The solute and particulate chemistry of background versus a polluted, black snowfall on the Cairngorm mountains, Scotland

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Vol. 23, No. 2,9~. 3954JLl989.

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THE SOLUTE AND PARTICULATE CHEMISTRY OF BACKGROUND VERSUS A POLLUTED, BLACK SNOWFALL ON THE CAIRNGORM MOUNTAINS, SCOTLAND SHELDON LANDSBERGER,* TREVOR D. DAVIES, MARTYN TRANTER,tll ABRAHAMS~

and

PETER W.

JOHN J. DRAKES

School of Environmental Sciences, University of East Anglia, Norwich, Norfolk NR4 7TJ, U.K., * Department of Nuclear ~n~n~~ng, University of Illinois, Urbana, IL 61801, U.S.A. and t ~pa~ment of Oceanography, The University, Southampton SO9 SNH, U.K. (First received 27 August 1987 and received for publication 8 August 1988) Abstract-The solute (Al, Br, Ca, Cl, Cu, Fe, I, Mg, Mn, Na, Pb, S and V) and particulate (Al, Ba, Br, Ca, Cl, Co, Cu, Dy, I, Mn, Na,Ti, U and V) chemistry of a relatively unpolluted snowfall, associated with a maritime airmass, is presented, to characterise background conditions for the region. The variability of the concentration of solute and the chemical composition of particulate material is investigated on an intra- and inter-site basis. The seasalt solute component is less variable than the terrigenous component. Hence, the aerosol scavenged by the snow is assumed to be a mixture of at least two components. The solute content of a relatively polluted, ‘black’ snow is distinctly different from background snowfall. However, there is little difference in the chemistry of particulate material with diameter >0.45 pm. Most lithophiles have enrichment factors (EF) close to 1, whereas only the chalcophiles and halogens have EF > 10. At most, the EF of each of the 14 elements considered differs by a factor of 5 between potluted and background snow. Particulate material gathered from within snowpack in the Same region has a similar range of EF values to those obtained from both snows. There is the potential for toxic effects associated with trace metal release during snbwmelt of both polluted and marine snows. Key word index: Snow, chemistry, INAA, particulates, solute, enrichment factors, environmental pollution.

INTRODUCTION

METHODOLOGY Sampling

In the Scottish Highlands, the solute content of snowfall is dominated by two components. One is predominantly marine in origin, the seasalt component while

the other is pr~ominantly polluted, an acidic component (Brimblecombe et al., 1985; Tranter et al., 1986). Individual snowfalls may show a range of compositions, reflecting different proportions of marine and polluted components (Brimblecombe et al., 1988). In this paper, we examine the composition of solute and particulate material contained within a fresh, relatively unpolluted snowcover, associated with a maritime airmass, and make comparison with the composition of a polluted black snow. In so doing, we aim to indicate the differences in the proportional chemical composition of both solute and particulate material which may be encountered over a sampling season. Implications of the toxic metal content of marine and polluted snow are also considered.

I/To whom all ~~~nden~ should be addressed. Present addresses: t Department of Geography. University College of Wales, &er&vyth SY23 3I%,*U:K., $Dep&tment of Geography, McMaster University, Hamilton, Ontario, Canada IX 4MI. 395

All sites are located within a 60 x 60 km area of the Scottish Highlands (see Fig. l), and are relatively remote from large pollution sources (Davies et al., 1984). All samples were collected at least 100 m upwind of roads, which are mainly little used country lanes. Sites varied in altitude between 220 and 590 m, and slopes varied from 0 to 25” (azimuth 32-324”N), except for site 8, at N 1080&m,which is a research site at Ciste Mhearad, in the Caimgorm Mountains (Tranter et al., 1986). A snowfall occurred on 23 March 1986, which was associated with a maritime trajectory (see Fig. 2). Within 2-3 h of the fall, a sampling programme was executed to establish the degree of small-scale variability in the particulate composition of the event at Site 1. A similar study had been previously undertaken to determine the small-scale variability in the major ion composition of snowfall at Site 8 (Tranter et al., 1987); this earlier study had highlighted the relatively high degree of spatial variability in the composition of snowfall in mountainous areas, and had outlined some of the implications for sampling and interpretation. For the present study, a lower level site, of altitude 290 m, of shallow slope (slope= lo”, azimuth

396

SHELDONLANVSBERGER et al.

