Polar ice cores offer datable past snow deposits in the form of annual ice layers, which reflect the past atmospheric composition. Trace substances in the cores are related to the past mid-tropospheric impurity load, this being due to the vast extent of the polar ice sheets (or ice caps), their surface elevations and remoteness from most aerosol sources. Volcanic eruptions add to the rather low background impurity load via their eruptive products. This paper concentrates on the widespread influence on atmospheric impurity loads caused by the acid gas products from volcanic eruptions. In particular the following subjects are discussed: acid volcanic signals in ice cores, latitude of eruptions as derived by ice-core analysis, inter-hemispheric dating of the two polar ice sheets by equatorial eruptions, volcanic deposits in ice cores during the last glacial period and climatic implications.

Techniques have been developed for the airborne bulk sampling of cloud and precipitation, including snow, for later chemical analysis. The techniques include the use of cyclones, meshes, riming rods, scoops and slotted collectors. The role of sampling location on the aircraft, flying attitude, collection efficiency, and sampling procedures for clean samples and mixed clouds with respect to the design and performance of the collectors will be discussed. The polyethylene meshes, with thread diameters of 545 and 345 microns, and the riming rods, with diameter of 1.0 cm, collect supercooled droplets with different efficiencies and can be used to examine the dependence of chemistry on the droplet size. Airborne snow samples were collected for the first time, with the cyclone being the most effective collector.

Atmospheric general circulation models are believed to be appropriate tools for studying airborne impurity cycles in order to supplement observations and to improve our knowledge of gaseous and particulate pollutant cycles in the atmosphere. The main aspects of the modelling of tracer cycles are reviewed and illustrated by two particular examples: desert dust particles in the 1 μm range and water isotope species HDO and H218O. Some results from a first simulation including desert dust and water isotope cycles using the model developed at the Laboratoire de Météorologie Dynamique (LMD) are presented and compared to observations, with particular emphasis on ice-sheet data. The relatively good agreement with observations obtained so far is encouraging and should stimulate further applications to other types of tracers.

We briefly review the application of cosmogenic 10Be measurements in ice and polar atmospheres to: (i) the dating of ice cores, (ii) the deduction of past accumulation rates, (iii) information on the influx of stratospheric aerosols in polar regions, and the mechanism of incorporation of aerosols into the ice. We find that at high latitudes (>74°), the 10Be deposition rate in the ice is more constant than the 10Be concentration.

This isolated snowpack shows wide spatial variability in composition, with 15 samples from a 700 m transect giving coefficients of variance of more than 50% for most of the major ions. Cations showed somewhat less variability than anions. Preferential elution, i.e. the more rapid loss of some ions from the pack than others, was more readily observed in the composition of meltwaters because of the high variability in snowpack composition. Preferential elution could be found in the snowpack where it was possible to resample a well-defined surface during a period of ablation. The elution sequence established from these surfaces was although the position of the hydrogen and ammonium ions may be affected by buffering. The sequence compares reasonably well with a generalized one assembled from earlier studies, where is usually eluted quickly and sodium and chloride rather slowly. The evolution of the snowpack composition towards solute depletion, but relative richness in chloride, is clearly represented in triangular diagrams.

The variations in grain size and internal surface area in a seasonal snow cover have been studied near Schefferville, Quebec. Sieve analysis was applied to snow in very cold conditions and gave closely reproducible results.

The studies show that grain-size distributions vary greatly with depth but show a pattern that is closely repeatable between sites in similar environments and which is also closely repeatable from one year to the next. Different environments produce variations on the general pattern.

The measurements enable estimation of the internal surface area and its variations with depth in different snow covers. The effects of this variation in surface area on contaminants of the snow cover are discussed.

An ice core 103 m long was extracted in 1980 from an altitude of 5340 m on the icefield plateau of Mount Logan, Yukon Territory (lat 60°35ˈN, long 140°30ˈW). The firn-ice transition occurs at a depth of 65 m, corresponding to about the year 1880.

The chemistry of this upper 65 m is apparently dominated by acid-ion species, the peaks in which are provisionally identified with several documented volcanic events. Although the analyses cover only selected discontinuous intervals, it appears that there is no significant long-term trend in the background acidity level of the precipitation at this location over the past century, in contrast to the results from the North American Arctic and Greenland.

Nitrate ion concentration shows pseudo-seasonal variations, which may be associated with stratospheric-tropospheric interactions, although other seasonally linked mechanisms are possible. This result has also been reported for ice-core sequences from Greenland. Other nitrate pulses are tentatively associated with local volcanic events and a possible meteorite event (the entry of Tunguska in 1908). One of the largest short-term sources of sulfate ions is probably from volcanic activity on the north Pacific rim. Background volcanically-quiet nitrate and sulfate ion concentrations are compared with similar Greenland data in an attempt to throw further tight on the origin of the acids.

Since the moisture for this precipitation originates primarily in the Gulf of Alaska, the data has particular relevance to that region. Short-term climatic changes, as reflected by the oxygen isotope (δl8O) record, show some response to the major volcanic-acid events. The influences affecting the δl8O record are listed but not discussed.

