Polycyclic aromatic hydrocarbons in the polar ice of greenland. Geochemical use of these atmospheric tracers

Pergamon

Am,osphm

Enwonmmr Vol 28. No. 6, pp. II39 1145. 1994 Copyright c’ 1994 Elsev~cr Swncc Ltd Primed in Great Britain. All rights reserved 1352-2310/94 56.W+O.C@

POLYCYCLIC AROMATIC HYDROCARBONS IN THE POLAR ICE OF GREENLAND. GEOCHEMICAL USE OF THESE ATMOSPHERIC TRACERS J. L. JAFFREZO,*t M. P. CLAIN~ and P. MAXLETS *Department of Civil Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, UXA; tNow at Laboratoire de Glaciolopie et Geophysique de I’Environnement, 54 rue Molitre. Domaine Universitaire BP 96, 38402 St Martin d’H&es. Cedex, France; and SLaboratoire d’Etudes des Systtmes Atmospheriques Multiphasiques. Universitt de Savoie. Campus Scientifique, BP 1104. 73 376 Le Bourget du Lac Cedex, France (f-‘irst received 4 June 1993 and in jina//orm

24 Ocroher 1993)

Abstract Sampling of surface snow for PAH analysis took place at the Summit of the Greenland Ice Sheet in summer 1991. with 24 samples collected in a 3-m snowpit covering the previous 4 years ofdeposition. All concentrations were below detection limits (a few pg kg-‘) in the soluble phase. while concentrations of 13 PAHs were determined in the insoluble fraction. These are essentially the same as those which are present in the aerosol at this location. The nature ofthe PAHs in the ice shows that the contamination is due essentially to fossil fuel combustion aerosols coming from industrial zones, and also to biomass burning aerosols. The total amount of PAH is on the order of a few hundred pg kg- ’ (1360 pg kg- ‘) on average. The concentration profiles in the ice indicate summer minima for all species, attributed 10 lower emissions and reactions in the atmosphere at that time of the year. The maxima take place in spring for many compounds, in phase with that of sulfate, and is tentatively attributed to the influence of Arctic Haze. However, concentration increases are already seen in winter, particularly for 3-ring species that peak at that time. This situation could reflect the larger emissions in winter, but indicates also differential scavenging among the compounds and specific transport pathways. Finally, the snowpit profile shows that some modification occurs after deposition, with for example a 90% decrease of benzopyrene concentrations in the course of 4 years. The rate of change seems in rough agreement with the atmospheric reactivity index of the compounds. This study shows that PAHs are good geochemical anthropogenic tracers: they can be used to know the composition of the atmosphere since the beginning of the industrial period. Kry lord inde.x: PAH. Greenland Ice Sheet, surface snow, seasonal variations

INTRODUCTION

Recent interest in the global environment allowed a considerable development of research in the polar regions. It is now generally agreed that high latitude regions play an important role in the biogeochemical cycles of many chemical species (Barrie and Hoff, 1985; Barrie, 1986) and may present the largest variations for many parameters during climatic changes (Walsh and Crane. 1992). Understanding these cycles is necessary to investigate the links with climatic variations but necessitate a good knowledge of longrange transport of gaseous and particulate chemical species from source regions. Alternatively. information about the variation ofconcentrations observed in ice cores constitute an excellent tool for studying atmospheric changes over many different scales of time ranging from seasonal to climatic (Davidson et ~11..1991). when direct measurements are not available. Particularly, with regards to the last 200 years, the reconstruction ofconcentration trends for compounds of anthropic origin makes it possible to determine the modifications of the atmospheric environment induced by human activities. However, this approach is still limited by inadequate understanding of the air-snow transfer function

of these compounds. It is generally considered that the variation of concentrations observed in the ice directly reflects atmospheric changes. Seasonal changes of source regions, physico-chemical modifications during transport and deposition processes, and the ultimate state of these components in the deep ice are poorly known. These mechanisms are many and complex, and the simultaneous study of all the components of the atmosphere does not necessarily lead to their better understanding. It is necessary to study only good geochemical tracers. Among the compounds of anthropic origin existing in the atmosphere, polycyclic aromatic hydrocarbons (PAH) can be extremely interesting with regard to this type of studies. These carcinogenic compounds (Natusch, 1978)come exclusively from the combustion of fossil fuels (Pistikopoulos et al., 1990) and from biomass burning (Masclet et al., 1993). They are emitted on all conticents but emissions in the Northern Hemisphere dominate (Masclet ef ol., 1986). The absence of any natural sources (oceanic or volcanic) of PAH makes this class of compounds an excellent anthropogenic tracer. After condensation and coagulation, they are mainly associated with small aerosol particles between 0.1 and 1 jirn (Van Vaeck and Van Cauwenberghe, 1985; Beak et al., 1991). Therefore, 1139

