Aerodynamic and deposition effects of street trees on PM2.5 concentration: From street to neighborhood scale
Introduction
Elevated concentrations of fine particulate matter (PM2.5) in urban areas are a growing concern due to its effect on human health and climate [1,2]. Traffic emissions generally constitute an important source of aerosol precursor gases and primary ultrafine particles in the ambient air [3,4]. Under unfavorable meteorological conditions (e.g., weak wind speeds and shallow planetary boundary layers), the interplay of local traffic emissions and regional emissions can result in severe pollution episodes in megacities such as Beijing, China [5,6].
One possible remediation strategy to help improve the ambient air quality in urban regions is to increase the urban vegetation coverage. This strategy is based on the collection capability of vegetation because it provides large surface areas for the absorption and deposition of gases and particles [[7], [8], [9], [10]]. Numerous field studies have provided evidence that the increase in vegetation coverage is related to lower particle pollution in urban areas [[11], [12], [13], [14], [15], [16]]. In contrast, a few field studies have reported contradictory results showing that urban vegetation such as forests or barriers have an insignificant effect reducing PM concentrations [17,18]. Particularly, increasing PM2.5 levels were observed within street canyon by Jin et al. [19]. There are two possible reasons for this: (i) vegetation influences airflow via drag forces, which may contribute to the accumulation of PM in specific locations within the urban environment; and (ii) as particle removal by deposition varies widely depending on the characteristics of vegetation, particles, and wind, this effect may not be very efficient at times. The complexity of this problem calls for an accurate assessment of the effects of urban vegetation on PM concentrations in urban environments.
In recent decades, many studies have been undertaken to investigate the effects of urban vegetation on PM. []. Janhäll [20], Abhijith et al.[21] and Buccolieri et al.[22] have provided comprehensive reviews. Britter and Hanna [23] suggests that the flow and dispersion in urban area can be addressed at four scales: regional (up to 100–200 km), city (up to 10–20 km), neighborhood (up to 1–2 km), and street (less than 100–200 m). At street scale, the problematic air pollution within street canyon is a research hotspot because of the potentially high traffic volume and poor ventilation conditions. Wind tunnel and computational fluid dynamics (CFD) studies indicate that the aerodynamic effect of vegetation reduces ventilation and circulation and favors increased pollutant concentration within the idealized two-dimensional (2D) canyon, especially on the leeward side [[24], [25], [26], [27], [28], [29]]. Recently, a few numerical studies sought to consider both aerodynamic and deposition effect. Vos et al. [30] and Vranckx et al. [31]found that aerodynamic effect appears to overshadow the pollutant removal capacity of vegetation at single street scale. Meanwhile , Xue and Li [32] and Santiago et al. [33] suggested that deposition can also be a major effect both for street canyon geometry and for cube geometry. In city-scale studies, Jeanjean [34,35] argued that the aerodynamic effect is stronger than the deposition effect and highlighted the importance of local meteorology. However, all of these studies are based on Reynolds-averaged Navier–Stokes models and use constant deposition velocity Vd, which is decoupled from local wind velocities. Further, most studies only considered the condition with local traffic emission at street scale. In fact, background pollution can dominate severe air pollution scenarios.
Additionally, although the microscale CFD simulations provide detailed information and help understand the dispersion and deposition process, the computational cost makes it unfeasible for practical applications. Typically, simplified models based on street canyon models or street network models are used for operational purposes [[36], [37], [38]]. To the best of our knowledge, however, most of them have not considered the effect of trees within streets canyons.
Therefore, the objectives of this study are:
- (1)
to evaluate the aerodynamic and deposition effect of trees in canyons under different emission scenarios;
- (2)
to examine the effect of leaf area index (LAI) on the dispersion and deposition at the street and neighborhood scales; and
- (3)
to devise a strategy to include the effect of trees in operational urban pollution model (OUPM).
To better represent turbulence, the simulations are conducted with large eddy simulation (LES) models. The canonical 2D street canyon is employed, which has been widely adopted to study both ventilation and pollutant dispersion in wind tunnel [[39], [40], [41], [42]] and CFD studies [29,[43], [44], [45]]. To evaluate the effect of vegetation on different sources of pollution, we investigated three emission scenarios: (i) only local traffic emissions; (ii) only background field pollution originating from upwind canyons; and (iii) a combination of scenarios (i) and (ii).
The rest of this paper is organized as follows: Section 2 describes the numerical model and simulation set-ups. Model validation is briefly presented in Section 3. The results and discussion are presented in Section 4, and in Section 5, we draw the conclusions.
Section snippets
Flow over street canyon with vegetation
The LES model together with an immerse boundary method (IBM) is employed to simulate the flow over an urban geometry and a drag force is used to represent the effects of vegetation. For brevity, only important features of the numerical model are presented here and the readers may refer to Ref. [46,47] for more details on the IBM and [48] for the canopy model. In the present study, only the neutrally stratified boundary layer is considered, and the turbulence caused by vehicle motion is assumed
Effects of trees on canyon particle concentration with different emission conditions
Before evaluating the effects of trees on canyon particle concentration, we first explore how the trees affect the flow within canyons. With an aspect ratio of 1, the skimming flow (as noted by Oke [57] drives an isolated clockwise mean flow vortex within the canyon in the control case. The trees, treated as a sink of momentum, strongly weaken the vortex and reduce the circulation within the canyon (Fig. 4).
The profiles of horizontally averaged turbulence statistics are presented in Fig. 5.
Conclusions
In this study, LES were performed using ENDLESS to evaluate the effect of street trees on air quality under different emission scenarios and at different scales. For local emissions, increasing vegetation reduces flow within the canyon, increasing concentrations in the vicinity of the leeward wall and reducing concentrations in the windward wall. The deposition effects are smaller than aerodynamic effects. On the other hand, vegetation has almost no effect on the transport of background
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was supported by National Natural Science Foundation of China (NSFC) under grant No. 11732008. MC was supported by the National Science Foundation (NSF) grant AGS-1644375. The calculations were were performed on the supercomputing center of Fujian Province installed at the Fuzhou University
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