Environ. Sci. Technol. 1992, 26, 2428-2435
Pandis, S. N.; Seinfeld,J. H. J . Geophys. Res. 1989,94,1105. Neta, P.;Huie, R. E. Enuiron. Health Perspect. 1985,64, 209. Kotronarou, A. Ultrasonic Irradiation of Chemical Compounds in Aqueous Solutions. Ph.D. Thesis, California Institute of Technology, 1992. Neta, P.; Huie, R. E.; Ross, A. B. J . Phys. Chem. Ref. Data 1988,17,1027. Byme, G. D.; Hindmarsh, A. C. ACM Trans. Math. Software 1975,71. Higashihara,T.;Saito, K.; Yamamura, H. Bull. Chem. SOC. Jpn. 1976,49,956. Bradley, J. N.; Dobson, D. C. J . Chem. Phys. 1967,46,2865. Roth, P.;Lohr, R.; Barner, U. Combust. Flame 1982,45, 273.
(51) Rozenberg, L.D.Sou. Phys.-Acoust. (Engl. Transl.) 1965, 11, loo. (52) Margulis, M. A. Sou. Phys.-Acoust. (Engl. Trawl.) 1969, 13, 135. (53) Goodwin, T.J. In Chemistry With Ultrasound, Critical Reports on Applied Chemistry; Mason, T. J., Ed.;Elsevier Applied Science: New York, 1990;Vol. 28. (54) Reynolds, G. T.; Walton, A. J.; Gruner, S. M. Rev. Sei. Instrum. 1982,53,1673.
Received for review March 30,1992. Accepted July 7,1992. This work was funded in part by the County Sanitation Districts of Los Angeles County (Contract 3082) and by the U.S. EPA (ExploratoryResearch Office R815041-01-0). We are grateful for their support.
Resuspension of Particulate Chemical Species at Forested Sites Yee-Lln Wu,*?+Cliff I . Davidson,+ Steven E. Llndberg,t and Armlstead 0. Russell§
Department of Civil Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, Oak Ridge Natlonal Laboratory, Oak Ridge, Tennessee 37831, and Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
I The resuspension rates of particulate chemical species
have been estimated using aerodynamic surrogate surfaces (symmetrical airfoils) at a mixed deciduous forest site, and also using throughfall sampling at a loblolly pine forest site. The airfoils were placed above, within, and below the crown of the deciduous canopy to determine the vertical variation of deposition and resuspension. The resuspension rates of SO:-, C204'-, and Ca from the airfoils are estimated to be on the order of 10-6-10-5s-l. The resuspension rates within the crown are only slightly less than those above the canopy. This is in contrast to the mean wind speed, which is much smaller within the crown than above the canopy. The smaller differences in the resuspension rate compared with differences in mean wind speed may be due to the effects of wind gusts which penetrate deep within the canopy. Based on the throughfall data, the resuspension rates for Ca, Mg, and Na are also in the range 10-6-10-5 s-l. Overall, these results suggest that resuspension may need to be taken into account when particle deposition to vegetation is estimated.
Introduction Resuspension of particulate species has been of interest for several decades. Most of the early work concerned soil erosion from desert and agricultural areas (1-3). Since the 19609, resuspension of deposited material from fallout from nuclear weapons tests and nuclear plant accidents has been brought into attention (4-7). These previous field studies for resuspension from soil have been conducted primarily in arid or semiarid areas (8, 9). The results have shown great variability in resuspension rates, varying over 9 orders of magnitude from to s-l due to the influence of factors such as wind speed, particle properties, relative humidity, soil properties, and duration of exposure. The
* Current address: Department of Environmental Engineering, National Cheng Kung University, Taiwan. t Department of Civil Engineering, Carnegie Mellon University. t Oak Ridge National Laboratory. 5 Department of Mechanical Engineering, Carnegie Mellon University. 2428
Environ. Sci. Technol., Vol. 26, No. 12, 1992
resuspension rate is defined as the fraction of previously deposited particles resuspended per unit time. The effects of resuspension on deposition of acids and other potentially toxic species have not been investigated until recently (10). Resuspension of dry-deposited S042and NO, has been suggested for measured airborne concentrations that are greater than model predictions in southern California ( 1 1 ) . Resuspension has resulted in significant airborne concentrations of alkaline soil particles that have been shown to neutralize a large portion of acid precipitation precursors in the western United States (12). Holsen et al. (13) found that particulate PCBs account for more than 80% of the total dry deposition flux,although most of the airborne PCB concentration is associated with the gaseous phase. The dry deposited material may be mainly from resuspension of previously deposited PCBs from urban surfaces (14). Although resuspension has been identified as one of the major processes in atmospheresurface exchange of particles, the mechanisms of resuspension are still not well understood ( 1 5 ) . Wu et al. (16) found that deposition velocities decrease as exposure times increase for particle deposition to potted plants and aerodynamic surrogate surfaces in field experiments. Data on resuspension from forest canopies are much more limited. In a previous study by Orgill et al. (17), the resuspension of DDT was measured from a forested area in the Colville Indian Reservation in northcentral Washington. Resuspension rates were estimated to be on the order of s-l based on vertical profiles of airborne concentration measured by aircraft-mounted high-volume samplers. However, the DDT data were complicated by high vapor pressure and possible chemical reactions. In this study, resuspension rates of particulate chemical species from forests are estimated by two different methods: surrogate surfaces and throughfall. The surrogate surfaces were placed at different heights to estimate the vertical variation in resuspension. The throughfall measurements, which provide the chemical input from the atmosphere to the forest floor after interacting with the canopy, are used to estimate the resuspension rates for the entire canopy.
