Environ. Sci. Technol. 1989, 23, 1396-1401
Atmospheric Transport and Deposition of Polychlorinated Dlbenzo-p -dioxins and Mbenzofurans Brian D. Eitzert and Ronald A. Hites'
School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Atmospheric transport and depositional processes of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) were studied by determining their concentrations in ambient air and rain samples. Total concentrations in ambient air range from tens of pg/m3 for urban locations to tenths of pg/m3 for rural locations. The average, total,concentration in rain from a suburban area is 90 pg/L. Comparison of the air and rain data gives washout ratio and Henry's law constant estimates. The total washout ratio ranges from 9300 to 90000, and the Henry's law constants range from 1.5 X 10" to 9.0 X lo-* atm.m3/mol. Sediment fluxes compare favorably with those predicted from atmospheric concentrations. Comparisons of PCDD/F profiles with other environmental compartments show a consistent and systematic change from the combustion source profile to the sedimentary sink profile. Introduction Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) are produced by combustion processes such as waste incineration (1-13). Once produced, these compounds are injected into the atmosphere, where they can be transported long distances before they are deposited (14). Because of the high toxicity of some of the individual PCDD/F (i.e., PCDD + PCDF) compounds, it is important to understand the transformations these compounds undergo during the transport and depositional processes. It is known that transformations are occurring. For example, there are noticeable changes in the PCDD/F profile (the relative amounts of the compounds grouped by level of chlorination). Combustion sources show a large range in concentrations and relative amounts of the various PCDD/F; even a single source will vary with changes in fuel and combustion temperature (13). Conversely, sediment samples (the ultimate environmental sink) consistently show the same relative amounts of the various PCDD/F (15-17). Though there is clear evidence that there are transformations occurring during transport and deposition, we do not fully understand the processes causing these transformations. Our understanding of these processes is limited by our lack of information on PCDD/F concentrations in the environmental compartments that link the combustion sources to the sedimentary sinks. These compartments are ambient air and rain. Furthermore, PCDD/F can be present in various phases in each compartment: they can be found in particle-bound and vapor phases in air and in particle-bound and dissolved phases in rain. These phases will interact with each other and undergo degradation processes (such as photodegradation) at different rates. Therefore, to fill this void, we have undertaken a study of PCDD/F in these environmental compartments. The primary goal of this project was to develop an understanding of PCDD/F characteristics that control the 'Current address: Connecticut Agricultural Experiment Station, 123 Huntington St., New Haven, CT 06504. 1396
Environ. Sci. Technol., Vol. 23, No. 11, 1989
interactions between these compartments. Thus, our study primarily focused on a single location where these compartments could be studied over a several-year period. The secondary focus was to examine other locations to estimate the extent of geographic variability. Finally, the study compared our atmospheric measurements to literature information on sources (1-13) and sinks (14-17)in order to explain the transformations that take place between them. This paper reports on the depositional processes. Thus, it examines the interaction between air and rain samples, assesses the geographic variability, and explains some of the source to sink transformations. The interactions between the vapor and particle-bound phases in the atmosphere are discussed elsewhere (18). Experimental Section Sampling Sites. The primary study location was Bloomington, IN. Bloomington is a city of approximately 50 000 people and the home of Indiana University. This location was chosen because there is a possibility that a municipal incinerator will be built that will also be used to combust PCB-contaminated materials. Thus, thisstudy also provides base-line data against which the performance of this incinerator could be evaluated. Air was sampled