Environ. Sci. Technol. 2004, 38, 3011-3018
Wet Deposition of Polychlorinated Biphenyls in the Eastern Mediterranean MANOLIS MANDALAKIS AND EURIPIDES G. STEPHANOU* Environmental Chemical Processes Laboratory (ECPL), Department of Chemistry, University of Crete, GR-71409 Heraklion, Greece
The concentrations of polychlorinated biphenyls (PCBs) were measured in rain samples collected from a semiurban and a marine background site of the eastern Mediterranean. The concentration of ∑PCB (sum of 54 PCB congeners) in the city of Heraklion (2.9 ( 1.9 ng L-1) was not significantly higher than the corresponding concentration measured at the background sampling station of Finokalia (1.9 ( 0.9 ng L-1). In both sites, the sum of tri- and tetrachlorinated congeners accounted for more than 55% of ∑PCB in rainwater. For all samples, the percentage of particle-bound PCBs ranged between 6.6% and 63.8%, providing an average value of 31 ( 18%. The washout ratios of particulate PCBs (WP) were constant for individual congeners regardless the degree of chlorination. Average WP values ranged between 1.9 × 105 and 5.2 × 105 while a value of 2.7((1.3) × 105 was deduced for ∑PCB. The corresponding washout ratios for gaseous PCBs were substantially lower and ranged between 7 × 103 (PCB 99) and 1.3 × 105 (PCB 180). Washout ratios of gaseous PCBs were also calculated based on Henry’s law, and they were found to be 30-920 times lower than those obtained from field measurements. On the basis of our data, the wet deposition flux of ∑PCB in the eastern Mediterranean should approach 820 ng m-2 yr-1. This flux is similar with the values recently reported for several background sites of the United States and Europe, but it is 1 order of magnitude lower than the flux of PCBs measured in the western Mediterranean 16 yr ago.
Introduction Although polychlorinated biphenyls (PCBs) production and use were phased out and banned by the mid-1970s in most countries, PCBs are still emitted into the atmosphere through primary (e.g., vaporization or open burning of products containing PCBs) (1) or secondary emissions (e.g., sea-air exchange) (2). In addition, it has been known for several decades that the atmosphere serves as an important compartment for distributing anthropogenic contaminants on a global level (3). Several studies have shown that PCBs and other semivolatile organic compounds (SOCs) can be transported from polluted areas to remote regions through the movement of air masses (4-6). Subsequently, atmospheric deposition should be the major process by which PCBs may enter pristine * Corresponding author telephone: +30 2810 393628; fax: +30 2810 393678; e-mail:
[email protected]. 10.1021/es030078q CCC: $27.50 Published on Web 05/05/2004
2004 American Chemical Society
regions and native ecosystems (7-10). Eisenreich et al. (7) were the first to show that atmospheric deposition should contribute more than 85% of total PCBs input to Lake Superior. Pollution throughout the Mediterranean was always under special concern. The relatively higher concentrations of PCBs and other xenobiotics observed in resident marine mammals (11, 12) indicated that the pollution of the western Mediterranean has reached alarming levels. Contaminants such as DDT and PCBs were regarded as responsible for the mass die-offs of monk seals (11) and striped dolphins (12) along the Mediterranean coasts. Recently, it was demonstrated that several pollutants such as O3, CO, etc. are formed in European air and subsequently transported toward the Mediterranean (13). Studies dealing with the mass balance of PCBs in the western Mediterranean Sea have shown that this region is continually acting as a reservoir of organochlorine compounds, while the atmospheric flux of PCBs due to dry and wet deposition account for most of the input (14, 15). However, the deposition fluxes calculated in these models were based on a limited number of field measurements (16). Indeed, several chlorinated hydrocarbons including PCBs were formerly detected in a limited number of dry and wet deposition samples collected in south France (16). Since PCBs are semi-volatile compounds, they exist in both gas and particulate phases of the atmosphere (17). Thus, both gaseous and particle-associated PCBs will be deposited on earth’s surface through precipitation scavenging (17, 18). The dissolution of gaseous PCBs into rain and cloud droplets is generally assumed to be a reversible, equilibrium process that depends on Henry’s law constant. Wet deposition of particulate PCBs takes place through particle scavenging, and it will depend on the chemical and physical properties of the aerosols (17). The gas and particle washout ratios are usually used in order to define the scavenging efficiency of a semi-volatile compound (17). In addition, washout ratios are frequently used to estimate probable PCB concentrations in rain, especially in the context of mass balance modeling (8, 19). Until now, total washout ratios (gas plus particles) for individual PCB congeners and ∑PCB (sum of all PCB congeners) have been estimated from field measurements, and they have been reported in several previous studies (7, 20-24). However, studies dealing with the separate estimation of particle and gas washout ratios of PCBs directly from field data are limited (25, 26). In a recent publication (27), we presented the study of the presence of PCBs in the atmosphere of the eastern Mediterranean, which has been attributed mainly to long-range transport. The present study aimed to determine the concentrations of individual PCB congeners in the dissolved and particulate phases of precipitation samples collected from an urban site and from a background marine site of the eastern Mediterranean. By combining the results of the present study with the atmospheric concentrations of PCBs measured at the same time period (27), the washout ratios of individual PCBs were calculated on the basis of data collected under real environmental conditions. In addition, the annual precipitation flux of PCBs to the eastern Mediterranean was estimated.
