Scanning Electron Microscope Study of Iron-Containing Particles on Foxtail Cliff 1. Davidson" and Liyang Chut Departments of Civil Engineering and Engineering and Public Policy, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213 ~
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Samples of Setaria viridis (wild foxtail) were collected from the side of a heavily traveled road near a steel manufacturing region in Pittsburgh. SEM-EDS techniques were used to locate and size 127 iron-containing particles deposited on awns near the tops of the plants. The particles were nearly perfect spheres of somewhat uniform diameter: 3.7 f 2.2 pm (mean f standard deviation). Application of deposition theories from the literature suggests that inertial impaction in low Reynolds number flow is responsible for transport of these particles onto the vegetation.
gradually increased until the single particle filled the entire field of view. A bulk scan was then conducted to determine the presence of other elements in the particle. This procedure was also used to look for lead-containing particles. I t was found that no such particles large enough to be discernible above background scatter (roughly 0.5 pm) were deposited on the awns. While this does not discount the existence of large lead-containing particles, it demonstrates that supermicron particles containing iron are more abundant in this region. Experimental Results
Introduction Evidence of relatively high trace-metal levels on vegetation, soil, and artificial collectors near roads and stationary sources has been well documented. However, only a limited number of studies have attempted single-particle analysis to obtain size and chemical composition data on deposited particles. For example, Heichel and Hankin ( I )used an electron probe microanalyzer to detect lead, chlorine, and bromine-containing particles on tree bark near a heavily traveled road. The reported mean diameter of the deposited particles containing lead was 6.7 pm. In another microprobe study, Davidson et al. ( 2 ) reported lead-containing particles as large as 15 pm deposited on flat carbon disks exposed to the free atmosphere in the Los Angeles urban area. The present study involves detection of iron-containing particles deposited on natural vegetation. The data are used to determine the physical deposition mechanisms applicable to atmosphere-surface transport of these particles. Experimental Procedure During fall and winter 1977 and spring 1978, samples of Setaria uiridis were collected a few meters from the curb of Boulevard of the Allies, a main thoroughfare in Pittsburgh, in central Schenley Park. This site is several hundred meters in the predominant downwind direction from the Monongahela River Valley, where several steel manufacturing plants are located. I t was expected that this site would be heavily influenced by a number of anthropogenic sources, so that a significant number of particles could be easily detected. Only the top few centimeters of the plants were cut. The specimens were placed in cIeaned beakers and transported back to the laboratory, where each stalk was removed and awns (fine hairlike fibers) a t the top were carefully cut off. The awns were placed on cleaned carbon-coated aluminum studs (10-mm diameter). After drying in an oven for a few hours, the samples were placed in a vacuum chamber where a thin film of carbon was deposited over the fibers. The completed samples were subjected to SEM-EDS analysis using a JEOL JSM-35U scanning electron microscope and a Kevex 5500 energy dispersive X-ray spectrometer system. Iron-containing particles were located on the fibers by performing X-ray mappings a t l@OOxmagnification. When the presence of iron was indicated, the magnification was
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Table I shows the arithmetic mean, the standard deviation, and the standard error of the mean for various parameters associated with 127 iron-containing particles deposited on 31 separate awns. The parameters in this table are defined as follows: d, = geometric particle diameter (pm);df = awn diameter (pm); Re, = particle Reynolds number = ud,/v, where u = wind speed (cm/s) and u = kinematic viscosity of air (cm2/s);Ref = awn Reynolds number = udf/u; St = Stokes number = Cup,dp2/9pdf, where pp = particle density (g/cm3), p = viscosity of air g/(cm s), C = slip correction factor = 1 (2X/dP)(1.257 0.4 exp(-0.55dPlX)) and X = mean free path of air molecules; R = interception parameter = d,/df. Definitions of these parameters have been taken from Friedlander (3). A value of u = 140 cm/s has been used for these calculations. This is the wind speed a t a 50-cm height (corresponding to the top of the vegetative canopy) based on the Pittsburgh annual average wind speed of 420 cm/s at 10-m elevation ( 4 ) .A zero plane displacement of 25 cm and a roughness height of 4 cm have been assumed, after Davidson and Friedlander ( 5 ) . Values of p,, p , v, and X are taken to be 7.8 g/cmg, 1.8 X g/(cm s), 0.15 cm2/s, and 0.065 pm, respectively. The value of pp assumes a solid iron particle which is probably an rlpper limit. Most of the iron-containing particles were spherical, with dot mappings indicating iron as the primary constituent. Bulk scans showed the particles to be similar to elemental composition, containing Si, s, C1, K, and Ca in addition to Fe. Detectable but lower levels of Sc, Ti, Mn, Cu, Zn, and Br were also present in many of these particles. No attempt was made
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Table 1. Values of Parameters Associated with 127 Iron-Containing Particles Deposited on 31 Awns of Setaria viridis parameter
dp,
Pm
df. F m
Re,
Rei St R
arithmetic mean
std devlalion
std error
3.7 57 0.35
2.2 16 0.20
0.19 2.9 0.018
5.3 24 0.070
1.5 33 0.043
0.27 2.9 0.0038
Present address: GCA Technology Division, Bedford, MA 01730.