at four locations in Bloomington over a 3-year period (18). Samplers were located on the roofs of buildings to provide convenient access with some security. Rain samples were collected at one of these locations during the final year of this study. Two sites outside of Bloomington were chosen for samples with higher and lower degrees of urbanization. These were Indianapolis, IN, a city with a population of about 800 0o0, and Trout Lake, WI, a forested area 100 km south of Lake Superior. These sites were only sampled for 2week periods: Indianapolis in November 1987 and Trout Lake in August 1987. Although we obtained only a few samples from these sites (four for Indianapolis and three for Trout Lake), they enabled us to assess the geographic variability in the data. Sample Collection. Air samples were collected with high-volume air samplers (Sierra-Misco, Berkeley, CA, Model 650) modified to pass the air through a glass fiber filter, to collect particles, followed by a polyurethane foam plug, to collect vapor-phase compounds. Rain samples were collected with a stainless steel 1-m2 wet-only collector. Rain sensors were used to open the sampler and start a pump. The pump drove the rain through a series of three precleaned, l-pm, glass fiber fdters into a 20-Lglass carboy. Filters were used in series because the action of the pump often destroyed the first fdter. The filters were Soxhlet extracted in a 1:l mixture of acetone and methylene chloride for 24 h. The rainwater was liquid-liquid extracted with methylene chloride by stirring vigorously with a stir bar for 24 h. The methylene chloride layer was removed and passed through 60 g of anhydrous sodium sulfate to remove any residual water. Sample Analysis. Prior to extraction, samples were spiked with two carbon-13 labeled isotopic compounds: 1,2,3,7,8-pentachlorodibenzofuranand octachlorodi-
0013-936X/89/0923-1396$01.50/0
0 1989 American Chemical Society
Table I. Average PCDD/F Concentrations for Ambient Air and Rain Samples Taken in Bloomington, IN
PCDD/F F4' F5 F6 F7 F8 D4 D5 D6 D7 D8
air concn, fg/m3 vapor particle 280 140 35 5.2 1.4 0.6 32 36 5.9 4.1
22
83 98 70 26 0.6 11
120 370 590
rain concn, pg/L dissolved particle 4.5 1.3 0.3 0.3 0.3 0.1 0.2 0.2 1.6 11
1.2
1.5 1.0 2.1
0.3 0.2 0.2 1.4 22
43
F refers to dibenzofurans and D to dibenzo-p-dioxins; the number refers to the level of chlorination. For example, F4 refers to the concentration of all the tetrachlorodibenzofurans summed together regardless of the positions of the chlorine substituents.
benzo-p-dioxin (Cambridge Isotope Laboratories, Woburn, MA; and KOR Isotopes, Cambridge, MA). These were used as internal standards. Sample extracts were taken through a two-step column chromatographic cleanup procedure described elsewhere (18,19). The purified extracts were concentrated to a final volume of approximately 50 pL prior to analysis by gas chromatographic mass spectrometry (GC/MS). Quantitation. All samples were analyzed by gas chromatographic low-resolution mass spectrometry operated in the electron capture negative ion mode with methane as the reagent gas. Selected ion monitoring was used to enhance sensitivity. Detection limits were as low as 1 fg/m3 for air samples and 0.1 pg/L for rain samples. The methods have an accuracy of f25% as compared to high-resolution mass spectrometry. For details on these quantitation procedures, see the companion paper (18). Data Reduction. The large number of samples in several phases required the use of statistical procedures to systematically reduce the data so that conclusions could be drawn. Use of the Statistical Analysis System (SAS Institute Inc., Cary, NC) determined that, within a given level of chlorination, all GC/MS peaks correlated from sample to sample. Therefore, concentrations summed over PCDD/F with a given level of chlorination were used in data analysis. Our statistical analyses also showed that the data distribution more closely approached log-normal than normal; thus, geometric means were used. If no PCDD/F of a given level of chlorination were detected, values of 0.05 fg/m3 for air or 0.1 pg/L for rain were used in the geometric average. When principal-component procedures were used to compare sample profiles, it was necessary to normalize the data to remove concentration effects; thus, the data for each level of chlorination were normalized to the total PCDD/F concentration in that sample.
