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Investigations of the deposition of lead-bearing aerosols on the surfaces of vegetation
The Science of the Total Enviroment, 14 (1980) 265--278 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
265
INVESTIGATIONS OF THE DEPOSITION OF LEAD-BEARING AEROSOLS ON THE SURFACES OF VEGETATION
R O B E R T W. ELIAS* and JUDITH C R O X D A L E * *
Biology Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (U.S.A) (Received August 13th, 1979; accepted October 1st, 1979)
ABSTRACT SEM and X-ray microprobe analysis were combined with clean lab technology atomic absorption spectroscopy to observe the deposition of aerosols on undisturbed pine needle surfaces, to obtain physical and chemical information from the observed particles and to chemically remove these particles for analysis. Comparisons were made among sites with traffic densities of 100, 1000, 10000 cars per day. Correlations were made with periods of heavy rainfall. The results show that the particle size distribution, shape and surface chemistry are possibly characteristic of automotive exhausts, that both the chemical washing procedure and rainfall efficiently remove most particles from the surface, including those bearing lead, and that the rate of deposition is constant between rainfalls and partly a function of traffic density. Deposition rates of 0.3 ng P b c m -2 day -1 were observed at 100 cars per day, 2--3 ng Pb cm -2 day -1 for 1000 and 10000 cars per day.
INTRODUCTION
The impact of atmospheric inputs of heavy metals into an ecosystem and their subsequent cycling through the system is of critical importance to biogeochemical investigations of terrestrial ecosystems. This is particularly true of lead, which is a major metallic component of automotive exhaust. One primary mode of lead inputs into an ecosystem is the dry deposition of aerosol particles on vegetation surfaces (Elias et al., 1978). The value of direct measurements of deposits on vegetation would be that long term (1--6 years) deposition rates for any geographical area could be estimated by measuring the accumulation on vegetation surfaces such as pine needles or tree bark. Estimates of inputs are currently based on artificial surfaces exposed for brief periods of time (1--20 days). Studies of the methods for measuring surface deposits on vegetation have raised the question of whether these surfaces, which are subjected to seasonal climatic variation, would yield accurate measurements of the deposition rates. * Member, Society for Environmental Geochemistry and Health. ** Present address: Department of Botany, University of Wisconsin, Madison, WI 53706, U.S.A.
266
A major difficulty with the use of natural surfaces is the effectiveness of the m e t h o d of particle removal for lead determination. Based strictly on chemical evidence, the procedure developed by Elias et al. (1976) appeared to remove all of the lead on pine needles and sedge leaves. In that study, no visual evidence of cleaning efficiency was obtained, and there was some uncertainty as to whether the procedure merely leached lead from the internal tissues of the needles or removed the particles themselves. Utilizing pine needles from sites with known lead pollution, the objectives of the present study were: (1) by SEM and X-ray microprobe analysis, to determine the presence and elemental nature of particulate matter on needle surfaces and the efficiency of the chemical cleaning procedure for removing these particles; (2) by SEM and atomic absorption analysis, determine if the lead-bearing aerosol particles are removed during a rainstorm; and (3) by atomic absorption analysis, to verify that the deposition rate of lead aerosols is constant between rainfalls.
MATERIALS AND METHODS Needles of Virginia pine (Pinus virginiana Mill.) were collected from roadside trees at sites in the vicinity of Blacksburg, Virginia, having traffic densities of 100, 1000, 10000 cars per day (Sites 1, 2 and 3, respectively). These sites were within 10 km of a major interstate route (50--70000 cars per day) but more than 50 km from major industrial sources of air pollution. Except where indicated, only needles which had emerged during the spring of 1977 were collected. Collections were made during March--May, 1978. Thus the needles had an estimated growth period of 300--360 days. To avoid contamination of the samples, needles were collected using polyethylene gloves and acid-cleaned stainless steel forceps and transported in wrapped and sealed acid-cleaned polyethylene containers. Further details of the collection procedure are described in Elias et al. (1976). Generally, triplicate samples from the designated trees were analyzed, except where the entire sample was required for a single, low-lead analysis.
Washing procedure and chemical analysis The sample containers were reopened in a Class 10000 clean room (Zief and Mitchell, 1976). Needles to be washed were grasped at the base with FEP (fluorinated ethylene propylene) teflon forceps and manually swirled, successively in methanol, distilled water, 1N HC1 and again in the same distilled water (10 seconds in each solution). The solutions were combined and are hereafter referred to as the surface washings. This solution was evaporated to dryness, the residue dissolved in double distilled HNO3, evaporated and dissolved with 0.1N HNO3, then diluted a working volume of a b o u t 4 ml. The length and width of the washed needles were measured, and the samples were then placed on a stub for SEM analysis or into a clean FEP teflon beaker for chemical analysis. For chemical analysis of the washed and unwashed
267 needles, each sample was dissolved in double distilled HNO3 and evaporated to dryness; the residue was dissolved in 0.1N HNO3 and diluted to a working volume of about 4 ml. Samples were analyzed for lead using an atomic absorption spectrophotometer (Perkin Elmer 460) equipped with a graphite furnace and a deuterium arc background corrector. The sensitivity of this technique was 0.3 ng sample -1 or 0.06 ng m1-2 , with an average blank value of 25 ng Pb/sample and 200 ng Pb per washing solution; sample values ranged from 200 to 1000 ng Pb sample -1 . The m e t h o d of standard additions was used for several samples. An NBS standard reference material was also used (NBS SRM No. 1575, pine needles) and agreement was within 5%.
SEM preparation SEM stubs were cleaned with acetone and covered with double sticky tape which had not been exposed to the air. Unfixed washed and unwashed needle tips, 1 cm in length, were placed four to a stub, two with the fiat (adaxial) side up, two with the convex (abaxial) side up. Silver paint was used on the ends of each needle segment to establish electrical contact with the stub. The needles were observed unfixed and uncoated. Observation and analysis was done using an AMR 900 scanning electron microscope operating at 20 kV, 100 #amp with an EDAX 707A X-ray microprobe unit. For elemental analysis of selected leaf surfaces and individual surface particles, the size of the areas scanned and the scan time were approximately constant. Under these conditions, the presence or absence of certain elements can be detected. Absolute concentrations are n o t available under this EDAX system, but relative concentration, based on peak height, of one element compared to another can be determined.
