Environ. Sci. Technol. 1985, 19, 238-244
Aggett, J.; Roberts, L. S.,unpublished data, 1983. Reay, P. F. “Proceedings of the Pollution Research Conference, Wairakei, New Zealand June 1973”;New Zealand Government Printer: Wellington, New Zealand, 1973; p 315. Aggett, J.; Aspell, A. C. N. Z. J. Sci. 1980, 23, 77. Reay, P.F. J. Appl. Ecol. 1972, 9, 557. Fadrus, H.; Maly, J . Analyst (London) 1975, 100, 549. Aggett, J.; Aspell, A. C. Analyst (London) 1976,101,341. Aggett, J.; Kadwani, R. Analyst (London) 1983,108,1495. ”Bathymetric map of Lake Ohakuri”. New Zealand Oceanographic Institute, 1976. Magadza, C. Ph.D. Thesis, University of Auckland, Auckland, New Zealand, 1973.
(12) Davison, W. Nature (London) 1981,290, 241. (13) Stumm, W.; Morgan, J. J. “Aquatic Chemistry”, 2nd ed.; Wiley: New York, 1981. (14) Andreae, M. 0.Limnol. Oceanogr. 1979,24, 440. (15) Holm, T. R.; Anderson, M. A.; Stanforth, R. R.; Iverson, D. G. Limnol. Oceangr. 1980,25, 23.
Received for review January 3,1984. Revised manuscript received J u n e 5,1984. Accepted August 23,1984. This work was supported by a n award of aPost Graduate Scholarship to G. 0.from the New Zealand University Grants Committee and by the New Zealand Department of Scientific and Industrial Research for field work.
Field Measurements of Particle Dry Deposition Rates to Foliage and Inert Surfaces in a Forest Canopy Steven E. Llndberg” and Gary M. Lovett
Environmental Sciences Division, Oak Ridge Natlonal Laboratory, Oak Ridge, Tennessee 37831 We measured dry deposition rates of particulate SO4”, NO3-, Ca2+, and K+ to leaves and inert surfaces in the upper canopy of a deciduous forest at Walker Branch Watershed, TN. During eight dry periods of several days duration each, polycarbonate Petri dishes were exposed adjacent to growing leaves in a chestnut oak tree at heights at 15-20 m above the ground. Following exposure, plates and leaves were extracted in the laboratory to determine surface accumulation of the above ions. Studies of dissolution kinetics were used to determine optimum extraction conditions for foliage. Mean dry deposition rates to leaves were as follows (pg m-2 h-l): SO:-, 48 (SE = 11); NOs-, 5.7 (1.6); Ca2+,15 (2); K+, 5.6 (2.6). These values are subject to large uncertainties because of considerable spatial variability in leaf surface concentrations. Deposition rates to inert surfaces were generally higher, except for SO:-. Mean deposition velocities to these inert surfaces ranged from 0.13 (0.02) for SO:-to 1.1(0.1) for Ca2+. Introduction There are no widely accepted methods for routine measurement of dry deposition of polydisperse aerosols to forest canopies in complex terrain. Micrometeorological methods are strongly advocated ( I ) but are difficult to apply routinely (2,3) and do not result in a direct sampling of the surface of interest. The need for direct analysis of environmentalsurfaces for accumulated dry deposition has been often cited as a major research need ( I , 3 , 4 ) . Previous research suggests that some inert surfaces can provide useful estimates of particle deposition; however, these may or may not be related to the flux to natural surfaces (4-6). Both of these approaches have their advantages and disadvantages ( I ) . The major problems of micrometeorological methods are an inability to directly sample the actual surface to which deposition has occurred and a limitation in space and to a very small fraction of the particle-borne constituents for which dry deposition may be important (3,6,7). Inert surface methods address these problems to some extent. They involve a reproducible surface that can be readily used in large spatial arrays to develop an understanding of deposition processes within canopies and that can be extracted for analysis of a wide range of deposited compounds (6-8). The critical problem with this approach is relating the deposition to inert 238
Envlron. Sci. Technol., Vol. 19, No. 3, 1985
surfaces to that to actual foliage ( I ) . Hence, there is a need for development of methods for direct analysis of foliage for dry deposition ( I , 3) and for a comparison of particle accumulation rates on inert and foliar surfaces (5). Research into the detailed features of mass transfer of particles to plant canopies requires direct in-canopy sampling. Data on the distribution of deposited material on surfaces within canopies can provide useful information on deposition processes for the interpretation of micrometeorologicalmeasurements. Similarly, surface studies on a forest stand scale utilizing natural processes within the canopy such as dry deposition washoff by precipitation may be useful in providing limits on dry deposition rates for comparison with micrometeorologicalresults. We have previously described a model for the estimation of whole canopy dry deposition rates based on the chemistry of rainfall collected above and below a deciduous forest (9). This paper describes the development and application of a method for the extraction and analysis of dry deposited material accumulated on individual growing leaves and a comparison of dry deposition rates of particulate SO:-, NO3-, K+, and Ca2+to adjacent leaves and inert surfaces exposed simultaneously to ambient dry deposition. The research was performed at the Walker Branch Experimental Watershed, a mixed oak-hickory forest located in ridge/valley terrain in rural eastern Tennessee, within 22 km of two coal-fired utility power plants (IO). Although our observations are limited and interpretation of field measurements of dry deposition in general is inadequate, we present our results to describe the problems with such approaches and to encourage further research in an important area where few observations exist. A preliminary analysis of a portion of this data has been presented (II), and a second paper in preparation will consider meteorological effects on spatial and temporal trends in deposition to surrogate surfaces in this same canopy over a 2-year period. Methodology Experimental Design. From July 1981 to October 1982, deposition measurements were made to foliage and inert surfaces in a mature chestnut oak canopy (Quercus prinus) at a ridgetop site [330 m mean sea level (MSL)]. Average canopy height at the site was 19-20 m, and data were collected at four points: 15.2 m high at SW orien-
0013-936X/85/0919-0238$01.50/0
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0 1985 American Chemical Society
TlMElminl
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Flgure 1. Dlssolutlon kinetlcs of several Ions from particles deposited on Inert plates and on leaves.
