M. Tranter

The composition of snowfall, snowpack and meltwater in the Scottish Highlands ― evidence for preferential elution

Abstract Acidic snows in a small, remote, high-altitude snowpack in the Cairngorms, Scotland, give rise to meltwaters which are proportionally rich in sulphate and nitrate. As a consequence, the within-pack snows become proportionally rich in chloride, even though depleted in solute. Preferential elution appears to be a major process in the chemical evolution of snowfall and snowpack.

Observations of the preferential loss of major ions from melting snow and laboratory ice

Abstract Two types of laboratory experiments were undertaken to understand changes in the ionic composition of solutions from melting snows: (1) a set of field-laboratory experiments which involved partially melting recently collected snows and (2) experiments on the controlled melting of artificial ices of known composition. Sulphate and nitrate ions are preferentially lost with respect to chloride during the melting process so apparent elution sequence may be written SO 4 2− > NO 3 − > Cl − . Of the cations, sodium appears to be removed least readily giving an elution sequence Mg 2+ = K + > Na + . In the laboratory the differences in the efficiency of removal of ions from ice are small in the first stages of melting, but pronounced by the end of the melt. Sodium and chloride are expected to be proportionally enriched in residual leached snow packs. This is generally born out by field observations. The preferential loss of some ions is more evident in aged laboratory ice than in the freshly made ice. The changes in the ice and meltwater composition throughout the process of melting may be understood to arise from the mixing of two types of solutions: (i) an intergranular surficial brine with high solute concentration that occupies the ice grain boundaries and (ii) the meltwater from the ice grains.

The Removal of Soluble Ions from Melting Snowpacks

Field and laboratory observations have indicated that the “fractionation” mechanism, whereby the bulk of the solute in seasonal snowcover is removed in the early meltwater fractions, is a differential process. Some soluble ions may be removed at faster rates than others. This process is known as “preferential elution”. Since the process is not yet universally accepted amongst scientists who engage in research into snowmelt chemistry, in this chapter we review the pertinent experimental work. Varying experimental conditions in real snowpacks often militate against reproducible elution sequences, although sodium and chloride are consistently found to be the least mobile ions upon melt. Laboratory experiments have provided more controlled conditions under which to examine the relative removal rates of soluble ions in meltwater. Those ions which are more soluble in ice are seen to be less mobile upon melt, whereas the ions which are eluted more rapidly may be regarded as being more strongly partitioned into the solute-rich surficial liquid, or quasi-liquid, layer. In real melting snowpacks, preferential elution of some ions by the meltwater can be masked by snowpack-scale hydrology.

Trace elements in snow samples from the Scottish Highlands: sources and dissolved/particulate distributions

Abstract Fresh snow samples were collected throughout a season at a high altitude (1080m) site in the NE Scottish Highlands and analysed for 14 trace elements in addition to the major element composition. Enrichment factors suggests three significant sources of the different elements, i.e. crust, seawater and anthropogenic inputs. Elements associated with anthropogenic and marine sources are in general soluble upon thawing (e.g. Br 98% dissolved) while elements from crustal sources are insoluble (e.g. Al 18% dissolved). Elements with mixed sources tend to show intermediate behaviour with average solubilities ranging from 27 to 96% for different elements. These solubilities are rationalized in terms of sources of the elements, their chemistry and the overall rainwater chemistry.

Spatial variability in the chemical composition of snowcover in a small, remote, scottish catchment

Abstract Fresh snowcover in a remote upland catchment is chemically heterogeneous. Coefficients of variation (Cof) for individual major ions range from 5 to 113%, assuming a normal distribution, or 1 to 144 % assuming a lognormal distribution. Aged snowcover is slightly more chemically variable, giving rise to Cof of 19–140 %, assuming a normal distribution, or 2–200 %, assuming a lognormal distribution. In general, the distribution of the concentration of major ions in snowcover is better described by the lognormal distribution. Both vertical and horizontal variability is observed in the chemical content of within-pack snows. Such heterogeneity has important implications for snowpack sampling. We suggest that rigorous methodologies should be devised for the sampling of fresh snowcover. The sampling of within-pack snows should also be performed under tight statistical constraints if representative estimates of the solute content of snowcover are to be obtained.

The weathering of aeolian dusts in alpine snows

Abstract The chemistry of precipitation (snow and rainfall), snow cover and meltwaters was studied at a French alpine site during the winter-spring seasons of 1986–1987 and 1987–1988. Both acid ( pH 5.6) deposition events occurred. The strong-acid anions, SO 4 and BO 3 , contributed to the acidity of precipitation but NO 3 was the principal anion associated with acidic snowfall. Many alkaline snowfalls are associated with airborne calcareous dusts from regional sources. The most alkaline snowfall, however, was associated with dust from the Sahara Desert. The weathering of dusts in the snow cover during melt leads to the consumption of acidity and an increase in the pH of meltwaters. The results of both field and laboratory experiments show that inputs of calcareous dusts by local aeolian erosion and transport can contribute significantly to the neutralization process. The laboratory experiments also show that the size and distribution of dusts in the snow cover have an effect on the degree of neutralization. Dust in the lower strata of snow cover is more efficient in neutralizing the acidity of meltwaters than dust in the upper strata. The relationship between the distribution of dust and its efficiency to neutralize the acidity of meltwaters can be explained by the kinetics of calcite dissolution under conditions of progressive decreases in the acidity of leached snow and the partial pressures of CO 2 within the snow column during the melt process.