SNOW

SAMPLING

SITES

l

Sample

Site

PITLOCHRIE

0

-4

Fig. 1. Location map of Sites l-10. Samples from Site 7 were not included in this

study.

the next 30 h, single samples were collected at Sites

Fig. 2. Back trajectories (6-h intervals) for the ‘black snow’ (denoted 13)and the unpolluted ‘marine’events (denoted 23 and 24), which were observed in the Cairngorm Mountains, Scotland. The trajectories were constructed using the surface geostrophic technique.

= 122” N), and with no major obstacles, was chosen (Site 1). Three samples were collected at intervals of 10 m on each of two parallel transects, 30 m apart. The location of the first transect was randomly located. This study is called hereafter the intra-site suroey. On the following day, 24 March 1986, there was another snowfall in the early morning, which was associated with a similar trajectory (see Fig. 2). Over

2-10. These samples comprise the inter-site survey, and represent relatively fresh snowfall collected over an area of -3600 km*. The intra-site and inter-site surveys consist of relatively unpolluted snow. To examine the likely range of composition of particulate material in different snowfalls over a relatively short period (i.e. variability between snowfalls), we also examine one of the most polluted snowfalls which occurred in the area in 1986. On 13 March 1986, there was a pronounced ‘black’ acidic snowfall (pH 3.4) associated with a trajectory which originated in Eastern Europe (see Fig. 2). These heavily-polluted snowfalls are not uncommon in the Cairngorm Mountains (Davies et al., 1989b), and represent the most acidic snowfalls of the season (Davies et al., 1984). The sample was collected within a few hours of falling at Site 8. Samples of fresh snow were collected with small, acid-washed, PTFE-coated, plastic scoops. Care was taken to collect only the recently-fallen snow throughout the complete depth of accumulation (- 5 cm). Between 2.5 and 3 kg of snow was collected at each site, and was stored in large, polyethylene bags. The samples remained frozen until treatment, up to 3 days later. Melting was achieved by partially immersing the polyethylene bags in water at 40°C. Holding experiments show that the polyethylene bags do not release significant quantities of trace metals to the solutions examined (Blackwood, pers. comm.). Immediately after complete melting, which required approximately 60 min, the meltwater was vacuum-filtered through acid-washed. , 0.45 8 urn Millinore filter membranes. The 1

397

Solute and particulate chemistry of background vs polluted snowfall

filtrate was collected in acid-washed flasks, which were initially rinsed with 100-150 ml aliquots of filtrate to avoid contamination. A 50 ml aliquot of filtrate was transferred to an acid-washed, polythene bottle, and acidified with 1 ml of Aristar HNO,. The filter membranes were placed in acid-washed plastic containers and sealed. All samples were shipped to the Nuclear Reactor Plant, McMaster University, for analysis. In view of the following discussion of the particulate chemistry of the marine snows and the polluted, black snow, samples were collected from previous heavilycontaminated coloured events which had fallen in mid-February 1986 (Davies et al., 1989a), and were accessed by digging a snowpit at Site 8. These snowfalls were associated with back-trajectories originating in Eastern Europe. Some limited leaching of these layers had occurred, but inspection of the snowpit revealed that particulate removal had been limited. Six samples, representing subsections over a depth of _ 1.3 m, were collected using acid-washed, PTFEcoated scoops. The two subsections nearest the top of the profile consisted of relatively clean, recent snowfall; the next three subsections consisted of greycoloured snow; the last subsection consisted of clean, metamorphosed snow. A seventh subsection, representing the bottom 0.3 m of the profile, was not analyzed, because of contamination during transit. Little grey or coloured material had penetrated the lowest two subsections. Analysis

Both Neutron Activation Analysis (NAA) and Inductively-Coupled Plasma Analysis (ICP) were used to determine the soluble fraction. The particulate material, typically weighing 2-10 mg, were analyzed using only NAA. Standard reference materials, NBS 1643 B and NRC SLRS for water, and NBS 1632A and NBS 1632B for particulate material, were also analyzed and results were within 10% of the certified or information values. A detailed description of the analytical procedures is to be published at a later date. Typical detection limits and the precision of the measurements at the concentrations encountered during this study are presented in Table 1.

