Environ. Sci. Technol. 1997, 31, 2120-2124
Observations on PAH, PCB, and PCDD/F Trends in U.K. Urban Air, 1991-1995 PETER J. COLEMAN,† ROBERT G. M. LEE,‡ RUTH E. ALCOCK,‡ AND K E V I N C . J O N E S * ,‡ National Environment Technology Centre, AEA Technology plc, Culham, Abingdon, Oxfordshire, OX14 3DB U.K., and Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, LA1 4YQ U.K.
Ambient air concentrations at London and Manchester are reported for PCBs, PAHs, and PCDD/Fs from 1991 to 1995. This data set suggests that urban PAH concentrations are trending downward in the U.K. ∑PCB concentrations (∑ ) congeners 28, 52, 101, 118, 138, 153, and 180) were usually ∼0.2-2 ng m-3 over the study period, with concentrations of the seven PCB congeners (∑PCB) in London generally 2-3 times higher than in Manchester. PCB concentrations have remained quite stable year-on-year over the study period. PCDD/F trends at the two sites were similar, with a decline through 1991-1994, followed by an upturn in 1995. Summer:winter seasonality of PCDD/Fs at both sites has been highly erratic year-on-year and compound-by-compound, far more so than for PAHs and PCBs. This is perhaps best interpreted as a sign that various sources (presumably combustion-derived) have contributed different mixtures of PCDD/Fs of different source strengths at different times to the air at these urban sites. Possible causes are discussed.
Introduction There is little information on the recent trends of semivolatile organic contaminants (SVOCs) measured in air. Trends usually have to be inferred by comparing old data sets with new data sets orsfor the longer termsfrom the analysis of dateable cores (sediments/peat) or the retrospective analysis of archived samples (e.g., refs 1-3). Hites and co-workers have presented some data for polychlorinated biphenyls (PCBs) in air from Indiana, the Great Lakes, and Bermuda (4-6). Halsall et al. (7) compared contemporary data for polycyclic aromatic hydrocarbons (PAHs) in London air with that obtained in the 1950, 1960s, and 1970s by other workers using different methods, while Heister et al. (8) compared the concentrations of polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) sampled in air from four German cities during 1987/1988 and 1993/1994. However, there is a clear need for reliable trend data from ongoing monitoring programs to provide context to present levels and to assess the effectiveness of source reduction strategies. Good quality compound-specific data may also ultimately be useful in helping to determine whether there are underlying changes in the relative importance of different types of sources. In the United Kingdom (U.K.), the Department of the Environment (DoE) has funded the Toxic Organic Micro* Author for correspondence, e-mail:
[email protected]. † AEA Technology plc. ‡ Lancaster University.
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Pollutants (TOMPS) Program to monitor ambient concentrations of PAHs, PCBs, and PCDD/Fs. We have previously reported on typical levels, seasonality, and gas/particle partitioning data arising from the measurements made in 1991/1992 (7, 9, 10). In this paper, we summarize the trends at London and Manchester (sites having a full 5 yr of data) from the first 5 yr of the program (1991-1995) and briefly comment on the implications of these trends with respect to sources/source abatement. To our knowledge, this is the most comprehensive continuous and ongoing data set of direct air measurements for these SVOCs. We stress that, given the inherent ‘noise’ in air concentrations, a 5-yr data set is still insufficient to identify long-term trends with confidence, although some patterns in the behavior of atmospheric concentrations are emerging.
