Evaluation of impacts of trees on PM2.5 dispersion in urban streets
Introduction
Traffic emissions generally constitute the major source for air pollution in urban areas. Particulate matter (PM) is one of the most important components in traffic emissions. Resulting from combustion process of fossil fuels in vehicles and road dust resuspension, particulates contribute to the deterioration of air quality, and have a large adverse impact on the health of central city population. Recently epidemiology studies have found that PM, especially PM2.5 (PM with aerodynamic diameters of 2.5 μm or less), can induce respiratory and lung disease, immune-system problems and even premature death (Heal et al., 2012, Zhang et al., 2012) and affects more people than any other airborne pollutants.
Critical situations usually emerge in densely built-up city areas with street canyons suffering from poor ventilation and high pollutant concentrations. Particularly in urban street canyons with avenues of trees, large crowns occupy considerable space of street canyons and separate the lower street level from the upper roof level (Gromke et al., 2008). Therefore, tree planting may hinder the ambient air exchange and vehicle exhaust dispersion, and increase PM2.5 concentration in the lower region. So research on the influence of tree canopy on PM2.5 concentration has great significance in terms of environmental condition in urban street canyon.
As an international metropolis, Shanghai has large population and intensive urban land use. Green space is fairly limited in central city area. To increase the green area, the municipal government has pursued a “green streets” program for several decades. In this context, “green streets” refer to urban secondary roads or minor roads with a street tree canopy density (CD, ratio of the projected ground area of tree crowns to the street canyon ground area, %) greater than approximate 50% (Zhang, 2012). The data of 50% is mainly based on the average canopy density of the existing green streets in summer in central Shanghai. In addition, designers consider “green streets” to be an integrated system consisting of road, street trees, architecture, and public facilities, typically in the form of high-density street canyons. Thus, besides transportation, ecological benefits, the green street also provides important aesthetic and recreational functions. However, since tree canopies intensify the accumulation of airborne PM2.5 and increase the health risk of pedestrians and residents in street canyons, it is important to study the dispersion of PM2.5 in street canyons with different street tree structures to find optimal tree-planting strategies, and to maximize the comprehensive benefits of the green street.
A great deal of literature exists on atmospheric quality in tree-free street canyons. Air flow field and turbulence in street canyons with different geometrical characteristics, traffic exhaust concentrations and temporal-spatial distribution, as well as traffic-induced turbulence, model studies and photochemical transformation of air pollutants have been investigated in field studies(Bady et al., 2011, Chan and Kwok, 2000, Kourtidis et al., 2002, Kukkonen et al., 2001, Richmond-Bryant and Reff, 2012, Weber et al., 2006, Wehner et al., 2002, Xie et al., 2003), laboratory wind-tunnel (Carpentieri et al., 2012, Kastner-Klein and Plate, 1999, Pavageau and Schatzmann, 1999, Uehara et al., 2000) and numerical stimulations (Gidhagen et al., 2004, Gousseau et al., 2011, Karakitsios et al., 2006, Nikolova et al., 2011, Solazzo et al., 2009, Xie and Castro, 2009, Zhang et al., 2011b). Further comprehensive overviews on these topics are given in reviews of Vardoulakis et al. (Vardoulakis et al., 2003), Ahmad et al.(Ahmad et al., 2005), Holmes (Holmes and Morawska, 2006) and Carpentieri et al. (Carpentieri et al., 2011). Moreover, PM2.5 measurement and pedestrian exposure to air pollutants by means of field investigation have been addressed in quite a few articles (Chan and Kwok, 2000, Foster and Kumar, 2011, Huang et al., 2007, Kaur et al., 2005, Kristensson et al., 2004, Li et al., 2007, Ye et al., 2003a, Ye et al., 2003b, Zhang et al., 2006).
In the last few years, flow and pollutants dispersion in street canyons with trees have been becoming new interests of air quality studies. By contrast, relatively little research can be found in literature. Gromke et al. (Gromke et al., 2008, Gromke and Ruck, 2007, Gromke and Ruck, 2009, Gromke and Ruck, 2012) investigated the flow and concentration of pollutants in an isolated street canyon with avenues of trees by means of wind tunnel measurement and CFD simulation. They found a decreasing pollutant concentration with increasing height at the facades in the middle of street canyon. They also found that increasing crown diameters and decreasing the tree spacing both led to a noticeable concentration increase at the street canyon wall. For some cases, a variation of trunk heights resulted in a modification of the concentration pattern on the walls. In another study, Buccolieri et al.(Buccolieri et al., 2011) showed the combined influence of building morphology and vegetation on flow and dispersion and assessed the effect of vegetation on local concentration levels. It was believed that for tree-free street canyons under inclined wind directions the larger the aspect ratio the lower the street-level concentration. However, in presence of trees the expected reduction of street-level concentration with aspect ratio was less pronounced. Balczo et al. performed numerical simulations of the impact of tree planting on pollutant dispersion in street canyons by using the CFD code MISKAM. They found the presence of trees lead to increased pollutant concentration inside the canyon, especially the upstream side (Balczó, 2009). Similar results were obtained in several other pieces of researches (Gu et al., 2010, Salim et al., 2011, Wania et al., 2012).