Experimental Section Sampling. Precipitation samples were collected from the suburbs of Heraklion (n ) 8) and the marine background sampling station of Finokalia (n ) 6) between April 2000 and May 2001 (Table 1). Both sites are located at the north part VOL. 38, NO. 11, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Sampling Period, Meteorological Parameters (Average Values), ∑PCB Concentration (Sum of 54 PCB Congeners), and Percentage of PCBs in Particulate Phase for 14 Precipitation Events sampling site
sampling period
wind speed (m s-1)
wind direction
temp (°C)
∑PCB (ng L-1)
particulate PCBs (%)
total vol (L)
total rainfall (mm)
Finokalia
Apr 14-21, 2000 Apr 21-May 5, 2000 Nov 27-29, 2000 Nov 29-Dec 18, 2000 Feb 11-13, 2001 Apr 30-May 13, 2001
2.8 4.1 6.2 4.8 4.0 5.7
N-NW N-NW NW N-NW N-NW W
16.8 17.3 14.8 15.2 10.6 18.8
1.9 3.6 1.5 1.7 1.7 1.0 1.8 ( 0.4a
46.2 52.8 23.2 22.5 10.9 25.6
1.1 1.5 2.0 1.6 1.0 2.6
8 11 14 11 7 19
Heraklion
Mar 17-18, 2000 Mar 22, 2000 Apr 15-20, 2000 Apr 21-May 5, 2000 Nov 27-29, 2000 Nov 29-Dec 20, 2000 Jan 27-Feb 11, 2001 Feb 11-12, 2001
5.2 12.5 3.0 4.2 6.2 4.6 4.6 1.9
N-NW N NW N-NW NW N-NW S-SW S
11.6 9.6 17.2 17.2 14.6 15.1 13.4 10.1
4.5 5.3 3.6 4.6 1.0 1.0 0.7 2.2 2.3 ( 0.9a
42.2 48.4 63.8 35.6 10.9 11.3 28.9 6.6
1.1 1.4 1.5 1.2 2.7 2.8 2.3 2.8
8 10 11 9 20 20 16 20
a
Volume-weighted mean concentration of ∑PCB and the corresponding standard error calculated according to Offenberg and Baker (29).