0013-936X/92/0926-2428$03.00/0
0 1992 American Chemical Society
Experimental Methods A. Surrogate Surfaces. Experiments using surrogate surfaces were conducted during September 1990 at the Forest Meteorology research site of the Atmospheric Turbulence and Diffusion Division (ATDD) of NOAA. This site is located in Walker Branch Watershed in a semirural area near the Oak Ridge National Laboratory (ORNL), TN, and is within 22 km of three coal-burning power plant^. The site is on a ridge in moderately complex terrain and is forested for several kilometers in all directions. The dominant overstory species include chestnut oak (Quercus prinus L.), white oak (Quercus alba L.), red oak (Quercus rubra L.), tulip poplar (Liriodendron tulipifera L , ) , and several species of hickory (Carya). The height of the canopy is -23 m and the leaf area index (LAI) is -5. Further details of this canopy are presented by Hutchison et al. (18) and by Johnson and Van Hook (19). Samples were collected at 5.7, 17.0, and 45.6-m heights on a tower, corresponding to locations below the heavily foliated crown of the canopy, within the crown, and above the canopy, respectively. At each height, there were three aerodynamic surrogate surface devices for measuring deposition fluxes (20). Two of these had ungreased surfaces, and one had a greased surface. Each device consisted of a solid piece of Teflon-coated aluminum in the shape of a low-speed airfoil with a Teflon film on top; the airfoil was symmetric about its vertical axis, having an overall “frisbee” shape. Thus,the variations in wind direction had negligible effect on the deposition process. These airfoils were used to obtain control over boundary layer and surface conditions. A Teflon Zefluor filter for measuring airborne concentrations was also used at each height. For some runs, Andersen impactors were used at each height to measure the particle size distributions. At the top of the tower, the wind speed, wind direction, and relative humidity were monitored continuously. There was no precipitation during these experiments. Particle deposits on the Teflon films were stored in polyethylene bags at 4 OC from immediately after collection until extraction with water and toluene 10-12 weeks later. Analysis was performed on the same day as extraction, using ion chromatography (IC) with a Dionex series 4500i equipped with an AS4A column. The resulting chromatograms provided reliable data for Sod2-, PO4%,and C2O4*. The particle deposits were also analyzed by flameless atomic absorption spectrophotometry (AA) for Ca with a Perkin-Elmer 703. All of these analyses were performed at Carnegie Mellon University. B. Throughfall. Event measurements of wet deposition and throughfall were conducted in a 30-year-old loblolly pine (Pinu taeda) forest for a full 3 years (January 1986-December 1988) as part of the Integrated Forest Study, operated by ORNL (22). Airborne concentration and particle dry deposition were also measured at the same location. The pine canopy is -27 m high and the average LA1 is 3-4. This canopy is located -10 km southwest of the Walker Branch site. Airborne concentrations were measured at the top of a 43-m meteorological tower by filter packs with Teflon, nylon, and carbonate-treated cellulose filters in series. Dry deposition samples were collected on the tower with polycarbonate Petri dishes in automatic collectors. More than one dry deposition run might be conducted during each dry period between rain events. Incident precipitation events were sampled in adjacent clearings by automatic wet-only collectors. A network of five replicate throughfall samplers of the same
design were established within a 0.1-h plot in the forest. The samplers were roughly uniformly spaced. The plot was 100 m from the 43-m tower. Micrometeorological data were measured continuously at the top of the tower. Details of the sampling methods are given by Lindberg et al. (22). Data Analysis. Data from the surrogate surfaces at the Walker Branch site were used to determine both the resuspension rate and the fraction of rebound, while the throughfall data from the pine canopy were used to determine resuspension only. Note that the former results are for single surfaces; results from the throughfall experiments are for the canopy as a whole. Different data analysis methods were used because of the different sampling techniques for these two categories of experiments. A. Surrogate Surfaces. Methods of calculating the resuspension rate /3 and fraction of rebound F from the three airfoils have been discussed by Wu et al. (16). Essentially, one ungreased surface, denoted as component 1, is used for two consecutive exposure times (0 < t < r1 and r1 < t < 4,while a second ungreased surface, denoted as component 2, is exposed continuously over the period 0 < t < r2. A third surface, which is greased and is denoted as component 3, is used for the same two consecutive exposure times as component 1. The accumulated masses on components 1and 2 are the same theoretically during the period 0 < t < 71. However, during the period r1 < t < r2,there is more deposited material on component 2 than component 1 because a new clean Teflon film is placed on the airfoil representing component 1 at r l . Therefore, there is more material resuspended from component 2 than from component 1during the period r1 < t < r2. The average resuspension rate is estimated from the difference in accumulated masses on component 1and component 2. After taking resuspension into account, the average fraction of rebound is obtained by comparing accumulated masses on components 1 and 3. For some runs, three ungreased surfaces were exposed simultaneously with the same exposure period to provide replicate data. The differences among the three surfaces at the same location were less than 15%. This value has been used to determine the validity of each run: if the difference between the sum of the masses in component 1and the overall mass in component 2 was less than 15?%, then the difference was considered to be less than the measurement uncertainty, and the data for that run were not used to compute the resuspension rates. B. Throughfall. The amount of chemical species in throughfall is the net effect of deposition via incident precipitation, dry deposition between rain events, and canopy exchange. The last of these processes includes uptake of a chemical species by plants and by microbes on the plant surfaces, as well as release of the species from the plant material (leaching). The amount of net throughfall, which is equal to the sum of dry deposition and canopy exchange, was obtained by subtracting the concentrations in rain from concentrations in the throughfall samples (TF), since the TF data were collected on an event basis. Lovett and Lindberg (23) have shown that the contribution of canopy exchange to the net throughfall can be estimated by determining an empirical canopy exchange coefficient [in mequiv m-2 (cm of rain)-’]. The coefficient is determined by multiple regression techniques using the duration of each dry period and the amount of each rainfall as independent variables; net throughfall is the dependent variable. The contribution of canopy exchange is calculated as the product of the canopy exchange coefficient and the amount of rainfall. N
