::::: growth :::::::::: contributes to the evolution of PN from roadside to the neighborhood scale. The simplified pa40 rameterization of aerosol processes can predict :::::: predicts ::: the :::::: change :: in : particle number concentrations between roadside and the urban background with an inaccuracy of ∼ 10 :::: urban ::::::::: background :::::: within ::: 10 % , compared to :: of :::: that :::::::: predicted :: by the fully size-resolved MAFOR model. 45