Journal of Hydrology 409 (2011) 737–748

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Journal of Hydrology

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Snowpack and precipitation chemistry at a high altitude site in the CanadianRocky Mountains

Melissa J. Lafrenière a,⇑, Kate E. Sinclair b,1

a Dept. of Geography, Queen’s University, Kingston, Ontario, Canada K7L 3N6b Dept. of Geography, University of Calgary, Calgary, Canada T2N 1N4

a r t i c l e i n f o s u m m a r y

Article history:Received 11 November 2010Received in revised form 2 September 2011Accepted 3 September 2011Available online 12 September 2011This manuscript was handled by LaurentCharlet, Editor-in-Chief, with the assistanceof Martin Beniston, Associate Editor

Keywords:Precipitation chemistrySnowpack chemistryCanadian Rocky MountainsRainfallSeasonal variationsAtmospheric aerosols

0022-1694/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.jhydrol.2011.09.007

⇑ Corresponding author. Tel.: +1 613 533 6000x787E-mail addresses: Melissa.lafreniere@queensu.ca (M

s.cri.nz (K.E. Sinclair).1 Present address: National Isotope Centre, GNS Sci

5010, New Zealand. Tel.: +64 04 570 4646; fax: +64 04

This study presents a first quantitative study of inorganic ion concentrations and loading in precipitationin the Canadian Rocky Mountains. The 2005/2006 winter snowpack and July 2006 rainfall are examinedto define the magnitude, and altitudinal and seasonal variability of atmospheric deposition at high alti-tude sites. Little is currently known about the sources and concentrations of inorganic ions in watershedsin this region, and careful measurement of snowpack and rainfall chemistry are required to accuratelyassess the total annual deposition and the potential environmental effects of atmospheric pollution onhigh elevation ecosystems and snowmelt-dominated catchments on both sides of the continental divide.

Snowpit profiles from three sites between 2100 and 2750 m above sea level (masl) show little variationin ion concentrations with altitude, but all sites show strong seasonal variations. Air-mass back trajecto-ries and the identification of storms in snowpits using high-resolution oxygen isotope (d18O) data showthat the increases in acid anions in the surface (spring) layers and early-winter snow layers are associatedwith southwesterly storm systems that swept moisture across the northwestern US into western Canada.July rainfall ion concentrations were 1.5–3.0 times higher than winter snowpack concentrations and werealso primarily associated with southwesterly storms. The chemical composition of precipitation in thisregion, and the relative contribution of snow and rain to annual precipitation, suggest that rates of nitro-gen (N) deposition in the Canadian Rocky Mountains may be comparable to the US Rocky Mountainswhere N deposition is altering ecosystem function. These results lend support to increased monitoringof precipitation chemistry in this environmentally sensitive region.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

High altitude, snowmelt-dominated environments are sensitiveto changes in atmospheric chemical deposition because of highprecipitation rates and watershed characteristics, such as dilutewater bodies, large areas of exposed bedrock, and sparse and thinsoils (Baron et al., 2000; Campbell et al., 1995). All these factorslimit an ecosystem’s capacity to adsorb or buffer atmospheric in-puts of pollutants and nutrients such as sulphate and nitrogen,and aquatic ecosystems are particularly sensitive to the seasonalflux of pollutants into watersheds since the chemical compositionof the annual snowpack is transferred directly into lakes andstreams with minimal interaction with terrestrial biota or geologicmaterials (Clow et al., 2002; Lafrenière and Sharp, 2004).

ll rights reserved.

20; fax: +1 613 533 6122..J. Lafrenière), k.sinclair@gn-

ence, Lower Hutt, Wellington570 4600.

The Rocky Mountain snowpack accumulates atmosphericallydeposited chemicals over most of the year (November–May) andreleases them over a relatively short snowmelt period. However,warming air temperatures at high elevations not only reduce thefraction of precipitation falling as snow, but can lead to the earlieronset of spring melt. Thus continued warming will result in lesswater being stored as snow over the winter months, and the earlierrelease of this water in spring (e.g. Mote et al., 2005; Regonda et al.,2005; Schindler, 2001). Because snowmelt is an important event inthe annual water budget in mountain streams and lakes, thesewater bodies are particularly sensitive to the chemical compositionof snowfall and the effect of climate-driven changes on the hydro-logical cycle and downstream loading of atmospheric chemicals instreams and lakes. For example, atmospheric N and S are the maincontributors to acid precipitation which causes regionally persis-tent ecological problems in some alpine areas and enhanced Ndeposition contributes to major ecological problems such as eutro-phication, and changes in species composition, diversity and resil-ience to climatic and environmental stressors (Fenn et al., 2003).This study therefore aims to improve our understanding of themagnitude and the spatial and temporal variability of snowpack

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Fig. 1. Site map illustrating the location of the AWS, the rain sampler, and thesnowpit sites, AWS, PG, RG and HG.

738 M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748

and precipitation chemistry at high altitude site in the CanadianRocky Mountains, and to quantify atmospheric inputs to theseecosystems.

Reliable estimates and long term records of snowfall and pre-cipitation chemistry are necessary to determine the impact of ouractivities on these high altitude environments and to assess eco-system sensitivity to atmospherically deposited pollutants (e.g. asnitrate, ammonium and sulphate). However, there are currentlyno intensively studied sites in high elevation areas in Canadawhere precipitation chemistry, and thus atmospheric deposition,is measured. Although Canada has a relatively comprehensivenetwork of climate stations which measure temperature and ratesof precipitation across the country, there is little data available forhigh altitude sites. There are currently only two federal govern-ment stations located above 2000 masl that measure both sum-mer and winter precipitation in the Canadian Rocky Mountains(Environment Canada, 2010). There are, however, five provincial(Alberta Environment) sites above 2000 masl on the easternslopes of the Rocky Mountains that are monitored for snowpackand precipitation. In the United States several organizations (theUS Geological Survey, National Park Service, US Department ofAgriculture, and US Forest Service) have been monitoring snowchemistry and the ecological effects of atmospheric N and S depo-sition at a large network of sites since the early 1990s, with thegoal of identifying spatial and temporal trends in chemical depo-sition to the high-elevation snowpack (Ingersoll et al., 2008), butthere is no comparable monitoring program in the Canadianalpine.

The goals of this research are four fold: (i) to assess the chemicalcomposition of precipitation and the magnitude of atmosphericdeposition of ions (with an emphasis on nitrogen species and sul-phate) to the winter snowpack at a high altitude location in theCanadian Rocky Mountains; (ii) to determine whether there areseasonal or altitudinal trends in the chemistry of precipitation atthis site; (iii) to compare the ion concentrations and loads of thesnowpack to summer rainfall in order to estimate the total annualatmospheric deposition and the relative importance of winter vs.summer precipitation, and (iv) using oxygen isotope (d18O) datato identify storm layers in the seasonal snowpack, model air-massback trajectories to identify the source regions and transport path-ways of pollutants to the site.

