2.1 Aerosol sampling

Two low-volume sampling systems were deployed simultaneously to collect the aerosol onto membrane filters (37 mm diameter). A Sartorius collector (42 1 min⁻¹ displacement) powered by a 500 W generator drew air through two sampling probes in parallel, at a flow rate of 11 ± 1 1 min⁻¹ for a 0.45 μm pore-size filter (face velocity, 21 ± 2 cm s⁻¹). In the second system a Gast rotary-vane air pump (242 1 min⁻¹ displacement) powered by a 1 kW generator was connected to three sampling probes giving a flow rate of 16 ± 2 1 min⁻¹ per filter. In both cases the air was exhausted via gum rubber tubing through a dry gas meter and a flow-rate meter. Both the temperature of the exhausted air and the atmospheric pressure were monitored. The generators were modified for use with propane to avoid leaded fuel.

The filters were taken into the field in disposable styrene filter holders. These were mounted face downwards in acid-washed, heavy-duty polypropylene jars to prevent blockage by snowfall or drift. The probes were suspended about 1.5 m above the snow surface from nylon lines which were supported by acetal poles. The pumps and metering systems were sited 12 m downwind inside a snow shelter to reduce contact with the ambient air. The generators and camp site were in turn 200 m further downwind and the generators were connected to a wind-speed and direction switching unit which turned the system off if the wind slackened or moved into an unfavourable sector. The limits were usually 0.7 m s⁻¹ or ±90° vector from the direct line between the camp site and the sampling area. Throughout, the sampling and pumping areas were approached only by personnel wearing full clean-area garments. There were no snowmobiles at either sampling site. Filter changeover could be achieved rapidly with minimal risk of contamination. After exposure the filter holders were double-packed in pre-cleaned polyethylene bags and returned to the laboratory under deep freeze. Blank filters were treated in exactly the same way and left exposed for the duration of a normal run alongside authentic samples. In these cases air was pumped through the filter for a few minutes only.

2.2 Snow sampling

Areas were set aside cross-wind from the air sampling probes for the collection of both fresh and aged surface snow. Samples were scooped upwind of the operator directly into pre-cleaned polyethlene (LPE) 2 1 bottles. These were sealed into polyethlene bags and returned under deep freeze to the laboratory. During prolonged snowfall, fresh snow was collected at regular intervals from the upper 2 to 3 cm in an attempt to monitor changes in composition during the course of the snowfall.

3.1 Filter selection and pre-cleaning

Standard aerosol collections were made on 0.45 μm pore size, tortuous pore, membrane filters. Their collection efficiency is essentially quantitative for particles larger than 0.02 μm in diameter (Reference Megaw and WiffenMegaw and Wiffen 1963). In order to reduce the filter contribution to the blank the trace-element content of several filter types was determined. The optimum filters were then pre-cleaned before mounting them in their disposable filter holders for shipment.

The heavy-metal content of eight types of filter and three types of support pad were determined under similar conditions used for analyzing authentic samples. Each filter was placed in a PTFE beaker using PTFE tweezers. 5.3 ml of “purest water” was pipetted into the vessel and acidified to pH 2.7 with “Aristar” HNO₃. After intermittent agitation for 30 min, aliquots of the solution were transferred to 30 ml FEP bottles. The samples were analyzed immediately for Cd, Cu, Pb and Zn by direct-injection flameless AAS. Analyses of 12 filters and six support pads of each type were carried out and the mean trace contents of the filters with their standard deviations are presented in Table II.

Table II

Composition of Different Filter Types (concentrations in ng per filter)

Although the PTFE filters appeared to give the most favourable blanks this may simply be a reflection of their poor wetting properties. The use of a wetting agent such as ethanol has been suggested (Reference Derrick and MoyersDerrick and Moyers 1981) but this would increase the risk of contamination. Nucleopore polycarbonate filters eluted small concentrations of the heavy metals but pre-cleaning proved difficult due to problems with handling the filters whilst wet. Finally it was decided to standardise on Sartorius SM111 06 filters (0.45 μm pore, cellulose acetate) which offered a good compromise, with low initial heavy-metal content combined with ease of handling whilst wet. Millipore AP 1003700 support pads were used as supplied since tests showed that contact between the filter and the support pad and the filter holder did not significantly raise the blank.

