Abstract
This paper discusses the combined influence of building morphology and trees on air pollutant concentrations in the Marylebone neighbourhood (central London). Computational Fluid Dynamics (CFD) simulations are performed with OpenFOAM using the k-ε model. Aerodynamic and deposition effects of Platanus acerifolia trees are considered. While aerodynamic effects are treated as typically done in the literature, i.e. as a porous media, for the deposition an enhanced model with an additional sink term was implemented. CFD results are compared with UK AURN (Automatic Urban and Rural Network) station concentrations. Several meteorological conditions are analysed based on London City Airport weather station data, with attention to prevailing winds.CFD simulations show that trees trap air pollution by up to about 7% at the Marylebone monitoring station in the spring, autumn and summer seasons, suggesting that the aerodynamic effects are similar over the different leaf seasons. Aerodynamic effects are more important at lower wind speeds causing little turbulent dispersion. Deposition effects are found to be 4 times less important with reductions of up to about 2%, with more deposition in summer due to a greater leaf area density. Furthermore, for winds parallel to Marylebone Road, the aerodynamic effects decrease concentrations suggesting that in such cases trees could be considered as a mitigation measures. This is different from perpendicular winds for which trees exacerbate trapping, as found in previous studies. The analysis of concentration levels obtained from CFD simulations across the whole street confirms a beneficial aerodynamic dispersive effect of trees of 0.7% in summer time for all wind directions averaged at a wind speed of 5m/s (yearly average wind speed observed in the area). Results highlight the need to account for both aerodynamic and dispersion effects of trees in CFD modelling to achieve a comprehensive evaluation and help city planners with a sustainable design of trees in urban environments.
Highlights
Many municipalities have shown a renewed interest in “urban forestry” by incorporating green space and vegetation into the urban environment
CFD simulations are used to provide a comprehensive evaluation of concentration levels and of the effects of trees over the whole study area, which is not possible for data monitored at a single point (Subsections 4.4)
- this study confirms previous findings that the street air quality is altered by trees, with increases of 7% for typical meteorological conditions at the monitoring site, and an additional benefit of 2% reduction of PM2.5 via deposition;
Summary
Many municipalities have shown a renewed interest in “urban forestry” by incorporating green space and vegetation into the urban environment. Researchers have been looking into potential benefits of green space and vegetation, including lower energy use, reduced air pollution (Gallagher et al, 2015; Gromke et al, 2016; Li et al, 2016), protection from harmful exposure to ultraviolet rays, heat island mitigation, decreased storm water runoff, potential reduced pavement maintenance (Roy et al, 2012; Maggiotto et al, 2014; Di Sabatino et al, 2015; Hsieh et al, 2016), improved wellbeing of the urban population (White et al, 2013; Van den Berg et al, 2015) and reduced traffic noise levels (Kalansuriya et al, 2009). Jeanjean et al / Urban Forestry & Urban Greening 22 (2017) 41–53
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