Although there have been studies examining the deposition oforganic contaminants in mountain snowpacks and glaciers (Blaiset al., 1998; Donald et al., 1999; Lafrenière et al., 2006), this isthe first study that reports on the spatial and temporal variationsin aerosol chemistry of precipitation at a high altitude sites inthe Canadian Rocky Mountains. Furthermore, although previousstudies have used air mass trajectories to examine the transportpaths associated with snowpack chemistry (e.g. Dong et al.,2010; Ming et al., 2007; Yalcin et al., 2006) and the relationshipsbetween trace element concentrations, snowpack stable isotopesand storm paths (Purdie et al., 2010), there has been no previousresearch that ties high-resolution d18O data and precipitationchemistry to identify the source regions of air masses responsiblefor the deposition of acid anions in alpine regions. In light of thepaucity of data on the composition of precipitation in alpine re-gions, the inherent sensitivity of these environments to changeswater cycles and chemical inputs (Baron et al., 2000; Dyurgerov,2003), the observed and anticipated climate-driven changes towater storage in alpine environments (Demuth et al., 2008; Mooreet al., 2009) and in the intensity of anthropogenic activities in theregion (Fenn et al., 2003), this research represents an importantcontribution to our understanding of the exposure and sensitivityof alpine environments in western Canada to atmosphericdeposition.

2. Field site and methods

2.1. Field site

All snowpit samples were collected on either Robertson or HaigGlaciers (115�200W, 50�450N) in Peter Lougheed Provincial Park, Al-berta, Canada. Approximately 100 km west of Calgary, this park ispart of Kananaskis Country and is bordered by Elbow-Sheep Wild-land Provincial Park, Spray Valley Provincial Park, Banff NationalPark and the province of British Columbia to the west. RobertsonGlacier is located immediately to the north of the significantly lar-ger Haig Glacier (Fig. 1), which is currently the largest icefield inthe southern Rocky Mountains (�3 km2) and the two glaciers aredivided at the continental divide by a steep col. Roberston Glacieroccupies a north facing valley, surrounded by steep walls to theeast and west, and spans elevations from approximately 2400 to2900 masl. The local bedrock is Upper Devonian carbonates and in-cludes the Pedrix and Mount Hawk, Palliser and Sassenach forma-tions. The Pedrix and Mount Hawk formation is composed ofcalcareous shale and limestones, and is overlain by the SassenachFormation, which includes sandstones, dark shale, and limestones.The younger Palliser Formation is composed of various types oflimestone (Hein and McMechan, 1994).

Local meteorological data were obtained from an automaticweather station (AWS) located at 2100 masl, approximately 2 kmaway and 200 m elevation down valley from Robertson Glacier ter-minus (Fig. 1). Shielded air temperature (±0.5 �C) was recordedwith a CS500 temperature and relative humidity probe, precipita-tion was recorded at 0.2 mm resolution with a TE525 tipping buck-et gauge, and snow depth (±1 cm) was recorded by means of an SR-50 sonic ranging sensor. All parameters were logged with a Camp-bell Scientific CR1000 datalogger.

M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748 739

2.2. Sample collection

A series of three snowpits along an altitudinal transect in the al-pine (above tree line) is investigated. The lowest-altitude snowpitwas sampled at the AWS site (�2100 masl; Fig. 1) on April 20th2006, a snowpit on Robertson Glacier (RG) at �2500 masl wassampled on April 24th 2006, and the highest-altitude snowpitwas sampled on April 23rd at the Haig Glacier (HG) site at�2750 masl. Avalanche hazards prevented sampling from a higheraltitude location on Robertson Glacier. Although a snowpit wasdug at a site approximately 200 m from the snout of the RobertsonGlacier (2300 masl), the data are not reported as several wet layerswere found in the snowpack. Ion concentrations were 20–50% low-er than other sites, confirming that the snowpack chemistry hadbeen affected by meltwater percolation. Return visits to this loca-tion in the summer of 2006 suggests that the snowpit was likelylocated within a meltwater channel, which would explain the un-ique snowpack conditions at this site. All other sampling sitesexhibited cold (<0 �C), dry snowpacks and the AWS air temperaturedata confirms that there was limited energy available for melt evenat the lowest-elevation site during the period prior to sampling(Fig. 2).

In order to link variations in ion chemistry to synoptic-scalestorm events and make a preliminary analysis of the source regionsof atmospheric pollutants, the April 23rd HG snowpit was alsosampled for stable isotopes of oxygen (d18O) at 5-cm resolution.Additional detailed d18O profiles were collected from snowpits atthe HG site on October 5th 2005, and February 21st and March18th 2006.

Repeat snowpits over the 2005/2006 winter provide a ‘time-stamp’ in the d18O stratigraphy, so that snow accumulation data(converted to snow water equivalent (SWE)) can be used to iden-tify the stable isotope signal of major storm events over the studyperiod. Knowing the location of these storms in the snowpackmeans that predominant changes in snowpack chemistry can alsobe dated and associated with individual storm events. The method-ology of identifying the snow layers associated with each storm isdescribed in detail by Sinclair and Marshall (2009) who use themean d18O from storms over three winters to identify the majorcontrols on stable isotope ratios in this region.

For all major ion samples, snowpits were dug to the ground orto glacier ice, and the sampling wall of each snowpit was scrapedclean. To avoid contamination from the soil or vegetation, weavoided sampling the bottom 5 cm of the snowpits that weredug to the ground. The snowpits were sampled in layers of varyingthickness according to the visual changes in snow stratigraphy(crystal type and density). Layer thicknesses represented by a gi-ven sample varied from a few centimetres (usually for fresh snow)up to 30–40 cm depending on the stratigraphy at the site. For deepsnowpacks, where changes in stratigraphy were not obvious, thesnowpit was sampled at regular intervals and snow density, thick-ness and temperature were determined for each layer. Densitysamples were taken using a fixed-volume (100 cm3) aluminium

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sampler and the mass of each sample (±0.01 g) was measured inorder to calculate the SWE of each layer.

Samples collected for chemical content were taken with glovedhands from the clean face of each snowpit, using polyethylenescoops that were precleaned in deionized (DI) water. Each samplewas double-bagged in a clean zipper sealed polyethylene bags andthen transported and stored frozen until analysis. Prior to analysis,samples were melted at room temperature and vacuum filteredthrough a 0.45 lm PVDF membrane filter using a polycarbonate fil-ter apparatus. pH was measured on the thawed sample immedi-ately following filtration, using a pH meter with an electrode andbuffers especially designed for low ionic strength solutions. The fil-tration apparatus was rinsed three times with DI water and threetimes with sample water between each sample.

In order to better understand the relative importance of snowand rainfall in terms of the annual atmospheric loading at the site,summer precipitation was collected and analysed for ion concen-trations. Rain samples were collected on an event basis from July5th until August 2nd 2006, from a bulk precipitation sampler(wet and dry deposition) located next to the AWS. Rain sampleswere collected in white 1 L polyethylene bottles that were fixedto a large plastic funnel. The opening at the base of the funnel stemhad an inner diameter of less than 3 mm to minimise evaporation.In addition, a plastic (ping-pong) ball sat at the base of the funnelto seal the funnel opening between rainfall events, thus furtherlimiting the possibility of evaporation from the bottle. All appara-tus were cleaned by scrubbing, soaking and rinsing three timeswith DI water. The rain samples were collected and filtered, as de-scribed for the snow samples above, within 24 h of the end of eachrainfall event.

