The influence of trees on air quality in a street canyon
Investigating the effect of leaf area density and seasonality on the dispersion of particulate matter emitted in a street canyon
S.I. van der Windt (TU Delft - Civil Engineering & Geosciences)
S.J.A. van der Linden – Mentor (TU Delft - Civil Engineering & Geosciences)
C. Garcia Sanchez – Mentor (TU Delft - Architecture and the Built Environment)
A. Patil – Graduation committee member (TU Delft - Architecture and the Built Environment)
Jorge Sousa – Mentor (VITO-Energyville)
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Abstract
Long-term exposure to hazardous pollutants in the air is a problem in urban areas all over the world. In European countries, where regulations have been sharpened over the years, an estimated 200,000 to 300,000 people die prematurely every year due to bad air quality. Many more people experience negative health effects, like respiratory and cardiovascular diseases. In the Netherlands as well, air quality has improved over the years. Yet still, most of the country exceeds the WHO standards for most pollutants. In an effort to bring the negative impact of air pollution down to 0, many urban planners suggest planting trees as a potential mitigation strategy. However, the impact of trees on air quality is not fully understood.
This thesis investigates how the leaf area density, or LAD, of trees in a street canyon affects the dispersion of pollutants emitted in said canyon. It does so with a Reynolds-Averaged Navier-Stokes model in the OpenFOAM v7 software. A street canyon with an aspect ratio of 1:1 is considered, with a row of trees running through the middle. For four different LAD values, the impacts on canyon concentrations are examined. The results for mean concentrations on the facades and within the canyon are computed and visualised for three different wind directions: one parallel to the canyon, one perpendicular to the canyon, and one at a 45° angle. This was also done for multiple street lengths to investigate the impact of the street boundaries.
The results for this LAD investigation are used to determine monthly averages and seasonal effects. Therefore, the results are subjected to daily KNMI meteorological data. This method utilises wind parameters, direction and speed, to estimate canyon concentrations. Two different types of trees are considered: deciduous trees, which lose their leaves in winter, and coniferous trees, which are evergreens.
It is found that an increase in LAD causes an increase in pollution for parallel wind and angled wind, especially near the boundaries. This is mainly due to the reduced wind speeds within the canyon, limiting dilution. For parallel wind, recirculation zones caused by trees cause the canyon concentrations to accumulate. For perpendicular wind, lower pollutant concentrations are found with higher LAD, due to enhanced vertical transport and reduced accumulation resulting from limited lateral transport.
The seasonal impact of deciduous and coniferous trees is very LAD-dependent when taking yearly averages. Depending on the chosen LAD value, deciduous trees are up to 17-23% worse than a case with no trees, and coniferous trees are up to 28-36% worse, although for low LAD values the impact is small.