Nicholson 1988 - Bibliotheek T» D W Uitsluitend VOOI‘...

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Unformatted text preview: Bibliotheek T» D W Uitsluitend VOOI‘ eigen gebruik / for own use only /////A Prometheusplein 1 Datum: 1 5 — S ep— 0 3 Postbus 98 2600 MG DELFT Bonnummer: 7 0 0 1 5 4 Telefoon: 015 - 2784636 Fax: 015 - 2785673 Email: Klanten-Service@Library.TUDelft.NL Aan; T.N.O. 'I'ECHN. PHYSISCHfi DIfiNSi POSTBUS 155 2600 AD DELFT NfiD *. RTAND Tav; A. van de Runstraat Aantal kopieén: 13 Uwreferentie(s): 008.80013/01.69 Artikelomschrijving bij aanvraagnummer: 7 0 0 l 5 4 Artike]; A review of particle resuspension Auteur; Nicholson Tijdschrift; ALMOSPH«.RIC «.NVIRONMENT J aar: l 9 8 8 Vol. 2 2 Aflevering: l 2 Pagina(s): 2 6 3 9 ‘2 6 5 1 Plaatsnr,; 8 4 O 2 D Met ingang van 1 april 2003 zullen de prijzen v00rf0t0k0pie levering baitenland stijgen met 6‘ 0,05 per pagina From April I 2003, prices for photocopy delivery abroad will increase by 6‘ 0.05 per page Atmospheric Environment Vol. 22, No. 12, pp. 26392651, 1988. Printed in Great Britain. 0004 6981/88 $3.00+0.00 Pergamnn Press plc I{13\7IIE\A/ I&I{1?I(3I.IZ A REVIEW OF PARTICLE RESUSPENSION . K. W. NICHOLSON Env1ronmenta1 and Medical Sciences Division, Harwell Laboratory, Oxfordshire OX11 ORA, UK. (First received 10 November 1987 and received for publication 13 June 1988) Abstract—«Some of the various types of studies on particle resuspension or re-entrainment are summarized along with shortcomings. General experimental aspects have been considered, rather than focusing on the numerlcal values of results, and research on erosion and resuspension by mechanisms other than wind has beenmcluded. It is evident that experiments have been performed in a wide range of environmental conditions but that additional research is required, in many areas, if a quantitative assessment of resuspension is to be achieved. Key word index: Erosion, particles, re-entrainment, resuspension. 1. INTRODUCTION The entrainment or suspension of particulate material, into the atmosphere, has long been of interest and most of the early scientific work was concerned with erosion and soil transport (e.g. Bagnold, 1941; Chepil, 1943). More recently, especially since the advent of nuclear technologies, interest has also been on health aspects due to the resuspension of deposited material from nuclear weapon tests or possible future acciden- tal releases from the nuclear industry. Romney and Wallace (1977) concluded that foliar contamination, due to the deposition of entrained particles, was more significant than root uptake for contributing to the plutonium (Pu) levels of vegetation at the Nevada Test Site. Bulman (1976) also noted that actinides were not readily transferred through food chains nor were they readily absorbed through the gut. As a consequence, the resuspension and subsequent inhalation of pluto- nium and other actinides has been considered to be the limiting hazard in' the aftermath of a contamination episode (Anspaugh et al., 1975; Bulman, 1977; Cham— berlain, 198.3). Nielsen (1981) noted the uncertainties in assigning representative rates for the resuspension of particles and Lassey (1980) suggested that inhala- tion of resuspended particles, in the first few weeks after a contamination event, might be at least as important as the direct inhalation of the contami- nating cloud. Lassey also noted the sparsity and variability of data on which a description of resuspen- sion may be based on and Corbett (1977) observed that the resuspension route perhaps contained the mast serious uncertainty in predicting the effects of ground contamination. Lateral translocation, as a consequence of resuSpen- sion, is responsible for spreading contamination to areas adjacent to a source, as observed after the recent Chernobyl reactor accident, when it was discovered AB 22:12-A that decontaminated zones quickly regained measur- able amounts of radioactivity (Eggleton, personal communication, 1986). A detailed knowledge of re— suspension mechanisms and estimates of their signifi— cance must be essentiai in the consequence assessment of potential accidental reactor releases and if predic- tive models are to be validated. Other interests in resuspension have included con- cern with the fates of industrial spills (Langer, 1983), the transport of chlorinated hydrocarbon pesticides (Orgill et al., 1976), the spreading of crop disease by fungal spores (Aylor, 1976) and the transmission of human disease (Hereim and Ritchie, 1976). In this review, it is intended to draw on all available sources of information in order to elucidate the complexities and problems that surround resuspension. Where applicable, attention is drawn to deficiencies in experi— mental design and concluding suggestions are added for future research. It is not intended to compile an inventory of results but to focus on the more general experimental aspects. A detailed summary of re— suspension results has been assembled by Sehmel (1980a) and one of his comprehensive illustrations is' shown in Fig. 1. A major limitation of many of the resuspension studies, is that they have been located in arid or semi— arid regions and that an extrapolation of conclusions, to other climates, cannot be endorsed easily. As a result, information for regions with a more humid climate and dense vegetation cover are more sparse. Cohen (1977) suggested that humidity would be likely to reduce resuspension when studies undertaken in arid or semi-arid regions were compared with loc- ations in the more humid eastern U.S.A. Other studies have compared rates of soil erosion with resuspension rates and, because many regions are not vulnerable to Aeolian weathering, it might be conceived that this comparison is irrelevant. These types of studies have 2639 Review Article 2640 NIOr nlOr .Amomo. .EEnum E9: 833% 56:32.3. 