Environ. Sci. Technol. 2000, 34, 2952-2958
Insights into the Global Distribution of Polychlorinated Dibenzo-p-dioxins and Dibenzofurans DIANE M. WAGROWSKI RONALD A. HITES*
#
AND
School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Polychlorinated dibenzo-p-dioxins and dibenzofurans were measured in 63 pairs of tree bark and soil samples. Maps of lipid-adjusted concentrations in bark and fluxes to soil indicated that Vancouver Island, the Midwestern United States, Germany, and Hong Kong were areas of high PCDD/F deposition. Concentrations and fluxes in the regions north of the 60th parallel, particularly the Canadian Arctic, were low, indicating that PCDD/F do not move appreciably from warm to cold latitudes. Linear regressions of the PCDD/F concentrations in tree bark versus fluxes to soil showed that total concentrations in bark can be used to estimate total fluxes to soil in a particular region. Comparison of the homologue profiles for each pair of bark and soil samples indicated that the pairs fell into three categories: 1. bark and soil both resembled source profiles; 2. bark and soil both resembled sink profiles; and 3. bark resembled a source profile but the soil resembled a sink profile. This variation in homologue profiles may be due to the proximity of sampling locations to sources. We found that anthropogenic NOx emissions are highly correlated to PCDD/F soil fluxes, and we used this regression to estimate global PCDD/F fluxes to soil on the same spatial scale as the NOx data. Multiplying these fluxes by the corresponding land areas, we estimated that total PCDD/F deposition to the earth’s land surface is about 2-15 t/yr.
Introduction Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) are well-known, toxic environmental contaminants that were never intentionally manufactured. Rather, they are byproducts of the synthesis and combustion of chlorinated organic chemicals such as chlorophenols and other chlorinated aromatic compounds. Although the manufacture and use of such chemicals is banned or heavily regulated in most countries, these products and their contaminants are ubiquitous and persistent in the environment (1). Combustion processes are believed to be the source of most PCDD/F to the environment (2, 3). Emissions from municipal waste incinerators, iron and steel production, cement kilns, secondary copper smelters, medical waste incinerators, and automobiles all contribute to the release of PCDD/F into the environment (2). Once emitted, * Corresponding author e-mail:
[email protected]. # Current address: Dow AgroSciences LLC, 9330 Zionsville Rd., Indianapolis, IN 46268. 2952
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PCDD/F move from these sources, some react in the atmosphere, some deposit to the earth through wet and dry deposition, and some eventually accumulate in human tissue (4-6). Clearly, knowledge of the global behavior and fate of PCDD/F is important. Measuring PCDD/F atmospheric deposition to the earth directly is difficult; instead, deposition is commonly estimated from concentrations determined in other matrices such as air, precipitation, and sediment (7-12). Recently, soil (which is easier to collect than the above matrices and is found globally) has been used to provide a record of PCDD/F deposition similar to that of sediment cores (13). Brzuzy and Hites (2) determined PCDD/F concentrations in 107 soil samples from around the world and calculated depositional fluxes to the soil. They then divided the earth into climate zones and calculated mean fluxes to soil in each zone. Based on these data, Brzuzy and Hites calculated that 13 ( 2 tonnes of PCDD/F were deposited per year from the atmosphere to the earth, while total annual emissions were 3 ( 0.6 tonnes/ yr. This lack of mass balance suggests that deposition is approximately four times greater than annual emissions (2) or that there may be significant uncertainties in both emission and deposition estimates. For example, it is possible that all sources have not yet been identified, resulting in the low emission estimate. Furthermore, emission factors (quantity of PCDD/F formed per amount of material combusted) have not been adequately determined for many types of combustion sources, and the effect of new regulations and emissions controls on each source is also difficult to estimate (14, 15). Estimating deposition of PCDD/F from the atmosphere to the earth using soil can be tricky. Collecting soil samples is often difficult due to obstructions such as roots and stones, and the mixing of soil layers is common. Another limitation of the Brzuzy and Hites global soil study was the scarcity of samples collected from Canada and Asia, leading to potential errors in depositional estimates to those areas (2). In addition, since PCDD/F are the products of combustion in industrialized