Environ. Sci. Technol., Vol. 26, No. 12, 1992
2420
Table I. Summary of Vertical Variations of Airborne Concentration C, Deposition Flux J , and Deposition Velocity Vd at the Walker Branch Site at Locations above, within, and below the Crown of the Canopya
C,rg/m3 J,ng/b m2) vd, cm/s
1ocation
SO$-
PO?-
czo42-
Ca
above within below above within below above within below
8.5 f 5.6 5.5 f 1.8 7.2 f 1.4 8.1 f 7.1 5.5 f 5.2 3.6 f 2.1 0.54 & 0.54 0.12 f 0.06 0.056 f 0.028
0.030f 0.007 0.055 f 0.024 0.031 f 0.009 0.022 f 0.011 0.031 f 0.005 0.032 f 0.019 2.2 f 1.6 1.6 f 1.3 1.1 f 0.83
0.16 f 0.06 0.14 f 0.08 0.11 f 0.06 7.2 f 7.7 4.8 f 4.8 1.6 f 1.4 5.0 f 3.0 2.6 f 1.6 1.4f 0.83
0.60 f 0.30 1.1 f 0.71 0.69 f 0.35 8.0 f 4.6 8.9 f 6.8 7.5 f 5.8 3.5 f 3.1 1.6 f 1.6 2.1 f 2.2
"The deposition data are for greased surfaces on the airfoils. The results are presented as arithmetic mean f standard deviation for 10 sets of experiments.
This procedure has been used in the current study to separate the effects of dry deposition and canopy exchange in the net throughfall data. Net throughfall data were obtained for eight particulate chemical species (Ca, K, Mg, and Na by AA; NH4+,NO3-, PO4$, and Sod2by IC), but the PO-: data were not used for further analyses since a large portion of analyses for Po43-were under the detection limit. However, for some of these species, the data may have been influenced by interference from dry deposition of gaseous species as well as from canopy exchange. Most of the data analyses to estimate resuspension focused on Ca, Mg, and Na, where gas-phase interference should not be as serious. Due to the variations in T F values from location to location, the average of five simultaneously collected samples was used. Unlike the surrogate surface measurements where the airfoils were exposed simultaneously in a separate experiment for each determination of P, the throughfall experiments involved comparisons among sequential time periods to get an estimated overall value of P. To account for changing airborne concentrations during the sequential periods, the net throughfall data have been expressed as deposition velocities rather than accumulated masses. Analysis of the net throughfall data begins with an expression for the accumulated mass of contaminants on the surface of the vegetation M (g/cm2):
I\ 0.8
0.2
F\ I
0.0 0
200
(2) where C is the airborne concentration of the chemical species (g/cm3) and v d o is the deposition velocity without the effect of resuspension (cm/s). For PAt >> 1,eq 2 can be simplified to (3)
Therefore, the deposition velocity is inversely proportional to the exposure time. If @At is not much greater than 1, the deposition velocity is smaller than that implied by eq 3. Figure 1shows the hypothetical data for P equal to lo+ s-'. If eq 3 is used with a linear regression to and determine the resuspension rate for these hypothetical data, the results are 3.9 X lo+ and 1.1 X s-l, respec2430
Environ. Scl. Technol., Voi. 26, No. 12, 1992
I
,
I
,
I
400 600 800 Exposure time, hr
,
1000
Flgure 1. Hypothetical data showing deposition velocity vs exposure tlme at resuspension rates of lo-' and s-'.
tively, if the minimum exposure time is 100 h. The differences are also dependent on the exposure time: for fl = lo4 s-l, the calculated estimates of j3 increase from 2.4 X lo4 to 6.6 X lo4 s-l as the exposure times decrease from 200 to 50 h. The minimum exposure time for the TF data is -75 h. Therefore, the maximum overestimate is a factor of 4.9 for 0 = IO4 s-l. Results
A. Surrogate Surfaces. In order to provide reliable estimates of bounce off and resuspension, exposure times for component 1me 1day even though shorter exposure periods are sufficient for flux measurements (20). This exposure period leads to samp1e:blank ratios from component 1 greater than 1O:l for all four species. Table I summarizes the airborne concentrations, fluxes, and deposition velocities for the greased surfaces in the 10 experiments (mean f standard deviation). The standard deviations show the variability from run to run. The deposition fluxes of SO:- and CzOt- at the top of the tower are always greater than those within the crown of the canopy, and those within the crown are generally greater than those below the crown. The deposition fluxes of Ca within the crown are slightly greater than those at the other two locations, although the differences are not statistically significant. The patterns for PO4$ deposition are variable: the deposition fluxes within and below the crown are generally greater than those above the canopy. The deposition velocity is computed as the ratio of deposition flux to airborne concentration. The greater deposition velocities of Po43and Ca are likely due to the greater particle sizes. The mass median aerodynamic PO?-, C2O4'-, and Ca are diameters (MMAD) for Sod2-, shown in Figure 2. A minimum diameter of 0.05 pm and a maximum diameter of 25 pm have been used in the computation of MMAD from the impactor sampling. These MMAD values may have been slightly underesti-
-
where J is the deposition flux in the absence of bounce off and resuspension [g/(cm2s)] and At is the exposure time (8). Equation 1 assumes that J,F, and are all constant. The average deposition velocity over the antecedent dry period is
,
Table 11. Summary of Vertical Variations of Resuspension Rate B and Fraction of Rebound F from the Surrogate Surface Expesriments at the Walker Branch Site" 8 X 106, 8'.
