Global Mass Balance for Polychlorinated Dibenzo-p-dioxins and

Global Mass Balance for Polychlorinated Dibenzo-p-dioxins and Dibenzofurans. Louis P. Brzuzy, and Ronald A. Hites*. School of Public and Environmental...
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Research Global Mass Balance for Polychlorinated Dibenzo-p-dioxins and Dibenzofurans LOUIS P. BRZUZY AND RONALD A. HITES* School of Public and Environmental Affairs, and Department of Chemistry, Indiana University, Bloomington, Indiana 47405

Chlorinated dioxins and dibenzofurans (PCDD/F) were measured in 107 soil samples globally, and depositional fluxes were determined. Deposition to land surfaces was estimated by dividing the earth into five depositional zones based on climatic and geographical factors. Mean depositional fluxes to these zones ranged from 18 to 610 ng m-2 yr-1. Low fluxes were observed in most zones not impacted by industrialization. Total global deposition from the atmosphere to land was estimated to be 12 500 ( 1300 kg/yr. Based on limited data, deposition to the oceans was estimated to be about 610 ( 1500 kg/yr, yielding a total global deposition of 13 100 ( 2000 kg/yr from the atmosphere. Emissions of PCDD/F to the global atmosphere were estimated by determining emission factors and production rates for the major PCDD/F sources. The major sources considered in this study were municipal waste incineration, biomass combustion, steel and copper mill emissions, cement kiln emissions, medical waste incineration, and emissions from automobiles Total annual emissions were estimated to be 3000 ( 600 kg. Global deposition (see above) is roughly four times greater than annual emissions. This suggests that sources of PCDD/F are not well-characterized. More data are needed on emission factors (particularly from developing countries) and on introduction rates of PCDD/F to the global atmosphere.

Introduction Chlorinated dioxins (PCDD) and dibenzofurans (PCDF) are well-known environmental pollutants, which have been extensively studied for several years. Most of this research has focused on the toxicity of PCDD/F to animals and people (1-3). Less attention has been given to identifying the * E-mail address: [email protected].

S0013-936X(95)00714-0 CCC: $12.00

 1996 American Chemical Society

major sources of PCDD/F to the environment and to determining the processes by which PCDD/F are transported from their sources to various environmental sinks. Studies of PCDD/F in lake sediment cores have shown that PCDD/F were not present to any large extent before about 1935 (4, 5). This finding has suggested that combustion, particularly the combustion of chlorinated waste, is the major source of PCDD/F to the environment (6, 7). In fact, PCDD/F have been detected in the emissions from several combustion sources, including municipal waste incinerators, steel mills, copper smelters, hospital waste incinerators, automobiles, and several other smaller sources (8-13). However, representative emission factors (quantity of PCDD/F formed per amount of material combusted) have not been adequately determined for most sources, with the possible exception of municipal waste incineration. In spite of this knowledge about the sources of PCDD/F to the environment, there is a substantial mass balance problem: Deposition of PCDD/F from the atmosphere (in the form of wet and dry deposition) seems to exceed the estimated emissions into the atmosphere. This disagreement is substantial. The observed deposition [either measured directly or estimated from PCDD/F concentrations in air and precipitation (14, 15)] seems to be 10-25 times higher than emissions from known sources (7, 16, 17). This poor mass balance, if true, may indicate that only a very small fraction of the sources are knownsan important conclusion. It is also possible that total atmospheric deposition is considerably overestimated. This research addresses the latter possibility. Most of the deposition estimates now in the literature are based on air and rain samples collected from regions of high anthropogenic activity, such as the Great Lakes basin and western Europe. Although this approach may give reasonable deposition estimates for developed countries, it would give inappropriately high deposition estimates for the rest of the world. To avoid this bias, we have determined PCDD/F deposition on a worldwide basis. In principle, we could have collected air and precipitation samples from many locations around the world, measured PCDD/F, and calculated depositional fluxes from these data. However, given the vast geographical scope of this project, this strategy was not practical. Instead, we have used soil samples, which act as passive collectors for atmospheric deposition and which give depositional fluxes that agree well with fluxes calculated from lake sediments (18). By estimating atmospheric depositional fluxes in this way, we have obtained a more realistic rate at which PCDD/F are removed from the atmosphere; this rate can then be compared to worldwide emission estimates. If the two rates agree, then we have a good accounting of the major PCDD/F sources.

