The effects of snow and ice on the environmental behaviour of hydrophobic organic chemicals

ENVIRONMENTAL POLLUTION

Environmental Pollution 102 (1998) 25±41

The e€ects of snow and ice on the environmental behaviour of hydrophobic organic chemicals F. Wania a,*, J.T. Ho€ b, C.Q. Jia c, D. Mackay d a

WECC Wania Environmental Chemists Corp., 280 Simcoe Street, Suite 404, Toronto, Ontario, Canada M5T 2Y5 b Department of Earth Science, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 c Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3E5 d Environmental and Resource Studies, Trent University, Peterborough, Ontario, Canada K9L 1N6 Received 1 October 1997; accepted 24 March 1998

Abstract A review is presented of the roles of snow and ice as they in¯uence the environmental fate of hydrophobic organic chemicals (HOCs). Measurements of HOC concentrations in snow are reviewed and present information on the partitioning and depositional and post-depositional behaviour of HOCs in snow is described and implications for environmental monitoring and assessment of fate are discussed. It is concluded that snow is an ecient scavenger of HOCs from the atmosphere both by adsorption of gaseous HOCs to the ice interface, and by particle scavenging. The post-depositional fate of HOCs in ageing snow packs is poorly understood. Suggested structures of quantitative models describing HOC interactions with ice and snow are presented. Key parameters in these models include the interfacial area of snow and the extent of HOC sorption to the ice surface. Recent laboratory determinations of these parameters are reviewed. Finally, research needs and gaps are identi®ed with a view to compiling more accurate estimates of net atmospheric wet deposition of HOCs, establishing their fate in snow packs, developing reliable sampling protocols and assessing the usefulness of the glacial record as an indicator of past atmospheric compositions. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Hydrophobic organic chemicals; Snow; Ice; Deposition; Interface; Adsorption

1. Introduction Snow and ice are critically important environmental components of the ecosystem in temperate and polar latitudes. They a€ect energy balances and hydrologic cycles and can thus directly a€ect the behaviour of chemicals in the environment on the local, regional and global scales. Of particular interest and concern are the relatively high levels of hydrophobic organic contaminants (HOCs) in marine wildlife of the Arctic which have recently stimulated more general interest in the behaviour of organic chemicals in cold regions (AMAP, 1997). Some persistent organic chemicals such as certain organochlorine pesticides and polychlorinated biphenyls (PCBs) may preferentially deposit and accumulate in cold regions (Wania and Mackay, 1993). The fate of organic chemicals is likely to be profoundly in¯uenced * Corresponding author. Tel.: +1-416-977-8458; fax: +1-416-9774953; e-mail: [email protected] 0269-7491/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S0269 -7 491(98)00073 -6

by the unique characteristics of high latitude ecosystems, especially the low temperatures, the prolonged snow cover and precipitation which occurs in the form of snow, rime, graupel, or hail. In the terrestrial environment the land is covered by a seasonal snow cover, some areas by permanent glaciers and ice caps, whereas water in the ground is permanently frozen (permafrost). The sea, lakes and rivers may be ice-covered either permanently or for parts of the year. Coverage by snow and ice limits the extent of direct dry particle deposition and di€usive gas exchange with water, soil and vegetation. Snow packs are themselves subject to di€usive gas exchange. The timing and extent of contaminant delivery to marine and terrestrial systems are in¯uenced by snow and ice melting. Snow fall has the potential to signi®cantly contribute to the deposition of airborne contaminants by washing out the aerosol and sorbing the vapour (Franz, 1994). In a snow pack, the large speci®c surface area of ice crystals has the potential to sorb appreciable quantities of

26

F. Wania et al./Environmental Pollution 102 (1998) 25±41

HOCs (Ho€ et al., 1995). Snow may be a valuable medium for monitoring contaminant levels in a region because it is less transient than rain. Our present understanding of how HOCs interact with frozen water is relatively limited and has only recently become subject to detailed investigation. In contrast to inorganic snow chemistry, which has generated a wealth of information in the past several decades, the study of the fate of organic chemicals in association with snow and ice has been largely neglected. The limited understanding of the physics and chemistry of these systems, and diculties in conducting ®eld studies under reproducible and controllable conditions has retarded the development of quantitative models describing snow±contaminant interactions. In order to assess and evaluate the environmental fate and behaviour of HOCs in cold ecosystems it is of particular importance to gain an extensive and, if possible, a quantitative understanding of: 1. the eciency and nature of snow scavenging of HOCs from the atmosphere; 2. the behaviour of organic chemicals in snow packs, especially as they age; 3. the release of organic chemicals from the snow pack into the ecosystem during melting; and 4. the potential preservation of a depositional record of organic chemicals in glacier ice. This paper aims to summarise the present state of knowledge and desirable future studies of the environmental behaviour of HOCs as in¯uenced by snow and ice. 2. Field studies quantifying the presence of HOCs in snow and ice 2.1. Snow fall and snow pack Several ®eld studies have been reported involving direct sampling of snow in precipitation samplers and/ or the sampling of a snow pack, and the subsequent analysis of organic chemical concentrations in the snow melt water. The emphasis here is on HOCs which tend to be persistent and which often bioaccumulate. Organic contaminant concentrations in snow have been reported for the Canadian Arctic (Stengle et al., 1973; McNeely and Gummer, 1984; Gregor, 1990; Hargrave et al., 1988, 1989; Gregor and Gummer, 1989; Patton et al., 1989, Welch et al., 1991), Siberia (Smagin et al., 1987) and the European Arctic (Lunde et al., 1977; Paasivirta et al., 1985; Marklund et al., 1991). Table 1 gives an overview of most studies quantifying organic contaminants in snow which have been published so far. There have been several determinations of the concentration of 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane (DDT), PCBs and hexachlorocyclohexanes (HCHs) in snow

from Antarctica. However, there are large discrepancies between the levels found by various researchers and some of the early determinations may be subject to analytical error (Peterle, 1969). The focus of many early studies was the search for evidence of long range transport of anthropogenic organic substances to remote areas such as Antarctica (Peterle, 1969; Stengle et al., 1973; Peel, 1975; Risebrough et al., 1976; Tanabe et al., 1983) and the Arctic (Paasivirta et al., 1985; Hargrave et al., 1988; Gregor and Gummer, 1989). In temperate areas the focus was rather on using snow packs as a tool to investigate regional concentration di€erences (Lunde et al., 1977; Herrmann, 1978; Schrimp€ et al., 1979; Kawamura and Kaplan, 1986), or the quanti®cation of depositional ¯uxes (Meyers and Hites, 1982; Marklund et al., 1991; Franz and Eisenreich, 1993; Franz, 1994; Rahm et al., 1995; Gregor et al., 1996). There have been several investigations of single, exceptional snow events that resulted in unusually high contaminant deposition in remote areas (Welch et al., 1991; Davies et al., 1992; FranzeÂn et al., 1994). For example, the grey or `black' colour of snow falls in the Cairngorm Mountains was caused by high concentrations of black carbon (Davies et al., 1992). The `yellow' snow observed in northern Fennoscandia was thought due to the scavenged dust that had migrated from several thousands kilometres away (FranzeÂn et al., 1994). 2.2. Glacier ice In contrast to inorganic chemical species (Wol€ and Peel, 1985), measurements of HOC concentration pro®les in glacier ice are rarely attempted as a means of detecting the historical development of depositional ¯uxes and thus atmospheric concentrations. Polycyclic aromatic hydrocarbons (PAHs) (Peters et al., 1995), PCBs (Gregor et al., 1995) and organochlorine pesticides (Gregor, 1990) were analysed in pits and snow cores from ice caps in Arctic Canada, and PAHs in the Greenland ice cap (Ja€rezo et al., 1994). In snow samples older than 1 year, Gregor (1990) found no consistent long term trend in concentrations of several organochlorinated pesticides in the Agassiz ice cap from 1986±87 to 1970±71. Neither did PCBs show a clear temporal trend over a 30-year period (Gregor et al., 1995). A study in the Antarctic region showed no signi®cant di€erence in HCH, DDT, and PCB concentrations between 1-year-old surface snow and 20-year-old deep snow (Tanabe et al., 1983). Peters et al. (1995) observed a sharp decrease in PAH concentrations in snow layers of the Agassiz ice cap during the 1970s, and stable values since then. The Greenland ice pit was shallow and covered only a period of 4 years. PAH concentrations showed seasonal variability with higher values in winter/spring and a decrease with depth

F. Wania et al./Environmental Pollution 102 (1998) 25±41

27

Table 1 Studies on organic compounds in snow (DDTs: 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane and related compounds, PAHs: polycyclic aromatic hydrocarbons, PCBs: polychlorinated biphenyls, HCHs: hexachlorocyclohexanes, TVOC: total volatile organic compounds, PCC: polychlorinated camphenes, PCDD/Fs: polychlorinated dibenzo-p-dioxins and dibenzofurans) Location

Sampling

Sampling medium surface snow

Compounds detected DDT

Reference

Plateau Station, Antarctica

Jan. 1967

Peterle, 1969

Halley Bay, Antarctica

1969±70

snow

DDT

Peel, 1975

Mt. Logan, YT

1970

15-m ice core

±

Stengle et al., 1973

Norway

winter 1974±75

17 snow packs

alkanes, PAHs, phthalic acid esters, fatty acids, fatty acids ethyl esters, PCBs, etc.