2.3. Analytical methods

Dissolved inorganic anions and cations (Cl�, SO2�4 , NO�3 , Na+, K+,

NHþ4 , Mg2+, Ca2+) were measured by ion chromatography. The an-ions and cations were determined simultaneously using a Dionex3000 ICS system. Anions were separated by gradient elution with23–40 mM potassium hydroxide (KOH) eluent, with a1.0 mL min�1 flow rate, AS18 analytical and guard columns, andself regenerated suppression (ASRS-ULTRA II). Cations were mea-sured isocratically with 23 mM methanesulphonic acid (MSA) elu-ent, flowing at 0.5 mL min�1, CS12A analytical and guard columns,and self regenerated suppression (CSRS-ULTRA II). Both systemswere equipped with 225 lL sample loops. The precision, basedon 10 replicate analyses of samples, was better than 0.04 leq L�1

for NO�3 and SO2�4 ; better than 0.11 leq L�1 for Na+, Ca2+ and

NHþ4 , and 0.16 leq L�1 for Mg2+ (or better than 0.002 ppm for allanalytes). Method detection limits for the analytes were betterthan 0.030 leq L�1 for Cl�, SO2�

4 , NO�3 , and K+; 0.041 leq L�1 forNa+; 0.111 leq L�1 for NHþ4 , and better than 0.4 for Mg2+ andCa2+. PO3�

4 was undetectable in all samples and is not reported.The detection limits are defined as three times the standard devi-ation of replicates of the lowest reproducible standard.

The volume-weighted mean snowpack concentrations were cal-culated as the sum of the concentration of the analyte in each layermultiplied by the fraction of the total snowpack SWE of the layer.Total snowpack ion load (i.e. total deposition) was determined bymultiplying the concentrations of each layer by the respectiveSWE of these layers; the values for each of the layers were thensummed to determine total deposition in each snowpit. Mass fluxwas determined and expressed as mass per unit area in kilogramsper hectare (kg ha�1).

Snow samples for isotope analyses were transported frozen tothe University of Calgary Stable Isotopes Laboratory where d18Oanalysis was conducted using CO2 equilibration, whereby theoxygen isotopes of gaseous CO2 are equilibrated with those of

740 M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748

the water sample in a vacuum at a constant temperature (Epsteinand Mayeda, 1953). Data are reported relative to the internationalstandard, Vienna Standard Mean Ocean Water (VSMOW), with anestimated uncertainty of ±0.2‰.

Air-mass back trajectories were modelled for the major stormevents identified in snowpit d18O stratigraphies using the US Na-tional Ocean and Atmosphere Administration’s (NOAA) HYSPLIT_4,version 4.9 (HYbrid Single-Particle Lagrangian Integrated Trajec-tory) model (Draxler and Rolph, 2003). Storm onset and end timeswere obtained from snow accumulation data, and gridded (32-kmresolution) North American Regional Reanalysis (NARR) data wereused to compute back trajectories at 3-hourly intervals over theduration of each event. For each trajectory, the model was initiatedat 3000 masl (250 m above the highest-altitude, HG, site) and runbackwards for 120 h.

3. Results

3.1. Variations in snow chemistry with altitude

The bulk composition of the snowpack chemistry is relativelyconsistent across the three sites between 2100 and 2750 masl,with the standard deviations of the mean weighted concentrationsof most ions varying less than 0.35 leq L�1 (or less than 15%)(Table 1).

SO2�4 and NO�3 are the dominant anions and have similar con-

centrations across sites. Only Ca2+, Mg2+ and acidity (H+) exhibitsignificant variability across the three sites. Ca2+ is the highest-concentration cation at all sites, but its dominance diminishes sig-nificantly with increasing altitude, as indicated by the large coeffi-cient of variation (CV = 92%; Table 1). Mg2+ concentrations alsodecrease with altitude and H+ exhibits increasing concentrationswith altitude (CV = 87%). Although the availability of only threesnowpits precludes an unequivocal analysis of variations in con-centrations with altitude, Ca2+ and Mg2+ have strong negativetrends across the three sties (r = �0.99 for both), and H+ is the onlyion to show a significantly positive trend with altitude (r = 0.91, Ta-ble 1).

There are also notable changes in Cl� along the altitudinal tran-sect. Although the mean concentrations of Na+ and Cl� vary by lessthen 15% across the three sites, the Cl� concentrations decreasesignificantly between the AWS site at 2100 masl and the HG siteat 2700 masl (r = �0.87). Na+ concentrations are also slightlyhigher at the two glacier sites but this trend is not strong(r = 0.74). The trend for Na+, although weak, is notable because itis opposite to the other prominent lithic cations, Ca2+ and Mg2+,

Table 1Mean weighted ion concentrations (leq L�1) and total ion loads (kg ha�1) for the AWS, RMean � 100) are also shown.

Site Altitude (masl) SWE (mm) Concentrations (leq L�1)

H+ Cl� SO2�4

AWS 2100 560 0.34 0.73 1.99RG 2500 1395 0.78 0.70 2.68HG 2750 1492 2.15 0.55 2.25

Mean 1149 1.090 0.66 2.31SD 512 0.945 0.10 0.35%CV 45% 87% 0.15 15%

Load (kg ha�1)

AWS 2100 560 0.002 0.14 0.53RG 2500 1395 0.011 0.34 1.80HG 2750 1492 0.029 0.24 1.31

Mean 1149 0.014 0.24 1.21SD 512 0.014 0.10 0.64%CV 45% 98% 41% 52%

and also because the combined result of these Na+ and Cl� trendsis a strong increase in the Na+:Cl� ratios with altitude (r = 1.00).

The high degree of variability in total snow accumulation be-tween sites means that there is (with the exception of Ca2+ andMg2+) a greater degree of variability between loads than concentra-tions at the different sites, with ion loads between sites generallyvarying around 50%. Overall, the ion concentrations and loads atRG and HG are similar and higher than concentrations at theAWS (with exception of Cl�, Mg2+ and Ca2+) suggesting that con-centrations for most non-lithic ions increase with altitude acrossthe study area. However, since there are only three sites to com-pare and the variability in mean concentrations across sites is rel-atively small, this conclusion is tenuous.

3.2. Seasonal variations in snowpack ion concentrations

Although the 400–650 m elevation difference between the AWSand RG and HG glacier sites, and steep valley topography, causes alarge orographically-driven difference in snow accumulation(560 mm SWE at the AWS compared to 1395 and 1492 mm SWEat RG and HG, respectively), the ion concentration profiles showsimilar patterns (Fig. 3 and Table 2). Because the snow accumula-tion differed substantially across the sites, the concentrations areplotted against depth as a percentage of total SWE (%SWE = layerSWE depth (mm)/total snowpack SWE depth (mm) � 100), suchthat the top of each graph represents the surface layers of thesnowpits (Fig. 3). Overall the seasonal variability in concentrationsis much greater than the variability in mean snowpack concentra-tions across sites (Tables 2 and 3). The concentrations of K+ and Na+

typically vary least, while NHþ4 and Ca2+ vary most over the courseof the season.