areas, climate (as used by Brzuzy and Hites) may not be a satisfactory surrogate for linking PCDD/F sources to sinks. Because PCDD/F have low vapor pressures (10-6 to 10-9 Torr), it is likely that these compounds do not move very far from sources or do not move from warm to cold latitudes (16, 17). There are, however, few environmental field data to support this suggestion. This lack of field data makes it difficult to estimate how far PCDD/F might travel and to calculate global depositional fluxes. To address these issues, we chose to examine tree bark instead of soil as a sink for PCDD/F. Bark is an excellent scavenger of semivolatile organic compounds due to its high lipid content and large surface area (18, 19). Bark is found throughout the world and is easily collected and shipped. Research on the global distribution of organochlorine pesticides indicates that, even in remote locations, trace amounts of these compounds are found in tree bark (19). We obtained paired bark and soil samples from global locations, including 18 pairs of samples from western Canada. Our goals were to (a) compare the total PCDD/F concentrations in bark and soil, (b) determine if bark instead of soil can be used to investigate the global distribution of PCDD/F, (c) determine the extent, if any, of long-range transport from sources, (d) determine if global inventories of anthropogenic NOx emissions (20) are a good surrogate for PCDD/F source emissions, and (e) recalculate the deposition of PCDD/F from the atmosphere to the earth. 10.1021/es991138o CCC: $19.00
2000 American Chemical Society Published on Web 06/08/2000
Experimental Section Sample Collection. Locations that appeared to be untouched by humans were selected as sampling sites. Permission from landowners or national park administrators was obtained before sample collection. Samples were collected between 1996 and 1998. Tree bark was collected at a height of 1 m from the ground in an area of the tree that did not contain moss or lichens. A 5 cm × 15 cm area of bark was chiseled onto a piece of aluminum foil. Soil was collected directly under the tree canopy and on the same side of the tree as the bark sample. Soil was collected with a hand trowel, which was used to dig out an area of 10 cm × 10 cm. The sample was dug out to a depth that corresponded to the organic layer depth of the soil type, up to 15 cm in depth. Previous research indicated that greater than 80% of PCDD/F are found in the upper 15 cm of a soil core (13). The sample was placed onto a piece of foil that was subsequently sealed in a clean plastic bag. Bark and soil import permits were obtained from the United States Department of Agriculture, Animal and Plant Health Inspection Service, Plant Protection and Quarantine Programs. All samples were shipped to USDA inspection stations, inspected, and then shipped to our laboratory at Indiana University. In the summer of 1997, we collected samples from western Canada (Alberta, British Columbia, and the Yukon Territory) from the U.S./Canadian border to the Arctic Circle. A permit for sample collection in the Yukon Territory, Canada, was obtained from the Heritage Branch, Department of Tourism, Whitehorse, Yukon Territory. Geographical locations in western Canada were obtained using an Eagle Explorer global positioning system (GPS). A 5 cm × 15 cm area of tree bark was chiseled into a clean glass jar. Soil samples were obtained by using a soil corer (Clements Associates Inc., Newton, IA) fitted with a 30-cm long, clean PETG Copolyester sample tube or with a hand trowel as previously described. The tubes were capped with vinyl lids, taped shut with electrical tape, wrapped in aluminum foil, and sealed in a plastic bag. In the laboratory, all samples were stored at -20 °C until extraction. Sample Extraction and Cleanup. We extracted replicate bark samples in a variety of solvent systems to determine the most efficient extraction system. We found that 50% hexane in acetone gave the best compromise between recovery, reproducibility, and speed of evaporation. Additionally, to address stability issues during transport from sampling locations to our laboratory, we collected bark and soil from a backyard in Bloomington, IN. A portion of each sample was frozen immediately after collection; the remainder of the sample was stored in the trunk of a car for 3 weeks in early summer. The relative percent difference between the frozen and car-stored samples (averaged over the 10 homologue concentrations) was < 25%, which is about the precision of the measurements. Therefore, we know that shipping and storing of the sealed samples at ambient temperatures does not affect the PCDD/F concentrations. In the laboratory, 10-20 g of bark were broken into small pieces (dimensions of a few millimeters) with a precleaned hammer and chisel. These pieces were then placed into a glass Soxhlet thimble plugged with glass wool, weighed, and directly spiked with an internal standard solution consisting of known amounts of 13C12-1,2,3,4-TCDF, 13C12-1,2,3,7,8PnCDF, 13C12-1,2,3,6,7,8-HxCDD, 13C12-1,2,3,4,6,7,8-HpCDD, and 13C12-OCDD (Cambridge Isotope Laboratories, Inc., Andover, MA). Between 20 and 35 g of soil were weighed into a beaker and mixed with an equal amount of anhydrous sodium sulfate. This mixture was allowed to sit for 30 min. Samples were then mixed, directly spiked with the above internal standard solution, mixed again, and placed into a glass Soxhlet thimble stopped with glass wool. All samples were