FIGURE 1. Map showing the location of the sampling sites. of the Island of Crete, Greece (Figure 1). Heraklion (35°21′ N, 25°10′ E) is the largest urban center of Crete with 150 000 inhabitants and rather limited industrial activity. Finokalia (35°20′ N, 25°40′ E) is a coastal background marine site 70 km eastward of Heraklion at the top of a hilly elevation (130 m above sea level), facing the sea within a sector of 270-90°. No human activities occur at distance shorter than 20 km from the Finokalia station. Stainless steel funnels (42 cm diameter) connected to amber glass bottles were used for the collection of precipitation. Prior to sampling, both funnels and bottles were washed with HPLC quality grade acetone and then with Milli-Q water. The rain collectors were mounted at the top of a sampling platform (6 m above ground) located at Crete University campus and at the roof of the Finokalia sampling station (2.5 m above ground). To minimize the effect from dry deposition, the installation of the samplers was conducted 1 day before the beginning of a rain event, based on the SKIRON weather forecasting system, which covers the region of Greece and is characterized by high time and spatial resolution. This weather forecasting system has been developed by University of Athens and Hellenic Meteorological Service; it is available through the Internet (http://forecast.uoa.gr). The samples were collected and analyzed as soon as possible after the end of a rain event (within 24 h). In cases when the amount of the collected rain was not enough for the analysis, the top of the sampler was covered until the next rain event. Meteorological data during sampling periods (including air temperature, wind speed, and wind direction) were provided by the Meteorology Service of the North Kazantzakis Airport of Heraklion. 3012
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Materials. Standard solutions of PCBs 54, 155, 185, and 116 were purchased by Dr. Ehrenstorfer GmbH (Augsburg, Germany). Anhydrous sodium sulfate, silica gel, and n-hexane (pesticide-free grade) were purchased from Merck (Darmstadt, Germany), while glass fiber filters (diameter 47 mm, nominal pore size 0.7 µm) were purchased from the Pall Corporation (Ann Arbor, MI). Analytical Procedure. Samples were filtered through glass fiber filters to isolate particles (operationally defined) from the dissolved phase. Subsequently, each phase was separately analyzed for PCBs. The extraction of the dissolved phase was performed using a liquid-liquid technique while particulate PCBs were extracted by sonication in an ultrasonic bath. The dissolved phase and 60 mL of n-hexane were placed in a separatory funnel, spiked with known amounts (∼3 ng) of surrogate standards (PCBs 54, 155, and 185), and then the funnel was shaken for 5 min. After phase separation, the organic layer was drained through a funnel containing anhydrous Na2SO4. The same procedure was repeated two more times with 60-mL portions of n-hexane. The total extract was concentrated to 2 mL by rotary evaporation and then to 1 mL under a gentle nitrogen stream at ambient temperature. The sample was then applied onto a 1.5-g silica gel column and eluted with 12 mL of n-hexane. The eluted fraction was reduced to 1 mL and then loaded onto a disposable Pasteur pipet packed with the following components: 1 cm of dehydrated NaSO4, 1 cm of H2SO4-impregnated silica, 1 cm of KOH-impregnated silica, and 1 cm of deactivated silica in series. PCBs were subsequently eluted with 6 mL of n-hexane. The final extract was reduced by rotary evaporation to 0.5 mL, transferred into a 1-mL vial, and further evaporated to almost dryness under a gentle nitrogen stream at -9 °C. Then, a known amount of internal standard (PCB 116) was added, and 2 µL of the sample was injected to a Finnigan GCQ gas chromatograph directly interfaced to a Finnigan GCQ ion trap mass spectrometer (GC-ITMS) operating in MS/MS mode. The GC was equipped with a 30 m × 0.25 mm i.d., 0.25 µm film thickness, HP-5MS fused silica column. Fifty-four PCB congeners (corresponding to 41 chromatographic peaks) were quantified during this study. More information about the analysis and detection of PCBs has been reported in a previous study (28). A different extraction method was used for the analysis of the particulate phase: The filter was spiked with a certain amount of surrogate standard (∼3 ng) and then placed in a flask containing 25 mL of n-hexane. The flask was
sonicated for 5 min, and the extract was drained through a funnel containing anhydrous Na2SO4. The same procedure was repeated two more times with 25-mL portions of n-hexane. Quality Control and Assurance. Before the onset of the sampling program, PCB recovery studies were undertaken to verify the method. This involved spiking two GFF filters and two portions of Milli-Q water with 3 ng of PCBs 54, 155, and 185 and treating them as real samples. These congeners cover a wide range of physicochemical properties, and thus they will adequately represent the properties of several PCB congeners analyzed in this study. The recoveries of spiked PCB standards from filter samples were similar with those observed in Milli-Q water samples, and the average recoveries of PCBs 54, 155, and 185 calculated from all standard samples (both GFF and Milli-Q water) were 77 ( 7%, 72 ( 8%, and 89 ( 10%, respectively (the errors correspond to 1 SD). The recoveries of the surrogate standards were also calculated in all the samples analyzed. For the case of GFF filters (particulate phase), the average recoveries of PCBs 54, 155, and 185 were 71 ( 21%, 78 ( 10%, and 84 ( 