location
SO,%
above
7.9 (0.81-15) 3.6 (0.28-12) 3.5 (0.28-6.7) 0.70 (0.30-0.83) 0.23 (0.038-0.68) 0.24 (0.062-0.63)
within
below above within below
F
GO,*
Ca
37 (8.2-84)
9.0 (4.1-21) 2.2 (0.43-4.4) 4.2 (1.0-7.6) 0.41 (0.26-0.53) 0.24 (0.20.29) 0.16 (0.048-0.31)
21 (15-29) 1.6 (1.1-2.2) 0.64 (0.33-0.92) 0.39 (0.072-0.86) 0.34 (0.25-0.80)
"Values shown are arithmetic means and ranges of values for 10 sets of experiments.
i . mmp. 1
20 25
= mmp.2 mmp. 3
I Sulfate
Phosphate
Oxalate
248
Cal~ium
Species
FIQIW 2. Mass medlan diameters of SO,". PO,%, C20,", and Ce at heights below. within, and above the crown of tha canopy.
mated since the largest particles were not collected due to nonisokinetic sampling. Note that the fractions of particles greater than 10 pm are about 4.0%, 12%, and 27.8% for SO,'-, NO,, and total suspended particles, respectively, as measured in southern California (24), although the size distributions at Walker Branch Waterahed may have been different. The posaible sampling errors due to particles smaller than 10pm for the filter and impactor sampling are believed to be small. The average values of MMAD for SO4'- and CzO,z- are 0.67 and 0.60 pm, respectively, while t h e for PO4%and Ca are 4.3 and 5.8 w, respectively. The value for S04z- is consistent with a previous study at the same location showing 0.7 pm (25) and with a recent literature review showing an average value of 0.77 pm for many studies at widely separated locations (26). However, the value for Ca is slightly greater than previously published estimates showing 3.0 and 4.6 pm (25,27). The differences in concentration measured by fdters and impactors run simultaneously in this study are always less than 18% with means of 11% and 7.5% for SO4'- and Ca, respectively. The particle size distributions for both and CzOa2-are bimodal. The two peaks are around 0.8 and 5 pm for PO," and around 0.5 and 4 pm for CZO4'-. The upper mode in each instance is probably responsible for the bulk of the mass deposition. It is recognized that MMAD values are only of limited usefulness in interpreting dry deposition data. This is because the small fraction of large particles, rather than the bulk of the mass, is responsible for most of the mass dry deposition flux. MMAD values can be used to help understand dry deposition, however, because a larger MMAD is suggestive of a greater fraction of airborne mass associated with large particles. The greater deposition fluxes of SO4* and CzO:' above the canopy are caused by the higher airborne concentrations and wind speeds. The average ratio of wind speed above the canopy to that within the crown is 15.6 with a standard deviation of 10.7. Note that the large ratio of wind speeds is in part due to the height of the tower, which is 46 m compared with the average canopy height of 23 m; this large ratio is consistent with results of Baldocchi and
249
250
251
Date
m
e 3. Example of tha results fw SO-:
horn
OM)
set of tha
ihree&nnpn%nl experiments.
Meyers (a), who found a ratio of 12.5 for a maximum height of 34 m. The absence of a strong positive gradient for PO4%and Ca deposition may be caused by local source such as soil and plant tissue debris (2S31). The importance of local sources is also suggested by the decreasing value of MMAD with increasing height for these species, shown in Figure 2. The vertical variation in airborne concentrationsof S02-and C2O4'; which show maximum values a t the top of the tower, suggests that the major sources of both species are outside the canopy. This is in agreement with Kawamura and Kaplan (32) and Norton et al. (33),both of whom suggested that sources of Cz0,2are outside of the canopy. Similar profiles for airborne concentrations of SO2' and Ca have also been observed by Lovett and Lindherg (34) on a nearby tower. An example of results for one run of the three-component experiments is shown in Figure 3. The fluxes for component 3 are always greater than those for component 1 with the same exposure periods. The fluxes for component 2 are always smaller than the average of those for component 1 within the same time period. The resulting resuspension rates and fractions of rebound are listed in Table 11. The ranges of values rather than standard deviations are given, due to the wide variations in these parameters. Note that only the accumulated masses on surrogate surfaces are needed to estimate the values of j3 and F. The concentrations of PO:- from component 1 within and below the canopy crown are usually below the detection limits. Thus the resuspension rates and fractions of rebound of cannot be determined at these two heights. The resuspension rates for PO4%at the top of the s d with a mean tower vary from 1.4 X lo4 to 5.3 X s-', and the fractions of rebound vary from of 1.3 X 0.43 to 0.64 with a mean of 0.49. The resuspension rates of and Ca summarized in Table I1 are on the same order as those measured in an open field in Gettysburg, PA, using the same three-component method (16). The resuspension rates and the fractions of rebound for SO4'-, Cz04z-,and Ca above the canopy are greater than those at the other two heights, but these vertical variations are not as great as the variations in wind speed. This may be due to the importance of large hut infrequent turbulent Environ. Sci. Technol.. Vol. 26, No.
