A study carried out at the Institute for Atmospheric and Earth System Research (INAR) of the University of Helsinki modeled 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.
"In that case, the air was one-fifth cleaner compared to the least favorable option, that is the one with four rows of trees of equal height," says Associate Professor Leena Järvi, head of the research group.
Järvi considers the difference surprisingly marked. Another find that can be considered a surprise is the fact that meter-high hedgerows located under the tree row closest to the edge were seen to have no practical impact on the air quality of pavements.