J. L. JAFFREZ~er ol.

1140

they can be transported over long distances (Daisey et al., 1981; Bjorseth and Olufsen, 1983: Masclet et al.. 1988) and their residence time in the atmosphere is very long, up to several months for some species (Jaenicke, 1987). At high latitudes, there is a further displacement of the gas-particle equilibrium due to a temperature effect (Masclet et al., 1988; Patton et al., 1991) and they are present mostly in the particulate form. These compounds are mostly hydrophobic, not very reactive in the particulate form and can reach zones very remote from the emission sources without major modification of their chemical properties (Finlayson-Pitts and Pitts, 1985). This is not the case of lighter hydrocarbons such as VOC, since most of them are severely degraded by chemical aggressors like OH radicals. PAH could therefore constitute a homogeneous population for which the transfer function between air and snow would be more easily understandable. The study of these compounds, of the variability of their concentrations and their fate in the ice, can therefore answer some of the general questions concerning the sources, the redistribution

in the polar atmo-

sphere, and the incorporation in the ice of chemical species at large. We present here results from snowpit samples collected in central Greenland during the ATM program. The focus of this preliminary study was to determine the feasability of PAH measurement in surface snow at this remote location and to gather seasonal variations of the different relation with that of major chemical

information compounds species.

Seppak cartridges are eluted with 5 ml of acetonitrile; this volume is then also reduced in a stream of nitrogen. Extracts from both fractions are extended to 500 ~1 with methanol and analysed by HPLC coupled with a variable wavelength fluorimetric detector, using a Vydac 201 TP column (granulometry 5 pm; length 25 cm; diameter 4.5 mm). The ternary elution gradient can separate 22 PAHs in less than 25 min. including the 15 usually measured in the standard EPA method. This protocol is described in detail elsewhere (Bresson er al., 1984). The excitation and emission wavelengths are adjusted lo obtain the highest sensitivity for each compound. Concentrations were considered measurable for a signal/noise ratio above 5; it corresponds lo concentrations of a few picogrammes per sample for most of the species. Four separate analyses were performed on each sample. The corresponding standard deviations are given in Table I in the case of sample No. 15. The reproductibility of these measurements is about 20% for the least abundant compounds, and 8% for the most abundant compounds, such as fluoranthene and pyrene. respectively. Two other series of samples are used in this study for dating the main snowpit. One series extending down to 2 m comes from a snowpit dug on 18 July 1991.200 m apart from the main pit for PAH analysis, and was sampled according lo the stratigraphy of the snow layers. Another series come from a snowpit dug on 15 July 1992, 1.5 km away east, extending down to 5 m. The snow accumulation between the two dates is 66+-7 cm, as recorded at the deposition network of the ATM site (JafTrezo et al.. 1994); it is taken into account for synchronizing the two series on Fig. 1. Samples from these pits were collected and analysed for majors ionic species according IO protocols described elsewhere (Jaffrezo et al.. 1994). The sulfate concentrations are used in this work for locating spring layers.

on in

EXPERIMENTAL

The samples for PAH analysis were collected during the summer of 1991. in the course of the ATM program (Jaffrezo et al.. 1991) that took place at the Summit of the Greenland Ice Sheet as part of the deep ice drilling programs GISP-2 and GRIP. A 3-m snowpit was dug on 5 August 1991,2.5 km away from the ATM camp, itself 28 km SSW from the main GISP-2 drilling camp (72 ‘20’N: 38 45’ W: elevation 3270 m) (Mayewski et al., 1990 and JafTrerezo er al.. 1994). Snowblocks approximately 40 x40 x 10 cm in size were cut from the sampling wall of the pit using a clean stainless steel snow saw. leading to 28 samples extending between 20 and 300 cm in depth and covering approximately 4 years of accumulation. Samples were then transported lo the main camp double bagged in dust-free polyethylene bags, where they were allowed lo melt at room temperature in the atmospheric laboratory. The samples were filtered under a laminar flow hood within 24 h of melting. using prewashed Durapore filters (Millipore Corp.. 0.1 pm porosity). The dried filters were then wrapped in aluminum foil and stored double bagged at subfreezing temperature. After this first filtration. the remaining water was extracted at low flow rate (3 ml min- ‘) for the collection of the soluble fraction of the PAH using Seppak cartridges precleaned with methanol; the cartridges are then stored in the same way than the filters, before being sent back lo France in the frozen state. For technical reasons, only 24 samples were workable. Back in the main laboratory, filters are extracted in. a Soxhlet with a 2: 1 mixture ofdichloromethane and cyclohexane. The extract is reduced in a Kuderna Danish evaporator, then evaporated to dryness in a stream of nitrogen. The