Experimental Section Soil Collection. Bulk soil samples were collected in 475mL (8.25 cm diameter by 8.25 cm high) precleaned glass jars with Teflon-lined lids. The jar opening was pressed into the soil covering an area of 53.4 cm2. Alternatively, an

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area of known dimension was outlined. The area was dug out to a depth that corresponded to the organic layer depth of the particular soil type (called the A horizon). Generally, this depth was between 5 and 15 cm. In areas where an appreciable A horizon could not be identified, soil was collected to a depth of 15 cm. Because PCDD/F could accumulate in vegetation from atmospheric deposition, the vegetation layer was not removed from the soil prior to or after collection. Once samples were collected, they were stored in a cool dark place until delivered to our laboratory. In the laboratory, they were stored at -18 °C until analyzed. Landscape considerations were taken into account. Erosional areas were avoided (such as steeply sloped areas). Flat, highly vegetated areas were targeted when possible. Over 250 soil samples were collected over a 3-yr period by individuals traveling to various locations who were willing to collect soils for this project; 107 were analyzed as described below. Sample Preparation. The sample preparation methods have been described in detail elsewhere (18); only a summary will be presented here. All samples were mixed and, if necessary, ground with a mortar and pestle. About 25 g of each sample was transferred to a beaker and mixed with enough pre-extracted Na2SO4 to make a loose, friable mixture. Between 0.04 and 0.1 ng of [13C12]-1,2,3,7,8pentachlorodibenzofuran and between 0.8 and 1.6 ng of [13C12]octachlorodibenzo-p-dioxin (Cambridge Isotopes, Cambridge, MA) were added to the mixture to serve as internal standards. The samples were Soxhlet extracted with 2-propanol for 24 h and then with dichloromethane for an additional 24 h. The two fractions were combined, rotary evaporated to less than 2 mL, solvent-exchanged into hexane, and concentrated to less than 5 mL. Fractionation procedures used liquid-solid chromatography on silica gel. The sample was eluted with hexane, 15% dichloromethane in hexane, and dichloromethane. PCDD/F eluted in the second fraction. The solvent from this fraction was exchanged into hexane and reduced to less than 1 mL total volume. The PCDD/F in this fraction were isolated on an alumina microcolumn; the sample was eluted with hexane, 2% dichloromethane in hexane, and 40% dichloromethane in hexane. The PCDD/F eluted in the third fraction, which was concentrated to less than 50 µL by slowly passing purified nitrogen over the sample. Analysis. The sample analysis methods have been described in detail elsewhere (18); only a summary will be presented here. All analyses were performed on a gas chromatographic mass spectrometry system operated in the electron capture mode. Chromatographic separations were achieved on 30 m × 250 µm i.d. (0.25 µm film thickness) DB-5MS fused silica column with helium as the carrier gas. Samples were injected into the column through a split/ splitless injector operated in the splitless mode. A twostep linear temperature program was used (18). For maximum sensitivity, care was taken to ensure that the system was leak free to avoid the formation of (M - Cl + O)- ions. PCDD/F were quantitated by homologues, which are all of the isomers at a particular level of chlorination. Thus, there are 10 homologues for the tetrachloro- through octachlorodioxins and dibenzofurans. For increased sensitivity, selected ion monitoring was used. Quantitation was performed by finding the ratio of the appropriate peak area to that of the internal standard and correcting for relative response factors that were obtained from running

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a standard mixture of PCDD/F. The [13C12]pentachlorodibenzofuran internal standard was used to quantitate the tetra through hexa homologues, and [13C12]octachlorodioxin was used to quantitate the hepta and octa homologues. Overall reproducibility of the method was (30% (18). The justification for using soils to estimate PCDD/F deposition from the atmosphere has been previously described in detail elsewhere (18). Briefly, the depositionfluxes of PCDD/F to soils are calculated by the following equation:

flux )

cm At

where flux is the depositional flux from the atmosphere (ng m-2 yr-1), c is the PCDD/F concentration in soil (ng/g), m is the mass of soil collected (g), A is the area of sample collected (m2), and t is the time of accumulation (yr). It is clear from the sediment record that PCDD/F accumulation in the environment began around 1935 (4, 5). From these data, t in the above equation was set at 60 yr. This equation generates a time-averaged depositional flux over the entire period during which PCDD/F have accumulated in the environment. The sediment record indicates that the present-day depositional fluxes of PCDD/F from the atmosphere are about 20% higher than this time-averaged flux (4, 5). Therefore, soil-derived fluxes were adjusted upward by this amount.