Soxhlet extracted with a 50% mixture of acetone in hexane (EM Science, Gibbstown, NJ) for 24 h. After extraction, the extracts were rotary evaporated to 25 mL, solvent exchanged two times with 50 mL of hexane, and rotary evaporated to 10 mL. These extracts were transferred to a glass centrifuge tube and diluted to 25 mL. To remove polar interferents, 25 mL of HPLC grade water (EM Science, Gibbstown, NJ) was added to the centrifuge tube, and the tube was shaken and and then centrifuged for 3 min. The hexane layer was removed and retained, while the water layer was discarded. Fresh water (25 mL) was added to the original hexane layer, and this procedure was repeated. The procedure was repeated until the water layer was no longer turbid (a total of 3-5 times). The hexane layer was transferred to a flask and rotary evaporated to 1 mL for subsequent silica gel column chromatography. PCDD/F were isolated from the above bark and soil sample extracts by silica gel column chromatography. Silica gel (Grace Davison, Baltimore, MD) was Soxhlet extracted for 24 h with dichloromethane. The silica gel was then activated at 160 °C for 24 h, deactivated with 1% water by weight, and equilibrated for 24 h. The silica was loaded into a 1.5 cm i.d. × 25 cm long column in a hexane slurry to a height of 20 cm and capped with 1 cm of anhydrous sodium sulfate. The sample was transferred onto the column and eluted with 75 mL each of hexane, 15% dichloromethane in hexane, and dichloromethane. PCDD/F eluted in the first two fractions. These fractions were combined, rotary evaporated to 25 mL, solvent exchanged two times with 50 mL of hexane, and concentrated to less than 1 mL for subsequent alumina column chromatography (to remove interfering organochlorine compounds). Alumina (ICN Biomedicals, Inc., Costa Mesa, CA) was activated for 12 h at 160 °C. The alumina was dry-loaded into a 0.5 cm i.d. × 9.5 cm Pasteur pipet to a height of 6.5 cm. The column was capped with 0.5 cm of anhydrous sodium sulfate and wetted with hexane. The sample was transferred onto the column and eluted with 8 mL each of hexane, 2% dichloromethane in hexane, and 40% dichloromethane in hexane. The PCDD/F eluted in the third fraction, which was concentrated to less than 20 µL under a gentle stream of nitrogen. Procedural blanks (no sample) were run with every set of extracts. When PCDD/F were found in the blanks, data for that set of extracts were not used. Recoveries of known amounts of PCDD/F were 65-110% for the entire procedure for both matrices. Previous work (13) showed that duplicate soil samples gave PCDD/F measurements that differed by, on average, (21%. Moisture and Lipid Determination. Between 10 and 20 g of bark or soil were weighed into beakers, dried at 105 °C for 24 h, and reweighed to obtain the percent moisture. A 50% mixture of hexane in acetone was added to cover each sample, and the samples were sonicated once for 1 h. The solvent was filtered, decanted into a preweighed drying pan, and allowed to evaporate. The procedure was repeated. The pans were reweighed, and the percent lipids was calculated (21). Sample Analysis. Samples were analyzed for PCDD/F on a Hewlett-Packard 5973 gas chromatographic mass spectrometer equipped with a 30 m × 250 µm i.d. DB-5MS capillary column with a 0.25 µm film thickness (J & W Scientific, Folsom, CA). The sample (2 µL) was injected with a 25 psi pulsed injection in the splitless mode. The injection port was held at 285 °C. The GC temperature program started at 110 °C for 2 min, ramped at 30 °C/min to 210 °C, and ramped at 2 °C/min to 285 °C, where it was held for 10 min. The mass spectrometer was operated in the electron capture, negative ionization mode with the ion source temperature at 150 °C. The flow of the reagent gas, methane, into the ion source was maintained at 40% of the total flow, or at 2 cm3/ VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. (A) Map of the world with soil sampling locations from this study and from Brzuzy and Hites (2). Colored dots indicate PCDD/F total flux ranges in ng‚m-2 yr-1. (B) Map of the world with tree bark sampling locations. Colored dots indicate the total concentration ranges in ng of PCDD/F per gram tree bark lipids. min, which gave a manifold pressure of 2 × 10-4 Torr. Selected ion monitoring and relative response factors were used to quantitate all congeners. Detailed quantitation methods have been described elsewhere (12, 13, 22). The PCDD/F detection limits were 0.02-0.2 pg/g wet weight of soil and 2-10 pg/g lipids for bark. We have elected to quantitate and report PCDD/F concentrations as the sum of all congeners in each homologue group. While we recognize that many researchers are interested in only those congeners that are toxicologically significant (the seventeen 2,3,7,8-substituted congeners), we 2954