9...... van $0.5...qu ES. 39:... .56.... coinagmom .. .mE 7E .mO.~O<m 20—wzumw3wwm 7.: $9 7.: To. claw Olav av 16' mmwmm—flhm 20.m2m&m3mm¢ J<O.2<10m1 AI; 02.5.5.5» 23:96: 2. Echo: .355: .10 .555... 93355 in: 53> SEE I 0.258. §E<E 60:00 .5045. ><a « I — figu.’ 9.33:: mg E0 .83! >3 EL 159:9. aide-ll lass...» un 98.9.. $252.! b3... .50! 3!..th 3 into: 3023. win... 23 x: .5929. EB EL. .3019. 9.5:, :33th no 5.6521. .96 5.59m: 05 gas §< Suin— a: :5. Buguzw a.qu >38 9334! 19.535qu 62 s 3. 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Similarly, laboratory studies which have been useful in determining particle adhesion are also included be- cause they can help to identify some of the mechan- isms that are important in resuspension. Most studies have examined resuspension by wind. However, there is a range of processes, including raindrop impact, mechanical disturbances by pedes- trians or vehicles, and agricultural operations, which also causes surface material to enter the atmosphere. All of these influences are addressed in this review. 2. DEFINITION OF RESUSPENSION TERMS The term resuspension has been used, so far, to mean the re-entrainment, into the atmosphere, of a previously deposited material. The term suspension or entrainment ought to be used if the material was not previously deposited by an atmospheric process. However, because the subsequent transport of depos- ited particles, in air, is often associated with host soil particles (Sehmel, 1976a, 1978) it is not easy to distin~ guish between resuspension and suspension (Sehmel, 1980a). For this reason and for consistency with other work, the term resuspension is used here to include suspension or entrainment. Measurements of resuspension have been often expressed as either resuspension factors or resuspen— sion rates and these are discussed in the following sections, along with some comments on their suit- ability. 2.1. Resuspensionfactor Early workers on resuspension (e.g. Stewart, 1967) usually expressed results as a resuspension factor (K or RF), defined as: airborne concentration C,( gm”) K(m'1)=——————————7—%. (1) surface contamination, S, (ugm ) The airborne concentration is measured at some reference height above ground level. The resuspension factor has been criticized because it fails to take into' consideration upwind resuspension and material ad— vected into the measurement zone (e.g. Horst, 1976a; Sehmel, 1977). The suitability of the resuspension factor, as an indicator of resuspension, is dependent on the deposition velocity of the resuspended material with a strongly depositing species having little influ— ence on the downwind concentration (Horst, 1976b). Another drawback with using K is that it is assumed that the airborne material is totally surface derived and this is not the case for materials with alternative sources (e.g. Pb, 801”). The inclusion of atmospheric concentration in Equation (1) introduces two complications into the determination of resuspension, even if advection from an upwind source is neglected. Firstly, the atmos- pheric concentration will depend on the rate of dilu- 2641 tion of the resuspended material and, consequently, the resuspension factor would be a function of wind speed irrespective of whether there was a change in the amount resuspended in unit time. Secondly, the at- mospheric concentration used to calculate K should be representative of resuspension. For large particles, which quickly fall back to the ground and are moving in saltation (section 3), the height of atmospheric, concentration. measurement is important. Anyhow, there is a dependence of resuspended particle concen- tration on height (Sehmel, 1980a, 1984). The value of K will depend on the depth of surface material sampled in order to determine surface con- tamination. Since the processes of transfer, up into the atmosphere and down into the soil, are poorly under— stood there is no obvious basis for choice and some of the variations in the values of K reported in the literature might be due to the adoption of different. sampling depths. 2.2. Resuspension rate An alternative expression of resuspension, to the resuspension factor, is the “resuspension rate” (A) which is defined as the fraction of a surface species removed in unit time (eg. Slinn, 1978). A(s”‘)=—-—-——-—R(flg nil—2571), (2) where R is the resuspension flux. Sehmel (1984) commented that the inclusion of surface concentration suffered similar shortcomings to those found when determining the resuspension fac— tor—only the amount of material available for re- suspension may be of interest and any contaminant material below a certain depth might only confound experimental results. On the other hand, if respiration doses from a contaminated surface are to be assessed over time, then species transport within the surface layer of soil must be considered. As previously noted, one of the major reasons for the study of resuspension is to assess potential radiation doses from the inhala- tion of resuspended material. The determination of resuspension rate does not essentially require a meas- urement of atmospheric concentration, although this is, anyhow, usually necessary. Another problem in determining the resuspension rate, also found in deter- mining the resuspension factor, is the requirement of steady state conditions, so that the short range move- ment of large particles, which are not fully resuspen- ded, can be neglected. 