What kind of an effect do trees have on aerosol particle concentrations on the pedestrian level of city boulevards? Modelling carried out at the University of Helsinki revealed that the air was cleanest on the street level with three rows of trees of variable height situated along a boulevard-type city street.
A study carried out at the Institute for Atmospheric and Earth System Research (INAR) of the University of Helsinki modelled how different street-tree alternatives and the location of the trees affect air quality on the pedestrian level. The study was carried out in collaboration by the University of Helsinki, the City of Helsinki and the Finnish Meteorological Institute.
In the end, the best option was found to be a row of taller common lime trees (Tilia x vulgaris) in the middle of the street canyon and lower Swedish whitebeams (Sorbus intermedia) lining the sides.
The cleanest air in street canyons would be achieved without any trees at all, as street trees reduce wind speed and, thus, impair the airing out of traffic pollutants from the street canyon. At the same time, vegetation is needed, for example, for ensuring wellbeing, which makes a treeless option unrealistic for planning.