Environ. Sci. Technol. 1994, 28, 1459-1466
Northern Hemispheric Organic Lead Emissions in Fresh Greenland Snow Rysrard Loblhski,’9t Claude F. Boutron,*p§ Jean-Pierre Candelone,* Sungmin Hong,* Joanna Szpunar-Loblhska,t and Freddy C. Adamst
Department of Chemistry, Micro- and Trace Analysis Centre, University of Antwerp, Universiteitsplein 1, 26 10 Wilrijk, Belgium, Laboratoire de Glaciologie et Ghophysique d e I’Environnement du CNRS (LGGE), Domaine Universitaire, 54 rue Molidre, B.P. 96, 38402 St. Martin d’Hdres, France, and UFR de Mecanique, Universit6 Joseph Fourier d e Grenoble, Domaine Universitaire, B.P. 68, 38041 Grenoble, France Ionic alkyllead species have been determined in fresh or slightly aged surface snow from Greenland relying on ultraclean sampling, high-resolution separation of individual species, and ultrasensitive determination procedures. The samples were collected from January to August 1989 on a precipitation event basis at Dye 3 (south Greenland) and in late spring and summer 1987 and 1989 at or near the Summit site (central Greenland). Evidence for long-range transport of organolead compounds into the very remote environment is given, and the nature and extent of the global pollution by organolead compounds are evaluated. Triethyl- and diethyllead species were found in all the samples, whereas methyllead species were absent in the late spring and summer samples. Monoalkyllead, mixed ionic organolead, and tetraalkyllead species were always found to be below 10-20 fg/g. The average total organolead concentrations form January to April was very high (476 fglg, values between 100 and 800 fglg, 0.32% of the total Pb), whereas the average summer concentration (May-August) was a factor of 5 lower (82.6 fg/g, values between 15 and 220 fg/g, less than 0.4 7% of the total lead). The variations in the concentrations measured are interpreted in terms of backwards air mass trajectories. Source regions influencing Dye 3 at different times are indicated. Organolead compounds in Greenland are established to be valuable indicators of the global character of the lead pollution originated specifically in the use of leaded gasoline. Introduction Long-term low-levelexposure of living organisms to lead has been associated with a wide range of metabolic disorders and neuropsychological defects and is believed to be responsible for subclinical poisoning of several million people (1). The main source of lead in the environment is gasoline-related emissions, which can be unequivocally accounted for by considering alkyllead species (2, 3). Tetraethyllead (TEL), tetramethyllead (TML), and sometimes mixed methylethyllead compounds are added to gasoline in varying proportions in order to prevent the spontaneous premature combustion of the fuel mixture (knocking). During combustion, they undergo a variety of degradation processes which continue in the troposphere under the influence of sunlight and some atmospheric constituents (2-6). Washed out, the degradation products, ionic di- and trialkyllead species, provide valuable tracers in rain and snow which can be unambiguously assigned to the gasoline-related pollution and often to its geographical origin (2-6).
* E-mail address:
[email protected]. + University
of Antwerp. t Domaine Universitaire, St. Martin d’H8res. 8
Domaine Universitaire, Grenoble.