RESULTS

Table 1. Detection limits and precision at concentrations presented herein Solute Detection limit (@I_‘) Al Br Ca Cl cu Fe I Mg Mn Na Pb SO, V

Precision (%)

4

4

3 110 50 4 2 1 220 1 40 I 12 0.1

3 8 4 25 5 10 2 9 3 10 1 14

Particulates

Al Ba Br Ca Cl co cu DY I Mn Na Ti U V AS Ce Cr Eu Fe In La Lu Ni Rb SC Se Sm Ta Th Yb Zn

Detection limit fppm) 60 8 2 110 10 0.1 0.1 1 0.004 0.4 7 20 0.1 0.3 4 4 8 0.3 500 0.004 2 0.1 150 10 0.1 4 0.5 1.2 0.6 0.5 33

Precision W) 3 20 10 15 5 10 10 10 20 10 3 10 30 10 I5 5 5 IO 3 IO 10 10 20 IO 10 10 10 15 5 20 3

Units of concentration denote weight of species per unit weight of particulate material.

Chemistry of the solute Unpolluted snow. Table 2 shows the mean and coefficient of variation (hereafter CV) of the concentration of 13 elements. Results of both regional (or intersite) and intra-site surveys are presented. The solute appears to be derived largely from marine aerosol. This is consistent with the trajectory analysis. The Cl, Na, Mg, SO, and Br ions are within l-6% of their seasalt proportions, using Na as the reference element (see discussion in Keene et al., 1986). The intra-site samples have averaged Cl : Na : Mg : SO, : Br

of 1.02 : 1: 0.95 : 0.99 : 0.94 (1.00 = standard seawater concentrations; Wilson, 1975). Similarly, the inter-site samples have ratios of 0.97: 1: 1.03 : 0.96: 0.94. This suggests that there is little fractionation of Na, Mg, Cl, SO4 and Br during the formation, deposition and dissolution of the aerosol. Br is depleted by 6%, possibly indicating some loss of Br from the original marine aerosol (Raemdonck et al., 1986). In contrast, all other ions are in excess, including Ca (by a factor of 2) and I (by a factor of 60), and are probably ratios

398

SHELDON LANDSEIERGERet

derived from the dissolution of terrigenous or anthropogenic aerosol. Cu is enriched in marine aerosol (e.g. Buat-Menard, 1983), and hence a proportion of the excess Cu could be derived from the dissolution of marine aerosol. The concentrations of Cu, Pb, Mn and V in the marine snow at each site fall between the median values for rural and remote sites (c.f. Tables 2 and 3). The CV of the seasalt ions (Br, Na, Mg, Cl, SO,) in the inter-site survey ranges from 31 to 35%, while the CV of the ions more likely to be derived from the dissolution of terrigenous material (Al, Ca, Cu, Fe, Mn, Pb, V) is more variable, i.e. 1879% (Cu and V are close to detection limit and their low CV should be treated with caution). This could result from the relatively homogeneous distribution of marine aerosol some 70 km inland. In contrast, terrigenous material is likely to be derived from more local, and therefore relatively less homogeneous sources. Aerosols which are mixtures of marine, anthropogenic and terrigenous components are well documented (Heidam, 1985; Schnider, 1987).

Table 2. The solute content of Scottish snowfall

Element Al Br Ca Cl cu Fe I Mg Mn Na Pb SO, V

Inter-site survey Concentration CV (pg(-‘) W) 19.2 30.6 419 9,110 4.36 10.8 3.98 641 3.63 5,210 4.49 1,260 0.18

78 31 47 35 18 55 25 31 79 35 64 31 25

n = 8 (or 7 for Mn and Cu)

Black snow, 14 March Element Al Br Ca Cl cu Fe I Mg Mn Na Pb SO, V Cd Cr Ni

Intra-site survey Concentration CV W) (!JgP_‘)

Concentration

52.2 33.2 310 10,600 5.38 6.13 4.75 673 1.46 6@0 2.79 1,430 0.47 n=6(or5forMnand3 for V)

1986. (fig/-‘)

84k3 65+2 630 k 50 3,200+ 110 12+3 199+ 10 9.7 +0.5 254k3 27+2 827+28 122*5 33,100&201 15+1 <3 <4 < 10

70 3 29 4 57 21 32 4 78 4 66 3 49

al.

