Factors Governing the Atmospheric Deposition of Polycyclic Aromatic

Atmospheric deposition of PAH to these remote sites appears to be independent of .... Mosses as an integrating tool for monitoring PAH atmospheric dep...
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Environ. Sci. Technol. 2003, 37, 3261-3267

Factors Governing the Atmospheric Deposition of Polycyclic Aromatic Hydrocarbons to Remote Areas P I L A R F E R N AÄ N D E Z , * GUILLEM CARRERA, AND JOAN O. GRIMALT Institute of Chemical and Environmental Research (ICER-CSIC), Jordi Girona, 18, 08034-Barcelona, Catalonia, Spain MARC VENTURA Department of Ecology, Faculty of Biology, University of Barcelona, Diagonal, 645, 08028-Barcelona, Catalonia, Spain L L U IÄ S C A M A R E R O A N D J O R D I C A T A L A N Centre for Advanced Studies of Blanes (CEAB-CSIC), Acce´s Cala St. Francesc, 14, Blanes 17300, Catalonia, Spain ULRIKE NICKUS Institute of Meteorology and Geophysics, University of Innsbruck, Innrain, 52, A-6020 Innsbruck, Austria HANSJO ¨ RG THIES AND ROLAND PSENNER Institute of Zoology and Limnology, University of Innsbruck, Technikerstrasse 25, A-6020 Innsbruck, Austria

Polycyclic aromatic hydrocarbons (PAH) were measured in bulk atmospheric deposition collected in three remote areas of Europe during 1997-1998. Mean total PAH fluxes over a period of 18 months were 1560 ( 750 and 1150 ( 630 ng m-2 mo-1 in the Pyrenees and the Alps, respectively. In the Caledonian mountains (Scandinavia) the observed mean fluxes were 1900 ( 940 ng m-2 mo-1 (6 month collection). Similar qualitative PAH compositions (p values -5). HMW-PAH: parent compounds from benz[a]anthracene to coronene (log pLo < -6). The calculations are based on 6, 19, and 44 samples collected at Øvre Neådalsvatn, Estany Redo´ , and Gossenko1 llesee, respectively. Error bars refer to one standard deviation. For simplicity only the upper interval is represented but they should be considered to refer to mean ( standard deviation. Winter results for Øvre Neådalsvatn are not considered as they only correspond to December 1997. the higher discrepancies corresponding to the more volatile compounds (15-24%) (18).

Results and Discussion The mean PAH depositional fluxes measured in the three remote mountain areas are shown in Figure 2. Total PAH encompass the sum of the 23 parent compounds included in Figure 3 (from fluorene to coronene), low molecular weight PAH (LMW-PAH) are those with log pLo higher than -5 Torr at 20 °C, and high molecular weight PAH (HMW-PAH) are those with log pLo lower than -6 Torr at 20 °C. The latter two groups comprise PAH mainly present in the atmospheric gas phase (six parent compounds from fluorene to pyrene) and those associated to the particulate matter (13 parent compounds from benz[a]anthracene to coronene). The first feature to remark is the uniformity in total PAH fluxes between sites, with mean values ( standard deviation of 1150 ( 630 ng m-2 mo-1 and 1560 ( 750 ng m-2 mo-1 in the Alps and Pyrenees, respectively. Slightly higher flux levels have been found in Norway (1900 ( 940 ng m-2 mo-1), but the difference between means is not statistically significant according to the Mann-Whitney nonparametric test. Furthermore, they correspond to samples collected over 6 months, mainly in spring-summer. Calculation of deposition in the Alps and Pyrenees for the same period gives values of 1210 ( 605 ng‚m-2‚mo-1 and 1620 ( 580 ng‚m-2‚mo-1, respectively. These differences among sites are mainly related to the HMW-PAH (Figure 2) which are predominantly bound to