2.3. The choice between resuspension factor and re- suspension rate The adoption of either the resuspension factor or resuspension rate, as a means of expressing experi— mental results, suffers distinct disadvantages. The resuspension factor is strongly influenced by a geogra— phic variation of a deposit and non-representative results may be gained in many circumstances. How- 2642 ever, for a location where local resuspension domi- nates air concentration, the resuspension factor is a useful measurement of resuspension which can be determined easily. Horst (1976a) pointed out that downwind concentration cannot be predicted from the resuspension factor and, therefore, advocated the use of resuspension rate. Garland (1983) argued that K increases slowly with the scale of contamination and, thus, experimental values should be adequate for the planning of an emergency reSponse to an accident where concentrations in the immediate area of con- tamination are of concern, although it will not predict airborne concentrations downwind. A, generally, cannot be directly measured in field experiments and can only be deduced by fitting the results to a numerical model of airborne dispersion and deposition of resuspended material. A is suitable for use inkmodels to predict airborne concentrations throughout a region of varying contamination levels, where K might only be useful at indicating where air concentrations are a maximum or minimum. 2.4. Resuspension relating to soil concentration A third method of describing resuspension is appli- cable for widely distributed substances, well mixed with the surface soil. This is likely to be true of deposits which have had a period of, at least, some years to become distributed in the soil. For such tracers, results of measurements are often expressed in terms of the equivalent concentration of suspended soil. If the concentrations of a species in soil and air are Cs (mass per unit mass) and Ca (mass per unit volume), respect— ively, then the equivalent soil concentration, SE (mass per unit volume of air) is: S C“ (3) E — Cs- The concentration of a second tracer (denoted by ’), expected in air, can then be predicted as: C; = SEC; (4) For many species there are industrial sources of atmospheric material which cause interference. A major problem is that upwind sources affect atmos- pheric concentrations (Healy, 1977), as found in the determination of a resuspension factor. Hamilton (1970) investigated atmospheric U concentrations in southern England and noted the probability of trans- port from distant sources although he was unable to estimate their likely importance. This method has limited applications although it could be useful where long-term consequences of resuspension are to be predicted. It, at least, has the added advantage of avoiding the problem of choosing a depth of soil for tracer measurements. 3. SOIL EROSION Most previous reviews (cg. Linsley, 1978; Sehmel, 1980a, 1984) have included some reference to soil Review Article erosion. For most locations in temperate regions, the effects of soil erosion are usually restricted to a few areas. Usually, a flat terrain, a dry climate and a general absence of obstructions to airflow all assist in the removal of soil from ploughed fields. The studies of soil erosion are important when considering resuspen— sion from soil surfaces but the mechanisms and surface influences on resuspension could enable a better un- derstanding of resuspension. Some of the earliest work on soil transport was concerned with the movement of desert sand (Bag— nold, 1941). There are three recognized mechanisms of soil and sand movement. Surface creep occurs when particles roll along the ground and is associated with particles of the size 500—1000 nm (Newman et al., 1976). Much smaller particles, < ~ 100 um, are sus— pended by the wind and the very smallest of these (< ~ 10 [,tm) may be suspended for a considerable amount of time. It is these particles moving in suspen— sion, which are of the greatest interest to the nuclear industry because they can travel large distances and, if inhaled, they can be deposited in the respiratory tract (Anspaugh et al., 1975). Gillette at al. (1974) concluded that particles, with a settling velocity (V,) sufficiently smaller than the vertical velocities present in the turbulent boundary layer, would remain in suspen- sion. The friction velocity (u*), commonly measured in micrometeorology, is a convenient measure of the fluctuating vertical velocities and Gillette et al. found that the threshold lay in the region 0.12 < Vs/u* < 0.68 for suspension. As u* rarely ex- ceeds l m 5", particles with VS > ~ 0.5 ms'1 (~ 100 um) rarely become suspended and a value for V5 ~ 0.05 m s" 1 (~ 20 ,um) is probably a more com- mon limit. The third type of particle movement is saltation, which can be described as a skipping action. Particles in the size range 100—500 um may saltate, being ejected almost vertically into an airstream where they gain considerable horizontal momentum and fall back to the ground. Saltating particles may also rotate at high speeds (Sehmel, 1980a). It is saltating particles that have been found to be of great importance in the erosion process. Although the amount of soil moving by each of the three mechan- isms must be dependent on the nature of the soil, saltation usually accounts for the movement of the greatest mass (eg. Sehmel, 1984). This material, how- ever, rarely rises more than a fraction of 1 m above a surface and consequently there is no appreciable vertical flux in comparison to the horizontal flow (Anspaugh et (11., 1975). It is the action of the falling saltating particles that is of significance in erosion and resuspension. Firstly, the particle may strike a ground- borne particle which might then either saltate or roll along the surface. Secondly, on impact, the saltating particle might induce resuspension due to a ‘sandblast’ effect (e.g. Gillette, 1976). In addition, the particle might once again saltate possibly also causing either or both of these actions. Large particles have also been observed to shed pm or sub-um sized secondary Review Article particles on impact with an obstruction (Rosinski et al., 