On the Impact of Trees on Ventilation in a Real Street in Pamplona, Spain

Jose-Luis Santiago,Riccardo Buccolieri,Fernando Martín,Elisa Gatto,Esther Rivas,Beatriz Sanchez,Alberto Martilli

doi:10.3390/atmos10110697

Abstract

This paper is devoted to the quantification of changes in ventilation of a real neighborhood located in Pamplona, Spain, due to the presence of street trees Pollutant dispersion in this urban zone was previously studied by means of computational fluid dynamic (CFD) simulations. In the present work, that research is extended to analyze the ventilation in the whole neighborhood and in a tree-free street. Several scenarios are investigated including new trees in the tree-free street, and different leaf area density (LAD) in the whole neighborhood. Changes between the scenarios are evaluated through changes in average concentration, wind speed, flow rates and total pollutant fluxes. Additionally, wind flow patterns and the vertical profiles of flow properties (e.g., wind velocity, turbulent kinetic energy) and concentration, horizontally-averaged over one particular street, are analyzed. The approach-flow direction is almost perpendicular to the street under study (prevailing wind direction is only deviated 4º from the perpendicular direction). For these conditions, as LAD increases, average concentration in the whole neighborhood increases due to the decrease of wind speed. On the other hand, the inclusion of trees in the street produces an increase of averaged pollutant concentration only within this street, in particular for the scenario with the highest LAD value. In fact, the new trees in the street analyzed with the highest LAD value notably change the ventilation producing an increase of total pollutant fluxes inward the street. Additionally, pollutant dispersion within the street is also influenced by the reduction of the wind velocity along the street axis and the decrease of turbulent kinetic energy within the vegetation canopy caused by the new trees. Therefore, the inclusion of new trees in a tree-free street should be done by considering ventilation changes and traffic emissions should be consequently controlled in order to keep pollutant concentration within healthy levels.

Highlights

Besides ecosystem services such as micro-climate regulation, carbon sequestration, rainwater drainage, noise reduction, psychological and recreational values [1,2,3,4,5,6], trees and green infrastructures, in general, are often used in the urban environment as a pollution mitigation strategy [2,7]
We extend the analysis by quantifying the variation of street ventilation in several scenarios, without trees and considering trees with different leaf area density (LAD)
The average concentration depended on LAD and, in particular, it increased from 105 μg m−3 to 113 μg m−3 as LAD rose from 0.1 m2 m−3 to 0.5 m2 m−3

Summary

Introduction

Besides ecosystem services such as micro-climate regulation, carbon sequestration, rainwater drainage, noise reduction, psychological and recreational values [1,2,3,4,5,6], trees and green infrastructures, in general, are often used in the urban environment as a pollution mitigation strategy [2,7]. The wind environment, and the final levels of pollutant concentration is determined by the atmospheric processes inside the urban canopy layer (UCL). Within the UCL, the interaction between the atmospheric flow and urban obstacles (e.g., buildings, trees, etc.) induces complex wind flow patterns which, in general, reduce the city ventilation and increase the pollutant concentration. With the help of computational fluid dynamics (CFD), field and wind-tunnel experiments, ventilation indices have been extensively developed and applied to assess the ventilation, and the distribution of pollutants around buildings (see the recent special issue on urban ventilation by Buccolieri and Hang [9]). Street ventilation and pollutant dispersion are influenced by urban obstacles, and in particular, by urban vegetation. Two main effects are induced by vegetation: Aerodynamic effects, i.e., trees modify the wind flow around them changing the distribution of pollutants

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