0013-936X/94/0928-1459$04.50/0
0 1994 American Chemical Society
Long-range atmospheric transport of anthropogenic pollutants from mid-latitude source regions (mainly North America and Europe) to central Greenland has been attracting considerable interest during the last two decades (7,8). The lack of understanding of transport pathways and the mechanisms by which atmospheric heavy metals are incorporated into central Greenland snow and ice hampers a proper interpretation of the ice core historical profiles. Several tracers for anthropogenic emissions have been proposed (9, 10) but organolead compounds as indicators of global automotive and aircraft pollution have not been considered. These compounds, however, offer some interesting features for a climatologist such as a different composition of North American (11, 12) and European emissions (13-16) with respect to the use of methyl- or ethyllead compounds, a year-to-year variable and known production since 1923 (17, 18), and different transport mechanisms of ionic (partly alkylated) and nonpolar (fully alkylated) species (19-22). The literature reports present a strong case for the existence of a natural biogeochemicalcycle of organic lead, which is animated by the atmospheric transformations and interacts with that of inorganic lead. Sink and source processes for organic lead in the environment are both still poorly understood despite a number of experimental studies (4-6). Studies of organolead in environments far away from direct automotive emissions have been scanty (20, 21). The magnitude of the organolead impact on a hemispheric or global scale is unknown. No data are available on the presence and atmospheric chemistry of organolead in polar environments. It seems probable that atmospheric aerosols that play a key role in biogeochemical cycles of many substances (23) are likely to transport organolead compounds over long distances before their removal by clouds, precipitation, and dry deposition to the surface. The aim of this work was to verify this assumption and to investigate the global pollution by organolead with respect to the extent, sources, and transport pathways. Materials and Methods
Sampling. Eighteen samples of fresh snow were collected from 12 precipitation events throughout the period from January 7 to August 6,1989. Sampling was conductednear Dye 3, South Greenland (65’11’N, 43’50’ W), elevation 2479 m, approximately 150 km from the coast, and mean annual snow accumulation rate about 50 g/cm2 year. To avoid contamination, which might have originated from the station itself, the samples were collected as soon as possible after the event, taking into account the wind direction during and after the event. The distance from the sampling site to the station was chosen to be as large as possible but was dependent upon the difficulty of traveling long distances during poor Environ. Sci. Technoi.,Vol. 28, No. 8,
1994
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Table 1. Results of Organolead Speciation Analysis in Surface Snow Sampled near Dye 3 (as Pb)
sampling date 1/7/89 2/23/89 4/10/89 4/15/89 4/19/89 4/22/89 4/25/89 4/29/89 6/5/89 6/10/89 6/15/89 6/25/89 6/26/89 7/2/89 7/18/09 8/5/89 8/6/89 7/16/89
sitea (km) 1 1 10 1 23 1 1 23 1 10 1 1 1 1 23 10 23
delayb (days) 0 0 2 0
ngc ng 0 0 0 -4 0 -10 0 0 0 0 0
EtaPb+ (fdg)
Et2Pb2+ (fg/g)
Me3Pb+ (fg/g)
MezPb2+ (fg/g)
concn ratio, Etd'b2+/EtsPbt
158 208 42 25 75 53 59 6 5 5 7 55 15 9 22 149 138 17
145 308 38 147 703 560 304 51 101 61 26 14 11 55 26 75 92 32
30 37
54 260 18 89
0.92 1.48 0.91 6.00 9.42 10.6 5.14 8.43 20.2 12.7 3.55
20
5.96 1.18 0.51 0.67 1.85
Pbinorg (Pg/g) 39 43 33 2700 47 218 18 55 118 293 129 300 110 52 13 26 93 355e
concn ratio, PborgIPbinorg (%) 0.99 1.89 0.29 0.01 1.65 0.29 2.02 0.10 0.09 0.02 0.03 0.01d 0.01d 0.12 0.37 0.86 0.25 0.01
ng ng Distance from the station. b Time between the end of snowfall and the sampling (0 indicates that sampling was done just at the end or no more than half a day after the end of the snow fall). ng, we have no clear indication on the delay. Approximate value. e This sample was possibly contaminated with inorganic Pb. a