18%, respectively, while the corresponding recoveries in water samples (dissolved phase) were 60 ( 11%, 68 ( 14%, and 80 ( 21%, respectively. Blank values of PCBs were also examined. The filtrate and the particles (collected in clean GFF filters) from 2 L of Milli-Q water were analyzed to quantify blank levels. The levels of individual PCB congeners in the filtrate and GFF filters were very low and in most cases not detectable. The average total amount of 54 PCB congeners (∑PCB) in blank GFF filters was 500 pg, while no PCBs were detected in the filtrate of Milli-Q water. As a quality assurance procedure, we participated in an intercalibration study where six different laboratories analyzed the same sample. The concentrations that we measured for PCBs 28, 52, 70, 90 + 101, 105, 110, 118, 149, 153, 160 + 158, 180, 194, and 199 were very close to the average values obtained by all laboratories. The average relative standard deviation between our results and the interlaboratory average values was 17%. For PCBs 18, 123, 132, and 138, which coelute with other congeners or their concentration is very low, the deviation was higher and approached 40%. In the present study, t-statistics (two-tailed t-test) were used to compare the concentrations of PCBs between Finokalia and Heraklion and with those reported in the literature. In all of these cases, mean values were compared to identify significant differences between them, and the statistical significance (p-value) was calculated. To compare PCB homologue group profiles between two different sites, the relative contributions of PCB homologue groups observed at a specific site were regressed against the corresponding contributions of another site, and the linear correlation coefficient (r) was calculated. Subsequently, the t-test was used to evaluate the statistical significance of the correlation coefficient. To decide on the use of arithmetic versus geometric mean values for the washout ratios of particulate and gaseous ∑PCBs (WP and WG), normality tests were performed using the Statistica 5.1 software package. For WP (165 data points), the Kolmogorov-Smirnov test gave d ) 0.15065, p < 0.01; the Shapiro-Wilks W test gave W ) 0.87365, p < 0.00001. The corresponding values for WG (142 data points) were d ) 0.24816, p < 0.01 and W ) 0.70459, p < 0.0001. In recent years, the Shapiro-Wilks W test has become the preferred test of normality because of its good power properties as compared to a wide range of alternative tests (Statistica 5.1 Electronic Manual). On the basis of this test, both WP and WG values of different PCB congeners were clearly welldistributed (p < 0.0001), and for this reason arithmetic mean values are reported in the present study.
Results and Discussion PCBs in Rainwater. Total PCB concentrations measured during 14 rain events (dissolved plus particulate phase) are summarized in Table 1, along with data on average wind speed, wind direction, air temperature, total sample volume, and rainfall. In addition, the percentage of PCBs detected in the particulate phase is also presented in Table 1. The total concentration of PCBs (∑PCB) ranged between 1.0 and 3.6 ng L-1 at the Finokalia sampling station and from 0.7 to 5.3 ng L-1 in the city of Heraklion. Even if the average concentration of ∑PCB in Heraklion (2.9 ( 1.9 ng L-1) was higher than the corresponding value calculated for Finokalia (1.9 ( 0.9 ng L-1), the difference between these two sites was not statistically significant (p ) 0.26). Moreover, in most of the samples collected from both sites, the fraction of PCBs detected in rainwater particles (e.g., filter-retained) was lower than the corresponding fraction in the dissolved phase. Namely, the percentage of particle-bound PCBs ranged between 6.6% and 63.8%, and its average value was 31 ( 18%. The large variation in ∑PCB concentrations could not be explained by the variability of the meteorological parameters (e.g., air temperature, wind speed, and direction). Previous studies conducted near to remote, coastal sites have shown that the levels of PCBs in rainwater may be significantly affected by the wind direction and air mass origin (16, 23, 29). In general, elevated concentrations of PCBs are expected when the sampling site is influenced by air masses that stem from urban centers or highly industrialized areas. In addition, Agrell et al. (22) observed a negative relationship between concentration of PCBs in rainwater and ambient temperature, while a lack of such relationship or an opposite correlation has been found in other studies (10, 30, 31). The concentration of PCB at Finokalia and Heraklion did not exhibit any significant correlation with air temperature, but this could be due to the small range of temperatures (10-19 °C) observed during this study. Moreover, most of the samples collected in our study were associated with north-northwestern winds, and thus an effect of wind direction could not be evaluated. A large part of the variability in PCB concentrations in precipitation may result from the “dilution effect” in which particle-bound PCBs are scavenged during the early phases of precipitation, with the subsequent “clean” precipitation diluting the contaminant concentration (22, 23). Due to this effect, an inverse relationship between the concentration of PCBs and the volume of precipitation is frequently observed (22, 23). Although the concentration of ∑PCB decreased with increasing volume of precipitation, the correlation between these two parameters was not statistically significant for the case of Finokalia (p ) 0.5). However, this was probably due to the limited number of rain samples