Environ. Sci. Technol. 2002, 36, 2372-2378
Polychlorinated Dibenzo-p-dioxin and Furan (PCDD/F) Uptake by Pasture GARETH O. THOMAS,* JOANNE L. JONES, AND KEVIN C. JONES Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster LA1 4YQ, U.K.
Uptake of airborne PCDD/Fs by a native pasture sward was studied. The concentrations of the less chlorinated PCDD/ Fs (up to and including the Cl5DD/Fs) in pasture harvested on the same day after 2, 6, and 12 weeks exposure were the same, implying that air-pasture steady-state was reached within 2 weeks of exposure. The implications of these observations for the relative importance of input (atmospheric deposition, soil resuspension) and loss (photolysis, degradation, volatilization, cuticular shedding, and growth dilution) processes are discussed and inferences made about the controlling factors. The concentrations of the more chlorinated PCDD/Fs were more variable. We infer that they were influenced by wash-off of particleand surface-bound chemical. Up to 4% of the Cl2-4DD/F and 4-13% of the Cl5-8DD/F loading on the pasture was estimated to have been supplied by adhering soil particles, with the remainder supplied by atmospheric deposition. Between 0.04 and 0.66 kg of each homologue group (excluding Cl2DFs) is estimated to be transferred annually from the atmosphere to pasture in the U.K.
Introduction Vegetation is a key environmental compartment with a constantly renewed lipophilic surface area, which has an important role in “buffering” air-soil exchange and, hence, the global transport of semivolatile organic compounds (SOCs). The consumption of vegetation is also the main route of entry to terrestrial (including wildlife and farm) foodchains for these compounds. Pasture grassland is of particular importance, because of the large land area it covers, and its role as a food for grazing farm animals, the products from which account for the bulk of typical average human intake of polychlorinated dibenzo-p-dioxins and furans (PCDD/ Fs) and other SOCs. Uncertainties remain about the kinetics of air-plant transfer of SOCs, which is an issue of fundamental importance. If the transfer of gas-phase SOCs into plants is “slow”, air-plant steady-state may not be reached during the growing season/lifetime of certain plants and the onward foodchain transfer of SOCs would be “kinetically constrained” by airplant transfer (1). McLachlan et al. (2) observed this for airpasture transfer of PCDD/Fs at a field site in Germany, for example, where non-steady-state conditions prevailed after many weeks of exposure. However, we have observed “rapid” attainment of air-plant (pasture; deciduous tree leaves) steady-state in our studies with PCBs (3, 4) and PAHs (5, 6), which have a similar range of physicochemical properties * Corresponding author phone: +44 1524 593922; fax: +44 1524 593985; e-mail:
[email protected]. 2372
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and octanol/air partition coefficients (KOA) to the PCDD/Fs. In each case, this steady-state occurred in a matter of days. Other experiments show that air concentrations can be buffered over even shorter time intervals (diurnally) by dynamic changes in plant concentrations (7). These issues are complicated, of course, by differences in plant type, environmental conditions, and compounds. SOCs are present in the air in both the vapor phase and the particulate phase, the relative proportions of which are influenced by compound physicochemical properties and the nature of particulate material present in the air. Uptake can occur by dry gaseous, dry particulate, dissolved (in precipitation) deposition, or wet particulate deposition, the relative importance of each of which have been discussed by McLachlan (1) and Smith and Jones (8). Because of the differences between studies as to the time required to attain air-pasture/plant steady-state, the study presented here was conducted to investigate the kinetics of uptake of PCDD/Fs by a pasture sward over a whole growing season and to make inferences about the transfer processes.
