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Resuspension of small particles from tree surfaces
Atmospheric Environment 35 (2001) 3799–3809
Resuspension of small particles from tree surfaces Zitouni Ould-Dada*, Nasser M. Baghini EAS, T.H. Huxley School, Imperial College at Silwood Park, Ascot, Berkshire, SL5 7TE, UK Received 1 November 2000; received in revised form 15 February 2001; accepted 7 March 2001
Abstract A detailed study of resuspension of 1.85 mm MMAD silica particles from five horizontal layers within a small scale spruce canopy was carried out in a wind tunnel in which saplings were exposed to a constant free stream wind speed of 5 m s1. This provided quantitative estimates of the potential for a tree canopy contaminated with an aerosol deposit to provide (i) an airborne inhalation hazard within the forest environment and (ii) a secondary source of airborne contamination after an initial deposition event. Resuspension occurred with a flux of 1.05 107 g m2 s1 from spruce saplings initially contaminated at a level of 4.1 102 g m2. An average resuspension rate (L) of 4.88 107 s1 was obtained for the canopy as a whole. Values of L were significantly different (ANOVA, p50.001) between canopy layers and L was markedly greater at the top of the canopy than lower down although there was a slight increase in L at the base of the canopy. The resuspended silica particles deposited onto the soil surface at an average rate of about 5.3 108 mg cm2 s1. It is concluded that resuspension under wind velocities similar to that used in the reported experiments is likely to pose a relatively small inhalation hazard to humans and a relatively minor source of secondary contamination of adjacent areas. Furthermore, resuspension rates are likely to diminish rapidly with time. The results are discussed in relation to the growing interest in the tree planting schemes in urban areas to reduce the impacts of air pollution. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Aerosol particles; Spruce canopy; Wind tunnel; Resuspension rate; Inhalation hazard
1. Introduction Following an accidental or planned airborne release and after deposition, resuspension of small particles is an important mechanism which may pose an inhalation hazard and lead to redistribution of contaminants. Contaminated forests for example may act, during dry windy periods, as sources of resuspended material subject to secondary wind transfer to adjacent areas. In the case of radionuclides and other carcinogens this process could represent a long term pathway of environmental exposure, especially for long lived iso*Corresponding author. Present address. Food Standards Agency, Radiological Safety Unit, Contaminants Division, Aviation House, 125 Kingsway, London, WC2B 6NH, UK. Tel.: +44-207-276-8774; fax: +44-207-276-8788/8789. E-mail address: zitouni.ould-dada@foodstandards. gsi.gov.uk (Z. Ould-Dada).
topes and organic compounds. Resuspension is influenced by various factors including the physical charact eristics of the contaminated surface, the physicochemical nature of the contaminant and meteorological conditions which together make measurement of resuspension rates under natural conditions a very difficult task. Control over some of these variables can be achieved by studies carried out in wind tunnels using artificial surfaces, or uniform ‘natural’ surfaces such as bare soil and grass swards. Several such studies have indicated resuspension rates and factors to be a function of particle size, wind speed, surface type and time (Fairchild and Tillery, 1982; Garland, 1983; Nicholson, 1988, 1993; Braaten et al., 1990; Wu et al., 1992; Giess et al., 1994). Most field data available on resuspension originated from observations at nuclear and weapon test sites (see Nicholson, 1988) which were situated mostly in arid or semi-arid conditions where vegetation cover was
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incomplete. Resuspension studies in the relatively moist, temperate conditions typical of Britain and northern Europe are still limited. More particularly, resuspension rate estimates of small aerosol particles contaminating complex natural surfaces such as forest canopies have been very rare within the literature on environmental aerosol behaviour. It was thus intended to redress this deficiency by taking detailed measurements of particle resuspension fluxes within a small scale ‘model’ Norway spruce (Picea abies) canopy contained within a wind tunnel. This has provided quantitative estimates of the potential for a tree canopy to provide a source of aerosol particles which may represent (i) an inhalation hazard to people within the forest environment and (ii) a redistribution problem in which previously deposited particles may be made available for atmospheric transport to previously uncontaminated areas. In the present study, resuspension is referred to as the mechanism by which deposited particles are removed by wind from tree surfaces under controlled dry conditions and after initial deposition has ceased.
2. Materials and methods 2.1. Aerosol particles Insoluble silica particles of 1 mm nominal physical diameter were used in the present study (Separations Ltd, Clywd, UK). Subsequent measurement with an aerodynamic particle sizer (APS, model 3300, TSI inc. Minnesota, USA) revealed a mass median aerodynamic diameter (MMAD) of 1.85 0.5 mm standard deviation. Silica particles, used more commonly in chromatographic separation, are highly porous and provide sites for labelling by a variety of active or inactive tracers. Rare earth elements such as dysprosium (Dy), indium or europium have been used as activatable tracers in previous aerosol studies at Imperial College (Watterson, 1989; Giess, 1993; Giess et al., 1994). Soluble Dy [Dy(NO3)3] was chosen as a tracer for this study because of its relatively low concentration in plant and soil materials which makes its detection by cyclical activation less prone to interference by the natural background signal. The level of labelling was sufficient to detect easily the amount of resuspended particulate. 2.2. Airflow characterisation Resuspension experiments were carried out in the MAFF/Imperial College wind tunnel, Silwood Park, which is of a closed circuit type and which has a working section 6 m long, 1.2 m high and 0.8 m wide. The entire length of the wind tunnel was first filled with uncontaminated Norway spruce saplings (40 cm in height) for wind velocity profile measurements. The wind tunnel’s
fan speed was adjusted until a free stream air velocity of 5 m s1 was established above the canopy. After establishing the desired free stream wind velocity an automated hot wire anemometry system was used to make wind velocity measurements at 10 mm intervals from 15 mm above ground level up to a height of 1.015 m. These measurements revealed the wind velocity profile within and above the canopy and allowed calculations of friction velocity, u * , (using the eddy correlation method) and turbulence intensity, iu , (using measured fluctuations on the mean wind speed) to be made. After air flow characterisation was complete five trees from the trailing end of the canopy were taken away for contamination with Dy labelled silica particles and then carefully replaced for resuspension measurements. 2.3. Contamination of spruce saplings Contamination of spruce saplings by aerosol deposition was achieved using a Perspex chamber (80 cm 50 cm 65 cm) and associated equipment (Fig. 1). The air within the chamber was actively exchanged with filtered laboratory air during the deposition process using a vacuum pump operating at a rate of 200 l min1. This exchange of air ensured thorough mixing of the atmosphere within the chamber and facilitated uniform contamination of the trees with aerosol. A powder dispersion generator (RBG 1000, Palas GmBH Partikel, Germany) was used to liberate the aerosol which was then carried to the contamination chamber by a nitrogen gas stream at 30 l min1 providing a constant input of aerosol during the deposition phase of the experiment. A cylinder containing a 74 MBq 85Kr source was attached to the RBG-1000 outlet to reduce electrostatic charge build up on the particles. The aerosol was injected into the top of the chamber via an aluminium manifold which consisted of two rows of four evenly spaced outlets. To avoid the formation of aggregates of aerosol within the manifold the latter was rinsed with deionised water and air dried between deposition experiments. A scanning electron micrograph of silica particles deposited on a spruce needle is shown in Plate 1. In order to avoid any accidental contamination of the tunnel interior surfaces by Dy tracer, contamination of spruce saplings was carried out away from the wind tunnel in a separate laboratory. After contamination, trees were placed in a large cardboard box and carefully transported to the wind tunnel where resuspension experiments took place. Contaminated trees were positioned at the trailing end of the experimental bay of the wind tunnel covering a floor area of 0.76 m2 (0.95 m 0.8 m). The remaining floor area of the wind tunnel (i.e. upwind of the contaminated saplings) was occupied by uncontaminated saplings to provide the fetch required to ensure the development of a
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Fig. 1. Enclosed Perspex chamber and associated equipment used for the contamination of spruce saplings with aerosol prior to resuspension experiments.