collected from this area because of the scarcity of rain events. On the contrary, a significant correlation was evident for the samples collected from Heraklion (p ) 0.004). Average concentrations of individual PCB congeners in rain samples (dissolved plus particulate phase) collected from Heraklion and Finokalia are shown in Figure 2. The average concentrations of PCBs 8 + 5, 18, 31, 28, 22, and 33 + 20 were the highest observed in both sites (>100 pg L-1). In addition, PCBs 101 + 90, 70, and 52 significantly contributed to the total amount of PCBs in rainwater. Higher relative concentrations of the above congeners have been recently reported for the atmosphere of Finokalia (27). Volume-weighted mean (VWM) concentration of ∑PCB in rain samples collected from Heraklion was 2.3 ( 0.9 ng L-1, and it was higher than the corresponding VWM value that was calculated for Finokalia samples (1.8 ( 0.4 ng L-1) (the standard error of the VWM values were calculated VOL. 38, NO. 11, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Average concentrations of individual PCB congeners in rain samples (dissolved plus particulate phase) collected from the city of Heraklion and the remote sampling station of Finokalia.
TABLE 2. Volume-Weighted Mean Concentrations of Total PCBs in Rainwater (ng L-1) at Various Remote and Urban Locations around the World
a
sampling site
characterization
∑PCB (ng L-1)
ref
Chicago, IL Menton, South Francea Camden, NJ Lake Michigan Jersey City, NJ Madison, WI Kiel, Germany New Brunswick, NJ Heraklion, Greece Cedar Creek, MN south Sweden (9 sites) Baltic Sea (16 sites)b Green Bay, WI(3 sites) Chesapeake Bay, MA Pinelands, NJ Tuckerton, NJ Finokalia, Greece
urban coastal, suburban urban background of Chicago urban urban urban suburban semi-urban rural regional background marine background coastal coastal, remote forest coastal (light residential) coastal, remote
29.3 25.1 13 5.8 3.9 3.5 1.5 1.3 2.3 2.6 2.4 2.3 2.2 1.2 0.38 0.35 1.8
29 16 20 29 20 32 21 20 this study 33 23 22 30 34 20 20 this study
Mean concentration was only reported in this study.
b
Annual median concentration was only reported in this study.
according to the method outlined by Offenberg and Baker; 29). A comparison between our results and those reported in other sites around the world is presented in Table 2. The VWM concentration of ∑PCB at Heraklion was slightly higher than the corresponding values observed in the city of Kiel, Germany (21), and in suburban New Brunswick, NJ (20), while it was substantially lower than the values reported for urban centers of the United States (20, 29, 32). For example, the VWM concentration observed at the city of Chicago, IL (29.3 ng L-1; 29), was more than 1 order of magnitude higher than the corresponding value measured at Heraklion (Table 2). The VWM concentration of ∑PCB at Finokalia fell in the range of values observed in rural and coastal sites of the United States (30, 33, 34), in background sites of southern Sweden (23), and in other coastal sites of the Baltic Sea (22), but it was statistically higher (p < 0.001) than those recently measured by Van Ry et al. (20) at Pinelands and Tuckerton, NJ, which are imbedded in a region of high PCB emissions. It is interesting to note that the VMN concentrations of ∑PCB at Heraklion and Finokalia, which are both located in the eastern Mediterranean, were more than 1 order of magnitude lower than the concentration of ∑PCB measured at a coastal site of the western Mediterranean (Menton, south France; 16) 16 yr ago. The sampling and analytical procedures utilized in both studies were quite similar, and they cannot explain the 10-fold difference in PCB concentrations. The relatively higher concentrations measured in southern France might 3014
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be explained by the significant high production and consumption of PCBs in western and southwestern European countries until the middle 1980s (15). Alternatively, the significantly lower PCB concentrations observed in our study might also reflect a temporal decline of PCBs in the broad region of the Mediterranean due to removal/destruction processes. The difference between our results and those presented by Villeneuve and Cattini (16) suggest a 92% decline of PCBs in a period of 16 yr. By assuming that the removal processes follow first-order kinetics, it was estimated that the half-life of PCBs in rainwater should approach 4.4 yr. This half-life is within the range of values reported for PCBs in the atmosphere of United Kingdom (between 1.7 and 11 yr; 35). Simcik et al. (36) reported congener-specific atmospheric half-lives ranging from 0.5 to 5.9 yr, while Jeremiason et al. (2) and Pearson et al. (37) have provided evidence that concentrations of PCBs in water from the Great Lakes have been reducing with a half-life between 2.5 and 9 yr. PCB Homologues Distribution and Partitioning. Table 3 presents the average percent contribution of different homologue groups to ∑PCB (particulate plus dissolved phase) for each one of the sampling sites. The same PCB homologue group profile (% ∑PCB) was observed at both Heraklion and Finokalia (p < 0.001), and the congeners containing three and four chlorine atoms were present in higher relative abundance for all analyzed samples. The profiles presented in our study were statistically the same (p < 0.001) with the
FIGURE 3. Particle washout ratios of individual PCB congeners calculated from field measurements at Finokalia. The dots correspond to average values, while the error columns and the error bars correspond to (1 standard error and (1 standard deviation, respectively.