Table 3. Median concentrations (from Galloway, Urban cu Mn Pb V

41 23 44 42

of metals in wet deposition 1982)

Rural

Remote -__

5.4 5.7 I2 9.0 (Units are pg(‘)

0.06 0. I 0 0.09 0.16

The recommended upper limits for metal concentrations water (from Galloway et al., 1982). Potable cu Mn Pb V

water

Aquatic

1000 50 50 No standard (Units are PgT-‘)

organism

in

toxicity

20.0 1000 10 500

Rather higher concentrations of the major seawater ions are found in samples collected for the intra-site survey. The CV of the seasalt ions is again less than those of the terrigenous ions, ranging from 3 to 4%. The CV of the terrigenous ions ranges from 21 to 78%. The CV of I is dissimilar to the seasalt ions in both surveys, which may indicate a more complex geochemical cycle of I. Highly polluted ‘black’ snow

In comparison to the relatively unpolluted snows described above, the polluted black snow contains elevated concentrations of all ions, apart from the major seawater ions, Na, Mg and Cl (see Table 2). Normalizing to Na, non-seasalt or excess SO,, Cl and Br is present, which is diagnostic of anthropogenic emissions (Likens and Bormann, 1974; Kallend et al., 1983; Sturges and Harrison, 1986). The concentrations of Mn and Pb are greater than median values for urban sites, while those of Cu and V lie between median values for rural and urban sites (c.f. Tables 2 and 3). Chemistry qfparticulate

material

Marine snow. The range in concentration of particulate material was 1.1-2.7 mg T- ’ for the intra-site

survey and 1.2-3.0 mg L ’ for the inter-site survey (although one sample contained 14 rng!- ’ ). These concentrations are an order of magnitude greater than those found in the Greenland Ice sheet (Davidson et al., 1985). The mean chemical composition and the enrichment factor (hereafter, EF) of 14 elements can be found in Table 4. Some of the variability in concentration may arise from weighing errors and dilution with an Si-rich or a C-rich component. Enrichment factors are derived from the following expression: EFi, rm=(Ci, p/Crs. ,)/‘(Ci, ,,/‘C,,, ,a)

(1)

where C denotes concentration and the subscripts have the following definitions; i=species of interest,

399

Solute and particulate chemistry of background vs polluted snowfall Table 4. The elemental composition and enrichment factors of particulate matter found within Scottish snow

Table 4 (Contd.) Concentration (ppm)

Inter-site survey

Al Ba Br Ca Cl co

cu DY I

Mn Na Ti U V

Concentration (ppm) 20,600 237 14 2,400 199 7.13 118 2.25 5.15 135 2,940

1.350 2 29

CV (%) 12 44 21 52 96 20 20 14 24 66

1 3 30 0.3 1 2 10 3 200 0.6 59 0.6 79 1 0 2 87 1 n=8 (for U, n=3)

64

61 31 19 71 19 52 12 26 36 19 34 12

co cu DY I

Mn Na Ti U V

21,100 215 17.3 2,160 142 4.40 * 4.50 5.33 135 2,850 1,575 1.8 24

Mean concentration (ppm) Al Ba Br Ca Cl co

cu 1 2 30 0.2 8 0.9 * 2 200 0.5

15 36 64

1 2 1

Black snowfall, 14 March 1986 Concentration (ppm) Al Ba Br Ca Cl

I Mn Na Ti U V

52,300 430 110 3,530 2,060 30 170 9.6 131 600 3,370 3,260 2.8 142

As Ce Cr Eu Fe In La Lu

47 79 240 1.3 34,600 1.6 38 0.3

co cu DY

DY I

EF 68 54 16 32 43 20 * 67 36 70 16

EF 1 1

70 0.1 20 2 5 2 1000 0.9 0.2 2 2 40 2 4 2 1 40 2 0.8

4 0.8 0.8 400 1 1 1 0.2 20

Seasonal snowpack

Intra-site survey Concentration (ppm) Al Ba Br Ca Cl

200 44 10.5 14 5.0 1.5 1.3 0.5 1038

Ni Rb SC Se Sm Ta Th Yb Zn

EF

EF

0

Mn Na Ti U V

23 61

7 59 11 9

3 4 5

10 51 5

5 3 5

As E; ;,u In La k Rb SC Se Sm Ta Th Yb Zn

CV (%)