1976). Particles moving in saltation extract wind energy and apply it to disrupt the surface. As they move across a field, a cascade process is created as more and more particles are ejected into the air. Methods for absorbing this energy, for example short crops such as short stubble laid in strips, effectively minimize erosion (Chepil, 1945) and, presumably, resuspension. The concept of threshold velocity, used in wind erosion (e.g. Gillette, 1983a), describes a minimum wind speed below which erosion does not take place. The threshold velocity has been defined as either the wind speed to initiate erosion, termed the impact threshold velocity, or the wind speed to main— tain erosion, termed the fluid threshold velocity. The first of these is the greater and illustrates that the presence of saltating particles is important in maintaining erosion, since once erosion has started it may continue at lower wind speeds. A soil erosion equation has been developed (e.g. Skidmore, 1976) based on the nature of the surface and climatic conditions. Details of the equation have been widely published (e.g. Smith et (11., 1982) and are commented upon here. Parameters are included to allow for the effects of exposure, surface roughness and vegetational cover. It is, perhaps, not surprising that these affect the rate of erosion, but it is important to note that erosion will vary, even over small areas, and will be least in the lee of obstructions (Sehmel, 1984), because of the increasing abrasion effect of the moving soil further downwind. A parameter is also included to allow for the erodibility of the soil. A climatic factor allows for the strength of local winds and the presence of surface moisture. Surface moisture is known to reduce the erodibility of soil by binding soil particles together and forming aggregates (Chepil, 1943, 1956). In addition, a crust may form on drying which reduces the rate of erosion, although no allowance is made for this in the wind erosion equation, The possible importance of dust devils in the re- suspension of particulate material has been previously noted (Sinclair, 1976; Phelps and Anspaugh, 1977). These might be of most significance in desert climates, because of the large degrees of convection required to form the dust devil. They occur rarely in temperate regions, but are examples of episodic events which happen on all timescales and which are likely to be of importance in determining mean resuspension rates and atmospheric concentrations due to resuspension. The episodic nature of soil erosion and resuspension is due to a dependence on a high power of wind speed (e.g. Garland, 1983; Sehmel, 1983b; section 4.3), so that rare events of high wind speed and very dry surface conditions would be expected to result, briefly, in intervals of high rates of resuspension and soil move- ment. Wind storms and gusts during a storm might be expected to make large contributions to annual aver~ ages and it is important that any sampling programme or predictive model includes such episodes in a rep resentative manner. 2643 There are, presumably, close similarities between resuspension from exposed soil or wind erosion and resuspension from vegetated surfaces. However, much work remains to be carried out on the identification of these similarities. It must be noted that significant areas of the earth’s surface are covered by trees. Initial indications are that material that originated from the Chernobyl reactor accident could have remained in the canopies of coniferous forests for significant periods (Bjurman et LIL, 1987). Res'uspension from such areas has been believed to significantly elevate airborne concentrations, over relatively large areas. Thus, the relative importance of source areas may substantially differ according to surface type and a better generalization of erosion work is still needed if future work on erosion is to lead to a better under- standing of resuspension from vegetated surfaces. 4. PARAMETERS AND PROCESSES INFLUENCING RESUSPENSION BY WIND Numerous parameters and processes influence re- suspension. Many observations of these effects are non-systematic and allow only general indications of the significance of a factor. For a few factors, it is possible to make, at least, a first attempt at assessing their importance. These are discussed in the following sections. 4.1. Adhesion and removal of particles at the surface Contaminant particles may become attached to resident surface particulate material. If this is the case, then both the affinity of surface and contaminant particles, for one another, and the magnitude of forces required to resuspend particles from the surface are important factors. The first of these is poorly under- stood (e.g. Gillette, 1983b) and most useful informa- tion on the second has been gained from wind erosion studies. However, there have been some important laboratory studies that are also relevant to both factors. The main adhesive forces are van der Waal’s force, electrostatic force and the surface tension of absorbed liquid films (Hinds, 1986). Various laboratory techni- ques have been used to measure the adhesion of particles to surfaces. Usually the removal force is determined by a technique such as ultracentrifuging (e.g. Larsen, 1958), subjection to sonic vibration (Walker and Fish, 1967) or, perhaps appropriately for resuspension measurements, by blasting with com— pressed air. Jordan (1954) reported removal efficien- cies at different jet velocities and Corn and Stein (1965) compared removal efficiency with calculated air drag. The efficiency of particle removal was reported by Corn and Stein to increase with air velocity, as well as duration of exposure, which they explained was be- cause there was either a penetration of the laminar sublayer by turbulent eddies or there was a presence