12, 1992 2431
eddies influencing particle interactions with surfaces, and in part due to the exclusion of some of the runs with small differences in accumulated mass between components 1 and 2. The estimated magnitudes of the average resuspension rates are reduced by 20-30% if the resuspension rates of those excluded runs are assumed to be zero. Baldocchi and Meyers ( 2 8 ) suggested that momentum transport within this canopy is dominated by large infrequent sweeps. Similar findings for momentum transport in other canopies have been reported by Shaw (35)and by Baldocchi and Hutchison ( 3 6 ) . The importance of the large turbulent eddies for particle deposition to smooth surfaces has been suggested by Owen (37) and has been illustrated by Wu et al. (38).Braaten and Paw U (39) and Wu et al. (40) have found that the resuspension of Lycopodium spores is dominated by large turbulent eddies in wind tunnel studies. Note that the ratio of wind speed above the canopy to that within the canopy has a coefficient of variation (ratio of standard deviation to mean) of 0.68; this large value suggests wide variations in the ratio, implying that turbulent gusts can penetrate deep within the canopy. Shaw and McCartney (41) have found that the frequency of large sweeps within the canopy is greater than that above the canopy, and they suggested that the mean wind speed may not be a sufficient parameter to characterize deposition and resuspension within the canopy. Our data are consistent with their hypothesis. For these different chemical species, Table I1 and Figure 2 suggest that there is no clear relationship between resuspension rate (or fraction of rebound) and particle mass median aerodynamic diameter. This is consistent with other data. For example, in the wind tunnel study of Wu et al. ( 4 0 ) ,the resuspension rate of Johnson grass pollen was smaller than that of Lycopodium spores even though the former was twice as large in diameter. Zimon (42) has also shown that particle size is not sufficient to explain the removal of particles of different chemical species from surfaces. The use of MMAD to characterize resuspension is apparently not appropriate, probably because the process is affected by a complexity of factors. For example, the shape of the particle is likely to affect the resuspension rate: a spherical particle is more easily resuspended than a flat particle with the same aerodynamic diameter. Surface characteristics (e.g., smooth vs rough) and water content can also play major roles. More studies are needed to characterize the effects of particle size on resuspension. B. Throughfall. Lovett and Lindberg (23) found that for Ca, canopy exchange contributes -40% and 75% to throughfall for chestnut oak and white oak, respectively. Therefore, the amount of Ca in throughfall from canopy exchange is not negligible and must be taken into account when resuspension is determined. The contribution of canopy exchange for Na is negligible because of its limited biological necessity (43),while the effect for M g is not clear. Based on previous work ( 2 3 ) , the canopy exchange coefficientsvary from 0.05 to 0.78 mequiv m-2 (cmof rain)-' for Ca from chestnut oak and white oak. If the same technique is used for the throughfall data for loblolly pine in this study, the calculated exchange coefficients, based on a regression of accumulated mass vs duration of dry period and amount of rainfall, are 0.17,0.055, and 0.0068 mequiv m-2 (cm of rain)-l for Ca, Mg, and Na, respectively. The amount of dry deposition is obtained after subtracting the estimated canopy exchange (product of the exchange coefficient and the amount of rainfall) from the net throughfall. There is 1sample out of 34 showing greater canopy exchange than the net throughfall, and it is not used for further analysis. The deposition velocity is as2432
Environ. Scl. Technol., Vol. 26, No. 12, 1992
Table 111. Summary of Correlation Coefficients for the Deposition Velocities of Ca, Mg, and Na with Various Meteorological Parameters in the Loblolly Pine Canopy for 33 Pairs of Data
mean wind species speed, m/s Ca Mg Na
-0.057 0.133 0.243
surface wetness,
RH,
%
%
0.275 0.055 -0.234
0.231 0.353 -0.291
rain rainfall duratn, amt, h cm 0.330 0.675 0.078
0.492 0.258 -0.421
Table IV. Summary of Resuspension Rates for Ca at Different Values of Exchange Coefficients"
exchange coeff, mequiv m-* 0 0.05 (cm of rain)-' fl x 106, a'' 2.3 9.8
0.1 0.17 12
9.9
0.3 0.5
I1
6.9
" Based on the throughfall data for the loblolly pine canopy. sumed to be independent of the duration of dry period in the regression analysis, even though such an assumption is inconsistent with later calculation of resuspension rate. If the deposition velocity is a function of the dry period, the estimated value of the exchange coefficient may be different from the true value. The effects of different meteorological parameters on deposition velocity were analyzed by determining the correlations between them after correcting for the effects of canopy exchange. The correlation coefficients for deposition velocity with wind speed, surface wetness, relative humidity, duration of rain, and amount of rainfall are listed in Table I11 ( N = 33). Surface wetness is the fraction of time that the surface was wet with dew or fog, which was measured once per minute during the dry period between rain events. Except for the duration of rain for Ca and Mg and the amount of rainfall for Ca, these meteorological variables are not significantly correlated with the deposition velocity at the 95% confidence level. However, some of the correlations are significant at lower confidence levels. In a recent sensitivity analysis involving several variables, Gould and Davidson (44) found that the most sensitive variable influencing the particle deposition velocity is the wind speed. The effects of wind speed are not significant for this data set, probably because the range of wind speed is only 1.36 0.33 m/s (arithmetic mean f one standard deviation) and because the mean wind speed is used to characterize quite long and variable dry periods. The effects of duration of rain and amount of rainfall can affect both the efficiency of washing and the canopy exchange. Lindberg and Lovett (30) found that deposited Ca on leaves can be washed off within several minutes with only limited leaching. The shortest rain event in this data set is 15 min, and the corresponding amount of rainfall is 0.127 cm. Thus the data show that the washing of deposited particles by rain is very efficient. However, many of the data pertain to rain events lasting for hours, which may result in significant leaching. In order to estimate the effects of canopy exchange, Table IV summarizes the resulting resuspension rates for Ca at different exchange coefficients. The resuspension rates increase by a factor of -4 (from 2.3 to 9.8) when the results for zero canopy exchange are compared with those for a minimal exchange coefficient [0.05mequiv m-2 (cm of rain)-l]. But the resuspension rates only vary by -30% for the exchange coefficients in the range 0.05-0.5 mequiv m-2 (cm of rain)-l. Therefore, the uncertainty in the resuspension rate of Ca from the loblolly pine canopy is probably -30%. The value of /3 is estimated to be 9.9 X
*
-
I 4
0
lo
,011 0
'
'
'
100
'
200
'
'
'
'
' " I
400
300
500
'
'
600
Exposure time, hr
Fwe 4. Depositionvelocity of Ca vs exposure time for the throughfail data. Table V. Summary of the Calculations for /3 Based on the Throughfall Data for the Loblolly Pine Canopy" species Ca Na Mg
vd,//3
x lo6, cm 0.424 0.414 10.2
vdo,
cm/s
0.42 0.17 2.26
/3 x 104, s-l 9.9
4.1 2.2
The results for Ca have been adjusted using a canopy exchange coefficient of 0.17 mequiv m-2 (cm of rain)-* while the results for Na and ME assume no canopy exchange.