Particle size classes The number and size classes (maximum linear diameter) of surface particles were determined from enlarged SEM photograhs with a microcaliper and template. For each needle, all observable particles in each size class were visually counted in a r a n d o m l y selected area equivalent to 4 m m 2 of actual needle surface. Particles smaller than 0.5 microns were not measured.
RESULTS
SEM and X-ray microprobe analysis Surfaces of unwashed pine needles show that some contain microflora, and that there is an increase in the number and size of particles associated with sites of increased traffic density (Figs. 1--6). At the lowest traffic density site (100 vehicles day -1 ), micro-organisms on the needle surface are relatively c o m m o n (Figs. 1 and 2). In this material, mycelial growth of fungi is predominant, some fungal hyphae even invade the stomatal cavity (Fig. 2). Even though the age of the needles at all three sites is the same, needles from sites 2 and 3 (1000 and 10000 vehicles day -1 ) rarely have microflora on their
268
Fig. 1 (left). Unwashed surface of pine needle collected at site 1 (100 vehicles per day) showing both particulate and organic matter, x 41. Fig. 2 (right). Enlarged surface view of unwashed needle from site 1 with numerous colonies of fungi;note that some hyphae have penetrated stomata and that particulate matter is sparse. X .164.
Fig. 3 (left). Unwashed surface of pine needle collected at site 2 (1000 vehicles per day) showing a large increase in particulate matter. × 41. Fig. 4 (right). Enlarged surface view of unwashed needle from site 2 showing the range of particle size classes and shapes. X 164.
269
Fig. 5 (left). Unwashed surface of pine needle collected from site 3 (10000 vehicles per day) showing extensive accumulation of particulate matter. × 41. Fig. 6 (right). Enlarged surface of unwashed needle from site 3 with many large, irregularly shaped particles in addition to numerous small particles, x 164.
surfaces, although the a m o u n t of particulate matter has increased (Figs. 3--6). X-ray microprobe analysis of needle surfaces (area scanned, 4 m m 2 ), shows these areas contain conspicuous peaks corresponding to A1, Si, Cl, K and Ca (Figs. 7--9). Although numerous individual particles, both large and small, were also scanned (Figs. 10--12), none contained lead at or above the detection limits of the instrument. Note that the relative amounts of A1 and Si in particles from the three sites decreases with increasing traffic density, compared to C1, K and Ca. In the case of K and Ca, increasing amounts of these O3
s,
cn
~
7
Si
8
Si
9
W cr
ENERGY LEVELS
Fig. 7 (left). EDAX displays of needle surface scans showing the relative concentrations of the elements indicated by the chemical symbols. Fig. 8 (centre). EDAX displays of needle surface scans showing the relative concentrations of the elements indicated by the chemical symbols. Fig. 9 (right). EDAX displays of needle surface scans showing the relative concentrations of the elements indicated by the chemical symbols.
270
CO ,1,," W ,"n ~o':,
$i
Ca
S* AI
Ca
AI
Si
CI
KCa U L,IJ rr"
ENERGY LEVELS
Fig. 10 (left). EDAX displays of individual particle scans showing the relative concentrations of the elements indicated by the chemical symbols. Fig. 11 (centre). EDAX displays of individual particle scans showing the relative concentrations of the elements indicated by the chemical symbols. Fig. 12 (right). EDAX displays of individual particle scans showing the relative concentrations of the elements indicated by the chemical symbols.
elements in particles are associated with increasing traffic density. The shape of the particles tend to be angular on site 1 needles and more globular to spherical at sites 2 and 3. Washed needles from all three sites (Figs. 13--18) have clean and intact surfaces. The majority of the microflora on needles from site 1 have been removed (Figs. 13 and 14) and the number of particles at all sites has been significantly reduced (Figs. 15--18). Needles from sites 2 and 3 which were exposed to heavy rainfall (3.2 cm
4
Fig. 13 (left). Surface of chemically washed needle collected at site i showing an absence of mieroflora and a decrease in the number of particles. × 41. Fig. 14 (right). Small amount of particulate matter is present after chemical cleaning on this site I needle. × 164.
271
Fig. 15 (left). Surface of chemically washed needle from site 2 showing that the leaf is intact and there is no apparent structural damage resulting from the cleaning procedure. * x 41. Fig. 16 (right). Enlarged surface view of chemically washed site 2 needle showing nearly complete removal of particles. X 164.
Fig. 17 (left). Surface of chemically washed site 3 needle. × 41. Fig. 18 (right). Enlarged surface view of chemically washed site 3 needle. Note the effectiveness of particle removal by comparing with Fig. 6. × 164.
272
O Fig. 19 (left). Surface view of rain washed site 2 needle. Compare with Fig. 3, 4, 15, 16 for the effectiveness of this natural means of particle removal. × 82. Fig. 20 (right). Surface view of rain washed site 3 needle. Note nearly complete absence of particles (cf. Figs. 5, 6, 17, 18). × 82. in 24 h) also have relatively clean surfaces (Fig. 19, 20). Only the smallest particles (less than 0.5/~m) are present in any number on leaves from these two high traffic density sites. Rain-washed needles from site 1 were not available for analysis.
Chemical analyses Table 1 contains data for the total measured lead of needles collected during a rainstorm (control), and one day, six days and 36 days following a rainstorm (3.2 cm in 24 h). No surface lead could be detected on needles collected during rainfall, thus their values are due entirely to internal lead. Following rain, needles show a systematic increase in the a m o u n t of total lead with time, but with no significant difference in values for any given exposure time between sites 2 and 3. Rain-washed needles from site 1 were not available for comparison. A comparison of the surface and internal lead values of chemically washed and rain-washed needles show close agreement of data. The a m o u n t of surface lead increases markedly at sites with high traffic density; surface lead on site 2 and site 3 needles after 36 days exposure is approximately four times and seven times greater than on needles at site 1. These increases can be seen in measurements from actual surface washings (chemically removed) or in recalculated data of total lead determinations (Table 2). In this latter case, surface lead was calculated as the difference between the control (rain washed) and exposed needles of Table 1, expressed per unit area. The close agreement of the surface lead values based on actual and calculated measure-
273
TABLE 1 I N C R E A S E S IN T H E T O T A L L E A D C O N T E N T OF N E E D L E S A S A F U N C T I O N O F TIME. E X P O S U R E IS IN D A Y S SINCE T H E L A S T R A I N F A L L , V A L U E S A R E IN gg Pb P E R g D R Y WT. V A R I A T I O N IS O N E S T A N D A R D D E V I A T I O N , R E P L I C A T E S A R E I N D I C A T E D IN PARENTHESES. Exposure
Site 1
Site 2
Site 3
Control a 1 day 6 days 36 d a y s
---4.4 + 0.6(3)
3.9 4.4 6.2 8.7 + 1.4(3)
2.0 2.4 4.7 13.7 + 1.8(3)
a T h e s e n e e d l e s were c o l l e c t e d d u r i n g a r a i n s t o r m . T h e chemical washing procedure removed no further detectable lead f r o m t h e i r surfaces.