Experimental Section Field Study. A field plot previously used for a similar PCB uptake experiment (3) was used. The plot had been isolated on an established sward of unimproved upland pasture at a semirural field station site owned by Lancaster University, in the northwest of England. The field site is on a windy, exposed, westerly sea-facing hillside with prevailing southwesterly wind which averaged 4.0 m/s over the period of June-October 1998. Meteorological and air sampling equipment were available at the field station; meteorological data for the experimental period are given in the Supporting Information. The sward comprised Lolium perenne (∼30%), Holcus lanatus (∼30%), Agrostis capillaris (∼10%), Poa pratensis (∼10%), and Cynosurus cristatus (∼10%) and a range of other grasses and forbs. The field plot was divided into 1 m2 subplots on which three management regimes were practiced; “simulated grazing”, in which triplicate subplots were harvested every 2 weeks through the sampling period (June-October 1998); “simulated silage production”, in which triplicate subplots were harvested at 6 week intervals (i.e., June, July, September, and October); and “long-term growth”, in which three sets of triplicate subplots were prepared for harvesting after 10-12 weeks growth (in June, September, October, and January, with January samples being collected from the October harvested subplots). Pasture yields (g/m2) were always recorded. Pasture samples were placed into bags in the field, immediately sealed and frozen until required. Air samples were also taken over 2 week periods throughout the experimental period, coinciding with pasture harvesting. A total of 1000-2000 m3 of air was sampled using a hi-vol sampler fitted with a filter and polyurethane foam (PUF) plug. The glass-fiber filters and PUF plugs were spiked with 13C-labeled PCDD/F recovery standards before sampling commenced. Three soil depth cores were taken for PCDD/F analysis from a 1 m2 area adjacent to the field plot used for the pasture uptake study. Each core was taken to a depth of 13 cm and divided into 0-2, 2-4, 4-6, 6-8, and 8-13 cm depth sections. The corresponding sections of the three cores were combined and frozen wrapped in aluminum foil for analysis. Soil Particle Adhesion Estimation. The importance of soil particle adhesion to the PCDD/F loading of pasture at this field site was estimated by measuring the concentrations of metals which are known to be subject to inefficient root uptake and translocation by grass. Cary et al. (9) showed that 10.1021/es010176g CCC: $22.00
2002 American Chemical Society Published on Web 05/03/2002
Ti is found in the soil but not translocated within pasture and that the amount of soil particles attached to the pasture (through particle suspension/deposition) can be estimated as a ratio of the concentration of this metal in the soil and the pasture. These authors also showed that Fe and Al, used to perform the same estimate, give good but slightly less reliable results. Two samples of pasture (100 g) and two samples of soil were taken adjacent to the study plot for Ti and Al analysis. Soil samples were taken from the top 2 cm of the soil. Pasture was sampled from a sward of approximately 4 cm length in June, collected in the same way as samples for PCDD/F analysis. All samples were immediately stored in plastic bags and analyzed the same day. Analysis. (i) Pasture. Samples for PCDD/F analysis (50 g wet weight) were frozen with liquid nitrogen, ground with powdered sodium sulfate (100 g), spiked with 13C-labeled internal standards, and Soxhlet-extracted overnight with 400 mL dichloromethane. On one occasion for the 2 week growth samples (October 19), the yield of pasture was too low to analyze each of the triplicate samples separately; two of the samples were therefore combined before analysis, providing duplicate samples overall on this date. Dry weight determination was carried out at the time of extraction, on a separate subsample of each sample, at 95 °C overnight. Pasture extracts were evaporated to 20-30 mL and initially cleaned up by passing through 20 g of silica gel in a 5 cm i.d. column eluted with 250 mL hexane. After concentration to approximately 2 mL, a multilayer silica gel column (silica gel, silica gel treated with concentrated sulfuric acid and silica gel treated with 1 M sodium hydroxide) was used to further clean the samples, followed by gel permeation chromatography (using BioBeads SX-3 and hexane/DCM as