The filters were cleaned by soaking in three changes of “purest water” acidified to pH 2.7 for periods of 1 h, rinsing with “purest water” between soaks. Finally they were rinsed twice in “purest water” and dried overnight on a PTFE grid inside a polycarbonate dessicator. Full procedural blanks for the heavy metal contribution from the pre-cleaned filters are given inTable II. They are some four times smaller than the values for un-cleaned filters and ten times smaller than the blanks reported by Reference Peel and WolffPeel and Wolff (1982). The corresponding blanks for other elements studied here were: A1 (9.6 ± 5.0 ng per filter), Ca (9.6 ± 1.9 ng per filter), Na (10.1 ± 7.0 ng per filter) and K. (12.1 ± 9.8 ng per filter).

3.2 Filter analysis

Filters were transferred to a PFTE beaker using PTFE tweezers and eluted with 7.42 ml “purest water” acidified to pH 2.7 with “Aristar” HNO₃ for 1 h. There was a 10 min period of sonication at the beginning and end of the process. Then 4 ml of the eluate was pipetted into a 30 ml FEP bottle and immediately frozen. These samples were melted within 48 h and analyzed for Al, Cd, Cu, Pb and Zn by flameless AAS using multiple injections. The residual eluate was pipetted into a second FEP bottle, weighed and diluted with 3 ml “purest water” and then frozen. Within 72 h these samples were analyzed for Ca by flameless AAS and for Na and K by flame emission spectrometry. Calibration sequences using mixed standards were distributed within authentic analyses. Simulated samples analyzed before and after freezing showed no significant change in composition. The rather mild acid conditions used in the elution step paralleled conditions required in our procedures used for analyzing heavy metals in snow samples (Reference Wolff and PeelWolff and Peel 1985). At the same time they avoid dissolution of the filter matrix which may increase the blank contribution. There is however a risk that a significant proportion of the heavy metals and A1 may not be released.

A check on the efficiency of extraction and acid-releasable elements was made by carrying out a second identical elution step immediately following the first one. Six exposed filters and three blank filters were examined. More than 90% of available Zn and Cu was removed in the first elution of five filters; concentrations in the sixth filter lay close to detection limits (blank levels). The levels of Pb and Cd were undetectable above the blank in these samples. The marine ions were never detected in the second eluate above the amount carried over in the wet filter (~10%).

Several investigations (Reference Nguyen, Valenta and NürnbergNguyen and others 1979, Reference Nürnberg, Valenta, Nguyen, Georgii and PankrethNürnberg and others 1982, Reference Rohbok, Georgii and PankrethRohbok 1982) on rural rainwater have shown that more than 90% of available Cd, Cu, Pb and Zn is dissolved at around pH 3. Consequently we believe that the analytical conditions release into solution a high proportion of the heavy metals involved in long-range transport. A small component will remain bound firmly as insoluble silicate complexes.

The efficiency of extraction of aluminium was investigated on filters exposed near Cambridge. Filters were exposed in pairs. One filter was taken through two elution stages following our normal procedures; the other was analyzed by neutron activation analysis (INAA), at the Atomic Energy Research Establishment, Harwell. More than 92% of A1 was analyzed in the first elution stage using HNO₃ at pH 3. There was a good correspondence between the data obtained from INAA and from flameless AAS.

3.3 Snow sample analysis

Working in a laminar-flow work station at −22°C subcores were taken from the snow samples using a PTFE tube (20 mm inside diameter). In general two sub-samples were taken from each container. The first sampled the top 1 to 2 cm of the sample only whilst the second penetrated the centre. In the one case (run 22) where elevated concentrations were detected in the upper sub-sample the results from the central sub-sample are quoted as upper limits. For one sample, three sub-cores were taken across the bottle from the outer surface towards the centre. Constant levels of Na, K, Al and Ca were measured, indicating that there was no significant contamination of the sample by the container walls. The sub-cores were transferred to ultra-clean PTFE beakers where they were melted in a microwave oven. Following acidification to pH 2.7 with “Aristar” HNO₃ the samples were transferred to 30 ml FEP bottles for analysis. Al and Ca were analyzed by direct injection on a flameless AAS. Na and K were analyzed by flame emission spectrometry.

4.1 Trace elements in the Antarctic aerosol

Most of the aerosol data from Antarctica available for comparison have been obtained from central areas of the ice sheet. We must consider whether data collected from coastal sites are representative of a wider area. Long-range transport of primary aerosol from distant sources to Antarctica seems to take place in the middle troposphere generally by a process of eddy diffusion (Reference ShawShaw 1979). Once over the continent slow vertical mixing processes transfer the aerosol to cloud levels where it is removed rapidly (in 1 to 2 d) by scavenging processes. Relatively long residence times (1 to 5 weeks) of aerosol in the Antarctic troposphere above the boundary layer, in combination with vigorous zonal mixing of air masses surrounding the continent, should yield a rather constant concentration of aerosols across the continent. This is supported by direct observation of a nearly constant total aerosol mixing ratio in the middle troposphere between the South Pole and the coast at McMurdo station (Reference HoganHogan 1979). Hence the contribution of “background” aerosol in the Antarctic Peninsula should be similar to that found in the interior. Superimposed on this there may be more localized intrusions of aerosol from the neighbouring ocean and from exposed rock.