atmospheric particles, with fluxes ranging between 980 ng m-2 mo-1 at Øvre Neådalsvatn and 410 ng m-2 mo-1 at Gossenko¨llesee (intermediate levels of 820 ng m-2 mo-1 at Estany Redo´). The most volatile compounds exhibit similar values, 710, 660, and 680 ng m-2 mo-1 at Øvre Neådalsvatn, Estany Redo´, and Gossenko¨llesee, respectively. These results are generally in agreement with the PAH concentrations measured in the atmosphere over these lakes (19), which exhibit similar levels of the more volatile LMW compounds that are typically present in the gas phase. Total PAH deposition is dominated by the LMW compounds, namely phenanthrene, pyrene, and fluoranthene in the Pyrenees and the Alps (Figure 3), without significant qualitative differences between seasons (Mann-Whitney nonparametric test, p < 0.05). Similar PAH distribution in deposition has been reported in other European (20, 21) and the United States sites (22). A higher proportion of the HMWPAH has been observed in Øvre Neådalsvatn, with significant contributions of chrysene, benzofluoranthenes, benzo[e]pyrene, indeno[123-cd]pyrene, and benzo[ghi]perylene to the total PAH burden. These results parallel those observed in snow (10), water (13), and sediment samples (14) from these remotes lakes, suggesting that the PAH composition of the mixtures deposited from the atmosphere do not undergo major transformations during water column transport and early diagenesis. As a result, PAH accumulated in remote aquatic systems reflect the atmospheric inputs to each site. Direct comparison of PAH deposition fluxes measured in these areas with values reported in the literature is difficult since most studies were focused on PAH concentration in precipitation, e.g. wet deposition. On the other hand, dry deposition is usually estimated from atmospheric particulate PAH and dry deposition velocity. In any case, comparison with previously reported data shows that the total PAH atmospheric fluxes measured at these remote mountain lakes are more than an order of magnitude lower than those described in previous studies. For example, the fluxes of the 14 main contributing PAH compounds considered in this study range from 16.6 µg m-2 mo-1 in the Chesapeake Bay (23) to 6.3-33 µg m-2 mo-1 in Northern Greece (24), 28 µg m-2 mo-1 in the Swedish west coast (25), 9.5 µg m-2 mo-1 in Siskiwit Lake (6), 19 µg m-2 mo-1 in Galveston Bay (26), 15-20 µg m-2 mo-1 in Narranganssett Bay (27), and 35.5 µg m-2 mo-1 in a rural lake in the United Kingdom (20). Volume weighted mean (VWM) levels for total PAH calculated from wet-only samples taken at Estany Redo´ during warm periods, 14 ( 2.3 ng L-1, are also an order of magnitude lower than the values reported in the above-mentioned studies. In addition, comparison of benzo[a]pyrene deposition measured in Øvre Neådalsvatn, Estany Redo´, and Gossenko¨llesee, 47.8, 62.1, and 28.6 ng m-2 mo-1, respectively, shows lower values than those estimated by the Cooperative Program for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe (EMEP; average deposition for 1998 in the grids corresponding to these lakes of 139, 497, and 1420 ng m-2 mo-1, respectively (28)). Interestingly, the discrepancy between measured and EMEP estimated values increases with lake altitude and situation closer to Central Europe, with ratios of 3, 8, and 50 in Øvre Neådalsvatn (720 m asl), Estany Redo´ (2240 m asl), and Gossenko¨llesee (2417 m asl), respectively. Although a dilution effect with height is taken into account in the EMEP model calculations, they lead to an overestimation of PAH deposition fluxes to high altitude sites according to our results. Seasonality. Independently of interannual differences higher PAH deposition to these remote areas is observed during the warmer seasons (spring-summer; Figures 2 and 4). The ratio of the total PAH deposition between warm and cold seasons ranges between 1.8 (Estany Redo´) and 2.8 VOL. 37, NO. 15, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Average relative PAH composition in the atmospheric deposition samples collected in remote mountain areas. Error bars correspond to one standard deviation. Compound identification: Flu, fluorene; Phe, phenanthrene; A, anthracene; Fla, fluoranthene; Ace, acefenantrilene; Pyr, pyrene; B(a)Flu, benzo[a]fluorene; Ret, retene; B(ghi)Fla, benzo[ghi]fluoranthene; Cp(cd)Pyr, cyclopenta[cd]pyrene; B(a)A, benz[a]anthracene; Chr+Triph, chrysene+triphenylene; B(b+j)Fla, benzo[b]fluoranthene + benzo[j]fluoranthene; B(k)Fla, benzo[k]fluoranthene; B(a)Fla, benzo[a]fluoranthene; B(e)Pyr, benzo[e]pyrene; B(a)Pyr, benzo[a]pyrene; Per, perylene; InChrys, indeno[7,1,2,3cd]chrysene; InPyr, indeno[1,2,3-cd]pyrene; B(ghi)per, benzo[ghi]perylene; DB(ah)A, dibenz[ah]anthracene; Cor, coronene. The calculations are based on the same data as in Figure 2. (Gossenko¨llesee). Mann-Whitney nonparametric tests (since data series differ significantly from a normal or log-normal distribution Lilliefors test) show that these seasonal differences are significant for all PAH groups considered (95% and 99% confidence level for Estany Redo´ and Gossenko¨llesee, 3264