RESULTS Ah’D DISCUSSION

First. the results indicate that all PAH are below detection limits (a few pg kg- ‘) for all the samples in the case of the soluble phase, while their actual solubility is in the range of ng ml - ’ and up. Essentially the same result is presented by Cape1 el al. (1991) in the case of urban fog samples in Switzerland, with much higher concentrations. It is tentatively attributed to processes taking place during combustion, making the PAH unavailable for solubilization. However, some other studies at the same site in Switzerland are not as definite, showing that a substantial contribution of the soluble fraction can exist for rain and Table 1. Concentrations and standard deviations PAHs in sample No. 15 (pg kg- ‘) Naphthalene Phenanthrene Fluoranthene Pyrene Benzouanthracene Chrysene Benzobfluoranthene Benzokfluoranthene Benozoapyrene Benzoghiperylene Coronene Sample 15 (depth: 160-170cm).

182& 10 460 f 32 668_+65 470+80 83+15 205+37 17+ 3 21* 2 3.5kO.5 27k 1.5 8&- 5

for 11

PAH in the polar ice of Greenland

snow events, and light compounds (Leuenberger et al., 1988). The PAH are hydrophobic compounds and the small fraction present in the soluble fractions does not reach the high latitude regions. Conversely, many PAHs are found in the insoluble fraction. Table 2 gives, for the 24 samples analysed, the concentrations of the 6 major compounds detected in the insoluble phase: naphthalene (NAP), phenanthrene (PHE), fluoranthene (FLA), pyrene (PYR), benzo-a-pyrene (BaP), benzo-ghi-perylene (BghiP). It also gives the sum of the concentrations for all PAH measured. Significant amounts of benzo-a-anthracene, chrysene and coronene were detected, on top of the 6 mentioned above. These compounds can have various origins. Phenanthrene and fluoranthene come mainly from the combustion of coal and fossil fuels (Bresson et al., 1984). Benzo-ghi-perylene, coronene and indenopyrene come mainly from combustion of gasoline in vehicles (Nikolaou et al., 1984) while pyrene is found in large quantities during biomass burning (Masclet et al., 1993). All types of combustions occurring in the Northern Hemisphere can therefore contribute on an integrated time scale to the PAH content of the polar ice at this location, meaning that the potential source regions include the industrial regions of North America and Europe, without neglecting the subArctic regions of Russia and Canada. The size resolution of the sampling, that mixes many snow layers in the same sample, is probably not sufficient to differentiate specific origins on the basis of chemical profiles of PAH only. Analysis of samples collected at the same location in 1993 with a finer resolution

(5 cm) is currently

under way to verify this

hypothesis.

1141

It is noteworthy that the most abundant PAH found in the ice have 3 or 4 rings. These compounds are relatively volatile and they are usually found in the gaseous phase in the atmosphere of continental and marine regions at low and mid latitudes (Marty et al., 1984; Masclet et al., 1988; Li and Kamens, 1993), with larger concentrations than the heavier PAH of the particulate fraction (Germain and Gonthier, 1993). But the gaseous fraction condenses or is adsorbed on the aerosols during transport towards cold regions where PAH are mainly found in the particulate phase (Patton er al., 1991: Jaffrezo et al., 1993), following the variation of the gas-particle distribution with the temperature (Handa et al., 1980; Masclet et al., 1988; Baek et al., 1991). The relative distribution of the concentrations in this fraction is therefore shifted towards lighter compounds compared to that measured in more temperate areas. This shift is due mainly to the temperature parameter, then to the condensation of the gaseous fraction during transport. Comparison of average composition of the aerosol for PAH between temperate and polar regions (Jaffrezo et al., 1993) show that surface snow in central Greenland globally records that process, with a composition close to that of the aerosol measured in cold regions. Table 2 shows that the sum of the PAH concentrations is in the order of a couple of ng kg-’ (range 0.60-2.37 ng kg- ‘, mean value 1.36 ng kg- ’) for the 24 samples. These values can be compared with results from snow events in North America. Welch et al. (1991) indicate a total PAH concentrations of 12.6 ng kg- ’ in northern Canada during an episode of “brown snow”. McVeety and Hites (1988) measured total PAH concentrations

of 19 ng kg -’ in snow from the region of

Table 2. Concentrations of the main size particulate PAH (pg kg- ‘). Column 9: sum of all PAHs except naphthalene and phenanthrene. Column IO: total concentration of all PAHs Sample 2 3 4 5 6 8 IO I1 12 13 14 I5 16 18 19 21 22 23 24 25 26 27 28