Lunde et al., 1977

Doumer Island, Antarctica

Jan./Feb. 1975

6-m snow pit

DDTs, PCBs

Risebrough et al., 1976

Chicago

winter 1975±76

snow fall

PCBs

Murphy and Rzeszetko, 1977

Isle Royale, Lake Superior

1976

snow fall

PCBs

Swain, 1978

Great Lakes, Canada

Feb. 1976

snow packs

PCBs, HCHs, DDTs, endosulfan, dieldrin, methoxychlor, HCB

Strachan and Huneault, 1979

NE Bavaria, Germany

winter 1978±79

snow packs

four PAHs, HCHs, dieldrin

Schrimp€ et al., 1979

Southern Indiana

winter 1979±80

snow fall

n-alkanes, fatty acids

Meyers and Hites, 1982

Ellesmere Island, NWT

1979±81

snow pack

HCHs, DDT, dieldrin

McNeely and Gummer, 1984

Herrmann, 1978

Antarctica

May/Nov. 1981

snow pack

HCHs, DDTs, PCBs

Tanabe et al., 1983

Southern California

1982±83

freshly fallen snow

n-alkanes, PAHs, fatty acids, benzoic acids, phenols

Kawamura and Kaplan, 1986

Siberian Arctic Seas

1982±85

snow packs

HCHs, DDTs

Smagin et al., 1987

NE Bavaria, Germany

winter 1984/85

two ageing snow packs

a-HCH, g-HCH, two PAHs

Simmleit et al., 1986

Isle Royale, Lake Superior

Jan. 1984

one snow pack

PCBs

Swackhamer et al., 1988

Isle Royale, Lake Superior

Feb. 1984

snow pack

11 PAHs

McVeety and Hites, 1988

North Pole and Finland

May 1984

surface snow

chlorophenolic compounds

Paasivirta et al., 1985

Stockholm, Sweden

spring

roadside snow

halogenated PAHs

Haglund et al., 1987

ZuÈrich, Switzerland

Jan./Feb. 1985

snow fall

alkylbenzenes, PAHs, TVOC

Czuczwa et al., 1988

NWT

Apr./May 1986

12 snow pack samples

HCHs, chlordanes, DDTs, PCBs, dieldrin, endosulfan

Gregor and Gummer, 1989, Gregor, 1990

Agassiz ice cap, NWT

spring 1986

2.2-m snow pit

HCHs, chlordanes, dieldrin, heptachlorepoxide

Gregor, 1990

spring 1987

7-m snow pit

summer 1986

old and new snow

HCHs, HCB, dieldrin, chlordanes, DDTs, PCBs

Hargrave et al., 1988, Hargrave et al., 1989

Ice Island, Canada Ice Island, Canada

Jun. 1987

old and new snow

HCHs

Patton et al., 1989

California

winter 1987±88

snow from three storms

carbonyls, carboxylic acids

Gunz and Ho€mann, 1990

Keewatin District, NWT

spring 1988

`brown' snow event

PAHs, PCB, DDTs, PCC, tri¯uralin, methoxychlor, endosulfan, HCHs

Welch et al., 1991

Atlantic Canada

1980±89

snow fall

PCBs, PAHs, HCHs

Brun et al., 1991

Green Bay, Lake Michigan

winter 1989±90

snow fall

PCBs

Franz and Eisenreich, 1993

Northern Scandinavia

spring 1991

`yellow' snow event

seven PCBs, 11 PAHs, HCHs

FranzeÂn et al., 1994

Bothnian Bay

Mar. 1991

10 snow samples

PCBs, HCHs, DDTs

Rahm et al., 1995 (Table continued on next page)

28

F. Wania et al./Environmental Pollution 102 (1998) 25±41

Table 1Ðcontd Location

Sampling

Sampling medium

Compounds detected

Reference

Summit Station, Greenland

summer 1991

surface snow, snow pit covering 4 years

13 PAHs

Ja€rezo et al., 1994

Northern Sweden

winter 1991

urban and remote snow

PCDD/Fs

Marklund et al., 1991

North NWT

winter 1990±91

snow fall

PCBs

Gregor et al., 1996

Minnesota

winter 1991±92

three snow fall events

PCBs, PAHs

Franz, 1994

Minnesota and North Michigan

1982±92

snow packs

PCBs, PAHs

Franz, 1994

Agassiz ice cap, NWT

1993

8-m snow pit

PCBs

Gregor et al., 1995

PAHs

Peters et al., 1995

NWT and YT

1990±94

snow packs, weekly snow fall

HCHs, chlordanes, DDTs, HCB, PCBs

Barrie et al., 1997

Cornwallis Island, NWT

spring 1993

ageing snow pack, snow melt water

HCHs, endosulfan, DDT, PCBs, chlordanes, dieldrin

Barrie et al., 1997

(Ja€rezo et al., 1994). There remains some doubt about the reliability of such pro®les as records of past deposition (e.g. Jaworowski, 1994), because of the possibility of vapour phase di€usion and evaporation. 2.3. Sea ice and lake ice Analyses of sea ice samples for HOCs are sparse. Table 2 lists some concentration data for sea ice in the Arctic (Smagin et al., 1987; Gaul, 1989; Hargrave et al., 1989) and Antarctic (Tanabe et al., 1983; Desideri et al., 1991). Concentration data for sea ice from the Russian Arctic are quoted in P®rman et al. (1995). Hargrave et al. (1989) and Desideri et al. (1991) analysed organic chemicals both dissolved in the ice melt water and associated with ®lterable particles. Generally, most chemical was in the dissolved phase, unless the sample had a high content of biological material. Hargrave et al. (1989) sampled the bottom of an ice core which included epontic algae and found signi®cant levels of PCBs, 1.1-dichloro-2,2-bis-(4-chlorophenyl)-ethylene (DDE) and hexachlorobenzene (HCB) in the particle fraction of the melt water. Gaul (1989) reported high levels of DDT and extremely high levels of a PCB isomer in sea ice that contained appreciable amounts of particles. Chlordane was detected by Thorne (1996) in ice cores and sur®cial sediments from the ice pack of the Arctic Ocean. When sea water from the same location was sampled simultaneously the concentrations of organic contaminants were higher in ice melt water than in sea water, indicating that sea ice becomes a source of chemical to the sea water upon melting. There are, however, exceptions. At an ice island HCH concentrations were higher in sea water than in ice (Hargrave et al., 1989). In samples from the Norwegian Sea a-HCH levels in sea water exceeded those in ice (Gaul, 1989). Fuoco et al.

(1991) measured PCB levels in sea water from Terra Nova Bay, Antarctica before and after pack ice melting, and observed a small concentration increase indicating contaminant input with the ice melt water. Concentrations in sea ice are generally in the same range as in snow sampled at the same location. In the only study to measure HOCs in lake ice, Tanabe et al. (1983) reported concentrations of 2.0 ng/litre HCHs, 0.01 ng/litre DDTs and 0.31 ng/litre PCBs in ice sampled from Lake Nurume (close to Syowa Station) in November 1981. These levels were almost an order of magnitude higher than those in the lake water sampled at the same time. Clearly, snow and ice can serve, at least, as valuable qualitative monitoring media for detecting the presence of HOCs in cold climates. Their value for quantitative purposes is less certain. 3. Field studies of the deposition of HOCs by snow HOCs are present in the atmosphere in both gaseous and aerosol sorbed forms, and both forms become associated with hydrometeors (snow ¯akes, rain drops, fog particles) and are thus transferred from the atmosphere to the ground. The eciency of scavenging and the atmospheric concentrations presumably determine the concentrations of HOCs in snow fall and, therefore, the ¯ux by deposition. In addition, chemicals identi®ed in snow pack samples may include a contribution of direct dry deposition of aerosols and adsorption of gaseous HOC. The most comprehensive study has been that of Franz (1994) who reported simultaneously measured organic contaminant concentrations in snow melt water and the atmosphere. The ratio of these concentrations is termed the total scavenging ratio WT. He assumed that the

F. Wania et al./Environmental Pollution 102 (1998) 25±41

29

Table 2 Concentrations of selected hydrophobic organic chemicals (HOCs) measured in sea ice (ng litreÿ1). (HCB: hexachlorobenzene, HCH: hexachlorocyclohexane, DDTs: 1,1,1-trichloro-2,2-bis-(4-chlorophenyl)-ethane and related compounds, PCBs: polychlorinated biphenyls) Location Norwegian Sea

Period a

n

HCB

a-HCH

g-HCH

DDTs

PCBs

2

0.08

1.1/1.3

2.2/2.3

1.64/2.08

15.5/20.3

Aug. 1985

3

0.04

0.27

0.46

0.25

1.0 b

Aug. 1985

3

0.05

1.0

5.2

0.1

2.5 b

±

Aug. 1979

Reference b

Kara Sea

1982±85

?

0.66

0.42

0.19

Canadian Ice Island

May 1986

3 <0.002

1.31

0.18

0.012

0.032 c

Jun. 1987

3 <0.006

0.69

0.08

0.101

0.11 c

3

0.58

0.085

0.114

0.093 c

Jun. 1987

a

0.021

±

Gaul, 1989

Smagin et al., 1987 Hargrave et al., 1988, Hargrave et al., 1989

Tottuki Point, Antarctica

Jul./Sep. 1981

1

±

2.2

0.01

0.61

Tanabe et al., 1983

Terra Nova Bay, Antarctica

Summer 1988±89

3

±

0.34±1.29

0.65±0.94

±

Desideri et al., 1991

a b c

Contained visible amounts of particles or algae. Only PCB-138. Quanti®ed as Aroclor 1254.

