Factors affecting the chemical composition of snowpack in the Kilpisjärvi area of North Scandinavia

Atmospheric Environment 118 (2015) 211e218

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Factors affecting the chemical composition of snowpack in the €rvi area of North Scandinavia Kilpisja Valle Raidla*, Enn Kaup, Jüri Ivask Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086, Tallinn, Estonia

h i g h l i g h t s
The
The
The
The

snow's chemistry showed notable variations in space and time on a narrow area. pH of snow is mainly controlled by carbonate dust. occurrence of Ca2þ and SO2 4 in snow can be linked to aerosol modification. snow's chemistry depends on local topography.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 February 2015 Received in revised form 29 July 2015 Accepted 30 July 2015 Available online 8 August 2015

Kilpisj€ arvi is a remote area in northwest Finland, almost without any local human impact on the environment. The design of the study was to investigate the chemical composition of local precipitation in €rvi region and determine the factors influencing it. To this aim, we collected 29 snow samples the Kilpisja from the Lake Saana catchment and its surroundings in spring of 2008 and 2009. Already within the radius of a few hundred metres, significant chemical heterogeneity could be detected in the snow cover, caused by mixed marine and terrestrial precipitation and the impact of local rock dust. Based on the Cl/ Ca2þ ratio in snow, this could occur in localised areas, where marine or terrestrial aerosols predominate. Clear correlation was noted between SO4 2 and Ca2þ, induced possibly by recombination of SO4 2 with carbonaceous dust. Other factors, such as nitrification in snow cover, could also have affected the pH levels of snow, which are mostly controlled by carbonate dust. © 2015 Elsevier Ltd. All rights reserved.

Keywords: pH of snow Sea salts Terrestrial dust Arctic

1. Introduction Kilpisj€ arvi is the sole region in Finland that extends to the Scandinavian Mountains. The region is sparsely populated and has €rvi area a weak local and regional human impact. Since the Kilpisja is considered the least contaminated region in the European Union (Rühling et al., 1992), it provides a wide range of sites for studying of topics such as natural evolution of a geotope, or global impact on this relatively isolated area. The best-examined object in the Kil€rvi area is by far Lake Saana. While earlier studies have pisja concentrated on the lake and its bioevolution (Blom et al., 2000; €m et al., Sorvari et al., 2000; Rautio et al., 2000, 2011; Forsstro 2005, 2007), the present study focused on wintertime precipitation chemistry in the lake catchment. Snow is an efficient scavenger of aerosols in the atmosphere,

* Corresponding author. E-mail address: [email protected] (V. Raidla). http://dx.doi.org/10.1016/j.atmosenv.2015.07.043 1352-2310/© 2015 Elsevier Ltd. All rights reserved.

and thus even moderate falls deposit a substantial proportion of elements from the atmosphere (regardless of their origin) (de Caritat et al., 2005). These aerosol contaminants reach their maximum concentration in late winter (e.g., Rahn, 1981; Xie et al., 1999; Polissar et al., 2001), at about the same time as the snow pack reaches its maximum depth (Douglas and Sturm, 2004). Later, during melt period, all scavenged aerosols are released into the environment. Arctic lakes feed mainly on springtime meltwater and thereby the chemical composition of snow directly affects the conditions of the lake. It is therefore crucial to understand the chemical composition of the snow pack in detail, which in turn helps better comprehend the formation of chemical composition of water in local lakes, and how it influences their ecosystems.

2. The study area €rvi region is located approximately 120 km from the The Kilpisja Arctic Ocean (except closer fjords), in northwestern part of the

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Finnish Lapland (Fig. 1). The area is part of the Scandinavian mountain range formed during the Caledonian orogeny (450e400 million years ago). Sandstone, claystone, conglomerate, dolomite and quartzite layers lie on the 2.8-billion-year-old metamorphic bedrock, sporadically covered with patches of weathering-crust (Atlas of Finland, 1986). Due to the Arctic climate, the local soil is weakly developed, being covered by just a thin humus layer on the Quaternary sediments, while bare rock surfaces and rock piles prevail. Climate in the Kilpisj€ arvi area is mostly affected by the region's altitude and its northern, relatively close location to the Atlantic €rvinen, 1987). Winter in Kilpisja €rvi lasts for about nine Ocean (Ja months. Approximately 60% of precipitation (annual mean ca. 420 mm) occurs as snow, which normally covers the ground for 210e220 days per year. In the open fell area, where wind and local topography shape the snow distribution, snow beds covered by several metres of snow as well as nearly snowless areas can be found. Snow cover is characterised by wind packed dense layers (Solantie, 2000). Most of the snow melts quickly by the beginning of June. Surface water runoff is minimal, except during the spring melt period when heavy floods are common (Sorvari et al., 2000). Lake Saana is situated 679.4 m a.s.l. Its surface area is 62 ha and maximum depth 24 m. The lake is 1400 m long and 750 m wide, its shores are mostly steep and stony, and its catchment covers 460.6 ha of mainly mountainous and rocky landscape. The lake basin is generally well shielded by high mountains on the western and northeastern sides. Other parts of the catchment are lower and flatter. The water of this subarctic lake is clear and has low nutrient content (Rautio et al., 2000). The lake is nevertheless subjected to the long-range atmospheric pollution related to fossil fuel combustion (Korhola et al., 2002). 3. Material and methods Samples were collected twicedin spring of 2008 and 2009