A1 is often used as an indicator of crustal material. The average airborne concentration of A1 from the two sites on the Antarctic Peninsula (Table V) is similar to that reported by Reference Maenhaut, Zoller, Duce and HoffmanMaenhaut and others (1979) and by Reference Cunningham and ZollerCunningham and Zoller (1981) for aerosol collections made during several summer seasons at the South Pole. Insoluble microparticles, which are mainly of crustal origin, have been examined by Reference Thompson and Mosley-ThompsonThompson and Mosley-Thompson (1982) in firn cores collected from interior and coastal areas of Antarctica including two sites from the Antarctic Peninsula. A close similarity in the size distribution of microparticles at such widely separated sites suggests that they are derived from well-mixed and long-range sources. Hence local rock exposure in the Antarctic Peninsula does not seem to make a significant contribution to the input of crustal elements to large snowfields which have been studied in the area south of 70°S.

Table V

Mean Trace Element Concentrations in Antarctic Air (concentrations in pg m⁻³ STP)

Airborne concentrations of Na, K and Mg at the South Pole are on the average three to ten times smaller than values reported here (Table V). Our data from both sites show high levels of correlation between these elements (Na:Ca, r=0.996; Na:K, r=0.992) and the ratios between the concentrations of these elements conform closely with that of bulk sea-water. In the interior of Antarctica there is evidence for a crustal contribution to both Ca and K (Reference BoutronBoutron 1982) which is masked in the Antarctic Peninsula by the proximity of the oceanic source. This source would however not be expected to make a significant contribution of Cd, Cu, Pb and Zn to the aerosol. Although an enrichment of Pb in the surface layers of the ocean and in sea spray has been reported, Reference Boutron and PattersonBoutron and Patterson (1983) estimate that this would lead to a concentration of only 0.025 pg g⁻¹ Pb in snow in the Antarctic interior. Allowing for an input of marine aerosol that is ten times greater to the Antarctic Peninsula the associated contribution of Pb would be 0.25 pg g⁻¹ (around 4% of the present-day concentration of Pb in Antarctic snow reported by Reference Boutron and PattersonBoutron and Patterson (1983) and Reference Wolff and PeelWolff and Peel (1985). Unless this oceanic source has been grossly underestimated we therefore believe that similar concentrations of these elements should exist in the Antarctic Peninsula and in the interior.

Although we are still able to present upper limit values only for the airborne concentrations of Cd and Pb (Table III) it is evident fromTable V that the concentration or upper limits of Cd, Cu, Pb and Zn here are substantially lower than values reported from the South Pole by Reference Maenhaut, Zoller, Duce and HoffmanMaenhaut and others (1979) and by Reference Cunningham and ZollerCunningham and Zoller (1981). They are also smaller than values previously reported from the Antarctic Peninsula (Reference Peel and WolffPeel and Wolff 1982). In all these studies it was necessary to apply large and uncertain blank corrections to the data. Peel and Wolff reported that from 25 to 50% of the total mass of heavy metals collected was contributed by the blank. Although we have been able to reduce both the size and variability of the filter blank it still makes a significant contribution to the sample. It is possible that improvement in techniques may reduce these concentrations even further.

Reference Boutron and PattersonBoutron and Patterson (1983) cited evidence that recent measurements from American Samoa have shown lower airborne concentrations of Pb than any values previously reported from Antarctica. Antarctic air should contain lower concentrations of Pb since the continent is much more remote and is protected from the mid-latitude circulation by the polar convergence zone around 50°S. Based on their data from recent snow they suggested an upper limit of 5 pg m⁻³ for Pb in air over the Antarctic ice sheet. Our data tend to support this suggestion and we must conclude that previous literature values for heavy metal concentration in Antarctic air are probably strongly influenced by contamination.