relative contributions of gas and particle scavenging could be estimated by measuring separately the dissolved fraction and the fraction of chemical attached to suspended particulate matter. The conditions in the melt water may, however, re¯ect these mechanisms of scavenging only if the kinetics of adsorption and desorption of organic chemicals to particles in cold aqueous solution are too slow for re-partitioning to occur between the dissolved and particle-adsorbed phases during and after melting. It is possible that there is no direct method of distinguishing accurately in what form chemicals have been scavenged from the atmosphere. With these limitations in mind, Franz (1994) observed that: 1. Measured scavenging ratios (WT) for HOCs are highly variable between snow events and range from 104 to 107 for PAHs and 104 to 106 for PCBs. 2. Scavenging ratios for di€erent compounds are obviously correlated, i.e. a snow event with high WT for one HOC tends to have a high WT for other chemicals of that type. 3. Snow scavenging ratios for PAHs and PCBs tend to be higher than rain scavenging ratios. In Minnesota, this is valid for chemicals which are scavenged mostly in the vapour phase (e.g. phenanthrene) and for completely particle-sorbed PAHs (such as benzo[a]pyrene, indeno[c,d]pyrene, and benzo[g,h,i]perylene). However, McVeety and Hites (1988) observed higher scavenging ratios in rain compared to snow for entirely particle sorbed chemicals. There have been relatively few ®eld measurements devoted to obtaining a mechanistic understanding of the processes involved in atmospheric snow scavenging of organic chemicals. This is understandable considering the diculties associated with such investigations. Pre-

cipitation scavenging eciencies for HOCs measured in the ®eld tend to be highly variable: the atmosphere may not be well mixed at the time the precipitation occurs, which implies that ground based air concentration measurements may not be representative of the conditions in and below the cloud where the scavenging actually takes place. Scavenging eciencies have been shown to vary within the course of one wet deposition event (e.g. Czuczwa et al., 1988; Schumann et al., 1988; Collett et al., 1991). This may be caused by air mass changes which are often associated with frontal precipitation systems, by partial depletion of contaminants from the atmosphere by the scavenging process or by a change of precipitation type (e.g. from sleet to rain) during the event. Field measurements of the scavenging eciencies of HOCs are likely to yield average values integrating the conditions of an entire wet deposition event, because of the large air sampling volumes required. 4. Studies of the post-depositional behaviour of HOCs in snow packs After deposition with falling snow, HOCs presumably undergo a number of processes such as repartitioning and translocation within the snow pack, volatilisation and drainage with melt water. There have been relatively few studies of the behaviour of HOCs during snow pack metamorphosis and melting. 4.1. Translocation within snow packs and between soil and snow pack In ®eld experiments involving snow packs accumulating over contaminated soil surfaces (Hogan and Leggett, 1995; Leggett and Hogan, 1995) it was shown that HOCs move within snow packs by vapour di€usion.

30

F. Wania et al./Environmental Pollution 102 (1998) 25±41

More volatile chemicals tended to accumulate in the colder parts of a snow pack, whereas less volatile chemicals developed concentration pro®les with an exponential decrease with distance from the source (Hogan and Leggett, 1995). HOC vapour mobility within the snow pack increased with vapour pressure and temperature. More water soluble HOCs may be translocated within the snow pack with percolating melt water, as is discussed later. 4.2. Evaporation from snow packs and ®rn In the Canadian Arctic a concentration decline of relatively volatile HOCs during snow pack metamorphosis has been observed by several investigators. A marked decrease in the concentrations of a range of organochlorine chemicals has been observed during repeated sampling of snow packs after several weeks or months. Snow sampled twice during 1986 from the Canadian Ice Island o€ the coast of the Canadian Arctic Archipelago showed a decrease in organochlorines, namely HCB and chlordanes (Hargrave et al., 1988). Similar results were obtained during the repeated sampling of a seasonal snow pack on Cornwallis Island in Arctic Canada (Barrie et al., 1997). The change was smallest for the HCHs, and largest for the less volatile chemicals such as DDT. On a longer time scale, substantial loss of several organochlorine compounds from the Agassiz Ice cap in Arctic Canada, presumably by volatilisation, was observed during the ®rst year after snow deposition (Gregor, 1990; Gregor et al., 1995). In contrast to the investigations on Cornwallis Island, HCH concentrations decreased most, and the decrease of the less volatile PCBs was less pronounced. Ja€rezo et al. (1994) attributed a 40 and 35% decrease of ¯uoranthene and pyrene in the Greenland ice cap during the four years following deposition to degradation, but evaporation may also have occurred. Although HOCs may be lost to the atmosphere from a snow pack they may also be added by dry deposition, i.e. absorption of gaseous chemicals and dry deposition with particles. By comparing the load of PCBs and PAHs in seasonal snow packs collected prior to melting and in cumulative snow fall samples in Northern Michigan, Franz (1994) concluded that the contributions by dry deposition are minor. However, high levels of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in older snow pack, as reported by Marklund et al. (1991), suggest that dry deposition to the snow pack is very important for some HOCs. 4.3. Horizontal transport of HOCs with drifting snow Pomeroy and Jones (1996) pointed out that ``in polar regions snow does not normally fall in a simple fashion

from the atmosphere to the surface to be buried by subsequent snow falls, but more often falls and moves horizontally, carried by the wind, to be either sublimated, resuspended or deposited''. As blowing snow sublimates rapidly, this may a€ect chemical concentrations in the snow. It is likely that less volatile organic chemicals associated with particles will experience a concentration increase in blowing snow because of particle scavenging and sublimation loss. More volatile organic chemicals, however, may volatilise more quickly in drifting than in resting snow, resulting in a concentration decrease. Uneven snow accumulation due to snow drifting may result in uneven contaminant input on a local scale. Contaminant deposition to the ecosystem may be elevated at places where snow tends to accumulate. 4.4. Snow melt processes A snow pack represents an integration of precipitation and associated contaminant over a period of time, possibly many months. When the pack melts, the water and accumulated contaminant are released in a relatively short pulse. This pulse may result in contaminant exposure being focused into a short time period in the spring, when biological systems are in a state of increased activity. Spring concentration peaks of PCBs and DDT coinciding with the snow melting period have been measured in the St. Lawrence River and four of its tributaries (QueÂmerais et al., 1994; Pham et al., 1996). SchoÈndorf and Hermann (1987) explained an observed fractionation of organic contaminants in the melt water of a column of snow, which had been exposed to repeated freeze±thaw cycles, with the extent to which the HOCs were associated with particles. Water soluble substances such as HCHs eluted with the ®rst melt water fractions, whereas less soluble substances were associated with the last melt water fractions, which also contained the particles. Simmleit et al. (1986) measured daily the concentrations of selected HOCs in two melting snow packs. In both cases a rapid loss of chemical occurred with the ®rst melt water fractions, resulting in a decrease in concentration in the remaining snow pack. In one case, however, snow concentrations of HOC increased again strongly at the end of the melting period. In general, less soluble chemicals became enriched in the remaining snow pack relative to the more water soluble HOCs such as the HCHs. Monitoring of Karst spring water during snow melting periods indicated the importance of particulate material in transporting the less water soluble HOCs (Simmleit and Herrmann, 1987). The same conclusion was drawn from measurements of snow melt running-o€ from roofs and streets (Daub et al., 1994). Similar behaviour was observed when the concentration of various HOCs was monitored in several creeks of the Amituk Lake basin in the Canadian Arctic

F. Wania et al./Environmental Pollution 102 (1998) 25±41

31

Archipelago during the snow melting period. Concentrations of endosulfan and HCHs were highest in the ®rst melt water samples, then steadily decreased during the melting period. Concentrations of the sparsely soluble HOCs, e.g. PCBs and DDT, were more likely to increase during the melting period (Semkin, 1996). This preferential elution or `®rst ¯ush' melting behaviour of relatively water soluble HOCs matches that of water soluble inorganic compounds (e.g. Johannessen and Henriksen, 1978; Colbeck, 1981; Tranter et al., 1986; Semkin and Je€ries, 1988).

well as the thickness of the liquid layer. The presence of organic and inorganic solutes usually lowers the freezing point of a solution and, therefore, may stabilise the liquid phase at low temperatures and increase its volume. For the partitioning between the air phase and the water or ice surface an interface±air partition coecient kia can be de®ned as the ratio of the equilibrium concentrations on the interface and in the air phase, the former being expressed on a amount per area basis (Ho€ et al., 1993). kia-values thus have units of length (i.e. m or [mol/m2]/[mol/m3]).

5. Sorption of HOCs to air±water and air±ice interfaces

5.1. Interfacial sorption coecients kia for water

Several studies (Jellinek, 1967; Fletcher, 1973; Granat and Johansson, 1983; Sommerfeld et al., 1992; Conklin et al., 1993) have shown that there is a quasi-liquid layer on the surface of solid ice. It has been suggested that gases can both adsorb to the air±liquid layer interface or absorb in the bulk phase of the liquid layer (Orem and Adamson, 1969; Ocampo and Klinger, 1982; Goss, 1992; Ho€ et al., 1995; Brimblecombe and Conklin, 1996). Clearly, the relative fractions adsorbed and absorbed depend on physical±chemical properties including water solubility and interfacial partition coecient as

Coecients kia for sorption on the interface of liquid water have been measured by Hartkopf and Karger (1973), and more recently by Ho€ et al. (1993) for a number of relatively volatile organic chemicals (Table 3). Goss (1993a) suggested that extrapolating measurements of adsorption on certain mineral surfaces at variable relative humidity to values of 100% will approximate adsorption coecients at the bulk water surface, and found good agreement between estimated and measured kia-values (Goss, 1994). Pankow (1997) used this assumption when estimating kia-values for n-alkanes and PAHs from adsorption measurements on quartz.

Table 3 List of chemicals for which adsorption at the air±water or air±ice interface has been measured Surface Water

Chemical

Reference

n-pentane, n-hexane, n-heptane, n-decane, 2,2,4-trimethylpentane, benzene, toluene, ethylbenzene, chlorobenzene, methylformate, dichloromethane, trichloromethane, tetrachlormethane, 1,2-dichloroethane

Hartkopf and Karger, 1973 Ho€ et al., 1993

n-octane, n-nonane, 2-methlyheptane, 2,4-dimethylhexane, cycloheptane, cyclooctane, cis-2-octene, trans-2-octene, ¯uorobenzene, n-propyl ether, ethylformate

Hartkopf and Karger, 1973

cyclohexane, isopropylbenzene, 1,3-dichlorobenzene, 1,1,1-tricloroethane, trichloroethene, tetrachloroethene, 1-bromobutane, ethyl ether, ethylacetate, acetone

Ho€ et al., 1993

Quartz (extrapolated to water)

heptadecane, nonadecane, eicosane, heneicosane, docosane, tricosane, 2-methylphenanthrene, ¯uoranthene, pyrene, benzo[a]¯ourene, benz[a]anthracene

Pankow, 1997

Cold ice

n-pentane, n-hexane

Orem and Adamson, 1969

Ice

n-nonane, p-xylene, m-xylene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 2,3-benzofuran, anisole

Goss, 1993b

n-hexane, n-heptane, n-octane, benzene, chlorobenzene, 1,4-dichlorobenzene, dichloromethane, trichloromethane, tetrachloromethane, 1,1,1-trichloroethane, trichloroethene, tetrachloroethene