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respectively), although they are more pronounced among the less volatile compounds (ratios up to 3.2). This seasonal trend is not expected a priori since higher atmospheric PAH concentrations have been detected in these sites during colder than warmer periods (19), namely the compounds associated

FIGURE 4. Time-series of measured PAH deposition (bars) at remote mountain lakes. Precipitation, particle deposition fluxes, and air temperatures measured during PAH collection are represented by straight lines. PAH are grouped as in Figure 2. The calculations are based on the same data as in Figure 2. with the air particulate matter. In addition, at lower temperatures, a larger fraction of the atmospheric PAH will be associated with the air particles, which have been described to be more efficiently scavenged than those in the gas phase (29-31). In this sense, positive linear correlations between total washout and PAH particulate fraction have been reported (32). Furthermore, exponential increases in the PAH gas washout coefficients at decreasing temperature are expected due to the exponential dependence of the PAH

Henry law constants with temperature (30). Different seasonal trends for PAH deposition have been reported in the literature. In Atlantic Canada higher PAH concentrations in wet deposition were observed during the colder months (33) being attributed to increased emission and lower atmospheric mixing heights in winter (22), and consequently, higher pollutant concentrations in the atmosphere close to the ground in this season. Similar seasonal trends have been reported in other sites of Europe and the VOL. 37, NO. 15, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Parametric (Linear Regression) and Nonparametric (Rho Spearman and Tau Kendall) Correlation Coefficients (r) of PAH Deposition Fluxes (ng m-2 mo-1) and Mean Air Temperature (T, °C), Precipitation (mm mo-1), and Particle Deposition (mg m-2 mo-1) for Estany Redo´ , Gossenko1 llesee, and the Pooled Data linear regression coefficients (r) Gossenko1 llesee (n ) 20) total PAH vs

LMW-PAH vs

HMW-PAH vs

T precipitation particles particles + T precipitation + T particles + precipitation particles + precipitation + T T precipitation particles particles + T precipitation + T particles + precipitation + T T precipitation particles precipitation + T particles + precipitation precipitation + T + particles

0.816a 0.662a 0.665a 0.808a 0.672a 0.437 0.485b 0.681a 0.784a 0.758a 0.725a 0.904a

precipitation vs T particles vs T particles vs precipitation

Estany Redo´ (n ) 18) 0.540b 0.609b 0.748a

nonparametric regression coefficients (r)

all data Rho Spearman Tau Kendall (n ) 38) all data (n ) 38) all data (n ) 38)

