Environ. Sci. Technol. 2004, 38, 1941-1948
Atmospheric Deposition of Polycyclic Aromatic Hydrocarbons to Atlantic Canada: Geographic and Temporal Distributions and Trends 1980-2001 GUY L. BRUN,* OM C. VAIDYA,† AND M A R T I N G . L EÄ G E R Environmental Conservation Branch, Environment Canada, P.O. Box 23005, Moncton, NB, E1A 6S8, Canada
Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants. The semivolatile organic compounds may disperse into the atmosphere by direct input from several sources such as the burning of fossil fuels, from motor vehicle emissions, and forest fires. Once in the atmosphere, they may travel great distances before being deposited to the earth’s surface by the scavenging action of rain and snow. Up to 14 PAHs were determined in wet precipitation samples collected monthly from five sites in the four Canadian Atlantic Provinces during 19802001. The relatively more volatile PAHs (phenanthrene, fluoranthene, naphthalene, and pyrene) were predominant in the samples. Significant (P < 0.05) spatial variations in the deposition of some PAHs were observed among sites, but there were no consistent geographic patterns. Seasonal patterns were discernible with peak deposition for Σ6&14 PAHs occurring during the colder months of the year (December to March) and coinciding with higher energy consumption for heating and transport. The monthly volume weighed mean concentration for Σ6 PAHs has declined steadily since the mid-1980s at Kejimkujik National Park in southwest Nova Scotia, with a calculated halflife of 6.4 ( 0.3 years. The maximum annual deposition flux of 20 µg m-2 yr-1 reached in 1985 for Σ6 PAHs decreased approximately 1 order of magnitude by the year 2000. The decrease in Σ6&14 PAHs for the region was found to be correlated (P < 0.05) with decreasing sulfate ion concentrations in the precipitation. The implementation of air pollution abatement programs in Canada, the United States, and elsewhere, switching to cleaner sources of energy and improved technology during the past few decades is most likely responsible for the observed decline.
Introduction Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants present in air, water, soil, and vegetation (1-3). They are found in remote and pristine areas such as the Arctic (4). Belonging to a group of compounds commonly known as persistent organic pollutants (POPs), they have attracted much attention in recent years because * Corresponding author phone: (506)851-2366; fax: (506)851-6608; e-mail:
[email protected]. † Current address: 15th Floor, Queen Square, 45 Alderney Dr., Dartmouth, NS, Canada, B2Y 2N6. 10.1021/es034645l CCC: $27.50 Published on Web 02/19/2004
Published 2004 by the Am. Chem. Soc.
of their inherent toxicity and ability to disperse in the environment by direct emissions to the air and consequently long-range atmospheric transport. Many PAHs are potentially carcinogenic and mutagenic (1). There is growing evidence that priority PAHs and their derivatives have dioxin-like potency, and some may be estrogenic (5). PAHs are mostly produced by the incomplete combustion of organic matter, while smaller amounts of alkyl substituted PAHs are produced naturally or by diagenesis. Anthropogenic sources include the burning of fossil fuels for residential heating, industrial processes, power generation, motor vehicles, and the burning of refuse. Forest, prairie, and agricultural fires and volcanoes constitute natural sources. The physical and chemical properties of PAHs make them suitable candidates for dispersion by long-range atmospheric transport. In the atmosphere, PAHs may be present in the gas phase, adsorbed on aerosols, or partitioned between the two phases depending on temperature, vapor pressure, solubility of the compound, and size and surface area of suspended particulates (2). Deposition to the Earth’s surface occurs in the form of dry precipitation or wet precipitation by the scavenging of vapor phase contaminants and aerosols by rain or snow (6, 7). Air-water and air-soil exchange processes, revolatilization, and atmospheric transformation are important mechanisms that control the fate of these compounds in the environment (1, 8). In recent years, several studies have been conducted in various countries on the atmospheric deposition of PAHs. Several studies focused on short-term deposition at specific urban (9) and rural (10) areas. In Canada and the United States, the Integrated Atmospheric Deposition Network (IADN), which is mandated to measure the deposition of toxic substances to the Great Lakes (11), reported on organic contaminant concentrations in precipitation collected between 1991 and 1997. A study closer to Atlantic Canada focused on 16 PAHs in wet and dry deposition during 19982000 at rural sites in Maine and Massachusetts near the New England coast in the U.S.A. (12). Atlantic Canada, with a small industrial base and population density, is generally considered an area relatively free of pollution. However, ecosystems in Atlantic Canada situated downwind of major Canadian and U.S. industrial/urban centers remain susceptible to the deposition of POPs from long-range atmospheric transport. Atmospheric emissions were indicated as a source of trace metals and selected trace organic contaminants, including PAHs, observed in blue mussels around the Gulf of Maine coastline and the Canadian Bay of Fundy during 1991-1998 (13, 14). A study looking at organochlorine compounds and PAHs in wet precipitation in Atlantic Canada was initiated in 1980, and results for the period 1980-1989 were reported in our earlier publication (15). Rain and snow samples were collected on a monthly basis and localized, and long-range atmospheric source inputs were critically examined. The study, which is still ongoing, is the subject of the current paper. The data set for the main site at Kejimkujik National Park, Nova Scotia, is considered unique, spanning over 20 years of continuous data collection. To the best of our knowledge, this is the first study ever reporting PAH deposition in precipitation over this long a period of time. Since 1989, two new sampling sites were implemented to provide greater spatial coverage, new and more efficient collection apparatus were deployed, and the number of individual target analytes was increased. In the present paper, PAH data are interpreted to assess geographic distribution and seasonal and long-term trends VOL. 38, NO. 7, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Observations and Median Monthly PAH Concentrations (ng L-1) at Five Sites in the Atlantic Region Kejimkujik (Oct 80-Aug 01)
Npc Acec Fc Anc Phc Fld Pyc B[a]An c B[b]Fld B[k]Fld B[a]Pyd B[ghi]P ed I[1,2,3-cd]Pyd dB[ah] Anc Σ6 PAHs
Kouchibouguac (Nov 97-Aug 01)
#PO a
% PO
med b
#PO
% PO
med
85 4 29 21 118 184 82 27 83 71 50 19 27 12
62.0 3.0 23.4 14.1 77.6 75.4 53.6 18.2 34.3 29.5 20.5 7.9 11.2 7.8
3.6 1.2 3.0 0.6 2.2 4.0 0.8 0.3 1.0 0.8 1.0 0.7 1.9 1.9 9.4
27 0 1 5 30 29 27 18 7 5 5 19 7 4
90.0 0.0 25.0 33.3 93.8 90.6 87.1 60.0 53.8 50.0 16.1 59.4 23.3 12.9
1.9
a
0.3 0.6 1.7 1.1 0.8 0.3 0.8 0.7 0.4 0.4 0.3 0.4 3.7
Ellerslie (Oct 80-Oct 86) #PO
% PO
med
48
77.4
9.0
29 27 25 5 10
45.3 42.9 39.1 7.9 15.6
3.0 2.0 3.0 11 9.0 37.0
Jackson (Nov 84-Apr 96)
Gros Morne (Nov 93-Mar 00)
#PO
% PO
med
#PO
% PO
med
51 7 21 8 87 120 43 9 55 38 30 7 11 4
47.7 6.2 19.6 8.5 76.3 75.9 38.1 8.0 35.3 24.4 19.1 4.5 6.9 3.5
20 4.0 6.0 19 8.4 6.0 3.0 0.6 1.5 1.0 1.0 4.0 5.0 0.6 18.5
45 1 3 5 49 49 47 16 22 20 16 12 14 6
91.8 2.3 8.6 8.9 80.3 80.3 77.0 27.1 37.3 32.8 26.2 19.7 23.0 9.8
2.0 1.4 1.7 1.3 1.4 1.6 1.1 0.6 2.1 0.9 0.6 1.7 1.5 1.7 8.4
b
Number of positive observations. Median monthly concentration derived from positive observations (i.e., monthly VWM concentration). Eight PAHs added to list in March 1988. d Original six PAHs measured starting in 1980. Np (naphthalene), Ace (acenaphthene), F (fluorene), An (anthracene), Ph (phenanthrene), Fl (fluoranthene), Py (pyrene), B[a]An (benzo[a]anthracene), B[b]Fl (benzo[b]fluoranthene), B[k]Fl (benzo[k]fluoranthene), B[a]Py (benzo[a]pyrene), B[ghi]Pe (benzo[ghi]perylene), I[1,2,3-cd]Py (indeno[1,2,3-cd]pyrene, dB[ah]An (dibenz[ah]anthracene). c
collector was initially installed at Berry Hill from 1993 to 1996, just a few kilometers from the Park Administration area to which the collector was relocated in 1996 for logistical reasons. Because of the short distance between sites, the data were combined to facilitate analysis. Kouch National Park is located on the east coast of New Brunswick in a secluded area. The nearest significant industrial activities are approximately 50 km to the northwest. The sampler is located in a meteorological station compound approximately 100 m from the main Park Administration area.