Ho€ et al., 1995

32

F. Wania et al./Environmental Pollution 102 (1998) 25±41

Four correlations have been suggested for estimating kia from available physical±chemical properties. For non-polar organic chemicals, Valsaraj (1988) and Valsaraj et al. (1993) suggested an empirical relationship between kia and the octanol±water partition coecient: ln kia …12:5 C† ˆ 0:68
ln KOW ÿ 19:63 ‡ ln KWA ;

…1†

where kia is the interfacial adsorption coecient (m) KOW is the dimensionless octanol±water partition coecient and KWA is the dimensionless water±air partition coecient. Valsaraj (1994) subsequently obtained correlations for ®ve compound classes (aliphatic hydrocarbons, alcohols, acids, alkyl benzenes, chloromethanes). Ho€ et al. (1993, 1995) have suggested that kia for non-polar chemicals can be estimated from the aqueous solubility and Henry's Law constant: ln kia …20 C† ˆ ÿ0:769
ln Cws ÿ 13:75 ‡ ln KWA ;

…2†

where C sw is the aqueous solubility (mol/m3) they give a separate correlation for polar chemicals based on surface tension. Goss (1994) suggested a correlation involving vapour pressure and hydrogen bond acceptor for adsorption of polar and non-polar organic vapours to a bulk water surface: ln kia …50 C† ˆ ÿ0:615
ln pL ‡ 7:86
ÿ 10:41;

…3†

where pL is the vapour pressure of the (sub-cooled) liquid at 25 C (Pa) and is the hydrogen bond acceptor, measuring a chemical's ability to form hydrogen bonds. For hydrophobic substances is approximately 0, and kia can be estimated from vapour pressure alone. Pankow (1997) recently suggested similar empirical relationships based on vapour pressure for speci®c groups of non-polar compounds, namely PAHs and n-alkanes: ln kia …20 C† ˆ ÿ1:20
ln pL ÿ 7:53 for PAHs

…4†

ln kia …20 C† ˆ ÿ0:93
ln pL ÿ 5:63 for n-alkanes:

…5†

The temperature dependence of kia can be expressed as:  
HS 1 1 ÿ ; …6† ln kia …T † ˆ ln kia …Tref † ÿ T Tref R where
HS is the enthalpy of adsorption, R is the gas constant, T is the temperature (K), and Tref is the reference temperature (K). Based on a regression for 22 non-polar compounds (Hartkopf and Karger, 1973), Ho€ et al. (1993, 1995) suggest that the enthalpy of adsorption
HS was 87.8%

of the enthalpy of condensation of the sub-cooled liquid
HC. Goss (1994) suggested a correlation involving vapour pressure and the hydrogen bond acceptor parameter:
HS ˆ 3:20
ln pL ÿ 5:02
ÿ 55:0:

…7†

No evaluation of the relative accuracy of these estimation methods has apparently been reported. 5.2. Interfacial sorption coecients kia for ice Some kia values for ice surfaces have been established experimentally (Orem and Adamson, 1969; Goss, 1993b; Ho€ et al., 1995). From these measurements three signi®cant conclusions have been reached: 1. there is no major discontinuity of kia at the freezing point: i.e. at 0 C the air±water and air±ice partition coecients are approximately equal; 2. the enthalpies of adsorption to the water and ice interface are di€erent, but similar in magnitude: with the enthalpy to ice being greater; and 3. to a ®rst approximation partitioning at the air± ice interface can be estimated by extrapolating adsorption constants for the air±water interface (Ho€ et al., 1995). These assertions are consistent with the concept of the ice surface being covered by a quasi-liquid layer at temperatures close to 0 C. 5.3. The speci®c surface area of snow and ice It is obvious that for any quantitative treatment of the sorption of HOCs on ice surfaces, surface area has to be known. Unfortunately, there have been few measurements of the speci®c surface area of snow and ice in the environment (Jellinek and Ibrahim, 1967). Recently, Ho€ et al. (1998) have developed a method based on the nitrogen adsorption technique. The values obtained for the speci®c surface area of snow samples collected during six snow fall events during January and February 1995 in Waterloo, Ontario, ranged from 0.06 to 0.37 m2/g. These data are comparable with surface areas estimated from the dimensions of snow crystals obtained by microscopy techniques. Thus on the basis of snow surface areas and size distributions for snow crystals, the surface area of fresh snow is probably in the range of 0.05 to 0.5 m2/g, or 5104 to 5105 m2/m3 of melt water. This corresponds to an equivalent spherical particle diameter of 12 to 120 mm, which is much smaller than a typical raindrop of 1000 to 2000 mm. Snow thus presents a much larger area to the atmosphere, resulting in enhanced adsorption.

F. Wania et al./Environmental Pollution 102 (1998) 25±41

6. Quantifying and modelling snow±HOC interactions There are three general areas in which there is a need for quantitative treatment of their interaction. First is the group of depositional processes by which snow acts as a vector to convey a HOC present in the atmosphere in gaseous and sorbed forms to the land, water, or ice surface. Second is the group of post-depositional processes by which the HOC present in depositional snow packs evaporates, reacts (degrades) or leaches in melt water. A third area which is not treated here is the role of ice cover in preventing or impeding air±water exchange in the fresh and marine waters and the corresponding role of snow in terrestrial systems. In this section we focus on the ®rst two areas, presenting an account of the structure of predictive equations which can represent the phenomenon, and be incorporated into mass balance models of HOC fate in cold environments. 6.1. Depositional processes (scavenging) Fig. 1 illustrates the scavenging processes. To quantify snow scavenging, the concepts conventionally applied to rain scavenging have been adopted, notably the total scavenging ratio WT which is de®ned as the ratio of concentrations: CS ; WT ˆ CA

…8†

where CS is the total mass of chemical/volume of melt water and CA is the total mass of chemical/volume of air.

33

Numerically, WT can be interpreted as the number of volumes of air which is scavenged of chemical by one volume of snow melt water. Relatively few data for WT exist for snow scavenging of HOCs. Typical measured values range from 105 to 107. To obtain a more mechanistic explanation of the deposition phenomena and ultimately assemble a predictive model which can be applied to untested chemicals it is necessary to break the above expression into separate components. The total concentration of chemical in air, CA, is the sum of the gaseous component CAG, and the particle sorbed component CAP. The proportions are dictated by a sorption partition coecient which is generally correlated with the chemicals subcooled liquid vapour pressure or with the octanol±air partition coecient (Finizio et al., 1997). The total concentration in the fallen snow can be expressed as the sum of the quantities dissolved in the quasi-liquid layer CSW, sorbed to the snow±air interface CSI and associated with particles which have been trapped by the falling snow CSP. Assuming equilibrium to apply between CAG, CSW and CSI it follows that: CSW ˆ

CAG
vSW ˆ CAG
vSW
KWA KAW

and CSI ˆ CAG
A
kia ;

…9†

where vSW is the volume of water in the snow per unit volume of melt water, KAW is the dimensionless air±water partition coecients, and A is the snow area (m2/m3 melt water). The group A.kia thus plays the same role as the partition coecient KWA in the description of rain scavenging. The simplest approach for estimating CSP is to invoke an empirical scavenging coecient WP such that CSP ˆ WP
CAP :

…10†

WP can be regarded as the volume of air which is e€ectively scavenged of particles per unit volume of melt water. For rain WP is believed to be about 2105 (Mackay, 1991) but the available data for snow indicate a larger value of possibly 5105±1106. A more mechanistic treatment of WP is given later. It follows that: CSW ‡ CSI ‡ CSP CAG ‡ CAP CAG
…vSW
KWA ‡ A
kia † ‡ WP
CAP ˆ : CAG ‡ CAP WT ˆ

Fig. 1. Scavenging of gaseous and particle bound hydrophobic organic chemicals (HOCs) by snow.

…11†

It is instructive to examine the magnitude of each term in the numerator. If the snow surface area A is 0.1

34

F. Wania et al./Environmental Pollution 102 (1998) 25±41

m2/g (i.e. 105 m2/m3) and the liquid layer is 10 nm (i.e. 10ÿ8 m2/m3) thick, the volume of water vSW will be 10ÿ3 m2/m3. Since KWA is typically 102 to 104 the group vSW.KWA is typically 0.1 to 10 which usually proves to be negligible, i.e. little HOC is actually dissolved in the quasi-liquid layer. Based on the correlations given earlier kia ranges from 10ÿ4 for volatile HOCs to >10 m for less volatile HOCs, thus A.kia varies from 10 to 105. Clearly for contaminants of relatively high vapour pressure in which CAGCAP the group A.kia controls WT, there being little sorption to aerosol particles. For contaminants of low vapour pressure CAPCAG, WT approaches WP and particle scavenging controls deposition. It is noteworthy that kia is also large for these substances, thus sorption to the interface may also play an important role. Clearly the optimal strategy for determining the parameters in the deposition equation is to measure WT for a series of compounds which vary in the ratio CAP/CAG and for which estimates are available for kia. Assuming the group vSW.KWA to be negligible, WT is expected to range from A.kia to WP depending on the relative magnitudes of CAP and CAG. The above analysis assumes that equilibrium applies between CAG and the HOC sorbed to the snow ¯ake. It is instructive to examine the kinetics and extent of this process. Scavenging of gaseous HOC may be treated as a mass transfer process from bulk air to the surface of ice crystals in falling snow ¯akes. The driving force is the fugacity di€erence between bulk air and the ice surface. This process presumably consists of three steps: external (i.e. extra-snow ¯ake) di€usion, internal (intrasnow ¯ake) di€usion and surface accommodation (adsorption). External di€usion is the mass transfer across an exterior boundary layer surrounding a falling snow¯ake, the rate of which can be characterised by a mass transfer coecient which can be regarded as a ratio of di€usivity to boundary layer thickness. The internal di€usion is gas-phase pore di€usion which can be quanti®ed using an e€ective di€usivity and di€usion distance. Surface accommodation allows for the e€ect that not every molecule which arrives at the ice surface after the journey of external and internal di€usion will be adsorbed by the ice surface. Analysis of the characteristic times of equilibration for g-HCH (Ho€ et al., 1997) showed that surface accommodation is very fast compared to external and internal di€usion. It can thus be assumed that local equilibrium is reached at the air±ice interface. For snow¯akes with diameters about 1 mm the time scale of external di€usion and internal di€usion are comparable. Di€usion in such relatively small snow ¯akes is probably fast enough for equilibrium to be established between atmospheric vapour concentrations and snow ¯akes. For snow ¯akes larger than 10 mm internal di€usion controls the overall rate of gas uptake and the time a