Depth

NAP

PHE

FLA

PYR

BaP

~~hiP

CPAH

XtPAH

20-30 30-40 40-50 50-60 60-70 70-80 100-110 1 IO-120 120-130 130-140 140-150 150-160 160-170 170-180 190-200 200-210 220-230 230-240 240-250 250-260 260-270 270-280 280-290 290-300

I69 I83 285 183

291 302 708 300 177 366 376 692 757 237 388

522 413 308 342 206 174 418 453 402 270 165 682 668 387 315 241 193 248 313 411 315 249 195 100

373 338 220 259 144 239 342 324 285 I32 118 595 470 284 385 455 138 176 190 365 267 228 125 111

22 15 10 33 5.5 6.5 17 I4

84 8 46 117 I9 49 105 56 16 34 32 26 27 35 32 39 71 50 I3 51 39 19 42 36

1516 918 875 1111 625 712 1170 IO81 909 537 474 1450 1503 930 895 998 750 617 714 1269 1113 636 582 301

1976 1403 1868 1594 802 1120 1741 2051 2037 774 988 1585 2145 1981

42 I95 278 371 I26 135 I82 420

460 631

336 301 212 I17 72 498 95 92 42

293 380 470 242 175 756 107 197 255

4.5 5.5 3 3.5 5 4 3.5 6 3.5 5 2.5 2.5 I.5 2

1627 1431 1299 1073 1516 2367 838 871 598

1142

J. L. JAFFREZOer 01.

Lake Superior. Thus, surface snow in central Greenland contains at least 6-20 times less PAH than polluted snow samples from North America. Such a comparison is approximate, since the same sets of species are not necessarily taken into account and since our data are averages over several snow episodes, as opposed to specific events. The relatively small range of concentrations for the sum of PAH compared to species like sulfate (varying in snowpits across the year from 20 to 330 ppb at this location, Fig. 1) is also induced by the poor time resolution of our samples. But larger seasonal variations are seen for specific compounds, indicating that the seasonal pattern of the various PAH is not strictly the same and introduces some smoothing for the sum of the concentrations It also means that larger differences probably exist when individual events are taken into accounts, and that PAH profiles could be closely associated with specific source regions. Seasonal variation

of the PAH

concentrations

Figure 1 presents the variations ofconcentration for fluoranthene, pyrene, naphthalene and phenanthrene in the snowpit samples. The variation of sulfate concentrations in the two pits from nearby areas are also indicated in this figure. The covariations observed for fluoranthene and pyrene present a timing of the maxima that is globally in phase with that of sulfate. Part of the apparent time lag for spring 88 (at 230-260 cm) may be due to compaction of deep snow in 1992 compared to 1991. However, the increase in concentration leading to spring maxima is not as sharp as that of sulfate or crustal species (J. L. Colin, personal communication) and seems to begin earlier (during the winter). The yearly minima are reached during summer. The pattern seems slightly different for naphthalene and phenanthrene, that also covary: their maximum ofconcentration is taking place before that of the previous compounds, putting it in winter (Fig. 2). while the minimum is still occurring during summer. The summer minimum can be explained by several phenomena whose effects are superimposed:

0

50

IOU

150

zoo

250

cm

(i) physiochemical degradation of several PAH (particularly the most reactive like benzo-a-pyrene) during their transport in the atmosphere; (ii) lower emissions during summer in the mid latitude regions (Germain and Gonthier, 1993) and may be, (iii) lower transfer from sources regions located at mid latitudes, as seen with most of the chemical species in south (Davidson et al., 1993) and central Greenland (Whitlow et al., 1992). The spring maximum coincides with major inputs in the surface snow of the ice sheet for many chemical species from different origin and is attributed to better transport from source regions and from the Arctic Basin due to changes in the meteorological regime (Shaw and Kahlil, 1989). Higher atmospheric concentrations of PAH at sea level in spring compared to summer are reported for Barrow, Alaska (Daisey et al., 1981). Similar behavior of PAH and other chemical species of anthropogenic origin with respect to transport during Arctic Haze episodes are supported by high correlation between PAH and total S concentrations during spring at Alert (Patton et al.. 1991). High correlations between concentrations of PAH coming from different sources (fluoranthene, pyrene and phenanthrene) are found as well in the study at Alert, indicating that transport processes may have more influence than source origin on the chemical profile in the atmosphere at this time of the year, when long residence time in the Arctic favors mixing of species in an aging aerosol. With this respect, closer examination of snowpit data with a better time resolution is necessary in order to clearly verify the timing of the inputs of PAH, particularly during winter when most of the other species present low concentrations. Specifically, would the timing of naphthalene and phenanthrene inputs in the surface snow be significantly different, as suggested by this preliminary study, it would imply that transport processes do not impact light (3 rings) and heavier PAH in the same way during winter, despite the condensation processes that should take place at low temperature.

