1 Introduction

Air pollution is a great cause of concern around the globe. Increase in levels of air pollution has been linked to increase in mortality, increase in hospitalization due to respiratory diseases and cardiovascular problems. Pollution from particulate matters (PM) in the atmosphere is of great concern for human health not only in the developing countries but also in developed ones. Mitigation of air pollution has emerged as one of the challenging tasks for environmental management agencies in different parts of the world. Increase in population followed by urbanization caused increase in sources of air pollution day by day. Increase in individual mode of transport has counterbalanced the technical improvements in vehicle and fuel technologies. Preventive measures taken do not help much either to control or to reduce the amount of emissions. However, in recent decades, use of plants for air pollution abatement is being given priority. Trees act as a sink for both particulates and gaseous pollutants. Use of plant canopies to reduce atmospheric particle concentration was reported by Lohr and Pearson-Mims (1996). It has been estimated in a study that plant canopies can remove 711, 000 metric tons of air pollutants from the cities of United States (Nowak et al. 2006). It has been established that leaves and exposed parts of plants act as persistent absorbers in a polluted environment (Nowak et al. 2006; Samal and Santra 2002). Tree leaves act as efficient filters of airborne particles because of their large size and high surface to volume ratio of foliage. Depending on the morphology, leaves can act as biological adsorbers of air pollutants (Beckett et al. 1998; Wittig 1993). Foliar features that play important role in determining the efficiency of plants to trap dust particulate are roughness of leaf surface, frequency & length of trichomes and size & frequency of stomata (Meusel et al. 1999; Neinhuis and Barthlott 1998). Plants differ significantly in their ability to capture dust, due to differences in their leaf surface characteristics such as epicuticular wax, cuticle, epidermis, stomata and the trichomes. Thus, selection of appropriate plants having suitable morphological features is an important aspect of air pollution abatement strategy.

Plants such as: neem (Azadirachta indica), peepal (Ficus religiosa), banyan (Ficus benghalensis), almond (Terminalia catapa) have the potential to serve as excellent quantitative and qualitative indices of pollution level (Wagh et al. 2006). The hypothesis that plants are important particulate traps is supported by the evidence obtained from studies dealing with trace elements, pollen, spore, salt, dust and unspecified particles (Das et al. 2006; Lorenzini et al. 2006). The most common pollutants associated with fine particles in contaminated atmosphere are trace elements especially heavy metals. A significant burden of heavy metals accumulated on vegetative surfaces has been demonstrated by trace element investigations conducted in roadsides, industrial and urban environments (Aksoy and Ozturk 1997; Rossini Oliva and Rautio 2004; Rossini Oliva and Valdes 2004; Sawidis et al. 1995). High lead content in banyan leaves exposed to vehicular exhaust was reported by Datta and Ghosh (1985).

Varieties of techniques have been employed to estimate the efficiency of particulate matter interception by tree leaves. Some indirect methods compare dust fall measurements in collectors deployed beneath tree canopies with those in adjacent open spaces, ascribing the difference to the influence of the vegetation (Beier and Gundersen 1989; Dochinger 1980). Other indirect approaches rely on atmospheric aerosol monitoring, meteorological measurements and the use of predictive deposition velocity models (Bache 1979; Wiman et al. 1985). In this study, we have applied washing of dust from leaves for subsequent characterization.

Plant canopies provide a naturally absorbing surface with a favorable surface area/volume ratio and a long life-span. Leaves have been shown to be able to capture coarse, fine and ultra-fine particles, including transfer of ultra-fine particles to the inner layer by diffusion. Dust from the leaves can be collected and characterized in the laboratory or key properties of a leaf-loaded with dust can be directly measured. Therefore in the present work we have tried to validate the use of leaves of avenue trees as absorbers of traffic generated dusts in urban areas. The present study also aims to probe into morphological features that help the canopies to adsorb dusts and analyse distribution of trace elements in dust over the leaves. In essence the overall objective of the present study is to establish dual nature of plant canopies as bio-monitor as well as an intervener of air pollution.

2 Materials and methods

2.1 Sampling site

Kolkata is one of the oldest, largest and most densely populated metropolitan cities of India (22° 82′N and 88° 20′E). The city has approximately 16 million people and thus a sizeable auto and truck traffic exists. The study site for sampling of leaves was Karunamoyee (22° 35′ N and 88° 25′ 0″ E), Kolkata, India. This site is located in Salt Lake region of Kolkata, near a busy road with high traffic density (5,000–6,000 vehicles/hour, during peak hours). Being the software industry hub, the Salt Lake region of Kolkata is undergoing urbanization at a faster rate than expected. Constructions of buildings, bridges, roads as well as increase in transportation have resulted in discharge of gaseous and particulate contaminants into the atmosphere. The annual average concentration of SPM and RSPM at the study site, exceeds the national standard limit (Table 1) as per West Bengal Pollution Control Board (WBPCB) air-quality data.