the solvent). Samples were finally fractionated using basic alumina chromatography (PCDD/F fraction elution with 1:1 hexane/DCM) and evaporated to 15 µL of nonane with 37Cllabeled injection standard added. (ii) Air. PUF plug and glass-fiber filters were Soxhletextracted separately after the addition of 13C-labeled internal standards. PUF plugs were extracted with DCM overnight, and glass-fiber filters were extracted with toluene overnight. After extraction, all samples were evaporated to near dryness and resuspended in hexane before cleanup using the same multilayered silica chromatography and basic alumina chromatography methods as were used in the previous pasture analysis. Finally, the samples were evaporated to 15 µL of nonane with 37Cl-labeled injection standard added. (iii) Soil. Samples for analysis (20 g wet weight) were mixed with powdered sodium sulfate (50 g), spiked with 13C-labeled internal standards, and Soxhlet-extracted overnight with 250 mL of dichloromethane. Dry weight determination was carried out at the time of extraction, on a separate subsample of each sample, at 95 °C overnight. Soil extracts were evaporated to 20-30 mL and initially cleaned up by passing through 10 g of silica gel and 10 g of neutral alumina in a 2.5 cm i.d. column eluted with 150 mL 1:1 hexane/DCM. The samples were then evaporated and cleaned up by gel permeation chromatography (as given previously). After concentration to approximately 2 mL, a multilayer silica gel column (silica gel, silica gel treated with concentrated sulfuric acid and silica gel treated with 1 M sodium hydroxide) was used to further clean the samples. Samples were finally fractionated using basic alumina chromatography (PCDD/F fraction elution with 1:1 hexane/DCM) and evaporated to 15 µL of nonane with 37Cl-labeled injection standard added. (iv) Instrumentation. All samples were analyzed by GCHRMS using a HP6890 GC and a VG Autospec Ultima mass spectrometer in SIR mode at 10 000 resolution. Each sample was separately analyzed for the PCDD/F total homologue groups (Cl2DD/F to Cl8DD/F) and the 2,3,7,8-substituted congeners using a 30 m HP5MS capillary column and a 60
TABLE 1. Concentrations of 2,3,7,8-Substituted PCDD/Fs and Homologues Found in Pasture Exposed for 2 Weeks, Harvested in June average average 2,3,7,8 as concn concn % of total (fg/g DM) homologues (fg/g DM) homologue 2,3,7,8-Cl4DF 1,2,3,7,8-Cl5DF 2,3,4,7,8-Cl5DF 1,2,3,4,7,8-Cl6DF 1,2,3,6,7,8-Cl6DF 2,3,4,6,7,8-Cl6DF 1,2,3,4,6,7,8-Cl7DF 1,2,3,4,7,8,9-Cl7DF
68 93 55 110 58 95 930 17
2,3,7,8-Cl4DD 1,2,3,7,8-Cl5DD 1,2,3,4,7,8-Cl6DD 1,2,3,6,7,8-Cl6DD 1,2,3,7,8,9-Cl6DD 1,2,3,4,6,7,8-Cl7DD
27 29 13 82 60 1000
Cl4DF Cl5DF
1800 1000
Cl6DF
1000
Cl7DF
1800
Cl8DF Cl4DD Cl5DD Cl6DD
790 3500 1400 1400
Cl7DD Cl8DD
2200 5500
4 9 5 11 6 9 52 1 1 2 1 6 4 47
m SP2331 column, respectively. Quantification was achieved using an isotope dilution method. (v) Metal Determination. A “glass” disk was produced with ashed (baked at 450 °C for 8 h) pasture or soil samples and “spectroflux” (Johnson Matthey) at 1200 °C for analysis by X-ray fluorescence spectroscopy. Analysis was performed on a Philips PW1400 XRF spectrometer using a Rhodium X-ray tube, calibrated using geological reference material. QA/QC. Laboratory blanks were included as at least 1 in 12 of all samples analyzed and consisted of anhydrous sodium sulfate for pasture and soil analysis and unused PUF plug and glass-fiber filters for air samples. Blank levels were subtracted from the samples. The criteria for the quantification of analytes were a retention time found within 2 s of the standard, isotope ratio found within 20% of standard, a signalto-noise ratio of at least 3, and recoveries of 2,3,7,8-labeled congeners between 50% and 130%. All 2,3,7,8-labeled recovery standards in all samples fulfilled the last criterion. Method detection limits were calculated as 3 times the standard deviation of the concentrations found in the analytical blanks. If the concentrations in the blanks were below the instrumental detection limit (1 pg/sample), then the method detection limit was defined as equal to the instrumental detection limit. Method detection limits (pg/ sample) for the pasture and soil analysis were 1.0-1.1 (Cl2-6DD), 1.9-2.2 (Cl7DD, Cl4-6DF, Cl8DF), 3.1 (Cl8DD), 3.9 (Cl3DF), 4.5 (Cl7DF), and 11 (Cl2DF). Method detection limits (pg/sample) for air analyses were 1.0 (Cl4-6DD, Cl5-8DF), 1.8 (Cl3DD), 2.2-2.4 (Cl7-8DD), 4.0 (Cl3-4DF), 20 (Cl2DD), and 32 (Cl2DF).