EF

27,200 341 53.3 2,500 850 17.7 161 4.59 30.9 284 2,140 1,740 2.21 75.1

78 32 63 40 89 56 14 62 140 83 63 71 51 74

1 4 90 0.3 30 3 20 4 400 0.8 0.4 1 6 2

0 100 64 63 112 54 15 19 89 22 65 23 92 48

6 6 6 6 5 6 6 6 6 6 6 6 5 6

28.7 51.3 165 1.17 20,760 0.63 23.7 0.25 198 35.1 5.97 8.00 3.03 1.43 7.18 0.50 1,220

50 51 56 23 70 93 68 36 21 34 75 59 68 24 44 22 48

50 2 5 4 1 30 2 2 10 2 0.7 500 1 4 3 0.7 70

43 38 41 50 I 44 36 51 66 44 14 39 25 79 71 60 60

5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

Enrichment factors are calculated by normalizing with respect to crustal material, using Al as the reference species. Units of concentration denote weight of species per unit weight of particulate material. * Denotes unavailable.

material (e.g. shale or seawater), rs =reference species (e.g. Al or Na) and p denotes the particulate material. It should be noted that the EFs reported are for bulk particulate material which are ~0.45 pm in diameter. High EFs (> 10) are found for I, Br and Cu. Cl is also enriched to some extent. Low EFs (< 1) are found for Ca, and perhaps Mn and Na. For the inter-site samples, elements with EF in the range 0.3 to 2 have CV of 19-36%, whereas those with EF in the range 3-200 have CV of 52-19% (Co, with an EF of 2, lies in rm =reference

400

SHELDON

LANDSLIERGER et al.

the latter grouping). This suggests that the lower EF elements co-vary more with Al than with the halogens, and provides additional support for the hypothesis that at least two types of aerosol are scavenged by the marine snow. In general, the same characteristics are found in the inter-site samples, although both Co and U show higher CV than the other elements with EFt2. It seems likely that most of the readily soluble aerosol will dissolve on contact with snowmelt during sample treatment (approximately 60 min). Hence the high enrichment factors for the halogens, Br, Cl and I, are unlikely to reflect characteristics of marine aerosol. It seems more likely that the excess halogen is relatively insoluble, and is derived from other anthropogenie or terrigenous sources, e.g. sulphide smelting, coal and diesel combustion and organic-rich soil (Sturges and Harrison, 1986; Fuge et al., 1986). Highly-polluted ‘black’ snow. The concentration of particulate material was 16 mg P- I. Compared to the relatively unpolluted aerosol described above, the EFs for Ba, Ca, Co, Cu, Dy, Na, Ti and U are slightly lower, while only the halogens, Br, Cl and I, and perhaps Mn show any slight increase (see Table 4). At most, the EF value for the same species in marine and polluted snows differ by a factor of approximately 5. Apart from Co and Cu, these elements are predominantly lithophilic in nature (Mason and Moore, 1982). Large variations in EF would not be anticipated, since the lithophiles are relatively unreactive elements, residing largely in silicate, aluminosilicate or refractory oxide lattices (Goldschmidt, 1954). An anthropogenic or terrigenous origin for the halogens, which are lithophiles, is required to explain their high EF. The EF of 17 elements not determined in the intraand inter-site surveys are also shown in Table 4. High EF values are observed for As, In, Se and Zn. All four are chalcophiles (Mason and Moore, 1982), which are elements preferentially incorporated into sulphide phases (Goldschmidt, 1954). Chalcophiles are likely to be concentrated onto particulate material derived from the combustion of fossil fuels or the smelting of sulphides (Raask and Goetz, 1981). Elements with EF near 1 are predominantly lithophiles. Co, Ni, SCand U may also be enriched in coal ash relative to crustal material (Mason and Moore, 1982). Particulates in the black snow give rise to EF values (normalized to crustal material) > 1 for Co, Ni and U. Seasonal snowpack. The concentration of particulate material ranged from 3.3 to 10 mgk- ‘. The range and average EFs for particulate material recovered from the snowpit are shown in Table 4. The values are within a factor of 4 of the equivalent values for the polluted, black snow or the marine snow. The halogens, the chalcophiles, Co, Ni and U all have EF > 1. DISCUSSION