tation from the tree trunk; 18.0 m, NW orientation; 18.3 m, SE orientation; 19.8 m, NE orientation. At each point two inert surface deposition plates (described below) were situated adjacent to living foliage near ends of branches, suspended in free air on a support clamped to the branches (ref 11; see also Figure 2), or attached to a meteorological tower extending above the canopy. There were no branches or leaves directly above the inert surfaces or above the leaves which were collected for analysis. Access to each point was accomplished with minimal canopy disturbance by using a hydraulic lift vehicle. During the two successive summers we completed eight leaf extraction experiments (232 leaves sampled) and 24 inert surface experiments (110 plates exposed). The leaf experiments were concurrent with eight of the inert surface experiments (53 plates) and ranged in duration from 51to 166-h duration (mean 100 h), for a total of a 33-day exposure, all during dry weather. The separate inert surface collections ranged in duration from 81 to 340 h (mean 140 h) for a cumulative exposure of 132 days, or 20% of the total period. These longer duration inert surface experiments experienced precipitation or fog for brief periods during -40% of the experiments, but the surfaces were kept dry as described below. During 15 of these experiments we also sampled suspended particles by air filtration through 0.4-pm Nuclepore filters in stainless steel holders. Replicate collectors were positioned at 19.8 m, facing into the prevailing wind with a 16-cm2opening through which air was pumped at a rate of -3 L/min. Air filters were extracted in distilled deionized water for 2 h on a reciprocating shaker, and the extract was analyzed for S042-and NO3- by standard colorimetric autoanalyzer methods and for Ca+2and K+ by flame atomic absorption. Sequential extractions confirmed the completeness of the 2-h leaching procedure. Foliage. Estimation of deposition from foliar analysis is a difficult task because of the natural variability of both deposition rates and leaf surface characteristics and the possible flux of many elements into and out of the foliage (2). We developed an extraction technique designed to distinguish between surface-deposited and internal material based on kinetics of dissolution of several ions into distilled water from inert and foliar surfaces similarly exposed in the forest canopy. The results of several extractions are shown in Figure 1. Leaching of all ions from particles deposited on inert surfaces was rapid, with constant concentrations attained within 1-4 min. The kinetics of dissolution of Ca2+and NO3- from leaves were similar to dissolution from inert surfaces, with stable levels reached within 3 min. Calcium and nitrogen are generally considered to exist in foliage predominantly in immobile forms (12). However, and K+, which exist in mobile forms in foliage (13,14), exhibit notably different kinetics of dissolution from leaves. The rapid initial increases in concentration occurred, as for the inert surface extractions, but were followed for several hours by continued increases in concentration at slower rates. These subsequent increases in concentration
may represent internal material that enters solution once the leaves have been sufficiently wetted. In contrast, the rapid initial increases in concentration may represent element dissolution primarily from surface-deposited particles and gases. Further kinetics experiments were done with 10 deposition plates collected over a 1-year period, 20 upper-canopy chestnut oak leaves collected in June and July 1981, and 20 leaves collected in May 1982; results were similar to those shown in Figure 1. We conclude that for these four ions, 3-min extractions result in efficient removal of surface-deposited constituents while introducing limited internal material to solution. Dry deposition to foliage was estimated from the net change in the concentration of a given ion extracted from leaf surfaces exposed during dry periods of several days. Because of spatial variability, estimation of mean surface concentrations and their variances at the beginning and end of a dry period required collection and analysis of at least 10 leaves and a 2-day exposure to provide a sufficient quantity of deposited material for analysis. It should be noted that, because of natural variability and the other problems mentioned above, this technique was not always successful; -30% of our estimated dry deposition fluxes to leaves were not significantly different from zero (see later discussion). In practice, we used a hydraulic lift to access the upper canopy of the chestnut oak, where we collected 16 leaves (blade plus petiole to the abscission layer), four each near each inert surface