weather conditions. Eight of the samples were collected 1 km from the station (in the upwind direction of the corresponding event); the remaining ones were collected at established sampling sites 10 and 23 km southwest of the station. In addition, five samples of aged snow that appeared to be related to specific fresh snow events were also collected mainly at the 10or 23 km sites. Each sample was obtained by operators wearing full clean-room garb and shoulder-length polyethylene gloves pushing widemouth 1-Lconventional (low-density)polyethylene (CPE) bottles horizontally into the distinct fresh or aged snow layer to be sampled. The cleaning of the bottles is described elsewhere in detail (24). After being filled with fresh snow, the bottles were immediately capped again and put in the original acid-cleaned polyethylene bags. They were transported back to the LGGE laboratory frozen. Eight samples of fresh and aged surface snow were collected on two different days (2 weeks apart) in May 1987 in the Summit area in Central Greenland at two different sites (72'21' N, 40'13' W and 72'59' N, 37'42' W). Four samples of fresh snow were collected on 4 different days in June and July 1989 at the Summit site. The snow was sampled in precleaned CPE bottles (24). They were pushed horizontally into the snow to collect snow layers from the surface down to a few centimetres without touching the sample with any tool, about 1.7 km upwind from any other field activities. The scientists wore full clean-room clothing and CPE gloves. The bottles were put in triple CPE bags, transported frozen to the LGGE laboratory, and stored frozen until analysis. Sample Handling and Analysis. The samples (surface snow or snowfice core) were allowed to thaw inside the LGGE ultraclean laboratory (25),and aliquots of the samples were prepared and refrozen for organolead analysis. Blanks of MQ water were prepared by exposure to the lab atmosphere during the preparation of aliquots. Upon freezing, they were transported together with the sample aliquots packed in triple CPE bags in an air-tight box to the MiTAC Laboratory in Antwerp and stored at -20 'C prior to analysis. Samples and the blanks were handled further under class 100 clean-bench conditions. 1480 Envlron. Sci. Technol., Vol. 28, No. 8, 1994
Organolead compounds were determined using capillary gas chromatography with a microwave-induced plasma atomic emission spectrometric detection according to the procedure described in detail elsewhere (26). The method has allowed quantification of organolead in Greenland snow down to the 10 fglg level on the basis of 50 g of sample. A typical standard deviation at the 50 fg/g level was about 10-15%. Contamination Control. Sampling bottles cleaning protocols applied in ultratrace analysis for metals are also valid for organolead (26). The probability of atmospheric contamination via diffusion through CPE storage bags and bottles is considered very small since methyllead speciespresent in the atmosphere of Grenoble and Antwerp are usually not found in Greenland snow samples. Nevertheless, in parallel to the preparation of aliquots of snow samples, several blanks were included that accompanied the samples batch during transport to Antwerp and further storage. A chromatogram for such a sample did not differ from that of a MQ water blank, which was on a level of 10 fgfg. A possible organolead contribution from the ultraclean laboratory air was controlled by exposing an ultrapure water sample to the laboratory atmosphere in parallel with the samples. No sizable deposition was observed under clean bench contrary to samples exposed to the uncleaned laboratory atmosphere. This indicates that, contrary to vapor-phase mercury, organolead compounds are efficiently removed by the filters. To allow for contamination control during analysis, procedural blanks must be run in parallel before and after the sample. An extensive discussion on contamination sources can be found elsewhere (26).
Results Surface Snow from Dye 3. The description of the samples analyzed and the concentrations of organic and inorganic lead found are given in Table 1. Ionic ethyllead species [diethyllead (EtzPb2+)and triethyllead (Et3Pb+)l were found at different concentrations in all the samples, whereas methyllead species were found in only a few of them. Monoalkyllead, mixed ionic organolead, and tet-
40
Me,Pb'
. cn
800
En
c
.-c0 c al
600
400
0
c 0
200
0 0
. 01
cn P
.-c-
600 400
0
e
2
900
c
ti0
200
8
100
c
0 400
P
P)
c
900
2 0
.-
C
2
c
200
C
8 8
100
0
Na+
Sampling date Flgure 1. Concentrations of (A) Et3Pb+, (6) Et2Pb2+,(C) Me3Pb+,and (D) Me2Pb2+found in surface snow from Dye 3.
raalkyllead species were found to be below 10-20 fg/g in all samples. The total ionic alkyllead concentration in the Dye 3 snow for the successive sampling periods shows a certain periodicity, with low levels in summer and much higher concentrations in winter and spring. In winter and early spring (January-April), the average concentrations of the total organolead is very high (476 fg/g, values between 100 and 800 fg/g), which accounts for 0.3-2% of the total Pb. On the contrary, the average summer concentration (MayAugust) is a factor of 5 lower (82.6 fg/g, values between 15 and 220 fg/g), which accounts for less than 0.4% of the total lead. These seasonal variations are much stronger
Sampling date Flgure 2. Concentrations of (A) total organolead, (6) total lead (7),(C) sulfate (7),and (D) sodium (7).