Experimental Section Four sites are currently operated on the TOMPs network; three are urban (London, Manchester, and Middlesborough [northeast England]), and one is rural (Hazelrigg near Lancaster, northwest England). Two other urban sites have been run in the past (Stevenage and Cardiff) but are no longer operational. Only two of the sites, London (southeast England) and Manchester (northwest England), have been operated over the full 5 yr to date, so the trends at these sites are the subject of this paper. Full details of the sites, analytical procedures, and QA/QC have been given elsewhere (7, 9-11), but brief information is given here for completeness. High-volume air samplers (equipped to sample particulate and gas-phase SVOCs using a glass fiber filter and two polyurethane foam plugs) are operated at roof top level on a DoE building in Westminster (London) and on the law courts in Manchester; both are city center sites. From January 1991 to March 1994, PAHs, PCBs, and PCDD/Fs were sampled every other week, and each sample was analyzed separately. After March 1994, the individual samples were bulked and analyzed to give quarterly samples. Each air sample (PUF and filter) for PCB and PAH determination was Soxhlet extracted with hexane for 18 h followed by chromatographic separation and solvent exchange on a Florisil column. PAHs at Manchester have been analyzed throughout by HPLC with fluorescence detection, while the London data have been obtained by GC-MSD (7, 11). The following compounds constitute the ∑PAH reported: acenaphthene, fluorene, phenanthrene, anthracene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[a]pyrene, and benzo[ghi] perylene. PCBs at Manchester have been quantified by GC-ECD and at London by GCMS. Congeners 28, 52, 101, 118, 138, 153, and 180 make up the ∑PCB reported here, although data are available on additional congeners for Manchester (9). For PCDD/Fs, combined filters and PUF plugs were Soxhlet extracted in toluene for 16 h. The toluene extract was screened for the removal of co-extractives and rotary evaporated to ∼1 mL volume, and the solvent was exchanged to hexane. The extract was then chromatographed on an acid/base-modified silica multicolumn and then eluted with hexane directly onto a Florisil column. This column was successively eluted with increasing eluent strengths to yield the analyte-containing fraction. The 17 2,3,7,8-substituted PCDD/Fs have been analyzed by GC-MS (10, 11). A comprehensive regime of quality control was employed in each laboratory. This included the use of a working standard mixture of PCB congeners to confirm calibration, the running of blank samples with each sample batch, and the use of sampling analyte spikes and method analyte spikes. A certified reference material (SRM 1649 Urban Dust/Organics
S0013-936X(96)00953-4 CCC: $14.00
1997 American Chemical Society
FIGURE 1. Annual average concentrations of ∑PAHs at London and Manchester, 1991-1995. supplied by the U.S. National Institute of Standards and Technology) was run for PAHs and for selected PCBs. Although the material is not certified for PCBs, it was analyzed with each batch of samples.
Results and Discussion Air concentrations of SVOCs are influenced by several factors, notably the mixture and strength of sources, meteorological/ atmospheric factorsssuch as the atmospheric mixing height, frequency of precipitation events, intensity of sunlight, temperature, air mass movement and lifetime, and behavior of the compounds in question. As a result, air concentrations will vary both geographically and seasonally. Temporal changes in atmospheric concentrations will include longterm trends, seasonally periodic fluctuations, and short-term non-periodic fluctuations along with a measure of uncertainty associated with analyte measurement. The sampling regime and emphasis on data presentation used here is intended to enable year-on-year differences to be discussed by reference to two geographically distinct (ca. 300 km apart) cities. The complete data set is available elsewhere (11). PAHs. The ∑PAH air concentrations for London and Manchester have been in the general range ∼20-150 ng m-3, in line with that reported for other urban centers (12, 13). London concentrations have generally been higher than those at Manchester; the London conurbation has a population of ∼12 million people, while that of the Manchester conurbation is ∼2.6 million. Lower molecular weight (LMW) compounds (notably phenanthrene and fluorene) have dominated the total at both sites, again consistent with other studies (see ref 7). At both sites, the annual average concentrations have trended downward (see Figure 1). In London, concentrations declined as follows: 1991 (151) > 1992 (115)/1993 (119) > 1994 (51)/1995 (75) (units in parentheses are ng m-3), while in Manchester it was 1991 (115) > 1992 (71)/1993 (69) > 1994 (45) > 1995 (19). On average, for the two sites over the 5 yr, the ∑PAH concentrations therefore declined by ∼30%/yr. A similar trend of 1993 > 1994/1995 was also noted for ∑PAHs at the two other TOMPs sites that ran over this period (i.e., Middlesborough and Hazelrigg) (11). Interestingly, the mixture of individual compounds appears to have changed little over the 5 yr, although the LMW compounds make a bigger contribution to the total in the summer. There is a clear seasonality in the data, as revealed by Table 1, with winter ∑PAH concentrations higher than summer concentrations. Two compoundssacenaphthene and phenanthrenesdiffer from this pattern and have average summer concentrations > winter concentrations.
