Greening the City

Biodiversity, the city’s ally

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Purifying and cooling the air

A certain number of plants, especially conifers, are able to capture the harmful fine particles emitted in car exhaust. However, for maximum efficacy in towns, where should they be planted, and in what form? Researchers from the Image, Ville, Environnement Laboratory (1) in Strasbourg (France) are trying to find the answer by using numerical simulations. “In narrow streets lined with high buildings, our work shows for instance that hedges capture more particles at chest height than a line of trees,” explains Christiane Weber, senior researcher at LIVE. “According to our model, the crowns of the trees tend to block air circulation, which concentrates the particles at ground level.”

Plants can also cool down air by evapotranspiration. As part of the Descartes group for the study of the ‘Greater Paris’ project, the Groupe d’étude de l’atmosphère météorologique (2) (GAME) simulated the impact in 2030 of a 30% increase in woodland in the Paris Region, the replacement of cereals by market gardening in a 50 km radius round Paris, and the creation of extensive reservoirs. “According to our results, this scenario could bring down nighttime temperatures in central Paris by as much as 2 °C during heat waves,” explains Valéry Masson, a researcher at GAME. “This is an encouraging avenue of research, since the persistence of high temperatures at night plays a decisive role in the excess mortality observed during heat waves.”

1 - CNRS Laboratory/Université Strasbourg 1 (France).
2 - CNRS Unit/CNRM (Météo France, Toulouse).