Results and Discussion Rain Washout. Our sample collection procedures define our phases. Thus, any material found on the filters was considered particle bound for either air or rain; material found in the polyurethane foam plug or in the rainwater was considered vapor phase or dissolved, respectively. There are obviously some limitations. In the air, PCDD/F can vaporize from the filters and be trapped on the plug; while in the rainwater, the compounds could dissolve from or adsorb to the filtered material. We have already shown that the four Bloomington sites are essentially equivalent, and therefore, we have averaged them together (18). The average concentrations of the
Table 11. Washout Estimates Based on Average Bloomington Air and Rain Concentrations'
PCDD/F F4 F5 F6 F7 F8 D5 D6 D7 D8
washout ratio -log H," gas CW,) part. CWJ total (w) % P b atmms/mol 16000 55OOO 19OOO 21 4.83 9300 18000 12 OOO 51 4.58 10OOO 8600 9800 77 4.56 58 000 32 OOO 30 OOO 87 5.39 210 000 12 OOO 21 000 52 5.77 18000 50 4.42 6300 9300 5600 10OOO 12 000 88 4.37 59 OOO 270 000 62 OOO 92 6.06 2700000 72 OOO 90 OOO 80 7.05
'The tetrachloro-PCDD are not listed because they were rarely detected. bPercent washout due to the particle phase, calculated by dividing W,,4 by W. CHenry'slaw constant. tetra- through octachloro-PCDD/F in each of the four phases for Bloomington air and rain are presented in Table I. The less chlorinated PCDD/F tend to have a greater proportion of their concentrations in the vapor and dissolved phases, while the more chlorinated PCDD/F tend to prefer the particle-bound phases. For example, the ratio of vapor to particle-bound concentrations for the PCDF in air ranges from 13 for the tetrachloro down to 0.05 for the octachloro compounds. The total concentrations (vapor plus particle for air or dissolved plus particle for rainwater) show the same trends in air and rainwater: The PCDF decrease in concentration with increasing level of chlorination while the PCDD increase in concentration with level of chlorination. We can compare these air and rain concentrations to one another using the concept of washout (20). This is expressed as the ratio between the concentration in rain and the concentration in air on a volume to volume basis. This ratio, usually given by the symbol W, is a measure of scavenging efficiency. As a rain event occurs, compounds are scavenged from the air and transferred into the rainwater. Depending on the intensity and duration of the rain event, the amount transferred from the air to the rain varies. Thus, washout measurements are highly variable (2&22), and they are ideally measured by determining the concentrations of the compound in air and rain samples taken simultaneously. We could not use this approach because the large sample volumes necessary for air measurements required us to sample for longer time periods than the duration of most rain events. We did, however, estimate washout by using the average concentrations measured in our ambient rain and air data. There are two distinct and separate phenomena to be examined. Gas scavenging is the equilibrium process in which a compound partitions between vapor and aqueous phases; its measure is W,,which is the ratio of the raindissolved phase to air vapor phase concentrations. Particle scavenging is the process by which rainfall removes particles and the compounds bound to those particles from the atmosphere; its measure is W,,which is the ratio of the rain particle phase to air particle phase concentrations. Our calculated W,and W,values are given in Table 11, second and third columns. The differences between the two processes are demonstrated in Figure 1, which plots the washout ratios as a function of the vapor pressure averaged for each level of chlorination. Note the strong correlation for W,(significant at the 1%level) and the lack of correlation for W,. The latter indicates that particle washout is a physical process acting on the particle, and therefore, all of the compounds bound to the particle are effected similarly. Environ. Sci. Technol., Vol. 23,No. 11, 1989
1397
920 fc/&
IUD14NAPOLIS
WP r = 0.10 --E-
wg
r = 0.79 .... ....
F1 F 5 F 6 F 7 F 8 D 4 D 5 D 6 D 7 D 8
~
590 fgim3
BLOOhllUGTOI
s4iI
-9
-8
-7
-6
I -5
Log (vapor p r e s s u r e ) Figure 1. Washout ratios (except for the tetrachloro-PCDD) as a function of average vapor pressure. The latter are from ref 18. The data have been averaged over each level of chlorination.