Concentrations of the primary lithogenic ions (Ca2+ and Mg2+)are much lower in the upper 30% of the snowpack relative to thedeeper layers at the AWS site, and to a lesser extent at the RG site(Table 2). There are no seasonal trends for Ca2+ and Mg2+ at the HGsite at 2700 masl. The concentrations of Ca2+ and Mg2+ are alsostrongly correlated with each other across samples from all sites(r = 0.85, Table 2).

At both glacier sites there are marked increases in the concen-trations and loads of SO2�

4 , NO�3 , and NHþ4 near the top of the snow-pack, but the increase is somewhat more pronounced at the RG sitethan the higher-altitude HG site. These peaks in concentrationsoccur between �85% and 90% SWE depth at HG and between92% and 98% SWE depth at RG. A less pronounced increase in theconcentrations of these ions is visible in the top sample at theAWS site, although the depth of snow in this layer is very low

G and HG snowpits. The mean, standard deviations, and coefficient of variation (SD/

NO�3 Na+ NHþ4 K+ Mg2+ Ca2+ Na+:Cl�

2.62 1.56 1.23 0.36 1.81 23.59 2.152.92 1.91 1.79 0.42 1.01 8.10 2.742.52 1.79 1.64 0.38 0.74 3.07 3.27

2.69 1.76 1.55 0.39 1.19 11.6 2.70.21 0.18 0.29 0.03 0.56 10.7 0.68% 10% 19% 8% 47% 92% 21%

0.91 0.20 0.12 0.08 0.12 2.65 1.402.53 0.62 0.45 0.23 0.17 2.26 1.791.91 0.51 0.35 0.18 0.11 0.75 2.12

1.78 0.44 0.31 0.16 0.14 1.89 1.80.82 0.22 0.17 0.08 0.03 1.00 0.446% 49% 54% 47% 24% 53% 20%

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Fig. 3. Profiles of select ion concentrations in the snowpits located at the AWS (2100 masl), the Roberston Glacier (RG, 2500 masl) and the Haig Glacier (HG, 2700 masl)according to depth as percent of total SWE (%SWE = SWE depth of sample (mm)/total snowpack SWE (mm) � 100).

M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748 741

(Fig. 3). The aerosol ions (NO�3 , NHþ4 and SO2�4 ) are also all wel cor-

related across all samples (r > 0.73, Table 3), with SO2�4 and NHþ4

having the strongest correlation (r = 0.87) of any ion pair in thesnowpack (Table 3).

3.3. Summer precipitation

Six rain events sampled at the AWS throughout July 2006yielded a total 51 mm of precipitation (Table 4). The total rainfallfrom a given storm ranged from 1 to 26 mm and rainfall intensitiesvaried between 0.4 and 3.0 mm per hour (Fig. 4). Ion concentra-tions in these rain samples are inversely correlated to the magni-tude (volume) of rainfall events whereby very large volumeevents (e.g. 15–25 mm) yielded significant ion loads despite thelow concentrations (Fig. 4). Neither concentrations nor loads wererelated to rainfall intensity (Fig. 4). The dominant ions are the samefor rain as snow, with Ca2+ dominating the cations and NO�3 andSO2�

4 dominating the anions. The weighted mean concentrationof ions in the six rain samples collected at the AWS (Table 4) are

between 1.5 and 3.0 times higher than the weighted mean ion con-centrations in the end of winter snowpack at this same location(Table 4).

3.4. Identification of storm systems using snowpit d18O and air massback trajectories

Synoptic-scale frontal systems associated with low pressurecells that originate over the Pacific Ocean sweep across westernCanada regularly throughout winter and spring, bringing marinemoisture and orographic precipitation to the Rocky Mountain re-gion. In order to estimate the timing of the elevated concentrationsof atmospheric aerosols observed in mid-winter and spring, d18Ostratigraphies from four consecutive snowpits at the HG site wereused to identify individual storm systems (Sinclair and Marshall,2009). d18O stratigraphies from each snowpit and the resultingstorm boundaries are shown in Fig. 5. Both ion chemistry andd18O samples were collected from the April 23rd snowpit so that

Table 2Ion concentrations (leq L�1) by sample (depth in mm SWE) for the AWS, RG and HG snowpits. The mean and coefficient of variation (%CV = SD/Mean � 100) are also shown.

H+ Cl� SO2�4

NO�3 Na+ NHþ4 K+ Mg2+ Ca2+ Na+:Cl�

AWS depth552–560 0.30 1.45 5.47 3.48 1.82 3.51 0.44 2.28 11.8 1.3468–552 0.35 0.59 2.59 1.54 0.92 1.68 0.38 1.08 5.78 1.6364–468 0.35 0.40 2.10 3.17 0.98 2.15 0.32 1.23 6.56 2.5256–364 0.13 1.16 1.42 2.13 3.18 0.03 0.37 2.65 46.3 2.796–256 0.43 0.77 1.39 2.66 1.48 0.45 0.33 1.93 31.1 1.90–96 0.40 0.57 2.67 3.39 1.02 2.27 0.42 1.90 20.5 1.8

Mean 0.32 0.82 2.61 2.73 1.57 1.68 0.38 1.85 20.3 2.0%CV 32 49 58 28 55 76 13 33 78 28

RG depth1368–1395 2.09 0.44 3.20 3.14 2.48 0.96 0.38 1.08 4.67 5.61278–1368 1.91 0.48 3.39 1.92 2.07 2.32 0.40 1.16 6.63 4.41136–1278 2.04 0.81 8.23 7.76 1.71 7.23 0.48 1.53 6.87 2.1994–1136 0.85 0.71 1.94 2.77 2.28 1.59 0.43 0.66 3.08 3.2824–994 0.21 0.86 1.07 2.03 2.23 0.60 0.38 0.73 5.12 2.6668–824 1.00 0.60 0.67 1.18 1.59 0.41 0.33 0.57 3.25 2.6486–668 0.60 0.93 1.77 2.90 1.93 0.88 0.40 1.24 13.99 2.1332–486 0.13 0.89 3.79 3.58 1.70 0.02 0.35 1.04 10.54 1.9166–332 0.63 0.42 1.96 2.21 1.47 1.56 0.42 1.29 16.10 3.50–166 0.17 0.51 2.34 2.11 2.21 2.55 0.58 0.93 5.70 4.3

Mean 0.96 0.67 2.84 2.96 1.97 1.81 0.41 1.02 7.59 3.2%CV 81 30 75 62 17 114 17 30 59 38