3al

Fig. I. Seasonal variation of Ruoranthene pyrene and sulfate concentrations in snowpit layers at Summit, Greenland. Surface as of 15 July 1991.

0

SO

loo

150

zoo

250

cm

300

Fig. 2. Seasonal variation of naphthalenc and phenanthrene at Summit.

PAH in the polar ice of Greenland Post-deposition

1143

variations

The comparison of the 4-year pattern of the variation of PAH concentration indicate that post-deposition process occurs to some extent on this time scale. Figure 3 shows the variation of concentrations in the snowpit samples for phenanthrene, fluoranthene and benzoapyrene; the regression lines drawn on this figure are indicative of tendencies that are superimposed to the seasonal cycle described above. The different compounds can be grouped according to three cases:

500

400

300 : 200

100

(i) Benzo-b-fluoranthene, chrysene, benzo-k-fluoranthene, benzo-ghi-perylene,with a pattern similar to 0 100 200 300 that of phenanthrene, showing no significant tendency over the four years of observation. It indicates that the degradation of these compounds in the ice is negligible. This result is in agreement with the very low reactivity of benzo-b and -k fluoranthenes, chrysene and benzo-ghi-perylene in the particulate form. (ii) Benzo-a-pyrene; this compound presents a regular and highly significant decreaseof its concentration, averaging 90% over 4 years. The sources of benzo-a-pyrene are not specific and should have been roughly constant during that time period. To a first approximation, transport processesimpact BaP in the same way as species from the previous group, and apparently did not change drastically between 1988 and 1991. Therefore, the decreaseof BaP concentration clearly reflects a degradation of this compound in the ice after deposition. It is known that BaP is the most reactive of the PAHs present in the aerosol, y = 19.362 - 6.69670-2x R”2 = 0,556 involved in photodegradation and reactions with radicals (Pitts et a/., 1978;Nielsen, 1983).The mechanisms taking place in the ice are not identified, although OH radicals produced by degradation of H20, present in 30 the ice could be a likely candidate for liquid-phase reaction. BaP could therefore potentially constitute a tracer for the evolution of reactive organic compounds P 20 in the ice archives. Nevertheless, more investigations lz are needed in order to identify the mechanismsof this degradation. 10 (iii) Fluoranthene and pyrene. These compounds represent an intermediate class, with a mean decrease of the concentrations lessimportant than that of BaP, 0 whilst apparently still significant (40 and 35% in 4 100 200 IO 0 years, respectively).These compounds are somewhat Fig. 3. Four-yeartrendsfor the concentrationsof phenanreactive in the atmosphere, both in the gaseous form threne,Ruorantheneand benzo-o-pyrene PAH at Summit, (Arey er al., 1986)and in the particulate form (NikolGreenland. aou et al., 1984)and could experiencethe same type of reaction as BaP. Further studies are needed,however, to confirm the tendency over longer time scales,and exclusively in the particulate form, while the soluble quantify the rate of decrease. fraction is much lower than that theoretically possible. Concentrations are lower than previous measurements at remote places at sea level in North America and Canada, a result comparable to that obtained for CONCLUSION major chemical speciesand attributed to the altitude This study presents the first results of PAH meas- of the Ice Sheet. The concentration profiles in a urements in the surface snow of the Greenland Ice snowpit covering the last 4 years indicate pronounced Sheet. It shows that these compounds are found seasonal variations. The minima of concentration

,,I

J. L. JAFFREZO et al.

1144

occurs in summer for all PAH, while the maxima are in phase with that of sulfate, or slightly earlier depending on the compound. Earlier maxima for 3-ring species may indicate that processes occuring during transport are at play in winter to introduce differential scavenging. The 4-year trends of the snowpit show that post deposition processes take place on this scale, particularly with a decrease in concentration of 90% for BaP, the most reactive species in the atmosphere. More studies are needed to confirm these observations, particularly with a finer time resolution in the snowpit sampling, and longer time scale. Such a work has already been initiated during the past ATM season. Acknowledgements-The authors would like to thank all personnal in the field (the GISP-2, PICO and GRIP communities) for their general support during the ATM program. Special thanks are due to P.A. Mayewski, J.E. Dibb and S. Weyenberg.

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