snow ¯ake is typically suspended in the atmosphere is probably not long enough to achieve equilibrium between vapour phase and snow ¯ake surface (Ho€ et al., 1997). In that case the expression CAG.A.kia in above equation has to be complemented by a factor indicating the extent of equilibrium achieved during the descent of the snow ¯ake. Particle scavenging by snow is a complex process occurring both in and below cloud. Its contribution to wet deposition of HOCs depends on many factors, including the concentration of aerosols, the size distributions of both snow¯akes and aerosols, the hygroscopic nature of the particulate matter and the ambient conditions (Schumann et al., 1988; Mitra et al., 1990; Sparmacher et al., 1993). Barrie (1991) has reviewed in-cloud scavenging processes in which particles can serve as seeds for condensation nuclei, a process called nucleation scavenging. Smaller particles can become attached to hydrometeors via Brownian di€usion. Di€erential sedimentation can also lead to particle removal when hydrometeors move in cloud at di€erent velocities. The latter two mechanism are often termed impaction scavenging. Riming, the capture of super-cooled cloud droplets by snow crystals, can be viewed as an in-cloud impaction scavenging process. Ice crystals growing by vapour deposition tend to be rather clean (Borys et al., 1988). However, it is evident that rimed snow contains more particles than unrimed snow (Scott, 1981; Collett et al., 1991). It has been shown that within cloud impaction scavenging plays a more important role than nucleation scavenging (Borys et al., 1988; Mitra et al., 1990). In-cloud scavenging is usually the dominant mechanism for sub-micron particles (Davidson, 1989). Below-cloud scavenging is more ecient for larger particles, with impaction scavenging being the only major mechanism (Baltensperger et al., 1993). According to Murakami et al. (1983), in-cloud and below-cloud scavenging could contribute equally to the overall particle scavenging. The below-cloud scavenging of particles may be viewed as a physical process in which the falling snow ¯akes act like `®lters'. Mitra et al. (1990) de®ned a dimensionless collection eciency, EC (which can be regarded as a component of WT) using the following equation. EC ˆ

mAP ; AF
H
WAP

…12†

where m is the mass of aerosol in a snow¯ake (g), AF is the cross-sectional area of the snow ¯ake (m2), H is the height of the atmosphere over which aerosol is collected by the snow¯ake (m), and WAP is the aerosol concentration (g aerosol/m3 of air). EC is, therefore, the fraction of particles collected by snow¯akes from the air column of volume AF.H (m3). Mitra et al. (1990) compiled EC data measured by

F. Wania et al./Environmental Pollution 102 (1998) 25±41

Knutson et al. (1976), Murakami et al. (1985), Lew et al. (1986) and Sauter and Wang (1989), and gave a range of 10ÿ4 to 1, suggesting that EC increased with an increase in Stokes number, and was dependent on temperature and independent of ¯ake size for snow ¯akes with a diameter from 6 to 26 mm. In their experiments, the diameter of about 90% of aerosols was in the range 0.2 to 0.5 mm. A ®brous snow¯ake of diameter d (m) falling Y (m) will sweep out an air volume of p.d 2Y/4 m3. If the concentration of particle bound contaminant is CAP (ng/m3) and the eciency is EC, the amount of contaminant in the snow¯ake will be EC.CAP.p.d 2.Y/4 ng. On melting the concentration in the melt water (CSP) will be this quantity divided by the melt water volume, namely p.d 3/ 6 (S/W) where W and S are the densities of water and snow and WP will be: WP ˆ

CSP 3
EC
Y
W ˆ : CAP 2
d
S

…13†

In reality the ¯ake is ®brous and not solid or spherical; however, this can be accounted for by assigning a low bulk density to the snow of perhaps 0.25 g/ml. As pointed out by Pruppacher (1981) and Schumann et al. (1988), WP is a function of many factors, including height of the cloud base, the size distribution of both hydrometeors and aerosol, the hygroscopic nature of the particulate matter and the ambient temperature and so on. The maximum possible WP (EC=1) for a Y of 2000 m, d of 10 mm, rW of 1 g/ml and rS of 0.25 g/ml is 1.2106. This value is based on below-cloud scavenging only. If the in-cloud scavenging contributes another 50% as indicated by Murakami et al. (1983), the overall WP may reach a magnitude of 2106. WP is proportional to EC which is determined by the microphysics of particle±snow¯ake interactions. The microphysics of particle scavenging by snow crystals has been modelled (Martin et al., 1980a, b; Wang and Pruppacher, 1980; Wang, 1985, 1989; Miller and Wang, 1989, 1991). In these studies, the eciencies of snow scavenging of particles of radii 0.001 to 10 mm were estimated based on Brownian di€usion, inertia and electrostatic, thermophoretic, and di€usiophoretic forcing. The models suggest that particle size is an important factor; for larger particles (>1 mm) inertial impaction, and for smaller particles (<0.01 mm) Brownian di€usion predominates. For medium size particles thermophoresis and di€usiophoresis play key roles in determining scavenging eciency. Since the phoretic e€ects are normally weaker than Brownian and inertial e€ects, the collection eciency is low for particles of medium size. A minimum collection eciency is predicted for particles in the size range of 0.01 to 1.0 mm radius, i.e. the Green®eld gap (Green®eld, 1957) where

35

Brownian di€usion and inertia become insigni®cant. Miller and Wang's (1991) more recent theoretical study suggests that the minima are dependent of crystal shapes: at 0.04 and 3 mm for the plate-like and columnar crystals, respectively. The limited number of studies on size fractionation in the atmosphere (e.g. Kaupp et al., 1994; Poster et al., 1995) suggest that HOCs may be mostly associated with the small particles (<1 mm) with the lowest collection eciency. Another important factor is relative humidity (RH). Since unsaturated conditions enhance the phoretic e€ects, a lower RH should result in a higher collection eciency, particularly for particles in the Green®eld gap. Miller (1990) predicted that a 5% change in relative humidity would lead to a change in the scavenging rate up to one order of magnitude. Experiments by Sparmacher et al. (1993) suggested, however, a weaker dependence of scavenging eciency on relative humidity. Treating a snow¯ake as a spherical ®brous ®lter falling freely in air, Redkin (1973) studied the mechanics of air ¯ow across snow¯akes. He concluded that in the absence of electrostatic forces snow¯akes were more ecient than rain drops in scavenging aerosols of all sizes due to the seepage ¯ow through the ®brous structure. His model predicts that snow¯akes are ®ve to eight times more e€ective than rain drops for particles 0.2 to 2 mm in radius. Particles carried by the seepage ¯ow will collide with and attach to the branches of ice crystals. Contribution of the seepage ¯ow to aerosol scavenging may not be linear. With an increase of permeability the enlarged seepage ¯ow will reduce the drag, and speed up the falling process. Higher terminal velocities will generate stronger turbulence and promote aerosol collection. On the other hand, very permeable snow¯akes may not function well as `®lters' because their pores are too large compared to the particles. The heterogeneous mass distribution of ice in snow ¯akes may play an important roles in determining collection eciency as observed by Li and Logan (1997) in aqueous systems. The general conclusion is that snow is a very ecient vector for scavenging both gaseous and particulate HOC from the atmosphere with values of WT being expected to reach approximately 2106. An implication is that a heavy snow fall has the potential to remove a very large fraction of the HOC present in the atmosphere. This is illustrated by a simple mass balance calculation. If snow falls through an atmosphere of height Y m and horizontal area A m2, with a WT of 106 at a rate of S m3 of melt water per hour, the rate of HOC removal will be WT.CA.S ng/h where CA is the total concentration in air. The amount in the air column is Y.CA.A ng, thus the change in mass of HOC in the well-mixed air will be given by:

36

F. Wania et al./Environmental Pollution 102 (1998) 25±41

d…Y
CA
A† ˆ ÿWT
CA
S: dt

…14†

Integration of this ®rst order equation gives: CA ˆ CA0
e

WT
S Y
A
t

t

ˆ CA0
eÿ ;

…15†

where  is the time for removal of 63% of the HOC and CA0 is the initial concentration CA.  is thus Y.A/S.WT. A heavy snow fall at a rate of 4 to 5 cm bulk snow per hour is approximately 1 cm of melt water per hour, i.e. S/A is 0.01 m/h. For Y of 2000 m and WT of 106,  is thus 0.2 h implying that a sustained snow fall through an air column can remove essentially all the HOC in a period of an hour. It is thus likely that measured values of WT will be very variable, depending on the history of air±snow contact. It is not surprising that periods of excellent visibility with an almost total absence of haze are observed after snow fall. A further implication is that there may be little merit in quantifying the deposition processes with high accuracy if the ®nal conclusion is simply that all the HOC is deposited. Testing this hypothesis requires that measurements be made of snow and air concentrations during snow fall under conditions of well-de®ned meteorology. 6.2. Partitioning and fate in the snow pack Fig. 2 illustrates the processes which may occur in a snow pack on land, while Fig. 3 shows the corresponding processes for water. As the result of gas and particle

scavenging processes, a snow pack contains HOC in four forms: bound to the ice surface, sorbed to particles, as vapour in the interstitial air, and dissolved in liquid water. Their relative contributions are determined by physical and chemical properties of the chemical and the snow pack. Assuming the snow pack is in equilibrium with air containing CAG (ng/m3) of HOC vapour, the total amount of HOC in 1 m3 of snow pack is. M ˆ …A
kia ‡ vSP
KPW
KWA ‡ vSA ‡ vSW
KWA †
CAG ;

…16†

where vSP is the volume fraction of aerosol (m3/m3), vSA is the volume fraction of air (m3/m3), vSW is the volume fraction of water (m3/m3), A is the speci®c surface area of ice crystals in the snow pack (m2/m3), and KPW is the particle±water partition coecient. In fresh snow, ice surface-partitioning usually dominates, particularly for less volatile HOCs (Ho€ et al., 1995) and is quanti®ed by the A.kia group. For the particle-bound HOC, vSP and KPW, which often should be related to KOW, are determinants. The contribution of the interstitial air depends on vSA which is a function of porosity. It is probably erroneous to assume that HOC which is particle bound in the freshly fallen snow remains as such in the pack. It is likely that there is a continuous redistribution between the four forms as the snow pack ages. Since the area, volume and volume fractions change, the sorptive capacity of a snow pack is time dependent.