Results and Discussion The data for these analyses are extensive, consisting of 10 homologue fluxes for each sample. The full data set is available as supporting information or from the authors in an electronic spreadsheet format. Figure 1 represents the total PCDD/F fluxes as color coded dots on a map of the world. Note that these fluxes are highest (red dots) in the northern United States, southern Canada, western Europe, and southeastern Australia. Other high flux areas are southern Japan, coastal China, and Taiwan. These are all highly industrialized areas and probably contain many PCDD/F sources. To convert from fluxes to flow rates, we needed to multiply the fluxes by the area appropriate for each particular sample. Given the high number of samples, the unequal geographic spacing of these samples, and the uncertainty in delineating regional industrial areas, this proved impossible. Thus, we sought a simple means of compositing or averaging the flux data before multiplying by area. Inspection of our data indicated that fluxes were related to climate zones, which are broadly defined by average temperature and precipitation. Figure 2 is a map of our depositional zones, roughly based on the climate zone designation of Trewartha (19), with our color-coded PCDD/F data representing different flux levels. Fluxes were highest in the subtropical and temperate zones and lowest in the dry or arid zones. This is reasonable given that the industrial sources are likely to be concentrated in temperate zones and that deposition of chlorinated organics is likely to be minimal in dry areas (20). Based on these observations, our strategy was to average the fluxes in each depositional zone, multiply by the area of that zone, and add the resulting flow rates together. This procedure gave us an overall estimate of deposition from the atmosphere. There are two caveats to this approach. First, this approach does not consider deposition over oceans, where

FIGURE 1. Map of the world showing soil sampling locations. Color coded dots indicate the flux ranges in ng m-2 yr-1. Exact data are given in the supporting information.

FIGURE 2. Map of the world showing climate zones. The symbols indicate the flux ranges in ng m-2 yr-1.

depositional behavior may be different. Second, we have modified the areas for some of the climate zones to simplify data reduction. The true Trewartha classification (19) consists of seven major and 12 minor zones. We have reduced this number to five. No samples were obtained from polar climates, and this zone was aggregated into the boreal zone. Also highland climates were eliminated by aggregating them with their adjacent zones. Our data indicate that the subtropical zone of South America experiences lower deposition than the other subtropical zones of the world. Therefore, we have aggregated this zone with the tropical zone. The resulting five depositional

zones and their areas are summarized in the first two columns of Table 1. Data Reduction. Before presenting the results, it is appropriate to discuss our approach to averaging the data and to reporting the related uncertainty in the data. To apply the appropriate statistical averaging (geometric or arithmetic) and error analysis techniques to our data, the distribution function of the data had to be determined. The Univariate Procedure in the Statistical Analysis System (Cary, NC) clearly showed that the data were log-normally distributed, which indicated that geometric averaging was appropriate. However, the geometric mean is a biased

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a

TABLE 1

Climate Zone Names, Areas of Climate Zones, Average Total Fluxes of PCDD/F (in ng m-2 yr-1) to Each Zone, and Confidence Limit of Average Flux climate zone

area (1013 m2)