IO4 s-l, based on the average exchange coefficient and the amount of rainfall. A plot of deposition velocity vs time between rain events (i.e., exposure time) is shown in Figure 4 for Ca. It shows a negative slope: deposition velocities decrease as exposure times increase. Similar results are obtained for Mg, Na, NO3-, S042-,and NH4+ (not shown), although data for NO3-,SO:-, and NH4+are more difficult to interpret due to the effects of plant uptake and complications from gaseous compounds. The remaining species K shows different patterns, probably due to strong canopy exchange. Table V summarizes the results for the throughfall data with the assumption of a canopy exchange coefficient of 0.17 mequiv mT2(cm of rain)-' for Ca, and a negligible contribution from canopy exchange for Mg and Na. The values for v d O / p are obtained using a linear regression according to eq 3. The values of v d o are taken as the averages of the deposition velocities for the three shortest exposure times 78, 88.5, and 98 h. The value of this average Vdo is smaller than the actual value due to the length of exposure time. The estimated resuspension rate will be underestimated by -15% for p = lo4 s-' and by -70% for ,6 = s-'. The results show that the resuspension rates for all three species are on the order of 104-10-5 s-', very similar to results for S042-,C2042-,and Ca in Table 11,and similar to results for S042-at Gettysburg, PA (16). Deposition velocities to polycarbonate Petri dishes were determined simultaneously with the throughfall measurements and are shown in Figure 5 for Ca. Similar results were obtained for the other six chemical species. In contrast to the results for throughfall, there is no significant trend of decreasing deposition velocity for increasing exposure time. Values of the correlation coefficient ? for least squares linear regressions through these points for the seven chemical species are all less than 0.12. Thus the deposition velocity is not correlated with exposure time at a 95% confidence level. The apparently negligible effect of resuspension on deposition to Petri dishes may be due to several possible factors, e.g., electrostatic forces, fluid dynamic interference by the rim of Petri dish, and shorter exposure times.
0
riod Ati, and where there are n such time periods in the year. The effect of resuspension on the deposited mass per unit area can be seen in the following simplified case. We assume a constant airborne concentration and a constant deposition velocity throughout the year and complete washoff by each rain event. The dry periods between rain events in the throughfall experiments described above were used in the computation. The mean values of Ati for 1986, 1987, and 1988 were 119.6,129.7, and 138.4 h, respectively. If we assume a resuspension rate of lo4 s-l, the deposited mass per unit area calculated without accounting for resuspension is overestimated by 17% for measurements where Ati is 1day. By the same calculation, the deposited mass per unit area is underestimated by 8.1% for measurements where At, is l week. If we assume a resuspension rate of s-l, the same calculations shown an overestimation of 106% where Ati is 1day and an underestimation of 49% where Ati is 1week. With the estimated magnitudes of the resuspension rates in this study, the errors are greater than 20-30% for both 1-day and 1-week measurements. Greater differences are expected in the western United States where the dry periods are longer. Conclusions
The extent of resuspension for particulate chemical species at forest canopy sites has been examined using aerodynamic surrogate surfaces (symmetrical airfoils) and throughfall. The airfoils used Teflon film surfaces, and they were placed above, within, and below the crown of a mixed deciduous canopy to determine the vertical variation of deposition and resuspension. Except for the airborne concentration of C2042-,the airborne concentrations, deposition fluxes, and deposition velocities of both SO-: and C2042-are significantly higher above the canopy than those within and below the crown. The higher values of deposition velocity are due to the greater wind speeds above the canopy; the higher airborne concentrations at the top height suggest that sources of these species are outside of the canopy. In contrast, significant sources of P043-and Ca are within the canopy as shown by the maximum airborne concentrations within the crown, and the negative gradient of mass median diameter with height. The resuspension rates of SOP, C2042-,and Ca from the airfoils are on the order of 104-10-5 s-'. The resuspension rates within the crown are only slightly less than those above the canopy despite the much smaller mean wind speed within the crown. The similar magnitude of the resuspension rates at all three heights may be due to the effects of wind gusts penetrating deep within the canopy. Thus, the mean wind speed may not be a good parameter for estimating particle resuspension within the canopy. The resuspension of particles from a loblolly pine canopy has been estimated using throughfall data. Ca, Mg, and Na are used in this study to avoid the interference from gaseous phases and plant interactions. The resuspension rates for these species are also in the range of 104-10-5 s-'. Although more than one-third of the Ca in throughfall is from canopy exchange, the effect of canopy exchange on estimated resuspension rates is not as significant. Deposition to polycarbonate Petri dishes in the pine canopy has also been measured simultaneously with the throughfall collection, although for shorter exposure times. No significant resuspension of Ca, Mg, and Na is observed. The negligible effects of resuspension for the polycarbonate Petri dishes may be due to electrostatic forces, effects of the 1-cm rim of the dishes, and/or shorter exposure times. With these resuspension rates, the estimated annual dry deposition rate from daily and weekly measurements may 2434
Environ. Sci. Technol., Vol. 26, No. 12, 1992
be off by at least 20-30% in the eastern United States without taking resuspension into account. Greater differences are likely in the western United States because of the longer dry periods between rain events. Acknowledgments
We thank Ray Hosker, Tilden Meyers, Randy White, Mark Halls,and their colleagues at ATDD and Jim Owens and his colleagues at Oak Ridge National Laboratory for their help with the field experiments and laboratory measurements. Registry No. H02CC02H, 144-62-7;Ca, 7440-70-2;Mg, 7439-95-4;Na, 7440-23-5.