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0013-936X/81/0915-0198$01,00/0 @ 1981 American Chemical Society
to determine possible sources of the particles, hut the similarities in shape and chemical composition suggest a single source or source type. Photographs showing deposited particles and an associated iron mapping for a 65-pm-diameter fiber are shown in Figures 1 and 2. Airborne size distributions of Fe and P b measured with inertial impactors demonstrate that the former element is associated with larger particle sizes. Figure 3 shows ambient distributions for both elements, obtained during summer 1979 as part of a study characterizing several trace elements in Pittsburgh (6). Sampling was conducted near steelmaking facilities -1 km upwind of the foxtail collection site. An Andersen low-volume impactor was used; details of the sampling and analysis procedures are given elsewhere (7). Figure 4 presents a size distribution of Fe collected later in the summer, -2 km from the collection site (8). A parallel stage impactor was used for this measurement (9).
significant for supermicrou particles. The importance of these mechanisms can he determined by applying appropriate models with mean values of the parameters in Table I. When one uses equations for sedimentation (121,a particle with d, = 3.7 pm and pp = 7.8 g/cm3 has an equivalent aerodynamic diameter of 10 p n and a settling velocity of 0.3 cmls. This
Determination of Transport Mechanisms Since nearly all of the iron-containingparticles detected had d, > 1pm, it is likely that transport by convective diffusion was negligible. Similarly, the role of interception was probably minimal as shown by examining the small mean value of R in Table I and the data of Wong e t al. (10).Detailed calculations showing that these mechanisms were unimportant are given by Chu (11). Sedimentation and wind eddy transport above the canopy, and inertial impaction within the canopy, are likely to he
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Figure 1. Scanning electron microscope photograph 01 particles deposited on a 65-pm diameter awn of Setarla vrrldis. Two of these particles contained appreciable amounts of iron. Although some of the awns had two or more iron-containing particles in a single field of view. many awns contained none. No attempt was made to determine the total number of deposited particles per unit area of surface.
Flgure 3. Distributions of Fe and Pb airborne mass concentration with respect to particle size. AC represents the concenbation in each size range, G is the total concentration in all size ranges, and A log dp represents magnitude of each size interval based on logarithms of the particle diameters. dp refers to the aerodynamic diameter in this case. The graphs show the average and standard deviation of two runs near Pittsburgh steelmaking facilities, conducted during July 17-22 and August 31-September 5, 1979.An Andersen low-volume sampler using adhesivecoated substrates was positioned horizontally into the wind to generate these distributions (6, 7).Mass mean aerodynamic diameters are 8 and 0.6 pm, corresponding to average airborne concentrations of 5000 and 600 ng/m3, respectively.