4.2 Crustal enrichment factors for heavy metals in the aerosol

Crustal weathering is probably the principal natural source of heavy metals in the aerosol. In order to identify whether additional sources exist it is common practice to calculate enrichment factors (EF_crust) for each element relative to major reference elements (in this case A1) of exclusive crustal origin. Here these have been calculated using crustal abundances estimated by Reference TaylorTaylor (1964) as follows:

where X is the concentration of the heavy metal element. Averaged EF_crust values from all our air sampling runs are given inTable VI in comparison with previous data from Antarctica. Our data can only be regarded as upper limits yet they still show that the EF_crust values for these elements may be very much lower than previously reported. Reference Wolff and PeelWolff and Peel (1985) report EF_crust values for the heavy metals in recent summer snowfall from the Antarctic Peninsula which are somewhat lower than the values given here for the aerosol. Their values (with the exception of Cd) are similar to estimates of the ratio of present-day anthropogenic emission rates to natural airborne emission rates for these elements. Hence it seems likely that there may be no need to seek elaborate mechanisms for natural enrichment of these elements in the aerosol which have proved difficult to substantiate in the past (Reference Ng and PattersonNg and Patterson 1981).

Table VI

Mean Crustal Enrichment Factors for Heavy Metals in Antarctic Air

4.3 Short-term variations in the aerosol

Within the limits of experimental error no significant differences in the heavy metal content of the aerosol were detected between individual sample collections at each site or between collections at the two sample sites (Table III). In contrast, large variations were observed for both A1 (up to a factor of 9) and for the marine cations (up to a factor of 64). This suggests that crustal weathering and ocean spray are not major sources of heavy metals in the present-day aerosol.

The highest concentrations of A1 were all recorded during runs undertaken under clean-air conditions where there was no precipitation (runs 6, 12, 21, and 26) and these values may be representative of the background aerosol. The marine cations showed much more variability, probably due to the proximity of the source. The aerosol burden in this case will be sensitive to local weather conditions over the sea and to the particular track taken by an air mass approaching the sampling site.

Three groups of sampling runs spanned periods of continuous snowfall (3-4-5, 14-15-16 and 22-23-24). A systematic decrease in the concentration of both A1 and the marine cations was observed during the most stable event (3-5) and during the first two days of events 14-16 and 22-24. In these latter cases a further increase in concentration occurred on the third day, possibly associated with a change in wind direction and a new weather system. These decreases in the aerosol concentration probably reflect a progressive scavenging of the aerosol by snowfall. The effect has been noted by several authors for precipitation events in less remote regions (Reference Nürnberg, Valenta, Nguyen, Georgii and PankrethNürnberg and others 1982).

4.4 Relationship between aerosol and snowfall composition

Reference JungeJunge (1977) has considered that four major processes are responsible for removing aerosol from the atmosphere: nucleation of cloud droplets, scavenging of unactivated aerosols within cloud, scavenging of aerosol below cloud by precipitation, and dry deposition. He concluded that nucleation was the dominant removal mechanism for the clean-air conditions which occur over the polar ice sheets. Reference ShawShaw (1980) has examined the size distribution and vertical variation of the total aerosol over the Antarctic ice sheet and calculated that 90% of the total aerosol could be removed by nucleation. For this process Junge showed that the concentration of a given element in air (C_air) should be proportional to its concentration in snowfall (C_snow) according to the relationship:

where L is the liquid water content of the cloud, ε is the mass fraction of aerosols used for condensation (expected to lie in the range 0.5<ϵ<1.0) and η is a factor (>1) to allow for evaporation of snowflakes below the cloud base. ϵ, which is effectively a nucleation efficiency, may be sensitive to the size, shape and chemical properties of the aerosol. η is dependent on both the height of the cloud base and the relative humidity of the air below the cloud.

It is important to determine how far ϕ can be considered constant for a given trace element over a range of weather conditions and across a wide geographical area. If fractionation is a significant factor then different values of ϕ may be expected for different types of element. In Table VII we give values of ϕ for A1, Na, Ca and K from individual runs in which air and snowfall were sampled simultaneously. In all cases, during snowfall events the stratus cloudbase lay within 100 m of the surface and the relative humidity near the surface was in the range from 86 to 100% so that the possibilities for below-cloud scavenging and evaporation of snowflakes were very limited. The averaged ϕ values for A1, Na and Ca are similar, indicating that there is no important fractionation between crustal and marine-derived aerosol. A value for L (Equation (1)) in the range from 0.21 to 0.62 g m⁻³ is implied which is reasonable for stratiform clouds at the temperatures (−1 to −16°C) encountered during sampling.

Table VII

Values Of ϕ (Concentrations in Air pg m⁻³ STP/Concentrations in Snow pg g⁻¹) For Major Elements from Simultaneous Sampling in the Antarctic Peninsula. Comparison with Averaged Values Obtained from Separate Sampling in the Antarctic Peninsula¹ and at the South Pole²

The only values of ϕ available for comparison from simultaneously sampled air and snow from Antarctica were reported by Reference Peel and WolffPeel and Wolff (1982) from three different sites (marked by triangles in Fig.1) in the Antarctic Peninsula. Their average value for the marine ions Na, Mg and K was 0.54±0.37, which is in good agreement with the value of 0.57±0.29 obtained here for Na and Ca.