2.4. Sampling and analysis
Plate 1. Scanning electron micrograph of silica particles deposited on a spruce needle.
characteristic boundary layer. Peat-based compost was used as a sub-canopy soil surface which was kept moist throughout all experiments. Cardboard sheets were used as a platform for this compost layer under the contaminated trees in order to achieve as level a surface as possible.
Four resuspension experiments were carried out in this study: five contaminated trees were used for each experiment. The trees were exposed to a constant free stream wind velocity of 5 m s1 and the mass of resuspended Dy derived from five horizontal canopy layers was determined during seven consecutive time intervals, viz. 90, 1110, 2400, 3600, 7200, 21,600 and 50,400 s (i.e. the total cumulative time allowed for resuspension to occur within a single experiment was 24 h). The initial surface contamination (S0 ) of spruce saplings was determined within the five horizontal layers of the canopy prior to each experiment after saplings were placed in the wind tunnel. Two samples of spruce tissue were collected from the topmost layer and six samples (three new tissues and three old tissues) were collected from each of the remaining four layers of each tree. Each sample (needles+stems) was placed in a polyethylene bag and oven dried at 708C for 24 h after which time the tissue dry weight was recorded. Samples were then analysed (see below) and S0 was expressed in g Dy g1 dry weight (DW) for each canopy layer. This value was converted into g Dy m2 of the canopy area (including needles and stems) using the leaf area index (LAI). LAI was calculated using the mass–area relationship for each tissue category and an estimate of the total mass of that tissue category obtained by destructively sampling and weighing selected trees (Ould-Dada, 1996). Samples of resuspended particles were obtained within and above the canopy using isokinetic air samplers. The position and the flow rate of each sampler were
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determined according to the wind velocity profile. During experiments 1 and 2 aerosol particles were sampled from each of the five canopy layers using two isokinetic air samplers within each layer and from above the canopy using five isokinetic samplers positioned at different heights within the boundary layer. During experiments 3 and 4, resuspended particles were sampled only from within the canopy using three isokinetic samplers within each layer. Particles reaching the soil surface were also collected using four 4.7 cm diameter Whatman 542 filter papers randomly placed between the contaminated saplings and four under the crowns. Samples were analysed for total Dy content by instrumental neutron activation analysis (INAA) using the Imperial College reactor at Silwood Park. These were packed into polyethylene capsules and placed in an automatic cyclical irradiation and analysis system constructed around the reactor. This cyclical activation system (CAS) permits samples to be irradiated in a thermal neutron flux of 1.29 1012 neutrons cm2 s1 and then rapidly counted on a Ge(Li) semiconductor gamma spectrometry system. Samples were counted for 90 s after being irradiated for 90 s followed by a 1 s decay. A spectral analysis program (Spectran AT, Canberra, UK), run on an IBM personal computer, was used to determine the mass of Dy on each filter paper and spruce tissue sample. 2.5. Determining resuspension rate Resuspension was expressed as a resuspension rate (L) which is defined as the fraction of particulate contaminant initially present on a ground or vegetation surface resuspended per unit time: Lðs1 Þ ¼
resuspension flux R ðg Dy m2 s1 Þ : surface contamination S0 ðg Dy m2 Þ
ð1Þ
models to predict airborne concentrations of contaminants above soil, vegetation and urban surfaces subjected to acute or chronic deposition events.
3. Results and discussion 3.1. Micro-meteorology of ‘model’ spruce canopies within the wind tunnel The wind velocity profile obtained for the canopy in this study (Fig. 2) was similar to those observed in numerous other forest and agricultural canopies (Oliver, 1971, 1975; Wedding and Montgomery, 1980; Raupach and Thom, 1981; Grace, 1983; Slinn, 1982; Bache and Johnstone, 1992). The mean wind velocity fell from a free stream value of 5 m s1 to less than 1 m s1 near the base of the canopy, with a secondary maximum occurring within the ‘trunk space’ adjacent to the ground surface. The similarity of this velocity profile to those obtained in full scale tree canopies gives confidence that the boundary layer for the ‘model’ canopy was adequately developed for the purposes of the resuspension experiments. The downward flux of momentum from the boundary layer to the first layer of the canopy was characterised by a friction velocity, u * of 0.35 m s1 estimated using the eddy correlation method. The turbulence intensity (iu ) of the air flow within the canopy (where it varied between 0.1 and 0.38) was higher than above it (free stream trubulence intensity was 50.05; Fig. 3). Measurements from a full scale forest canopy (Parkin, 1987) and a model spruce canopy in a wind tunnel (Kinnersley et al., 1994) showed similar values and distributions of turbulence intensity. According to Raupach and Thom (1981) turbulence
The resuspension flux (R) was determined considering wind speed at the sampling position, the measured timeaveraged air concentration at that position, and the dimensions of the wind tunnel: R¼
MDy ½DC WWSi ti Cai ¼ ; TLAi ti ½LAIi 0:76 m2 ti
ð2Þ
where MDy is the total mass of Dy resuspended, TLAi is the total layer area at layer i, LAI is the leaf area index, Dc is the depth of the canopy layer, W is the wind tunnel width, WSi is the midpoint wind speed of layer i, t is the sampling duration, and Cai the time-averaged air concentration of aerosol within layer i. In practice L is very difficult to evaluate with confidence under field conditions and it can normally only be measured under controlled conditions in wind tunnel or laboratory studies. The resuspension rate is suitable for use in dynamic atmospheric transport
Fig. 2. Typical wind velocity profile within a ‘model’ spruce canopy contained in the MAFF/Imperial College wind tunnel.
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Fig. 3. Typical turbulence intensity profile within a ‘model’ spruce canopy.
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in conditions of high humidity hygroscopic aerosols such as silica particles may grow and changes in their shape and structure may occur. The adhesion of particles to tree surfaces can be enhanced in conditions of high humidity and surface wetness thereby preventing their dislodgement from the plant surfaces. High humidity will enable the formation of films of moisture around particles and hence favour adhesion and suppress resuspension (Garland, 1983). This implies the importance of the effect of such a factor on resuspension in temperate climates such as Britain and most of Europe. In the present experiments relative humidity did not exceed 50% and it is unlikely that any significant changes in particle size would have occurred under these conditions. In addition, the factors mentioned above indicate the further complications in resuspension when considering vegetation and in particular tree surfaces. 3.2. Resuspension rates ðLÞ
intensity is always higher within canopies than above them. The increase of turbulence within the canopy can be partially explained by needles and stems fluttering which converts kinetic energy from the mean flow to turbulent kinetic energy. Although turbulence can facilitate deposition of particles it can also provide an instantaneous lift force sufficient to detach and resuspend deposited particles. Deposition and resuspension may thus occur simultaneously and resuspension within a few seconds of deposition cannot generally be distinguished from the deposition process. The current study is relevant to resuspension commencing sometime after the initial deposition episode. In the field situation, resuspension will take place only in the advent of suitable circumstances such as surface drying, windy conditions or mechanical disturbance. The long term resuspension rate of particles is highly dependent on the stability of the fluid flow according to Hall (1989) who reported that flow changes as low as 1% of the mean flow may lead to changes in resuspension rates of a factor of 6. In their study on forest sites, Wu et al. (1992) reported that despite the mean wind speed being lower within the canopy, resuspension rates within the crown were only slightly less than those above the canopy. This was attributed to the effects of wind gusts penetrating deep within the canopy. During resuspension experiments, temperature and relative humidity measured inside the wind tunnel varied over the range 22–288C and 35–50%, respectively. These values were obtained over three days of intermittent operation. Variations of temperature and relative humidity with height were not established but the current experiments were assumed to be conducted under neutral conditions. Furthermore, particle size is one of the factors affecting the resuspension process and
Experimentally-derived L values (calculated using the initial surface contamination, S0 , for all time intervals) for 1.85 mm MMAD silica particles are shown in Figs. 4 and 5. These figures show a similar pattern for all experiments with a decline in L of approximately three orders of magnitude over the cumulative experimental period of 24 h. Resuspension experiments were intended to be conducted under steady state wind conditions although the start up of the wind tunnel at the beginning of each sampling interval produced a transient acceleration in wind speed (see Giess et al., 1994). This acceleration of wind speed from zero to the required steady state velocity approximates to a gust event according to Giess et al. During almost the entire initial 90 s sampling period, resuspension can be considered to be largely the result of a gust event during which a high proportion of material may be resuspended as the result of a rapid change in drag forces on particles on tree surfaces. This ‘gust’ may also cause the waving of trees and hence the dislodgement of particles from tree surfaces, although this process will also be dependent on the roughness characteristics of the surface. Caution was taken in interpreting resuspension rates measured over the shorter sampling intervals (90–7200 s) due to the effect of the tunnel start up described above. Average L values for each canopy layer and for the canopy as a whole were, therefore, calculated using the last two sampling time intervals (21600 and 50400 s) during which the period of accelerating wind speed at tunnel start up was small in relation to the total resuspension period. Resuspension occurred with a flux of 1.05 107 g Dy m2 s1 from spruce saplings initially contaminated at a level of 4.1 102 g Dy m2. Values of L from these two time intervals are summarised in Table 1. Geometric means of L, obtained
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Fig. 4. Variation of resuspension rate with time during resuspension experiments 1 and 2.
