enhanced the magnitude of the C8 compound response. There was only one sample, however, in which such a variety of acids was observed. Normally, very few components could be distinguished in samples directly injected onto Texax-GC. Because of the small rainfall dilution effect, small-volume precipitation events would be expected to exhibit relatively high concentrations of organic components. Retention times of known samples of the following additional acids were examined on Tenax-GC columns and compared with collected rain samples: normal C1-CS carboxylic acids, oxalic, succinic, citric, isocitric, glutaric, malic, maleic, fumaric, lactic, glycolic, palmitic, and stearic acids. Some of these acids are known to decompose under the conditions that obtained on the column, e.g., citric ( 1 7 ) .Palmitic and stearic acids did not elute, and, according to Ho et al. (18),one should not expect straight-chain acids above C12 to do so on Tenax-GC columns under the conditions employed. Although Galloway et al. (2) found evidence for some of these acids in rain, we did not. The components found by Lunde et al. (15) in incident rain samples contrast, particularly in numbers, with the relatively few components we have identified. In this Scandanavian study, wetfall plus dryfall samples were composited in bulk collectors over long time periods. The resulting solutions were adsorbed on charcoal and eluted with carbon disulfide. This charcoal pretreatment, in particular, seems a likely source for some of the compounds observed. Our choice to sample wetfall only by event, to isolate organic materials from individual storm event samples, and to avoid solid absorbent pretreatment procedures may account for the fact that compounds like polyaromatic hydrocarbons were not observed. Our failure to detect such compounds, coupled with the identifications that we have made, however, does suggest that the primary organic materials in throughfall are aliphatic saturates and neutral esters. Furthermore, a t least as much material appears to originate from sources external to the vegetation as from internal plant processes. Conclusion Plasticizers and chlorohydrocarbons were identified in this study and appear to be related to sources external to the deciduous trees examined (Quercus prinus L.). Organic acids and high molecular weight “waxy” materials also were detected and are likely derived from the metabolism, or the attrition, of the leaves themselves. Specific organic compound types present in single rain event samples are probably related to processes of deposition, leaching, and weathering in a forest system. Information about where the observed compounds originate, therefore, can provide a rational glimpse of chemical pathways in these biological systems. Event sampling provides sample quantities that are generally within the detection capabilities of commercial instruments and minimizes prob-
lems of contamination and time averaging that can arise with bulk sampling. Acknowledgments Thanks are due Ed McBay, Lloyd Brown, and I. B. Rubin of the Analytical Chemistry Division, Oak Ridge National Laboratory, for experimental assistance, S.E. Herbes and Dave Shriner for the use of instruments and equipment and for helpful comments on the manuscript, and Norb Goeckner of Western Illinois University for discussions concerning all aspects of this work. Literature Cited (1) Likens, G. E., Bormann, F., Johnson, N. M., Pierce, R. S., Ecology,
48,772-85 (1967). (2) Galloway, J. N., Likens, G. E., Edgerton, E. S., Science, 144,772-3 (1976). ( 3 ) Schlesinger, W. H., Reinprs, W. A., Knopman, D. S., Enuiron. Pollut., 6,39-42 (1974). (4) Hancock, J. L., Applegate, H. G., Dodd, J . D., Atmos. Enuiron., 4,363-70 (1970). (5) Flowers, E. C., McCormick, R. A., Kurfis, K. R., J . Appl. Meterol., 8,955-62 (1969). (6) Hoffman, W. A., Lindberg, S. E., Turner, R. R., J . Enuiron. Qual., 9,95-100 (1980). (7) Lindberg, S. E., Harriss, R. C., Turner, R. R., Shriner, D. S., Huff, D. D., “Mechanisms and Rates of Atmospheric Deposition of Trace Elements and Sulfate to a Deciduous Forest Watershed”. ORNL/ TM-6674, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1979. (8) Lindberg, S. E., Turner, R. R., Ferguson, N. M., Matt, D. M., in “Watershed Research in Eastern North America”, Vol. 1,Carroll, D. L., Ed., Smithsonian Institution, Edgewater, Md., 1977, pp 125-52. (9) Menzel, D. W., Vacarro, R. F., Limnol. Oceanogr., 9, 138-42 ( 1964). (10) Strickland, J. D. H., Parsons, T. R., “A Practical Handbook of Seawater Analysis”, Fisheries Research Board of Canada, Ottawa, 1972, p 153. (11) Hoffman, W. A., Jr., Anal C h e m , 50,2158-9 (1978). (12) Stenhagen, E., Abrahamson, S., McLafferty, R., “Registry of Mass Spectral Data”, Wiley, New York, 1974. (13) Miles, D. C., Briston, J. H., “Polymer Technology”, Chemical Publishing Co., New York, 1965, p 324. (14) Quackenbos, H. M., Jr., Ind. E ~ PChem.. . 46.1335-44 (1954). (15) cunde, G., Gether, J., Gjos, N., Luide, M. B. S.;Atmos. Enuiron., 11,1007-14 (1977). (16) Martin, S . T., Juniper, B. E., “The Cuticles of Plants”, St. Martins, New York, 1970. (17) Rodd, E. H., Ed., “Chemistrv of Carbon ComDounds”. Vol. IB. Elsevier, New York, 1952. (18) Ho, C. H., Clark, B. R., Guerin, M. R., J . Enuiron. Sci. Health, Ser A l l , 7,481-9 (1976).