av flux

90% CL

subtropical temperate polar/boreal tropical arid total land area

0.4 2.4 2.2 5.5 3.7 14

610 280 41 25 18

90 52 6 7 8

estimate of the central tendency of the data, under representing the higher values in a sample population. Therefore, we have chosen to arithmetically average our data and to represent the error as 90% confidence limits of the mean. The confidence limits were obtained by estimating the standard error of each mean and multiplying it by the appropriate Student’s t-value for the correct degrees of freedom. Flux Levels and Homologue Profiles. Figure 3 shows the average flux versus PCDD/F homologue for the five different depositional zones. The total flux and the associated error for each zone are given in the last two columns of Table 1. The highest average fluxes were in the subtropical and temperate zones; total flux in the other depositional zones is low, ranging from 18 to 40 ng m-2 yr-1. A single factor ANOVA performed on the data revealed that the between zone variance is significantly different than the within zone variance (P < 0.1). All homologue profiles are similar, except that there is a slightly more even distribution of the homologues in the boreal zone (Figure 3c) relative to the other zones. Overall, it appears that some PCDD/F are subjected to long-range atmospheric transport and are distributed globally. However, the majority of the emissions deposit in climate zones near to their sources. Deposition to the Earth’s Surface. By multiplying the average PCDD/F flux to each zone by the area of that zone, we can estimate a total deposition rate. Table 2 shows the deposition rate of the individual PCDD/F homologues for each zone. Total global deposition to land is estimated to be 12 500 ( 1300 kg/yr (90% confidence limits). There are no measurements of PCDD/F concentrations in air or rain over oceans from which to make a deposition estimate for this zone. Air collected on remote Black Island, near McMurdo Experiment Station, Antarctica, revealed no PCDD/F present at a detection limit of 5 fg/m3 (21). Using this detection limit as a lower limit for PCDD/F in air over oceans and assuming a depositional velocity of 0.2 cm/s [an accepted depositional velocity for semivolatile organic compounds in the ambient environment (22)], then deposition to the oceans would be about 0.3 ng m-2 yr-1. Unpublished results from our laboratory of PCDD/F air concentrations at Bermuda are about 10 times higher than at Black Island. This would yield a PCDD/F flux to oceans at about 3 ng m-2 yr-1. This is probably an upper bound estimate given the proximity of Bermuda to the eastern United States, a large regional source of PCDD/F. The average of these two values is about 1.7 ( 4.1 ng m-2 yr-1 (90% confidence limit). The fact that the confidence limit is larger than the mean does not indicate that the flux to the oceans is at times negative (upward). Instead, it reflects the large uncertainty in the estimate. Indeed, the flux to

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c

d

e

FIGURE 3. Average flux versus PCDD/F homologue for the different climate zones. The letter F refers to dibenzofurans; and the letter D refers to dibenzo-p-dioxins. The numbers next to the letters denote the level of chlorination. Error bars are the 90% confidence limits.

the oceans may be very low, approaching zero. Multiplying this flux by the area of the oceans (36 × 1013 m2) yields a flow rate to the oceans of 610 ( 1500 kg/yr, about 5% of the flow to land. Adding this estimate to the deposition to land given in Table 2 gives a total global deposition of about 13 100 ( 2000 kg/yr. We must comment further on the precision of this deposition estimate. Even though the statistical analysis indicates that the precision of this estimate is (15%, we feel that this is an underestimate of the true uncertainty in the deposition estimate. There are two reasons: First, we chose to represent the errors using statistical formulas that assume a normally distributed sample population, but we know that our data are not normally distributed based on the SAS analysis. Thus, the normality assumption is not truly valid. Second, the application of statistical techniques requires that samples are randomly drawn from a given population, but our experimental design for sample collection did not ensure randomization. Based on a subjective interpretation of our results, we feel that the error associate with our estimate is probably more on the order of ( 4000 kg/yr. Sources of PCDD/F to the Atmosphere. To determine if the PCDD/F atmospheric inputs are balanced by the PCDD/F atmospheric depositional outputs, we need a global accounting of PCDD/F emissions to the atmosphere. There are two broad categories of PCDD/F sources to the environment: chemical (minor) and combustion (major). PCDD/F were never manufactured by the chemical industry as marketable products. Instead, they were formed

12 500 ( 1300

2 570 ( 500 6 870 ( 1150 885 ( 130 1 450 ( 250 682 ( 170 1 330 ( 431 4 050 ( 1100 247 ( 70 878 ( 247 292 ( 137 6 800 ( 1220 299 ( 121 1 100 ( 261 222 ( 86 260 ( 45 149 ( 72 2 030 ( 312 133 ( 68 538 ( 150 62 ( 26 105 ( 24 48 ( 25 886 ( 170 37 ( 22 174 ( 61 23 ( 10 22 ( 6 7(5 263 ( 70 4(4 40 ( 12 16 ( 12 55 ( 15 8(2 123 ( 23 60 ( 34 80 ( 26 21 ( 6 25 ( 6 8(7 194 ( 44

a

The error for each estimate represents the 90% confidence limits.