from all five layers, were similar between all resuspension experiments and varied between 1.78 106 s1 ( 8.82 107 s.e.m) and 2.14 107 s1 ( 1.06 107 s.e.m) with an average value of 4.88 107 s1 ( 2.42 107 s.e.m). As shown in Table 1, the overall average canopy L was 4.88 107 s1. This was determined as the average of geometric means of L at each canopy layer obtained from all four experiments. Similar values (in the range 106–105 s1) were reported by Wu et al. (1992) who estimated L for SO4, C2O4 and Ca particulates using aerodynamic surrogate surfaces (symmetrical airfoils) at a mixed deciduous forest site. The magnitude of L was, however, similar at all three heights investigated by Wu et al. In the current study, resuspension rates were significantly different (ANOVA, p50.001) between canopy layers (Table 1). Values of L declined from around 106 s1 at the canopy top to about 108 s1 towards the bottom of the canopy. Fluxes of resuspended silica particles deposited onto the soil surface are summarised in Table 2. These were determined at two locations, between and beneath the trees, and were about 5 108 and 2 108 mg Dy cm2 s1, respectively. On average, resuspended silica particles deposited onto the soil surface at a rate
of about 5.3 108 mg Dy cm2 s1. Silica particles resuspended from tree surfaces were also detected within the boundary layer with average L values ranging between 2.63 107 and 2.01 107 s1 obtained during experiments 1 and 2, respectively. As shown in Fig. 6, L was greater at the canopy top (layer 1) where wind velocity was high, although a secondary increase in L also occurred near the base of the canopy. In fact, the pattern of L reflects the typical distribution of wind velocity within the canopy shown in Fig. 2. The vertical distribution of L values gives, at the very least, a clear qualitative indication of the relative magnitude of resuspension rates between the forest floor and the canopy top. Wu et al. (1992) reported that L measured at forest sites was nearly constant with tree height despite the variation of wind speed within the canopy. These authors attributed this observation to wind gusts penetrating deep within the canopy as mentioned above. When wind speed increases, the aerodynamic lift and drag forces increase and will begin to exceed the adhesive forces holding particles to the tree surfaces, inducing dislodgement of particles. However, particles may be unavailable for the resuspension process because of the importance of the boundary
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Fig. 5. Variation of resuspension rate with time during resuspension experiments 3 and 4. Table 1 Resuspension rates (s1) calculated for the 21600 and 50400 s time intervals for all resuspension experiments 1–4. Values in parentheses indicate s.e.m
Boundary layer Canopy layer Layer 1 Layer 2 Layer 3 Layer 4 Layer 5
Experiment 1
Experiment 2
2.63E07 (7.05E08)
2.01E07 (7.50E09)
1.41E06 (4.12E07) 2.65E07 (3.5E08) 1.41E07 (2.40E08) 1.59E07 (1.88E08) 5.15E07 (1.16E07)
2.45E06 (4.5E07) 4.01E07 (7.18E08) 1.82E07 (2.81E08) 1.02E07 (1.67E08) 4.05E07 (3.19E08)
Average
layer resistance around the individual collecting elements of the tree. To be resuspended, particles need to traverse this resistance zone. Resuspension of particles will be influenced to a greater or lesser extent by changes
Experiment 3
Experiment 4
Geometric mean
1.91E06 (1.43E07) 1.48E07 (1.76E08) 1.30E07 (2.37E08) 8.31E08 (3.41E09) 8.92E08 (1.85E08)
1.47E06 (1.64E08) 9.75E08 (2.99E09) 1.17E07 (1.27E08) 9.79E08 (3.33E08) 1.09E07 (2.95E08)
1.78E06 (8.82E07) 1.99E07 (9.89E08) 1.38E07 (6.86E08) 1.07E07 (5.36E08) 2.14E07 (1.06E07) 4.88E07 (2.42E07)
in the boundary layer as the wind speed changes. Turbulent flow provides for the penetration of this resistance layer by random bursts of turbulence and may assist the extraction of particles from the sites where
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they were retained. Such turbulent eddies may provide an instantaneous lift force sufficient to detach a particle from its surface. It should not be forgotten, however, that other factors discussed in Section 3.1 also play a role in inducing or supressing the resuspension of particles. In the present study, resuspension occurred from all layers of the canopy indicating that deposited aerosol particles resuspended even at wind speeds of less than 1 m s1 (i.e. within the lower regions of the canopy). However, not all deposited material was resuspended under the experimental conditions of the present study. Wind velocity varies with tree height, and particles deposited in regions where wind speed is low would be
Table 2 Fluxes of resuspended silica particles deposited onto the soil surface during experiments 3 and 4 Time interval (s)
Flux (mg Dy cm2 s1) Experiment 3
21,600
50,400
Mean s.e.m
Experiment 4
2.15E07 5.42E08 1.57E08 2.07E07 7.53E08 1.09E08 3.16E08 9.13E08
3.43E08 1.53E08 1.58E08 8.74E09 2.51E08 1.81E08 1.80E08 1.00E08
8.77E08 2.87E08
1.82E08 2.92E09
expected to be less susceptible to resuspension than those positioned in regions of high wind speed (e.g. canopy top). However, depending on the roughness characteristics of the surface, a gust event may cause the gross movement of trees and the consequent detachment of particles from the tree surfaces. According to Braaten et al. (1990), the forces which promote or resist resuspension depend on the physical characteristics of the particle, the roughness of the surface and the properties of the mean flow. Any sheltered particles will not be subject to resuspension unless they become exposed to the airflow as a result of a change in wind direction or other mechanical disturbances. Tree canopies are effective at capturing many pollutant particles due to their large surface roughness. In urban areas for example woodlands can play an important role in reducing the effects of particulate pollution. Results of low resuspension rates for small particles obtained in the present study support the growing interest in implementing urban forestry strategies to reduce the impacts of air pollution and improve the quality of urban life. A literature review carried out by Beckett et al. (1998) on this subject illustrated the importance of suburban woodlands in reducing concentrations of fine particles such as PM10 (particulate matter having an aerodynamic diameter of less than 10 mm) which can have significant health effects. McPherson et al. (1994) estimated that the trees of Chicago (approximately 90% deciduous and 10% coniferous) removed approximately 234 tons of PM10 in 1991, improving the average hourly air quality by 2.1% in heavily wooded areas. They estimated that 50% of the particles being deposited to the trees were
Fig. 6. Variation of mean resuspension rate with canopy height.