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are interested in examining the overall environmental fates of all the tetra- through octa-chlorinated PCDD/F congeners. The fluxes of PCDD/F to soils were calculated by
flux )
CM At
(1)
where flux is the depositional flux from the atmosphere (ng m-2 yr-1), C is the PCDD/F concentration in soil (ng/g wet), M is the total mass of soil collected (g wet), A is the surface area of the sample collected (m2), and t is the time of
accumulation (yr). Based on sediment core studies, PCDD/F accumulation in the environment began between 1935 and 1940 (2, 12). From these data, t in eq 1 is set at 60 yr. This equation generates a time-averaged depositional flux over the entire depositional period. In 1996, Brzuzy and Hites found that the sediment record indicated present-day depositional fluxes were about 20% higher than this average, and they adjusted their soil fluxes upward by this amount (2). However, recent research indicates that PCDD/F levels in the top layers of dated sediment cores are decreasing (10, 23-25). We integrated the area under the time-trend curves (from the year of sediment collection to approximately 1935) for lake sediment cores from Germany (23, 24), Finland (10), and the U.S. Great Lakes (25) and divided these areas by the number of years to obtain time-averaged data. Comparison of the present day flux or concentration to the time-averaged data indicated that in most locations, the present day data are nearly equal to the time-averaged data. Therefore, we have not adjusted our soil fluxes by any factor, and we decreased the Brzuzy and Hites (2) fluxes by 20% in order to combine our data sets. Bark concentrations are reported on a per lipid basis. We have normalized the tree bark concentrations on a lipids basis to reduce the interspecies variability of the data (18, 19, 21). Nondetects are set to zero. Excel, SigmaPlot, and SigmaStat (SPSS Inc., Chicago, IL) and ArcView GIS and ARC/INFO (ESRI Inc., Redlands, CA) were used for statistical and graphical manipulation of the data.
Results and Discussion Concentrations. The data set for the bark and soil samples is extensive (126 samples with 10 homologue concentrations per sample, or 1260 data points) and is available as Supporting Information. By way of summary, Figure 1A shows the soil sampling locations and total PCDD/F fluxes to soil from this study and from Brzuzy and Hites (2), and Figure 1B shows the tree bark sampling locations and total PCDD/F concentrations in the lipid fraction of the tree bark. Note that the fluxes to soil are highest (red dots) in samples from Idaho, the Midwestern United States, Virginia, Hong Kong, and Taiwan. Note that concentrations in bark are highest (red and pink dots) in samples from Vancouver Island, in the Midwestern United States, Germany, and Hong Kong. Both fluxes to soil and concentrations in bark are extremely low in samples from above the 60th parallel, particularly in the Canadian Arctic, indicating that significant amounts of PCDD/F do not travel from warm to cold latitudes, as is predicted by the global fractionation theory (16, 17). One of our original aims was to compare the total concentrations of PCDD/F in paired bark and soil samples. Because our data are log-normally distributed (determined by using the normality test in Sigma Plot), we plotted the logarithm of the total flux to soil versus the logarithm of the total lipid-adjusted concentration in bark, see Figure 2. From this plot, we see that as the total concentration in bark increases, there is a corresponding increase in the flux to the soil, indicating that the bark and soil proportionately accumulate PCDD/F. Regression analysis gives an equation for predicting the total flux to soil from concentrations in bark:
log flux ) 0.756(log ng/g lipids) + 1.78
(2)
The standard error for the slope is 0.090, and for the intercept, it is 0.07. The correlation coefficient for this line is 0.548, which is significant at the 99.9% confidence interval; see Figure 2. Thus, if soil is not readily accessible, the total PCDD/F concentration in bark may be used instead to estimate the total flux to soil in a given location.