Results and Discussion The 2,3,7,8-substituted PCDD/F congeners (up to Cl7DD/F) were found to contribute from less than 1% to no more than 10% of the total concentration for each homologue group in the 2 week pasture samples harvested in June (except the Cl7DD/Fs, see Table 1), and many of these congeners were found to be close to the detection limit; the data are therefore presented and discussed on a homologue group basis. Comments on Air Sampling and Artifacts. Measurements of particulate and vapor-phase chemical concentrations in air are operationally defined. In this study, they are defined as the amounts collected on a glass-fiber filter and PUF plug, respectively. Various sampling artifacts affecting the apparent distribution of chemicals between the vapor and particulate phases in air are possible, which have been discussed in VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Air and pasture homologue group patterns (June 25). detail by Pankow and Bidleman (10). Briefly, sampling artifacts include particulate-bound chemical stripped from the particles and measured as vapor phase, sorption of vaporphase chemical to the filter and particulate during operation, and breakthrough of vapor-phase chemical from the PUF plug. Each of these artifacts is temperature-related, and the first two are also dependent on both the nature of the particulate material and concentration changes during sampling. Previous work on PCDD/Fs at Lancaster with similar air volumes, sampling durations, and average ambient temperatures showed no breakthrough problems (11). Other artifacts cannot be ruled out, but their effects were minimized by collecting samples at the same time each day (reducing variability relating to the temperature at sample changeover). In addition, meteorological data indicate that the diurnal temperature range was comparatively low (this site being in close proximity to the coast), averaging only 6.7 °C and with an unusual maximum of 12.5 °C during the air sampling periods. PCDD/Fs in Air. A strong seasonal trend in concentrations (with a winter maximum) was seen for all PCDD/F homologue groups above dichlorinated, as previously reported in other studies in the northwest of England (12, 13). This has been related to the increase in diffuse combustion sources during the winter (13). In contrast, very large mid-summer peaks in the concentrations of Cl2DF and Cl2DD were observed, each amounting to almost an order of magnitude between the maxima and minima. The abundance of these homologues in air at this site has been noted previously (13, 14). Vapor-phase concentrations above the detection limit were always found for the homologues up to and including the Cl7DD/Fs (and usually for the Cl8DD/Fs), while particlebound concentrations were rarely measurable below the tetrachlorinated dioxins, although they were measurable for all of the furan homologues. The particle-bound fraction of each homologue group was calculated and broadly shifted by one to two homologue groups between summer and winter. To illustrate, in June, 50-60% of Cl6DD/Fs were found in the particle phase, whereas, in January, 50-60% of Cl4DD/Fs were found in the particle phase, with a corresponding shift in the other homologue groups. The combined particle and vapor-phase homologue group pattern for air on June 25 is shown in Figure 1, and the average percentage of each homologue group found in the particulate phase between June 25 and October 19 and between October 19 and January 21 are shown in Table 2. Pasture Concentrations and Factors Controlling Them. The concentrations of PCDD/Fs measured in pasture will be 2374