Although simultaneous sampling has not been reported at the South Pole, extensive aerosol data have been obtained over four summer and two winter seasons during the period 1970 to 1978. Cunningham and Zoller (I98I) presented averaged data for summer and winter concentrations for a range of trace elements over this period. These data can be compared with values reported by Reference BoutronBoutron (1982) for South Pole firn which cover the same period. The resultant values of ϕ for A1 and the marine ions are reported in Table VI. Values of ϕ obtained for the marine ions at the South Pole are slightly, yet consistently, larger than values obtained from simultaneous air and snow sampling in the Antarctic Peninsula. Reference Pourchet, Pinglot and LoriusPourchet and others (1983) have estimated that 40% of the radionucleide content of South Pole snow may result from dry deposition. If applicable to the marine ions, this would lead to a proportionate increase in the values of ϕ if air and snowfall were sampled simultaneously at the South Pole. For a remote site in south-central Greenland, Reference Davidson, Chu, Grimm, Nasta and QamoosDavidson and others (1981) have reported a substantially larger value for Mg (ϕ = 5.6) for samples collected simultaneously. A possible explanation is that in the Antarctic Peninsula the marine aerosol is much closer to its source and will have a larger size distribution. More efficient removal by nucleation (i.e. ϵ closer to 1 in Equation (1)) might therefore be expected. The two data sets yield similar values of ϕ for A1. This may indicate that dry deposition does not make a significant input to South Polar snow, in contrast to the findings of Reference Pourchet, Pinglot and LoriusPourchet and others (1983) for the radionucleides. Direct analysis of fresh and aged snowfall at the South Pole is needed to resolve this question.

We do not yet have data for the heavy metal content of the simultaneously sampled snowfall. However, Reference Wolff and PeelWolff and Peel (1985) have analyzed Cd, Cu, Pb and Zn in fresh summer snowfall collected during February 1980 from sites on the southern Palmer Land plateau. Similar concentrations for each element were recorded over the 100 km sampling line. Five samples drawn from a section of firn representing approximately 6 months snowfall from Spaatz Island, which lies close to the Beethoven Peninsula occupied here, gave concentrations of Pb within the range of values observed on the plateau. Hence it appears that the spatial and temporal variations of these elements may not be great. We have therefore estimated provisional values of ϕ for these elements by comparing average airborne concentrations observed here with average snowfall data from Wolff and Peel. The results are summarised in Table VIII along with ϕ values compiled by Reference Boutron, Davidson and NriaguBoutron (in press) for the South Pole based on non-simultaneous collections during the period 1970 to 1978.

Table VIII

Values of ϕ (Concentration in Air, pg m⁻³/Concentration in Snow, pg g⁻¹) For Heavy Metals in the Antarctic Peninsula and at the South Pole

Although our values are necessarily tentative they still show good agreement with values predicted theoretically by Reference JungeJunge (1977). Agreement with the South Pole data set is probably fortuitous since the snowfall concentration data from this site have subsequently been demonstrated by Reference Boutron and PattersonBoutron and Patterson (1983) to be erroneous and the atmospheric concentrations are also very much larger than concentrations reported here. For the Antarctic Peninsula the values of ϕ for the heavy metals are higher than those for the major crustal and marine elements, but this too may reflect a residual contamination problem in our air sampling data.

Reference Ng and PattersonNg and Patterson (1981) have given evidence from several remote areas that present-day Pb/Si dust ratios are, on the average, 10-fold smaller in the air than in rain or snow collected at the same time. They argue that industrially produced Pb is associated with particles much smaller than natural dust particles and can consequently be scavenged by diffusion much more efficiently onto cloud droplets or snowflakes. If this is a global phenomenon, as indicated by the authors, then it would have important implications for interpreting long time series of Pb concentrations in Antarctic ice cores in terms of atmospheric trends. However, the only data cited in support from Antarctica must now be considered suspect. We have used A1 as a crustal index and this would be expected to behave in exactly the same way as Si. If the enrichment mechanism were operating we would expect to see values of ϕ for Pb some 10-fold smaller than those for A1. This is not substantiated by the data available from the Antarctic Peninsula. The strongest support for Ng and Patterson’s proposition came from sites in the northern hemisphere. Anthropogenic aerosol arriving in Antarctica will have travelled much further in the atmosphere and is unlikely to have retained its original identity. More precise data from Antarctica are now required to help resolve this important question.