FIGURE 1. Wet precipitation collection sites in Atlantic Canada, 1980-2001. of PAH deposition to Atlantic Canada. Results indicate significant short- and long-term trends.
Experimental Procedures Sampling. Wet precipitation samples were collected at five sites in Atlantic Canada (Figure 1). Sampling at Kejimkujik (Keji) National Park, Nova Scotia (44°25′58′′ N, 65°11′59′′ W) and Ellerslie, Prince Edward Island (46°36′49′′ N, 63°54′34′′ W) began in October 1980 and is still ongoing at Keji. The latter site was decommissioned in October 1986. A third site established at Jackson, Nova Scotia (45°36′00′′ N, 63°49′59′′ W) in November 1984 was operational until April 1996. Another site was established in Gros Morne (Gros M) National Park, Newfoundland (49°34′56′′ N, 57°54′33′′ W) in November 1993 and was active until March 2000. A fifth site was commissioned at Kouchibouguac (Kouch) National Park in New Brunswick (46°46′16′′ N, 65°00′07′′ W) in November 1997 and is operational to date. Site descriptions for the first three sites and selection criteria were described earlier (15). The collection sites at Gros M and Kouch were established in National Parks operated by Parks Canada. Park employees were trained to tend and maintain the collectors. Gros M National Park is situated on the west coast of Newfoundland with prevailing winds off the Gulf of St. Lawrence. The 1942
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The sites at Keji, Ellerslie, and Jackson were initially fitted with modified Sangamo Type-A automated precipitation collectors. The sampling apparatus consisting of a collection area of 0.044 m2 was described in an earlier report (15). In June 1993, the collectors at Keji and Jackson were replaced with Type B1 automated rain collectors for trace organic substances (M. I. C. Company, Richmond Hill, Ontario, Canada). This sampler has a relatively larger collection area of 0.212 m2 and includes a thermostatically controlled, heated, and insulated aluminum cabinet for winter operation. Two 4.5-L amber glass bottles connected with Teflon tubing serve to collect the precipitation. The sites at Gros M and Kouch were equipped with this collector from the very beginning. The principle of operation and sampling procedures were similar to those described earlier (15). The precipitation collected from all events during the month was aggregated to form a discrete monthly sample. At the end of the month, samples were forwarded to the laboratory for analysis. Analytical Methods. The study was initially focused on the measurement of the six compounds identified in Table 1, the so-called Borneff list, and later in March 1988 was expanded to 14 compounds. The limits of detection (LOD) for the high-performance liquid chromatography (HPLC)s fluorescence detection and gas chromatographysmass spectrometry (GC-MS) methods are given in Table 1 of the Supporting Information (SI) and range between 0.4 and 20 and 0.9 and 7.0 ng L-1, respectively. The LOD are based on a fixed sample volume of 4 L and will change according to sample volume and instrument sensitivity. A few changes in procedures were made over the course of this study. In March 2001, the solvent hexane used initially in the extraction process was replaced with dichloromethane (50 mL L-1 of sample), and the analysis technique carried out initially by HPLC was replaced in January 1994 by GC-MS to improve
TABLE 2. Comparison of PAH Wet Deposition Fluxes (µg m-2 yr-1) with Other Studies
cmpd Np Ace F An Ph Fl Py B[a]An B[b]Fl B[k]Fl B[a]Py B[ghi]Pe I[1,2,3-cd ]Py dB[ah]An a
Leister et al. (20) Chesapeake Bay 1990-1991
2 1 9 15 20 2 8 5 2 2 2 1
Dickhut et al. (21) S. Chesapeake Bay 1991
Golomb et al. (22) Mass. Bay Truro/ Nahant 1992-1993
2 0.4 8 5 4 1 2 1 1 2 2 0.4
65/24 1.0/1.6 0.55/2.6 2.2/1.6 7.1/27 9.1/27 5.8/19.4 1.2/5.2 2.8/6.8 2.9/8.3 2.1/4.8 1.6/4.4 2.1/4.8 0.20/0.46
Buehler et al. (23) Great Lakes S/ H/M/O/Ea 1997-1998 (mean)
2.3/12.4/4.3/ 19/7.4 1.6/8.8/3.2/17/ 5.2 2.3b/8.6/5.3/12/ 7.8 0.7/4.1/1.8/4.2/ 2.9
Kiss et al. (10) Lake Balaton, Hungary 1995
19 4.2 92 96 94 11 22 8.1 13 19
1.3/6.6/2.9/6.4/ 3.5 3.5
this paper Keji./Jack 90-93/ Keji/ Gros M 97-98 31/43/2.0/0.9 1.4/1.7/-/11/7.7/1.0/0.4 20/23/0.2/0.6 13/14/1.4/0.9 4.8/6.2/1.4/ 1.0 4.7/5.2/0.7/ 0.5 0.7/0.4/0.2/ 0.4 1.4/1.1/0.6/ 1.7 0.5/0.6/0.2/ 1.1 0.9/0.7/0.3/ 0.4 0.9/4.9/0.3/ 1.2 4.2/2.8/0.4/ 1.6 -/0.5/0.1/0.2
S, Lake Superior; H, Lake Huron; M, Lake Michigan; O, Lake Ontario; and E, Lake Erie. Values for Great Lakes are sum of B[b + k]Fl.