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TABLE 2. Average Values for the Proportion of Each Homologue Group in the Particulate Phase during the Summer (June 25-October 19) and Winter (October 19-January 21) percent found in particulate phase
Cl2DF Cl3DF Cl4DF Cl5DF Cl6DF Cl7DF Cl8DF
summer
winter
1 2 9 23 51 77 87
2 8 24 58 86 91 90
Cl2DD Cl3DD Cl4DD Cl5DD Cl6DD Cl7DD Cl8DD
summer
winter
0 2 5 21 60 85 88
0 9 25 53 86 95 89
a sum of all input (deposition, soil resuspension) and loss (volatilization, photolysis, cuticle shedding, degradation, and growth dilution) processes, which will vary for each homologue group. The PCDD/F concentrations found in pasture and in air on June 25 are shown in Figure 1. The pattern is dominated by the Cl2DD/Fs, which are present at over 100 times the concentration of the next highest homologue group, reflecting the high concentrations of this homologue group in air. On three occasions throughout the growing season (AprilOctober, inclusive), samples were taken on the same day for all three exposure periods (i.e., pasture that had been growing for 2, 6, and 10-12 weeks was harvested together). Figure 2A shows the homologue concentrations for this occasion in June 1998. June is a period of active pasture growth. Concentrations were similar for each of the growth/exposure times for the homologues up to and including the Cl6DD/Fs. The Cl2-6DD/Fs were all predominantly (>50%) in the vapor phase in air at this time of year (Table 2). In a previous study (3), PCBs (found predominantly in the vapor phase in air) were also found to be at the same concentrations in 2, 6, and 12 week old pasture harvested at the same time. In contrast, Cl7-8DD/Fs, which were >75% particulate bound in the air, showed a downward trend in concentrations for the longer exposure times, although these differences were not statistically significant and caution must therefore be exercised in interpretation. Possible explanations for this observation include growth dilution, photodegradation of PCDD/Fs on the plant (15, 16), shielding of fresh pasture growth by the dense sward which develops after long growth periods, washoff of attached particles, reduced particle attachment (wet or dry deposition) with increased maturity of the sward or leaves, and cuticle shedding. More thorough discussion of these possibilities is made in the following section.
TABLE 3. Estimated Average Percent Contribution of PCDD/F Homologue Groups Found in Pasture Exposed for 2 Weeks between June and October (n ) 14) from Soil Particle Adhesiona
Cl2DF Cl3DF Cl4DF Cl5DF Cl6DF Cl7DF Cl8DF Cl2DD Cl3DD Cl4DD Cl5DD Cl6DD Cl7DD Cl8DD
mean
range
standard deviation
0.1 1.1 4.3 6.9 12 15 14 0.1 1.0 2.7 7.4 7.3 5.6 5.1
0.02-0.12 0.55-1.8 1.5-8.2 2.0-14 2.3-23 1.2-34 0.49-40 0.06-0.31 0.53-1.8 0.41-8.9 2.3-14 2.2-12 1.7-11 1.1-12
0.03 0.40 2.0 3.8 8.2 12 14 0.08 0.38 2.1 3.4 3.4 3.1 3.8
a Percent contribution from soil ) (100 × [PCDD/F] soil (pg/mg DM) × estimated amount of soil adhering to pasture (mg DM soil/g DM pasture)/ [PCDD/F]grass (pg/g DM).
