Environ. Sci. Technol. 2001, 35, 4046-4053
Atmospheric Distribution of Polychlorinated Dibenzo-p-dioxins, Dibenzofurans (PCDD/Fs), and Non-Ortho Biphenyls (PCBs) along a North-South Atlantic Transect R . L O H M A N N , †,§ W . A . O C K E N D E N , † J . S H E A R S , ‡ A N D K . C . J O N E S * ,† Department of Environmental Science, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster LA1 4YQ, U.K., and British Antarctic Survey, High Cross, Cambridge CB3 0ET, U.K.
Air samples were taken on board the RRS Bransfield (typically for 24-72 h), during an Atlantic cruise from the U.K. to Antarctica in October-December 1998, to investigate the global scale distribution of PCDD/Fs and coplanar PCBs. Highest concentrations of Cl2-8DD/Fs all occurred between 25 and 52 °N; lowest concentrations were measured around ∼ 60 °S and further south. Cl3DFs showed highest overall concentrations (up to 9800 fg/m3), followed by Cl2DFs (up to 5300 fg/m3) and OCDD (up to 1300 fg/m3). Lowest concentrations, measured in the remote Southern hemisphere, were generally 2 orders of magnitude lower than their highest concentrations over the North Atlantic. Concentrations of PCB-77 were higher in the northern hemisphere, while PCB-126 and PCB-169 exhibited highest concentrations around the equator. Evidence was obtained for substantial emissions of PCDD/Fs off west Africa and while in the port of Montevideo, Uruguay. Shifts in PCDD/F profile distribution were observed on increasing distance from source regions, such that those from the most remote locations were dominated by Cl3DF (∼40% of the total) and OCDD (∼20%). Gas-particle partition data was obtained for all samples. Cl4-6DD/Fs showed the widest range, varying between 10 and 90% of the total in the gas phase, depending on location/ temperature. The study gave limited evidence for the influence of OH-radical initiated depletion reactions of gaseous PCDD/Fs. The global atmospheric burden is estimated to be on the order of 350 kg ΣCl4-8DD/Fs and ∼3 kg ΣTEQ.
Introduction Polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) are ubiquitous contaminants that are released into the environment as byproducts of incomplete combustion or as chemical impurities. Diverse atmospheric emissions supply PCDD/Fs to the global atmosphere. Long-range atmospheric * Corresponding author phone: +44 1524 593-972; fax: +44 1524 593-985; e-mail:
[email protected]. † Lancaster University. ‡ British Antarctic Survey. § Present address: Ralph M. Parsons Laboratory, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology 48-114, Cambridge, MA 02145. 4046
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transport (LRAT) can move PCDD/Fs away from source regions to more remote locations. PCDD/Fs are among the Persistent Organic Pollutants (POPs) targeted for international source reduction by United Nations protocols because of concerns over their persistence and LRAT potential (1, 2). There are continuing uncertainties over the relative importance of different sources of PCDD/Fs; this includes primary and secondary source categories (3-7) as well as possible natural production and atmospheric reactions and formation mechanisms (7-9). There are also uncertainties about the loss mechanisms which control PCDD/F depletion from the environment (10). Attempts have been made to make preliminary global budget calculations for PCDD/Fs, based on concentrations in soils and tree-bark (11, 12) and by correlating PCDD/F emissions with the gross national product or CO2 emissions (11). However, these studies rely heavily on information from the relatively well-known and “data rich” industrialized regions of the northern hemisphere (NH). Very few data are available from remote parts of the world, the southern hemisphere (SH) and over the oceans. It is likely that the SH has generally fewer emissions of PCDD/Fs than the more industrialized NH (13). LRAT is considered as a possible vector to move emissions from the NH to the SH. Ambient measurements can be used to help assess atmospheric emissions and obtain clues about the relative importance of different sources and processes controlling ambient levels. They provide a snapshot of the balance between current emissions/loss processes, whereas soils and tree bark provide a picture of longer-term, integrated atmospheric deposition. Caution is needed, however, because there is potential for confusion in interpreting data on regional/global PCDD/F distribution due to their persistence and possible atmospheric weathering. The wide range of PCDD/F physicochemical properties makes them an interesting group to consider with respect to their gas/particle partitioning and susceptibility to undergo LRAT. Between the mono- and octa-CDD/Fs, their subcooled liquid vapor pressures (pL) range from 10-2 to 10-7 Pa and their octanol-air partition coefficients (Koa) from 108 to 1012 (10). The Cl1-3DD/Fs are almost exclusively gas phase under ambient conditions in the U.K., while the Cl6-8DD/Fs are predominantly particle bound (14). This has implications for their fate and behavior in the atmosphere, because the scope and importance of scavenging processes and reactions with OH-radicals depends on their phase distribution. Polychlorinated biphenyls (PCBs) are another group of semivolatile compounds and are also targeted by the POPs protocols. Concentrations of non-ortho substituted PCBs77, -126, and -169 are presented here. To obtain data on the atmospheric distribution of PCDD/ Fs in remote, oceanic locations, including the SH, air samples were collected on a British Antarctic Survey (BAS) research vessel as it travelled from the U.K. to Antarctica in OctoberDecember 1998 (see Figure 1). A total of 53 samples were taken, between 52 °N, 1 °E and 75 °S, 20 °W, as the ship sailed from Grimsby, U.K., passed parts of Europe, Africa, and South America, and visited islands in the southern Atlantic, before reaching the BAS Halley research station on the Brunt ice shelf in Antarctica.