Receiued for reuieu: November 19, 1979. Accepted March 31, 1980. Research sponsored by the Office of Health and Enuironmental Research, U.S. Department of Energy, under Contract W-7405eng-26 with Union Carbide Corporation. Publication No. 1516 from the Enuironmental Sciences Diuision, Oak Ridge National Laboratory.
Enrichment of Trace Elements in Remote Aerosols Fred C. Adams,’ Marc J. Van Craen, and Pierre J. Van Espen Department of Chemistry, University of Antwerp (U.I.A.),8-2610 Wilrijk, Belgium A method is described for comparison of the concentrations of anomalously enriched elements in air particulate material sampled with high-volume filtration sampling equipment a t remote locations. The applicability is tested with data obtained a t Chacaltaya, Bolivia and a t Jungfraujoch, Switzerland.
1002
Environmental Science 8, Technology
During the last decade a number of studies have been conducted in various locations to derive the extent of worldwide transport of air pollutants from urban and industrial areas to remote regions of the earth’s surface and to assess the atmospheric burden of trace elements in unpolluted background air. A number of elements were shown to be enriched in the atmosphere (2-3) and also in snow and ice ( 4 , s )by a factor
0013-936X/80/0914-1002$01 .OO/O
@ 1980 American Chemical Society
of 10 to 10 000 in relation to the amounts expected from bulk crustal material or sea salt. The total mass of these atmophile or anomalously enriched elements is estimated to range between 5 and 10%of the total atmospheric burden (6). Several explanations for this enrichment have been postulated such as: aerosol generation a t the sea surface layer ( 7 ) , physical or biologically mediated volatilization from the earth’s surface ( 8 ) ,release of metal-rich particulates from vegetation ( 9 ) ,volcanism ( 6 ) , burning of vegetation, and, finally, fossil fuel burning and industrial activities. An estimate of the relative importance of these sources of atmospheric input for 20 trace metals is given by Lantzy and Mackenzie (10). They state that, although anthropogenic emissions may be large, the total contribution of all natural emissions is more important. However, up to now insufficient data are available for pinpointing the sources of anomalous enrichment. To study the enrichment effect, air particulate material may be collected with high-volume filtration equipment in order to obtain sufficient amounts of material for quantitative analysis with reasonable accuracy. To quantify enrichment, Rahn (1 ) proposed the use of enrichment factors (EF), defined as: EF, - C, P Cref,c (1) Cref,p C x c where C x , pand Cref,pare the concentrations of the element and of a suitably chosen reference element in the atmospheric particles, respectively. and C, and Cref,care the concentrations in average crustal rock. Any crustally derived element of negligible enrichment (e.g., Al, Ti, Sc, or Fe) can be chosen, but the first element is generally preferred to study continental aerosols. Sodium is used as a reference to compare element enrichment due to marine influence. The lithophilic reference elements, which are predominantly generated by dispersion processes, are overwhelmingly present in the large particle fraction of the aerosol, whereas the anomalously enriched elements (AEEs) are associated with particles of submicron size ( I ) . This has important repercussions when employing EFs for the comparison of the aerosol a t different remote locations: EFs may vary over a wide range depending on the magnitude of the large particle dust concentration, irrespective of any variation of the AEEs in the fine particle aerosol fraction. Variations of the large particle component commonly occur a t remote continental locations, but also a t a number of remote marine sites, e.g., over the Atlantic Ocean, where they are caused by a major, large particle component originating in the arid and semiarid regions of North and West Africa ( I I ) , the so-called Saharan dust plume. Perhaps the only region of the world that is located far enough away from any sources of dispersion aerosols to be free from disturbances is the Antarctic continent. The residence time of aerosol particles depends on the aerodynamic diameter (12, 13);the size distribution of the particulate matter a t a given site hence significantly depends on its age and past history due to, e.g.. rain or snowout (14. 15). It is assumed in comparison of data obtained by different investigators that the large particle cutoff of the sampling equipment is identical T o take into account the natural variability of the aerosol concentration and of ihe EFs, a log normal distribution is frequently assumed (16). The geometrical mean concentrations of the AEEs or the EFs, as obtained a t different remote locations, can hardly be compared without the tacit assumptions that the large particle aerosol generation rate and the scavenging history are on the average identical. These are strictly speaking valid only for a well-mixed. aged aerosol. In all other cases differences in concentrations and enrichment