HG depth1454–1491 2.45 0.63 2.12 2.18 2.45 1.90 0.42 0.85 3.55 3.91350–1454 2.29 0.52 3.69 2.28 1.97 2.64 0.42 1.17 5.26 3.81249–1350 2.24 0.68 5.71 4.81 2.42 5.54 0.41 1.12 4.03 3.61128–1249 1.35 0.51 2.03 3.39 1.82 2.19 0.37 0.61 1.93 3.6998–1128 0.51 0.67 2.54 2.86 2.00 1.66 0.40 0.95 5.38 3.0862–998 3.47 0.82 1.24 2.28 1.92 0.78 0.33 0.57 2.15 2.3724–862 0.16 0.36 0.64 1.22 1.91 0.22 0.34 0.61 2.63 5.2577–724 3.63 0.65 1.15 2.25 1.85 0.75 0.42 0.55 1.93 2.9443–577 4.27 0.67 1.75 3.57 2.04 0.08 0.36 0.82 3.81 3.0296–443 1.91 0.31 1.52 1.21 1.45 1.13 0.34 0.56 1.64 4.7149–296 0.68 0.48 2.68 2.34 1.19 1.86 0.38 0.69 2.75 2.50–149 2.95 0.39 3.09 2.37 1.29 2.43 0.40 0.74 3.11 3.3

Mean 2.16 0.56 2.35 2.56 1.86 1.77 0.38 0.77 3.18 3.5%CV 60 28 58 39 21 82 9 28 40 25

Table 3Correlation coefficients for ion concentrations in the AWS, RG, and HG snowpits(n = 28). Italic text indicates coefficients where p < 0.05, bold text indicates p < 0.01,and bold with an asterisk indicates p < 0.0001.

H+ Cl� SO2�4

NO�3 Na+ NHþ4 K+ Mg2+ Ca2+

H+ 1Cl� �0.19 1.00

SO2�4

0.08 0.25 1.00

NO�3 0.15 0.29 0.81� 1.00Na+ 0.14 0.37 �0.01 0.04 1.00NHþ4 0.11 0.04 0.87� 0.73� �0.07 1.00K+ �0.04 0.10 0.45 0.31 0.22 0.56 1.00Mg2+ �0.39 0.60 0.34 0.29 0.14 0.19 0.14 1.00Ca2+ �0.42 0.47 �0.08 0.02 0.21 �0.19 �0.07 0.85� 1.00

Table 4Mean weighted ion concentrations (leq L�1) and total ion loads (kg ha�1) for the July rain

Precipitation (mm) Cl�

Concentrations (leq L�1) AWS rain 51 1.5AWS snow 560 0.7Rain:snow 2.1

Loads (kg ha�1) AWS rain 51 0.03AWS snow 560 0.14Snow:rain 5.1

742 M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748

d18O profiles could be used to determine the timing of changes inNO�3 , SO2�

4 and NHþ4 concentrations.The increase in NO�3 in the HG snowpack between 30% and 40%

of SWE (Fig. 3) is mainly associated with two consecutive events(Storms 1 and 2) that occurred 14 h apart in early-winter betweenOctober 29th and November 6th, 2005 (Table 5). Storm 1 (October29th–November 2nd) brought proportionally more accumulation(56 cm compared to 17.4 cm in Storm 2) and would therefore dom-inate the chemistry signal. Upper-air (500 mb) wind direction andspeeds from flow indices over the HG site (Shea and Marshall,2007), along with frequency distributions of 3-hourly back trajec-tories for these events (Fig. 6), show that these storms originatedfrom the west-to-southwest sector with mean wind directions of263� and 254� respectively (Table 5). Both events had a muchstronger westerly than southerly component over the field site (Ta-ble 5), although back trajectories for Storm 1 indicate that the

and end of winter snowpack at the AWS site.

SO2�4

NO�3 Na+ NHþ4 K+ Mg2+ Ca2+

5.8 9.2 0.9 4.2 1.5 5.1 30.52.2 2.6 1.6 1.2 0.5 1.8 23.62.7 3.5 0.6 3.4 3.2 2.8 1.3

0.14 0.29 0.01 0.04 0.02 0.03 0.310.53 0.91 0.20 0.12 0.08 0.12 2.653.8 3.1 18.9 3.2 3.4 3.9 8.5

0

10

20

30

40

50

60

Jul 10 Jul 13 Jul 24 Jul 25 Jul 28 Jul 310

5

10

15

20

25

30Cl-SO42-

NO3-

NH4+Mg2+

Rain

SW

W

SW

W

WSW

Con

cent

ratio

n µ

eq L

-1

Rai

n (m

m)

Rai

n (m

m)

SW

W

SW

W W

SW

Load

kg

ha-1

a

b

0.00

0.02

0.04

0.06

0.08

0.10

Jul 10 Jul 13 Jul 24 Jul 25 Jul 28 Jul 300

5

10

15

20

25

30

0.9 mm/hr

0.4 mm/hr

0.4 mm/hr

1.5 mm/hr

1.3mm/hr

3.0mm/hr

Fig. 4. The concentrations (a) and loads (b) of select ions in the July rain samples.The amount of precipitation per event (mm) as recorded at the AWS is also shownin (a) and (b). The rainfall rates (mm/h) are shown below the date of the event. Theoverall flow direction of each event (either west (W) or southwest (SW)) is alsonoted.

M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748 743

storm followed a predominantly southwest pathway en route tothe Rocky Mountains. Storm 2, by contrast, appears to have origi-nated in the north Pacific and travelled directly west beforecrossing the northern US (Fig. 6). Note that the back trajectories

Fig. 5. Four consecutive d18O profiles at the HG site, showing individual storm boundasnowpit (red lines). The zones of increased ion concentrations between 30–40% and 85–91–4 in the d18O profiles. (For interpretation of the references to colour in this figure leg

in Figs. 6 and 7 provide modelled pathways for a parcel of airmoved backwards in time from the study site. The upper-air winddirection and speed are used to compute the flow path (Table 5),but do not necessarily reflect the surface wind conditions at thefield site. By contrast, the wind sensor on the AWS records topo-graphically channelled flow, but does not tell us much about theoverall synoptic conditions associated with each precipitationevent. For this reason, we chose to report flow indices at 500 mbabove the field site (Table 5). This is generally considered to the‘steering level’ of synoptic-scale flow, and reflects the average pre-vailing wind direction during each storm event.

The high NO�3 , SO2�4 and NHþ4 in the upper layers of the snow-

pack (85–90% SWE in the HG snowpit) are primarily associatedwith two short-duration (<1 day) storms (Storms 3 and 4), that oc-curred between March 25th and March 31st, 2006 and bought 11.6and 8.2 cm of accumulation respectively. Storm 3 was dominatedby southerly flow, while Storm 4 originated directly from thesouthwest (225�) with westerly and southerly wind speeds of�15 ms�1 (Fig. 6 and Table 5). While all four storms crossed thenorthern US, Storm 3 appears to have penetrated further inlandthan the other three storms, approaching the Canadian Rockies di-rectly from the south (Fig. 6).

Flow paths associated with July rainfall events were also ana-lysed using 3-hourly back trajectories calculated for the durationof each storm (n = 29). These were clustered into three distinctgroups using the HYSPLIT algorithm (Fig. 7). Cluster 1 contains52% of trajectories, which are primarily short (slow-moving) wes-terly trajectories from July 9th–10th, along with a fast-moving tra-jectory from July 28th. Cluster 2 contains 41% of trajectories, whichoriginated from the west or northwest, and are associated with cy-clonic flow that moved moisture across the northern US. All Cluster2 trajectories are from July 12–14th, aside from one trajectoryassociated with July 30th. Cluster 3 contains trajectories fromtwo short events on July 24th and July 25th.