of wake generated turbulence due to particles in the 2644 laminar sublayer. Eddy penetration or “bursting” has been reported in studies of flow near smooth walls also (e.g. Wilmarth and Lu, 1972). Corn and Stein (1965) reported that adhesive force was found to increase with relative humidity (r.h.) and surface roughness but decreased with increasing particle size, The depen- dence of removal efficiency on particle size was partly explained by the smallest particles becoming enclosed in the viscid boundary layer. Corn (1961) used a microbalance to directly measure particle adhesion and he concluded that adhesive force increased with particle size and r.h. The apparent difference between his results and those of Corn and Stein (1965), on the effect of particle size, is probably explained by the differences in measurement technique and the argu— ment of Corn and Stein that small particles can be enclosed in the viscid boundary layer. Punjrath and Heldman (1972) studied particle en- trainment in a wind tunnel and found that it was dependent on two mechanisms. The first of these was the initiation of particle movement when shear stresses on the particle exceeded the frictional retarding force and this is in agreement with the concept of a thre— shold velocity. The second was the transfer of momentum from other moving particles, an observa- tiOn widely reported in studies on saltation and ero- sion mechanisms. Masironi and Fish (1967) observed particle movement on a smooth surface although, in this case, particles rolled very slowly and for short distances only. Mechanical agitation has been observed to induce resuspension (e.g. Garland, 1979) and might be ex- plained by momentum being imparted onto deposited material. The effect of rain, inducing resuspension, has been observed in a wind tunnel (Garland, 1979) and widely noted explanations have included the effects of both a mechanical agitation of the vegetation by the falling drop and a disturbance of the laminar sublayer (cg. Aylor and Parlange, 1975). Garland found that resuspension was only enhanced in his first appli- cation of artificial rain otherwise the effects were small and he added that, in his experiment, the rain did not appear to cleanse the grass under study. A Laboratory measurements of resuspension have been eSSential in elucidating the complex problems of resuspension but there is still much to be learnt. Resuspension from rigid smooth surfaces can give insight into the magnitudes of parameters required for resuspension and the nature of particle—surface inter- actions. However, resuspension from vegetation often involves movement, such as leaf-flutter, and the rela—, tive significance of this needs to be assessed. Thus; wind tunnels ought to be more commonly used in resuspension work, esPecially for studies relating to vegetation. 4.2. Time dependence of resuspension A decrease in the amount of resuspension occurring from asource has been observed with time. This so- Review Article called aging process has been considered by both modellers (e.g. Anspaugh et al., 1975) and experi- mentalists (e.g. Garland, 1979). Time-dependence is not explained by a reduction in the amount of material available for resuspension, but is due to deposited material becoming less erodible (Anspaugh et al., 1975). Most considerations have been for radioac- tively contaminated areas in arid regions, although Garland (1982) looked at grass in laboratory condit- ions. Garland observed a decrease in resuspension with time and commented that some of the deposited material became positioned towards the bottom of the grass canopy, which would be relatively sheltered from the wind, and'much more difficult to resuspend than the material on the top of the grass. Sehmel (1980a) summarized measurements of the airborne concentration half life, that is the time taken for an airborne concentration. due to a surface contami- nation, to halve and he noted a large range of values. In other measurements, Sehmel (1980b, 1983a) and some others have not observed a decrease in resuspension with time. Anspaugh et al. (1976) commented that radioactive surface contamination could still be a significant source of airborne radioactivity even after a couple of decades. Shinn et a1. (1983) found a very long half life for Pu. Many authors have described the time dependence or resuspension by exponential factors (e.g. Linsley, 1978), but Garland (1979) found that an inverse power law fitted his wind tunnel data for times varying from several minutes to a number of months. Previous theoretical studies have not addressed a time depen- dence, but Reeks et a1. (1985a,b,c) have described resuspension as a process whereby particles reside in potential wells formed by attractive (adhesive) and repulsive (elastic) forces. The particles are also subject to lift forces with mean and fluctuating components generated by turbulent flow over the surface. Re- suspension occurs when the particle acquires enough energy to escape from the potential well and deeper wells result in lower resuspension rates. Allowing for variations in the strength of adhesive force among the particles, resuspension rate is found to decrease ap- proximately inversely with time. , It seems evident that there are still uncertainties about the time dependence of resuspension and that these might be due to the large range of surface and environmental conditions, in field experiments and the complex interaction of particulate materials with sur— faces. 