for methyllead compounds that were found in the first 3 months of the year at considerable concentrations of 50200 fg/g but are virtually absent in the summer time. The changes in concentration of particular organolead species as a function of sampling date are shown in Figure 1. Figure 2 compares the concentration of the total organolead with that of the total lead and those of some ancilliary species: Na+ and S042- measured in the same samples. Surface Snow from Summit. The fresh and aged surface snow collected in the spring and early summer of 1987 and the summer of 1989 showed relatively high concentrations of both Et3Pb+ and Et2Pb2+with a clear domination of the latter (Table 2). It is not likely that ,Envlron.Sci. Technol., Vol. 28, No. 8, 1994
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Table 2. Results of Organolead Speciation Analysis in Surface Snow Collected in the Summit Area (as Pb)
sampling date
sampling site
5/13/87
72'21' N, 40'13' W
5128187
6/14/89 6/19/89 71251a9 7128189" a
72'59'N, 37'42'W
72'35'N, 37' 38' W
EtSPb+ (PPIg) 0.20 0.15 0.12 0.10 0.25 0.045 0.61 0.077 0.059 0.027 0.15 0.23
EtzPb2+ (PdP) 0.15 0.31
0.19 0.15 0.53 0.21 0.29 0.48 0.71 0.20 0.50 2.3
concn ratio, EtzPb2+lEt3Pbi
0.75 2.07 1.58 1.50 2.12
4.67 0.47 6.23 12.0 7.22 3.41 9.72
Pbinorg (PPIP) 36 52 43 41 33 64 36 36 98 190 12
29
concn ratio, Pborg/Pbinorg ( % ) 0.97 0.88 0.72 0.61 2.36 0.40 2.50 1.55 0.78 0.12 5.41 9.02
Also 51.4 MesPb+ and 99.3 MezPb2+.
they can be associated with contamination as rather low values for total lead were found in all the samples. It is worth noting that the surface snow sampled in 1989 at Summit also showed unusually high Hg concentration (27). Discussion The results indicate generally a higher ratio of organolead vs inorganic lead than that typically observed in the continental atmospheric deposits. Simultaneously, considerable variations in this ratio, in the distribution of particular organolead species, and in the absolute organolead concentrations occur. These variations are driven mainly by differences in source emissions, atmospheric chemistry of organolead, and transport pathways. Below, a summary of the current knowledge on the sources of organolead and its possible transport pathways to Dye 3 is followed by a detailed study of the variations observed. Source Emissions and Basic Transport Pathways. The primary emissions of organolead compounds show seasonal and geographical variations with respect both to the amount and to the composition. Data on the composition of antiknock additives are scarce and scattered, but tetraethyllead seems to be by far the dominant additive used in Canada (II), the United States (12), and China (28). In European gasolines, the distribution of TAL species may vary drastically from one brand to another (13),but TML is generally more abundant in winter than in summer (14). A study of Belgian gasoline in winter gives a TML/TEL average ratio of 1:l (14). The same ratio was reported in Germany (15),while in Denmark an average ratio of 1.9 was reported (16). A summer study in Belgium showed exclusively TEL and is supported by a dominating contribution of tetraethyllead reported in a Danish study (16). In the exhaust gas, TML is enriched vs TEL compared to the original gasoline in the vapor phase because of high volatility and higher thermal stability resulting in higher fractions of unburnt lead alkyls emitted (29). The release of organolead to the atmosphere in winter is larger than in summer because cold and choked engines emit ca. 10 times more organolead than those run under normal conditions (29). This organic lead is composed of 50-7076 of vapor-phase TAL compounds (presumably as unburnt fuel) which are more easily transported than when incorporated in the aerosol. The backward air mass trajectories indicate the Arctic troposphere as the direct source of pollution during the winter and the spring with a possibility of injections from 1462
Envlron. Scl. Technol., VoI. 28, NO. 8, 1994
eastern North America and Western Europe during the spring (30). There is a virtual absence of transport from the Atlantic at distances greater than 2000 km (30). Transport from the Arctic Basin is apparently not observed at Dye 3 height in winter (24, 31), but the North Polar Basin may impact Dye 3 in April giving rise to injections of highly polluted Arctic Haze (32). The Arctic Haze season lasts until the polar front migrates north late in spring, eventually isolating the Arctic from mid-latitude source emissions. With the arrival of summer, short-range transport (within 2000 km) starts dominating. The trajectories originate mostly in southeastern Canada or northeastern United States (30). Organic vs Inorganic Lead Transport Mechanisms. In contrast to Pb2+,of which concentrations in Greenland surface snow are considerably lower than those measured in rural areas on the continent, several organolead concentrations were relatively high compared to those reported earlier for some locations in the Outer Hebrides (20) and rural areas in Ireland (21). This confirms the enrichment of organolead vs Pb2+ observed in Europe during the transport of