Growing greenery in streets and building roofs, reduces energy consumption and improves air quality (Bourbia & Boucheriba, 2010; Tafvizi et al., 2022). Apart from the possible reduction of air pollution, Urban Green Infrastructure (UGI) provides benefits by decrease of Urban Heat Island (UHI) (Aflaki et al., 2017; Chen et al., 2014; Gago et al., 2013), providing less energy consumption (Berardi et al., 2014; Pérez et al., 2014) and less noise pollution (Berardi et al., 2014; Cohen et al., 2014; Salmond et al., 2016). Moreover, greenery in urban spaces controls storm (Berndtsson, 2010; Roy et al., 2012) and mitigates climate change (Matthews et al., 2015). In addition, the environmental services provided by green interventions improve the health and well-being of the urban population in several ways (Dean et al., 2011; Nowak et al., 2014; Tzoulas et al., 2007). A study in India ascertained that ambient air temperature, road surface humidity and air pollution are lower in tree-covered parts of roads (Vailshery et al., 2013).
Exposure to near-ground air pollution in urban street canyons poses a serious threat to pedestrians’ health. Several studies have also analyzed the relationship between the capacity of the natural environment to be exposed to air pollution and its implications for human health (Bowler et al., 2010). A field experiment study checked correlations between PM1, PM2.5, PM4, PM10, TSP, street morphology, and climatic conditions. The results showed that the morphology of street canyons and air humidity were two of the most important factors affecting the concentration of particulates in urban street canyons. Concentrations of PM1, PM2.5, PM4, and PM10 (TSP micrograms per cubic meter) in deep canyons were significantly lower than in medium and wide canyons. The concentration of pollutants distributed in the E-W or N-S street directions was lower than the pollutants in the NE-SW or NW-SE directions. In addition, pollutants concentration was significantly lower in areas of tall buildings than in areas of multilayer buildings (Miao et al., 2020). Studies have also confirmed the influence of wind speed and direction in improving air quality. Moreover, the presence of street trees mitigates pollutants concentration by increasing wind speed in different directions(Hofman & Samson, 2014; Wania et al., 2012). Typically, the main wind directions (i.e., perpendicular or 90°, parallel (aligned) or 0°, and diagonal or 45°) are included in street canyon studies compared to those without trees. Diagonal and parallel wind flow rise pollutants from both sides and intensify their concentrations to the outer end of the canyon (Abhijith & Gokhale, 2015; Buccolieri et al., 2011; Gromke & Ruck, 2012; Wania et al., 2012).
A study in London evaluated the effect of sycamore trees with different leaf densities on street ventilation and concentrations of NOx and PM2.5 in the canyons of Merrill Boone street and simulated the steady-state via OpenFOAM. Diverse scenarios have been used to assess the relative importance of different LAD trees’ aerodynamic and sedimentary effects for specific meteorological conditions. The wind speed of 3 m/s (leads to more effectiveness of trees on pollutants concentration) and 5 m/s (close to the average speed in 2014; i.e., 4.3 m/s) were considered. Two parallel and two perpendicular directions were also selected. The former led to the strongest decrease in pollutants concentration on the street (best scenarios), and the latter led to the highest exacerbation of concentration (worst scenarios). In general, the impact of trees varies depending on the height and location of the street. For perpendicular winds, trees reduce the wind speed, intensifying pollutants concentration. For parallel winds, trees increase the exchange rate on the roof, reducing pollutants concentration (Buccolieri et al., 2018).
Hosseini et al. (2019) studied the effect of vegetation on the level of pollution in urban canyons via Envi-me and by examining the local and sub-climatic air quality model using field studies in the urban context Isfahan. Their results showed that the increase of wind speed in shallow urban canyons is lower and when there are no natural obstacles (e.g., trees) in the canyons, the level of pollution decreases due to the higher wind speed. If the trees are located in the centerline of the canyon between the paths, the pollution and the distance between the trees will inversely correlate with each other; the longer the distance, the less pollution. In other words, smaller canopies trap fewer particles and reduce pollutants concentration.
Studies on removing air pollution through vegetation along urban highways report that vegetation barriers and trees along roads reduce the concentration of roadside pollutants (Brantley et al., 2014; Hagler et al., 2012; Lin et al., 2016; Tong et al., 2016). However, roadside vegetation might adversely impact air quality under certain conditions (Tong et al., 2015). Baldoff (2017) summarized roadside vegetation traits with positive and negative effects on air quality. The potential for removing air pollution through green roofs and walls (Joshi & Ghosh, 2014; Ottelé et al., 2010; Pugh et al., 2012) or combining green infrastructure with other passive pollution control methods have also been included in the literature (Baik et al., 2012; Baldauf et al., 2008; Bowker et al., 2007; Tan & Sia, 2005; Tong et al., 2016).
Using the CFD approach, Xue and Li (2017) evaluated the aerodynamic and sedimentation impacts of roadside trees on the air quality of street canyons and the sensitivity of the impacts to different tree parameters. They also studied the diffusion of released PM10 traffic in an ideal isolated street. Their results showed that both aerodynamic and sedimentation impacts might largely correlate with street canyon air quality, and vegetation parameters should be carefully considered when assessing street canyons pollution levels.