A physical and chemical study of snow mesostructure was carried out in a boreal forest site at Lake Laflamme, Quebec in spring 1983. The concomitant physical observations and δ18O‰ data on the upper layers led to the reconstruction of recent snowfall events and meltwater movements which influenced the chemistry of snow at different levels of the snowpack. Correlations between data relating the concentrations of atmospheric pollutants in snow strata were significant (H+, , r = 0.89; H+, , r = 0.81, P ≤ 0.001). The grouping of chemical species by factor analysis distinguished clearly between ionic species originating from the long-range transport of atmospheric pollutants and those due to local deposition phenomena and organic debris from the forest canopy. Mesostructural characteristics led to heterogeneous flow patterns of meltwater and rainwater through the snow cover. Both physical and chemical characteristics of the snow mesostructure change rapidly in a non-uniform manner under such conditions. This leads to increased difficulties in the interpretation of data gathered during snow sampling studies on pack decay during the spring run-off period.

An intensive snow-cover survey at Lake Laflamme, Quebec, during the spring of 1983 showed that wet deposition in the form of rain, which was a dominant phenomenon during the 1983 melt season, gave rise, according to the intensity and chemical quality of the precipitation, to both losses and gains of ion loads (meq m−2) in the snowpack. Mean values for the daily wet deposition loadings (meq m−2 d−1) of ionic species associated with atmospheric aerosols (H+, , ) were of approximately the same magnitude as the daily changes in gains recorded in the snow cover during the melt period. In contrast, the mean value for the contribution by wet deposition to the total loads of K+ and in the snow cover was far outweighed by the gains which were observed at the same time. The expected losses for the snowpack, calculated from the sum of the total loads stocked in the pack at the beginning of the melt period and the total loads in precipitation during the melt period, were lower than the sum of the actual losses observed for all ionic species except H+. The increases (%) in the loads for the major anions Cl−, and were comparable (25 to 32%). The results suggested that dry deposition either directly by aerosol interaction with the snow cover or indirectly by adsorption on organic material followed by leaching during the melt period, or by a combination of both, was a major factor in the increases observed. The values for the increases in loads for Ca2+, , Mg2+ and Na+ (50 to 287%) probably represented, in addition to leaching of local debris, the exudates of cellular material from the cell plasmolysis of detrital organic debris. High rates of in-pack production, however, were characteristic of Al3+. Mn2+, K+ and which showed substantial increases in pack loads (480 to 750%). These increases cannot be accounted for by any local phenomena other than the dissolution or microbiological degradation of organic debris. It is suggested that ion exchange capacity of both particulate and soluble organic material led to a decrease in pack acidity; this phenomenon should thus be considered as a major factor in all attempts to model acid rain fluxes through boreal forest systems.

Three multi-elemental techniques (neutron activation analysis, proton-induced X-ray emission and inductively coupled plasma-atomic emission spectrometry) are described in terms of their special advantages in determining sulphur and heavy metal pollution in urban snow. Environmental analytical interpretations, including wash-out factors, enrichment factors, inter-elemental correlations, mobilization factors, and toxicity potential, are also discussed.

The influence of advective and eddy transport of atmospheric moisture on the 18O content of atmospheric vapour and precipitation is discussed. Measurements of the 18O content of atmospheric vapour and precipitation have been carried out since July 1981 in Uppsala, Sweden. The removal of water from atmospheric vapour is accomplished by freezing it out of a stream of air passed at the rate of 60 1 h−1 through cooling traps maintained at a temperature of −60°C. The method is practically non-fractionating. The observed slopes of the regression equations defining the linear relationship between weighted monthly δ18O of precipitation and mean monthly temperatures at the ground level agree fairly well with the predicted slopes assuming that the transport of atmospheric moisture is due to eddy motion. Similarly the slope of the linear relationship between δ18O in atmospheric vapour and vapour density at ground level is also closely predicted by the eddy transport model. However, if it is assumed that the transport of atmospheric moisture is advective, the predicted slopes are nearly double those observed.

It was also observed that precipitation and atmospheric vapour are in isotopic equilibrium during summer, but not during winter. Results obtained confirm the strong large-scale eddy diffusive nature of the transport of atmospheric moisture.

The physical and chemical structure of the snowpack near North Bay, Ontario was examined during the winter of 1984. precipitation chemistry measurements were also made. A marked areal uniformity was noted in the layered structure of the snowpack and in the vertical variation of the chemical composition. In late January, pH values of the snowpack varied from ∼4.1 at the surface to ∼4.8 near the ground. After rain and a major thaw in February the pH was near 4.8 throughout. Nitrate concentrations exceeded those of sulfate in the snowpack. The molar concentration ratio of / was typically 0.5.