AND CONCLUSIONS

The sampling sites are best classified as rural, rather than remote, under the scheme of Galloway et al. (1982). This recognises the episodic anthropogenic

pollution of the Scottish Highlands and that earlier ‘rural’ trace metal concentrations may have been rather high (Barrie et al., 1987). The back trajectory of the air mass associated with the snowfall (Abrahams et al., 1988) and amount of precipitation (Ambe and Nishikawa, 1986) may also influence whether or not the event has a more urban or a more remote nature. Snowfall associated with maritime back trajectories has dissolved Cl : Na : Mg : SO, : Br ratios within 6% of their seawater ratio, suggesting that there is little fractionation of these elements during the formation, scavenging and dissolution of the marine aerosol. There may, however, be some depletion of Br. It seems that soluble marine aerosol is more homogeneously distributed and scavenged across the region than soluble aerosol of a more terrigenous nature. The concentration of ions in melted, filtered black snow is typically greater than in the marine snow, except for the major seawater ions, Na, Mg and Cl. However, the black snow contains excess Cl, SO, and Br when normalized to Na, accounting for 54%. 98% and 92% of the total ionic concentrations, respectively. It is instructive to contrast the trace metal concentrations of each type of snow, in terms of their toxicity potential. Galloway et al. (1982) define the toxicity potential, TP, of a metal ion as the ratio of the concentration in wet precipitation to the recommended upper limits for that ion (a) which is toxic to aquatic organisms or(b) in potable water (see Table 3). The recommended upper limit for toxicity to aquatic organisms is generally lower for each toxic metal than the upper limits for that metal in potable water (except for Mn). Strictly, this is not applicable to solute in snowcover, since, on melting, ions fractionate into the first melt-water fractions (Johannessen and Henriksen, 1978). The fractionation factor is defined as the ratio of the concentration ion in meltwater to the concentration of that ion in the coexisting snow or ice (Brimblecombe et al., 1986). Fractionation factors ranging from 3 to 12 have been reported for partial melting of between 5-10% of the parent snow (Davies et al., 1987). Hence the first meltwaters may contain up to 10 times the concentration of the ion in the bulk snowmelt. The TP with respect to aquatic organisms for Cu, Mn, Pb and V in the bulk marine snow is < 1. However, fractionation factors of 5 give rise to first meltwaters which have TP> 1. Therefore, these first meltwaters, even though derived from relatively unpolluted snow, could be toxic to aquatic organisms under some circumstances. The TP of Pb and Zn in the bulk, black snow are 12 and 1.5, with respect to aquatic organisms. The TP of Pb is 2.4 with respect to potable water. Fractionation factors of around 2 would give rise to TP for Cu and V which are > 1 for aquatic organisms. Although TP values close to 1 have not been recorded in our Cairngorm site during snowmelt (Abrahams et al., 1989; Tranter et al., 1988), probably due to soilmeltwater interactions (e.g. Cresser and Edwards,

Solute and particulate chemistry of background vs polluted snowfall

1987), under some conditions, there is clearly the potential for toxic effects promoted by trace metals during snowmelt. Particles >0.45 pm show little difference in their EF, normalized to crustal material, for the lithophiles, Ba, Br, Ca, Cl, Dy, Mn, Na, Ti, U and V and the chalcophile, Cu, when comparing relatively polluted and unpolluted snowfall. Of these elements, only the halogens (Br and I) and Cu have EF> 10 in the relatively unpolluted snow, and only the halogens, Br, Cl and I show such values in the relatively polluted snow. At most, the EF of these elements differ by a factor of 5 between the two snows. Similar values of EF, within a factor of 4, are exhibited by particulate material gathered from within snowpack. The chalcophiles, As, In, Se and Zn have EF> 10 in particulate material within black snow and seasonal snowcover. Acknowledgements-The

authors wish to thank the constructive comments of Drs T. Jickells, P. Statham and the referees. The work was supported by the NERC (grant nos. GR3/5144A and GST/02/205) and the EEC (grant no. ENV/782/UK).

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