First comprehensive estimation of particle number concentrations carried out in five European cities and for the whole of Europe

The Finnish Meteorological Institute participated in an extensive international study called TRANSPHORM to model the emissions and concentrations of particle numbers in the whole of Europe for the first time. Comprehensive information on particle size distribution and, for example, the number of particles has not been available before. Additionally, particle number concentrations were examined in more detail in five European cities, one of which was Helsinki.

"It has not been possible to present results as comprehensive as this earlier because there is a very limited amount of measured data on particle numbers and this kind of modelled information has not been available neither for these cities nor the whole of Europe at all. Based on these results, it is possible to assess the health effects of particles and to consider how particle numbers and the health effects they cause could be reduced most effectively," says Professor Jaakko Kukkonen from the Finnish Meteorological Institute. Comprehensive information about particle numbers has not been available in Europe or anywhere else earlier because there are only isolated measurement stations in cities. "It is not possible to obtain a comprehensive picture or spatially representative data on the sources of particle number concentrations on the basis of these measurement stations."

In addition to Helsinki, the target cities in the study were Oslo, London, Rotterdam and Athens. In all these cities, the local emission source that most affected particle number concentrations was vehicular traffic. The impact of shipping and harbours was also significant everywhere else, except in London. Aviation in turn had a considerable effect especially in Athens. In cities that were situated in densely populated and industrial areas, such as Rotterdam and London, particles transported from outside the city had an even greater impact than local sources.

Earlier estimates of the harmful effects of particles have mainly been based on assessing the mass concentration of particles. "However, the adverse effects and harmfulness of particles depends strongly on their size, and not so much on their total mass," Jaakko Kukkonen states.

The accuracy of the modelled results was estimated by comparing the calculated and measured concentrations, both on a European scale and in the target cities.