0.695a 0.512a 0.698a 0.821a 0.720a (bT) 0.759a 0.806a (bprec.) 0.784a 0.867a (bT) 0.860a 0.540a 0.604a 0.536a 0.207 0.748a 0.429a a 0.843 0.734a 0.896a (bprec.) 0.369 0.587a 0.619b 0.623a 0.669a 0.729a 0.694a 0.683a 0.800a (bprec.) 0.878a 0.273 0.489a 0.237

0.706a 0.562a 0.834a

0.496a 0.375a 0.643a

0.613a 0.408a 0.624a

0.420a 0.230b 0.428a

0.554a 0.723a 0.791a

0.394a 0.527a 0.605a

0.297 0.740a 0.497a

0.222b 0.543a 0.316a

a Significance at 99% confidence level. b Significance at 95% confidence level. Multilevel regression coefficients indicated only if all considered variables in the model show a significant correlation at least at 95% confidence level; in brackets, factors with lower significance.

United States (34, 35). No trend between PAH deposition and temperature has been observed in other cases (23). Consistently with the results of the present study, Dickhut and Gustafson (30) observed higher PAH particle washout in spring-summer than fall-winter. These authors attributed these results to the atmospheric PAH redistribution to larger particles at increasing temperature and higher intensity precipitation events and convective storms during the warm seasons. Factors Governing PAH Atmospheric Deposition. The observed variability in bulk atmospheric PAH deposition to these mountain lakes may be influenced by the type and amount of precipitation, concentrarion, and/or characteristics of atmospheric particles, and the thickness of the tropospheric boundary layer which differs largely between winter and summer. Comparison between sites shows that higher PAH levels are associated with higher particle deposition and precipitation (Figure 4). For example, higher precipitation and particle load have been measured at Estany Redo´ than at Gossenko¨llesee. Moreover, at both lakes higher precipitation and particle deposition is observed in springsummer than in winter. At both lakes, positive correlations can be observed between total PAH fluxes and temperature, precipitation, and particle deposition flux (99% or 95% significance levels; Table 1). Nonparametric Kendall and Spearman correlation confirms the results of the linear model, often with higher regression coefficients and significance. At Gossenko¨llesee, the most significant factor is air temperature accounting for 67% of the variance in total PAH deposition fluxes, whereas about 44% of the variance is explained by precipitation or particle deposition. At Estany Redo´ particle deposition accounts for 56% of the variance in PAH deposition, whereas precipitation and temperature account for 37% and 29%, respectively. Further insight into the significance of these terms can be assessed by multilevel regression analysis. While at Gossenko¨llesee the inclusion of other terms than temperature in the linear model does not explain a higher proportion of 3266

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variance in total PAH deposition, 75% of the data variability is accounted for by a multilevel regression analysis including precipitation, particle flux, and temperature at Estany Redo´. More evidence of the influence of the above-mentioned variables on total PAH deposition is obtained from linear regression models based on the pooled dataset of Gossenko¨llesee and Estany Redo´. About 74% of the observed variance in bulk deposition PAH is explained by multilevel linear regression including particle deposition, mean air temperature, and precipitation (n ) 36, p < 0.01; Table 1). Among these factors, precipitation, while statistically significant, is the term contributing less to the multilevel model (74% versus 67% of total variance including or excluding this term, respectively). Thus, the main factors controlling total PAH bulk deposition to the study sites are particle deposition and temperature (with contributions of 49% and 48% to the marginal sum of squares, respectively). These linear correlations are also observed when LMWPAH and HMW-PAH are considered separately. At Gossenko¨llesee, temperature is the factor explaining higher variance in both cases, 45% and 61%, respectively, whereas at Estany Redo the main factor is particle deposition accounting for 56% and 45%, respectively (Table 1). All coefficients of LMW-PAH parallel those described for total PAH. The smaller coefficients for LMW-PAH could be partially due to volatilization losses back into the atmosphere during sampling, which are more important for the more volatile compounds. Among HMW-PAH, the regression coefficients for temperature are lower than those for precipitation and particle flux evidencing a smaller influence of this variable. In this PAH fraction, combined particle deposition and precipitation accounts for 82% and 64% of the total variance at Gossenko¨llesee and Estany Redo´, respectively. These high regression coefficients suggest that washout by rain and snow is the main mechanism of HMW-PAH removal from the atmosphere which is consistent with the atmospheric speciation of this PAH fraction.