TABLE 2 T H E R E L A T I V E EFFICIENCIES O F R A I N W A S H I N G A N D C H E M I C A L W A S H I N G IN T H E R E M O V A L O F S U R F A C E LEAD. T H E S U R F A C E A R E A W A S 25--50 c m 2 F O R E A C H S A M P L E . V A R I A T I O N IS O N E S T A N D A R D DEVIATION, R E P L I C A T E S A R E I N D I C A T E D IN P A R E N T H E S E S .
Site 1
Surface lead (ng Pb c m -~ ) 36 days accumulationa 12 b chemically removed 19 Internal lead (pg Pb per g drywt.) rain w a s h e d chemically washed 2.4 + 0 . 1 ( 3 )
Site 2
Site 3
57 60
110 108
3.9 3.8 + 1.0(5) c
2.0 3.2 + 0.6(5) c
a C a l c u l a t e d as t h e d i f f e r e n c e b e t w e e n t h e c o n t r o l a n d 36-day needles o f T a b l e 1, e x p r e s s e d p e r u n i t area. b S i n c e t h e i n t e r n a l lead value for rain w a s h e d needles a t site 1 was n o t available, t h e c h e m i c a l l y w a s h e d value ( 2 . 4 ) was used. c D a t a f r o m t h e c h e m i c a l l y w a s h e d needles o f 6 a n d 36 d a y s e x p o s u r e are p o o l e d for these comparisons.
ments indicates that internal lead remains constant. This is verified by a comparison of the measured internal lead values of control (rain washed) and chemically washed needles. When the surface lead values are divided by the number of days of exposure, the rate of deposition may be determined, as shown in Table 3. These data show that the rate of accumulation of lead is nearly constant over the brief periods following rain washing. Although there is no apparent difference in deposition rates between sites 2 and 3, these sites have a much greater deposition rate than site 1, as indicated by the 36-day exposure values. Data
274 TABLE 3 C A L C U L A T E D D E P O S I T I O N R A T E S O F L E A D ON T H E S U R F A C E S O F N E E D L E S . S U R F A C E L E A D IS C A L C U L A T E D AS T H E D I F F E R E N C E BETWEEN T H E CONT R O L A N D T H E E X P O S E D V A L U E S O F T A B L E 1, E X P R E S S E D AS A F U N C T I O N OF AREA AND EXPOSURE PERIODS
Exposure
1 day 6 days 6 days a 36 d a y s
Site 1
Site 2
Site 3
ng c m -2
ng c m -2 d -1
ng c m -2
ng c m -2 d -1
ng c m -2
ng c m -2 d -1
---12
---0.32
3.3 16 12 57
3.3 2.6 2.0 1.6
2.4 17 8.0 110
2.4 2.9 1.3 3.0
a T h e s e data are f r o m n e w l y e m e r g e d needles (see t e x t ) .
for new needles which had emerged about 14 days before the rainstorm and were collected 6 days after the rainstorm show deposition rates comparable to those of fully expanded, one year old needles. Particle size classes The frequency of particles by size class (longest diameter) at each site shows a unimodal distribution with a peak at less than 5 microns, and no apparent peak in any size class above 5 microns (Fig. 21). Site 1 has the fewest particles, most of which are in the 1--5 pm size range. However, both sites have particles in size classes larger than those found at site 1.
SITE I
--
SITE 2
SITE 5
400 [.--
>,-
Z W W(~ 2 0 0 h
I00
L_ 5 I0 15 ~
25 30
5 I0 15 2025 3o 354o
PARTICLE SIZE CLASSES
I0 15 20 25 30 35 4 0 45 50 5
(MICRONS)
Fig. 21. Particles size class d i s t r i b u t i o n o f t h e t h r e e sites s h o w i n g t h e d i f f e r e n c e s in n u m bers o f small particles (1--5 m i c r o n s ) at t h e l o w e s t traffic d e n s i t y , Site 1.
