Abstract
The complex interaction between the Urban Heat Island (UHI), local circulation, and air quality requires new methods of analysis. To this end, this study investigates the multiple scale nature of the UHI and its relationship with flow and pollutant dispersion in urban street canyons with and without the presence of vegetation. Two field experimental campaigns, one in summer and one in winter, were carefully designed in two parallel urban street canyons in the city of Bologna (44°29′ N, 11°20′ E; Italy) characterized by a similar orientation with respect to the impinging background flow but with a different aspect ratio and a different presence of vegetation. In addition to standard meteorological variables, the dataset collected included high-resolution flow data at three levels and concentration data of several pollutants. The UHI has been evaluated by combining surface temperature of building facades and ground surfaces acquired during two intensive thermographic campaigns with air temperature from several stations in order to verify the presence of intra-city neighborhood scale UHIs additional to the more classical urban–rural temperature differences. The presence of trees together with the different morphologies was shown to mitigate the UHI intensity of around 40% by comparing its value in the center of the city free of vegetation and the residential area. To capture the multiple-scale nature of UHI development, a simple relationship for the UHI convergence velocity, used as a surrogate for UHI strength, is proposed and used to establish the relationship with pollutant concentrations. The reliability of the proposed relationship has been verified using a Computational Fluid Dynamics (CFD) approach. The existence of a robust relationship between UHI strength and pollutant concentration may indicate that the positive effect of mitigation solutions in improving urban thermal comfort likely will also positively impact on air pollution. These results may be useful for a quick assessment of the pollutant accumulation potential in urban street canyons.
Highlights
More than half of the world’s population live in urban areas, and especially in highly-dense cities, a proportion that is expected to increase to 68% by 2050 [1]
Besides the evolution of air temperature measured with the thermohygrometers, the Figure presents the pattern of temperature of the building façades, considering one reference building located on the west side and one located on the east side in both canyons
We have investigated the multiple scale nature of the Urban Heat Island (UHI) phenomenon and its impact on flow dynamics, have derived relationship
Summary
More than half of the world’s population live in urban areas, and especially in highly-dense cities, a proportion that is expected to increase to 68% by 2050 [1]. The identification of mitigation measures capable simultaneously to improve thermal comfort and air quality is currently needed In this framework, urban street trees have been recognized to provide many environmental, social, and economic benefits for our cities [23], among them the reduction of building energy use [24], reduction in high urban temperatures, mitigation of the UHI [25], mitigation of climate change [26], and improvement in thermal comfort [27] as linked to the enhanced shading and conversion of radiation into latent heat and evapotranspiration [28,29].
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