analytical performance and efficiency (16). The effect of these changes on the quality of results is explained next under the discussion for quality assurance/quality control (QA/QC). The GC-MS instruments consisted of Hewlett-Packard (HP) 5890 and 6890 gas chromatographs interfaced to HP 5971 and 5973 mass selective detectors (EI mode) in positive ion and single ion monitoring (SIM) mode. The GC-MS instruments were also equipped with an automated sample introduction system and splitless injector (2.0 µL injection, injector temperature set at 280 °C). The capillary column was an HP-5 MS or equivalent, 30 m × 0.25 mm i.d. × 0.25 µm film thickness, with helium as a carrier gas and flow rate of 0.8-1.0 mL/min. The column oven temperature program settings were initial temperature 115 °C, ramp (#1) to 150 °C at 25 °C min-1, ramp (#2) to 250 °C at 15 °C min-1, and ramp (#3) to 280 °C at 10 °C min-1. Details concerning the extraction and HPLC procedures are presented elsewhere (15). Quality Assurance/Quality Control. QA/QC procedures were established from the beginning of the study. Laboratory internal QC procedures consisted initially in the analyses of method blanks and spikes to evaluate target compound recovery. Details of QA/QC procedures have been described elsewhere (15). In the latter part of the study (1991-2001), extraction efficiency was also measured by the addition of surrogate compounds to the samples prior to extraction, and an internal standard was added to sample extracts to enable compensations for any instrument variation during analysis. The method blanks/spikes were also treated with the surrogates and internal standards. The protocol included aspects of QA/QC for field operations to ensure target analyte integrity over the sampling/transport period. This included the implementation of a system of field blanks/spiked samples prepared in the laboratory and shipped to one of the sampling sites (Keji) on a monthly basis. The field QA/ QC samples were handled and treated like the precipitation samples and were returned with the samples to the laboratory at the end of the month for analyses. Additionally, method performance was continuously assessed by the recovery of spiked samples and participation in a rigorous certification and accreditation program under the Canadian Association of Environmental Analytical Laboratories/Standards Council of Canada (ISO 25).
Results and Discussion Quality Assurance/Quality Control. Results for the analysis of spiked samples for the period 1991-2001 are summarized in Table 1 of the Supporting Information. Mean group recovery was 99.5% with individual compound mean re-
b
coveries ranging between 83.8 and 109%. The standard deviation (SD) ranged between 9.80 and 37.7% with a mean of 24.8%. The recovery for F was the lowest of the group at 83.8%; however, reproducibility was the best of the group with a standard deviation of 9.80%. At the other end of the range, the reproducibility for B[a]An was 37.7% with n: 146. These results are typical for POPs measured at trace concentrations and near the LOD. QC results on the initial part of the study were reported elsewhere (15) and were also typical of those expected at low concentrations. Additional statistical information, such as the number of less than LOD observations, and concentration range for each compound are available in Table 2 of the Supporting Information. In January 1994, the analytical method was changed from HPLC to GC-MS. A t-test (or Mann-Witney rank sum test) was performed on field QA/QC spiked sample results to see if there were any differences between methods. The results of this analysis are presented in Table 3 of the Supporting Information. The mean recoveries for the HPLC and GCMS methods were comparable at 115 and 113%, respectively. Individual PAH recoveries ranged between 90.0 and 134%, and 94.9 and 131% for the HPLC and GC-MS methods, respectively. Differences, however, were found for four of the 14 PAHs: Ace, An, Ph, and B[a]Py, albeit very small. Small variations such as these do not compromise data quality or the ability to make comparisons between data sets. Another modification involved the replacement of hexane as the extraction solvent with dichloromethane in 2001. A statistical comparison of QA/QC field spike results before and after the switch indicates that there is no variability associated with the use of the two solvents. In June 1993, the Sangamo precipitation collectors were replaced with the MIC collectors. At Keji, the old collector was operated for an additional four months to compare results with the new collector. A t-test performed for all PAHs failed to indicate any differences between the two samplers. The other modification to the study was the relocation of the Gros M precipitation collector to the Park Administration area just a few kilometers from the original site at Berry