Fig. 2. Post-depositional processes a€ecting hydrophobic organic chemicals (HOCs) fate in a terrestrial snow pack.

F. Wania et al./Environmental Pollution 102 (1998) 25±41

37

Fig. 3. Post-depositional processes a€ecting hydrophobic organic chemicals (HOCs) fate in a marine or aquatic system.

Depending on ambient conditions, fallen snow undergoes many physical changes, such as subliming, compacting, sintering, freezing and melting. In a dry snow pack when temperatures are below the freezing point, sintering can lead to a continuous reduction in speci®c surface area and porosity with the corresponding increase in grain size and the decrease in grain population by vapour transfer. The storage capacity of snow pack for ice surface-bound and vapour HOC decrease with time. The rate of sintering is very sensitive to temperature change; at ÿ10 C a signi®cant change in surface area was found within hours while almost no change was determined after days at ÿ35 C (Jellinek and Ibrahim, 1967). Temperature may have other e€ects; e.g. at higher temperatures both kia and KWA are smaller, which translates into smaller sorptive capacities. The rate of sintering is also a function of initial surface area; a larger initial surface area results in a faster sintering process, since smaller particles are less stable. If the particle phase is non-volatile and nonreactive, the capacity for particle-bound HOCs should be time independent. Overall, the capacity of the pack is expected to decrease with time at a rate controlled by temperature. Over a period of months, and in polar climates over years, the pack undergoes continued metamorphosis with an increase in density and formation of an impermeable or non-di€usive zone (Gray and Male, 1981; Schwander, 1996). There may be percolation by melt water and refreezing (LaChapelle, 1969). Wania (1997) has developed a model which provides a ®rst quantitative treatment of the processes a€ecting HOC fate in an ageing homogenous snow pack. The

model simulates how HOCs partition between four snow pack compartments (air-®lled pore space, liquid water, organic matter, air±ice interface) and estimates losses by volatilisation and by drainage with the melt water in the course of di€erent scenarios of snow pack ageing. Illustrative calculations suggest that relatively volatile HOCs such as chlorobenzenes are likely to evaporate rapidly to the atmosphere, relatively water soluble chemicals such as HCHs are lost predominantly with the drainage melt water, whereas involatile and very hydrophobic chemicals such as DDT are associated with the organic matter throughout the entire snow metamorphosis. The model calculations make assumptions about the changes in physical snow pack properties, especially surface area. There is a need to expand this modelling e€ort to treat a `multi-layer' snow pack and deduce how HOC levels will vary with depth in the snow pack. Obviously there is a need for complementary measurements of concentrations in the ageing and sintering pack as a function of time. Such measurements and models should cast light on the issue of what fraction of the deposited HOC evaporates back to the atmosphere, an important factor in regional modelling and in assessing the local impact of HOCs deposited with snow. Also important is the identi®cation of the time period during melting when there may be a pulse or a ¯ush of high HOC levels in melt water. This period of high concentration and exposure may occur at critical times in the growth and reproduction of certain species. Whereas in temperate regions the steady input of HOC leading to chronic low level exposure may be tolerable,

38

F. Wania et al./Environmental Pollution 102 (1998) 25±41

in colder regions the exposure may be more focused in time and thus more severe. 7. Future research needs and conclusions From a consideration of the present state of knowledge, we suggest a number of priority areas for research: 1. measurement of partitioning of HOCs (especially less volatile HOCs) to ice surfaces; 2. measurements of snow speci®c area and its changes with time in the snow pack; 3. measurement of air and snow concentrations simultaneously and over time during snow-fall events; 4. continued attempts to model the scavenging process and the overall e€ect of scavenging on levels in the a€ected air mass; 5. measurements and models of the fate of HOCs in ageing snow packs especially to determine rates and proportions evaporated and leached in melt water; and 6. assessment of the feasibility of using snow as a monitor of past and present HOC levels. Whereas there is a need for continued measurement of concentrations of HOCs in snow, the resulting data are of relatively little value unless this work is supplemented by a deeper understanding of the prevailing physical and chemical processes which control these concentrations. The available evidence is compelling that snow is a major vector for HOC transport between the lower troposphere and the ground. It is clear that snow is more e€ective in scavenging airborne HOCs than rain. Adsorption to the ice surface is a major mechanism of scavenging gaseous HOCs. The speci®c surface area of ice crystals and the air±ice partition coecient are the two key parameters in this process. Snow is also an ecient scavenger of aerosols and thus aerosol-bound HOCs. Snow pack metamorphosis is expected to play a key role in determining the post-depositional behaviour of HOCs. Although much has been accomplished, much remains to be done before the full role of snow and ice as determinants of HOC fate in cold climates can be assessed and quanti®ed. Acknowledgements The authors are grateful for ®nancial support from the Natural Science and Engineering Research Council of Canada, the Atmospheric Environment Service of Environment Canada, the Waterloo Centre for Groundwater Research, the Northern Contaminants

Programme of the Canadian Department of Indian A€airs and Northern Development and the consortium of chemical companies which support the Environmental Modelling Centre at Trent University. References AMAP (Arctic Monitoring and Assessment Programme), 1997. Arctic Pollution Issues: a State of the Arctic Environment Report. Arctic Monitoring and Assessment Programme, Oslo, Norway. Barrie, L.A., 1991. Snow formation and processes in the atmosphere that in¯uence its chemical composition. In: Davies, T.D. et al. (Eds.), Seasonal Snowpacks NATO ASI Series Vol. G 28. Springer, Berlin, pp. 1±20. Barrie, L., MacDonald, R., Bidleman, T., Diamond, M., Gregor, D., Semkin, R., Strachan, W., Alaee, M., Backus, S., Bewers, M., Gobeil, C., Halsall, C., Ho€, J., Li, A., Lockhart, L., Mackay, D., Muir, D., Pudykiewicz, J., Reimer, K., Smith, J., Stern, G., Schroeder, W., Wagemann, R., Wania, F., Yunker, M., 1997. Chapter 2. Sources, occurrence and pathways. In: Jensen, J., Adare, K., Shearer, R., (Eds.), Canadian Arctic Contaminants Assessment Report, Indian and Northern A€airs Canada, Ottawa, Canada, pp. 25±182. Baltensperger, U., Schwikowski, M., GaÈggeler, H.W., Jost, D.T., Beer, J., Siegenthaler, U., Wagenbach, D., Hofmann, H.J., Synal, H.A., 1993. Transfer of atmospheric constituents into an alpine snow ®eld. Atmospheric Environment 27A, 1881±1890. Borys, R.D., Hindman, E., Demott, P.L., 1988. The chemical fractionation of atmospheric aerosol as a result of snow crystal formation and growth. Journal of Atmospheric Chemistry 7, 213±239. Brimblecombe, P., Conklin, M.H., 1996. Thermodynamics of the solute layer on the surface of ice. In: Wol€, E.W., Bales, R.C. (Eds.), Chemical Exchange Between the Atmosphere and Polar Snow. NATO ASI. Series Vol. I43. Springer, Berlin, pp. 517±526. Brun, G.L., Howell, G.D., O'Neill, J.J., 1991. Spatial and temporal patterns of organic contaminants in wet precipitation in Atlantic Canada. Environmental Science and Technology 25, 1249±1261. Colbeck, S.C., 1981. A simulation of the enrichment of atmospheric pollutants in snow cover runo€. Water Resources Research 17, 1383±1388. Collett, J.L., Jr., PreÂvoÃt, A.S.H., Staehelin, J., Waldvogel, A., 1991. Physical factors in¯uencing winter precipitation chemistry. Environmental Science and Technology 25, 782±788. Conklin, M.H., Sommerfeld, R.A., Laird, S.K., Villinski, J.E., 1993. Sulfur dioxide reactions on ice surfaces: implications for dry deposition to snow. Atmospheric Environment 27A, 159±166. Czuczwa, J., Leuenberger, C., Giger, W., 1988. Seasonal and temporal changes of organic compounds in rain and snow. Atmospheric Environment 22, 907±916. Daub, J., FoÈrster, J., Herrmann, R., Robien, A., Striebel, T., 1994. Chemodynamics of trace pollutants during snow melt on roof and street surfaces. Water Science and Technology 30, 73±85. Davidson, C.I., 1989. Mechanisms of wet and dry deposition of atmospheric contaminants to snow surfaces. In: Oeschger, H., Langway, C. (Eds.), The Environmental Record in Glaciers and Ice Sheets. Wiley, Chichester, pp. 29±51. Davies, T.D., Tranter, M., Jickells, T.D., Abrahams, P.W., Landsberger, S., Jarvis, K., Pierce, C.E., 1992. Heavily-contaminated snow falls in the remote Scottish highlands: a consequence of regional-scale mixing and transport. Atmospheric Environment 26A, 95±112. Desideri, P., Lepri, L., Santianni, D., Checcini, L., 1991. Chlorinated pesticides in sea water and pack ice in Terra Nova Bay (Antarctica). Annali di Chimica 81, 533±540.