factors may as well be due to variation of the reference element as to differences in concentrations of the AEEs. A number of researchers realize the problems described and circumvent them by using size-fractionated sampling of the aerosol. Sampling with cascade impactors is inherently much more difficult than high-volume filtration, and the amount of material collected is often insufficient for the accurate determination of many important elements. Method
In the argument which follows, an approach is developed and illustrated that allows the comparison of data obtained with high-volume sampling equipment a t different locations. I t is based on the temporal variation of the total suspended particulate matter concentration, which generally occurs a t any sampling site. It is assumed that the AEEs have two general sources in the aerosol: a large particle source associated with dispersion processes and a submicron particle source, responsible for a concentration, Cmin,which is assumed to be constant a t a given sampling site. The submicron aerosol originates in a gas-to-particle condensation process or any other, up to now unknown, generation process for small particle aerosols. It is further assumed that the refractory reference element is only generated through the dispersion process. The following relations then hold for every AEE,:
and
& - Cx,rnin I Cx,disp (2) Cref.p Cref,disp Cref,disp Dividing by the concentration ratio in average crustal rock: C x , m i n Cref,c (3) EF, =--k EFdisp Cref.disp C x , c where EFdisp is the enrichment factor for the large particle aerosol. Regression analysis using the concentration of the reference element as an independent variable leads to Cmin and EFdisp for a number of AEEs. The linear correlation coefficient r provides an indication of the validity of the assumptions made in the rather crude model.
Experimental
The model is tested on two data sets from continental mountain locations. (a) Chacaltaya Mountain, at 5230 m Altitude in the Bolivian Cordillera, near La Paz. The site is amply described elsewhere (1 7). High-volume samples (23) were taken over 10- to 21-day periods, during 1977 and 1978. About 40 elements were determined in all aerosol samples. The results are published elsewhere (18). (b) Jungfraujoch, a Mountain Location at 3750 m above Sea Level in the Swiss Alps. The chemical composition of the aerosol was described by Dams and De Jonge (19).Individual concentrations of ca. 20 elements in 18 samples, obtained from August 1973 to March 1975, are available. At both sites, total suspended particulate concentration and the individual element concentrations show a strong seasonal variation that can be correlated with precipitation characterist,ics a t Chacaltaya and snow cover a t Jungfraujoch. The variations are more pronounced for the lithophilic elements. The variation of the EF for arsenic and aluminum throughout the sampling campaign a t Chacaltaya is shown in Figure 1,as an example. Results a n d Discussion
When the EFs for both locations are plotted against the reciprocal iron concentration as a reference element, a linear Volume 14, Number 8, August 1980
1003
Table 1. Coniparison of Cmlnand EFdlsp(Equation 2) at Chacaltaya and Jungfraujoch Cmln (ne rn-3 STP)
e Ieme ni
Chacaltaya
Jungfraujoch
co
0.085f 0.018 0.21 f 0.03 1.6 f 0.14 -0.03 0.85 f 0.08 0.15 f 0.04 0.11 f 0.04
0.016f 0.02 0.48f 0.13 2.78 f 1.8 0.013 f 0.004 0.13 f 0.02 0.14 f 0.02 0.55 f 0.085
cu Zn
Se AS
I
Br a
Chacaltaya
EFdiso
1.38 f 0.39 3.7f 0.35 7 f 1.1 -120 175 f 25 182 f 0.02 70 f 8
corr coeff and probabllliy, % Chacaltaya Jungfraujoch
Jungtraujoch
0.73( P = 0.01) 0.81 ( P < 0.01) 0.92( P< 0.01) 0.74a 0.91 (P< 0.01) 0.75( P = 0.18) 0.53( P = 0.88)
1.5 f 0.26 10.7f 6.9 168 f 75 800 f 230 79 f 31 182 f 50 365 f 98
0.89 ( P < 0.01) 0.70( P = 0.25) 0.38( P = 15) 0.70( P = 0.4) 0.89( P< 0.01) 0.87( P < 0.01) 0.86( P < 0.01)
Detected in approximately half of the samples.