Samples of precipitation from six sites had pH values ranging from 3.4 to 5.2. to equivalent ratios were <1 in snow and near 1 or >1 in rain. The chemical composition of the precipitation was closely related to the airmass trajectory. Southerly trajectories yielded the lowest pH values and highest pollutant concentrations. The one-month period from 20 January to 21 February had a deposition of 0.2 g m−2 and a deposition of 0.35 g m−2. For , this deposition would be about one-twelfth the expected annual deposition and for about one-fifth.

The purpose of this study is to use the snowpack of the Saskatchewan portion of the Precambrian Shield as a passive collector of atmospheric total deposition. Although the focus is on the anions of sulphate and nitrate, the hydrogen ion and various metals are also considered.

Twenty-six small lakes on the Precambrian Shield area of northern Saskatchewan were used as sites. The lakes were all approximately 1 square km in area and serviced by aircraft. The study was conducted in late March 1982 and 1983 just before the major snowmelt period.

Concentrations of sulphate ranged from 0.26 to 7.3 mg 1−1. The concentrations were typical of large areas in northern Saskatchewan with sharp transitions in composition occurring as the treeline boundary was encountered. Typical sulphate deposition was approximately 30 mg m−2 in the Precambrian areas and 150 mg m−2 in the other areas. The distribution of conductivity in the snow cores ranged from 6 to 13 μS cm−1. Most of the snow on the Precambrian Shield areas can be described as acidic, with pH values of the meltwater being below 5.6.

Metals such as Ca, Mn, Mg, Zn and As were measured across the region. The metals Ca and Mg show a direct correlation with high sulphate levels south of the treeline. Metals such as Zn and As show anthropogenic influence particularly within 100 km of industrial sources. Large areas of the study region show uniform and low deposition to the snowpack. It is suggested that natural atmospheric processes are dominating deposition in this area of Canada.

It has been recognised for many years that the first fraction of meltwater from a melting snowpack contains a much higher concentration of ions than that of the bulk (or average) snow of which the pack is comprised (Foster 1978, Johannessen and Henriksen 1978). This process leads to the so called “acid flush” in the spring and can have severe ecological effects on lakes and streams (Hagen and Langeland 1973, Leivestad and Muniz 1976, National Research Council of Canada 1981). We have studied elution of ions through a snowpack on Folgefonna near Bergen, Norway, and in the laboratory. In the field we collected meltwater samples at hourly intervals for a period of 4 d at the start of the melt season, and analyzed them for pH, , Cl−, Na+, Mg2+, Ca2+ and K+. 2 m cores were also collected at the beginning and end of the experiment in order to study bulk changes in snow composition. The meltwaters showed a diurnal cycle with high ionic concentrations around noon, with and levels rising by factors of up to four and six-fold compared to the average concentrations on the first day. Cl− levels peaked much later, after the and levels had decreased, and only reached 2.3 times their lowest concentrations. pH values were strongly correlated with and levels, suggesting that a significant proportion of these anions were in the form of strong acids.

The laboratory experiments involved slow melting of snow samples collected in the Cairngorm mountains, Scotland, and also showed that and (and also Mg2+ and K+) ions were removed from the snow preferentially whilst Na+ and Cl− tended to remain longer. The position of H+ within the ion elution sequence is unclear due to uncertainties in the absolute determination of pH in the field measurements, but the laboratory experiments confirm the differential rates of elution shown in the Norwegian snowpack and reaffirms its importance in the acidification of streams during spring.

The stratigraphic variability of ion concentration in lake ice was examined and related to the processes of lake-ice formation. Ice blocks obtained from Elizabeth Lake, Labrador and Knob Lake, Quebec showed marked vertical variation in the concentration of major ions, with peaks in conductivity occurring in the white ice and an overall increase in conductivity upwards from the black-ice/white-ice interface to the top of the block. Exsolution of ions from the black ice into the water body appears to be very efficient, producing low concentrations of ions in the black ice and high concentrations in the immediate sub-ice layer of water. This sub-ice concentration of ions appears to influence the entire water column, possibly via density-induced movements. Changes in snow depth, white- and black-ice growth, and the vertical distribution of conductivity in the water column were found to be related to slushing events. A controlled slushing experiment suggested that a marked concentration of ions does occur during white-ice formation.

Snow and ice played important roles in determining the occurrence and timing of pH depressions in the shallow (∼1 m) nearshore waters of a small lake of the Canadian Shield. The influences of the terrestrial snowpack and lake-surface snow on the chemical composition of under-ice water were distinctly different. On land, acidic meltwater from snow was enriched with lithological elements in the soil/litter layer during lateral movement to the lake shoreline. The resulting run-off water was of relatively high ionic strength (X conductivity 55 μS cm−1), low pH (X 4.1) and contained high concentrations of Al (X 1786 μg 1−1), an element potentially toxic to aquatic biota. Snow cover on the ice surface controlled thermal conditions in the underlying water, dictating the depth to which run-off water penetrated. Uniformly cold conditions (<2°C) allowed penetration of run-off water to the substrate while under-ice warming to 4°C resulted in surface layering of less dense run-off water. flooding of the ice with lake water and melting of the ice and snow-surface cover served to counteract the effects of the run-off water through dilution.