FIGURE 2. Logarithm of the total PCDD/F flux to soil versus the logarithm of the total PCDD/F lipid adjusted concentration in tree bark. The regression is significant at the 99.9% confidence level, and the 95% confidence limits of the regression are shown. Homologue Profiles. We compared the homologue profiles (each homologue group as a fraction of the total) for each pair of bark and soil samples. In 10 of the 63 pairs of samples, the soil fluxes were at the detection limits, as defined in the Experimental Section, and we removed these pairs from the homologue comparison. For each bark and soil sample, we classified the profile as either a sink (enriched in D6-D8 congeners) or a source (enriched in PCDF). These classifications as “source” or “sink” are based on homologue profiles in the literature (26). Based on these classifications, we examined each soil-bark data pair and found that each pair could be fit into one of three categories: In Category 1, both profiles resembled sinks; see Figure 3A,B. In Category 2, both profiles resembled sources; see Figure 3C,D. In Category 3, the bark resembled a source and the soil resembled a sink; see Figure 3E,F. Twenty-four pairs fell into the first category; 13 pairs fell into the second category; and 16 pairs fell into the third category. We tried to find a correlation between the lipid content and concentration of PCDD/F for each bark sample and did not find trends that could explain the above classifications. We plotted each pair on a map to determine if sampling location effected the homologue profiles; see Figure 4. Interestingly, for most of the European locations, both the bark and soil profiles resemble sources (Category 2), but for most of the United States locations, both the bark and the soil profiles resemble sinks (Category 1). The Category 3 data pairs are less clearly delineated, occurring in Canada and Asia. We believe that the following hypothesis, which is subject to test, can explain these observations. PCDD/F are emitted from a variety of stationary and mobile sources at unknown locations throughout the world. If the population density is high (for example, central Europe), it is likely that the sources and the environmental sinks will be relatively close to each other. In this case, the PCDD/F will have little time to degrade in the atmosphere as they move from the source to the sink, and thus, the profiles in both bark and soil will resemble the sources (Category 2). In areas of low population density (for example, most of the United States), the sources are likely to be much more distant from the sinks, and thus, the PCDD/F will have sufficient time to degrade in the atmosphere before deposition to bark or soil. In this case, the homologue profiles in both the bark and the soil will resemble sinks (Category 1). Both of these situations assume that the profile being emitted from the sources has been relatively constant over time and that the degradation of PCDD/F occurs only in the atmosphere. Let us imagine a situation in which these assumptions are not correct. If there had been a major change in the homologue profile being emitted during the last 20-30 years, the bark and soil profiles could be different because of the different integration times of the two different sampling media: years VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Homologue profiles (normalized to total) illustrating Categories 1 (both profiles resemble environmental sinks), 2 (both profiles resemble environmental sources), and 3 (the bark profile resembles sources but the soil profile resembles sinks). Bark (A) and soil (B) were sampled at Park Ridge, IL; bark (C) and soil (D) were sampled at Reilingen, Germany; and bark (E) and soil (F) were sampled at Tra Vinh, Vietnam. On the x-axis, F refers to polychlorinated dibenzofurans and D refers to polychlorinated dibenzo-p-dioxins, and the numbers indicate the degree of chlorination.