Storms that travelled from the southwest over the northern US(Clusters 1 and 2) were associated with high volumes of rainfalland low ion concentrations. However, overall these eventsdelivered 77–91% of the total solute load and deposited 92% ofthe rainfall in the six events sampled (Fig. 4). Therefore, southwest-erly flow systems dominated the total deposition and loading ofrainfall in July of 2006 and were also associated with the highion concentrations and loads in the 2005/2006 winter snowpackin the study area.

ries (dashed lines). The top of each stratigraphy was used to date the subsequent0% SWE are indicated by yellow boxes, which correspond to the locations of Stormsend, the reader is referred to the web version of this article.)

Table 5Start and end dates/times and total accumulation of Storms (1–4) that brought elevated atmospheric aerosol concentrations to the HG site, along with mean upper air winddirection and speed (westerly and southerly components, and total) for each event at 500 mb over the HG site.

Storm # 1 2 3 4Month October/November November March

Storm start: day–time (GMT) 29–1230 04–0600 25–1500 30–2030Storm end: day–time (GMT) 02–1600 06–0900 26–0900 31–0200Total accumulation (cm) 56.0 17.4 11.6 8.2Mean wind direction at 500 mb (�) 263 254 210 225Wind speed at 500 mb (ms�1)

West 36.2 25.7 14.6 15.5South 3.6 7.4 25.6 15.3Total 37.5 27.1 29.5 21.8

Fig. 6. The frequency distribution of all 3-hourly back trajectories over the duration of Storms 1–4.

744 M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748

4. Discussion

4.1. Variations in snow chemistry with altitude

Concentrations of ions in precipitation are generally expected todecrease at higher altitudes, in part because the depth of the air

column between the cloud and the surface is smaller, which limitsthe time and opportunity for falling precipitation to scavenge aero-sols out of the atmosphere below the cloud base (Tranter andJones, 2000). In addition, the concentrations of solutes in precipita-tion may decrease along a storm track (and hence with altitude) asthe air mass is cleaned with progressive scavenging (Pomeroy

Fig. 7. Clusters of 3-hourly back trajectories for July 2006 rainfall events. The mean for each cluster are shown by the coloured lines and other plots show all individualtrajectories in clusters 1–3. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748 745

et al., 2005). However, the opposite effect may also occur, or theremay be an absence of any trend with altitude, depending on thesource and pathway of the air mass delivering precipitation, andhow the trajectory relates to solute sources (e.g. Camarero and Cat-alan, 1996; Clow et al., 2002; Nickus et al., 1997).

The observed decrease in Ca2+ and Mg2+ concentrations, and in-crease in H+ with increasing altitude can be explained by the factthat carbonate mineral (CaCO3 and CaMg(CO3)2) dust is likely theprimary source of Ca2+ and Mg2+ in this environment, and becausethe local source areas for dust (i.e. exposed rock) diminishes withincreasing altitude and snow/ice cover, there is a correspondingdecline in Ca2+ and Mg2+ concentrations. The increase in acidity(H+) with altitude is likely also the result from a reduction in depo-sition of carbonate dust with altitude, as this would reduce acidneutralisation reactions.

The increasing acidity with altitude is also likely related to theincreasing Na+:Cl� ratios at the high altitude sites. Na+ and Cl� insnowfall at remote sites are normally derived primarily from sea-salt aerosols, however the Na+:Cl� ratios typically increase abovethat of seawater (0.85 using equivalent concentrations) at inlandsites as a result of Na+ enhancements from terrestrial sources(e.g. mineral dust) and/or else losses of Cl� due to reactions withacid aerosols (e.g. HNO3 and H2SO4) in the atmosphere (Maupetitand Delmas, 1994):

Hþ þ NaCl! Naþ þHCl ðgasÞ ð1Þ

The snowpack Na+:Cl� ratios increase from 2.2 at the AWS to 3.3at the HG site. The fact that Na+ increases only slightly at higheraltitude, while the other major lithic ions decrease dramatically

with altitude, suggests that the increasing Na+:Cl� ratios are notlikely due to Na+ enhancement from mineral dust sources (litholo-gies in this area are not rich in minerals containing sodium). Ratherthe variations of Na+ concentrations with altitude are similar tothose observed for the other aerosol species (NHþ4 , SO2�

4 , NO�3 ), withslightly more elevated concentrations at the glacier sites, relativeto the lowest altitude AWS site. The stronger decreasing trend inCl� with altitude (r = �0.87) combined with the increasing H+ sup-port that Na+ and Cl� are primarily derived from sea salt aerosolsand that the Na+:Cl� ratio is controlled by the loss of Cl� by reac-tion with acids at the higher altitudes where acid neutralisationby carbonate dust diminishes (Maupetit et al., 1995; Tranter andJones, 2000).

The lack of substantial differences in the concentrations of mostnon-lithic ions (e.g. SO2�

4 , NO�3 -, NHþ4 , K+) across the altitude gradi-ent suggests that factors such as a reduction in below-cloud scav-enging potential with higher altitude, and the persistentmovement of storm tracks either up or down the Robertson valleydo not have a significant impact on concentrations along the gradi-ent investigated here. This may be in part due to the narrowaltitudinal range observed here (�2100–2750 masl) and/or the ori-entation of the valley with respect to dominant storm systems. Yetother studies that have investigated variations in precipitationconcentrations across altitudinal gradients have also found littlevariations in precipitation chemistry across altitude (Camareroand Catalan, 1996; Lovett and Kinsman, 1990). This finding sug-gests that the origin and trajectory of the air mass may be thekey factor in determining the composition of precipitation acrossthe study site. The increases in the loads of most ions (except

746 M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748

Ca2+, and Mg2+) with altitude indicates that total deposition is pri-marily controlled by precipitation rates, which increase signifi-cantly (almost triple) across the study site.

4.2. Seasonal variations

4.2.1. Winter and spring snowpackConcentrations of reactive nitrogen species and sulphate ions in

the snow samples from these sites are similar to precipitation atother remote sites (Galloway et al., 1982), and at the lower endof range reported from a regional study of 17 sites at high altitudesin the European Alps (Kuhn et al., 1998; Nickus et al., 1997) andfrom a suite of 52 sites in the US Rocky Mountains (Clow et al.,2002). In terms of the loads, SO2�

4 , NO�3 and NHþ4 at the two glaciersites (RG and HG) are 50–62% of the mean loads for glacier snow-packs sampled by Nickus et al. (1997) and on par with the less pol-luted of their study sites.

The elevated concentrations of Ca2+ and Mg2+ in the lower halfof the snowpack at the lower-altitude RG and AWS sites suggestthat carbonate mineral dusts from local terrain are the dominantsource of these ions in the snowpack, since there is more exposedrock in the early winter. The low concentrations of these ions in HGsnow also supports this conclusion, as there is limited exposedrock in the vicinity of this site (Fig. 1).