sampler. Each leaf was placed in a separate 200-mL polypropylene bottle by using plastic gloves, returned to the laboratory within 30-60 min, and extracted on a slowly moving, reciprocating shaker in 50 mL of distilled, deionized water for 3 min. Extracts were analyzed as described earlier, and leaf surface areas were measured by a light attenuation method. Dry deposition was calculated from the difference in extracted ion concentrations (pg mL-‘) before and after the dry period, expressed on a single-surface leaf area basis (pg m-? and divided by the duration of the period b g m-2 h-9. Because deposition was calculated as a difference, a minor amount of consistent internal leaching should not have strongly biased the results, although it may have influenced the variability as discussed below. Inert Surfaces. Although the “bucket”commonly used for collection of dry deposited particles has several disadvantages (1, 2), it offers the important advantage of automatic protection from rainfall. Unattended field operation is necessary for experimental application of dry deposition collectors in areas of frequent precipitation or if such samplers are ever used in a monitoring network. We developed an inert surface sampler that minimized the aerodynamic effects of the large side-to-bottom surface area ratio of the bucket sampler and that was contained in a semiportable housing that provided automatic protection from rain (Figure 2). The inert surface was a polycarbonate Petri dish of 9.4 cm i.d. with a 1.3-cm rim, which was chosen based largely on chemical criteria. While this small-rimmed configuration did not eliminate possibly influential turbulence over the surface (151,this design provided several advantages: low substrate background for trace and major elements in deposition; minimized contamination during handling and in-canopy mounting due to presence of the rim, the outside of which was not analyzed; ease of efficient extraction by using a stirred water column, as described below; ready availability at low cost. In addition, the small rim precluded dew and fog drip, even small amounts of which result in loss of soluble components from rimless Environ. Scl. Technol., Vol. 19,
No. 3, 1985 239
Table I. Dry Deposition Rates ( r g m-2 h-l) Estimated from Extraction of Surface Material from Sequentially Collected Oak Leaves and Measured to Adjacent Inert Surfaces: All Surfaces Exposed in the Upper Canopy of a Mature Forest
NO,
5042expt
leaves
1 2 3 4 5 6 7 8 1-8' GS' ISILd
28, 43a 13, 14 50, 17O -8, 18 88, 46O 48, 14e 35, 8e 20, 5e 48, lle 0.66, 0.10
inert surface
leaves
inert surface
leaves
24, 4" 53, 4 41, 2 29, 3 33, 5 45, 7 20, 3 12, 2 30, 6 44, 3
20, 2 2.5, 1.5O 5.3, 2.ge 1.7, 0.4O 9.6, 1.3* 0.3, 0.4 2.7, 2.7 4.1, 1.5O 4.7, 1.6
11, 4 8.4, 1.6 21, 2 11, 1 4.4, 1.8 2.2, 2.2 13, 0.6 11, 1 13, 2 14, 1
-0.8, 3.1 1.0, 1.8 9.6, 2.5e 0.8, 0.4e 0.5, 0.6 6.5, 3.5O -0.5, 1.3 -0.1, 0.7 5.6, 2.6
3.1, 0.8
1.1, 0.5
K+ inert surface 1.0, 0.5 1.8, 0.3 2.8, 0.9 1.6, 0.8 2.5, 0.7 6.6, 2.9 1.8, 0.8 1.5, 0.4 3.7, 1.5 5.4, 1.1
leaves 12, 3e 17, 5e 22, 9e 9.0, 1 3 12, 2e 9.3, 6.8O 23, 4e 14, 3e 15, 2
Ca2+ inert surface 38, 5 19, 3 37, 6 16, 4 20, 1 22, 3 26, 4 13, 1 25, 2 23, 1
1.7, 0.3
"Mean and standard error; n = 32 leaves and five to eight plates per experiment, except for experiment 2 which included eight leaves and eight plates. *Means and standard errors calculated from all paired leaf and inert surface experiments for which leaf deposition values were significant (designated by e; see footnote below). Hence, n = 5 for Sod2-,n = 6 for NO3-, n = 3 for K+, and n = 8 for Ca2+. CGrandmeans calculated from all growing season (GS) data for inert surfaces (24 experiments, 110 samples). dMean ratio of inert surface deposition to leaf deposition for paired experiments with significant leaf deposition values. e Leaf deposition value significantly different from zero (p < 0.05). All values for plate deposition are significant, as are the overall means.
Two inert surfaces were exposed in each automatic collector for generally dry periods, and the moisture sensors were adjusted to close the samplers during all rain, fog, and heavy dew events. No such events occurred during the leaf/inert surface comparison experiments. On several occasions one of the two inert surfaces was exposed upside down to provide information on flux of nonsedimenting particles. Following exposure, all inert surfaces were sealed and returned to a laminar flow clean bench for extraction and analysis. Surfaces were extracted with 50 mL of distilled, deionized water by using Teflon stirrers to keep the particles in suspension for 30 min (as discussed earlier).
Flgure 2. Apparatus for exposure of deposltion plates in forest canopy with automatic protection from rain.
