NOTES
Some Observations of Organic Constituents in Rain above and below a Forest Canopy William A. Hoffman, Jr.'*, Steven E. Lindberg, and Ralph R. Turner Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830
Some organic components of rain have been determined in samples collected on an event basis above and below the canopy of a deciduous forest. Plasticizers and chlorohydrocarbons that were identified were attributed to sources external to the vegetation. Organic acids and high molecular weight "waxy" components identified were attributed to the foliage itself. Approximately equal amounts of material were ascribed to each of these sources. Low molecular weight organic acids were not found generally. Event sampling is advocated as the best approach to use for identifying sources of organic components in rain reaching the forest floor.
Forest canopies can be important sites of deposition for atmospheric gases and particulates. During dry weather, the forest canopy may exude components that accumulate on leaf surfaces along with dry-deposited atmospheric constituents. These accumulations are subsequently washed off, perhaps only partially, by precipitation events which follow. Rain, collected under a forest canopy (throughfall), may therefore contain constituents from several sources, among them the tree or leaf interior, the external environment, and the incident rain itself. Forest canopies are thus sites of physical and chemical interactions that affect the composition of rainfall which ultimately reaches a forest floor. A number of components have been identified and measured in precipitation both above and below foliar canopies, notably essential elements, acidity, trace metals, and organic carbon. Despite low concentrations and the attendant analytical difficulties, it is clear from the variety of studies that have been undertaken (1-5) that this quantitative approach is crucial to an understanding of the complexities of rain-leaf interactions. When we recently studied precipitation chemistry in this laboratory (6),individual event sampling of wetfall alone was found to be the best means to help clarify the mechanisms of foliar exchange with rain, since bulk sampling usually incorporates averaging over long time periods and many different kinds of deposition processes. A major conclusion of that study was that weak acids were a minor part of the organic material present in rain. The present study was designed to identify as much of the organic material as possible and to define its relationship to deposition and leaching phenomena. This research was a part of ongoing deposition studies a t the Walker Branch Watershed on the Oak Ridge National Laboratory Reservation in eastern Tennessee (7). Experimental Rain samples were collected on an event basis from August 1977 to June 1978. Lindberg et al. ( 7 , 8 ) described the Sam-
pling sites, sampling procedures, and the precipitation collectors (HASL type, wetfall only) used. Samples from above (incident rain) and below (throughfall) the forest canopy (Quercus prinus L., chestnut oak) were collected for 43 rain events representing >90% of the precipitation volume that occurred during the sampling period. Samples were retrieved within 24 h after a rain event and brought directly to the laboratory. An initial 10-mL aliquot was taken for analyses of pH and total acidity, which are reported in an earlier study (6).The remaining sample was filtered through a fresh plug of prewashed glass wool, and a second 10-mL aliquot for the analysis of dissolved organic carbon (DOC) was taken if there was sufficient volume. Dissolved organic carbon samples were stored a t 2 "C and analyzed using standard methods (9, 20) within 14 days of collection. If the remaining sample had a volume 1 1 5 mL, it was immediately extracted for 12 to 18 h. Extraction recoveries were made using 25 to 30 mL of redistilled dichloromethane (CHZC12) under reflux in a glass continuous extractor of our own design ( 2 2 ) and were compared with sample blanks. Extracts were allowed to evaporate to dryness a t room temperature (-22 "C), weighed as a measure of "organic content", and redissolved in 100 pL of CHZC12, from which 2 to 8pL was injected into the chromatograph. Sufficient volume (20.5 cm measured