Conversely, vapor washout appears to be highly dependent on the compounds vapor pressure and therefore must involve a different process than particle washout. The vapor washout process involves the dissolution of the gas phase into water. If it is assumed that, during rainfall, equilibrium is reached between the gas and dissolved phases [which Slinn et al. (23)estimated to be the case], then W,can be used to estimate the Henry’s law constant for a compound. This constant (H) is the equilibrium ratio of a compound’s vapor pressure to its solubility at a given temperature. Thus,for gas scavenging, as shown by Ligocki et al. (21) H = RT/ W, (1) where R is the gas constant and T is the absolute temperature. Vapor pressure and solubility are both temperature-dependent variables, and thus, Henry’s law constants show large variations with temperature (24). Because our data cover a broad range of temperatures, our Henry’s law constants can only be considered estimates. The Henry’s law constants were calculated by using an average temperature of 290 O K and are given in the last column of Table 11. Shiu et al. (25)reported that Henry’s law constants for several PCDD ranged from 3.7 X lod to 1.3 X lo*, which compare quite favorably with our data. These estimated Henry’s law constants are a factor of 10-100 lower than those measured for PCB with the same number of chlorines at 25 “C (24). The measured particle scavenging ratios, presented in Table 11, are quite similar to those measured by Ligocki et al. (22) for semivolatile organic compounds, but they are lower than those reviewed by McMahon and Denison (26) and Slinn et al. (23). Lower particle scavenging ratios might reflect either below cloud scavenging (22) or the possibility that PCDD/F are associated with the smaller particles, which are scavenged less efficiently (23, 27). Total washout is given by (20, 22) (2) w = Wg(l - 9)+ WP$ where q5 is the fraction of the total air concentration bound to particles. Dividing Wp$ by W allows us to determine which type of scavenging is the dominant process for a given compound. Total washout values and percent particle scavenging are presented in Table 11. These data are in the same range (though on the low side) as several experimental and predicted values for organochlorines as reviewed by Bidleman (20). From these measured washout variables we draw several conclusions: (a) For PCDD/F particle scavenging is gen1398 Environ. Sci. Technol., Vol. 23, No. 11, 1989
~
F4F5F6F7F8 D4D5D6D7D8
-9
T R O L T LAkE
-&
F 4 F 5 F6 F7 F8 D 4 D 5 D 6 D 7 D 8 Figwe 2. Geometric avConcentration of the tetra-, penta-, hexa-, hepta-, and octachloro-PCDF (F4-F8, respecttvely)and tetra-, penta-, hexa-, hepta-, and octachloro-PCDD (D4-D8, respectively) in ambient air from Indianapolis, IN;Bloomington, IN;and Trout Lake, WI. The solid portion of the bar is partlcle bound whlle the blank portion is vapor phase. The total concentration (vapor phase particle) of octachloroPCDD is given as a scaling factor.
erally a somewhat more important process than gas scavenging. (b) Total scavenging efficiency of PCDD/F generally increases with level of chlorination. (c) Henry’s law constants for the less chlorinated PCDD/F are in a range where there might be some volatilization of these compounds from water in lakes (28). These observations suggest that the more chlorinated PCDD/F have a greater tendency to reach the sedimentary sinks than the less chlorinated PCDD/F. In other words, wet depositional processes favor a sedimentary profile that shows higher concentrations of the more chlorinated PCDD/F. This is, in fact, what is seen. Geographic Variability. Analysis of air samples from locations other than Bloomington allows us to assess geographic variability and its causes. Figure 2 shows the average PCDD/F profiles for air samples at our three sampled locations. Even though the Indianapolis and Trout Lake locations were only sampled for 2-week periods, it is clear that there are some significant differences between the locations. The primary difference is in concentrations. Average PCDD/F concentrations from Indianapolis are 1.6-6 times higher than in Bloomington and 7-30 times higher than at Trout Lake. However, when differences in the total suspended particle concentration at the three sites are taken into account, the factors are much smaller. Thus, the ratio of the total PCDD/F concentration to the total suspended particle concentration is only 2.5 times greater in Indianapolis than Trout Lake. That urban sites such as Indianapolis have more PCDD/F in their air is not surprising; there are clearly more numerous and more intense sources of these compounds in urban locations than in rural locations. This concentration trend can also be found in the limited PCDD/F ambient air concentration data in the literature (29-33). These data are complied in Table III. Note, that the concentrations of our three sampling locations fall correctly on the continuum created moving from industrial to urban to suburban to rural. This assures us that our
Table 111. Comparison of This Study’s Air Concentration Measurements with Those of Previous Workers”
sampleb
location type
a b
industrial auto tunnel industrial urban urban and industrial
C
d e f
concn, pg/m3 total PCDF total PCDD 15 13 10 8.8
13 16 9 8.6 3.2
Indianapolis
5.5 2.6
g
suburban
0.74
1.8
h
Bloomington
0.78
i
suburban
0.50
j
Trout Lake
0.18
1.1 1.0 0.24
2.5
“Samples in bold face are from this study. bReferences: a, average of sites 6 and 7 from Tables 2-7 in ref 32; b, average of site 8 from Tables 2-7 in ref 32; c, average of CAM site from Table 2 in ref 33; d, determined from Figure 4 in ref 31; e, from Table 9 in ref 30; g, average of sites 1 and 2 from Tables 2-7 in ref 32; i, average of HRB site from Table 2 in ref 33.