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resuspended to the atmosphere. This estimate is much larger than results obtained in the present study. In their study, McPherson et al. recognised, however, that their estimate should be considered first-order approximation because of many limitations to their results that have unknown bounds on the error of estimation. For example, their 50% resuspension value was estimated as mid-value based on limited literature and they considered that a resuspension rate of 20% would be reasonable. Furthermore, the authors indicated that their estimates are specific to 1991 conditions in the Chicago area and that any extrapolation to other years or other cities must consider specific pollution concentrations, tree configuration and local meteorology. It is known that in the urban environment very high pollutant concentrations can be highly localised, both temporally and spatially (Croxford et al., 1996). This draws attention to the need for research on the resuspension of particles in urban areas. It should be noted, however, that under field conditions particles deposited on trees can be removed from tree surfaces through resuspension, wash off by rain, or through leaf and twig fall. Tree canopies thus constitute only a temporary retention site for atmospheric particles and should not be considered as replacement to emission controls. This highlights the need for more research on how trees can be most effectively utilised in the urban environment to reduce the amount of pollutants inhaled by humans.
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Tree canopies are morphologically complex and consist of surfaces of varying micro-roughness and a wide range of adhesive sites. Their dense and complex structure can be expected to strengthen the adhesion of particles to the tree surfaces. For example, deposited material may become associated with the epicuticular wax or caught in sheltered parts of the tree and hence may be prevented from being resuspended. The adhesion of particles to tree surfaces can be enhanced in conditions of high humidity and surface wetness thereby preventing their dislodgement from the plant surfaces (Garland, 1983). Particles thus sheltered will not be subject to resuspension unless they become exposed to the airflow as a result of a change in wind direction or other mechanical disturbances. This may not occur in the wind tunnel where winds are unidirectional. Under natural conditions, however, wind speeds and direction can change frequently and unidirectional winds produced in the tunnel may not be considered representative of field conditions. However, there are areas in the UK such as coastal locations (e.g. West Cumbria) where wind is predominantly unidirectional for long periods. The above factors indicate the further complications in resuspension when considering vegetation and in particular tree surfaces.
3.4. Uncertainties 3.3. Application of wind tunnel data to the natural environment Measurements of mean wind velocity and turbulence quantities were carried out with respect to height within and above the canopy in order to compare the flow regime in the wind tunnel with that likely to be found in the field. Results showed that a characteristic boundary layer (Fig. 2) was developed above the canopy within the wind tunnel and wind velocity and turbulence intensity profiles (Figs. 2 and 3) were typical of ‘real’ forest canopies and similar to those observed in numerous other forest and agricultural canopies. This similarity gives confidence that the boundary layer for our ‘model’ canopy was adequately developed for the purposes of resuspension experiments and confirms the adequacy of the experimental set-up and the realism of the wind tunnel study with respect to the field situation. It should be born in mind, however, that a complete simulation of atmospheric wind is generally difficult to achieve in the wind tunnel and wind tunnel conditions can be expected to be different from those of the field by some degree. Furthermore, resuspension is likely to vary significantly in time and space because of the variety of surface types of trees and environmental conditions.
As mentioned above resuspension is a very difficult process to measure in the field particularly for complex surfaces such as tree canopies. In the present study, small trees of about 45 cm height and a controlled environment provided the only method to obtain a detailed high resolution study of vertical profiles of resuspension rates within the canopy. Potential sources of uncertainty associated with wind tunnel measurements reported in this study include, inter alia, difference between wind tunnel and field conditions and errors in measurements. In preparing data for this study, efforts have been made to reduce these uncertainties where possible and to provide realistic measurements. For example, contamination of trees was carried out away from the wind tunnel (in the laboratory) in order to avoid any accidental contamination of the wind tunnel. In addition, caution was taken in interpreting resuspension rates measured over the shorter sampling intervals (90–7200 s) due to the effect of the tunnel start up (i.e. gust effect) described above. Average L values for each canopy layer and for the canopy as a whole were therefore calculated using the last two sampling time intervals (21600 and 50400 s) in which the tunnel start up gust effect was small in relation to subsequent resuspension.
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4. Conclusion Resuspension rates (L) of small aerosol particles were estimated using wind tunnel derived measurements of particle fluxes from five horizontal layers within a small scale spruce canopy where trees were exposed to a constant wind speed of 5 m s1. Values of L were significantly different (ANOVA, p50.001) between canopy layers and L was markedly greater at the top of the canopy than lower down although there was a slight increase in L at the base of the canopy. The results obtained during the course of this study revealed that resuspension rates of small particles from tree surfaces under the action of wind are very small. Therefore, for situations similar to the present experimental conditions the inhalation hazard to the human population, which will probably diminish with time, and the secondary contamination of other sites are likely to be small. These results support the growing interest in implementing forestry strategies in urban areas to reduce the amount of pollution inhaled by humans. Following an accidental release of pollutants into the environment, it is essential to predict accurately the expected impact to members of the public in order to evaluate its consequences and to establish adequate countermeasures. Environmental assessments currently rely on the use of models in order to predict the impacts on the human population of accidental releases of pollutants into the environment. The experimental data reported here can be used to calibrate such models and test the accuracy and precision of their predictions. This application may be appropriate for radionuclides as well as pollutant aerosols such as SO4, NO3 and NH4 which are characterised by particle sizes in the range used in the present study. Given the difficulties in making resuspension rate measurements in full-scale tree canopies, the wind tunnel derived data presented here are extremely useful in assigning order of magnitude rate values to the process of resuspension in forest canopies. Furthermore, the vertical distribution of L values observed in these experimental ‘model’ canopies gives, at the very least, a clear quantitative indication of the relative magnitude of resuspension rates between the forest floor and the canopy top.
Acknowledgements The financial support of this work by the Commission of the European Communities is gratefully acknowledged (Contract Number FI3PCT920016). The MAFF/ Imperial College environmental wind tunnel was constructed with partial funding from the Ministry of Agriculture, Fisheries and Food (MAFF), UK.
Special thanks are due to Dr Rob Kinnersley for his technical help with airflow characterisation and to Dr George Shaw for his useful advice throughout the project.
References Bache, D.H., Johnstone, D.R., 1992. Microclimate and spray dispersion. Ellis Horwood, London. Beckett, K.P., Freer-Smith, P.H., Taylor, G., 1998. Urban woodlands: their role in reducing the effects of particulate pollution. Environmental Pollution 99, 347–360. Braaten, D.A., Paw, U.K.T., Shaw, R.H., 1990. Particle resuspension in a turbulent boundary layer}observed and modeled. Journal of Aerosol Science 21, 613–628. Croxford, B., Penn, A., Hillier, B., 1996. Spatial distribution of urban pollution: civilizing urban traffic. Science of the Total Environment 190, 3–9. Fairchild, C.I., Tillery, M.I., 1982. Wind tunnel measurement of the resuspension of ideal particles. Atmospheric Environment 16, 229–238. Garland, J.A., 1983. Some recent studies of the resuspension of deposited material from soil and grass. In: Pruppacher, H.R., Semonin, R.G., Slinn, W.G.N. (Eds.), Precipitation Scavenging, Dry deposition and Resuspension, Vol. 2. Elsevier, Amsterdam, pp. 1087–1097. Giess, P., 1993. Factors affecting the resuspension of monodisperse particles from grass swards. Ph.D. Thesis, Imperial College of Science, Technology and Medicine, University of London. Giess, P., Goddard, A.J.H., Shaw, G., Allen, D., 1994. Resuspension of monodisperse particles from short grass swards: a wind tunnel study. Journal of Aerosol Science 25, 843–857. Grace, J., 1983. Plant-atmosphere relationships. Chapman and Hall, London, New York. Hall, D., 1989. The time dependence of particle resuspension. Journal of Aerosol Science 20, 907–910. Kinnersley, R.P., Farrington-Smith, J.G., Shaw, G., Minski, M.J., 1994. Aerodynamic characterisation of model tree canopies in a wind tunnel. The Science of the Total Environment 157, 29–33. McPherson, E.G., Nowak, D.J., Rowntree, R.E., 1994. Chicago’s Urban Forest Ecosystem: Results of the Chicago Urban forest Climate Project. USDA General Technical Report NE-186. Nicholson, K.W., 1988. A review of particle resuspension. Atmospheric Environment 22, 2639–2651. Nicholson, K.W., 1993. Wind tunnel experiments on the resuspension of particulate material. Atmospheric Environment 27A, 181–188. Oliver, H.R., 1971. Wind profiles in and above a forest canopy. Quaternary Journal of the Royal Meteorological Society 97, 548–553. Oliver, H.R., 1975. Ventilation in a forest. Agricultural Meterology 14, 347–355. Ould-Dada, Z., 1996. Dynamics of aerosol behaviour and fate within spruce canopies. Ph.D. Thesis, Imperial College of Science, Technology and Medicine, University of London.