In most environments NO�3 and SO2�4 in precipitation are pri-

marily derived from acid aerosols (e.g. HNO3 and H2SO4) formedin the atmosphere as a result of reactions involving oxides of nitro-gen (NOx) and sulphur (SO2) from industrial and vehicle emissionsand from the burning of fossil fuels (Fenn et al., 2003; Heuer et al.,2000; Mast et al., 2001). The primary source of NHþ4 is usually agri-cultural activities (animal wastes and fertilizers) (Fenn et al.,2003). Concentrations of SO2�

4 in samples are highly correlated(r > 0.79, p < 0.0001) with those of NHþ4 , NO�3 , NHþ4 and NO�3 con-centrations are also well correlated (r = 0.66, p < 0.0003). Thesetrends support the hypothesis that these pollutants have similarsources. The stronger correlation between NHþ4 and SO2�

4 relativeto NHþ4 and NO�3 is likely a reflection of the fact that the ammoniumsulphate aerosol is more stable than ammonium-nitrate, andtherefore NHþ4 tends to react more readily with sulphuric acid thannitric acid in the atmosphere (Maupetit and Delmas, 1994).

The pattern of high concentrations of SO2�4 , NO�3 and NHþ4 in the

upper portions of the snowpack at all sites is similar to the patternsobserved in other studies of alpine snowpack chemistry (Kuhnet al., 1998; Maupetit et al., 1995). This has been argued to reflectan increase in the vertical extent of the boundary layer, and thusconvective mixing of the atmosphere, with warming surface tem-peratures in the spring. The stable stratification of the atmospherein alpine valleys over the winter months (from cold air ponding inthe valleys) typically prevents the vertical transport of pollutantsfrom low elevations to the alpine zone during most of the winter(Kuhn et al., 1998; Maupetit et al., 1995). The data here indicatethat this process may affect the chemistry of spring precipitationat this site, and that regional pollution from lower altitudes arebeing transported to high elevations, leading to higher concentra-tions of NO�3 , SO2�

4 , and NHþ4 in precipitation at higher altitudes.The more limited increase in ion concentrations in the top of theAWS snowpit may be due to the lower rate of precipitation hererelative to the higher-altitude, RG and HG, sites combined with acoarse sampling interval (total of six samples).

Although it is possible that changes in the vertical mixing play arole in delivering pollutants to higher altitudes in the spring, theair mass back trajectories suggest that changes in the source andpathway of air masses bringing moisture to the site are also animportant control on seasonal changes in the chemistry of snow-fall. Air mass back trajectories indicate that both the early-winterand spring peaks in ion concentrations are associated with

southwesterly storms, such as Storm 3 (Fig. 6), that originate inthe Pacific and typically flow northeast over the US (California,Oregon, and/or Washington). These storms entrain additional acidaerosols relative to westerly or northwesterly flow systems, whichflow over the less populated and industrialised west coast of Can-ada and British Columbia.

Another possible mechanism for elevated NO�3 and SO2�4 concen-

trations in winter months are upslope storms (easterly flow up theeastern slopes of the Rockies) that would deliver pollutants fromthe urban and industrial areas east of the mountains. Sinclair andMarshall (2009) found that total accumulation at the HG site wasdominated by southwesterly (49%) and westerly (36%) events in2005/2006, and that there was no significant upslope flow. This isin contrast to the neighbouring two seasons examined in their study,when upslope events contributed a significant component of the an-nual snowfall total (25% and 7% in 2004/2005 and 2006/2007 respec-tively). Southwesterly flow was, however, dominant in all threeseasons investigated in this study and contributed between 34%and 49% of the total accumulation. It is therefore likely that south-westerly systems persistently deliver atmospheric pollution to win-ter snowpacks, and it is also possible that upslope snow could alsobring elevated concentrations of these atmospheric aerosols, but amulti-year snow chemistry program is required to investigate thismore closely.

4.2.2. Summer precipitationThese results indicate that summer precipitation may represent

a significant proportion of annual deposition in the catchment de-spite the dominance of snowfall in the annual water budget atthese altitudes. Summer precipitation is often found to carry high-er concentrations of certain solutes relative to winter snow as a re-sult of relationships between aerosol concentrations and thegrowth of water droplets and ice crystals in clouds (Baltenspergeret al., 1998). In their study of precipitation at high altitudes in Col-orado, Heuer et al. (2000) also found mean summer precipitationconcentrations to be at least twice as high as winter precipitation,and attributed much of the differences to changes in weather pat-terns between winter and summer, with westerly or southwesterlyflow dominating in the winter and easterly flows dominating in thesummer months.

4.3. First order approximation of total annual deposition

The solutes in the July rain were 2–3 times higher than the con-centrations found in the winter snowpack. However, since the totalend-of-winter SWE was 11 times higher than the volume of rainthat fell in July 2006, the total load of solutes delivered from thewinter snowpack was still anywhere from 3 to 9 times higher thanin bulk deposition in July (Table 4). While subject to a number ofuncertainties, a first order approximation of the total annual atmo-spheric deposition in this region and the relative importance ofrain vs. snow for annual deposition can be made using someassumptions with respect to the chemical composition of summerprecipitation and knowledge of the proportion of annual precipita-tion that falls as rain in this area. The tipping bucket at the AWS(Fig. 1) was not installed until the summer field season began inJuly of 2006 and was removed at the end of the study period inAugust. Therefore, the relative proportion of precipitation fallingas rain vs. snow is estimated from precipitation and snow pillowdata from the nearby Three Isle Lake site (2160 masl, 50�3705000N,115�1604000W, approximately 14 km south east of the AWS) moni-tored by Alberta Environment (Alberta Environment, 2010). Be-tween 2000 and 2009, the mean total annual precipitation atThree Isle Lake was 980 mm, and rain accounted for 21–47% ofthe annual total precipitation. Over the same period of observation,

Table 6Approximation of total annual precipitation and solute loads at the AWS and RG sites assuming rainfall accounts for anywhere from 25% to 45% of total annual precipitation. Otherassumptions include that April snowpack was representative of annual snowfall, and that mean weighted concentrations of ions in July rainfall is representative of the mean ofrainfall throughout the rest of the year (i.e. June–September). The load of ‘‘N’’ (last column) is the total mass of Nitrogen from NO�3 and NHþ4 .