Hill. Examination of the time series for Σ14 PAHs before and after the move in July 1996 does not indicate any noticeable change in deposition as a result of moving the collector. The upgrade of apparatus and modification of methods for a project of such long duration is very much inevitable, although it is always desirable to minimize changes as much as possible. Nonetheless, there is no evidence to suggest that any of the modifications made during the course of this project have impacted the time series. VOL. 38, NO. 7, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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PAH Composition of Wet Precipitation. A summary of PAH analytical results in wet precipitation samples for the five sites is presented in Table 1 (also refer to Table 2 of the Supporting Information). Approximately 6000 individual PAH measurements were made over the course of the study. The concentration for Σ6 PAHs (defined in Table 1) varied between 3.7 ng L-1 at Kouch and 37.0 ng L-1 at Ellerslie. In retrospect, 245 precipitation samples were collected at Keji, 159 at Jackson, 64 at Ellerslie, 61 at Gros M, and 32 at Kouch for their respective sampling periods. It should be noted that the sum of positive and negative observations in Table 1 and Table 2 of the Supporting Information rarely add up to the total number of samples taken at each site. There are a number of reasons for this: for example, in the initial phase of the project, only six PAHs were determined as compared to 14 after 1988; results for some parameters were not reported by the laboratory for various reasons (i.e., poorly resolved peaks); or in some instances (rare), outliers were flagged and rejected (only in exceptional cases when justified, such as sample contamination). With the exception of Ellerslie, reporting only six of the 14 compounds, results indicate that the compounds with the highest frequency of observation were Ph, Fl, Np, and Py. The data also indicate that the profiles of the frequency of observation for these four PAHs are similar between Keji, Kouch, and Jackson. Comparatively, the profile at Gros M is slightly different with Np being the most frequently observed PAH. During the period of November 1993 to April 1996, three stations were running concurrently, presenting an opportunity to compare atmospheric deposition in the same time scale to see if there were variations in the composition of PAHs. Thus, PAH deposition for 14 compounds at Keji, Jackson, and Gros M are reported for the period in question as median monthly deposition (µg m-2 month-1) in Table 4 (Supporting Information). Median monthly deposition for each compound was calculated from all monthly deposition results generated for that compound at one site over the entire period. Monthly deposition (i.e., in µg m-2 month-1) for each compound was calculated from the initial PAH concentration (i.e., by determining the absolute amount of compound from the total volume in the collector for the month) for the month and dividing by the collection area of the sampler. The total deposition and ranking for the three sites indicate that Np was the most predominant PAH deposited in that period, followed by Fl, Ph, and Py. This is somewhat different to Ph as the most predominant PAH in terms of frequency of observation reported previously. The difference is most likely attributable to an inherent bias in using the whole database covering the 20 year period for describing the frequency of observation as opposed to median deposition at three sites in a narrower time scale. Profiles may also vary slightly with time at certain locations. In contrast, the compounds least present, both in terms of deposition and in terms of frequency of observation, are dB[ah]An, B[a]Py, B[ghi]Pe, and I[1,2,3-cd]Py. Our profiles are dissimilar to profiles reported in wet precipitation in 1996-1999 at Wielkopolski National Park and the City of Poznan in Poland, where the prominent PAHs identified were An, Np, B[a]An, Ace and An, Np, B[a]An, and Fl, respectively (17). The dominant PAHs (Np, Ph, Py, and Fl) observed in rain and snow samples collected at Lake Balaton in Hungary during 1995 were similar in profile as compared to our results (18). Significant differences in profile were evident, however, in wet precipitation samples collected from the City of Gdansk, Poland, which is in a typical urban/industrial setting (19). The prominent PAHs were B[b]Fl, Cry, Fl, and An, representing more of a midrange suite of PAHs associated both with the gaseous and with the particulate phases as compared with our results. These comparisons indicate that variations in the composition of PAHs in wet precipitation 1944
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can vary due to geographical area, meteorological conditions, and type of sources in an area. The dominance of low molecular weight PAHs (i.e.,