Z. Ould-Dada, N.M. Baghini / Atmospheric Environment 35 (2001) 3799–3809 Parkin, C.S., 1987. Factors affecting the movement of spray droplets above a forest canopy. Proceedings of Symposium on the aerial application of pesticides in forestry, National Research Council, Canada, pp. 69–79. Raupach, M.R., Thom, A.S., 1981. Turbulence in and above plant canopies. American Review of Fluid Mechanics 13, 97–129. Slinn, W.G.N., 1982. Predictions for particle deposition to vegetative canopies. Atmospheric Environment 16, 1785–1794.
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Watterson, J.D., 1989. Interception and retention of aerosols by vegetation in relation to their surface cuticular characteristics. Ph.D. Thesis, Imperial College of Science, Technology and Medicine, University of London. Wedding, J.B., Montgomery, M.E., 1980. Deposition velocities for full-scale corn and soybean canopies: a wind tunnel simulation. Environment International 3, 91–96. Wu, Y.L., Davidson, C.I., Lindberg, S.E., Russell, A.G., 1992. Resuspension of particulate chemical species at forested sites. Environmental Science and Technology 26, 2428–2439.
Resuspension of small particles from tree surfaces Zitouni Ould-Dada*, Nasser M. Baghini EAS, T.H. Huxley School, Imperial College at Silwood Park, Ascot, Berkshire, SL5 7TE, UK Received 1 November 2000; received in revised form 15 February 2001; accepted 7 March 2001
Abstract A detailed study of resuspension of 1.85 mm MMAD silica particles from five horizontal layers within a small scale spruce canopy was carried out in a wind tunnel in which saplings were exposed to a constant free stream wind speed of 5 m s1. This provided quantitative estimates of the potential for a tree canopy contaminated with an aerosol deposit to provide (i) an airborne inhalation hazard within the forest environment and (ii) a secondary source of airborne contamination after an initial deposition event. Resuspension occurred with a flux of 1.05 107 g m2 s1 from spruce saplings initially contaminated at a level of 4.1 102 g m2. An average resuspension rate (L) of 4.88 107 s1 was obtained for the canopy as a whole. Values of L were significantly different (ANOVA, p50.001) between canopy layers and L was markedly greater at the top of the canopy than lower down although there was a slight increase in L at the base of the canopy. The resuspended silica particles deposited onto the soil surface at an average rate of about 5.3 108 mg cm2 s1. It is concluded that resuspension under wind velocities similar to that used in the reported experiments is likely to pose a relatively small inhalation hazard to humans and a relatively minor source of secondary contamination of adjacent areas. Furthermore, resuspension rates are likely to diminish rapidly with time. The results are discussed in relation to the growing interest in the tree planting schemes in urban areas to reduce the impacts of air pollution. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Aerosol particles; Spruce canopy; Wind tunnel; Resuspension rate; Inhalation hazard
1. Introduction Following an accidental or planned airborne release and after deposition, resuspension of small particles is an important mechanism which may pose an inhalation hazard and lead to redistribution of contaminants. Contaminated forests for example may act, during dry windy periods, as sources of resuspended material subject to secondary wind transfer to adjacent areas. In the case of radionuclides and other carcinogens this process could represent a long term pathway of environmental exposure, especially for long lived iso*Corresponding author. Present address. Food Standards Agency, Radiological Safety Unit, Contaminants Division, Aviation House, 125 Kingsway, London, WC2B 6NH, UK. Tel.: +44-207-276-8774; fax: +44-207-276-8788/8789. E-mail address: zitouni.ould-dada@foodstandards. gsi.gov.uk (Z. Ould-Dada).
topes and organic compounds. Resuspension is influenced by various factors including the physical charact eristics of the contaminated surface, the physicochemical nature of the contaminant and meteorological conditions which together make measurement of resuspension rates under natural conditions a very difficult task. Control over some of these variables can be achieved by studies carried out in wind tunnels using artificial surfaces, or uniform ‘natural’ surfaces such as bare soil and grass swards. Several such studies have indicated resuspension rates and factors to be a function of particle size, wind speed, surface type and time (Fairchild and Tillery, 1982; Garland, 1983; Nicholson, 1988, 1993; Braaten et al., 1990; Wu et al., 1992; Giess et al., 1994). Most field data available on resuspension originated from observations at nuclear and weapon test sites (see Nicholson, 1988) which were situated mostly in arid or semi-arid conditions where vegetation cover was
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incomplete. Resuspension studies in the relatively moist, temperate conditions typical of Britain and northern Europe are still limited. More particularly, resuspension rate estimates of small aerosol particles contaminating complex natural surfaces such as forest canopies have been very rare within the literature on environmental aerosol behaviour. It was thus intended to redress this deficiency by taking detailed measurements of particle resuspension fluxes within a small scale ‘model’ Norway spruce (Picea abies) canopy contained within a wind tunnel. This has provided quantitative estimates of the potential for a tree canopy to provide a source of aerosol particles which may represent (i) an inhalation hazard to people within the forest environment and (ii) a redistribution problem in which previously deposited particles may be made available for atmospheric transport to previously uncontaminated areas. In the present study, resuspension is referred to as the mechanism by which deposited particles are removed by wind from tree surfaces under controlled dry conditions and after initial deposition has ceased.