Literature Cited
Bagnold, R. A. The Physics of Blown Sand and Desert Dunes; Chapman & Hall: London, 1941. Chepil, W. S. Soil Sei. 1945, 60, 305. Gillette, D. A. In Precipitation Scavenging,Dry Deposition, and Resuspension, Proceedings of the Fourth Intemtional Conference;Pruppacher, H. R., Semonin, R. G., Slinn, W. G. N., Eds.; Elsevier: New York, 1983; pp 1047-1057. Anspaugh, L. R.; Shinn, J. H.; Phelps, P. L.; Kennedy, N. C. Health Phys. 1975,29, 571. Lassey, K. R. Health Phys. 1980,38, 749. Shinn, J. H.;Homan, D. N.; Gay, D. D. In Precipitation Scavenging, Dry Deposition, and Resuspension, Proceedings of the Fourth International Conference; Pruppacher, H. R., Semonin, R. G., Slinn,W. G. N., Eds.; Elsevier: New York, 1983;pp 1131-1145. Garland, J. A. In Precipitation Scavenging and Atmosphere-Surface Exchange Processes; Schwartz, S. E., Slinn, W. G. N., Eds.; Hemisphere: Washington, DC, 1992;pp 1605-1614. Sehmel, G. A. Environ. Int. 1980, 4 , 107. Nicholson, K.W. Atmos. Environ. 1988, 22, 2639. Davidson, C. I.; Wu, Y.-L. In Acid Precipitation: Sources, Deposition, and Campy Interactions; Lmdberg, S. E., Page, A. L., Norton, S. A., Eds.; Springer-Verb New York, 1990; Vol. 111, pp 102-216. Analyais of the SCE Precipitation Chemistry Data Base for Southern California Document P-D57&300; Prepared for Southern California Edison Co. by Environmental Research and Technology, 1986. Emissions Involved in Acidic Deposition Processes. In Acidic Deposition: State of Science and Technology; S/N 040-000-00571-4; National Acid Precipitation Assessment Program, Washington, DC, 1991;Vol. I, Report 1. Holsen, T. M.; Noll, K. E.; Liu, S. P.; Lee, W. J. Environ. Sci. Technol. 1991, 25, 1075. Gatz, D. F. In Precipitation Scavenging and Atmospheresurface Exchange Processes; Schwartz, S. E.; Slinn, W. G. N., Eds.; Hemisphere: Washington, DC, 1992;pp 1195-1212. Proceedings of the Dry Deposition Workshop of the National Acid Precipitation Assessment Program. Hicks, B. B., Wesely, M. L., Lindberg, S. E., Bromberg, S. M., Eds.; NOAA/ATDD: Oak Ridge, TN, 1986;Chapter 5. Wu, Y.-L.; Davidson, C. I.; Dolske, D. A.; Sherwood, S. I. Aerosol Sci. Technol. 1992, 16, 65. Orgill, M. M.;Peteraen, M. R.; Sehmel, G. A. In Proceedings of the Atmosphere-Surface Exchange of Particulate and Gaseous Pollutants;ERDA Symposium Series 38;National Technical Information Service: Springfield, VA, 1974; pp 813-834. Hutchison, B. A.; Matt, D. R.; McMillen, R. T.; Gross, L. T.; Tajchman, S. J.; Norman, J. M. J . Ecol. 1986, 74,635. Johnson, D. W., Van Hook, R. I., Eds. Analysis of Biogeochemical Cycling Processes in Walker Branch Watershed; Springer-Verlag: New York, 1989;Chapters 1-3. Wu, Y.-L.; Davidson, C. I. Air and Waste Management Association Annual Meeting, Anaheim, CA, 1989; Paper 89-140.6.
Envlron. Sei. Technol. 1992, 26, 2435-2439
Johnson, D. W.; Lindberg, S. E. Atmospheric Deposition and Forest Nutrient Cycling; Ecosystem Study Series 92; Spring-Verlag: New York, 1992. Lindberg, S. E.; Bredemeier, M.; Schaefer, D. A.; Qi, L. Atmos. Environ. 1990, 24A, 2207. Lovett, G. M.; Lindberg, S. E. J. Appl. Ecol. 1984,21,1013. Noll, K. E.; Fang, K. Y. P.; Yuen, P.-F. Air and Waste Management Association Annual Meeting, Anaheim, CA, 1989; Paper 89-140.7. Coe, J. M.; Lindberg, S. E. J . Air Pollut. Control Assoc. 1987, 37, 237. Milford, J. B.; Davidson, C. I. J. Air Pollut. Control Assoc. 1987, 37, 125. Milford, J. B.; Davidson, C. I. J. Air Pollut. Control Assoc. 1985, 35, 1249. Baldocchi, D. D.; Meyers, T. P. Boundary-Layer Meteorol. 1988, 43, 345. Hendry, C. D.; Brezonik, P. L.; Edgerton, E. S. In Atmospheric Pollutants in Natural Waters; Eisenreich, S. J., Ed.: Ann Arbor Science Publishers: Ann Arbor, MI, 1981; pp 196-215. Lindberg, S. E.; Lovett, G. M. Enuiron. Sei. Technol. 1985, 19, 238.Shaw, R. D.; Trimbee,A. M.; Minty, A.; Fricker, H.; Prepas, E. E. Water, Air, Soil Pollut. 1989, 43, 119. Kawamura, K.; Kaplan, I. R. Environ. Sei. Technol. 1986, 21, 105. Norton, R. B.; Roberta,J. M.; Huebert, B. J. Geophys. Res. Lett. 1981, 10, 517. Lovett, G. M.; Lindberg, S. E. Atmos. Environ. 1992,26A, 1469.