4.3. Wind speed An expected rapid increase in K and A with wind speed (u) has been noted by several authors (e.g. Sehmel, 1983a) and usually a power relationship has been fitted where: KorA ecu“. (5) However, the values of a have been found to vary for both K and A and Garland (1983) noted that he found Review Article a to decrease with time. Values of a have been found to be usually in the range 1—6 (e.g. Sehmel, 1984) and most estimates are 2 3. Unlike saltation and wind erosion, which are associated with a threshold velocity (Gillette, 1983), resuspension has been observed to occur at all wind speeds and resuspension effects have been observed in the clear absence of saltating par- ticles (Garland, 1983). The relationships between wind speed and resuspension rates or factors will probably be dependent on the type of surface and environ- mental conditions but, to date, measurements have indicated too large an amount of scatter to make any quantitative assessments rather more than specula- tive. 4.4. Particle size A variation of resuspension rate with particle size would be expected because aerodynamic lift force will increase with wind speed and particle area. The adhe- sive force would also increase with particle size via area of contact. The difficulty of resuspending small particles has been widely noted in laboratory studies over smooth surfaces (e.g. Corn and Stein, 1965) and vegetation (Garland, 1983). As noted earlier, a particle projecting from the viscid boundary layer would be likely to be more rapidly resuspended than a particle enclosed in stagnant or slow moving surface air. Particles that are transported by saltation (> 100 um) are sufficiently large that gravitation is a more effective restraint than adhesion. 5. FIELD STUDIES OF RESUSPENSION The most obvious feature of the field studies of resuspension is the wide range of reported results. Sehmel (1980a, 1984) summarized a large number of reported resuspension factors and found values in the range 10"“——10‘1°m’l for wind resuspension and this range was extended to 10"2—10"‘°m'1 if the effects of mechanical disturbances were also included. One of Sehmel’s comprehensive illustrations demon— strates the large range of reported resuspension factors and is shown in Fig. 1. Reported resUspension rates were also found by Sehmel to cover a large range (10'6—10' ‘3 s‘ 1). Obviously, resuspension processes are complicated and are likely to depend on a number of environmental and meteorological factors, but these factors must be identified and measurements must be explained if assessment and predictive models are to be validated. Many of the field studies of resuspension have been carried out in arid or semi-arid locations in the USA. (eg. Healy and Fuquay, 1959). Other studies outside North America have been conducted often in semi— arid locations also (e.g. Stewart, 1967). These studies, because they are usually associated with dusty areas, might be considered to be more relevant to wind erosion investigations. At best, the results can be representative of resuspension in temperate regions 2645 for an extremely limited range of conditions only. Many studies have been located at sites chosen be- cause of the presence of radioactive contamination that has arisen due to industrial leakage (Langer, 1983, 1986), atmospheric discharges (Carlson et al., 1983) and nuclear weapons tests (Anspaugh et al., 1976). In most cases of this kind, contamination of the surface is highly non-uniform and the consequent difficulty in the interpretation of measurements may explain part of the variability in results. Much of the work has been carried out to assess inhalation doses (e.g. Volchok et al., 1972) and Pu has often received the greatest attention (e.g. Sehmel and Lloyd, 1976a; Sehmel, 1983b). In general, most studies are in agreement that resuspension increases as some power of wind speed and this has also been indicated in field studies using inert tracers at similar locations (Sehmel, 1983a) and in wind tunnel studies over grass (Garland, 1983). However, there is some disagreement about whether the source ages and resuspension decrease with time. Of the information that is available on resuspension from sources other than those located in dry regions, from exposed soils and vegetation (Aylor, 1976; Orgill et al., 1976; Makhon’ko and Rabotnova, 1982; Shinn et al., 1983 and others), the variety of experimental designs makes any comparisons speculative. Sehmel and Lloyd (1976b,c,d) have measured resuspension using towers and a grid system which has the advan- tage that some information is found on the variations in concentration of resuspended material with height. The measurement of atmospheric concentration at a single height is of limited value because of interpret— ation difficulties. Slinn et al. (1979) performed some useful experiments over grass but, as they also noted, more experimental data are required. Some other research has considered the removal and translocation of particulate material from plant surfaces (Witherspoon and Taylor, 1969, 1970, 1971) although, mostly, large particles have been investi- gated and, generally, relatively high rates have been found for resuspension. Cataldo and Vaughan (1977) noted that sub-pm particles were very difficult to remove from leaf surfaces. 