Lake Jeäkasnjoaskejavri

Finland

sampling point where 2+ Cl /Ca <1.7 sampling point where 2+ Cl /Ca >1.7 catchment area clint

800 760

920

720

840

Mount Saana 920

Lake Saana

1000

600

760 680

500 m

€hkasnjoaskejavri catchments with the Fig. 1. A schematic map of Lake Saana and Jea positions of sampling points in 2009.

(Table 1). All sample sites were chosen far from ski tracks, in order to minimize potential contamination. Pits were dug in snow pack, using a spade, the wall was cleaned and integrated samples through the entire cross-section of the snow pack were taken, using a pre-cleaned plastic shovel. Samples were immediately placed inside pre-cleaned plastic grip-bags and transported to the Helsinki €rvi. University's Biological Station in Kilpisja The samples were melted in closed containers in a cool room, in order to avoid evaporation. pH and conductivity were measured in €rvi Biological Station. The pH was the laboratory of the Kilpisja measured at room temperature (i.e. 20  C) after the sample bag had been open for 10 min in a well-ventilated clean room for achieving equilibrium with the atmospheric CO2. pH and electrical conductivity were measured with an IQ170 Scientific Instruments field pHconductivity meter. The instrument was calibrated with pH buffers of 4 and 7. Major ions were measured on Dionex ICS-1000 ion chromatograph at the University of Tartu. For anion analyses, AS14A 4  250 mm analytical column and AG14A 4  50 mm guard column were used with 8.0 mM sodium carbonate/1.0 mM sodium bicarbonate eluent at flow rate of 1 mL min1. For cation analyses, CS12A 4  250 mm analytical column and CG12A 4  50 mm guard column were used with 20 mM methanesulfonic acid eluent at flow rate of 1 mL min1. On the day of the sampling, NH4 þ and NO3  in the melted snow samples were also determined, using a HACH DR2000 spectrophotometer. Oxygen stable isotope ratio in water (d18O) was determined on a Thermo Fisher Scientific Delta V Advantage mass spectrometer at the Institute of Geology at Tallinn University of Technology. d18O reproducibility in water was better than ±0.1‰. The results were expressed in per mille deviation relative to Vienna Standard Mean Ocean Water (VSMOW). The Finnish Meteorological Institute provided meteorological data for the years of 2007e2009 from its two meteorological stations located near Lake Saana. Wind data were obtained from the automatic weather station (AWS) located on top of Mount Saana, ca. 1.5 km southwest of Lake Saana, at the altitude of 1060 m a.s.l. (i.e. 381 m above lake surface). The manned meteorological station in the village of Kilpisj€ arvi about 4 km west from the lake, at the elevation of 473 m a.s.l, provided the precipitation data. 4. Results and discussion 4.1. pH In 2008, the sampling campaign focused mainly on biogenes €rvi area, therefore, only a and d18O in the snow covering the Kilpisja few major ion analyses were done. In 2009, the data were collected from three sites: top of Mount Saana and the catchments of Lake €hkasnjoskejavri (Fig. 2A and B). Table 2 sums up Saana and Lake Jea correlations between major soluble mineral ions and pH. The pH values of snow vary considerably, especially in samples taken in 2009 (Table 1). Earlier studies have shown a short low-pH€rvi area, which is tied to the spring period in the lakes of the Kilpisja melt period in early spring when pH can drop below 6. The period only lasts a few days and its environmental influence is weak € m et al., 2007). Such acid pulse in (Sorvari et al., 2000; Forsstro spring has been observed in many Arctic lakes and rivers (Bishop and Pettersson, 1996; Moiseenko et al., 2001). The phenomenon is explained by dissolution of anthropogenic NO3  and SO4 2 into snowmelt, causing a rapid decline in the neutralising capacity of water (e.g., Molot et al., 1989; Petrone et al., 2007). In contrast, de Caritat et al. (2005) show no correlation between pH and SO4 2 in the meltwater of the Arctic snow and claim that only a weak local relationship exists between pH and NO3  in Sweden, Finland and Russia.