FIGURE 4. Map of the world with sampling locations for the bark and soil pairs. The location markers refer to the three categories outlined in Figure 3. for the bark and decades for the soil. If there were significant degradation of PCDD/F in the soil after deposition, then the bark profile could resemble a source and the soil profile could resemble a sink. It is possible that our observations in Category 3 are a result of some combination of these two effects. These observations suggest that multimedia sampling can give useful hints about the behavior of PCDD/F in the environment. A few soil samples showed homologue profiles that were dominated by OCDD. In the normal soil profile, OCDD was always less than about 80% of the total; see Figure 3 for typical examples. However, in six soil samples (3 from Hong Kong, 1 from Ohio, and 2 from Indiana), 85-99% of the total flux to the soil was due to OCDD. Interestingly, the bark samples corresponding to these six soil samples showed typical bark profiles. The homologue profiles for the Ohio bark (A) and soil (B) samples are shown in Figure 5; these profiles are typical of the patterns seen at the other five sites that were unusually high in fraction of OCDD. There are a few other reports in the literature of environmental samples that were unusually high in the 2956
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fraction of OCDD. Brzuzy and Hites found high relative contributions of OCDD in soil samples from Virginia (91% of total PCDD/F) (2) and in soil from Mitchell, IN (about 96% of total) (13), but no explanations were given. Rappe et al. found high relative OCDD contributions (about 90% of total) in lake sediments from southern Mississippi (27, 28). These researchers suggested that the natural formation of OCDD could account for the high relative levels. Additionally, pentachlorophenol (PCP), a once heavily used wood preservative, contains high levels of OCDD as an impurity (29). It is possible that these explanations can be extended to cover these OCDD anomalies in Ohio, Indiana, and Hong Kong soils. Deposition to the Earth’s Land Surfaces. To convert the soil-derived fluxes to flow rates out of the atmosphere, each flux must be multiplied by a depositional area representative of that particular flux. To divide the earth into specific depositional areas, we need to find a surrogate for PCDD/F emissions, and this surrogate needs to have an emissions inventory that is known with relatively high geographical resolution. PCDD/F are the byproducts of chemical produc-
FIGURE 5. Homologue concentrations of PCDD/F in bark (A) and in soil (B) from Felicity, OH; see Figure 3 for the code to the x-axis. tion and combustion, which are found predominantly in industrialized areas. Nitrogen oxides (NOx) are emitted from the combustion of fossil fuels, predominantly from the generation of electricity and from mobile sources (20). These
emissions are almost always found in heavily industrialized areas, and among their other sources, NOx are produced during municipal solid waste incineration, a known source of PCDD/F (30). A regional study of PCDD/F atmospheric transport in Sweden indicated that there was a good correlation between PCDD/F sources and NOx emissions (4). Furthermore, a gridded global inventory of anthropogenic emissions of NOx has been produced with 1° × 1° longitude/ latitude resolution, which is available via the Internet (31). We, therefore, selected NOx emissions as a surrogate for PCDD/F emissions. After downloading them from the Internet, we converted these 1° × 1° NOx emissions data into files usable by geographical information systems (GIS) software. We then overlaid our bark concentration and soil flux map layers on this NOx emissions grid. Because our sampling locations did not often fall directly in the center of a 1° × 1° grid cell, we averaged the adjacent grid cells for a 1° × 1° resolved NOx emissions estimate for each bark and soil data location. We took the same approach to obtain 3° × 3° (the 9 surrounding grid cells) and 5° × 5° (the 25 surrounding grid cells) resolved NOx emission estimates at each of our sample locations. Because these environmental data are log-normally distributed, we plotted the logarithm of the soil flux and the logarithm of the bark lipid-adjusted concentration versus the logarithm of the NOx emission for each of these three grid systems. For both the bark and the soil, the best correlation was found for the 1° × 1° resolved grid. Figure 6A (soil flux) and Figure 6B (bark concentration) show correlation coefficients (r2) of 0.207 and 0.459, respectively, both of which are significant at the 99.9% confidence level. Similar plots using the average emissions values for a 5° × 5° resolved grid are shown in Figure 6C,D. While the correlations are still significant (0.118 for soil fluxes and 0.368 for bark concentrations), they are not nearly as strong as the 1° × 1° grid.