surfaces high in the canopy. As discussed below we have compared dry deposition rates measured to adjacent rimmed and rimless surfaces in a grassy field (16);however, similar comparisons in the more turbulent flow near the forest canopy have not been done. In the forest canopy an electronically operated rain shield (30 X 18 cm) was positioned 4 cm above the inert surfaces in the closed (rain-protected) position. In the open position the horizontal distance between these surfaces and the cover was 50 cm, assuring a minimal effect of the cover on the airflow over the surfaces. In addition, the samplers were positioned so that the rain shield in the open position was within the leafy canopy as illustrated elsewhere (11). Electronic control of the rain shield proved to be a useful alternative to the fixed shield design (8) which was not readily adaptable to use in a forest canopy. Fixed shields may limit particle dry deposition and do not effectively exclude blowing rain (16). Our samplers sensed tainfall with a heated resistance grid. In the dry position, the shield was retained away from the inert surfaces by a linear-drive motor (as shown in Figure 2). When the first rain drop was detected by a change in sensor resistance, power to the motor was interrupted, allowing a constant-tension spring to return the rain shield to the closed position. The covers were retracted by the motor when rain ended. Because most rain events, in general, are preceded by scattered raindrops, the 70-cm2 moisturesensing grid provided excellent protection for the dry deposition plates. The sensing circuit also controlled a timer for recording the duration of the dry period. 240
Environ. Sci. Technol., Vol. 19, No. 3, 1985
Results Dry deposition rates estimated from the eight paired leaf/inert surface experiments are presented in Table I. The results of 24 inert surface experiments are also summarized in Table I as mean values representative of the forest growing season. Deposition rates estimated from leaf extractions are subject to large uncertainties, resulting from the high standard errors of the means of individual experiments (Table I). Two of the eight estimates of dry deposition to forest leaves for Sod2-(experiments 1 and 2) and K+ (experiments 2 and 5), although positive, are not significantly different from zero because of the variability in surface concentrations of the sets of 16 leaves collected before and after each experiment. In addition, three of the estimates of K+and one of SO>-flux suggest a decrease in surface concentrations over the course of the experiment, although the resulting negative fluxes are not significantly different from zero. It was not uncommon for the standard deviations of the mean leaf surface concentrations for a given set of 16 leaves to exceed the mean. For 61 sets of leaf analyses of 16 samples each the coefficients of variation (CV) exceeded 100% on 10 occasions, while the median CV was 50%. We have previously described comparable variability of leaf surface concentrations of trace metals; the CV's for Cd and Zn were on the order of 100% and were independent of the spatial scale over which the leaves were collected (10m2-103M) (7). Because of this degree of variability we were not able to successfully analyze the leaf deposition data for the four individual collection sites sampled during each experiment (four leaves at each site from 15-20 m above the ground) but had to pool all 16 leaves to describe more precisely the mean leaf surface concentration at any point in time. Significant influences of canopy position
DRY DEPOSITION RATES (pq ni' h-') 50
$33 e Z3 k20 w u)
2
' L o4
O0 0 NOso:
F
0 0
to
K+
-0
0 0
20
40 60 LEbVES
BO
F l p n 5. Scatter pkt of &pmMm-t m m & m-' h-') estimated Iran seqmnthty &acted leaw and fran adkmnt hwnt sufacss exposed simultaneously In me upper canopy of an oak fwest. The linear regression f a all p i n t s has an r 2 valw of 0.42 (p < 0.01). and the equation of line is Y = 12 0.40X.
+
on particle deposition were apparent from our inert surface data (17)and will be described in detail in another paper. Our results indicate reasonable similarity in mean deposition rates to leaves and inert Surfaces, consideringthe complexities discugsed above. For the paired experiments for which the leaf deposition ratea were significant (starred values in Table I), deposition rates estimated from forest canopy leaves averaged 48,5.7,5.6,and 15 pg m-2 h-', for SO,", NO3-, K*, and Ca2+,respectively, while deposition rates measured to inert surfaces adjacent to the collected leaves averaged 30, 13, 3.7, and 25 pg rn+ h-' for SOIZ-, NO3-, K+, and Ca2+,respectively. For comparison, the individual deposition rates measured to simultaneously exposed leaves and inert surfaces are plotted in Figure 3 for each ion analyzed,along with the overall regression line. The linear regression of inert surface deposition on leaf deposition is significant only when the data for all ions are combined (f= 0.42,p < 0.00,because of the variability in the data and the limited number of paired observations for individual ions. This regression is not beneficially influenced by the point at X = 88 and Y = 33 m-2 h-'. While we have no reason to suspect this highest leaf flux value, its deletion improves the overall regression f to 0.54. Without this point,the regression for the $40;- data alone is significant (f= 0.91,p < 0.05);with this point the f decreases to 0.30. The f values for the other ions range from 0.04 for K+ to 0.15 for NO; and Ca2+. It is apparent that a more extensive data set is required to quantify any possible relationships between dry deposition rates measured to both surfaces. Discussion Several factors account for the variability in deposition rates to foliage. The spatial variability of element concentrations on leaves (mean CV = 70%) is higher than that for similarly exposed inert surfaces (mean CV = 40%), because of the variability in leaf surface morphology and orientation in the canopy. Another complication is the large internal pool size of these elements in foliage. For chestnut oak we measured the ratio of internal/surface element content as ranging from 50 to 5ooo. Some fraction of this internal pool of Sol2-and K+, in particular, may be readily leached from the foliage upon wetting (24). Note that, for the deposition rates to leaves shown in Table I, the relative standard errors of the means are smaller for Ca2+and NO; than for K+ and SO;?-, in accordance with our expectations of the mobility of these ions in foliar tissue, as discussed earlier. Although we believe that the 3-min extraction dissolves primarily surface material, these
Figure 4. Scanning electron photomicrographs of particles deposlied on chestnut oak (Ouercus prlnus) leal (upper two photographs) and
inert deposition plate (lower two photographs) exposed in the upper canopy. Scale line furthest to tth3 left in the lower right-hand comer of each picture is as follows: 10 pm on both left-hand pictures and 1.0 itm on both right-hand pictures.