Table II. Comparison of EFdlspwith Enrichment Factor in Stages 1, 2, and 3 of Cascade Impactor, Chacaltaya
Table 111. Comparison of Cmln(Equation 3 ) with the Average Concentration in Stages 5 6 of Cascade Impactor (Chacaltaya)
+
stage and CUI-off EFdisp
Cu
1 > I 6 prn
3.7f 0.35 6.7f 3.6 15.2f 10 7fl 175% 25 33041300 51 f 7 105 f 60
Zn As
Pb
3 >4 pm
2
>8 prn
eIerne ni
-8 5.6f 3.2 7.6 f 4 8.5 f 6.5 220f 200 225f 120 44% 20 42f21
cu Zn As Pb
0.21 f 0.03 1.60f 0.14 0.85f 0.08 1.30f 0.16
1
Gl
' 1977
'
'
JAN 1978
'
'
'
JUL '
1578
'
Figure 1. Enrichment factor of aluminum and arsenic (normalized to iron and average crustal rock) throughout the sampling period at Chacaltaya dependence is obtained for nearly all the AEEs measured. Figure 2 shows the data obtained for arsenic. Table I summarizes the results for a number of AEEs that were systematically detected in both data sets. The percent probability, P,, that a correlation coefficient as large or larger than r would result if the variables were uncorrelated is also indicated. For the South American site a number of other elements (Cr, Ag, Cd, In, Ba, Sb, W, Au, Pb) follow Equation 3 with a probability of noncorrelation P , < 0.01%. The explanation of the systematics detected may be different a t both locations. The air a t Chacaltaya is representative for true remote conditions, as is proven by the very low concentrations of vanadium, nickel, and lead, which are considered to be of predominantly anthropogenic origin (10).The concentrations for these elements are among the lowest recorded. A considerable variation of the total suspended particulate concentration is the result of washout by rain and especially snow during the wet austral summer period. A t Jungfraujoch, variations are much less systematic throughout the year and may be affected by long-range transport of pollutants from the industrialized European regions and by a nonnegligible marine influence. Even then, the variance on both Cminand EFdisp obtained a t both sites is low. EFdisp re1004
Environmental Science & Technology
concn stages 5 prn)
+8
0.28 f 0.19 0.42f 0.19 0.75 f 0.84 0.60f 0.28
L
3.3195
2.238
"'Fe
o\;"
BY
Cmi ng prn-9' STP
m 5
3.130
s.2615 I IC
Fe
Figure 2. Enrichment factor of arsenic as a function of the reciprocal concentration of the normalizing element (left, Chacaltaya; right, Jungfraujoch) flects a remaining enrichment in the coarse particulate aerosol fraction. The differences of this parameter may be explained by pollution of the environment in Western Europe. I t is noteworthy that, although much larger anthropogenic influences should act upon the European aerosol, some Cminvalues are higher at Chacaltaya. This could point to larger trace metal fluxes a t the equator than at high latitude locations in support of an hypothesis put forward by Lantzy and Mackenzie (10). Thirty-six cascade impactor samples were taken a t the Chacaltaya sampling site between September 1977 and November 1978. Sampling was performed with a Batelle-type impactor (20) (flow rate 10 L min-l) during 7 - to 14-day periods, and a number of constituents were determined in the different stages. The results allow a test for the applicability of the assumptions. Table I1 summarizes the average enrichment factors obtained in the coarse particulate impactor stages for Cu, Zn, As, and Pb, and compares the results with EFdiSp.The standard deviations on the cascade impactor measurements reflect the temporal variation of the concentrations superposed on an analytical uncertainty which is of the order of 30-50%. EFdisp and the enrichment factors obtained for the large particulate fractions agree reasonably well; the uncertainty on EFdisp is for all elements considerably lower than for the one derived from cascade impactor samples. In Table I11 Cmina t Chacaltaya is compared with the av-
Table IV. Comparison of Cminat Chacaltaya with Concentration at South Pole ( 72) for Atmophile Elements element
Cr
co cu Zn AS
Se Ag
Cd In
0a Sb
w Au Pb
Cmin,Chacallaya, ng mP3 STP
concn at South Pole (X43, see text)
0.45 f 0.08 0.085 f 0.018 0.21 f 0.034 1.6 f 0.14 0.85 f 0.08 -0.03 0.076 f 0.022 -0.02 0.0072 f 0.001 -0.5 0.1 1 f 0.03 -0.08 0.0037 f 0.0006 1.30 f 0.16