275 DISCUSSION
Observed surface deposition SEM shows the occurrence of b o t h microflora and particulate matter on pine needle surfaces. The presence of microorganisms, predominantly fungi, is restricted to needles from the lowest traffic density site. Their absence on needles at other sites may indicate that changes in the local environment due to increased exposure to automotive enhaust exclude or impair colonization b y fungi. On the other hand, the a m o u n t of particulate matter increases with increasing traffic density. Sites 2 and 3 have comparable numbers of particles, significantly higher than site 1, with the largest increases associated with the smallest size class of particles. The unimodal distribution of size classes is indicative of a single source of particles. The absence of a bimodal distribution, typical of urban atmosphere (Fennelly, 1976), indicates that the relative influence of industrial emissions at these locations is low. However, since the sampled pine trees are in close proximity to the highways, the contribution of industrial particles may be masked by the abundance of automotive particles, thus resulting in an atypical unimodal distribution. The angular shape and elemental nature (high levels of A1 and Si) of particles from site 1 indicates they are of local or soil origin. The globular shape and decreases in A1 and Si of scanned particles at sites 2 and 3 may indicate a chemical origin, perhaps automotive exhausts. K/Ca ratios can also be used as indicators of particle origin. Biological tissues, other than those involved in Ca storage, rarely have a K/Ca ratio of less than 20. The crustal abundances of these t w o elements are approximately equal, although certain exposed soils and bedrock components may reach a ratio of two. Consequently, distinctions are possible between inorganic and biological particles. No particles with a high K/Ca ratio indicative of biological materials were observed. Because of the sensitivity limits of X-ray microprobe analysis, it was not possible to localize lead on particles of any size class by this method. However, since lead is believed to leave the automotive exhaust system as a lead bromochloride, the presence of C1 on particles at site 3 may indicate a recently c o m b u s t e d automotive aerosol. The bromine peak was somewhat obscured b y the aluminium peak and could not be used to support this argument. In some geographical areas, although not in these study areas, the use of chloride c o m p o u n d s in highway snow removal operations may account for the CI. The chemical washing procedure effectively removes both microflora and particulate matter from needles; the efficiency of removal is greater than 90%, as indicated by SEM. There is no apparent damage to the surfaces of the needles. Intense rainshowers also remove surface deposition on needles, leaving only particulate matter in the smallest size classes (less than 1 ~m). Chemical analysis o f surface and internal lead Because the visual evidence clearly indicates that the chemical washing procedure removes all or most of the surface particles without damaging the
276 surfaces of the needles, it is reasonable to assume that the lead in the washing solution is surface lead, whereas the remaining lead is internal. This assumption is supported by the fact that all washed needles from a given site have the same internal lead content, even though lead in unwashed needles varies by a factor of five. For internal lead, the small differences between sites are probably due to local soil conditions, rather than surface deposition. Since the internal lead remains constant, the variation is caused by the accumulation of surface lead between periods of rainfall. The question of foliar uptake is not within the scope of this study, but was considered in a related study by Elias et al. (1979). Natural removal mechanisms Previous work b y Elias et al. (1978), has shown that the apparent deposition rate of lead on natural surfaces such as pine needles and sedge leaves in a remote subalpine ecosystem in the High Sierra is significantly less than the observed rate of deposition on artificial surfaces normally used to determine inputs of lead and other atmospheric metals into the ecosystem. The results of the present study show that rain is a major removal mechanism and are consistent with the laboratory results of Carlson et al. (1976), even though the intensity of rainfall in our study is significantly lower. Although the intensity of the rain and its pH probably have the greatest influence on the removal process, other removal mechanisms, such as fog, dew, high relative humidity, resuspension and movement along surface films and direct foliar uptake, may theoretically alter the apparent deposition rates to a small degree. Some preliminary lead determinations from bud scales at the base of the pine needles indicate that much of the lead removed from the surfaces of needles is washed d o w n and trapped by these scales. Deposition rates Although the results of this study cast serious d o u b t on the feasibility of using leaf or needle surfaces as a means of determining rates of deposition over long periods of time, the rate of deposition between rainfalls appears to be constant. Even though no significant differences in deposition rates were observed between traffic densities 1000 and 10000 cars per day, these rates are significantly higher than those measured for 100 cars per day (0.32 ng Pb cm -2 day -1 ) in this study, and more than 20 times higher than the rate observed in a remote ecosystem (Elias et al., 1979). Knowledge of the true deposition rates is essential in determining the magnitude of input of metals into an ecosystem by dry deposition. Historically, this mechanism has generally been overlooked, causing errors in mass balance estimates. Inputs via precipitation are substantially overestimated as rain washes dry deposition particles from needle surfaces. Dry deposition may affect the major components of rainfall differently. Whereas throughfall may be significantly increased in lead content following contact with needles, the evidence for the fate of lead in stemflow is unclear. The fact that lead may
277
accumulate on bud scales or associated crevices suggests that much of the lead in stemflow may be removed before the water reaches the base of the tree.
CONCLUSIONS
The combination of SEM, X-ray microprobe analysis and clean lab technology represents a powerful research tool to investigators interested in difficult analytical problems. Using these techniques, it is possible to m o u n t and observe needles by SEM without disturbing their surfaces, to obtain some chemical information from the observed particles and to chemically remove these particles without disturbing the epidermal tissues of the needles. From the results of this study, we conclude that: 1. The size and number of particles deposited on needle surfaces vary according to traffic density at each roadside site during the periods of no rainfall. 2. Needle surfaces exposed to increasing amounts of automotive exhaust show qualitative changes in the kinds and amounts of elements present, in addition to showing increasing amounts of surface lead with increasing traffic densities. 3. The distribution of size classes of particles on needles exposed to increasing traffic densities shows a singular t y p e source, probably automotive, rather than a combination of industrial and automotive origin. 4. The K/Ca ratio of some surface particles 25--50 pm in diameter indicates they are of local or soil origin. The relative high amounts of A1 and Si seen in larger surface area scans also suggest a clay soil origin. 5. The inability of detecting specific lead aerosols by X-ray microprobe analysis is n o t inconsistent with the detection limits of the technique and the measured a m o u n t of lead per unit surface area (as high as 83 ng Pb cm -2 ). 6. The standard cleaning protocol effectively removed particles from the surface of needles without disrupting the integrity of the needle surface. 7. Rain also removes particles from the surface, including those particles bearing lead. 8. The rate of deposition of lead appears to be constant between periods of rainfall. 9. The use of SEM, X-ray microprobe analysis and AAS provides some visual and chemical information needed to infer the origin of aerosol particles. The implications of these findings are: 1. Data obtained from pine needles and probably most other plant surfaces must be viewed with caution since rain and perhaps surface moisture films will remove lead particles from leaf surfaces and significantly alter the apparent deposition rate. 2. The removal of particles during rainfall may substantially influence estimates of foliar uptake.
278
3. Precipitation samples taken beneath vegetation may be contaminated by throughfall which washes lead-bearing particles from needles or leaves. These measurements may give the appearance of leaching lead and other metals from the needles.
ACKNOWLEDGEMENTS
We gratefully acknowledge the field and technical assistance of Rodney E. Roth, and the expert technical assistance of Frank Mitsianis, Department of Forestry, VPI & SU, for the SEM portion of this research.
REFERENCES Carlson, R. W., A. Bazzaz, J. J. Stukel and J. B. Wedding, Environ. Sci. Technol., 10 (1976) 1139--1142. Elias, R. W., T. K. Hinkley, Y. Hirao and C. C. Patterson, Geochim. Cosmochim. Acta, 40 (1976) 583--587. Elias, R. W., Y. Hirao and C. C. P a t t e r s o n , in: D. C. Adriano and I. L. Brisbin (Eds.), Environmental Chemistry and Cycling Processes. CONF-760429. U.S. Dept. of Energy, 1978, pp. 691--699. Elias, R. W., H. Shirahata, K. Fujii and C. C. Patterson, Environ. Sci. Technol., manuscript submitted. Fennelly, P. F., Am. Sci., 64 (1976) 46--56. Zief, Morris and J. W. Mitchell, Contamination Control in Trace Element Analysis. John Wiley & Sons, New York, 1976, 262 pp.