2. Materials and methods 2.1. Aerosol particles Insoluble silica particles of 1 mm nominal physical diameter were used in the present study (Separations Ltd, Clywd, UK). Subsequent measurement with an aerodynamic particle sizer (APS, model 3300, TSI inc. Minnesota, USA) revealed a mass median aerodynamic diameter (MMAD) of 1.85 0.5 mm standard deviation. Silica particles, used more commonly in chromatographic separation, are highly porous and provide sites for labelling by a variety of active or inactive tracers. Rare earth elements such as dysprosium (Dy), indium or europium have been used as activatable tracers in previous aerosol studies at Imperial College (Watterson, 1989; Giess, 1993; Giess et al., 1994). Soluble Dy [Dy(NO3)3] was chosen as a tracer for this study because of its relatively low concentration in plant and soil materials which makes its detection by cyclical activation less prone to interference by the natural background signal. The level of labelling was sufficient to detect easily the amount of resuspended particulate. 2.2. Airflow characterisation Resuspension experiments were carried out in the MAFF/Imperial College wind tunnel, Silwood Park, which is of a closed circuit type and which has a working section 6 m long, 1.2 m high and 0.8 m wide. The entire length of the wind tunnel was first filled with uncontaminated Norway spruce saplings (40 cm in height) for wind velocity profile measurements. The wind tunnel’s
fan speed was adjusted until a free stream air velocity of 5 m s1 was established above the canopy. After establishing the desired free stream wind velocity an automated hot wire anemometry system was used to make wind velocity measurements at 10 mm intervals from 15 mm above ground level up to a height of 1.015 m. These measurements revealed the wind velocity profile within and above the canopy and allowed calculations of friction velocity, u * , (using the eddy correlation method) and turbulence intensity, iu , (using measured fluctuations on the mean wind speed) to be made. After air flow characterisation was complete five trees from the trailing end of the canopy were taken away for contamination with Dy labelled silica particles and then carefully replaced for resuspension measurements. 2.3. Contamination of spruce saplings Contamination of spruce saplings by aerosol deposition was achieved using a Perspex chamber (80 cm 50 cm 65 cm) and associated equipment (Fig. 1). The air within the chamber was actively exchanged with filtered laboratory air during the deposition process using a vacuum pump operating at a rate of 200 l min1. This exchange of air ensured thorough mixing of the atmosphere within the chamber and facilitated uniform contamination of the trees with aerosol. A powder dispersion generator (RBG 1000, Palas GmBH Partikel, Germany) was used to liberate the aerosol which was then carried to the contamination chamber by a nitrogen gas stream at 30 l min1 providing a constant input of aerosol during the deposition phase of the experiment. A cylinder containing a 74 MBq 85Kr source was attached to the RBG-1000 outlet to reduce electrostatic charge build up on the particles. The aerosol was injected into the top of the chamber via an aluminium manifold which consisted of two rows of four evenly spaced outlets. To avoid the formation of aggregates of aerosol within the manifold the latter was rinsed with deionised water and air dried between deposition experiments. A scanning electron micrograph of silica particles deposited on a spruce needle is shown in Plate 1. In order to avoid any accidental contamination of the tunnel interior surfaces by Dy tracer, contamination of spruce saplings was carried out away from the wind tunnel in a separate laboratory. After contamination, trees were placed in a large cardboard box and carefully transported to the wind tunnel where resuspension experiments took place. Contaminated trees were positioned at the trailing end of the experimental bay of the wind tunnel covering a floor area of 0.76 m2 (0.95 m 0.8 m). The remaining floor area of the wind tunnel (i.e. upwind of the contaminated saplings) was occupied by uncontaminated saplings to provide the fetch required to ensure the development of a
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Fig. 1. Enclosed Perspex chamber and associated equipment used for the contamination of spruce saplings with aerosol prior to resuspension experiments.
2.4. Sampling and analysis
Plate 1. Scanning electron micrograph of silica particles deposited on a spruce needle.
characteristic boundary layer. Peat-based compost was used as a sub-canopy soil surface which was kept moist throughout all experiments. Cardboard sheets were used as a platform for this compost layer under the contaminated trees in order to achieve as level a surface as possible.
Four resuspension experiments were carried out in this study: five contaminated trees were used for each experiment. The trees were exposed to a constant free stream wind velocity of 5 m s1 and the mass of resuspended Dy derived from five horizontal canopy layers was determined during seven consecutive time intervals, viz. 90, 1110, 2400, 3600, 7200, 21,600 and 50,400 s (i.e. the total cumulative time allowed for resuspension to occur within a single experiment was 24 h). The initial surface contamination (S0 ) of spruce saplings was determined within the five horizontal layers of the canopy prior to each experiment after saplings were placed in the wind tunnel. Two samples of spruce tissue were collected from the topmost layer and six samples (three new tissues and three old tissues) were collected from each of the remaining four layers of each tree. Each sample (needles+stems) was placed in a polyethylene bag and oven dried at 708C for 24 h after which time the tissue dry weight was recorded. Samples were then analysed (see below) and S0 was expressed in g Dy g1 dry weight (DW) for each canopy layer. This value was converted into g Dy m2 of the canopy area (including needles and stems) using the leaf area index (LAI). LAI was calculated using the mass–area relationship for each tissue category and an estimate of the total mass of that tissue category obtained by destructively sampling and weighing selected trees (Ould-Dada, 1996). Samples of resuspended particles were obtained within and above the canopy using isokinetic air samplers. The position and the flow rate of each sampler were
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determined according to the wind velocity profile. During experiments 1 and 2 aerosol particles were sampled from each of the five canopy layers using two isokinetic air samplers within each layer and from above the canopy using five isokinetic samplers positioned at different heights within the boundary layer. During experiments 3 and 4, resuspended particles were sampled only from within the canopy using three isokinetic samplers within each layer. Particles reaching the soil surface were also collected using four 4.7 cm diameter Whatman 542 filter papers randomly placed between the contaminated saplings and four under the crowns. Samples were analysed for total Dy content by instrumental neutron activation analysis (INAA) using the Imperial College reactor at Silwood Park. These were packed into polyethylene capsules and placed in an automatic cyclical irradiation and analysis system constructed around the reactor. This cyclical activation system (CAS) permits samples to be irradiated in a thermal neutron flux of 1.29 1012 neutrons cm2 s1 and then rapidly counted on a Ge(Li) semiconductor gamma spectrometry system. Samples were counted for 90 s after being irradiated for 90 s followed by a 1 s decay. A spectral analysis program (Spectran AT, Canberra, UK), run on an IBM personal computer, was used to determine the mass of Dy on each filter paper and spruce tissue sample. 2.5. Determining resuspension rate Resuspension was expressed as a resuspension rate (L) which is defined as the fraction of particulate contaminant initially present on a ground or vegetation surface resuspended per unit time: Lðs1 Þ ¼
resuspension flux R ðg Dy m2 s1 Þ : surface contamination S0 ðg Dy m2 Þ
ð1Þ
models to predict airborne concentrations of contaminants above soil, vegetation and urban surfaces subjected to acute or chronic deposition events.
3. Results and discussion 3.1. Micro-meteorology of ‘model’ spruce canopies within the wind tunnel The wind velocity profile obtained for the canopy in this study (Fig. 2) was similar to those observed in numerous other forest and agricultural canopies (Oliver, 1971, 1975; Wedding and Montgomery, 1980; Raupach and Thom, 1981; Grace, 1983; Slinn, 1982; Bache and Johnstone, 1992). The mean wind velocity fell from a free stream value of 5 m s1 to less than 1 m s1 near the base of the canopy, with a secondary maximum occurring within the ‘trunk space’ adjacent to the ground surface. The similarity of this velocity profile to those obtained in full scale tree canopies gives confidence that the boundary layer for the ‘model’ canopy was adequately developed for the purposes of the resuspension experiments. The downward flux of momentum from the boundary layer to the first layer of the canopy was characterised by a friction velocity, u * of 0.35 m s1 estimated using the eddy correlation method. The turbulence intensity (iu ) of the air flow within the canopy (where it varied between 0.1 and 0.38) was higher than above it (free stream trubulence intensity was 50.05; Fig. 3). Measurements from a full scale forest canopy (Parkin, 1987) and a model spruce canopy in a wind tunnel (Kinnersley et al., 1994) showed similar values and distributions of turbulence intensity. According to Raupach and Thom (1981) turbulence
The resuspension flux (R) was determined considering wind speed at the sampling position, the measured timeaveraged air concentration at that position, and the dimensions of the wind tunnel: R¼
MDy ½DC WWSi ti Cai ¼ ; TLAi ti ½LAIi 0:76 m2 ti
ð2Þ
where MDy is the total mass of Dy resuspended, TLAi is the total layer area at layer i, LAI is the leaf area index, Dc is the depth of the canopy layer, W is the wind tunnel width, WSi is the midpoint wind speed of layer i, t is the sampling duration, and Cai the time-averaged air concentration of aerosol within layer i. In practice L is very difficult to evaluate with confidence under field conditions and it can normally only be measured under controlled conditions in wind tunnel or laboratory studies. The resuspension rate is suitable for use in dynamic atmospheric transport
Fig. 2. Typical wind velocity profile within a ‘model’ spruce canopy contained in the MAFF/Imperial College wind tunnel.
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Fig. 3. Typical turbulence intensity profile within a ‘model’ spruce canopy.