Precipitation (mm) Cl� SO2�4

NO�3 Na+ NHþ4 K+ Mg2+ Ca2+ N

AWS (2100 m)Loads Snow 560 0.14 0.53 0.91 0.20 0.12 0.08 0.12 2.65 0.30

Rain (45%) 458 0.25 2.34 4.80 0.18 0.63 0.21 0.28 2.80 1.58Rain (25%) 187 0.10 0.52 1.07 0.04 0.14 0.08 0.12 1.14 0.35

Total annual load Rain (45%) 1018 0.40 2.88 5.71 0.38 0.76 0.28 0.41 5.45 1.88Rain (25%) 747 0.25 1.05 1.98 0.24 0.26 0.16 0.24 3.79 0.65

RG (2500 m)Loads Snow 1395 0.34 1.80 2.53 0.62 0.45 0.23 0.17 2.26 0.92

Rain (45%) 1141 0.63 3.18 6.52 0.24 0.86 0.51 0.70 6.97 2.14Rain (25%) 465 0.26 1.30 2.66 0.10 0.35 0.21 0.29 2.84 0.87

Total annual load Rain (45%) 2536 0.97 4.98 9.05 0.86 1.31 0.74 0.88 9.24 3.06Rain (25%) 1606 0.60 3.09 5.18 0.71 0.80 0.44 0.46 5.11 1.79

M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748 747

July rainfall represented 11–30% of all rain at Three Isle Lake(Alberta Environment, 2010).

A first approximation of the total annual deposition of solutes atthe AWS (2100 masl) and the RG glacier sites (2500 masl) is madeusing the following assumptions: (i) that April snowpack repre-sented total annual snow accumulation, (ii) that weighted meanconcentration of ions in July samples are representative of meanconcentration of summer precipitation, (iii) that the compositionof rainfall does not change with altitude, and (iv) that rain accountsfor anywhere from 25% to 45% of total annual precipitation. Theestimates and assumptions are presented in Table 6. For the RG site(at 2500 masl) total deposition of SO2�

4 is estimated to be anywherefrom 3.1 to 5.0 kg ha�1, and total annual deposition of N (from bothNO�3 and NHþ4 ) is between 1.8 and 3.1 kg ha�1 (Table 6). These cal-culations illustrate that total annual atmospheric deposition willbe higher if rainfall makes up a larger proportion of precipitation.The elevated concentration of solutes in spring snow and summerrainfall indicates that total annual deposition in this area will behighly sensitive to the amount of precipitation that falls in springand summer. It is clear from this study that a more comprehensiveknowledge of monthly snow and rainfall amounts at high altitudeare required in order to better assess the sensitivity of these sites toatmospheric deposition and how this loading might change as cli-mate in the region continues to warm.

It is useful to compare these estimates of annual deposition ofsolutes with deposition at the US National Atmospheric DepositionProgram (NADP) sites, as there is evidence that alpine ecosystemsin these alpine regions are at risk as a result of elevated atmo-spheric deposition of acids (e.g. H2SO4) and reactive nitrogen(NHþ4 and NO�3 ) species in snow (Heuer et al., 2000; Ingersollet al., 2008; Williams and Tonnessen, 2000). Studies from theRocky Mountains in the US reveal that the total annual depositionof sulphate and nitrogen deposition in wetfall at NADP sites in al-pine areas (>2500 masl) range from 1.0 to 5.0 kg ha�1 for S (or�3.0–13.0 kg ha�1 SO2�

4 ) and 1.0–3.5 kg ha�1 for inorganic N depo-sition (Hidy, 2003; Williams and Tonnessen, 2000). Thus, the esti-mates of total deposition rates for SO2�

4 and N at 2500 masl in thisstudy are low but still within the range of deposition observed forthese species at similar altitudes in the US Rocky Mountains, wheredeposition rates are argued to pose a threat to alpine ecosystems.

5. Conclusions

The ion loads in both snow and rain at high altitude sites in theRocky Mountains in 2005/2006 were dominated by ions from lithic

sources (Ca2+ and Mg2+) and anions from acid aerosols (SO2�4 and

NO�3 ). With the exception of Ca2+, Mg2+ and H+, ion concentrationsvaried by less than 20%. Also aside from Ca2+, Mg2+ and Na:Cl ratiosno strong altitudinal trends in chemical composition were revealedacross the three sites along the gradient (2100–2750 masl) exam-ined in this study. Since there are only three sites to compare,our conclusions about variation with altitude are somewhat tenu-ous and should be validated by further study. However, total ionloads increased dramatically at the two glacier sites, as a resultof higher rates of snow accumulation.

Snowpit profiles show a strong increase in acid anions in thesurface (spring) layers of the snowpack, which were similar tothe mean weighted concentrations of ions in July rainfall. Howevermean concentrations in July rain were 1–3 times higher than themean snowpack concentrations, and varied inversely with the vol-ume of rainfall. The elevated concentration of solutes in springsnow and summer rainfall indicates that total annual depositionin this area will be highly sensitive to the amount of precipitationthat falls in spring and summer.

The identification of winter storms using snowpit isotope pro-files, and back trajectory modelling of these events, indicate thatthe chemistry of precipitation (both rain and snow) is sensitiveto synoptic conditions. Elevated concentrations in the spring snow-pack is thought to be a result of both stronger convective mixingcompared to the relatively stable stratification of winter months,and southwesterly storms that sweep across the northwesternUS and bring elevated concentrations of acid aerosols to the RockyMountains. Southwesterly storm tracks were particularly impor-tant to the delivery of high ionic concentrations in July rainfall,with 77–91% of the load in July rainfall coming from these events.However, higher-resolution snow chemistry data, combined witheither stable isotope analyses or real-time sampling of wintersnow, is required to more accurately determine the extent towhich different synoptic conditions influence atmosphericdeposition at high altitudes in the region. A multi-seasonal studywould also enable further investigation of the extent to which ups-lope (easterly) air masses bring atmospheric pollutants fromindustrialised regions in central Canada. Additionally, a morecomprehensive and long-term record of precipitation chemistry(rain and snow) in spring and summer is required to provide abaseline for monitoring atmospheric pollutants in the hydrologicalcycle, and to assess both the impact of our activities on these highaltitude environments and ecosystem sensitivity to atmospheri-cally deposited pollutants. Finally, a better understanding of theorigin of these pollutants, using detailed synoptic-scale meteoro-logical models to characterise storm types that bring high

748 M.J. Lafrenière, K.E. Sinclair / Journal of Hydrology 409 (2011) 737–748

concentrations of pollutants to high elevation ecosystems is rec-ommended as a focus of future research.

A first order approximation of total annual deposition suggeststhat annual rates of deposition of SO2�

4 and N in the Haig and Rob-ertson Glacier valleys are at the low end of the range in concentra-tions found in the European Alps and the US Rocky Mountains.However, given that these regions are more densely populatedand industrialised, and the fact that the concentrations and loadsof acid ions and ammonia in snow in this relatively remote alpinesite are within the range found in these areas, there is strong evi-dence that a wider survey of chemical deposition in the snowpackin high alpine areas in the Canadian Rocky Mountains is warranted.

Acknowledgements

The authors would like to thank Shawn Marshall, Brad Thomasand Lindsey Nicholson for volunteering to help with field sampling.Thanks also to Stephanie Robbins for assistance in the field in thesummer of 2006, and to Tara Moran for assistance with winter iso-tope sample collection. The Biogeoscience Institute, University ofCalgary is acknowledged for use of field and laboratory facilities,Alberta Tourism, Parks and Recreation for permitting this researchwithin the Park, and NOAA Air Resources Laboratory for use of theHYSPLIT model. Funding for this work was provided by NSERC Dis-covery Grant to M.J. Lafrenière.

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