FIGURE 2. PCDD/F concentrations in pasture sampled simultaneously after 2, 6, and 10-12 weeks exposure in (A) June, (B) September, and (C) October. The same homologue patterns for the different exposure periods described previously were seen in October (Figure 2C), but the reduction in concentration with exposure time included the Cl6DD/Fs, which had a higher particulate-bound fraction at that time of year. In September (Figure 2B), however, the concentrations were similar in all of the treatments for all of the homologues. Again, none of these differences were significant at the 95% confidence limit, and these data are therefore interpreted cautiously. Discussion of Seasonal/Exposure Data. May-June is the major pasture growth period at the field site (3) and, therefore, the time when the 12 weeks exposure samples produced the longest, most dense sward. The dry matter yields (g/m2) of the plots related to Figure 2 (in the order 2, 6, and 12 week samples) were the following: (June) 51, 110, 450; (September) 30, 130, 220; (October) 11, 36, 120. Cumulative rainfall (mm) during the 2 weeks preceding sampling for June, September, and October samples were 82, 7.3, and 62, and the average temperatures in these weeks were 13, 12, and 10 °C, respectively. Cumulative rainfall over 6 and 12 weeks were similar for each sampling occasion. June yields were therefore similar (6 week samples) or about 2 times higher than those for September, and September yields were 2-4 times higher than those for October. Growth dilution and shielding are therefore unlikely to offer good explanations for the reducing concentrations of the heavier homologues in June and October. Cuticle shedding is also likely to be linked to growth rates and the age of the sward so is unlikely to offer a good explanation. Photodegradation can probably also be discounted because the most hours of sunlight, and highest light intensity, are experienced in June. It is possible that the
various factors described here could be contributing (both positively and negatively) to PCDD/F concentrations seen in pasture, but the most noticeable difference between the June and October samples and the September samples was the amount of rainfall in the 2 weeks preceding sampling. This leads us to speculate that rainfall is an important factor in determining the concentrations of the heavier PCDD/Fs in pasture. It is not possible to tell from these data whether the observations noted are caused by increased wet particulate deposition in June and October (affecting the shorter growth period, lower yield samples) or increased wash-off of already-attached particles (affecting the longer growth period samples). The pasture concentrations of the more chlorinated homologues show higher variability between replicate samples (as seen in Figure 2) than the less chlorinated homologues. We speculate that this is because particle/surface-bound SOCs are not retained uniformly within the sward or the transfer of the heavier homologues from attached particles to the plant cuticle (7, 17) is more variable. In other words, wash-off, particulate uptake, and transfer from attached particles are more variable, affected by rainfall, windspeed, leaf and sward morphology, and so forth, than processes controlling vapor-phase partitioning. Importance of Soil Particle Adhesion to PCDD/F Concentrations in Pasture. Ti and Al analysis showed that approximately 17 mg soil (DW) adhered to 1 g pasture (DW) at this site in mid-summer. Both metals gave similar results, which fell within the range found by other workers at different sites (tabulated in ref 8). Multiplying this with PCDD/F concentrations found within the top 2 cm of the soil at this site enabled an estimation of the proportion of the total PCDD/F concentrations found in pasture attributable to soil particle adhesion. (Note: This calculation assumes that the distribution of soil-bound PCDD/Fs on the leaves is the same as that in the surface soil and that the soil particle type and size distribution of the metals and PCDD/Fs is similar and does not include the impact of PCDD/Fs and metals contained in particles deposited from the atmosphere.) The average and ranges of percent contribution from soil adhesion for all of the 2 week samples (14 samples in total) are shown in Table 3. These estimates suggest that the Cl5DD/Fs and above had the highest percent contribution from soil particle adhesion (4.3-15%) and that the tetrachlorinated and lower homologue groups had less than about 4% of the VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Scavenging coefficient plot, derived using vapor-phase air concentrations of PCDD/F uptake in (A) June 25, (B) July 9, (C) July 23, (D) September 4, and (E) October 19 (with temperature corrected log KOA values). total pasture concentration