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We present an overview of the modelling of particle number concentrations (PNCs) in five major European cities, namely Helsinki, Oslo, London, Rotterdam, and Athens, in 2008. Novel emission inventories of particle numbers have been compiled both on urban and European scales. We used atmospheric dispersion modelling for PNCs in the five target cities and on a European scale, and evaluated the predicted results against available measured concentrations. In all the target cities, the concentrations of particle numbers (PNs) were mostly influenced by the emissions originating from local vehicular traffic. The influence of shipping and harbours was also significant for Helsinki, Oslo, Rotterdam, and Athens, but not for London. The influence of the aviation emissions in Athens was also notable. The regional background concentrations were clearly lower than the contributions originating from urban sources in Helsinki, Oslo, and Athens. The regional background was also lower than urban contributions in traffic environments in London, but higher or approximately equal to urban contributions in Rotterdam. It was numerically evaluated that the influence of coagulation and dry deposition on the predicted PNCs was substantial for the urban background in Oslo. The predicted and measured annual average PNCs in four cities agreed within approximately ⩽26% (measured as fractional biases), except for one traffic station in London. This study indicates that it is feasible to model PNCs in major cities within a reasonable accuracy, although major challenges remain in the evaluation of both the emissions and atmospheric transformation of PNCs. [ABSTRACT FROM AUTHOR]

We present an overview of the modelling of particle number concentrations (PNC's) in five major European cities, namely Helsinki, Oslo, London, Rotterdam and Athens, in 2008. Novel emission inventories of particle numbers have been compiled both on urban and European scales. We use atmospheric dispersion modelling for PNC's in the five target cities and on a European scale, and evaluate the predicted results against available measured concentrations. The concentrations of PN in the selected cities were mostly influenced by the emissions originated from local vehicular traffic; however, in some of the cities, also harbour and airport activities were significant. The highest values of the predicted PNC's were higher in the megacities, London and Athens, and also in Rotterdam, whereas these were lower in Helsinki and Oslo. It was numerically evaluated that the influence of coagulation and dry deposition on the predicted PNC's was substantial for urban background in Oslo. The predicted and measured annual average PNC's in four cities agreed within approximately ⩽36% (measured as fractional biases), except for one traffic station in London. The indexes of agreement (IA) for the comparisons of hourly measured and predicted time-series in Oslo and Helsinki ranged from 0.75 to 0.79. [ABSTRACT FROM AUTHOR]