Heavily-contaminated snowfalls in the remote Scottish Highlands: A consequence of regional-scale mixing and transport

Abstract Heavily-contaminated (black) snowfalls frequently occur in the Cairngorm Mountains (Scotland), in spite of their relative remoteness from major source regions. The frequency of such pronounced events has been characterized throughout one complete snow season. Daily samples of snow were collected at 1100 m under sub-arctic conditions, and the reported results from the analyses of major ions and a large number of trace elements (dissolved and particulate phases) represent a novel precipitation chemistry data set. This paper focuses on the major ion, black carbon and particulate concentrations, but formic acid concentrations are also reported. The heavily-contaminated snowfalls are associated with back-trajectories which originate in the east (Eastern Europe, European U.S.S.R. and the Baltic) and, in spite of providing a small proportion (30%) of the total snowfall, produce the bulk of the pollutant wet deposition (e.g. 70% of non-marine SO 4 2− ; 80% of black carbon). The covariances of the constituent concentrations indicate that airborne pollution is relatively aged and well-mixed before removal by the snowflakes. Back-trajectory, synoptic and upper-air analyses have been combined to produce a qualitative model of pollutant transport from source to receptor region. Once air is advected out of a (typically) long-lived anticyclone over Eastern Europe (which produces the regional-scale mixing), transport westwards of the ‘regional plume’ is usually associated with an elevated stable layer which can effectively decouple the plume from the surface and inhibit dispersion. Over the North-Sea-NE Scotland region, horizontal convergence, convection and topographic uplift can all contribute to precipitation formation and pollutant wet deposition. Although there are indications that the NE Scottish Highlands are particularly susceptible to these ‘black’ snowfalls, comparison with data from monitoring stations within 35 km of the study site, and 600–800 m lower elevation, does not indicate enhancement of ion concentration in snowfall with increasing altitude.

GEOCHEMICAL STUDIES IN A REMOTE SCOTTISH UPLAND CATCHMENT II. STREAMWATER CHEMISTRY DURING SNOW-MELT

Abstract‘Acid-flush’ events, monitored in an upland catchment in the Cairngorm Mountains (Scotland) at the time of the spring-thaw, are associated with an increase in stream discharge and raised concentrations of both major ions (Ca, Mg, Na, Cl, N03, and SO,) and trace-elements (Al, Cd, Cu, Fe, Mn, and Pb), in addition to H+. The streamwater chemistry is determined by the hydrological pathways which are operative in the catchment during these periods of snowmelt, and reflects both the meltwater composition and the influence of the soils within the catchment. Aluminium, in particular, is leached from the soils and high concentrations (up to 330 μg L−1) occur in the streamwaters. The presence of frozen soils, which result largely due to the influence of meteorological conditions prior to the accumulation of the snowpack, is likely to have a large impact on the Al concentrations in the streamwaters. The low concentrations of Ca monitored in the stream during the periods of snow-melt (<0.2 mg L−1) may promote subsequent toxic effects of the Al to aquatic life forms.[/p]

Chemical Composition of Snow in the Remote Scottish Highlands

Altitudinal surveys suggest an increase in ionic concentration in snow with increasing fall-path, although such studies are complicated by the marked spatial variability in the chemical composition of fresh snow-cover. This variability is characterised and the implications for snowfall/snowcover sampling are addressed. Nitrate concentrations in snow are relatively enhanced compared to concentrations in rainfall. The NO3 -/SO4 2- ratios in snowfall appear to be dependent on back-trajectory type and associated source region. We consider the relevance of the observations for studies of the chemical evolution of air masses. The Cairngorm range of mountains experiences, not uncommonly, deeply-coloured snowfalls with a large black carbon content which appears to be directly related to snowfall acidity. The events can represent relatively large contributions to the total impurity deposition in the mountains. The synoptic conditions which lead to the black deposition events have been identified.

Trace-element studies in a remote scottish upland catchment

Fresh and aged within-pack snow samples were collected and analyzed for 6 trace-elements during the winters of 1984 and 1985 at a remote site located within the Cairngorm Mountains, Scotland. All of the fresh snow samples were acidic and highly variable in composition. The variability of pH and trace-element concentrations of snowfall are demonstrated to be associated with different air trajectory categories, and the study catchment is susceptible to episodic pollution events due to long range atmospheric transport. Partial melt experiments have shown that both fractionation and preferential elution of trace-elements occur during melting, the concentrations being 1.3 to 5.4 times higher in the first 10% meltwater fraction than in the bulk snow. Cadmium and Mn appear to be preferentially eluted from the melting snow with respect to the remaining elements. Upon snowpack melting, the trace-elements may be mobilized and redistributed within the snow profile. The ions concentrate at depth from where they can be quickly removed with the early water runoff during the spring.