205 ( 118 221 ( 59 43 ( 13 54 ( 13 30 ( 20 553 ( 135 167 ( 101 178 ( 55 42 ( 15 41 ( 12 25 ( 16 453 ( 118 210 ( 112 322 ( 101 94 ( 31 38 ( 12 36 ( 27 700 ( 157 subtropical 124 ( 75 temperate 161 ( 45 polar/boreal 115 ( 40 tropical 62 ( 19 arid 79 ( 46 total deposition 541 ( 108 total global deposition to land

total climate zone deposition D8 D7 D6 D5 deposition (kg/yr) F8 D4 F7 F6 F5 F4 climate zone

Average PCDD/F Homologue-Specific Deposition Rates (in kg/yr) to Five Climate Zonesa

TABLE 2

as byproducts during the manufacture of other chemicals, such as chlorinated phenols. Another chemical source of PCDD/F is the bleaching of wood pulp using chlorine (23), and as a result, areas of sediment outside pulp mills that use this process are contaminated with PCDD/F (24). Additionally, paper products made from bleached wood pulp such as coffee filters and milk cartons have been found to be contaminated with PCDD/F (25, 26). Chemical sources of PCDD/F can cause local environmental contamination, but unless these chemical products are incinerated, they do not lead to widespread contamination. Therefore, chemical sources will not be considered in this mass balance. Combustion processes are thought to be the cause of the ubiquitous environmental contamination by PCDD/F (4). Once injected into the atmosphere, PCDD/F can be transported long distances from their sources and are ultimately removed from the atmosphere through dry and wet deposition (14, 27). Several combustion sources have been identified in recent years, although reliable emission factors for most of these sources do not yet exist, particularly for developing countries. We discuss below the major PCDD/F sources and their contributions to the load of these compounds to the atmosphere. Municipal Waste Incineration. The data from ref 28 were used to estimate an emission factor for municipal waste incineration (MWI). The emission factor estimates from 12 incinerators were averaged to give an average emission factor of 13 µg/kg of waste burned. From these 12 measurements, we can also estimate a confidence limit for the emission factor. At the 90% confidence limit, the mean emission factor ranges from 7 to 17 µg/kg; this is a factor of (40%. We will use this error estimate to represent the error of all the emission factors where there are not enough data to compute meaningful confidence limits. Total emissions from MWIs were calculated by multiplying the above emission factor by the estimated waste burned in 12 European countries, the United States, Canada, Japan, Australia, and New Zealand. Total waste incinerated for these countries in 1990 was about 8.7 × 1010 kg (29); multiplying this value by the above emission factor gives 1130 kg of PCDD/F per year. There are potentially large incineration sources in the former Soviet Union, China, and India, but emission data from these countries are not available. Therefore, our estimate of PCDD/F emissions from municipal waste incineration is probably low. Biomass Combustion. The combustion of biomass as a source of PCDD/F has been investigated (30-32), and it has been proposed as a major source of PCDD/F to the environment (33). The PCDD/F that are detected when biomass is burned may be formed by the combustion of chlorinated pesticides (such as silvex) used on the biomass, or they may simply be resuspended atmospheric deposition that had deposited onto the plant material. [Studies have shown that PCDD/F accumulate in pine needles (34) and in other types of vegetation (35).] An emission factor of 0.04 µg/kg was determined by taking the average of the data reported in ref 32 for wood stove emissions and multiplying it by 60 (28) to convert from TEQ to total PCDD/ F. In 1991, approximately 8.7 × 1012 kg of biomass was burned globally (36). Multiplying this value by the emission factor yields about 350 kg of PCDD/F. Ferrous Metal Production. Production of iron and steel has been reported to be the largest combustion source of PCDD/F in Europe (37). An emission factor of 0.5 µg/kg

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TABLE 3

Emission Factors, Global Production Rates, Average Emissions, and 90% Confidence Limits for Major PCDD/F Sources to Atmosphere sources

emission factor (µg/kg)

production (109) kg/yr

emissions (kg/yr)