At Øvre Neådalsvatn, the low number of samples does not allow for performing a detailed analysis of the influence of these factors. However, a good linear relation has been observed between HMW-PAH deposition fluxes and particle deposition + precipitation (r2) 0.77; p < 0.01). Multilevel regression analysis for HMW-PAH, including particle deposition, precipitation, and temperature as independent variables, accounted for 77% of the observed variability in the deposition data, with the particle flux as the main factor (71%), followed by precipitation (63%). In contrast, temperature has little effect on the regression model. These findings are consistent with the physical processes of particle scavenging which are controlled by meteorological conditions and particle characteristics (e.g. size, specific area, hydrophobicity, surface distribution of sorption sites), rather than by equilibrium partitioning. Even in the case of LMWPAH deposition, where the main factor is air temperature, deposited particles showed a significant linear correlation. In this sense, it has been reported that for the LMW-PAH the efficiency of particle washout by precipitation exceeds that of gas washout by more than a factor of 10. As a result, particle washout contributes significantly to the wet deposition of LMW-PAH despite the fact that they are mainly present in the gas phase of the atmosphere (30). In a previous study on atmospheric gas-particle partitioning of phenanthrene, fluoranthene, pyrene, and chrysene in high mountain regions it was shown that soot carbon was the main transport medium for the long-range transport of aerosol-associated PAH (19). The present results based on the examination of the deposited atmospheric materials are in agreement with this previous finding. Although multilevel regression analysis indicates no collinearity between independent variables, examination of the measurements for precipitation, particle flux, and temperature shows a dependence between the latter two involving 24% of the particle deposition variance related to temperature (Table 1). This direct dependence involves higher particle deposition at higher temperature and may be explained by the increase of the mixing layer height during warm seasons which may involve a higher influence of local sources in high mountain regions. Therefore, diffusive and vertical convective transport of aerosol particles from low altitudes, involving an increase of total suspended particles, would be more significant as air temperature increases. In addition, during spring-summer there is a higher incidence of long-range transported aerosols from arid areas in the Northern Hemisphere. In this sense, significant inputs of Saharan dust have been observed in the Iberian Peninsula, arriving as far as Central Europe (36), enhanced by the low atmospheric scavenging potential due to the low rainfall in South Europe in summer. The influence of dust transport over the Iberian Peninsula could explain the episodes of high particle deposition observed in spring in Estany Redo´ (Figure 4) and the higher influence of this factor on PAH deposition than in Gossenko¨llesee or Øvre Neådalsvatn. Overall, these results suggest that at high altitude environments with similar atmospheric PAH concentrations, in the range of 1.8-3.0 ng m-3 (19), the deposition fluxes of PAH mainly depend on particle deposition and, to a lesser extend, on air temperature and the amount of precipitation.

Acknowledgments We thank Torun Berg (NILU, Norway) and Leif Lien (NIVA, Norway) for Øvre Neådalsvatn deposition sampling. Technical assistance in instrumental analysis by Roser Chaler and Dori Fanjul is acknowledged. This work has been undertaken in the framework and financial support of EU, MOLAR (ENV4CT95-0007), and EMERGE (EVK1-CT-1999-00032) projects.

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Received for review July 8, 2002. Revised manuscript received April 30, 2003. Accepted April 30, 2003. ES020137K VOL. 37, NO. 15, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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