590 w m 3
h--J 4VERAGE AIR
F 4 F 5 F6 F7 F8 D 4 D 5 D 6 D 7 D 8 54 P d L
2 AVERAGE RAIN
F 4 F 5 F 6 F7 F8 D 4 D 5 D 6 D 7 D 8 1100 P d S
GREAT LAKES S E D I M E N T
.__.I F 4 F 5 F6 F 7 F 8 D 4 D 5 D 6 D 7 D 8
Flgure 4. Average concentratlons of PCDD/F in air and rain taken in Bloomington, IN (this work) and in surface sediments from the Great Lakes (from ref 18). See Figure 2 for the key. The solM portion of the bars represents the particiabound phases for the air and rain samples while the blank portion represents the vapor and dissolved phases, respectively. I
0 ~
(4)
(2)
0
2
4
6
Prin Comp 1 Figure 3. Principal-componentsanalysis of PCDD/F In Indianapolis, Trout Lake, and Bloomington a t samples; the first principal component vs the second principal component is plotted.
samples are truly representative of what is seen in ambient air. Other differences shown in Figure 2 are related to changes in the relative amounts of PCDD/F at the three locations. The limited sampling time for the Indianopolis and Trout Lake locations means that these locations did not have the broad range of sample temperatures seen in Bloomington. Because temperature is a primary determinant of a compound’s vapor to particle ratio (181,it would not be prudent to use vapor and particle-bound concentrations separately in this discussion. Thus, we will only consider the total (vapor plus particle) PCDD/F concentrations and patterns. The profiles (see Figure 2) show similarities: the tetrachloro-PCDD or octachloroPCDF levels are always low, and the hepta- and octachloro-PCDD levels are always high. There are also differences: the Trout Lake and Bloomington locations appear to be more similar to each other than to Indianapolis. To compare the PCDD/F profiles from the three locations in a more formal way, we performed a principalcomponents analysis on the data from each individual sample. The first two components accounted for 56% of the variability between the samples and are plotted in Figure 3. Note that the four Indianapolis samples all have principal component 1values greater than 1.5 and are on the edge of the Bloomington cluster. The three Trout Lake samples, however, all lie within the region where the majority of the Bloomington samples are seen; thus, they have essentially the same profile as Bloomington. Clearly, there is a difference in the PCDD/F profile between urban and suburban locations; suburban and rural locations seem to be more similar. The observed geographic variability suggests the following atmospheric transport scenario. Urban air is con-
taminated with PCDD/F by proximity to the combustion sources of these compounds. As the air mass moves away from the urban area, it is diluted with cleaner air, lowering the PCDD/F concentrations. As the air is transported, transformations occur changing the profile. One transformation is photodegradation of vapor-phase PCDD/F (34). The less chlorinated PCDD/F have greater proportions of their total concentration in the vapor phase. Thus, vapor-phase photodegradation during the transport process would have a greater effect on the less chlorinated PCDD/F. Like the washout process, these degradation processes would favor an ultimate PCDD/F profile with enhanced concentrations of the more chlorinated compounds. Depositional Fluxes. It is useful to compare the observed air and rain concentration data with flux data from sediments. This can be done qualitatively by comparing the profiles, as well as quantitatively by comparing depositional fluxes. The qualitative comparison is shown in Figure 4, which plots the profiles of Bloomington air and rain and of average Great Lakes sediments as reported by Czuczwa (16). Note that all three profiles show octachloro-PCDD to be the highest in concentration. There appears to be considerable similarity between the particle-bound portion of air,total rain, and sediment. Thus,it appears that particle and wet deposition are important processes, and they should be studied quantitatively. Deposition is best measured in terms of flux, which is the amount deposited per unit area per unit time. For rain, the flux can be easily calculated by multiplying the average rain concentration by the yearly rainfall (100 cm/year in the Great Lakes region). For sediments, flux is calculated by dating the sediment, determining the concentration, accounting for sediment focusing (not all sediment comes from the area directly above it), and determining the amount of compound deposited per year. Particle deposition is dependent on the deposition velocity of the particle, which varies with the nature of the particle (23). Fortunately, a recent study estimated the deposition velocity of combustion-type particles by deEnviron. Sci. Technol., Vol. 23. No. l l , 1989