TABLE 3. Average Percent Contribution of Different PCB Homologue Groups to ∑PCB Measured in Precipitation Samples (Particulate plus Dissolved Phase) from Finokalia and Herakliona PCBs homologues di-CBs tri-CBs tetra-CBs penta-CBs hexa-CBs hepta-CBs octa-CBs
% ∑PCB in rain Finokalia Heraklion 8(3 37 ( 5 24 ( 3 13 ( 3 12 ( 2 3(1 3(1
9(3 34 ( 8 22 ( 7 14 ( 4 13 ( 4 4(1 4(5
% on particles Finokalia Heraklion 27 ( 12 26 ( 15 30 ( 19 36 ( 22 34 ( 21 43 ( 28 39 ( 19
33 ( 26 31 ( 25 30 ( 22 33 ( 21 34 ( 22 42 ( 18 32 ( 11
a Average percent fraction of each homologue group in the particulate phase of rainwater is also presented. The errors correspond to 1 standard deviation.
profiles observed in rain samples collected from Madison, WI (32), while different profiles have been observed in a suburban site of Minnesota (26), in several coastal sites around Green Bay, WI (30), and throughout the state of New Jersey (20). In our case, the sum of tri- and tetrachlorinated congeners accounted for 61% and 56% of ∑PCB measured in rainwater (particles plus dissolved phase) of Finokalia and Heraklion, respectively (Table 3). On the contrary, the contribution of higher molecular weight congeners was constantly low. In fact, the sum of hepta- and octachlorinated biphenyls accounted for less than 10% of ∑PCB in both sites. Van Ry et al. (20) suggested that, at increasing distances away from emission sources, the relative importance of higher chlorinated PCBs in environmental samples should decrease because heavier congeners tend to be associated with aerosols and have correspondingly lower residence times. In our study, the PCBs profile from the urban site was not enriched in highly chlorinated congeners (compared to the remote site), and this is consistent with the fact that no significant sources of PCBs should exist close to Heraklion. The distribution of different PCB homologue groups between the “dissolved” and particulate phase of rainwater was also investigated. The fractions of homologue PCBs observed in the particulate phase are shown in Table 3. It has been thoroughly established that compounds of lower volatility (e.g., higher molecular mass) should have a higher tendency to be associated with suspended particles of the atmosphere (38, 39). Thus, a higher fraction of these
compounds should also be expected in the particulate phase of rainwater, compared with the lower molecular mass species. This aspect has been proved for the case of polycyclic aromatic hydrocarbons (34, 40), but it has never been confirmed for PCBs. In our study, the particle-bound fractions of heavier congeners were not statistically different from the corresponding values of lighter ones when all samples are taken into account. Indeed, four individual samples provided a correlation between particle-bound fraction and the degree of chlorination of PCBs (R 2 ranged between 0.75 and 0.89), but even in these cases the correlation was borderline statistically significant (p < 0.05). In general, the particle-bound fraction for all homologue groups of PCBs approached 30% (Table 3). Similar values have also been observed at a rural site adjacent to the Chesapeake Bay, MA (between 7% and 50%; 34), while significantly higher amounts of particle-bound PCBs have been measured in the urban area of Madison, WI (between 7% and 94%; 32). Washout Ratios of Particulate PCBs. Washout ratios describe the scavenging of semi-volatile compounds by precipitation (18, 20, 22, 40). The particle washout ratio (WP) is dimensionless and defined from the following equation:
WP ) CP,rain/CP,atm where CP,rain and CP,atm are the particle-bound concentration of the compound in precipitation and in the atmosphere, respectively. During the periods of precipitation sampling at Finokalia (except April 30-May 13, 2001), several 24-h air samples were simultaneously collected, and the concentrations of PCBs in the particulate phase of the atmosphere have been measured (27). In fact, the concentrations of particulate PCBs have been monitored from April 1999 up to March 2001, and these results have been reported in a previous study (27). Subsequently, the washout ratios of several PCB congeners were determined for each one of the five rain samples collected at Finokalia by using their corresponding atmospheric concentrations measured at the same time period. For some PCB congeners, the calculation of WP was not feasible because they were not detectable in the particulate phase of precipitation or aerosols. More details about the calculations of the washout ratios are given in the Supporting Information (Table S1). The average WP values of PCBs are presented in Figure 3. By taking into account the uncertainty of the measurements VOL. 38, NO. 11, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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in air (about 20% relative standard deviation) and precipitation samples (about 25% relative standard deviation), the propagated uncertainty in washout ratios is expected to approach 32%. Although PCBs 95 and 99 presented slightly higher WP values, the average particle washout ratios of all PCB congeners were similar and ranged between 1.9 × 105 (PCB 22) and 5.2 × 105 (PCB 95). The particle washout ratio for ∑PCB was also calculated, and it was found equal to 2.7((1.3) × 105. Mackay et al. (41) developed a fugacity model to predict the fate of organic chemicals in the environment, and they suggested that a total washout ratio of 2 × 105 should be representative for compounds that are primarily or totally in particulate form. On the basis of field measurements at a suburban site in Minnesota (26), the WP of ∑PCB approached 4 × 104, and this was about 7 times lower than the corresponding value measured in our study. In a recent publication, Offenberg and Baker (25) reported that the WP of individual PCB congeners (associated with large particles) varied about 8 orders of magnitude during their whole campaign (between 72 and 4.1 × 109), but no further information was given by the authors about the average WP values of individual congeners or ∑PCB. This range of values was extremely large, probably because the precipitation samples were collected from several locations around southern Lake Michigan (25). The WP of ∑PCB has also been indirectly calculated by using field measurements from several sites around New Jersey, and its values varied between 1.6 × 104 and 1.6 × 107 (20). Although these particle washout ratios ranged 3 orders of magnitude (because the sampling sites were located both close and away from evaporative sources of PCBs), they were consistent with those observed at Finokalia (Figure 3). We consider that the range of values observed in our study was more restricted because the results were obtained from only one sampling area. It is generally known that the scavenging of particle-bound compounds depends on the meteorological conditions as well as the chemical and physical properties of the particles (17, 18). In a previous study, the particle washout factors of volatile PAHs (as well as alkanes and phthalates) were about an order of magnitude higher than those of less volatile PAHs, and this discrepancy was attributed to the greater tendency of the more volatile PAHs to be associated with larger atmospheric particles (18). Our results indicated that the precipitation scavenging of particulate PCBs should be equally efficient for all congeners regardless of the degree of chlorination. Therefore, it can be suggested that all PCB congeners will have the tendency to be associated with atmospheric particles having the same physical and chemical properties (i.e., aerodynamic diameter, hydrophobicity, etc.). Washout Ratios of Gaseous PCBs. The gas scavenging ratio (WG) of a chemical is defined from the following equation:
WG ) CDis,rain/CG,atm where CDis,rain and CG,atm are the concentrations of the compound in the dissolved phase of precipitation and in the gas phase of the atmosphere, respectively. The data about concentrations of gaseous PCBs in the atmosphere of Finokalia have been presented in a previous study (27). Hence, the WG values of PCBs were calculated for each one of the rain samples collected at Finokalia (except April 30May 13, 2001),by using the atmospheric concentrations of PCBs measured at the same time period (Supporting Information, Table S2). The average gas scavenging ratios of congeners having two to five chorine atoms were similar and ranged between 7 × 103 and 47 × 103 (Table 4). On the contrary, hexa- to octachlorinated congeners provided substantially higher values ranging between 1.9 × 104 and 1.3 × 105, while the 3016