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in conditions of high humidity hygroscopic aerosols such as silica particles may grow and changes in their shape and structure may occur. The adhesion of particles to tree surfaces can be enhanced in conditions of high humidity and surface wetness thereby preventing their dislodgement from the plant surfaces. High humidity will enable the formation of films of moisture around particles and hence favour adhesion and suppress resuspension (Garland, 1983). This implies the importance of the effect of such a factor on resuspension in temperate climates such as Britain and most of Europe. In the present experiments relative humidity did not exceed 50% and it is unlikely that any significant changes in particle size would have occurred under these conditions. In addition, the factors mentioned above indicate the further complications in resuspension when considering vegetation and in particular tree surfaces. 3.2. Resuspension rates ðLÞ
intensity is always higher within canopies than above them. The increase of turbulence within the canopy can be partially explained by needles and stems fluttering which converts kinetic energy from the mean flow to turbulent kinetic energy. Although turbulence can facilitate deposition of particles it can also provide an instantaneous lift force sufficient to detach and resuspend deposited particles. Deposition and resuspension may thus occur simultaneously and resuspension within a few seconds of deposition cannot generally be distinguished from the deposition process. The current study is relevant to resuspension commencing sometime after the initial deposition episode. In the field situation, resuspension will take place only in the advent of suitable circumstances such as surface drying, windy conditions or mechanical disturbance. The long term resuspension rate of particles is highly dependent on the stability of the fluid flow according to Hall (1989) who reported that flow changes as low as 1% of the mean flow may lead to changes in resuspension rates of a factor of 6. In their study on forest sites, Wu et al. (1992) reported that despite the mean wind speed being lower within the canopy, resuspension rates within the crown were only slightly less than those above the canopy. This was attributed to the effects of wind gusts penetrating deep within the canopy. During resuspension experiments, temperature and relative humidity measured inside the wind tunnel varied over the range 22–288C and 35–50%, respectively. These values were obtained over three days of intermittent operation. Variations of temperature and relative humidity with height were not established but the current experiments were assumed to be conducted under neutral conditions. Furthermore, particle size is one of the factors affecting the resuspension process and
Experimentally-derived L values (calculated using the initial surface contamination, S0 , for all time intervals) for 1.85 mm MMAD silica particles are shown in Figs. 4 and 5. These figures show a similar pattern for all experiments with a decline in L of approximately three orders of magnitude over the cumulative experimental period of 24 h. Resuspension experiments were intended to be conducted under steady state wind conditions although the start up of the wind tunnel at the beginning of each sampling interval produced a transient acceleration in wind speed (see Giess et al., 1994). This acceleration of wind speed from zero to the required steady state velocity approximates to a gust event according to Giess et al. During almost the entire initial 90 s sampling period, resuspension can be considered to be largely the result of a gust event during which a high proportion of material may be resuspended as the result of a rapid change in drag forces on particles on tree surfaces. This ‘gust’ may also cause the waving of trees and hence the dislodgement of particles from tree surfaces, although this process will also be dependent on the roughness characteristics of the surface. Caution was taken in interpreting resuspension rates measured over the shorter sampling intervals (90–7200 s) due to the effect of the tunnel start up described above. Average L values for each canopy layer and for the canopy as a whole were, therefore, calculated using the last two sampling time intervals (21600 and 50400 s) during which the period of accelerating wind speed at tunnel start up was small in relation to the total resuspension period. Resuspension occurred with a flux of 1.05 107 g Dy m2 s1 from spruce saplings initially contaminated at a level of 4.1 102 g Dy m2. Values of L from these two time intervals are summarised in Table 1. Geometric means of L, obtained
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Fig. 4. Variation of resuspension rate with time during resuspension experiments 1 and 2.
from all five layers, were similar between all resuspension experiments and varied between 1.78 106 s1 ( 8.82 107 s.e.m) and 2.14 107 s1 ( 1.06 107 s.e.m) with an average value of 4.88 107 s1 ( 2.42 107 s.e.m). As shown in Table 1, the overall average canopy L was 4.88 107 s1. This was determined as the average of geometric means of L at each canopy layer obtained from all four experiments. Similar values (in the range 106–105 s1) were reported by Wu et al. (1992) who estimated L for SO4, C2O4 and Ca particulates using aerodynamic surrogate surfaces (symmetrical airfoils) at a mixed deciduous forest site. The magnitude of L was, however, similar at all three heights investigated by Wu et al. In the current study, resuspension rates were significantly different (ANOVA, p50.001) between canopy layers (Table 1). Values of L declined from around 106 s1 at the canopy top to about 108 s1 towards the bottom of the canopy. Fluxes of resuspended silica particles deposited onto the soil surface are summarised in Table 2. These were determined at two locations, between and beneath the trees, and were about 5 108 and 2 108 mg Dy cm2 s1, respectively. On average, resuspended silica particles deposited onto the soil surface at a rate
of about 5.3 108 mg Dy cm2 s1. Silica particles resuspended from tree surfaces were also detected within the boundary layer with average L values ranging between 2.63 107 and 2.01 107 s1 obtained during experiments 1 and 2, respectively. As shown in Fig. 6, L was greater at the canopy top (layer 1) where wind velocity was high, although a secondary increase in L also occurred near the base of the canopy. In fact, the pattern of L reflects the typical distribution of wind velocity within the canopy shown in Fig. 2. The vertical distribution of L values gives, at the very least, a clear qualitative indication of the relative magnitude of resuspension rates between the forest floor and the canopy top. Wu et al. (1992) reported that L measured at forest sites was nearly constant with tree height despite the variation of wind speed within the canopy. These authors attributed this observation to wind gusts penetrating deep within the canopy as mentioned above. When wind speed increases, the aerodynamic lift and drag forces increase and will begin to exceed the adhesive forces holding particles to the tree surfaces, inducing dislodgement of particles. However, particles may be unavailable for the resuspension process because of the importance of the boundary
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Fig. 5. Variation of resuspension rate with time during resuspension experiments 3 and 4. Table 1 Resuspension rates (s1) calculated for the 21600 and 50400 s time intervals for all resuspension experiments 1–4. Values in parentheses indicate s.e.m
Boundary layer Canopy layer Layer 1 Layer 2 Layer 3 Layer 4 Layer 5
Experiment 1
Experiment 2
2.63E07 (7.05E08)
2.01E07 (7.50E09)
1.41E06 (4.12E07) 2.65E07 (3.5E08) 1.41E07 (2.40E08) 1.59E07 (1.88E08) 5.15E07 (1.16E07)
2.45E06 (4.5E07) 4.01E07 (7.18E08) 1.82E07 (2.81E08) 1.02E07 (1.67E08) 4.05E07 (3.19E08)
Average
layer resistance around the individual collecting elements of the tree. To be resuspended, particles need to traverse this resistance zone. Resuspension of particles will be influenced to a greater or lesser extent by changes
Experiment 3
Experiment 4
Geometric mean
1.91E06 (1.43E07) 1.48E07 (1.76E08) 1.30E07 (2.37E08) 8.31E08 (3.41E09) 8.92E08 (1.85E08)
1.47E06 (1.64E08) 9.75E08 (2.99E09) 1.17E07 (1.27E08) 9.79E08 (3.33E08) 1.09E07 (2.95E08)
1.78E06 (8.82E07) 1.99E07 (9.89E08) 1.38E07 (6.86E08) 1.07E07 (5.36E08) 2.14E07 (1.06E07) 4.88E07 (2.42E07)
in the boundary layer as the wind speed changes. Turbulent flow provides for the penetration of this resistance layer by random bursts of turbulence and may assist the extraction of particles from the sites where
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they were retained. Such turbulent eddies may provide an instantaneous lift force sufficient to detach a particle from its surface. It should not be forgotten, however, that other factors discussed in Section 3.1 also play a role in inducing or supressing the resuspension of particles. In the present study, resuspension occurred from all layers of the canopy indicating that deposited aerosol particles resuspended even at wind speeds of less than 1 m s1 (i.e. within the lower regions of the canopy). However, not all deposited material was resuspended under the experimental conditions of the present study. Wind velocity varies with tree height, and particles deposited in regions where wind speed is low would be
Table 2 Fluxes of resuspended silica particles deposited onto the soil surface during experiments 3 and 4 Time interval (s)
Flux (mg Dy cm2 s1) Experiment 3
21,600
50,400
Mean s.e.m
Experiment 4
2.15E07 5.42E08 1.57E08 2.07E07 7.53E08 1.09E08 3.16E08 9.13E08
3.43E08 1.53E08 1.58E08 8.74E09 2.51E08 1.81E08 1.80E08 1.00E08
8.77E08 2.87E08
1.82E08 2.92E09
expected to be less susceptible to resuspension than those positioned in regions of high wind speed (e.g. canopy top). However, depending on the roughness characteristics of the surface, a gust event may cause the gross movement of trees and the consequent detachment of particles from the tree surfaces. According to Braaten et al. (1990), the forces which promote or resist resuspension depend on the physical characteristics of the particle, the roughness of the surface and the properties of the mean flow. Any sheltered particles will not be subject to resuspension unless they become exposed to the airflow as a result of a change in wind direction or other mechanical disturbances. Tree canopies are effective at capturing many pollutant particles due to their large surface roughness. In urban areas for example woodlands can play an important role in reducing the effects of particulate pollution. Results of low resuspension rates for small particles obtained in the present study support the growing interest in implementing urban forestry strategies to reduce the impacts of air pollution and improve the quality of urban life. A literature review carried out by Beckett et al. (1998) on this subject illustrated the importance of suburban woodlands in reducing concentrations of fine particles such as PM10 (particulate matter having an aerodynamic diameter of less than 10 mm) which can have significant health effects. McPherson et al. (1994) estimated that the trees of Chicago (approximately 90% deciduous and 10% coniferous) removed approximately 234 tons of PM10 in 1991, improving the average hourly air quality by 2.1% in heavily wooded areas. They estimated that 50% of the particles being deposited to the trees were
Fig. 6. Variation of mean resuspension rate with canopy height.