1399
Table IV. Comparison of Measured Sediment and Calculated Atmospheric Depositional Fluxes at Various Locationso location
type
F4
F5
F6
F7
F8
D4
D5
D6
D7
D8
total
Trout Lake Bloomington Bloomington Bloomington Indianapolis av Great Lakes Siskiwit Lake Bloomington
air part. rain air part. dry wet air part. sediment sediment dry/wet
0.27 0.57 0.69 1.3 5.7 1.2 0.47 1.2
0.38 0.28 2.6 2.9 18 1.9 0.16 9.3
0.38 0.13 3.1 3.2 21 1.8 0.06 24
0.21 0.24 2.2 2.4 7.9 11 0.63 9.2
0.16 0.07 0.82 0.89 2.3 4.0 0.13 12
0.002 0.03 0.02 0.05
0.03 0.04 0.35 0.39 3.8 1.7 0.38 8.8
0.91 0.16 3.7 3.9 16 3.1 0.32 23
2.3 2.3 11.7 14 25 15 2.2 5.1
2.8 5.4 19 24 29 94 18 3.4
7.4 9.2 44 53 130 140 23 4.7
+
0.01
1.2 0.83 0.70
Sediment fluxes were calculated by Czuczwa (16). All fluxes are pg cm-2 yr-’.
termining deposition velocities of polycyclic aromatic hydrocarbons in air (27). This estimate is 1 cm/s, and it is within the range of reported values of deposition velocities. Vapor-phase deposition will depend on the rate of mass transfer between two phases, and it is highly dependent on the nature of the surface. Thus, deposition to the surfaces of a smooth lake, a choppy lake, and a leaf are different. We have assumed that this is not an important process, and we have not made estimates of the associated flux. This assumption will be validated later. Depositional flux estimates for Bloomington rain, air particles from all three locations, and average Great Lakes and Siskiwit Lake sediments are presented in Table IV. Given the errors in the analysis of each compartment, the inherent approximations necessary to make the calculations, and the difference in geographic locations, the air and rain fluxes are quite similar to those actually measured in sediment. For example, Siskiwit Lake, a remote lake on Isle Royal about 55 km south of Thunder Bay, Ontario, shows fluxes similar to those calculated at Trout Lake, the rural location. The Great Lakes average fluxes are similar to the total Bloomington values and to the Indianapolis particle values. This seems correct: there are both highly industrial and rural areas in the Great Lakes’ airshed. The greatest discrepancy appears to be in the octachloro-PCDD flux, which is 3-5 times higher in the Great Lakes sediment than the fluxes from the atmospheric data. This could have several possible causes: (a) the inherent errors associated with these analyses, (b) additional sources of octachloro-PCDD to the sediment, or (c) other depositional processes that we do not fully understand. We can also use the Bloomington fluxes to estimate the relative importance of the wet and dry pathways. The geometric average dry to wet deposition ratio was 6:1, the total (essentially a weighted average) dry to wet ratio was 51. These data compare quite favorably with the 91 ratio reported by McVeety and Hites (27) for PAH and with the 1O:l ratio reported for chloroorganics by Villeneuve and Cattini (35).Clearly, the dry deposition of particles is the more important of the two depositional pathways. Source to Sink Transformation Conjecture. We compared profiles from the different compartments encountered between the sources and sinks of PCDD/F. Because the concentration data from each compartment are reported in different units, we normalized the data to the total PCDD/F concentration of each compartment. The normalized data were analyzed with a principal-components procedure. The first two principal components, which represent 57% of the variability, are plotted in Figure 5, which shows how the profiles change between the sources and the sinks. The various combustion sources are spread around the plot; this indicates that these sources generate a broad spectrum of profiles. The urban/industrial ambient air samples group together in the middle of the sources. Thus, the mixture of different emissions in an urban area produces a fairly consistent profile. The 1400
Environ. Sci. Technol., Vol. 23, No. 11, 1989
3.