supplied by soil particles adhering to the surface of the pasture leaves. Of note is that 12-15% of the Cl6-8DFs were contributed from soil, as compared to 5.1-7.3% of the Cl6-8DDs. In colder periods, the percent contributions from soil for each homologue group are much lower (by a factor of 2-10 for the samples taken in January) because the pasture concentrations are higher (due to both higher PCDD/F concentrations in the air and increased partitioning of vapor-phase compounds to pasture at lower temperatures). Scavenging Coefficients (SC). SC are defined as the concentration ratio between air (expressed as pg/m3) and pasture (expressed as pg/g DM) and have units of m3 of air/g of DM pasture (defined in refs 1 and 3). They have been calculated for the uptake of PCDD/Fs by pasture (after the estimated contribution from adhering soil particles was subtracted) from the vapor phase. They increased with increasing degree of chlorination, in line with previous studies and expectations (2-4, 18, 19). The log SC values for the growing season 2 week samples (June 25-October 19) are plotted against the temperature corrected KOA values (from ref 20), which also generally increase with increasing degree of chlorination, in Figure 3. Vapor-phase scavenging coefficients could not be calculated when vapor-phase concentrations were not detectable (affecting the octachlorinated congeners, see Figure 3, parts A and B). Figure 3A appears to contain two anomalous points (Cl4DD and Cl5DD) which 2376
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are due to low vapor-phase air concentrations measured at this time; these homologues are not anomalous at other sampling times and have been assumed to be outliers for the June sample in further discussion. Each of the plots in Figure 3 gives a positive linear relationship between log SC and log KOA up to and including the Cl5DD/Fs (log KOA ∼11) while the Cl7-8DD/Fs (log KOA ∼ 11-13) generally do not follow the same line. The Cl6DD/Fs appear to show an “intermediate” type behavior. These observations are indicative of the dominance of vapor- and particulate-phase deposition of the lower chlorinated and higher chlorinated homologues, respectively, as postulated by McLachlan (1) and discussed next. Linear regressions were performed on the data up to and including the Cl6DD/Fs. Statistically significant (at the 99% level) linear relationships were obtained between log SC and log KOA for all of the 2 week samples, with an average gradient of 0.485 (range 0.258-0.790), similar to previous studies on organochlorine compound uptake by plants (discussed in ref 4). McLachlan (1) postulated a scavenging coefficient model, describing vapor-phase uptake at lower log KOA values (linear SC/log KOA curve), followed by supply mediated uptake (zero slope) and then particulate dominated uptake at high log KOA values, which fitted data obtained at a rural site in southern Germany. The straight line between log KOA values of approximately 8.5 and 11 (see Figure 3) indicate that neither supply controlled partitioning (a plateau in the line) or
TABLE 4. Estimated PCDD/F Deposition to U.K. Landa congener
SCtotal air
SC σn-1
CPasture
CPasture σn-1
A
B
C
D
E
Cl2DF Cl3DF Cl4DF Cl5DF Cl6DF Cl7DF Cl8DF Cl2DD Cl3DD Cl4DD Cl5DD Cl6DD Cl7DD Cl8DD
12 27 45 60 110 1300 590 13 28 69 140 100 130 220
6.5 14 25 46 130 2800 1400 6.7 13 34 130 100 140 270
670 2.7 2.3 2.0 2.7 5.7 4.0 5.2 0.59 2.0 1.4 2.1 3.7 11
330 1.0 1.1 1.3 2.5 7.6 7.8 2.3 0.21 1.1 0.71 1.3 2.7 8.7
590 2.4 2.1 1.7 2.4 5.0 3.5 4.6 0.52 1.8 1.2 1.8 3.3 9.4
10 000 24 000 40 000 53 000 98 000 1 100 000 520 000 11 000 25 000 60 000 130 000 90 000 110 000 200 000
41 0.17 0.14 0.12 0.17 0.35 0.25 0.32 0.036 0.12 0.086 0.13 0.23 0.66
430 1.8 1.5 1.3 1.7 3.6 2.6 3.3 0.37 1.3 0.89 1.3 2.4 6.8
6.1 0.025 0.021 0.018 0.024 0.052 0.036 0.048 0.0054 0.018 0.013 0.019 0.034 0.10
a SC 3 3 total air ) total air concentration scavenging coefficient (m of air/g of DM pasture); SC σn-1 ) standard deviation on SCtotal air (m air/g DM pasture); CPasture ) average concentration found in pasture after the estimated contribution from soil particle adhesion has been subtracted (pg/g); CPasture σn-1 ) standard deviation on CPasture (pg/g); A ) uptake throughout growing season (ng/m2) {)CPasture × DM yield over growing season}; B ) air volume equivalent scavenged throughout growing season (m3/m2) {)SC × DM yield over growing season}; C ) total uptake by pasture over the U.K. per growing season (kg) {)B × U.K. land area under pasture}; D ) peak daily U.K. uptake (g) {)C × peak daily DM yield/DM yield over growing season}; E ) peak daily uptake per m2 (ng/m2) {)D/U.K. land area under pasture}; DM yield over growing season ) 8800 kg/ha; U.K. land area under pasture ) 7 million ha; peak daily DM yield ) 91 g/ha.