F. Wania et al./Environmental Pollution 102 (1998) 25±41 Finizio, A., Mackay, D., Bidleman, T.F., Harner, T., 1997. Octanol± air partition coecient as a predictor of partitioning of semi-volatile organic chemicals to aerosols. Atmospheric Environment 31, 2289±2296. Fletcher, N., 1973. The surface of ice. In: Whalley, E., Jones, S., Gold, L. (Eds.), Physics and Chemistry of Ice. Royal Society of Canada, Ottawa. Franz, T.P., 1994. Deposition of semivolatile organic chemicals by snow. PhD thesis, University of Minnesota, Minneapolis, MN. Franz, T.P., Eisenreich, S.J., 1993. Wet deposition of polychlorinated biphenyls to Green Bay, Lake Michigan. Chemosphere 26, 1767± 1788. FranzeÂn, L.G., Hjelmroos, M., KaÊllberg, P., BrorstroÈm-LundeÂn, E., Juntto, S., Savolainen, A.-L., 1994. The ``Yellow Snow'' episode of Northern Fennoscandia, March 1991Ða case study of long distance transport of soil, pollen and stable organic compounds. Atmospheric Environment 28, 3587±3604. Fuoco, R., Colombini, M.P., Abete, C., 1991. Evaluation of pack melting e€ect on polychlorobiphenyl content in sea water samples from Terra Nova BayÐRoss Sea (Antarctica). Annali di Chimica 81, 383±394. Gaul, H., 1989. Organochlorine compounds in water and sea ice of the European Arctic Sea. In: 8th International Conference of Comite Arctique International, Oslo, Sept. 18±22, 1989. Goss, K.-U., 1992. E€ects of temperature and relative humidity on the sorption of organic vapors on sand. Environmental Science and Technology 26, 2287±2294. Goss, K.-U., 1993a. E€ects of temperature and relative humidity on the sorption of organic vapors on clay minerals. Environmental Science and Technology 27, 2127±2132. Goss, K.-U., 1993b. Adsorption of organic vapors on ice and quartz sand at temperatures below 0 C. Environmental Science and Technology 27, 2826±2830. Goss, K.-U., 1994. Adsorption of organic vapors on polar mineral surfaces and on a bulk water surface: development of an empirical predictive model. Environmental Science and Technology 28, 640± 645. Granat, L., Johansson, C., 1983. Dry deposition of SO2 and NOX in winter. Atmospheric Environment 17, 191±192. Gray, D.M., Male, D.H., 1981. Handbook of Snow: Principles, Processes, Management and Use. Pergamon Press, Canada. Green®eld, S., 1957. Rain scavenging of radioactive particulate matter from the atmosphere. Journal of Meteorology 14, 115±125. Gregor, D.J., 1990. Deposition and accumulation of selected agricultural pesticides in Canadian Arctic snow. In: Kurtz, D. (Ed.), Long Range Transport of Pesticides. Lewis, Chelsea, MI, pp. 373± 386. Gregor, D.J., Gummer, W.D., 1989. Evidence of atmospheric transport and deposition of organochlorine pesticides and polychlorinated biphenyls in Canadian Arctic snow. Environmental Science and Technology 23, 561±565. Gregor, D.J., Peters, A.J., Teixeira, C.F., Jones, N.P., Spencer, C., 1995. The historical residue trend of PCBs in the Agassiz Ice Cap, Ellesmere Island, Canada. Science of the Total Environment 160/ 161, 117±126. Gregor, D.J., Teixeira, C.F., Rowsell, R., 1996. Deposition of atmospherically transported polychlorinated biphenyls in the Canadian Arctic. Chemosphere 33, 227±244. Gunz, D.W., Ho€mann, M.R., 1990. Field investigations on the snow chemistry in central and southern CaliforniaÐII. Carbonyls and carboxylic acids. Atmospheric Environment 24A, 1673±1684. Haglund, P., Alsberg, T., Bergman, AÊ., Jansson, B., 1987. Analysis of halogenated aromatic hydrocarbons in urban air, snow and automobile exhaust. Chemosphere 16, 2442±2450. Hargrave, B.T., Vass, W.P., Erickson, P.E., Fowler, B.R., 1988. Atmospheric transport of organochlorines to the Arctic Ocean. Tellus 40B, 480±493.

39

Hargrave, B.T., Vass, W.P., Erickson, P.E., Fowler, B.R., 1989. Distribution of chlorinated hydrocarbon pesticides and PCBs in the Arctic Ocean. Canadian Technical Report of Fisheries and Aquatic Sciences 1644. Hartkopf, A., Karger, B.L., 1973. Study of the interfacial properties of water by gas chromatography. Accounts of Chemical Research 6, 209±216. Herrmann, R., 1978. Regional patterns of polycyclic aromatic hydrocarbons in NE-Bavarian snow and their relationships to anthropogenic in¯uence and air ¯ow. Catena 5, 165±175. Hogan, A., Leggett, D., 1995. Soil-to-snow movement of synthetic organic compounds in natural snow pack. In: Biochemistry of Seasonally Snow-Covered Catchments. Proceedings of a Boulder Symposium, July 1995. IAHS Publ. No. 228, pp. 97±105. Ho€, J.T., Mackay, D., Gillham, R., Shiu, W.Y., 1993. Partitioning of organic chemicals at the air±water interface in environmental systems. Environmental Science and Technology 27, 2174±2180. Ho€, J.T., Wania, F., Mackay, D., Gillham, R., 1995. Sorption of non-polar organic vapors by ice and snow. Environmental Science and Technology 29, 1982±1989. Ho€, J.T., Jia, C.Q., Wania, F., Mackay, D., 1997. Scavenging of organic vapors from the Arctic atmosphere by falling snow¯akes. In: The AMAP International Symposium on Environmental Pollution in the Arctic, Tromsù, Norway, June 1±5, 1997, Extended Abstracts, pp. 242±243. Ho€, J.T., Gregor, D.J., Mackay, D., Wania, F., Jia, C.Q., 1998. Measurement of the speci®c surface area of snow using the nitrogen adsorption technique. Environmental Science and Technology 32, 58±62. Ja€rezo, J.L., Clain, M.P., Masclet, P., 1994. Polycyclic aromatic hydrocarbons in the polar ice of Greenland. Geochemical use of these atmospheric tracers. Atmospheric Environment 28, 1139± 1145. Jaworowski, Z., 1994. Ancient atmosphereÐvalidity of ice records. ESPRÐEnvironmental Science and Pollution Research 1, 161±171. Jellinek, H., 1967. Liquid-like (transition) layer on ice. Journal of Colloid and Interface Science 25, 192±205. Jellinek, H., Ibrahim, S., 1967. Sintering of powdered ice. Journal of Colloid and Interface Science 25, 245±254. Johannessen, M., Henriksen, A., 1978. Chemistry of snow melt water: changes in concentration during melting. Water Resources Research 14, 615±619. Kaupp, H., Towara, J., McLachlan, M.S., 1994. Distribution of polychlorinated dibenzo-p-dioxins and dibenzofurans in atmospheric particulate matter with respect to particle size. Atmospheric Environment 28, 585±593. Kawamura, K., Kaplan, I.R., 1986. Biogenic and anthropogenic organic compounds in rain and snow samples collected in Southern California. Atmospheric Environment 20, 115±124. Knutson, E.D., Sood, S.K., Stockham, J.J., 1976. Aerosol collection by snow and ice crystals. Atmospheric Environment 10, 395±402. LaChapelle, E.R., 1969. Field Guide to Snow Crystals. University of Washington Press, Seattle. Lew, J.K., Montagne, D.C., Pruppacher, H.R., 1986. A wind tunnel investigation of the riming of snow ¯akes. Part II: natural and synthetic aggregates. Journal of Atmospheric Science 43, 2410±2417. Leggett, D.C., Hogan, A.W., 1995. A preliminary experiment to examine chemical exchange at the soil±snow interface. Science of the Total Environment 160/161, 403±408. Li, X., Logan, B.E., 1997. Collision frequencies of fractal aggregates with small particles by di€erential sedimentation. Environmental Science and Technology 31, 1229±1236. Lunde, G., Gether, J., Gjùs, N., Stùbet Lande, M.-B., 1977. Organic micropollutants in precipitation in Norway. Atmospheric Environment 11, 1007±1014. Mackay, D., 1991. Multimedia Environmental Models, the Fugacity Approach. Lewis, Chelsea, MI.

40

F. Wania et al./Environmental Pollution 102 (1998) 25±41

Marklund, S., Tysklind, M., Andersson, R., Ljung, K., Rappe, C., 1991. Environmental deposition of PCDDs and PCDFs as determined by the analysis of snow samples from the Northern Sweden. Chemosphere 23, 1264±1359. Martin, J.J., Wang, P.K., Pruppacher, H.R., 1980a. A theoretical study of the e€ect of electric charges on the eciency with which aerosol particles are collected by ice crystal plates. Journal of Colloid and Interface Science 78, 44±55. Martin, J.J., Wang, P.K., Pruppacher, H.R., 1980b. A theoretical determination of the eciency with which aerosol particles are collected by simple ice crystal plates. Journal of Atmospheric Science 37, 1628±1638. McNeely, R., Gummer, W.D., 1984. A reconnaissance study of the environmental chemistry in East±Central Ellesmere Island, N.W.T.. Arctic 37, 210±223. McVeety, B.D., Hites, R.A., 1988. Atmospheric deposition of polycyclic aromatic hydrocarbons to water surfaces: a mass balance approach. Atmospheric Environment 22, 511±536. Meyers, P.A., Hites, R.A., 1982. Extractable organic compounds in midwest rain and snow. Atmospheric Environment 16, 2169±2175. Miller, N.L., 1990. A model for the determination of the scavenging rates of submicron aerosols by snow crystals. Atmospheric Research 25, 317±330. Miller, N.L., Wang, P.K., 1989. A theoretical determination of the eciency with which aerosol particles are collected by falling columnar ice crystals. Journal of Atmospheric Science 46, 1656± 1663. Miller, N.L., Wang, P.K., 1991. A theoretical determination of the collection rates of aerosol particles by falling ice crystal plates and columns. Atmospheric Environment 25A, 2593±2606. Mitra, S.K., Barth, U., Pruppacher, H.R., 1990. A laboratory study of the eciency with which aerosol particles are scavenged by snow ¯akes. Atmospheric Environment 24A, 1247±1254. Murakami, M., Kimura, T., Magono, C., Kikuchi, K., 1983. Observations of precipitation scavenging for water soluble particles. Journal of the Meteorological Society of Japan 61, 346±357. Murakami, M., Kikuchi, K., Magono, C., 1985. Experiments on aerosol scavenging by natural snow crystals, Part I and II. Journal of the Meteorological Society of Japan 63, 119±135. Murphy, T.F., Rzeszetko, C.P., 1977. Precipitation input of PCBs to Lake Michigan. Journal of Great Lakes Research 3, 305±312. Ocampo, J., Klinger, J., 1982. Adsorption of N2 and CO2 on ice. Journal of Colloid and Interface Science 86, 377±383. Orem, M.W., Adamson, A.W., 1969. Physical adsorption of vapor on ice. II. n-alkanes. Journal of Colloid and Interface Science 31, 278±286. Paasivirta, J., Knuutila, M., Paukku, R., Herve, S., 1985. Study of organochlorine pollutants in snow at the North Pole and comparison to the snow at North, Central and South Finland. Chemosphere 14, 1741±1748. Pankow, J.F., 1997. Partitioning of semi-volatile organic compounds to the air/water interface. Atmospheric Environment 31, 927±929. Patton, G.W., Hinckley, D.A., Walla, M.D., Bidleman, T.F., Hargrave, B.T., 1989. Airborne organochlorines in the Canadian High Arctic. Tellus 41B, 243±255. Peel, D.A., 1975. Organochlorine residues in Antarctic snow. Nature 254, 324±325. Peterle, T.J., 1969. DDT in Antarctic snow. Nature 224, 620. Peters, A.J., Gregor, D.J., Teixeira, C.F., Jones, N.P., Spencer, C., 1995. The recent depositional trend of polycyclic aromatic hydrocarbons and elemental carbon to the Agassiz Ice Cap, Ellesmere Island. Canada. Science of the Total Environment 160/161, 167±179. P®rman, S.L., Eicken, H., Bauch, D., Weeks, W.F., 1995. The potential transport of pollutants by Arctic sea ice. Science of the Total Environment 159, 129±146. Pham, T., Lum, K.R., Lemieux, C., 1996. Seasonal variation of DDT and its metabolites in the St. Lawrence River (Canada) and four of its tributaries. Science of the Total Environment 179, 17±26.

