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 0:68ln 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 ÿ0:769ln 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 ÿ0:615ln 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 ÿ1:20ln pL ÿ 7:53 for PAHs
…4†
ln kia …20 C† ˆ ÿ0:93ln 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 3:20ln 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 Akia ;
…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 ‡ Akia † ‡ 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 HWAP
…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 3EC YW
ˆ
:
CAP
2dS
…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…YCA A†
ˆ ÿWT CA S:
dt
…14†
Integration of this ®rst order equation gives:
CA ˆ CA0 e
WT S
YA 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 …Akia ‡ 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.
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