11 (the Cl6-8DD/Fs and above). This indicates that supplymediated and particulate uptake become more important at this site during the winter months (when lower temperatures favor greater partitioning of gas-phase compounds to plants and airborne particulates, as discussed previously). Estimating PCDD/F Deposition to the U.K. Assuming that air and pasture PCDD/F concentrations at the Lancaster site are typical for the U.K. and that the site is representative of national pastureland sward composition, it is possible to estimate the magnitude of deposition of PCDD/Fs to U.K. land (3).
FIGURE 4. Scavenging coefficient plot, derived using the vaporphase air concentrations, for the 12 week exposure samples harvested in summer (September) and winter (January). particulate deposition (which would cause a different slope in the latter parts of the line) are dominant factors in the uptake of Cl2-5DD/Fs at this site and that vapor-phase steadystate has been approached. However, particulate deposition dominates the behavior of the Cl7DD/Fs and Cl8DD/Fs, with the Cl6DD/Fs showing intermediate behavior. Seasonality. Figure 4 shows the scavenging coefficients for the 12 week exposure samples collected in early September and January, calculated using the vapor-phase air concentrations averaged over the growing period, plotted against temperature corrected KOA values. In September, a straight line is obtained (with gradient ) 0.539 and r2 ) 0.945, significant at the 99% level); in contrast, the January data does not follow a linear trend above a log KOA value of about
Using the typical pasture yield (8800 kg of DM/hectare for the whole growing season) and pastureland coverage (7 million hectares) for the U.K. (from ref 3) and the average total air concentration based scavenging coefficients calculated for the 2 week samples throughout the growing season (after subtraction of the estimated input from soil, calculated previously), the amount of PCDD/F deposition to pastureland during the growing season in the U.K. was estimated. This calculation assumes that the uptake of all PCDD/F homologues by pasture can be estimated directly from the total air concentration (i.e., that the same uptake rate applies to both vapor- and particle-phase compounds from the air); however, this is preferable to the alternative of assuming that uptake can be predicted purely from vapor-phase scavenging coefficients. The equivalent volume of air depositing PCDD/F to pasture and peak daily deposition rates (assuming a peak pasture yield of 91 kg of DM/ha) were estimated (shown in Table 4). For all homologues (apart from the Cl2DFs), between 0.04 and 0.66 kg of each homologue group are transferred to pasture each year in the U.K. This information may be usefully compared to estimates of annual emissions and transfers into the human diet.
Acknowledgments We thank Mrs. Vicky Burnett for performing XRF analysis. Financial support for this work came from the U.K. Food Standards Agency and the Environmental Diagnostics Programme of the Natural Environment Research Council.
Supporting Information Available Summarized concentration data for the air and pasture samples analyzed in this study are available in Tables SI1 and SI2, respectively. Meteorlogical data pertaining to the VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Received for review June 29, 2001. Revised manuscript received March 11, 2002. Accepted March 11, 2002. ES010176G