atmospheric pollution from urban in to the rural areas (21) explicable in terms of different atmospheric lifetimes of the different species emitted in the automobile exhaust: Pb2+,tetraalkyllead (TAL), and ionic alkyllead (IAL) (di- and trialkyllead). The lifetime of lead species in the atmosphere is controlled by their scavenging rate and in the case of organolead species by their atmospheric photodegradation. The gas-phase species (TAL and to a large degree IAL) are apparently scavenged less efficiently from the atmosphere through deposition processes than the inorganic lead aerosol (2022). The dominant role of gas-phase transport of organolead to Greenland is thus likely. The vapor-phase TAL is gradually converted to ionic trialkyllead and dialkyllead species in the troposphere which have a fairly long lifetime (5-10 days) and can be advected to remote areas before complete breakdown to inorganic lead has occurred. This lifetime, however, suggested in laboratory studies (22) does not justify such high concentrations to be measured in Greenland, and it must be longer in a real environment. To explain this, an analogy in atmospheric transport pathways between trialkyllead and SO2 (19) and an increased persistence of SO2 in the Arctic (a factor of 10higher compared to Europe) (33)are worth evoking. Interestingly, the SO2 lifetime on the way to the Arctic was shown to be 10-32 rather than a few days as observed on the continent (33). Vapor-
2.0
'.6
1.5
c L
;1.0 C
+ L m C
6
0.5
0
Sampling date Figure 3. Concentration ratio of PbWg/Pbhagfound in surface snow from Dye 3. 25 B Et,PbZ+IEt,Pb' 20
.-
0
e
m,
c '5
.-
c)
E 5
e
10
0
8
5
0
Sampling date Figure 4. Et2Pb2+/Et3Pbt concentration ratio found in surface snow from Dye 3.
phase alkyllead seems to play a key role in prolonging the occurrence of organolead in air similarly as SO2 does for sulfate. Variations Driven by Atmospheric Chemistry. The main sink of organolead is reaction with the hydroxyl (OH) radicals and ozone, the concentration of which in the atmosphere considerably increases as one proceeds from dark winter through polar sunrise into spring. The increasing photochemical atmospheric activity results in speeding up the degradation of organolead to Pb2+,which is reflected by a sharp drop of the Pborg vs Pbinorg ratio elevated in the first 3 months of the year (Figure 3) around mid-April. The lifetime of organolead drops in the summer and then matches that of Pb2+aerosol, so enrichment of organoleadvs Pb2+is no longer observed. It is worth noting that a similar winter maximum for organolead compounds was observed in the only all-year study available to date (14). Note that the drop on April 15is due to an abnormally high concentration of inorganic lead measured on this day. The Pb2+concentration was 3 orders of magnitude higher than that found in the neighboring precipitation events. The explanation of low levels of organolead in summer by a higher photochemical degradation rate is further supported by a rapid increase (a factor of 5-10) in the EtzPb2+/Et3Pb+ratio in April (Figure 4). Organolead Concentrations vs Sources and Transport Pathways. Maximum concentrations of organolead
are generally associated with the Arctic as the direct source which receives significant anthropogenic emissions, a consequence of the extension of the polar front well into mid-latitudinal populated areas in winter and spring (34). European sources are much more easily available to the Arctic than those in Northern America due to peculiar synoptic conditions which result in surges of polluted European air into the Arctic on the upper air (500 mb) level (35). In January and February, the frequency of surges of air from eastern and western Europe is fairly equal, but the latter becomes dominant from March on. Atmospheric pollutant concentrations in the Canadian Arctic show a similar winter-spring maximum (34). Whereas low concentration of the total organolead observed in summer can be explained by atmospheric degradation, the latter does not account for the virtual absence of methyllead while the less stable ethyllead species are present. The explanation for the phenomenon observed lies in the increasing contribution of the U.S. and eastern Canadian emissions, which are cleaner with respect to organolead emissions than Europe. Cleansing in June is less probable as the concentrations of other species remain high (24, 31). A deeper insight in the transport mechanisms can be gained by consideration of lead speciation in individual precipitation events. Speciation of Organolead in Particular Snow Events. There is a clear difference in the behavior of the Pb2+and organic lead concentrations in winter and spring. While low concentrations of Pb2+are measured in winter followed by high concentration episodes in April, high concentrations of organolead are continually measured during the winter-spring period. The lack of a winter maximum for lead and other heavy metals was explained by the fact that only two out of at least six precipitation events were sampled (24). Another plausible explanation may be different transport mechanisms and air mass trajectories (30). Because of a relatively high elevation, Dye 3 may receive the highly polluted Arctic Haze aerosol only via brief incursions when strong aloft air surface mixing occurs, which does not happen at this location before spring. Therefore Pb2+ and other metal species that are advected as aerosols do not show the winter pollution maximum characteristic for sea-level locations in the Arctic (31). However, the gas phase plays an important role in the transport of organolead. Due to the transport of vapor phase at high tropospheric or stratospheric levels, the range of influence of Arctic Haze with respect to the gas phase is extended beyond that which its particulate emissions alone would reach. The high elevation is also responsible for frequent delivery of air from the free troposphere to the surface of the ice sheet. In contrast to aerosol, vapor-phase pollutants are less subject to deposition and washout and consequently have a longer residence time in the atmosphere. That anthropogenic emissions reach Dye 3 in winter is confirmed by CO, which is used as a gas-phase transport indicator (31). Air mass trajectories in winter show central Canada as the source of air arriving at Dye 3 (30). This region is less abundant in Arctic Haze aerosol, which explains low concentrations of Pb2+. The difference in transport mechanisms of Pb2+and organolead is supported by the fact that the variations from one snowfall to another are not so extreme as in the case of inorganic lead, showing a maximum amplitude of 10 (comparing to BOO). Also, concentrations of sulfate (initially emitted and transported Envlron. Sci. Technol., Vol. 28, No. 8, 1994
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as SOz) behave similarly to those of organolead. An additional insight in the transport mechanisms can be gained by considering the distribution pattern of organolead compounds in the winter and early spring snows. If equal concentrations of TML and TEL in the original gasoline are assumed, MeSPb+,Me2Pb2+,Et3Pb+, and EtzPb2+are all present in the vapor phase, and EtzPb2+accounts for above 50% (29). The relatively uniform distribution of organolead species in winter snow closely follows that of emitted species in vapor phase. The distribution pattern changes in April when aerosol-rich highly polluted Arctic Haze may impact Dye 3 (30). The concentration of Pb2+ increases, and Et2Pb2+ becomes definitely the dominant species. This clearly reflects the composition of the aerosol phase of the exhaust emissions. Indeed, despite equal concentrations of TML and TEL in the original gasoline, EtzPb2+ accounts for 90% of the total ionic alkyllead in the emitted aerosol. Also, an enrichment of ethyllead toward methyllead by a factor of 5-20 is observed in the aerosol phase (29). Note that ionic alkylleads are emitted in roughly equal proportions in vapor phase and in aerosol while TAL is practically absent in aerosols. In contrast to the emissions, aerosol organolead seems to be insignificant in ambient urban air compared with organolead in the gas phase (19). This may be due to rapid deposition of large droplets and other particulate material from the exhaust fumes. The Arctic Haze aerosol is apparently more stable due to lower temperature. Contrary to sea-level Arctic sites where concentrations remain high for several days, the April episodes at Dye 3 are characterized by rapidly changing (over time periods of 1-2 days) concentrations of the constituents (32). Maxima occur at different times for different species,being a complex function of sources, long-range transport pathways and local conditions. Out of the April episodes, only the samples collected on April 15, 25, and 29 were fresh snow collected just after the event. Samples of April 19 and 22 were older snow of unknown age. Low overall concentrations found in the April 22 snow suggest,however, that this snow when sampled did not reach as far as the April 15deposition. On the contrary, the samples on April 19 may have been possibly mixed with the April 15 snow, patches of which became exposed when wind removed much of the fresh snow. The first April (on the 10th) snow does not differ significantly from the winter precipitation with respect to the distribution of organolead species. Indeed, air mass trajectories still show dominant transport from central Canada (30). The concentrations are lower probably because the polar front had moved further to the north restricting communication between the Canadian Arctic/ Polar Basin and central Canada. The meteorological conditions and back-trajectories are rapidly changing on April 15, indicating the Arctic Basin as the main source (30). This region regularly receives chemical constituents over the Pole from Eurasia and possibly even soil dust from the Sahara Desert transported over Europe (30).The April 15 episode results in very high concentrations of Al, Ca, Fe, Zn, and 'Be but not of S042- (24)or trialkyllead. The lower concentrations of sulfate may be due to scavenging by fog, but different sink mechanisms of the gas-phase organolead and SO2 comparing to particulate Pb2+ are likely to play an important role. The aerosol metal ionic species are readily scavenged, while the 1464