The above findings were mostly derived from wind tunnel or numerical simulation rather than field data. Due to various factors that may influence pollutant dispersion, wind tunnel or numerical simulation could not reliably reproduce real processes occurred in street canyons with trees. Only Salmond et al. presented data from a field study undertaken in New Zealand to determine the local impact of deciduous tree canopies on the distribution of the oxides of nitrogen within a street canyon. An increase in concentrations on the leeward side was observed during leaf-on relative to leaf-off conditions (Salmond, 2013). Hofman et al. (Hofman et al., 2012) demonstrated that biomagnetic leaf monitoring of crown deposited particles could be used to estimate ambient PM concentration and assess its spatial variations. In this context, the impact of tree-planting on the spatial PM2.5 dispersion was seldom quantified by field experiments.
The overall goal of this paper is to clarify the temporal-spatial distribution of PM2.5 concentration in street canyons, and the impact of street trees on PM2.5 concentration variation compared with street canyons without trees through direct measurement. For this reason, we performed a seasonal study in six typical street canyons with trees in the residential area of central Shanghai. We monitored the concentrations of PM2.5 at five different heights during wind direction perpendicular to street canyons. In addition, structural characteristics of street trees, instantaneous wind velocity, temperature and humidity were also measured to identify the crucial predictor that may affect pollutant contents. The practical intention of all these investigations is to provide city planners and designers with guidelines on how to plant trees in urban street canyons in regard to air quality management. Developing the optimum strategy is of great importance for the green street planning and management in cities in China and many other parts of the world where haze pollution has become a serious problem.
Section snippets
Study area
Shanghai, 31°12′ north latitude, 120°30′ east longitude, has a subtropical monsoon climate and experiences four distinct seasons. The prevailing wind direction is southeast in spring and summer, northwest in autumn and winter. Annual mean wind speed in central Shanghai is 2.8 m/s (at 10 m above ground level). Shanghai covers a total area of 6340.5 km2, of which 289 km2 is central area. The population was about 23.5 million including 7million within central area at the end of 2011.
Sampling sites
We conducted a
Structures of street trees during four seasons
The structure characteristics of street trees in six experimental plots are listed in Table 2. The height of street tree vary from 11.09 to 15.24 m, which is in the same range with building height, as seen in Table 1. The CBH and HLL values range from 3.08 to 4.43 m. Therefore, the four sampling heights at the sidewalk were uniformly set at 1.5 m, 4 m, 8 m and 12 m, respectively. The obtained LAI and CD vary greatly among different plots and seasons due to different pruning treatments in early
Factors affecting PMAC
The dispersion of airborne pollutants in street canyons is a complex process, which might be affected by various factors, including local atmospheric chemistry, meteorology, and characteristics of street canyons such as architecture and street trees (Gromke and Ruck, 2007, Kastner-Klein and Plate, 1999, Uehara et al., 2000). In this study, the main factors may influence PMAC include weather, initial pollution level, and structure of street trees such as height and canopy density. To be
Conclusions
To sum up, a seasonal campaign was conducted to monitor the PM2.5variation in six typical street canyons in the residential area of central Shanghai, China. The results show that the diurnal average PM2.5 statistically decreases with increasing height in tree-free street canyons. The decrease could not be found or is weakened in street canyons with trees due to the fact that the street trees hinder the dispersion of air pollutants. In addition, the attenuation coefficient of PM2.5 (PMAC) is
Acknowledgments
This work was funded by the National Key Technology R&D Program (2012BAC13B04-05) and the Shanghai Key Projects of Science & Technology Program (11231201002). All authors would like to give thanks to Shanghai Greening & Management Guidance Station for their help on street trees pruning. We would like to thank our team workers Bingqin Yu, Zhigang Li, Changkun Xie, Mingling Chen, Qiangqiang Li, Wenjuan Rui, Tao Chen, Zixin Shen, Hongjian Wei, Yuan Zhang and Yanling Zhao for their hard work on
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