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resuspended to the atmosphere. This estimate is much larger than results obtained in the present study. In their study, McPherson et al. recognised, however, that their estimate should be considered first-order approximation because of many limitations to their results that have unknown bounds on the error of estimation. For example, their 50% resuspension value was estimated as mid-value based on limited literature and they considered that a resuspension rate of 20% would be reasonable. Furthermore, the authors indicated that their estimates are specific to 1991 conditions in the Chicago area and that any extrapolation to other years or other cities must consider specific pollution concentrations, tree configuration and local meteorology. It is known that in the urban environment very high pollutant concentrations can be highly localised, both temporally and spatially (Croxford et al., 1996). This draws attention to the need for research on the resuspension of particles in urban areas. It should be noted, however, that under field conditions particles deposited on trees can be removed from tree surfaces through resuspension, wash off by rain, or through leaf and twig fall. Tree canopies thus constitute only a temporary retention site for atmospheric particles and should not be considered as replacement to emission controls. This highlights the need for more research on how trees can be most effectively utilised in the urban environment to reduce the amount of pollutants inhaled by humans.
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Tree canopies are morphologically complex and consist of surfaces of varying micro-roughness and a wide range of adhesive sites. Their dense and complex structure can be expected to strengthen the adhesion of particles to the tree surfaces. For example, deposited material may become associated with the epicuticular wax or caught in sheltered parts of the tree and hence may be prevented from being resuspended. The adhesion of particles to tree surfaces can be enhanced in conditions of high humidity and surface wetness thereby preventing their dislodgement from the plant surfaces (Garland, 1983). Particles thus sheltered will not be subject to resuspension unless they become exposed to the airflow as a result of a change in wind direction or other mechanical disturbances. This may not occur in the wind tunnel where winds are unidirectional. Under natural conditions, however, wind speeds and direction can change frequently and unidirectional winds produced in the tunnel may not be considered representative of field conditions. However, there are areas in the UK such as coastal locations (e.g. West Cumbria) where wind is predominantly unidirectional for long periods. The above factors indicate the further complications in resuspension when considering vegetation and in particular tree surfaces.
3.4. Uncertainties 3.3. Application of wind tunnel data to the natural environment Measurements of mean wind velocity and turbulence quantities were carried out with respect to height within and above the canopy in order to compare the flow regime in the wind tunnel with that likely to be found in the field. Results showed that a characteristic boundary layer (Fig. 2) was developed above the canopy within the wind tunnel and wind velocity and turbulence intensity profiles (Figs. 2 and 3) were typical of ‘real’ forest canopies and similar to those observed in numerous other forest and agricultural canopies. This similarity gives confidence that the boundary layer for our ‘model’ canopy was adequately developed for the purposes of resuspension experiments and confirms the adequacy of the experimental set-up and the realism of the wind tunnel study with respect to the field situation. It should be born in mind, however, that a complete simulation of atmospheric wind is generally difficult to achieve in the wind tunnel and wind tunnel conditions can be expected to be different from those of the field by some degree. Furthermore, resuspension is likely to vary significantly in time and space because of the variety of surface types of trees and environmental conditions.
As mentioned above resuspension is a very difficult process to measure in the field particularly for complex surfaces such as tree canopies. In the present study, small trees of about 45 cm height and a controlled environment provided the only method to obtain a detailed high resolution study of vertical profiles of resuspension rates within the canopy. Potential sources of uncertainty associated with wind tunnel measurements reported in this study include, inter alia, difference between wind tunnel and field conditions and errors in measurements. In preparing data for this study, efforts have been made to reduce these uncertainties where possible and to provide realistic measurements. For example, contamination of trees was carried out away from the wind tunnel (in the laboratory) in order to avoid any accidental contamination of the wind tunnel. In addition, caution was taken in interpreting resuspension rates measured over the shorter sampling intervals (90–7200 s) due to the effect of the tunnel start up (i.e. gust effect) described above. Average L values for each canopy layer and for the canopy as a whole were therefore calculated using the last two sampling time intervals (21600 and 50400 s) in which the tunnel start up gust effect was small in relation to subsequent resuspension.
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4. Conclusion Resuspension rates (L) of small aerosol particles were estimated using wind tunnel derived measurements of particle fluxes from five horizontal layers within a small scale spruce canopy where trees were exposed to a constant wind speed of 5 m s1. Values of L were significantly different (ANOVA, p50.001) between canopy layers and L was markedly greater at the top of the canopy than lower down although there was a slight increase in L at the base of the canopy. The results obtained during the course of this study revealed that resuspension rates of small particles from tree surfaces under the action of wind are very small. Therefore, for situations similar to the present experimental conditions the inhalation hazard to the human population, which will probably diminish with time, and the secondary contamination of other sites are likely to be small. These results support the growing interest in implementing forestry strategies in urban areas to reduce the amount of pollution inhaled by humans. Following an accidental release of pollutants into the environment, it is essential to predict accurately the expected impact to members of the public in order to evaluate its consequences and to establish adequate countermeasures. Environmental assessments currently rely on the use of models in order to predict the impacts on the human population of accidental releases of pollutants into the environment. The experimental data reported here can be used to calibrate such models and test the accuracy and precision of their predictions. This application may be appropriate for radionuclides as well as pollutant aerosols such as SO4, NO3 and NH4 which are characterised by particle sizes in the range used in the present study. Given the difficulties in making resuspension rate measurements in full-scale tree canopies, the wind tunnel derived data presented here are extremely useful in assigning order of magnitude rate values to the process of resuspension in forest canopies. Furthermore, the vertical distribution of L values observed in these experimental ‘model’ canopies gives, at the very least, a clear quantitative indication of the relative magnitude of resuspension rates between the forest floor and the canopy top.
Acknowledgements The financial support of this work by the Commission of the European Communities is gratefully acknowledged (Contract Number FI3PCT920016). The MAFF/ Imperial College environmental wind tunnel was constructed with partial funding from the Ministry of Agriculture, Fisheries and Food (MAFF), UK.
Special thanks are due to Dr Rob Kinnersley for his technical help with airflow characterisation and to Dr George Shaw for his useful advice throughout the project.
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