Qin (2018) examined the impact of green roofs and green walls on reducing air pollution and realized that cross-factors, namely the plant barrier morphology, vegetation types, planting density, coverage ratio and location of pollutant sources, act as the factors influencing air quality. Therefore, seasonal changes and meteorological conditions should be considered in eliminating air pollution. Elimination of air pollution by vegetation should be controlled locally and at the neighborhood and municipal scale. The removal of species-specific pollutants is influenced by external factors such as wind conditions or local pollutants concentrations (Badach et al., 2020). Using field surveys in Hong Kong, Xing (2019) investigated the efficacy of a tree planting scheme on pollutant diffusion. They considered a set of indicators correlated with tree and landscape morphology and examined their impact on the distribution of air pollutants in parks using ENVI-met. Tree surveys were conducted in 32 urban parks in Hong Kong metropolitan areas. The results showed that dense trees with low crown bases should be used as vegetation barriers near the roads with a width of 15 meters to improve air quality in parks, though vegetation should be avoided at large scales. Medium-tall trees with low crown bases and dense canopies can slow the airflow and local intensification of pollutant concentration. Tall trees with long crown bases slightly impact pollutants diffusion, making them unsuitable as barriers but more suitable for small urban parks near the road, where proper ventilation is a priority. Some research has discussed the extent of exposure of greenery to contaminants in street canyons with hedges and has reported a decrease in contaminants along the sidewalk due to low-level hedges (Gromke et al., 2016; Li et al., 2016).
These research collectively present an understanding of the complex relationship between vegetation and air pollution in urban environments (Janhäll, 2015). By discussing different strategies and methods for reducing pollutant exposure, enhancing the thermal performance of buildings and improving urban infrastructure, these studies offer a comprehensive overview of the potential benefits of green infrastructure in urban design e.g. deposition of pollutants in vegetation canopies (Litschke & Kuttler, 2008; Petroff et al., 2008), passive methods to reduce pollutant exposure (Gallagher et al., 2015), features of vegetation design (Baldauf et al., 2013) and the energy aspects of green roofs (Saadatian et al., 2013).
Yang (2015) developed a ranking method to evaluate PM2.5 removal efficacy, the negative impacts on air quality and the suitability of urban tree species in urban environments. Using i-tree software, they evaluated the trees in man-made landscapes such as streets, parks and residential areas and discussed the negative effects on air quality and its suitability for urban environments. Their results demonstrated that most tree species did not perform the best in removing PM2.5 in global cities. Further, trees with larger leaf areas have a higher removal efficacy of PM2.5. Evergreen cones are highly efficient throughout the year due to their high leaf area over the year. Trees with dense canopies and fine textures have much surface roughness that can facilitate PM2.5 interception (Freer-Smith et al., 2005; Petroff et al., 2008). At the leaf surface, leaves with complex structures and a rough, sticky or waxy surface can maintain PM2.5 more efficiently (Abdollahi, 2000bø et al., 2012; Wedding et al., 1975).
Using a CFD-based model and micro-climate-met 4 ENVI, Liyan Rui et al. (2018) studied the effect of green spaces on micro-climate and PM10 concentration in a typical residential area in Nanjing, China. They evaluated the effect of vegetation by analyzing wind velocity profiles and lines, temperature, relative humidity and PM10 concentration. According to the results, thermal comfort increases slightly with increasing greenery, especially trees. Moreover, the landscaping separation index influences cooling, but the ambient temperature around the central green space heightens even with greater values, leading to more public comfort. As the result of the complete separation of most trees, local ventilation rises while the average wind velocity decreases due to the impact of a single tree dispersion scheme on airflow. However, the effect on PM10 concentration is negligible. In sum, thermal comfort increases with more greenery, especially with more trees. Hence, greenery is much more important than vegetation design. However, higher pollutants concentration may result in a greater wind-blocking effect. Accordingly, a single tree is preferred to several ones located in the central part of the green space to improve the convenience of general heating, ventilation and air quality (Rui et al., 2018).
By reviewing the literature, it was concluded that green infrastructure carries positive and negative effects on air quality on the streets depending on urban features and vegetation. However, there is still a need to regularly review and summarize the findings published on different types of green infrastructure regarding the improvement of local air quality in diverse urban environments. Beyond the scope of existing literature, few studies on urban pollution are conducted in Iran; no study has focused on Shiraz as a megacity in the south-west of Iran as a case that could be cited in similiar districts by hot-arid areas. Moreover, attention to urban air quality in street canyons has been neglected widely. Given the differences in climatic conditions of any urban microclimate, some relevant research may provide significant results. Therefore, by further examining the factors included in previous works and comparing them, the present study develops a scenario with maximum coverage of streets and walls with suitable plants. To this end, Envi-Met scientific software was employed to examine effective planting front, correct planting method, proper selection of plants and trees for urban streets, building walls and green roofs, and analyze their impacts on reducing air pollutants and particulate matter. Furthermore, the study focuses on the effectiveness of vegetation and the extent of confinement on urban sidewalks for particulate diffusion in Shiraz, with emphasis on PM10, PM2.5, and NOX. Each variable was simulated separately in ENVI-met to analyze the impact of vegetation variables on reducing pollution particles in the streets.
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