results suggest some interference from internal sources for K+ and SO4", possibly exudation of internal SO;" and K+ from transpiring leaves during dry periods (9). Fluxes measured to inert surfaces generally equal or exceed those estimated for foliage, with the exception of SO>-where inert surface values are generally lower than leaf values (Table I). The ratio of the mean deposition rates (inertlleaves) for all significant results from the paired experiments range from 0.7 for SO;- to 1.1 for K*, to 1.7 for Ca2+,and to 3.1 for NO,. However, the ratios from individual experiments are much more variable (ranging from -0.3 for K+ to 6.5 for NO3-), reflecting the complexities of the deposition process and the biological surface. Although these ratios suggest different deposition rates to each surface, the deposited particles were similar in morphology and size (Figure 4). From scanning electron microscope analysis of particles on leaves, inert surfaces, and aerosol filters (over 4000 particles counted), we cald a t e d particle mass median diameters (MMD) assuming all particles were of equal density with volumes proportional to the cube of their diameters. The values for particles deposited on each surface were higher than that for particles suspended in the atmosphere (MMD = 0.5 pm) but were similar for leaves (3pm) and inert surfaces (5 pm). Despite the high fraction of deposited particle mass accounted for by large particles, small particles were common on both inert surfaces and leaves. Approximately 25% of the particles counted were less than 1 pm in diameter, generally 0.2-0.3 pm. Spheres typical of fly ash from fossil fuel combustion (7) comprised 3&50% of the total particle counts on both leaf and inert surfaces; the majority of the remaining particles appeared to be soil dust. From 5 to 20% of the identifiable submicrometer fly ash occurred in aggregate form. This is important because aggregates exhibit aerodynamic characteristics of large particles (deposited by sedimentation)while retaining chemical characteristics of small particles (element-rich surface coatings). We previously reported ratios of trace metal deposition to inert surfaces and leaves to range from 1.4 to 2.2,except for Ph (ratio = 8.8) (7).Other data suggested that the ratio for P b reflected irreversible uptake of dry-deposited P b at the leaf surface rather than a major difference in deposition characteristics of each surface (20). This illusEmlron. Sci. Tedmol.. VoI. 19. No. 3. 1985 241
trates another problem in the interpretation of estimated fluxes to vegetation. It would be reassuring to think that the relative deposition rates to inert and foliar surfaces reflect the particle capture characteristics of each surface. However, there are at least three other factors to consider: (1) biological uptake of dry-deposited material on the leaves, (2) differences in particle retention efficiencies of each surface, and (3) the influence of gas and vapor deposition on the S042- and NO< flux. Foliar uptake of numerous elements (including N, Ca, and Pb) applied as particles or in solution to leaf surfaces is well-known (18, 19). In the field, dry deposition may partially dissolve in leaf solution moisture (dew, intercepted fog, exudates), resulting in high localized concentrations (7). Both active and passive (e.g., diffusion) uptake may be involved in immobilization of this external material within the leaf. This same leaf moisture, if sufficient in quantity, can also result in loss of dry deposition during nonrain periods by water dripping from dew-laden or fog droplet laden leaves. The retention of deposited particles, particularly those in the supermicron size range, is also influenced by surface morphology. The rimmed inert surface may capture atmospheric particles at the same rate as an adjacent leaf but retain them more efficiently because the rim decreases wind resuspension and particle rebound losses. During several experiments 1.5 m above the ground in a grassy field in Illinois (16) we exposed both flat, rimless Teflon plates and polycarbonate Petri dishes. Dry deposition of Ca2+to rimless plates was lower than that to rimmed plates by a factor of 2.5, while that of 502- was lower by a factor of 1.4. This may reflect differences in particle transport to or particle retention by the surfaces, or both. Differences in the particle size distributions of these ions in depositing aerosols may also explain these trends. Sulfate is the only ion analyzed that occurs predominantly in the submicron size range. We measured the size distributions of these ions during four of the experiments (17). Airborne K+, Ca2+,and NO3- were dominated by large particles, with 34, 55, and 70% of the mass of each ion, respectively, associated with particles greater than 2 pm in diameter; only 12% of the S042- fell into this class. Because of the importance of leaf microroughness properties in scavenging small airborne particles (20,21), it is possible that the inert surfaces collect or retain submicron SO:- with a lower efficiency than do the leaves, while overcollecting larger particle K+, Ca2+,and NO3-. Interestingly, the mean ratios of dry deposition rates to inert surfaces and leaves for each ion (Table I) are in the same order as the relative contributions of large particles to the aerosol concentration of each ion: S042- < K+ < Ca2+< NO3-. All of the above factors may explain the tendency for somewhat higher deposition rates of Ca2+,K+, and NO3to inert surfaces relative to foliage, but they also further emphasize the complexities involved in comparisons of these methods. Interpretation of leaf extraction and inert surface results for S042-and NO3- as measures of particle dry deposition is further complicated by the behavior of SO2 and HN03 vapor, In fact, differences in the degree to which deposited vapors can be extracted from foliage may partially explain the different ratios of S042- and NO3- dry deposition to inert surfaces relative to leaves (Table I). While dry deposition of SO2 to an inert substrate is entirely a surface phenomenon, deposition to foliage occurs both to external surfaces and via stomates to internal membranes (22). We measured the dry deposition rate of SO2 to the polycarbonate Petri dishes under controlled conditions and found it to be several orders of magnitude lower than both 242
Environ. Scl. Technol., Vol. 19, No. 3, 1985