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Figure 2. EDS 001 n n p p m ~tor ron rnon rnq in" J-,r>~-oismeterpartic es, corresponfl ng IO !ne aan n k g ~ r e1 h e d , do c I tne ironcontaining pan c e=,nnecled in !his sluo) were ctwaclei I C O by a high dens ry 01 (101s over oacqrauno smner. ihowing tnai iron was a primary constituenl 01 Ihe panicles
Flgure 4. Sire distribution of Fe obtained in a single run in the central Oakland district of Pittsburgh. on September 12, 1979. A parallel stage impactor employing adhesive-coated surfaces was used (8. 9). The mass mean aerodynamic diameter is 8 p m with an airborne concentration of 15 pg/m3 Volume 15. Number 2. February 1981 199
represents a lower limit to the rate at which particles enter the canopy, since wind eddies will increase the transport rate over an$ above that caused by settling. The importance of inertial impaction can be seen by comparing the Table I data with Figure 5 . This plot shows the collection efficiency 77 of an individual cylindrical fiber in transverse airflow as a function of St. The efficiency may be defined as particles per second depositing on cylinder particles per second passing through area d f L where L is the length of the cylinder. Shown in Figure 5 are data from several independent studies (10,13-15). No literature values are available for Ref in the vicinity of 5.3, needed for exact comparison with the Table I information. However, 13 vs. S t data for 0.038 < Ref < 1.39,13.7 < Ref < 32.8, and Ref = 10 bracket the desired value. These curves suggest that impaction at St = 24 results in collection efficiencies near unity, showing the importance of this mechanism for the iron-containing particles. Even considering the wide range of S t indicated by the large standard deviation, the efficiencies of nearly all of the particles detected in the present study are fairly large. The combined effects of sedimentation, wind eddy transport, and in-canopy filtration result in an overall deposition velocity which may be estimated. Applying the model of Sehmel and Hodgson (16, 17) to these particles and surface and wind conditions suggests that the deposition velocity exceeds settling by a factor of 4. Thus sedimentation is probably not of great importance in transporting the particles from the atmosphere into the canopy. It is also likely that sedimentation is unimportant for deposition onto the awns because of their approximate vertical orientation. It is also possible to estimate the importance of the awns as a deposition surface relative to the importance of leaves and stems of foxtail. Miller (18) has used the filtration model concept ( 5 ) to calculate deposition velocity as a function of particle size for various parts of the foxtail plant. The results show that the awns collect more airborne material than the rest of the plant for the size range 1-20-pm aerodynamic di17=
ameter. This is true despite the small total surface area of the awns in the canopy compared with stems and leaves. The latter elements are characterized by df 0.25 cm, resulting in much smaller Stokes numbers and impaction efficiencies. According to these calculations, a particle with aerodynamic diameter of 10 pm has a deposition velocity of 3 cm/s to the canopy, based on awns alone. The total deposition velocity to leaves and stems is 1 cm/s. This last value is close to predictions of Sehmel and Hodgson (17) mentioned above. The presence of small fibers such as foxtail awns may thus significantly increase the overall dry deposition onto a vegetative canopy, with possibly a much smaller effect on the windfield. A similar conclusion was reached by Wells and Chamberlain (19)concerning microscale roughness elements on filter paper. Interpretations based on comparisons of field data with laboratory results, such as discussed here, must be viewed cautiously. Davidson and Friedlander ( 5 ) have shown that merely using a time-averaged wind speed to predict particle impaction is acceptable for periods of a few days, but undoubtedly more error is introduced when using an annual average. Furthermore, the likelihood of particle bounceoff from the awns cannot be overruled; Chamberlain (20) and Chamberlain and Chadwick (21) have shown that bounceoff may be significant under dry surface conditions. These and other factors require that the results of this study be viewed qualitatively.
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Conclusions
Iron-containing particles deposited on awns of Setaria viridis near an industrial area of Pittsburgh are spherical, and of roughly uniform size and elemental composition. Wind eddies are responsible for transporting these particles from the atmosphere into the vegetative canopy, while inertial impaction is responsible for deposition onto the awns. Attempts to locate lead-containing particles on the vegetation were unsuccessful. Ambient size distribution measurements showed that lead is found in predominantly smaller particles than iron, making detection by SEM techniques more difficult. In addition, lower airborne concentrations of lead relative to iron suggest that a greater effort would be needed to detect deposited lead. Acknowledgment
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The assistance of David Boufford of the Carnegie Institute, Botany Section, is greatly appreciated.
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Literature Cited
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(1) Heichel, G. H.; Hankin, L. Enuiron. Sci. Technol. 1972, 6,
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(2) Davidson, C. I.; Hering, S. V.; Friedlander,S. K. “The Deposition
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of Pb-Containing Particles from the Los Angeles Atmosphere”; Int. Conf.Enuiron. Sens. Assess., [Proc.],1975 1976, paper 6-3. (3) Friedlander, S. K. “Smoke, Dust, and Haze, Fundamentals of Aerosol Behavior”; Wiley: New York, 1977;pp 88-121.
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Figure 5. Plots of the collection efficiency of cylindrical fibers vs. Stokes number for three investigations. Closed circles represent the experimental data points of Wong et al. ( 73) for sulfuric acid aerosols depositing on 3.51-pmdiameter glass fibers. These points cover the range 0.43 < dp < 1.30 pm; 0.038