precipitation) was obtained in 24 of the 43 rain events to perform an organic analysis. For 15 events, paired analyses of the organic content in incident rain and throughfall were obtained by determining both DOC and the CHzClz-extractable material. Replicate analyses for DOC on one hand and extract recoveries on the other did not vary significantly using glass and polyethylene containers, when both types of containers were thoroughly washed, rinsed, and stored with double-distilled water. Spectra of extraction isolates were taken with commercial instruments: IR (Beckman IR-91, NMR (Varian TF-80), UV-fluorescence (Perkin-Elmer MPF-43). Gas chromatograms were obtained with a Perkin-Elmer 3920B FID instrument equipped with a 3 m X 0.32 mm column packed with 3% Dexsil300 on 80/100 H P Chrom G. The column was programmed from 60 to 356 "C at 4 "C/min with a 4-min initial temperature hold using helium carrier gas a t 40 mL/min. Other chromatograms were obtained with the same instrumentation using a 2 m X 0.32 mm column packed with Tenax-GC, programmed from 0 "C (using dry ice) to 200 "C a t 2 "C/min with a 4-min initial hold using a 30-mL/min helium carrier flow. The GC-MS data were obtained using an interfaced PE-3920/Du Pont 490B instrument pair equipped with a Hewlett-Packard 21-094B data system. The N,O-bis(trimethylsi1yl)trifluoroacetamide (BSTFA) was obtained from Pierce Chemical Co., Rockford, Ill., and direct silyl esterification was employed using recommended procedures. Results and Discussion
Permanent address, Department of Chemistry, Denison University, Granville, Ohio 43023. 0013-936X/80/09 14-0999$01.00/0
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1980 American Chemical Society
The interception of incoming rain by a fully foliated forest canopy produced a large increase in the organic content of Volume 14, Number 8, August 1980
999
Table 1. Comparative Analysis of Organic Content in Incident Rain and Throughfall for 15 Sample Pairs a incident
‘i
throughfall
leaves, 8 samples DOC, mg/L CH&Iz, mg/L
linear corr coeff regression line slope no leaves, 7 samples DOC, mg/L CHzClz, mg/L
linear corr coeff regression line slope
1.0 f 0.26
0.41
6f2 9&6 0.96 ( P I0.001) 0.55
1.5f 0.2 1.4f 0.4 0.62( P 5 0.01) 0.31
2.6f 0.5 3fl 0.68( P I 0.05) 0.32
1.4f 0.5 0.55 ( P 5 0.05)
a September 1977-May 1978. Mean, standard error. Correlation coefficient between concentration of DOC and concentration of material extracted by CHzCiz.
resultant throughfall compared to the organic content of incident rain. A direct quantitative comparison between dissolved organic carbon (DOC) analyses and extraction recoveries in incident rain and throughfall is indicated in Table I. In the absence of leaves the organic content of throughfall decreased substantially, although it remained above that of incident rain. Material recovered from dichloromethane extracts of incident and throughfall samples followed this pattern as well. The organic content of incident rain samples generally appeared to be unaffected by seasonal factors. A number of factors complicate a direct comparison between DOC and extraction recovery values. Highly polar carbon compounds, such as quaternary amines and saccharides, are unlikely to extract, which, coupled with the loss of compounds of high volatility, may lead to low recovery values. On the other hand, direct mass recoveries by extraction include all elements present in the compounds isolated and would tend to give higher values than DOC. A representative chromatogram of a throughfall extract is shown in Figure 1.Numbered peaks in the chromatogram are partially identified in Table 11. Some variation in the concentration of individual components from sample to sample was observed; however, all throughfall extracts were generally similar in composition. Constituents found in extracts of incident rain samples were invariably present in lower concentrations than those found in comparable throughfall. Some peaks, however, particularly 13 and 15, and occasionally 11, were found in incident rain samples in significant amounts. Virtually all throughfall constituents were observed a t some time in incident rainsample-extract chromatograms. Excepting dioctyl phthalate (15) and dioctyl adipate (13),however, they were generally a