265
INVESTIGATIONS OF THE DEPOSITION OF LEAD-BEARING AEROSOLS ON THE SURFACES OF VEGETATION
R O B E R T W. ELIAS* and JUDITH C R O X D A L E * *
Biology Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (U.S.A) (Received August 13th, 1979; accepted October 1st, 1979)
ABSTRACT SEM and X-ray microprobe analysis were combined with clean lab technology atomic absorption spectroscopy to observe the deposition of aerosols on undisturbed pine needle surfaces, to obtain physical and chemical information from the observed particles and to chemically remove these particles for analysis. Comparisons were made among sites with traffic densities of 100, 1000, 10000 cars per day. Correlations were made with periods of heavy rainfall. The results show that the particle size distribution, shape and surface chemistry are possibly characteristic of automotive exhausts, that both the chemical washing procedure and rainfall efficiently remove most particles from the surface, including those bearing lead, and that the rate of deposition is constant between rainfalls and partly a function of traffic density. Deposition rates of 0.3 ng P b c m -2 day -1 were observed at 100 cars per day, 2--3 ng Pb cm -2 day -1 for 1000 and 10000 cars per day.
INTRODUCTION
The impact of atmospheric inputs of heavy metals into an ecosystem and their subsequent cycling through the system is of critical importance to biogeochemical investigations of terrestrial ecosystems. This is particularly true of lead, which is a major metallic component of automotive exhaust. One primary mode of lead inputs into an ecosystem is the dry deposition of aerosol particles on vegetation surfaces (Elias et al., 1978). The value of direct measurements of deposits on vegetation would be that long term (1--6 years) deposition rates for any geographical area could be estimated by measuring the accumulation on vegetation surfaces such as pine needles or tree bark. Estimates of inputs are currently based on artificial surfaces exposed for brief periods of time (1--20 days). Studies of the methods for measuring surface deposits on vegetation have raised the question of whether these surfaces, which are subjected to seasonal climatic variation, would yield accurate measurements of the deposition rates. * Member, Society for Environmental Geochemistry and Health. ** Present address: Department of Botany, University of Wisconsin, Madison, WI 53706, U.S.A.
266
A major difficulty with the use of natural surfaces is the effectiveness of the m e t h o d of particle removal for lead determination. Based strictly on chemical evidence, the procedure developed by Elias et al. (1976) appeared to remove all of the lead on pine needles and sedge leaves. In that study, no visual evidence of cleaning efficiency was obtained, and there was some uncertainty as to whether the procedure merely leached lead from the internal tissues of the needles or removed the particles themselves. Utilizing pine needles from sites with known lead pollution, the objectives of the present study were: (1) by SEM and X-ray microprobe analysis, to determine the presence and elemental nature of particulate matter on needle surfaces and the efficiency of the chemical cleaning procedure for removing these particles; (2) by SEM and atomic absorption analysis, determine if the lead-bearing aerosol particles are removed during a rainstorm; and (3) by atomic absorption analysis, to verify that the deposition rate of lead aerosols is constant between rainfalls.
MATERIALS AND METHODS Needles of Virginia pine (Pinus virginiana Mill.) were collected from roadside trees at sites in the vicinity of Blacksburg, Virginia, having traffic densities of 100, 1000, 10000 cars per day (Sites 1, 2 and 3, respectively). These sites were within 10 km of a major interstate route (50--70000 cars per day) but more than 50 km from major industrial sources of air pollution. Except where indicated, only needles which had emerged during the spring of 1977 were collected. Collections were made during March--May, 1978. Thus the needles had an estimated growth period of 300--360 days. To avoid contamination of the samples, needles were collected using polyethylene gloves and acid-cleaned stainless steel forceps and transported in wrapped and sealed acid-cleaned polyethylene containers. Further details of the collection procedure are described in Elias et al. (1976). Generally, triplicate samples from the designated trees were analyzed, except where the entire sample was required for a single, low-lead analysis.
Washing procedure and chemical analysis The sample containers were reopened in a Class 10000 clean room (Zief and Mitchell, 1976). Needles to be washed were grasped at the base with FEP (fluorinated ethylene propylene) teflon forceps and manually swirled, successively in methanol, distilled water, 1N HC1 and again in the same distilled water (10 seconds in each solution). The solutions were combined and are hereafter referred to as the surface washings. This solution was evaporated to dryness, the residue dissolved in double distilled HNO3, evaporated and dissolved with 0.1N HNO3, then diluted a working volume of a b o u t 4 ml. The length and width of the washed needles were measured, and the samples were then placed on a stub for SEM analysis or into a clean FEP teflon beaker for chemical analysis. For chemical analysis of the washed and unwashed
267 needles, each sample was dissolved in double distilled HNO3 and evaporated to dryness; the residue was dissolved in 0.1N HNO3 and diluted to a working volume of about 4 ml. Samples were analyzed for lead using an atomic absorption spectrophotometer (Perkin Elmer 460) equipped with a graphite furnace and a deuterium arc background corrector. The sensitivity of this technique was 0.3 ng sample -1 or 0.06 ng m1-2 , with an average blank value of 25 ng Pb/sample and 200 ng Pb per washing solution; sample values ranged from 200 to 1000 ng Pb sample -1 . The m e t h o d of standard additions was used for several samples. An NBS standard reference material was also used (NBS SRM No. 1575, pine needles) and agreement was within 5%.
SEM preparation SEM stubs were cleaned with acetone and covered with double sticky tape which had not been exposed to the air. Unfixed washed and unwashed needle tips, 1 cm in length, were placed four to a stub, two with the fiat (adaxial) side up, two with the convex (abaxial) side up. Silver paint was used on the ends of each needle segment to establish electrical contact with the stub. The needles were observed unfixed and uncoated. Observation and analysis was done using an AMR 900 scanning electron microscope operating at 20 kV, 100 #amp with an EDAX 707A X-ray microprobe unit. For elemental analysis of selected leaf surfaces and individual surface particles, the size of the areas scanned and the scan time were approximately constant. Under these conditions, the presence or absence of certain elements can be detected. Absolute concentrations are n o t available under this EDAX system, but relative concentration, based on peak height, of one element compared to another can be determined.