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residence time of the vapor-phase species is apparently longer. This difference is reflected in a few days delay observed in the appearance of a concentration maximum in the precipitation. It is characteristic that the aerosolphillic species sod2-or dialkyllead (ethyl- and methyl-) are alreadyvery high (5 times higher than during the former event) on April 15while triethyllead peaks only somewhat later. The hypothesis of a delayed washout is plausible only on the condition that after the April 15 event relatively little new air arrives and organolead and sulfur from the old atmosphere may still be scavenged. Indeed, between April 16and 19,Dye 3 is under relatively constant pressure, which stabilizes the atmosphere and allows the air masses to spend 2 days in the proximity of Dye 3 (30). The air arriving is relatively wet and therefore able to scavenge the gas-phase organolead. On April 19, the 5-day 775and 700-hPa trajectories stay entirely over the ice sheet. This situation is reflected in high s04'- and organolead concentrations in the April 19th snow but not in Pb2+ which had already been removed earlier. On April 20, fresh air masses arrive from the north. It is reflected in increased concentrations of Pb2+,sulfate, and dialkyllead (both MezPb2+and EtzPb2+)in the April 22 sample, which is aged and not fresh snow. Further, the trajectories on April 22 suggest that Dye 3 may have been influenced by air passing over the west coast of Europe (Ireland and Scotland). These areas were reported to receive in April well-established easterly (continental)flow from overpopulated areas (20)with, interestingly, diethyllead as the dominant organolead species. Finally, the CO concentration confirms the ar.;hropogenic origin of pollution (31). The next snow event (April 25) is associated with oceanic air southeast of Greenland, which could have passed over western Europe. There is also an indication of high-level transport coming from the Canadian Arctic (30). This snow event shows the lowest concentration of most chemical species but not of organolead. This may be explained in terms of the considerable time spent by air arriving at Dye 3 over the ocean resulting in efficient removal of particulate matter by precipitation en route. Scavenging of alkyllead is less efficient than that of Pb2+, which is reflected by a very high organoleadllead ratio (2%) on this day. The smaller EtzPb2+/Et3Pb+ratio may be due either to the lower photochemical activity on this day or to easier scavenging of more aerosolphillic Et2Pb2+ during transport. It is also noteworthy that the accumulation was relatively large on April 25. It implies, on one hand, that any chemical constituents scavenged early in the storm would have been diluted by subsequent clean snow and, on the other hand, that less efficiently washed out organolead compounds get an opportunity to be scavenged more quantitatively. The summer short-range transport (within 2000 km) is reflected by the low concentrations of organolead found in the June snow, although those of other metallic species still remain at elevated levels due to the proximity of source regions. In July and August, air masses spend considerable time over the warm Atlantic before arriving at Dye 3. It is mirrored in the overall drop of the pollutant concentrations because air masses are well-cleansed en route when carried to high elevations. The July and August air mass trajectories are reflected by the higher Pbo,g/Pbi,o,g and
the tri/diethyllead ratios which indicate short-range transport and scavenging according to different sink mechanisms as discussed above. Similarly as in the case of SOz, the residence time of organolead in the Arctic atmosphere may also be controlled by dry deposition. Table 1 shows however that, in contrast to Pb2+,concentrations of organolead are generally lower in aged surface snow than in fresh snow. This may be due to negligible dry deposition of organolead but also to its decomposition during aging under exposure to ultraviolet radiation. Surface Snow from the Summit Site. Different source regions and transport pathways may influence central Greenland because of the spatial separation from Dye 3 and its even higher elevations. The Summit site is situated in the interior of a large ice sheet at high altitude (3150 m), which suggests that the upper troposphere and stratosphere should be relatively important as sources of chemical constituents. The Summit is an integral component of the zonal circulation of the Northern hemisphere, generally acting as an impediment to west-east air movement (36). The differences in concentrations among the snow samples from the same area can be seen. They are due to the lack of homogeneity not only in the horizontal but also in some cases in the vertical distribution of organolead as the snow samples analyzed could comprise several precipitation events (aged snow). A nonhomogenous horizontal distribution of contaminants in snow (37) was reported to hamper the interpretation of vertical concentration gradients. Note