t or below detection limits. If an absolute detection limit of 1 ng is assumed based on instrument specifications, no component below -0.5 pg/L in the rain could be identified. For the chromatograph shown in Figure 1,small peaks represent compounds at -1-5 pg/L, while larger peaks represent concentrations of 10-50 pg/L. The extracted material in throughfall samples proved to be largely aliphatic in character. Infrared absorptions associated with aliphatic C-H bonds were found a t 2965,1445, and 950 cm-’. Ester or ether C-0-C linkages were inferred from a weak to moderate absorption a t 1260 cm-l. Weak absorptions attributed to 0-H (with some hydrogen bonding) and carbonyl C-0 were observed at 3250 and 1665 cm-l, respectively. Very weak absorptions a t 1620 and 670 cm-l may be related to the presence of respective olefinic and C-S bonds. Samples in dichloromethane run against solvent blanks showed a weak 1000
Environmental Science & Technology
0
40
20
30
40 TIME (mini
50
60
70
80
Figure 1. Gas chromatogram of a typical throughfall sample: (-)
normal chromatogram;(. BSTFA; (- - -) blank
-
a)
chromatogram after sample treatment with
absorption in the ultraviolet a t 289 nm and a corresponding weak fluorescence emission at 330 nm, indicating very limited conjugate character. A relatively strong NMR signal a t 1.20 ppm was attributed to methylene hydrogens (CH2) and an extremely weak NMR signal a t 7.5 ppm to some aromaticity. No other NMR signals were observed. These spectra do not provide definitive support for aliphatic character when viewed individually, but together they provide a persuasive consonance. The throughfall constituents designated in Table I1 were identified by comparison of experimental mass spectra with reference spectra given by Stenhagen et al. (12) and, where available, with spectra of reference compounds. Octanoic acid and decanoic acid were the only organic acids found, and of these only octanoic (or isomeric CS acids) appeared consistently. Only dioctyl phthalate was present in all samples, incident and throughfall. Dioctyl phthalate and dioctyl adipate (or their 2-ethylhexyl isomers) are widely used plasticizers (13) and have been distributed in the environment for over 20 years (14). Dioctyl adipate was frequently present but was not usually predominant. Regularly spaced peaks (Figure 1, 17-25) were noted a t the high-temperature end of the chromatogram, particularly in autumn throughfall samples. In the spring, throughfall chromatograms were much simpler, traces of the high-temperature series were rarely seen, and fewer different low concentration components were observed. The 17-25 component series appears to be comprised of homologues that have much the same appearance in the chromatogram as some of the components identified by Lunde et al. (15)as hydrocarbons. An n-Cso hydrocarbon standard eluted at the approximate position of dioctyl phthalate on the chromatogram. Low-intensity, persistent, mass to charge ratio ( m l e ) fragments were observed of mass 33 (HS) and mass 47 (CHBS)in the mass spectra of the peaks of the series, which strongly suggest the presence of sulfur. Mass spectra of the series do provide a classic branched-chain hydrocarbon fragmentation pattern, however. The repetitiveness of the sulfur and hydrocarbon fragment patterns provides evidence that the components may be members of a homologous series, -(S)R(CHZ)n-, -(S)R(CHz)n+l-. Several other distinctive features of the mass spectra of this series are: (a) a consistent hydrocarbon fragment maximum a t C6 (m/e 85) and another hydrocarbon fragment maximum that “grows” from CS ( m / e 127) to about Cl2 (mle 169) in the series; and (b) a subseries of hydrocarbon mass fragments that begin at m/e 83 and continue by 14 additional mass units with each series member. The nature of functional groups within this molecular series is not clear, and the presence of simple
Table 11. Mass Spectral Data peak no.
max mass obsd
100% d e a line
2 3
179 162 222
83 83 83"
4
21 1
133'
5
144 254
60 97
9
172 245 274
60 167 125
10 11
223 305
109 193'
12
303
193'
13 14 15 16 17
24 1 344 279 279 300
129 111 149 149
18
300
85
19
35 1
141
20
407
141
21
379
155
22
379
169
23
42 1
155
24
393
169
25
393
155
1
6 7 8
a m / e , mass to charge ratio. character.