surface adsorption and stomatal uptake rates reported for leaves (22). Even if the adsorption of SO2by leaf and inert surfaces was comparable, and this SO2 was equally extractable in water, we would still predict higher S042concentrations in leaf leachates compared to inert surface extracts for samples exposed simultaneously. This is because a measurable fraction of internally absorbed SO2is leachable from foliage as S042- (23). The behavior of nitric acid vapor (HN03),which is a major component of the total nitrate dry deposition at our site (17, 24), may explain the relatively high inert surfaceJleaf ratios for NO3-. Because of its water solubility and reactivity, HN03 vapor is thought to deposit rapidly onto both foliar and inert surfaces (25). Nitric acid vapor deposited on an inert surface is readily extractable in distilled water; however, its extractability from foliage is questionable because of the well-known ability of forest canopies to absorb and metabolize NO, deposited from the atmosphere (26, 27). To the extent that biological uptake immobilizes dry deposited NO3- and "OB, as our own throughfall data indicate (9), water extraction of adjacent leaf and inert surfaces would be expected to result in higher NO3- values for the inert surfaces. Because of the unquantified influence of vapor deposition on the relationship between dry deposition rates of SO>-and NO3- to inert and foliar surfaces, a better comparison of particle deposition rates to each surface can be made with Ca2+or K+, which have no known vapor components. However, the dry deposition of K+-containing particles is complicated because much of the airborne K+ in the forest canopy originates within the canopy itself, as resuspended biological material (9,17).Hence, the Ca2+ data provide the most useful comparison of the deposition of particles to these surfaces for several reasons: all leaf experiments yielded significant positive fluxes, the range of measured ratios from individual experiments is relatively small, Ca2+deposition involves only particles, and Ca2+represents a component without a significant local source (resuspension of Ca2+-richsurface dust from litter-covered soils beneath a mature forest is unlikely). For large-particle constituents of the troposphere such as Ca2+, inert surfaces may provide dry deposition rates that can be reasonably compared with particle deposition rates to foliage as determined by surface extraction methods. Because of the variability in individual inert surface/leaf ratios (Table I, range from 0.93 to 3.2), the comparison may be useful when expressed as a mean value for the forest growing season. This value is 1.7 with a standard error of 0.3. Because of the large particle character of Ca2+in the atmosphere here (MMD r 3 pm, 17) and elsewhere (B), it will be important to expand these studies to include elements associated primarily with small particles such as Pb. However, recent reports suggest that even for smail-particle constituents in the atmosphere, dry deposition may be controlled by sedimentation of a relatively small number of large particles (25, 29). Hence, inert surface/leaf dry deposition ratios for small-particle constituents may be of the same order as those reported here for Ca2+. If particle dry deposition to leaves can be measured, deposition to the whole canopy may be calculated from knowledge of canopy structure and dry deposition mechanisms within the canopy. Unfortunately, the mechanisms of deposition are poorly understood (4). The factor relating deposition per unit horizontal area on an upper-canopy leaf to deposition on the whole canopy could range from 1, if particles were only sedimenting from above, to the full leaf area index of the vegetation, if all surfaces within the
canopy received equal deposition. This index is -5 m2 of upward-facing leaf area per 1m2 of ground area for our chestnut oak during the growing season (9). Two lines of evidence suggest that sedimentation is a dominant means of transport of atmospheric particles onto in-canopy surfaces on a mass basis. First, as discussed above, we used optical analysis (e.g., Figure 4) to determine particle size distributions on air filters (MMD = 0.5 pm), inert surfaces (MMD = 5 pm), and leaves (MMD = 3 pm). From another experiment involving simultaneous collection of both deposited and suspended particles, we calculated a MMD of 9 pm for particles on upward-facing surfaces and a MMD of 0.9 pm for suspended particles (measured with an eight-stage cascade impactor). This indicates that the largest particles in the air, those most subject to gravitational sedimentation, deposit more efficiently on the artificial surfaces than do smaller particles. The second line of evidence supporting sedimentation is the comparison of deposition to upward- and downward-facing inert surfaces. Ratios of particle flux to upvs. down-facing surfaces have been interpreted by others as an indication of the relative importance of large-particle sedimentation (30). Upward-facing surfaces should collect particles depositing by diffusion, interception, impaction, and sedimentation; downward-facing surfaces will not collect sedimenting particles. However, as we have noted elsewhere (16)such ratios must be interpreted with caution because sedimentation may also enhance submicron particle deposition to upward surfaces to some extent. Particles with very small sedimentation rates moving into the boundary layer over an upward collector can settle onto the surface, while those nearing the downward surface may settle into the airstream below. For six experiments in which such surfaces were placed side by side, the ratio of deposition to upward-facingldownward-facingsurfaces ranged from 8.9 for Sod2to 11.3 for K+, suggesting that sedimentation may be a major delivery mechanism. While the rim on our plates does interfere with airflow over the surface (15),similar results for upward- and downwardfacing rimless surfaces have been reported (31),including comparable ratios for K+ deposition (30). If sedimentation transports much of the aerosol mass to the canopy surfaces, however, this does not imply that the canopy is exposed to a vertical “rain” of particles. A more likely possibility is that turbulent eddies move particle-laden air into the canopy, where the particles settle out onto canopy surfaces.
Summary and Implications We have described a laboratory extraction method to selectively remove surface deposited material from living vegetation exposed to ambient levels of dry deposition in the field. Results of experiments in a deciduous forest canopy have been used to compare particle dry deposition rates to simultaneously exposed foliar and inert surfaces. Our data suggest that sedimentation may be an important process in the dry deposition of particle mass to both surfaces in this forest but that any direct comparison of results from these methods must be interpreted cautiously. The data for Ca2+ are most representative of the relationship for large atmospheric particles and indicate an inert surface/leaf deposition ratio of 1.7 f 0.3 (8f SE). However, an analysis of variance indicated that the measured variance in Ca2+deposition rates to leaves explained only 15% of the variance in Ca2+deposition to adjacent inert surfaces. Clearly more extensive data will be required to establish the true relationship between particle deposition rates to each surface. Interpretation of particle deposition measurements of SO:- and NO< is complicated by simultaneous deposition of SO2 and HNO, vapor.