Particle size classes The number and size classes (maximum linear diameter) of surface particles were determined from enlarged SEM photograhs with a microcaliper and template. For each needle, all observable particles in each size class were visually counted in a r a n d o m l y selected area equivalent to 4 m m 2 of actual needle surface. Particles smaller than 0.5 microns were not measured.
RESULTS
SEM and X-ray microprobe analysis Surfaces of unwashed pine needles show that some contain microflora, and that there is an increase in the number and size of particles associated with sites of increased traffic density (Figs. 1--6). At the lowest traffic density site (100 vehicles day -1 ), micro-organisms on the needle surface are relatively c o m m o n (Figs. 1 and 2). In this material, mycelial growth of fungi is predominant, some fungal hyphae even invade the stomatal cavity (Fig. 2). Even though the age of the needles at all three sites is the same, needles from sites 2 and 3 (1000 and 10000 vehicles day -1 ) rarely have microflora on their
268
Fig. 1 (left). Unwashed surface of pine needle collected at site 1 (100 vehicles per day) showing both particulate and organic matter, x 41. Fig. 2 (right). Enlarged surface view of unwashed needle from site 1 with numerous colonies of fungi;note that some hyphae have penetrated stomata and that particulate matter is sparse. X .164.
Fig. 3 (left). Unwashed surface of pine needle collected at site 2 (1000 vehicles per day) showing a large increase in particulate matter. × 41. Fig. 4 (right). Enlarged surface view of unwashed needle from site 2 showing the range of particle size classes and shapes. X 164.
269
Fig. 5 (left). Unwashed surface of pine needle collected from site 3 (10000 vehicles per day) showing extensive accumulation of particulate matter. × 41. Fig. 6 (right). Enlarged surface of unwashed needle from site 3 with many large, irregularly shaped particles in addition to numerous small particles, x 164.
surfaces, although the a m o u n t of particulate matter has increased (Figs. 3--6). X-ray microprobe analysis of needle surfaces (area scanned, 4 m m 2 ), shows these areas contain conspicuous peaks corresponding to A1, Si, Cl, K and Ca (Figs. 7--9). Although numerous individual particles, both large and small, were also scanned (Figs. 10--12), none contained lead at or above the detection limits of the instrument. Note that the relative amounts of A1 and Si in particles from the three sites decreases with increasing traffic density, compared to C1, K and Ca. In the case of K and Ca, increasing amounts of these O3
s,
cn
~
7
Si
8
Si
9
W cr
ENERGY LEVELS
Fig. 7 (left). EDAX displays of needle surface scans showing the relative concentrations of the elements indicated by the chemical symbols. Fig. 8 (centre). EDAX displays of needle surface scans showing the relative concentrations of the elements indicated by the chemical symbols. Fig. 9 (right). EDAX displays of needle surface scans showing the relative concentrations of the elements indicated by the chemical symbols.
270
CO ,1,," W ,"n ~o':,
$i
Ca
S* AI
Ca
AI
Si
CI
KCa U L,IJ rr"
ENERGY LEVELS
Fig. 10 (left). EDAX displays of individual particle scans showing the relative concentrations of the elements indicated by the chemical symbols. Fig. 11 (centre). EDAX displays of individual particle scans showing the relative concentrations of the elements indicated by the chemical symbols. Fig. 12 (right). EDAX displays of individual particle scans showing the relative concentrations of the elements indicated by the chemical symbols.
elements in particles are associated with increasing traffic density. The shape of the particles tend to be angular on site 1 needles and more globular to spherical at sites 2 and 3. Washed needles from all three sites (Figs. 13--18) have clean and intact surfaces. The majority of the microflora on needles from site 1 have been removed (Figs. 13 and 14) and the number of particles at all sites has been significantly reduced (Figs. 15--18). Needles from sites 2 and 3 which were exposed to heavy rainfall (3.2 cm
4
Fig. 13 (left). Surface of chemically washed needle collected at site i showing an absence of mieroflora and a decrease in the number of particles. × 41. Fig. 14 (right). Small amount of particulate matter is present after chemical cleaning on this site I needle. × 164.
271
Fig. 15 (left). Surface of chemically washed needle from site 2 showing that the leaf is intact and there is no apparent structural damage resulting from the cleaning procedure. * x 41. Fig. 16 (right). Enlarged surface view of chemically washed site 2 needle showing nearly complete removal of particles. X 164.
Fig. 17 (left). Surface of chemically washed site 3 needle. × 41. Fig. 18 (right). Enlarged surface view of chemically washed site 3 needle. Note the effectiveness of particle removal by comparing with Fig. 6. × 164.
272
O Fig. 19 (left). Surface view of rain washed site 2 needle. Compare with Fig. 3, 4, 15, 16 for the effectiveness of this natural means of particle removal. × 82. Fig. 20 (right). Surface view of rain washed site 3 needle. Note nearly complete absence of particles (cf. Figs. 5, 6, 17, 18). × 82. in 24 h) also have relatively clean surfaces (Fig. 19, 20). Only the smallest particles (less than 0.5/~m) are present in any number on leaves from these two high traffic density sites. Rain-washed needles from site 1 were not available for analysis.
Chemical analyses Table 1 contains data for the total measured lead of needles collected during a rainstorm (control), and one day, six days and 36 days following a rainstorm (3.2 cm in 24 h). No surface lead could be detected on needles collected during rainfall, thus their values are due entirely to internal lead. Following rain, needles show a systematic increase in the a m o u n t of total lead with time, but with no significant difference in values for any given exposure time between sites 2 and 3. Rain-washed needles from site 1 were not available for comparison. A comparison of the surface and internal lead values of chemically washed and rain-washed needles show close agreement of data. The a m o u n t of surface lead increases markedly at sites with high traffic density; surface lead on site 2 and site 3 needles after 36 days exposure is approximately four times and seven times greater than on needles at site 1. These increases can be seen in measurements from actual surface washings (chemically removed) or in recalculated data of total lead determinations (Table 2). In this latter case, surface lead was calculated as the difference between the control (rain washed) and exposed needles of Table 1, expressed per unit area. The close agreement of the surface lead values based on actual and calculated measure-
273
TABLE 1 I N C R E A S E S IN T H E T O T A L L E A D C O N T E N T OF N E E D L E S A S A F U N C T I O N O F TIME. E X P O S U R E IS IN D A Y S SINCE T H E L A S T R A I N F A L L , V A L U E S A R E IN gg Pb P E R g D R Y WT. V A R I A T I O N IS O N E S T A N D A R D D E V I A T I O N , R E P L I C A T E S A R E I N D I C A T E D IN PARENTHESES. Exposure
Site 1
Site 2
Site 3
Control a 1 day 6 days 36 d a y s
---4.4 + 0.6(3)
3.9 4.4 6.2 8.7 + 1.4(3)
2.0 2.4 4.7 13.7 + 1.8(3)
a T h e s e n e e d l e s were c o l l e c t e d d u r i n g a r a i n s t o r m . T h e chemical washing procedure removed no further detectable lead f r o m t h e i r surfaces.
