Reduced effectiveness of tree planting on micro-climate cooling due to ozone pollution—A modeling study

Abstract

Urban tree planting is an important measure to provide micro-climate cooling (MCC) in megacities. However all of the studies available in the literature were focused on an ideal and clean environment where trees are not under environmental stresses. Here, we studied the effects of ambient ozone on the leaf area index (LAI) of trees and the resultant reduction in the effectiveness of trees in providing MCC. The ENVI-met model, which was well validated against our bio-meteorological measurements, was used to quantify the effect of reduced LAI on the cooling performance of trees in two urban sites of Hong Kong. Under the influence of ambient ozone stress with a high sky view factor (SVF), the sensible heat fluxes and surface temperatures were calculated to be at least 40 Wm−2 and 5 °C higher than the stress-free case, respectively. For the low-SVF situation with the LAI value decreased by 25%, the physiological equivalent temperature (PET) values indicated that heat-related physiological stress increased by 1 grade in the planted areas. With the increasing surface-level ozone concentrations in the southern China region, the current study provides useful information for cities to refine design schemes and plan specifications for urban greenery to cope with the problem.