Table 1 Particulate matter pollution level at the study site and pollution control board standard standard
Full size table

2.2 Sampling and dust load measurement

Equally matured leaves (n = 5) facing the roadside at the selected site (Karunamoyee at Saltlake, Kolkata) were collected from the selected plants i.e. Ficus benghalensis (Banyan tree) and Polyalthia longifolia (Indian Fir tree) above a height of 10 ft from ground. Sampling was done during September–October 2009. Dusts from the individual leaves were washed with distilled water using a thin brush separately on different surfaces. Dusts collected in the distilled water were filtered onto nitrocellulose membrane filter of 0.2μm pore size using a vaccum filtration unit. Individual leaf area was calculated by tracing out the leaves on graph paper. Amount of dust per unit area was calculated using the equation W = (Wd-Wb)/A, where W is dust content (mg cm−2), Wd is weight of filter paper with dust, Wb is initial weight of blank filter paper and A is total area of the leaf (cm2).

2.3 Leaf morphology and trace element distribution analysis

Discs of 1 cm2 area were cut from unwashed leaves and were air-dried in a clean and closed chamber. Small strips were trimmed from areas between the margin and midrib of leaves. Each leaf strip was mounted on an aluminum stub, over double-sided stick tape. The specimens were coated with a thin conductive film of gold (about 200A°), in an ion sputter coater (Hitachi E-1010). Coated specimens were examined and photographed under a scanning electron microscope (Hitachi S-3400N) with attached energy dispersive x-ray fluorescence spectrometer at an accelerating voltage of 15 kV, at the magnification range of 200–1,4000X.

3 Results

3.1 Dust deposition

Annual average pollution level at the study site, Kolkata city and national standards are presented in Table 1. Air-quality at the study site and in the Kolkata city are exceeding the national air-quality standard. Amount of dust adsorbed per unit area of leaf is depicted in Table 2. Interspecies variation in dust deposition is observed. More dust deposition (1.89±0.09 mg/cm2) is observed on the upper surface of Ficus benghalensis leaves in comparison to that of Polyalthia longifolia (0.84±0.15 mg/cm2). Similarly, more dust deposition (0.47±0.065 mg/cm2) was observed on the lower surface of Ficus benghalensis leaves in comparison to that of Polyalthia longifolia (0.12±0.024 mg/cm2). The frequency of stomata in Ficus benghalensis, and Polyalthia longifolia leaves is found to be 25±3 and 21±2 per unit area respectively. Dust particulates on the lower surface are adsorbed along the trichomes (Figs.1a and 2a) in both the species. Although the dust is adsorbed on both surfaces of the leaves, the upper surface of leaves in both the plants (Figs.1b and 2b) has adsorbed more dust in comparison to that of lower surface (Figs. 1a and 2a). Dust particles on the lower surface are found to be inside the stomata (Figs. 1c and 2c). Size of stomata on Ficus benghalensis leaf ranges from 25μm*18μm to 30μm *25μm (Fig. 1c) where as that of Polyalthia longifolia leaf ranges from 18μm *15μm to 23μm *18μm (Fig. 2c). Number of trichomes are more in Ficus benghalensis in comparison to that of Polyalthia longifolia i.e. 4±1 and 2±1 per unit cross section area. Drawing of the canopy shapes of the studied plants are presented in Fig. 3.

Table 2 Morphological features of leaf and amount of dust deposition per unit area
Full size table
Fig. 1
figure 1

Scanning electron micrographs of leaf surfaces of Ficus benghalensis: a Lower surface showing dust particles, stomata and trichomes, b Upper surface showing dust particle deposition (c) Lower surface showing stomata blocked by dust particles

Full size image
Fig. 2
figure 2

Scanning electron micrographs of leaf surfaces of Polyalthia longifolia: a Lower surface showing dust particles stomata and trichomes, b Upper surface showing dust particle deposition (c) Lower surface showing stomata blocked by dust particles

Full size image
Fig. 3
figure 3

Sketch showing canopy shapes of (a) Ficus benghalensis and (b) Polyalthia longifolia

Full size image

3.2 Elemental distribution

The SEMEDS spectrum of the dusts on the leaf surface have revealed the chemical signature of Na, Mg, Al, Si, Cl, K, Ca, Fe, Zn and As (Figs. 4 and 5).