TABLE 2 T H E R E L A T I V E EFFICIENCIES O F R A I N W A S H I N G A N D C H E M I C A L W A S H I N G IN T H E R E M O V A L O F S U R F A C E LEAD. T H E S U R F A C E A R E A W A S 25--50 c m 2 F O R E A C H S A M P L E . V A R I A T I O N IS O N E S T A N D A R D DEVIATION, R E P L I C A T E S A R E I N D I C A T E D IN P A R E N T H E S E S .
Site 1
Surface lead (ng Pb c m -~ ) 36 days accumulationa 12 b chemically removed 19 Internal lead (pg Pb per g drywt.) rain w a s h e d chemically washed 2.4 + 0 . 1 ( 3 )
Site 2
Site 3
57 60
110 108
3.9 3.8 + 1.0(5) c
2.0 3.2 + 0.6(5) c
a C a l c u l a t e d as t h e d i f f e r e n c e b e t w e e n t h e c o n t r o l a n d 36-day needles o f T a b l e 1, e x p r e s s e d p e r u n i t area. b S i n c e t h e i n t e r n a l lead value for rain w a s h e d needles a t site 1 was n o t available, t h e c h e m i c a l l y w a s h e d value ( 2 . 4 ) was used. c D a t a f r o m t h e c h e m i c a l l y w a s h e d needles o f 6 a n d 36 d a y s e x p o s u r e are p o o l e d for these comparisons.
ments indicates that internal lead remains constant. This is verified by a comparison of the measured internal lead values of control (rain washed) and chemically washed needles. When the surface lead values are divided by the number of days of exposure, the rate of deposition may be determined, as shown in Table 3. These data show that the rate of accumulation of lead is nearly constant over the brief periods following rain washing. Although there is no apparent difference in deposition rates between sites 2 and 3, these sites have a much greater deposition rate than site 1, as indicated by the 36-day exposure values. Data
274 TABLE 3 C A L C U L A T E D D E P O S I T I O N R A T E S O F L E A D ON T H E S U R F A C E S O F N E E D L E S . S U R F A C E L E A D IS C A L C U L A T E D AS T H E D I F F E R E N C E BETWEEN T H E CONT R O L A N D T H E E X P O S E D V A L U E S O F T A B L E 1, E X P R E S S E D AS A F U N C T I O N OF AREA AND EXPOSURE PERIODS
Exposure
1 day 6 days 6 days a 36 d a y s
Site 1
Site 2
Site 3
ng c m -2
ng c m -2 d -1
ng c m -2
ng c m -2 d -1
ng c m -2
ng c m -2 d -1
---12
---0.32
3.3 16 12 57
3.3 2.6 2.0 1.6
2.4 17 8.0 110
2.4 2.9 1.3 3.0
a T h e s e data are f r o m n e w l y e m e r g e d needles (see t e x t ) .
for new needles which had emerged about 14 days before the rainstorm and were collected 6 days after the rainstorm show deposition rates comparable to those of fully expanded, one year old needles. Particle size classes The frequency of particles by size class (longest diameter) at each site shows a unimodal distribution with a peak at less than 5 microns, and no apparent peak in any size class above 5 microns (Fig. 21). Site 1 has the fewest particles, most of which are in the 1--5 pm size range. However, both sites have particles in size classes larger than those found at site 1.
SITE I
--
SITE 2
SITE 5
400 [.--
>,-
Z W W(~ 2 0 0 h
I00
L_ 5 I0 15 ~
25 30
5 I0 15 2025 3o 354o
PARTICLE SIZE CLASSES
I0 15 20 25 30 35 4 0 45 50 5
(MICRONS)
Fig. 21. Particles size class d i s t r i b u t i o n o f t h e t h r e e sites s h o w i n g t h e d i f f e r e n c e s in n u m bers o f small particles (1--5 m i c r o n s ) at t h e l o w e s t traffic d e n s i t y , Site 1.