6. RESUSPENSION FROM WATER SURFACES The formation of sea salt aerosol has received recent interest, notably because it has been recognized as an important mechanism for the sea to land transfer of other materials found in sea-water. Cambray and Eakins (1980) noted that they found elevated Pu concentrations near the cost of west Cumbria in the UK. which they suggested could possibly originate from sea spray. Eakins et al. (1981) confirmed that the Irish Sea was a source of the airborne Pu and Patten— den et al. (1980) confirmed that this had resulted from discharges to sea from the Sellafield reprocessing plant. Eakins et a1. (1982) noted that actinides are enriched in sea spray and that they are associated with 2646 suspended sediments. They concluded that bursting bubbles were the sources of the sea spray. Blanchard (1983) noted that the action of tearing droplets from the crests of waves was a probable mechanism for aerosol generation but, under conditions usually found at sea, the bursting of bubbles was probably the predominant particle production mechanism. Bubble bursting produces droplets by two mechanisms (Blan- chard, 1983). There are the film drops produced by the breaking of the liquid skin of the bubble on its arrival at the air—water interface and the jet drops which are simultaneously splashed upwards from the bottom of the bubble. Enrichment within the spray has been explained by the existence of a microlayer (Walker et al., 1986), but it is not certain which of the droplet production mechanisms of bubble bursting are most important in the marine resuspension of plutonium (McKay, personal communication, 1986). Pattenden er al. (1981) also found enrichment of some trace metals in aerosols collected over the North Sea. Similarly, Woollam (1984) reported the significant role of the microlayer in the radioactive enrichment of particulate material originating from the cooling ponds of nuclear power stations. 7. RESUSPENSION BY MECHANISMS OTHER THAN WIND 7.1. Resuspension by traffic The passage of a vehicle along a road can cause surface material to be resuspended either by the shearing stress of the tyres or by induced turbulence (Sehmel, 1973, 19760). An additional mechanism which could be important, in wet conditions, is the ejection, by the tyre, of spray droplets. Branson (personal communication, 1986) found highest Na“r and Cl‘ concentrations in air filter samples taken during the winter months in an urban environment and these could have been due to the use of impure NaCl (rock salt) as a de-icing agent on roads and resuspension by tyre spray. Smith (1970) reported salt damage to white pine trees along the side of a road and acknowledged the importance of salt resuspen- sion by traffic-generated spray, although he added that tyre stress and traffic generated turbulence should be of great importance in dry conditions. It seems probable that the presence of moisture on roads, after the application of salt, will be important in dictating the mechanisms for salt resuspension and the size and nature of the resuspended salt particles, even if the roadway dries before resuspension by the traffic oc- curs. Road salt may provide a suitable analogue for spills of soluble materials on roads. There has been some interest in the resuspension of surface material from unpaved roads (Dyck and Stukel, 1976). Usually, an air concentration of dust, often suitably labelled, is measured during and after the passage of a vehicle. Hodgin (1983) measured the resuspension of dust at Rocky Flats, which contained traces of Pu due to an industrial spill, although his study was aimed at identifying the influence of traffic Review Article on local atmospheric concentrations of Pu, rather than conducting an assessment of resuspension mech- anisms. Hereim and Ritchie (1976) investigated the resuspension of bacteria from a desert soil due to the passage of a truck, although a resuspension rate or resuspension factor was not quantified. Stewart (1967) reported resuspension factors for a vehicle driving along a graded soil road at two weapons test sites and Dyck and Stukel (1976) determined dust emissions from an unpaved road. These studies have yielded some useful information about resuspension in certain conditions but their relevance is very restricted for temperate regions. In most developed countries un- paved roads would be likely to act as significant dust sources during dry spells only. Sehmel (1973, 1976c) measured the resuspension rate of ZnS tracer particles due to traffic travelling along an asphalt road. Downwind atmospheric con- centrations of tracer were measured according to a grid system and the amount of surface depletion was determined to assess the resuspension rate. A vehicle was driven down the contaminated lane to give a resuspended fraction of 10“ 5«10‘ 2 per vehicle pass on the same day that the tracer was deposited. Sehmel also commented that the resuspension rates decreased with time and were two or three orders of magnitude less after 30 days. The resuspension rate was found to be higher if the vehicle was driven through the tracer rather than on the adjacent lane, which Sehmel ex- plained by differences in the patterns of turbulence, although the possibility of tyre stress, as an additional resuspension mechanism, was noted if the vehicle was driven through the tracer. Sehmel investigated the effects of different vehicles on resuspension, using a car and a truck, and concluded that the larger vehicle caused more resuspension. Sehmel (1976b) investi~ gated the resuspension effects of driving a truck over cheat grass using an identical method to that used by Sehmel (1973) for measurements made on an asphalt road, and concluded that resuspension was one to two orders of magnitude lower in the cheat grass because the vegetation provided cover for the tracer particles located at the base of the grass. Sehmel‘s results, represent the only comprehensive data set for traffic resuspension to date. His carefully conducted experiments allow surface concentrations to be estimated from atmospheric concentration and deposition measurements. The particles which he used were < 25 pm with a mass median diameter of 8 ,um. It would be expected that contaminant material if deposited onto a surface, may become associated with larger host particles. Sehmel’s results are pertinent, therefore, but an indication of resuspension rate with size of tracer could provide some useful information. Rahn and Harrison (1976) and Harrison et a1. (1976) examined street dust from Chicago, both noting the likely importance of resuspension by traffic, although neither quantified their results to imply resuspension rates or factors. Rahn and Harrison (1976) also repo— rted some size ranges of suspended particulates. Review Article The shortage of reported work on the resuspension of material from roadways, especially in cities, has been previously mentioned (e.g. Engelmann, 1976; Jensen, 1984). It is evident that additional research is required in this area and that efforts are needed to quantify urban resuspension. To date, most work appears to have been designed primarily for con» venience of measurement. Sehmel’s work on traffic resuspension is comprehensive but this needs to be endorsed. 