However, inert surface/leaf deposition relationships for small atmospheric particles could be determined by a similar analysis of other elements (e.g., Br, Se, and Sb). A useful parameter for comparison of dry deposition results is the deposition velocity ( v d ) , which is the flux to a surface divided by the concentration in the air a t some reference height (20). We calculated v d on the basis of several inert surface deposition and air concentration measurements taken just above our forest canopy, 20 m above the ground. The mean dry deposition velocities for the inert surfaces during the forest growing season were as follows (in cm/s, n = 15 experiments, SE in parO:-, 0.13(0.02);K+,0.75(0.24); Ca2+,l.l(O.1). entheses): S The deposition velocity for NO3- is not reported because of the difficulties in interpreting measurements of particulate NO3- air concentration (32). To our knowledge no similar measurements of particle deposition velocities to surfaces above a forest canopy have been published in the open literature. Deposition velocities for several crustal elements, including Ca2+and K+, to flat inert surfaces have been measured in an open field a t two heights above the ground: mean values were 1.2 cm s-l at 1.5 m and 1.7 cm c1at 10 m (30). We have measured the deposition velocity of airborne S042-to rimmed and rimless inert surfaces in an open grassy field (16) at 1.5 m above the ground. Mean values were 0.35 cm 8-l to rimmed and 0.22 cm s-l to rimless plates. All of these values may be higher than those reported in this study because of the location of the collectors within the resuspension layer near the ground (16). Continued development of foliar extraction methods and establishment of mathematical relationships between particle deposition rates to inert and foliar surfaces (if possible) and further investigations of deposition mechanisms in the canopy will eventually assist in the extrapolation of point measurements of deposition to the full canopy. Promising methods for quantifying these relationships include analysis of rain above and below the canopy and application of canopy deposition models. The former involves statistical analysis of rain and throughfall chemistry for well-characterized rain events that wash dry deposited material from the canopy (9,33). The latter uses detailed canopy and atmospheric measurements to calculate particle deposition from physical principles (29,34-36).
Acknowledgments We thank Jan Coe for help in the collection and analysis of samples and G. Taylor and B. Hicks for helpful comments on the manuscript. Publication No. 2395, Environmental Sciences Division, Oak Ridge National Laboratory. Registry No. Ca, 7440-70-2; K, 7440-09-7.
Literature Cited (1) Hicks, B. B.; Wesely, M. L.; Durham, J. L. (Eds.) “Critique of Methods to Measure Dry Deposition”; U.S. Environmental Protection Agency: Washington, DC, 1980; EPA60019-80-050. (2) Lindberg, S. E.; McLaughlin, S. B. In “Air Pollutants and Their Effects on Terrestrial Ecosystems”; Krupa, S. V.; Legge, A. H., Eds.; Wiley: New York, in press. (3) Hicks, B. B.; Garland, J. In ”Precipitation Scavenging,Dry Deposition, and Resuspension”; Semonin, R. G.; Pruppacher, H. R.; Slinn, W. G. N., Eds.; Elsevier: New York, 1983; Vol. 2, pp 1429-1434. (4) Hosker, R. P.; Lindberg, S. E. Atmos. Enuiron. 1982,16, 889-910. (5) Davidson, C. I.; Lindberg, S. E. In “Critique of Methods to Measure Dry Deposition”; Hicks, B. B.; Wesely, M. L.; Durham, J. L., Eds.; U.S. Environmental Protection Agency: Envlron. Scl. Technol., Vol. 19, No. 3, 1985
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Washington, DC, 1980; EPA-600/9-80-050,pp 66-70. (6) Davidson, C. I.; Goold, W. B.; Wiersma, G. B. In “Precipitation Scavenging, Dry Deposition, and Resuspension”;Semonin,R. G.; Pruppacher, H. R.; Slinn, W. G. N., Eds.; Elsevier: New York, 1983; Vol. 2, pp 871-882. (7) Lindberg, S. E.; Harriss, R. C.; Turner, R. R. Science (Washington, D.C.) 1982,215, 1609-1611. (8) Peirson, D. H.; Cawse, P. A.; Cambray, R. S. Nature (London) 1974,251, 675-679. (9) Lovett, G. M.; Lindberg, S. E. J . Appl. Ecol., in press. (10) Lindberg,S. E.; Harriss, R. C. Water, Air, Soil Pollut. 1981, 16,13-31. (11) Lindberg, S. E.; Lovett, G. M. In “PrecipitationScavenging,
Dry Deposition, and Resuspension”; Semonin, R. G.; Pruppacher, H. R.; Slinn, W. G. N., Eds.; Elsevier: New York, 1983; Vol. 2, pp 837-848. (12) Carlisle, A,; Brown, A. H.; White, E. J. J . Ecol. 1966, 54,
87-98. (13) Turner, J.; Johnson, D. W.; Lambert, M. J. Oecol. Plant. 1980, 1 (E),27-35. (14) Tukey, H. B., Jr. Annu. Rev. Plant Physiol. 1970, 21, 305-324. (15) Gregory, P. H.; Stedman, 0. J. Ann. Appl. Biol. 1953,40, 651-674. (16) Davidson,C. I.; Lindberg, S. E.; Schmidt,J. A.; Cartwright, L. G.; Landis, L. R. J. Geophys. Res., in press. (17) Lindberg, S. E.; Lovett, G. M.; Coe, J. M. Electric Power Research Institute, Palo Alto, CA, 1984, Acid Depos-
ition/Forest Canopy Interactions: Final Progress Report of Project RP1907-1; Oak Ridge National Laboratory,Oak Ridge, TN, ORNL-ESD Publication No. 2429. (18) Thimann, K. V. ”The Physiology of Forest Tress”; The Ronald Press Co.: New York, 1957; pp 1-678. (19) Krause, G. H. M.; Kaiser, H. Environ. Pollut. 1977, 12, 63-71. (20) Chamberlain,A. C. Proc. R. SOC.London Ser. A 1966, A296, 45-70. (21) Little, P. Environ. Pollut. 1977, 12, 293-305. (22) Taylor, G. E., Jr.; McLaughlin, S. B., Jr.; Shriner, D. S.; Selvidge, W. J. Atmos. Environ. 1983, 17, 789-796. (23) Garland, J. A,; Branson, J. R. Tellus 1977, 29, 445-454.