FIGURE 2. Annual average concentrations of ∑PCBs at London and Manchester, 1991-1995. PCBs. ∑PCB concentrations were usually ∼0.2-2 ng m-3 over the study period, typical of data reported in the last decade from cities throughout Europe (14) and North America (15). Concentrations of the seven PCB congeners (∑PCB) in London were generally 2-3 times higher than in Manchester. The concentrations in Manchester were, in turn, generally a factor of ∼4 higher than at Hazelrigg, the rural TOMPs site (11). These observations are consistent with the view that urban centers continue to act as important net sources of these compounds to the environment (9). The lower chlorinated species dominate the urban air mixture, with PCB congeners 28 and 52 the most abundant of the six constituting the ∑PCB represented here. Concentrations of PCBs at both sites were clearly seasonal, with summer concentrations averaging over double winter concentrations for all congeners (see Table 1). This observation of a correlation between concentrations and ambient temperatures has been noted previously in studies (9, 1619) and has been ascribed to the outgassing/volatilization of PCBs previously released and deposited to soils, water bodies, and vegetation (i.e., environmental recycling). Increases in mean concentrations from winter to summer were generally more pronounced for London than Manchester, possibly reflecting the slightly warmer conditions and lower rates of wet deposition in the south of England than the north, which could influence the rates of volatilization/outgassing/deposition. PCB concentrations at both sites were much more constant than those for PAHs (see Figure 2). Annual average concentrations in London were 1.45, 1.14, 1.13, 1.09, and 1.11 ng m-3 in 1991-1995, respectively. In Manchester, the concentrations were 0.44, 0.42, 0.46, 0.34, and 0.34 ng m-3. Studies by other workers in other locations also note the general recent stability in PCB air concentrations over time (4-6). For example, Panshin and Hites (4) compared air data from 1986/ 1987 with data from 1993 at Bloomington, IN, and reported no change. The same workers noted little or no change over more than 20 yr, up to the early 1990s when they compared separate data sets collected in Bermuda (5). However, retrospective analysis of archived air filters and herbage from the U.K. suggest that PCB air concentrations have remained relatively constant/declined only slowly since the mid 1980s in the U.K. but were substantially higher in the mid-/late1960s and early 1970s (2, 20). These studies may collectively suggest that environmental recycling and the persistence of PCBs is effectively ‘buffering’ air concentrations at a fairly stable level. Air concentrations probably declined more sharply in the 1960/1970s and are probably still slowly declining, but the rate of decline is difficult to discern (and prove statistically) above the seasonal/shorter term ‘noise’.
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TABLE 1. Typical Range Concentrations and Summer:Winter Ratios for PAHs, PCBs, and PCDD/Fs at London and Manchester, 1991-1995a typical range PAHs (ng acenaphthene fluorene phenanthrene anthracene pyrene benz[a]anthracene chrysene benzo[b]fluanthene benzo[a]pyrene benzo[ghi]perylene
0.4-4.7 25-90 21-74 4.5-12 0.4-2.5 0.5-3.5 0.5-3.5 0.9-2.8 0.3-1.8 0.7-5.4
PCB 28 PCB 52 PCB 101 PCB 118 PCB 138 PCB 153 PCB 180
58-1014 33-1350 17-252 5-120 8-37 11-95 2-33
2,3,7,8-TCDD 1,2,3,7,8-PCDD 1,2,3,6,7,8-HxCDD 1,2,3,4,7,8-HxCDD 1,2,3,7,8,9-HxCDD 1,2,3,4,6,7,8-HpCDD OCDD 2,3,7,8-TCDF 1,2,3,7,8-PCDF 2,3,4,7,8-PCDF 1,2,3,4,7,8-HxCDF 1,2,3,6,7,8-HxCDF 1,2,3,7,8,9-HxCDF 2,3,4,6,7,8-HxCDF 1,2,3,4,6,7,8-HpCDF 1,2,3,4,7,8,9-HpCDF OCDF