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TABLE 4. Average Gas Scavenging Ratios of PCBs Calculated from Field Measurements at Finokalia and Predicted Values from Henry’s Lawa PCB congener
WG(field) (×103)
WG(pred) (×102)
WG(field)/ WG(pred)
6 8 + 5* 12 + 13 18 24 + 27 16 + 32 31 28 33 + 20* 22 52 49 47 + 48 44 41 + 46 74 70 66 95 91 101 + 90* 99 153 132 138 + 163* + 164* 158 + 160* 174 180 170 + 190 196 + 203 194
35 ( 35 32 ( 36 20 ( 23 24 ( 27 16 ( 10 28 ( 26 32 ( 28 36 ( 37 39 ( 34 47 ( 29 15 ( 7 18 ( 7 23 ( 14 18 ( 10 21 ( 7 12 ( 4 15 ( 7 14 ( 4 9(4 8(5 11 ( 5 7(2 19 ( 10 25 ( 4 43 ( 13 86 ( 60 53 ( 33 134 ( 66 67 ( 38 77 ( 32 126 ( 61
1.8 ( 0.4 1.2 ( 0.2 3.0 ( 0.7 1.5 ( 0.2 1.3 ( 0.2 2.1 ( 0.4 1.6 ( 0.3 2.1 ( 0.3 1.1 ( 0.2 1.1 ( 0.2 0.9 ( 0.1 0.8 ( 0.1 0.7 ( 0.1 1.3 ( 0.1 0.7 ( 0.1 0.8 ( 0.1 1.3 ( 0.2 1.3 ( 0.2 0.5 ( 0.0 0.7 ( 0.0 0.9 ( 0.1 0.7 ( 0.0 1.1 ( 0.3 1.6 ( 0.4 1.6 ( 0.6 1.0 ( 0.3 15.0 ( 11.6 4.6 ( 3.1 11.3 ( 7.9 15.9 ( 12.6 36.8 ( 29.5
178 252 61 150 133 122 182 158 326 421 158 213 339 142 297 143 110 106 171 126 120 98 182 173 279 924 29 411 54 47 58
a An asterisk (*) indicates the PCB congeners that should slightly contribute to the peak of two or three coeluting isomers. The errors correspond to (1 SD.
highest values were observed for PCBs 180 and 194. The gas scavenging ratio of ∑PCB was found to be equal with 2.1((1.0) × 104. It is interesting to note that WG values between 1 × 103 and 1.8 × 104 have been previously observed for PAHs (42). Since the washout of organic vapors may be viewed as an equilibrium partitioning, a theoretical washout ratio can be calculated for a specific chemical by the following equation:
WG(pred) ) RT/H where WG(pred) is the predicted gas scavenging ratio, T is the absolute temperature, H is the Henry’s law constant, and R is the gas constant (7). Henry’s law constants of PCBs and their dependence on temperature have been accurately measured by Bamford et al. (43). These constants were corrected against the average temperature of each sampling period (43, 44), and the theoretical gas scavenging ratios of PCBs were subsequently calculated (Table 4). The average WG(pred) values of di-, tri-, and hexachlorinated congeners (range between 100 and 300) were slightly higher than those calculated for tetra- and pentachlorinated congeners (range between 50 and 130), but they were significantly lower than those calculated for hepta- and octachlorinated biphenyls (range between 460 and 3680). Although the same trend was also obvious for the field-determined gas scavenging ratios of PCBs, these values were always much higher than the scavenging ratios predicted from Henry’s law (Table 4). The average WG values of PCB congeners, obtained from field data, were 29-924 times higher than the corresponding WG(pred). This discrepancy could be explained by nonequi-
FIGURE 4. Seasonal variation in precipitation rate obtained from historical records (between 1995 and 2000). The calculated monthly wet deposition flux of ∑PCB is also shown. librium partitioning of PCBs between raindrops and the surrounding gas phase or sampling artifacts during the collection of precipitation and separation of the dissolved (operationally defined) from the particulate phase. Additionally, gas scavenging of PCBs by precipitation takes place in the total air column, and it is still not clear how well field measurements at the ground level can accurately describe this process. The enrichment of PCBs in the dissolved phase of rainwater has been observed in previous studies (32, 34). Murray and Andren (32) advocated that the dissolution of gaseous PCBs to rain is apparently larger than what is predicted by the congeners’ air-water partition coefficients due to the presence of nonfilter-retained particles (