(24) Huebert,B. J. In “PrecipitationScavenging,Dry Deposition, and Resuspension”; Semonin, R. G.; Pruppacher, H. R.; Slinn, W. G. N., Eds.; Elsevier: New York, 1983; Vol. 2, pp 785-794. (25) Garland, J. A. In “Acid Precipitation: Origins and Effects”; Thiel, W., Ed.; VDI Berichte 500, Verein Deutscher Inge-
nieure: Diisseldorf, West Germany.
(26) Verry, E.; Timmons, D. R. Can. J . For. Res. 1977, 7, 112-119. (27) Cole, D. W.; Rapp, M. In “Dynamic Properties of Forest
Ecosystems”;Reichle, D. E., Ed.; Cambridge University Press: London, 1981; pp 341-410. (28) Harris, F. S. “AtmosphericAerosols: A Literature Survey of Their Physical Characteristics and Chemical Composition”. National Aeronautics and Space Administration, Washington, DC, 1976, Report NASA CR-2626. (29) Davidson, C. I.; Miller, J. M.; Pleskow, M. A. Water, Air, Soil Pollut. 1982, 18, 25-43. (30) Elias, R. W.; Davidson, C. I. Atmos. Environ. 1980, 14, 1427-1432. (31) Dasch, J. M. In “Precipitation Scavenging, Dry Deposition,
and Resuspension”; Semonin, R. G.; Pruppacher, H. R.; Slinn, W. G. N., Eds.; Elsevier: New York, 1983; Vol. 2, pp 883-899. (32) Appel, B. R.; Tokiwa, Y. Atmos. Environ. 1981, 15, 1087-1089. (33) Lakhani, K. H.; Miller, H. G. In ”Effects of Acid Precipitation on TerrestrialEcosystems”;Hutchinson,T. L.; Havas, M., Eds.; Plenum Press: New York, 1980; pp 161-172. (34) Lovett, G. M. Atmos. Environ. 1984, 18, 361-371. (35) Slinn, W. G. N. Atmos. Environ. 1982, 16, 1785-1794. (36) Bache, D. H. Atmos. Environ. 1979,13, 1257-1262.
Received for review January 24, 1984. Revised manuscript received September 4, 1984. Accepted October 12, 1984. This research has been funded as part of the National Acid Precipitation Assessment Program in part by the Electric Power Research Institute under Contract RPl907-1 and in part by the Officeof Health and Environmental Research, U.S. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
Kinetics of the Gas-Phase Reactions of the Hydroxyl Radical with Naphthalene, Phenanthrene, and Anthracene Heinz W. Biermann, H M n e Mac Leod, Roger Atkinson,” Arthur M. Wlner, and James N. Pltts, Jr. Statewide Air Pollution Research Center, university of California, Riverside, California 92521
The two- and three-ring polycyclic aromatic hydrocarbons (PAH), which are emitted into the atmosphere from combustion sources, exist predominantly in the gas phase. In this work, a relative rate technique employing gas chromatography and long path-length differential optical absorption spectroscopy was used to determine rate constants for the gas-phase reaction of OH radicals with naphthalene, phenanthrene, and anthracene. By use of a rate constant for the reaction of OH radicals with propene of 4.24 X 10-12e644/T cm3 molecule-l sbl, the rate constants obtained were the following (in units of lO-ll cm3 molecule-’ s-l): naphthalene, 2.35 f 0.06 at 298 f 1 K; phenanthrene, 3.4 f 1.2 at 298 f 1 K and 2.8 f 0.6 at 319 f 1 K;anthracene, 11.0 f 0.9 at 325 f 1 K. When an OH radical concentration of 1 X lo6 cm-3 is assumed, these rate constants lead to atmospheric lifetimes of these PAH due to reaction with OH radicals of -12, -9, and -2 h for naphthalene, phenanthrene, and anthracene, respectively.
Introduction A variety of polycyclic aromatic hydrocarbons (PAH) 244
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are emitted into the atmosphere from combustion sources (1-4). These PAH exhibit a wide range of volatilities, with the smaller compounds such as naphthalene, phenanthrene, and anthracene existing predominantly in the vapor phase (2,5),while the larger PAH (e.g., five and six ring) exist predominantly in the adsorbed state. Furthermore, it has been shown that the two- and three-ring PAH such as naphthalene, phenanthrene, and anthracene [which have vapor pressures at 298 K of 7.8 X lov2,1.2 X and 6.0 X lo4 torr, respectively (S)]are among the most abundant PAH emitted from automobiles (2) and wood-burning stoves (1, 3). While numerous studies (7-13) have been carried out in recent years to investigate the reactions of a variety of PAH adsorbed on various substrates with atmospherically important species such as O3and NO2,little work has been carried out to date to investigate the reactions of PAH present in the gas phase under atmospherically realistic conditions. Recently we investigated the kinetics of the gas-phase reactions of hydroxyl (OH) radicals with naphthalene at room temperature (14) using a relative rate
0013-936X/85/0919-0244$01.50/0
0 1985 American Chemical Society
