A simulation study of the co-benefit of in-canyon trees for heat mitigation and air quality

The co-benefit of urban trees for  heat mitigation and air quality improvement is not fully understood, and conflicting recommendations on whether trees should be integrated into (core) urban areas exist in literature. For climate-sensitive urban design,  trees help mitigate urban overheating. However, in terms of air quality-related effect, there is conflicting recommendation depending on the perspective of the study: dispersion (aerodynamic effect) or deposition (removal of pollutants). For the former, trees minimize pollutant dispersion and increase concentration levels, thus not recommended for urban integration. On the other hand, trees have been found to remove particulate pollutants through their filtration and deposition mechanisms, thus, improving air quality – recommended. This study leveraged the Particle(gas) Dispersion and Deposition Module (PDDM) and atmospheric modules in the ENVI-met modelling system to investigate the relationship between 3D tree form, urban morphology, urban microclimate and air quality. Parametric models were developed with variable urban morphology (SVF:0 to 1) and different 3D tree form based on tree height, trunk height, crown diameter and foliage density; with fixed traffic and meteorological information as boundary conditions. Thereafter, the micro-climate and air-quality related output were analysed using a proposed integrated climate-air quality index that accounts for the impact of trees on pollutant dispersion, deposition, and heat mitigation to estimate the co-benefits of different trees forms in different urban morphology. Results show  Findings are useful in practice as it helps recognize a set of tree form (species) with similar and simultaneous heat mitigation and air quality improvement capacities thereby aiding decision making in selecting the right set of species for the right place.