Fig. 4
figure 4

Scanning electron micrograph (a) and EDS spectrum (b) of the dust adsorbed on leaves of Ficus benghalensis

Full size image
Fig. 5
figure 5

Scanning electron micrograph (a) and EDS spectrum (b) of the dust adsorbed leaves of Polyalthia longifolia

Full size image

4 Discussion

Present study shows both the plants under investigation are capable of adsorbing re-suspended dusts. However, variation exists in dust capturing capacity in different plants. Ficus benghalensis is more efficient dust adsorber than Polyalthia longifolia. It is already known that trees improve urban air quality by capturing airborne dust particulates (Freer-Smith et al. 1997). The air borne dust particulates get deposited on the canopies by three means i.e. sedimentation, impaction and/or precipitation process (Virginia and Pearson-Mims 1995). Our present findings clearly showed the deposition of dust particulates on both abaxial & adaxial leaf surfaces. However, there is variation in amount of dust deposition on different sides of surfaces. More dust was found to be deposited on the upper surface of leaves and this observation can be attributed to the fact that dust get settled on the upper surface by sedimentation, however on the lower surface dust gets impacted along with the wind current. This continuous process of dust deposition reduces the concentration of particulate matters in the ambient air. Paulsamy and Senthilkumar (2009) have reported that, plantation of species such as Aegle marmelos, Azadirachta indica, Ficus benghalensis, F. religosa, Holopetea integrifolia, Pongamia pinnata and Tamarindus indica in polluted areas reduced the effect of air pollution in Tirupur, India. According to Prajapati and Tripathi (2008) dust interception capacity of plants depends upon their shape and size, phyllotaxy, and leaf surface characteristics such as hairs, cuticle etc., height and canopy of trees. In our study larger and more frequent stomata were found in F. benghalensis leaves. More trichomes were found on the leaves of F. benghalensis, in comparison to that of P. longifolia. However the length of trichomes in P. longifolia is more in comparison to that of F. benghalensis. The number of stomata and trichome may be considered as the reason behind more dust on the leaves of F. benghalensis. Surface roughness, large contact area, foliar structures such as trichomes and stomata on the leaves helped to capture airborne dust. Prusty et al. (2005) have reported significant difference in dust accumulation among plant species. The variation in efficiency of dust adsorption over leaves of Ficus benghalensis than Polyalthia longifolia can also be attributed to the differences in canopy shape. As discussed by Chakre (2006), each plant has two sides wrt to the way the dust is carried by wind and captured by the plant canopies. The first is the front-side, where the air current diverts or redirected before entering the plant and is known as Luff-side. The second is Lee-side i.e. the back-side of the plant where the wind current falls after passing through the canopy. In plant canopies (thick) like that of Ficus benghalensis (Fig. 3a), the in-coming air-current can enter easily and settle the impurities inside the plant. However in case of thin canopies like Polyalthia longifolia (Fig. 3b), dust concentration falls rapidly before entering inside the plant, leaving the maximum of dust on the luffside and carrying rest to the lee-side. Our present work agrees with the fact that in addition to the foliar structures, canopy shapes also play important role in capturing air-borne dust particles.

The dust in the urban atmosphere is found to contain heavy metals such as V, Cr, Mn, Fe, Cu, Zn, Pb and Si. Distribution of trace elements over the leaf surfaces observed in the present investigation is in harmony with the earlier reports by Sudarshan et al. (2011) and Tomasevic et al. (2005). Among the elements detected in dusts adsorbed over leaf surfaces, most are of considerable concern for public health (Beckett et al. 1998; Borja-Aburto et al. 1998; Schwartz et al. 1996). Elements present in the dust over leaf surfaces are mainly made up of mainly two components i.e. wind blown ground dust and particulates arising out of vehicular exhaust. Trace elements present in the air-borne dust have various sources of origin such as anthropogenic, lithogenic, industrial and vehicular exhaust. Present study site, Saltlake is primarily a residential area and hence witnesses various anthropogenic activities such as open dumping of household solid wastes, burning of street garbage, construction works and domestic cooking fuels. These activities can be attributed as the potential source of the several elements detected in the dust over leaves. In addition to these, there is enough traffic movement that can release particulate pollutants into the atmosphere. In the present investigation, elemental profile of dust has shown the presence of Arsenic. Kolkata is one of the most arsenic polluted areas. Presence of Arsenic in the roadside dust of Kolkata metropolis has already been reported by Kar et al. (2006). Both these plants under investigation capture toxic metals from the atmosphere and survive in polluted environments. These plants are air pollution tolerant plants. Therefore, care must be taken to consider air pollution tolerant plants for air pollution management.

5 Conclusion

Major conclusions of the present study are as follows:

  • Dust particles are deposited on both abaxial and adaxial surfaces of leaves. This observation further supports the fact that vegetation cover acts as persistent adsorber of airborne dust particulates. Hence, plant canopies can be used for air pollution mitigation by creating green belts around the city under pollution threat.

  • Dust deposited over the leaf surfaces are contaminated with trace elements, which are of different origin and can deteriorate human health. Thus roadside plants can be used for passive air pollution monitoring. As bio-monitor of air pollution, plants will give better resolution in terms of sampling site as active monitoring is expensive in developing or emerging countries in Asia and Africa.

  • Moreover, policy makers around the globe, before selecting the plants for pollution abatement or for urban & peri-urban greening in the form green belts must consider morphology of leaves and canopy shapes.