Pomeroy, J.W., Jones, H.G., 1996. Wind-blown snow: sublimation, transport and changes to polar snow. In: Wol€, E.W., Bales, R.C. (Eds.), Chemical Exchange Between the Atmosphere and Polar Snow. NATO ASI Series, Vol. I 43, Springer, Berlin, pp. 453±489. Poster, D.L., Ho€, R.M., Baker, J.E., 1995. Measurement of the particle-size distributions of semivolatile organic contaminants in the atmosphere. Environmental Science and Technology 29, 1990±1997. Pruppacher, H.R., 1981. The microstructure of atmospheric clouds and precipitation. In: Hobbs, P.V., Deepack, A. (Eds.), Clouds: Their Formation, Optical Properties and E€ects. Academic Press, New York, pp. 93±186. QueÂmerais, B., Lemieux, C., Lum, K.R., 1994. Temporal variation of PCB concentrations in the St. Lawrence River (Canada) and four of its tributaries. Chemosphere 28, 947±959. Rahm, L., HaÊkansson, B., Larsson, P., Fogelqvist, E., Bremle, G., Valderrama, J., 1995. Nutrients and persistent pollutant deposition on the Bothnian Bay ice and snow ®elds. Water Air and Soil Pollution 84, 187±201. Redkin, Yu. N., 1973. Washout of atmospheric aerosols by snow in the surface layer. In: Voloshchuk, V.M., Sedunov, Yu. S., (Eds.), Hydrodynamics and Thermodynamics of Aerosols. John Wiley, New York, pp. 226±238. Risebrough, R.W., Walker II, W., Schmidt, T.T., de Lappe, B.W., Connors, C.W., 1976. Transfer of chlorinated bipheyls to Antarctica. Nature 264, 738±739. Sauter, D.P., Wang, P.K., 1989. An experimental study of the scavenging of aerosol particles by natural snow crystals. Journal of Atmospheric Science 46, 1650±1655. SchoÈndorf, T., Hermann, R., 1987. Transport of organic micropollutants and ions during snow melt. Nordic Hydrology 18, 259± 278. Schrimp€, E., Thomas, W., Hermann, R., 1979. Regional patterns of contaminants (PAH, pesticides and trace metals) in snow of Northeast Bavaria and their relationship to human in¯uence and orographic e€ects. Water, Air and Soil Pollution 11, 481±497. Schumann, T., Zinder, B., Waldvogel, A., 1988. Aerosol and hydrometeor concentrations and their chemical composition during winter precipitation along a mountain slopeÐI. Temporal evolution of the aerosol, microphysical and meteorological conditions. Atmospheric Environment 22, 1443±1459. Schwander, J., 1996. Gas di€usion in ®rn. In: Wol€, E.W., Bales, R.C. (Eds.), Chemical Exchange Between the Atmosphere and Polar Snow. NATO ASI. Series Vol. I43. Springer, Berlin, pp. 527±540. Scott, B.C., 1981. Sulphate wash out ratios in winter storms. Journal of Applied Meteorology 20, 619±625. Semkin, R.G., 1996. Processes and Fluxes of contaminants in aquatic systemsÐ1994/95. In: Murray, J.L., Shearer, R.G., Han, S.L., (Eds.), Synopsis of Research Conducted Under the 1994/95 Northern Contaminants Program. Indian and Northern A€airs Canada, Environmental Studies report No. 73, pp. 105±118. Semkin, R.G., Je€ries, D.S., 1988. Chemistry of atmospheric deposition, the snow pack and snow melt in the Turkey Lakes watershed. Canadian Journal of Fisheries and Aquatic Science 45(Suppl. 1), 38±46. Simmleit, N., Herrmann, R., 1987. The behaviour of hydrophobic organic micropollutants in di€erent karst water systems. 1. Transport of micropollutants and contaminant balances during the melting of snow. Water, Air and Soil Pollution 34, 79±95. Simmleit, N., Herrmann, R., Thomas, W., 1986. Chemical behaviour of hydrophobic micropollutants during the melting of snow. IAHS Publication Series 155, 335±346. Smagin, W.M., Melnikov, S.A., Wodowatowa, S.M., Wlasow, S.W., 1987. Investigation of the contamination of the snow and ice cover of the Arctic seas. In: Monitoring of Background Contamination of Natural Media, Vol. 4. Hydrometeor Press, Leningrad, pp. 246±255 (in Russian).

F. Wania et al./Environmental Pollution 102 (1998) 25±41 Sommerfeld, R.A., Conklin, M.H., Laird, K.J., 1992. NO adsorption on ice at low concentrations. Journal of Colloid and Interface Science 147, 569±574. Sparmacher, H., Fulber, K., Bonka, H., 1993. Below-cloud scavenging of aerosol particles: particle-bound radionuclides-experiments. Atmospheric Environment 27A, 605±618. Stengle, T.R., Lichtenberg, J.J., Houston, C.S., 1973. Sampling of glacial snow for pesticide analysis on the high plateau glacier of Mount Logan. Arctic 26, 335±336. Strachan, W.M.J., Huneault, H., 1979. Polychlorinated biphenyls and organochlorine pesticides in Great Lakes precipitation. Journal of Great Lakes Research 5, 61±68. Swackhamer, D.L., McVeety, B.M., Hites, R.A., 1988. Deposition and evaporation of polychlorinated biphenyl congeners to and from Siskiwit Lake, Isle Royale, Lake Superior. Environmental Science and Technology 22, 664±672. Swain, W.R., 1978. Chlorinated organic residues in ®sh, water and precipitation from vicinity of Isle Royale, Lake Superior. Journal of Great Lakes Research 4, 398±407. Tanabe, S., Hidaka, H., Tatsukawa, R., 1983. PCBs and chlorinated hydrocarbon pesticides in antarctic atmosphere and hydrosphere. Chemosphere 12, 277±288. Thorne, P., 1996. Preliminary trials of the use of immunoassay screening for chlordane in Arctic sea ice cores. 53rd Eastern Snow Conference, Williamsburg, Virginia 1996, pp. 177±180. Tranter, M., Brimblecombe, P., Davies, T.D., Vincent, C.E., Abrahams, P.W., Blackwodd, I., 1986. The composition of snow fall, snow pack and melt water in the Scottish HighlandsÐevidence for preferential elution. Atmospheric Environment 20, 517±525.

41

Valsaraj, K.J., 1988. Binding constants for non-polar hydrophobic organics at the air±water interface: comparison of experimental and predicted values. Chemosphere 17, 2049±2053. Valsaraj, K.T., 1994. Hydrophobic compounds in the environment: adsorption equilibrium at the air±water interface. Water Research 28, 819±830. Valsaraj, K.T., Thoma, G.J., Reible, D.D., Thibodoaux, L.J., 1993. On the enrichment of hydrophobic organic compounds in fog droplets. Atmospheric Environment 27A, 203±210. Wang, P.K., 1985. A convective di€usion model for the scavenging of submicron aerosol particles by snow crystals of arbitrary shapes. Journal de Recherches Atmospheriques 19, 185±191. Wang, P.K., 1989. A convective di€usion model for the scavenging of submicron aerosol particles by snow crystals of arbitrary shapes: some comments and corrections. Atmospheric Research 23, 195±198. Wang, P.K., Pruppacher, H.R., 1980. The e€ect of an external electric ®eld on the scavenging of aerosol particles by cloud and small raindrops. Journal of Colloid and Interface Science 75, 286±297. Wania, F., 1997. Modelling the behaviour of non-polar organic chemicals in an ageing snow pack. Chemosphere 35, 2345±2363. Wania, F., Mackay, D., 1993. Global fractionation and cold condensation of low volatility organochlorine compounds in polar regions. Ambio 22, 10±18. Welch, H.E., Muir, D.C.G., Billeck, B.N., Lockhart, W.L., Brunskill, G.J., Kling, H.J., Olson, M.P., Lemoine, R.M., 1991. Brown snow: a long range transport event in the Canadian Arctic. Environmental Science and Technology 25, 280±286. Wol€, E.W., Peel, D.A., 1985. The record of global pollution in polar snow and ice. Nature 313, 535±540.