Materials and Methods Air Sampling. The air sampler was installed on the monkey deck immediately above the bridge deck (ca. 20 m above sea level). For the majority of samples, air was collected while 10.1021/es010113y CCC: $20.00
2001 American Chemical Society Published on Web 09/18/2001
2,3,7,8-substituted PCDD/F congeners, 13C12-2,8-DiCDF, 13C -2,7-DiCDD, 13C -2,3,7-TriCDD, and 13C -PCBs-77, -126, 12 12 12 -169 (all at 100 pg each) and a range of 13C12-labeled PCBs (28, 52, 101, 153, 138, 180 each at 325 pg) were spiked onto the samples immediately prior to extraction. PUFs were extracted in DCM for 16 h; GFFs were extracted consecutively for 12 h in DCM, followed by 12 h in toluene. The DCM and toluene extracts were combined for the cleanup procedure. The extracts were put through a mixed silica column and were fractionated for PCDD/F and non-ortho PCB analysis on a basic alumina column (see ref 4 for details). All PCB and PCDD/F standards were obtained from Promochem (U.K.); solvents and reagents were pesticide grade /high purity. PCDD/Fs and non-ortho PCBs were analyzed by HRGCHRMS using an HP 6890 GC connected to a Micromass Autospec Ultima high-resolution instrument at a resolving power of ∼10 000 (for more details see refs 4 and 15).
FIGURE 1. Mean sampling location of each sample (ITCZ ) intertropical convergence zone). the ship was in the open ocean. Air samples were taken for 24 h periods (∼300 m3) from the U.K. to the Falkland Islands (51 °S, 58 °W); thereafter samples were taken for 48 or 72 h intervals (typically 560-950 m3). Additional samples were taken for the entire periods the ship was moored in port, namely at Montevideo (Uruguay, 34 °S, 55 °W), Port Stanley (Falkland Islands, 51 °S, 58 °E), Bird Island (54 °S, 37 °W), and South Georgia (54 °S, 37 °W). A week long sample was taken around Signy Island (60 °S, 47 °W). More details about the sample numbers, the average position for each sample, air volumes taken, and meteorological conditions are given in the Supporting Information. On three occasions, a glass fiber filter was installed on the stack outlet and exposed for ∼1 h, to assess whether the ship’s engine emissions could potentially contaminate the samples. Air samples were taken with a GPS-1 PUF air sampler (Andersen Instruments, Smyrna, GA), with a Whatman glass fiber filter (GFF, 10 cm diameter, grade GF/A, Maidstone, U.K.) collecting particle-bound compounds and two polyurethane foam plugs (PUF, 6.25 cm diameter, 5 cm length, density 0.035 g/cm3, Ziemer, Germany) retaining compounds in the gaseous phase. GFFs were fired at 350 °C overnight; the PUF plugs were twice pre-extracted in dichloromethane (DCM) for 12 h and vacuum-dried. Sampling modules were assembled and taken apart immediately prior to and after deployment. Samples, once taken, were handled with nitrile gloves and rinsed pairs of tongs only. PUFs and GFFs were stored in solvent-rinsed metal tins and amber glass vials, respectively. They were firmly sealed and stored at -25 °C until extraction. Of the 53 samples taken, 12 were blanks. Great care was taken to minimize contamination of the samples (see below). Analytical Procedures. The analytical method employed has been described elsewhere (15). However, it was improved to include the reliable detection of Cl1DFs for this study. PUFs and GFFs were extracted separately. Briefly, all 13C12-
Quality Control. For every five samples, a field blank was taken by assembling two PUFs and a GFF in a module next to the air sampler. For every 10 samples a laboratory blank was taken, representing the unopened PUFs and GFF. Field and laboratory blanks were incorporated in the analytical procedure and used to derive detection limits as mean plus 3 times the standard deviation of the mean concentration in the blanks. There was no significant difference between field and laboratory blank levels for the GFFs; for the PUFs, concentrations of Cl1-3DFs were consistently lower in the field blanks. The mean concentrations found in the laboratory and field blanks were subtracted from the samples; in the case of Cl1-3DFs the field blanks were used. For most homologues and PCBs-126 and -169, mean blank concentrations were 0.1-1.0 pg/sample, but higher for Cl2DFs, OCDD, and PCB-77 (see Table 1). In the gaseous phase, Cl2-5DD/Fs were in general at least 10× blank levels, with few (southern) samples 5× blank levels. In the particulate phase, Cl6-8DD/Fs were generally 5-10× blank levels, with fewer detects for OCDF. Detection limits were < 2 pg/sample for most homologues but higher for Cl1DFs (4.0 pg/sample), Cl2DFs (35), Cl2DDs (2.6), Cl3DFs (2.5), Cl3DDs (2.2), and OCDD (7.6) in the PUFs. Detection limits for the 2,3,7,8substituted Cl4-6DD/Fs (nondetected in the blanks) were derived as 3 times the standard deviation of the noise in the blanks. They were ∼0.5 pg/sample for the 2,3,7,8-Cl4-6DFs and 2,3,7,8-Cl5DD and 25 °C but increased to over 50% south of 50 °S. PCDD/Fs in the Particulate Phase. Concentrations of PCDD/Fs in the particulate phase were generally low, with most homologue groups rarely exceeding 50 fg/m3 (see Figure 4a for Cl7DF and OCDD concentrations). Apart from one or two samples, concentrations of Cl7/8DD/Fs on particles varied little along the north-south transect. These fairly constant concentrations of the particle-borne Cl7/8DD/Fs suggests a consistent background concentration of fine particles (