FIGURE 6. Logarithm of the total PCDD/F flux to soil (A) and logarithm of the total PCDD/F concentration in bark (B) versus the logarithm of total NOx emissions (as kg nitrogen per year) on a 1° × 1° longitude/latitude grid. Plots (C) and (D) are for soil and bark, respectively, versus NOx emissions in a 5° × 5° longitude/latitude grid. The 95% confidence limits of the regressions are shown. VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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The correlations on the 3° × 3° grid (0.183 for the soil and 0.353 for the bark) were also significant but weaker than those on the 1° × 1° grid. These generally good relationships indicate that NOx emissions are a good surrogate for PCDD/F emissions. Furthermore, the higher correlation at the 1° × 1° range compared to the 3° × 3° and 5° × 5° ranges indicates that PCDD/F travel through the atmosphere for relatively limited distances, probably on the order of 100-200 km. We can predict PCDD/F fluxes to soil from the NOx emissions using the equation derived from the regression shown in Figure 6A:
log flux ) 0.512 + 0.401 (log NOx)
(3)
The standard error of the slope is 0.079, and for the intercept, it is 0.253. We extracted the NOx emissions data and the corresponding land areas using GIS software; the input to this software was the 1° × 1° gridded information downloaded from the Internet. The program sums all land areas corresponding to a particular NOx emission value and creates a spreadsheet with the NOx values and the total land area for that value s in this case, 2500 rows of data. Using eq 3, we generated another column with the calculated PCDD/F flux to soil corresponding to each of the 2500 NOx emissions values. We multiplied this calculated flux times the surface area for each NOx emission value to give us a flow rate for PCDD/F deposition to each of the 2500 NOx emissions areas. This approach gave us a total surface area of 9.7 × 1013 m2 and a total PCDD/F flow rate to this area of 4.6 t/yr. The remaining land area, 5.1 × 1013 m2, consisted of 1° × 1° grid cells that had no or unknown NOx emissions. Thus, our estimate of 4.6 t/yr is probably a conservative estimate of PCDD/F deposition to land. To obtain a global deposition estimate we must also consider deposition to the oceans. This flow rate has recently been estimated by Baker and Hites (32) to be 0.9 t/yr. Adding this value to our estimate of deposition to land, we get a total PCDD/F deposition to the earth’s surface of 5.5 t/yr, which is about twice the global emission estimate of 3 t/yr. A simple comparison of this emissions estimate and this deposition estimate suggests that about twice as much PCDD/F are leaving the atmosphere as are entering it. If one believed this mass imbalance, one would be forced to conclude that we (and others) have missed about half of the global PCDD/F emission sources. Before drawing this conclusion, however, we should carefully consider the errors associated with these two estimates. Quantitative estimates for the uncertainties in the NOx data are currently not available (20), and a consideration of the errors associated with the emissions estimate is beyond the scope of this paper. We do know, however, that there is considerable error associated with the deposition estimate. The errors of the regression line in Figure 6A suggest that there is about a factor of 3 error in predicting the fluxes from the NOx emissions using eq 3. Applying this error to our global deposition estimate (5.5 t/yr) gives a range of global PCDD/F deposition of about 2-15 t/yr, a range that includes the emissions estimate of 3 t/yr. In addition to the errors associated with the quantitative estimates of emission and deposition, we must also note the qualitative differences in the homologue profiles for PCDD/F from sources and those in sinks. These profiles are significantly different from one another; see Figure 3. This difference in profiles must also be understood before we can achieve a good PCDD/F mass balance (33,34).
Acknowledgments We thank Jason Hobson and Jennifer Philips for help in collecting the samples from Western Canada, the many colleagues and friends who collected other samples for us 2958
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over the years, and Anna Radue in the SPEA GIS group. This project has been supported by the U.S. Environmental Protection Agency through Grant 825377.
Supporting Information Available Tables of soil and bark PCDD/F concentrations and locations are available free of charge via the Internet at http:// pubs.acs.org.
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Received for review October 4, 1999. Revised manuscript received March 13, 2000. Accepted March 14, 2000. ES991138O