90% CL

municipal waste incineration biomass combustion ferrous metals production cement kilns (burning haz. wastes) cement kilns (no haz. wastes) secondary copper smelting medical waste incineration unleaded fuel combustion leaded fuel combustion total

13 0.04 0.5 2.6 0.2 39 22a 320b 2800b

87 8700 700 260 1600 2 4 3800b 3800b

1130 350 350 680 320 78 84 1 11 3000

450 140 140 280 130 31 35 0.4 5 600

a Emission factor and production data apply to United States only; see text for how global emissions were estimated. pg/km; production units ) km/yr.

of steel produced was reported for steel mills in Sweden (38). A similar factor was also reported by Lahl (37) for steel mills in Germany. PCDD/F emissions from steel mills were estimated by multiplying this emission factor by the amount of iron and steel produced worldwide in 1990, which was 700 × 109 kg (39). We estimate that about 350 kg of PCDD/F are emitted from steel mills worldwide. Cement Kiln Emissions. Because of the large amount of cement produced worldwide, cement kilns are potentially large sources of PCDD/F. Cement kilns that burn hazardous waste as fuels are particularly high emitters (28). Emission factors for PCDD/F from nine cement kilns burning hazardous waste in the United States are given in ref 28. The mean is 2.6 µg/kg of raw material used with a 90% confidence interval of 1.5-3.7 µg/kg ((42%). Fewer data are available for cement kilns that do not burn hazardous waste. Ref 28 reports two measurements, of which the average is 0.2 µg/kg. It takes 1.6 kg of raw material to produce 1 kg of cement clinker; therefore, clinker production must be multiplied by this factor to estimate emissions (40). If we assume that only the United States allows the burning of hazardous waste in cement kilns (about 16% of all U.S. kilns), then about 1.6 × 1011 kg of cement clinker (2.56 × 1011 kg of raw material) are produced by these incinerators each year (41). Worldwide production of cement from kilns that do not burn hazardous waste was about 9.9 × 1011 kg (1.6 × 1012 kg of raw material) in 1990 (39). Emissions from incinerators that burn hazardous wastes are estimated to be about 680 kg/yr, and emissions from incinerators not burning hazardous wastes are estimated to be about 320 kg/yr. Secondary Copper Smelting. Stack emissions of PCDD/F from a secondary copper smelter were measured by the EPA during the National Dioxin Study (10). The tested facility recovered copper and other metals from copper and iron-bearing scrap; an emission factor of 39 µg/kg was measured. If we assume that 20% of the world’s copper production is from recycled copper scrap, then about 2.0 × 109 kg of copper is produced through recycling per year. Multiplying this value by the emission factor yields about 78 kg of PCDD/F emissions per year. Medical Waste Incineration. Due to the lack of optimal pollution control equipment and a high fraction of chlorinecontaining combustibles, medical waste incinerators generate relatively large amounts of PCDD/F. The average emission factor for the six hospital waste incinerators listed in ref 28 is 22 µg/kg, about twice as high as the emission

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b

Emission factor units )

factor for MWIs. In 1992, about 3.7 × 109 kg of hospital waste was incinerated in the United States (42), resulting in the emission of about 81 kg of PCDD/F. The emissions from other countries were not as high. For example, annual emissions from medical waste incinerators in Germany, Austria, the United Kingdom, the Netherlands, and Switzerland were approximately 0.3, 0.2, 2, 0.1, and 0.2 kg, respectively (32, 43-46). Thus, global emissions are estimated to be about 84 kg/yr based on the sum of emissions from the above countries. It appears that the data for United States’ medical waste incinerators might be anomalously high. On the other hand, emissions from the former Soviet Union, China, and India are not known. If hospital waste incinerators in these countries produce more PCDD/F than those in the United States, then global emissions from this source may be higher than estimated above. Leaded and Unleaded Fuel Combustion. In a study conducted by Marklund et al. (12), production rates of PCDD/F in automobile exhaust from selected light-duty vehicles burning leaded and unleaded gasoline were 2800 and 320 pg/km, respectively. We can use these data to estimate the global input of PCDD/F from automobiles. In 1988, there were approximately 3.8 × 108 passenger vehicles in the world (39); about half of these automobiles were in the United States. If we assume that all the vehicles in the United States now burn unleaded fuel and that the average automobile travels 20 000 km/yr, then about 1 kg of PCDD/F is emitted into the atmosphere from U.S. automobiles. Most other countries are just beginning to limit the amount of lead in gasoline, so we will assume that all other cars in the world burn leaded fuel. From these vehicles, we estimate an additional 11 kg of PCDD/F are emitted per year into the atmosphere. The total global estimate from vehicles is about 12 kg. Overall Mass Balance. The major sources of PCDD/F to the global atmosphere, emission factors, and annual emissions are summarized in Table 3. Total global emissions are approximately 3000 ( 600 kg/yr. Based on the same arguments presented above, we feel that this is an underestimate of the true uncertainty in the total emission estimate and that the error associated with our emission estimate is probably more on the order of (1000 kg/yr. This emission estimate can be compared to the deposition estimate given above of about 13 100 ( 4000 kg/yr. Note that the deposition estimate is about four times greater than the estimate of emissions, which is considerably less