85
in order, next most Intense d e
93, 71, 100, 125, 59, 165 93, 95, 70, 55, 110, 139 147*, l l l ' , 69, 99, 192, (36) 169', 107*, 143, 109, 56, (36) 73, 43, 41, 27, 55, 101, (45) 133*, 169', 146, 111, 56, 61, (36) 73, 43, 41, 57, 129, (45) 165, 143, 145, 107, 85 173', 137+,lOl*, 208*, 231, 65, (36) 69, 95, 83, 123, 153, 135 159', 230*, 267*, 123", 83, (36, 38) 207, 159', 123", 147, 111, 83, (36) 112, 147, 70, 57, 83, 101 57, 85, 71, 99, 221, 281, (33) 167, 113, 57, 71, 85, 99 113, 104, 123, 193 127, 71, 99, 141, 113, 57, 155, (33) 127, 141, 99, 155, 113, 7 1, 57, (33, 47) 127, 113, 115, 71, 85, 57, 169, (33, 47) 155, 169, 127, 133, 183, 85, 99, (33, 47) 141, 127, 113, 169, 183, 85, 71, (33, 47) 155, 127, 141, 183, 85, 57, 71, 99, (33,47) 169, 141, 183, 197, 85, 57, 113, 71, (33, 47) 155, 183, 197, 141, 85, 113, 71, (33,47) 169, 183, 197, 141, 85, 71, 57, (33, 47)
idenliflcalionlremarks
terpenoid?, linalool derivative? terpenoid?, furan related? chlorinated cyclohexane derivative?, 147 RCIp?, 83 CHClp chlorinated HC; 207 RCI4, 169 RC13, etc., 36 HCI
octanoic acidC chlorinated HC; RC12, etc.
169 RC13, 133
decanoic acid unidentified chlorine containing, at least RCI4 unidentified
isodrin?
11
+ CH2, isodrin homologue?
dioctyl adipate, (2-ethylhexyl)? branched HC, see series below dioctyl phthalate, (2-ethylhexyl)? unidentified, contaminated by peak
15?
homologous series (a) mle 33, HS; rnle 47, CHBS
(b) mle 14, branched HC chain (c) subseries masses at 83, 97, 111, . suggest cyclohexane
..
+ +CH2
postulate-branched-chain thioether with cyclohexyl group on chain?
An asterisk indicates the compound is part of a characteristic chlorine isotope group. Italics indicate confirmation of chemical
branched-chain hydrocarbons cannot be excluded; however, hydrocarbons would have to be > C ~ or O be more polar organic compounds to account for the retention times observed in the gas chromatogram. The persistence of masses that are difficult to assign to fragments other than ones containing sulfur suggests that one cannot eliminate more complex structural possibilities. Anthropogenic sources of this group of compounds are not apparent to us. Further, the increase in concentration of these compounds during leaf senescence suggests a biological origin, perhaps the cuticular wax. Some terpenoid-like materials and highly chlorinated compounds were observed in extraction isolates also. Mass spectra of the chloro compounds did not coincide with those of halogenated pesticides known to be used in small quantities in the area, Le., endosulfan, dieldrin, aldrin, and heptachlor. However, compounds very similar in spectra to isodrin, and an apparent homologue, were observed. Terpenoids are common atmospheric components derived from coniferous vegetation in the southeastern U.S. ( 5 ) and, in consequence, might be expected in low concentrations. Both chloro compounds and terpenoids seem likely to come from external sources rather than from the deciduous trees themselves.
N,O-Bis(trimethylsily1)trifluoroacetamide (BSTFA), a powerful derivatizing reagent that reacts with most active hydrogens, including acids, phenols, and alcohols, produced little change in derivatized constituent chromatograms. Treatment of the representative throughfall sample (Figure 1) with BSTFA brought three new "acid" products (31-33) into elution range and shifted peaks attributed to C8 ( 5 , 5a) and Clo (7, 7a) acids. BSTFA adducts of palmitic (c16)and stearic (CIS) acid standards eluted ahead of the new peaks. The new "acids" are therefore of higher molecular weight and/or of greater polarity than these reference compounds. The components may be related to the C16 and CIS trihydroxycarboxylic acids that Martin and Juniper (16)associate with the leaf cuticle. We previously discussed the possible degradation of these compounds in rain-leaf equilibrations (6). A single throughfall sample taken from a very small volume spring storm (-0.2 cm precipitation) was injected directly onto a 2-m Tenax-GC column and gave a series of responses corresponding to Cq, Cg, Cg, Cg, (210, and Clz acids. Retention times of (reference) normal C g , Cs, and Clo acids were confirmed on the same column, and the addition of an n-C8 spike Volume 14, Number 8, August 1980
1001
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
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