Shaw, R. H. In The Forest-Atmosphere Interactions; Hutchison, B. A., Hicks, B. B., Eds.; D. Reidel Publishing: Dordrecht, The Netherlands, 1985; pp 407-419. Baldocchi, D. D.; Hutchison, B. A. Boundary-Layer Meteorol. 1987, 40, 127. Owen, P. R. J . Fluid Mech. 1969, 39, 407. Wu, Y.-L.; Davidson, C. 1.; Russell, A. G. A stochastic model
for particle deposition and bounceoff.Aerosol Sei. Technol., in press. Braaten, D. A.; Paw U, K. T. In Precipitation Scavenging and Atmosphere-Surface Exchange Processes; Schwartz, S. E., Slinn,W. G. N., Eds.; Hemisphere: Washington, DC, 1992; pp 1143-1152. Wu, Y.-L.; Davidson, C. I.; Russell, A. G. Controlled wind tunnel experiments for particle bounceoff and reauspension. Aerosol Sei. Technol., in press. Shaw, R. H.; McCartney, H. A. Atmos. Environ. 1985,19, 827.
Zimon, A. D. Adhesion of Dust and Powder, 2nd ed.; Consultants Bureau: New York, 1982; Chapters 4 and 5. Schaefer, D. A.; Reiners, W. A. In Acid Precipitation: Sources, Deposition, and Canopy Interactions; Lindberg, S. E., Page, A. L., Norton, S. A., Fds.; Springer-Verlag: New York, 1990; Vol. 111, pp 241-284. Gould, T. R.; Davidson, C. I. In Precipitation Scavenging and Atmosphere-Surface Exchange Processes; Schwartz, S . E., Slinn,W. G. N., Eds.; Hemisphere: Washington, DC, 1992; pp 1115-1124. Braaten, D. A.; Paw U, K. T.; Shaw, R. H. J . Aerosol Sei. 1990, 21, 613. Wu, Y.-L.; Davidson, C. I.; Russell, A. G. In Precipitation Scavengingand Atmosphere-Surface Exchange Processes; Schwartz, S. E., Slinn, W. G. N., Eds.; Hemisphere: Washington, DC, 1992; pp 695-706. Received for review March 6,1992. Revised manuscript received June 15,1992. Accepted June 25,1992. This project is supported by National Science Foundation Grant ATM-8913723 and National Park Service Cooperative Agreements CA-0424-6-8002 and CA-0424-1-9005. Preliminary results of the surrogate surface experiments reported in this paper were first presented at The Fifth International Conference on Precipitation Scavenghg and Atmosphere-Surface Exchange Processes,Richland, W A (46).
Chloride Interference in the Analysis of Dissolved Organic Carbon by the Wet Oxidation Method George R. Alken US. Geological Survey, Water Resources Division, Box 25046, M.S. 458, Denver Federal Center, Denver, Colorado 80225-0046
The presence of Cl- in concentrations greater than 0.02 M is shown to interfere with the analysis of aqueous DOC concentrations by the wet oxidation method of analysis when a reaction time of 5 min is employed. Chloride competes with DOC for S2082-, lowering the overall oxidation efficiency. The resulting HOC1 from the oxidation of C1- reacts with DOC, producing significant amounts of chlorinated intermediate compounds in addition to COP. These compounds were found in the waste effluent from the reaction chamber and in the gas stream transporting C02to the detector. While a possible C1- effect has been noted for DOC measurements in the past, it has not previously been demonstrated to be a source of error a t the concentrations reported in this paper. The interference can be overcome either by increasing the digestion time or by diluting samples to contain less than 0.02 M C1-.
Introduction Since the early 1960s,dissolved organic carbon (DOC) concentration has become a common and important parameter measured in all types of water samples from saline to fresh waters. With the introduction of wet oxidation
methods for the analysis of DOC in seawater (I, 2), a number of papers have been published focusing on different technologies and problems inherent in the measurement of DOC (3-5). Presently, the most commonly employed methods for determining DOC concentrations involve the oxidation of organic matter to C02. These methods include high-temperature combustion (5),persulfate oxidation (2,4),and ultraviolet photooxidation (6). Difficulties arise in the analysis of DOC, in part, because of the nature of the samples themselves (3). The quantities of DOC present in a given sample may be low, as is the case in seawater and groundwater samples. In addition, DOC comprises a complex mixture of organic compounds that have a range of molecular sizes, weights, and reactivities. Other problems related to the method of analysis include the efficiency of oxidation, the possibility of sample contamination, and the difficulty of obtaining reasonable values for blanks (4). The method using wet oxidation with persulfate, for instance, is dependent on the efficiency of oxidation, which may not be the same for all compound classes or in all sample matrices. Other factors, such as the thermal degradation of persulfate at elevated temperature, also affect the reaction (7).
Not subject to U S . Copyright. Publlshed 1992 by the American Chemical Soclety
Environ. Scl. Technol., Vol. 26, No. 12, 1992 2435