275 DISCUSSION
Observed surface deposition SEM shows the occurrence of b o t h microflora and particulate matter on pine needle surfaces. The presence of microorganisms, predominantly fungi, is restricted to needles from the lowest traffic density site. Their absence on needles at other sites may indicate that changes in the local environment due to increased exposure to automotive enhaust exclude or impair colonization b y fungi. On the other hand, the a m o u n t of particulate matter increases with increasing traffic density. Sites 2 and 3 have comparable numbers of particles, significantly higher than site 1, with the largest increases associated with the smallest size class of particles. The unimodal distribution of size classes is indicative of a single source of particles. The absence of a bimodal distribution, typical of urban atmosphere (Fennelly, 1976), indicates that the relative influence of industrial emissions at these locations is low. However, since the sampled pine trees are in close proximity to the highways, the contribution of industrial particles may be masked by the abundance of automotive particles, thus resulting in an atypical unimodal distribution. The angular shape and elemental nature (high levels of A1 and Si) of particles from site 1 indicates they are of local or soil origin. The globular shape and decreases in A1 and Si of scanned particles at sites 2 and 3 may indicate a chemical origin, perhaps automotive exhausts. K/Ca ratios can also be used as indicators of particle origin. Biological tissues, other than those involved in Ca storage, rarely have a K/Ca ratio of less than 20. The crustal abundances of these t w o elements are approximately equal, although certain exposed soils and bedrock components may reach a ratio of two. Consequently, distinctions are possible between inorganic and biological particles. No particles with a high K/Ca ratio indicative of biological materials were observed. Because of the sensitivity limits of X-ray microprobe analysis, it was not possible to localize lead on particles of any size class by this method. However, since lead is believed to leave the automotive exhaust system as a lead bromochloride, the presence of C1 on particles at site 3 may indicate a recently c o m b u s t e d automotive aerosol. The bromine peak was somewhat obscured b y the aluminium peak and could not be used to support this argument. In some geographical areas, although not in these study areas, the use of chloride c o m p o u n d s in highway snow removal operations may account for the CI. The chemical washing procedure effectively removes both microflora and particulate matter from needles; the efficiency of removal is greater than 90%, as indicated by SEM. There is no apparent damage to the surfaces of the needles. Intense rainshowers also remove surface deposition on needles, leaving only particulate matter in the smallest size classes (less than 1 ~m). Chemical analysis o f surface and internal lead Because the visual evidence clearly indicates that the chemical washing procedure removes all or most of the surface particles without damaging the
276 surfaces of the needles, it is reasonable to assume that the lead in the washing solution is surface lead, whereas the remaining lead is internal. This assumption is supported by the fact that all washed needles from a given site have the same internal lead content, even though lead in unwashed needles varies by a factor of five. For internal lead, the small differences between sites are probably due to local soil conditions, rather than surface deposition. Since the internal lead remains constant, the variation is caused by the accumulation of surface lead between periods of rainfall. The question of foliar uptake is not within the scope of this study, but was considered in a related study by Elias et al. (1979). Natural removal mechanisms Previous work b y Elias et al. (1978), has shown that the apparent deposition rate of lead on natural surfaces such as pine needles and sedge leaves in a remote subalpine ecosystem in the High Sierra is significantly less than the observed rate of deposition on artificial surfaces normally used to determine inputs of lead and other atmospheric metals into the ecosystem. The results of the present study show that rain is a major removal mechanism and are consistent with the laboratory results of Carlson et al. (1976), even though the intensity of rainfall in our study is significantly lower. Although the intensity of the rain and its pH probably have the greatest influence on the removal process, other removal mechanisms, such as fog, dew, high relative humidity, resuspension and movement along surface films and direct foliar uptake, may theoretically alter the apparent deposition rates to a small degree. Some preliminary lead determinations from bud scales at the base of the pine needles indicate that much of the lead removed from the surfaces of needles is washed d o w n and trapped by these scales. Deposition rates Although the results of this study cast serious d o u b t on the feasibility of using leaf or needle surfaces as a means of determining rates of deposition over long periods of time, the rate of deposition between rainfalls appears to be constant. Even though no significant differences in deposition rates were observed between traffic densities 1000 and 10000 cars per day, these rates are significantly higher than those measured for 100 cars per day (0.32 ng Pb cm -2 day -1 ) in this study, and more than 20 times higher than the rate observed in a remote ecosystem (Elias et al., 1979). Knowledge of the true deposition rates is essential in determining the magnitude of input of metals into an ecosystem by dry deposition. Historically, this mechanism has generally been overlooked, causing errors in mass balance estimates. Inputs via precipitation are substantially overestimated as rain washes dry deposition particles from needle surfaces. Dry deposition may affect the major components of rainfall differently. Whereas throughfall may be significantly increased in lead content following contact with needles, the evidence for the fate of lead in stemflow is unclear. The fact that lead may
277
accumulate on bud scales or associated crevices suggests that much of the lead in stemflow may be removed before the water reaches the base of the tree.
CONCLUSIONS
The combination of SEM, X-ray microprobe analysis and clean lab technology represents a powerful research tool to investigators interested in difficult analytical problems. Using these techniques, it is possible to m o u n t and observe needles by SEM without disturbing their surfaces, to obtain some chemical information from the observed particles and to chemically remove these particles without disturbing the epidermal tissues of the needles. From the results of this study, we conclude that: 1. The size and number of particles deposited on needle surfaces vary according to traffic density at each roadside site during the periods of no rainfall. 2. Needle surfaces exposed to increasing amounts of automotive exhaust show qualitative changes in the kinds and amounts of elements present, in addition to showing increasing amounts of surface lead with increasing traffic densities. 3. The distribution of size classes of particles on needles exposed to increasing traffic densities shows a singular t y p e source, probably automotive, rather than a combination of industrial and automotive origin. 4. The K/Ca ratio of some surface particles 25--50 pm in diameter indicates they are of local or soil origin. The relative high amounts of A1 and Si seen in larger surface area scans also suggest a clay soil origin. 5. The inability of detecting specific lead aerosols by X-ray microprobe analysis is n o t inconsistent with the detection limits of the technique and the measured a m o u n t of lead per unit surface area (as high as 83 ng Pb cm -2 ). 6. The standard cleaning protocol effectively removed particles from the surface of needles without disrupting the integrity of the needle surface. 7. Rain also removes particles from the surface, including those particles bearing lead. 8. The rate of deposition of lead appears to be constant between periods of rainfall. 9. The use of SEM, X-ray microprobe analysis and AAS provides some visual and chemical information needed to infer the origin of aerosol particles. The implications of these findings are: 1. Data obtained from pine needles and probably most other plant surfaces must be viewed with caution since rain and perhaps surface moisture films will remove lead particles from leaf surfaces and significantly alter the apparent deposition rate. 2. The removal of particles during rainfall may substantially influence estimates of foliar uptake.
278
3. Precipitation samples taken beneath vegetation may be contaminated by throughfall which washes lead-bearing particles from needles or leaves. These measurements may give the appearance of leaching lead and other metals from the needles.
ACKNOWLEDGEMENTS
We gratefully acknowledge the field and technical assistance of Rodney E. Roth, and the expert technical assistance of Frank Mitsianis, Department of Forestry, VPI & SU, for the SEM portion of this research.
REFERENCES Carlson, R. W., A. Bazzaz, J. J. Stukel and J. B. Wedding, Environ. Sci. Technol., 10 (1976) 1139--1142. Elias, R. W., T. K. Hinkley, Y. Hirao and C. C. Patterson, Geochim. Cosmochim. Acta, 40 (1976) 583--587. Elias, R. W., Y. Hirao and C. C. P a t t e r s o n , in: D. C. Adriano and I. L. Brisbin (Eds.), Environmental Chemistry and Cycling Processes. CONF-760429. U.S. Dept. of Energy, 1978, pp. 691--699. Elias, R. W., H. Shirahata, K. Fujii and C. C. Patterson, Environ. Sci. Technol., manuscript submitted. Fennelly, P. F., Am. Sci., 64 (1976) 46--56. Zief, Morris and J. W. Mitchell, Contamination Control in Trace Element Analysis. John Wiley & Sons, New York, 1976, 262 pp.