MODELLING THE IMPACTS OF URBAN TREES ON AIR QUALITY IN STREETS

Models are commonly used to study air quality in urban areas, which are known to be areas of poor air quality. To model air pollutant concentrations at a fine scale (street) and over entire cities, fast running codes are developed such as the Model of Urban Network of Intersecting Canyons and Highways (MUNICH). They used simplified parameterizations especially for transport. Although they can not reproduce finely the street micrometeorology, they can reproduce the gradients of concentrations between the streets and the urban background. These transport parameterizations have been recently improved and the effect of the tree crown was included in MUNICH. Planting trees in cities has been widely developed nowadays mainly for their capacity to decrease temperatures and limit the urban heat island. Tree effects on air quality are various, they can have a positive impact by capturing pollutants by dry deposition. However, trees reduce air flow in the street and limit pollutant dispersion. They also emit volatile organic compounds (BVOCs) that can lead to ozone and secondary organic aerosol production. The objective of this study is to analyse and compare these three effects of trees on air quality based on simulation results from MUNICH. For the test case presented here, the tree aerodynamic effect is the main effect, it induces an increase of the concentrations (up to +10%). The decrease of concentrations due to dry deposition is very limited (less than 1%). The BVOC emission has a negligible impact on particle formation at the local scale.