a

other sources. To achieve homologue-specific mass balance, data must be reported for each homologue and not just as a toxic equivalent concentration. Third, there are no open ocean measurements of deposition. Based on the limited available data, we suspect that the deposition to oceans is low, but this needs to be verified.

Acknowledgments

b

The National Science Foundation (Grant BES 93-21274) provided support. We are also grateful to Staci Simonich and several others for assistance in sample collection. We thank Jerry Johnston, Anna Radu, and J. C. Randolph for GIS support and David Parkhurst for statistical advice.

Supporting Information Available

FIGURE 4. Homologue profiles (normalized to D8) for the total global deposition to soil (a) and typical municipal waste incinerator emissions (27) (b).

than the factor of 10-25 with which we started. A Student’s t-test comparing the deposition and emission estimates gives a t-value of 2.46, which is not significant at the 90% confidence level. Thus, it could be argued that the emission sources and the atmospheric deposition estimates are roughly in balance. However, the deposition estimate is substantially higher than the emission estimate. This difference may indicate that the initial problem with the PCDD/F mass balance was both an overestimation of deposition and an underestimation of emissions. Although the existence of one or several large unknown sources is a possibility, it is more likely that the mass imbalance is due to the poor characterization of the known PCDD/F sources to the atmosphere. For example, cement kilns have been identified in this study as an important source of PCDD/F to the atmosphere. Roughly 20% of global cement production takes place in the Peoples Republic of China (39), but we have no information regarding PCDD/F emissions from these kilns. A similar situation exists for ferrous metals production in the former Soviet Union. Another problem also remains unresolved. Figure 4a shows the average homologue profile for global soil deposition, and Figure 4b shows the homologue emission profile from a typical municipal waste incinerator (27). The two profiles do not match. The incinerator profile shows a roughly even distribution of the homologues (with the exception of F8 and D4), where the deposition profile is skewed toward the higher chlorinated dioxins. To resolve these problems, more work is needed. First, reasonable estimates of atmospheric lifetimes of the homologues need to be made. This will require data on the rates of reaction between hydroxyl radicals and PCDD/F as a function of degree of chlorination. Second, more work needs to be done on characterizing the emission factors and profiles from the major sources listed above. With the exception of municipal waste incineration, little is known about the emission profiles or production rates from the

Three tables containing sample numbers, dates collected, state/province/city, country, land use, latitude, longitude, climate zone, area collected, weight, site description, and fluxes for F4-F8 and D4-D8 plus total fluxes (15 pages) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supporting information from this paper or microfiche (105 × 148 mm, 24× reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St. NW, Washington, DC 20036. Full bibliographic citation (journal, title of article, names of authors, inclusive pagination, volume number, and issue number) and prepayment, check or money order for $30.00 for photocopy ($32.00 foreign) or $12.00 for microfiche ($13.00 foreign), are required. Canadian residents should add 7% GST. Supporting information is available to subscribers electronically via the Internet at http://pubs. acs.org (WWW) and pubs.acs.org (Gopher).

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Received for review September 22, 1995. Revised manuscript received February 22, 1996. Accepted February 23, 1996.X ES950714N X

Abstract published in Advance ACS Abstracts, April 15, 1996.