7.2. Resuspension by other mechanisms Sehmel (1986) measured resuspension due to pedes- trian activity finding that the fraction resuspended each pass was between 1x 10‘5 and 7x10“. His experimental technique was, presumably, identical to that used to measure resuspension by vehicles (Seh- mel, 1976c) and the surface was an asphalt road. Sehmel added that wind resuspension during the experiments was low being typically 5 x 10'9—6 x 10‘[3 s‘ 1. Schwendiman (1958) conducted some early work and investigated the probability of con- tamination by various activities including walking. Schwendiman also looked at the effects of resuspen— sion when vehicles, having been driven through a contaminated plot, were subject to various human operations. His results were directed towards assessing the probability of particle inhalation or skin contamination and the inference of resuspension rates or factors from his results must be largely speculative. Hereim and Ritchie (1976) assessed the resuspension of bacteria by men walking or crawling in a desert environment although it is not possible to carry out a quantitative interpretation of their results. Agricultural operations, including ploughing, have been found to cause resuspension. Milham et al. (1976) examined resuspension factors for Pu, due to wheat planting, in an agricultural field adjacent to a fuel reprocessing plant. Schwendiman (1958) considered the probability of contamination whilst digging al- though he reported low values. Some work has also been carried out on resuspension indoors (e.g. Brun~ skill, 1967; Glauberman et al., 1967) considering acti- vities such as walking (Jones and Pond, 1967) and sweeping (Fish et al., 1967). Sehmel (1980a) summar~ ized most major resuspension studies on mechanical resuspension both indoors and outdoors (see Fig. 1) and the large range of resuspension factors that he cites (10“ 1°40" m“) demonstrates the wide range of experimental conditions and the need for carefully documented experiments. 8. SUMMARY A wide variety of resuspension experiments has been reported. However, experimental design has of~ ten been based on convenience resulting in many measurements being taken under a limited range of conditions. Radioactive substances are comparatively easy to trace in the environment and, consequently, 2647 many resuspension measurements have been carried out in contaminated areas. Usually, these areas have been arid or semi-arid and the extrapolation of the results to more temperate locations is subject to a degree of speculation. Work carried out on wind erosion is often mostly restricted to arid regions also and does not lead directly to a quantification of resuspension. ' Some useful measurements have been made on mechanical resuspension but, because of a sparsity in the number of experiments, more work is required if past results are to be confirmed. The number of laboratory experiments on resuspension is also small and this must provide an area which requires further study, if the mechanisms of resuspension are to be better understood. In future, research needs to be desrgned in order that the results can be readily extrapolated to outdoor conditions. Subsequent to deposition, contaminant material may agglomerate, become associated with host par- ticles, or remain as separate particles. The nature of the inter-particle reaction is poorly understood and is likely to depend on the size, shape and chemical nature of the particles. Wherever possible, the size distribu- tions of resusp‘ended material should be given. The mechanisms of resuspension also require grea- ter study. The existence of a continuous laminar boundary layer, perhaps, presents a too simplistic model for resuspension. Eddy penetration has been observed by many authors and must play an import- ant role in resuspension. The effects of vegetation movement might also be likely to induce resuspension because of friction between leaves and a change in the structure of turbulence around the vegetation. The range of resuspension rates or factors which have been reported is very large. The effects of mech- anical disturbances increase this range even further. Because of the variety of experimental techniques and the diversity of environmental conditions, it is not possible to quantify, with any degree of certainty, the influences of environmental parameters on resuspen- sion. It is evident that the large spread of results requires a form of explanation if values are going to be predicted for localities and conditions other than those in which a particular experiment is taking place. Generally, there is a requirement for more work in the resuspension field. The experiments should be designed with the specific need to measure resuspen- sion rates or factors, identify deposition mechanisms and to endorse the few comprehensive resuspension studies that have been already undertaken. More emphasis should be placed on experiments which are representative of a wide range of conditions rather than relying on the convenience of radioactive con- taminant sources in dry and dusty areas. 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Nicholson 1988 - Bibliotheek T» D W Uitsluitend VOOI‘...

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