‘1
ni
’
*
1 t
Sediment I
t,
**
*
*
*
Urban Air A Suburban Air Particles
0
c
0
’
Vapor
(1)-
A
‘Z (2)a
Sources ~
(
( 2*)
0
I
2
4
Rain
1
Prin Comp 1 Flgure 5. Principakomponents analysis of PCDDIF in various environmental samples reported in this work and in the literature; the first principal component vs the second principal component Is plotted. Multiple samples from any locetbn were averaged to generate a single data set for each location. The data sources are as follows: surface sediment data, ( 76); urban and industrial ambient air, this study and ref 30-33; ambient air particles, this study: ambient air vapors, this study; combustion sources, ( 7 , 3, 5-8, 72, 73, 75); rain, this study.
suburban/rural samples form a second subgroup, which has undergone some changes so that the profiles are between the urban samples and the cluster of sediment samples. Of particular interest are the samples connected by lines. These are the averages for the sample locations of this study. Note, that when an air sample is split into its vapor and particle-bound constituents, the particle-bound portion is translated toward the sediment cluster while the vapor phase is moved away from the cluster. Similarly, when the rain sample is split into the dissolved and particle-bound constituents, the particle bound remains in the sediment cluster while the dissolved moves away from the cluster. Note also, that the vapor-particle-bound translation in air parallels the dissolved-particle-bound translation in rainwater. These translations indicate that particle deposition processes (wet and dry) control PCDD/F deposition because they are closer to the deposited (sediment) profile. This agrees with our data in Table I1 showing that particle washout is more important and validates our neglect of dry vapor deposition. To summarize, the data support the following scenario. A broad range of PCDD/F are injected into the atmosphere by numerous combustion sources forming a uniform, urban, ambient air mixture. As the air mass moves away from the sources, it is diluted with cleaner air and starts to “age”. Less chlorinated PCDD/F are found to a greater extent in the vapor phase, and thus, they seem to undergo a greater degree of photodegradation. This enhances the relative concentrations of the more chlorinated PCDD/F. The particles with their enhanced load of the more chlorinated PCDD/F are deposited by both wet and dry processes. Although the dry process dominates, the efficiency of the wet method improves for the more chlorinated PCDD/F. Once in the water column, Henry’s law constants predict greater vaporization to be occurring for the less chlorinated PCDD/F. This further
enhances the relative proportion of the more chlorinated PCDD/F that pass through the water column to the sediment. Every process that occurs favors a profile enriched in the more chlorinated PCDD/F. It is, therefore not surprising that the sediment profiles are most enriched in octachloro-PCDD and that the next most abundant are the heptachloro-PCDD and -PCDF. Although, the sedimentary octachloro-PCDFconcentration is somewhat less than might be expected, in most of the combustion sources it is one of the least abundant. Therefore, its sediment levels may still represent an enhancement over its original abundance. Our data indicate that only the most chlorinated PCDD/F are environmentally persistent. This finding may be of interest to policymakers because these PCDD/F tend to be the less toxic.
Acknowledgments We thank Monroe County, IN, the Monroe County Community School Corp., the state of Wisconsin Department of Natural Resources, Indiana UniversityPurdue University at Indianapolis, and Mr. Jerry Chasteen for allowing us to locate and operate our air samplers on their properties. We also thank R. Harless for helpful discussions and I. Basu for laboratory assistance.
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