that the majority of the 1987 results were obtained at the early stage of the method development, under suboptimized conditions so a precision as low as 20% is probable. In terms of the absolute values, the Pborg/Pbinorg ratio and speciation pattern and the concentrations of alkylleads found at the Summit site are similar to those at Dye 3 in the spring. The Arctic Haze aerosol containing emissions from Eurasia is the most probable source. The Euroasiatic origin of lead in the Summit snow is confirmed by isotopic measurements (18,38). There is, however, some shift in timing observed for the 1987 snow. It may be partly explained by the fact that aged snow was sampled possibly containing precipitation from the April events. However, this explanation is not valid for fresh snow sampled at the Summit in June and July 1989 for which a difference in geographical location should be considered. The latitudinal extent of Greenland increases the potential for northsouth variability in the timing and composition of the aerosols. In view of the similarity of organolead concentration and distribution, it is reasonable to assume that the Summit even in summer may be impacted by the injection of Polar Basin aerosol. This may be due to the more northern position of the Summit than Dye 3. Biomethylation. Relatively high ratios (about 1% comparing to an average of 0.1 % in continental deposits) of Pborg/Pbinorg found occasionally in the deposited snow are without doubt a reflection of even higher Pb,,,/PbinOr~ ratios in air. The elevated Pborg/Pbi,orE ratios measured in air (which had passed over clean sea) sampled in remote rural areas were often interpreted as evidence for natural production of organolead (biomethylation) (20, 39). Selective scavenging of Pb2+vs organic lead and consequent enrichment of the latter were proposed to account for these elevated ratios, but they failed to explain the presence of
organolead at a remote island site receiving masses which had not traversed any land surfaces (20). Long-range transport was not taken into account due, it was assumed to the instability of organolead. In view of our results, maritime biomethylation of lead is considered highly unlikely. None of the air masses which had been spending a long time over the ocean gave rise to a snow deposit with measurable concentrations of methyllead in mid-summer at the apogeum of ocean bioproductivity. On the contrary, methyllead species were measured in the winter and spring months, indicating a possibility of long-range transport of organolead from Europe. Note that there is an apparent correlation of methyllead concentrations with those of sodium species that is regarded as a marine aerosol indicator. The elevated Na concentrations observed in the winter and early spring are likely to be derived either from the winter Arctic aerosol (sea-salt aerosol can represent up to 50% of the total mass) (40) rather than from late winter cyclonic storms that pick up sea-salt aerosol when crossing the North Atlantic. Even if the latter explanation should be true, the elevated methyllead concentrations may be explained in terms of the preconcentration of organolead (also ethyllead) in the surface microlayer of the ocean enriched in oily material.
Conclusions The study has provided the first data on the levels of organolead compounds in the remote environment free from direct automotive emissions. Organolead compounds appear to be muchmore persistent in the atmosphere than expected on the basis of laboratory experiments. They are enriched vs inorganic lead during transport over long distances. The work has enabled us to identify long-range sources responsible for the Arctic pollution by organolead. Maximum concentrations of organolead are generally associated with low temperatures and north winds, indicating the Arctic Haze containing European emissions as the direct source of pollution. On the other hand, the low concentrations observed in summer originate in the eastern U.S.and Canadian emissions with the possibility of European injections well-cleansed over the Atlantic. The work contributes to improving our understanding of the fate and cycling of organolead compounds in the atmosphere. The absence of methyllead in the precipitation form of warm oceanic air masses makes the process of natural methylation of lead very improbable. A deeper insight in the pollution of Greenland by organic lead can be gained by the discussion of the historical record of the organolead pollution described in the followingpaper (41).
Acknowledgments This work was financially supported by the Commission of European Communities and Switzerland as part of the Eurocore program, by the Belgian government as part of the Global Change program, by the French Ministry of the Environment, and by the US. National Science Foundation. We thank C. I. Davidson, R. Delmas, J. L. Jaffrezo, and P. Mayewski for their participation in field sampling.
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Received for review October 8, 1993. Revised manuscript received March 24, 1994. Accepted April 11, 1994" Abstract published in Advance ACS Abstracts, May 15,1994.