V. Raidla et al. / Atmospheric Environment 118 (2015) 211e218

213

Table 1 Chemical composition of snow in the Kilpisj€ arvi area in spring 2008 and 2009.

d18O ‰

Depth of snow cm

El. cond. mS cm1

pH

Cl

Naþ

SO4 2

Mg2þ

Ca2þ

Kþ

NH4 þ

NO3 

1.0 3.5 0.0

0.7 1.9 0.0

3.3 5.3 1.8

1.0 2.1 0.5

mmol L1 2008 Average (n ¼ 31) Max Min 2009 Average (n ¼ 29) Max Min

14.7 11.1 17.3

76.9 132.0 50.0

6.4 7.7 5.2

76.6 160.0 39.0

6.9 9.0 5.4

A

6.2 11.5 2.8

34.3 88.3 17.2

7.3 7.7

59.8 124.0 33.1

5.6 12.8 2.5

6.4 6.2

6.1

5.8

6.5

6.4 6.3

7.3 6.7 7.2 6.4 9.0 8.7 7.7 6.6 8.6 5.6 6.1 8.4 5.4 8.4 6.3 7.2 5.5 7.4 Lake Saana 7.5 6.6 8.0 5.5 7.0 6.3 Mount Saana

6.0

5.8 6.9

7.2 6.3

5.6

5.4

6.6

7.3

S

2008/2009

5.8

Lake 5.9 Jeäkasnjoaskejavri

6.8

3.6 7.7 2.0

6.1

Lake 5.8 5.9 Jeäkasnjoaskejavri

6.8

23.2 39.9 8.5

N

6.2 8.0

B

6.1 6.1

9.2 13.5 4.4

6.1

5.2 Lake 5.5

Lake Saana

Mount Saana

Masetjarvi

Lake Kilpisjärvi 2007/2008 N

6.5 Lake Iso-Marjajarvi S

1 km

6.6 7.0

6.3

€rvi area. The winter wind roses are given in the corners of the plot. Fig. 2. A schematic map of pH values of snow cover in 2008 (A) and 2009 (B) in the Kilpisja

Table 2 Correlation coefficients between soluble ionic species and pH in snow samples. Cl (n ¼ 29)

Naþ (n ¼ 29)

SO4 2 (n ¼ 29)

Ca2þ (n ¼ 29)

Mg2þ (n ¼ 29)

Kþ (n ¼ 29)

NO3  (n ¼ 29)

NH4 þ (n ¼ 29)

0.23

0.08 0.88

0.30 0.35 0.68

0.34 0.06 0.49 0.81

0.01 0.76 0.91 0.73 0.59

0.04 0.26 0.39 0.45 0.30 0.24

0.15 0.27 0.19 0.04 0.13 0.16 0.23

0.12 0.08 0.10 0.10 0.18 0.25 0.18 0.03

pH Cl Naþ SO4 2 Ca2þ Mg2þ Kþ NO3 

Bold values signify the examples cited in the text.

We saw no correlation between pH and SO4 2 , NO3  or other measured ions either (Table 2). pH level of 5.65, though acidic, is not considered as acid precipitation. CO2 is the most common cause of acidity in water. pH level of natural, unpolluted precipitation (rain or snow) is close to 5.6, assuming there is a standard atmospheric CO2 concentration (Appelo and Postma, 1999). Only a single snow sample from 2008 to 2009 showed a pH below 5.6. Therefore, we can see a clear trend of increasing

alkalinity in snow. The snow in the western part of the Lake Saana catchment was in general markedly more alkaline than in the eastern part (Fig. 2B). This could be explained by the fact that Saana Cliff exposes a tens-of-metres-thick dolomite layer. In high wind conditions, small parts of the area not covered with snow could leave their own clear fingerprint on the ion composition of the surrounding snow (e.g., Hinkley, 1994). In winter, Saana Cliff is only partially covered with snow and it is more than likely that alkaline

214

V. Raidla et al. / Atmospheric Environment 118 (2015) 211e218

dolomite dust could spread from there. It is most probably local dolomite dust that has raised pH values of snow in the surroundings of Lake Saana. Above average snow conductivities in the Lake Saana catchment (6.8 mS cm1, as opposed to 4.5 mS cm1 in the €hkasnjoaskejarvi catchment) support this view. Moreover, Lake Jea in winter of 2007/2008, snowfall was heavier and hence the snow cover on the cliff much more extensive than in 2008/2009 (Finnish Meteorological Institute), resulting in lower dust emissions from the cliff. The one-year-difference is well reflected by lower and more homogenous pH values in the Saana catchment in 2008 (Fig. 2A). However, Ca2þ and Mg2þ as components of dolomite and possible tracers of local limestone dust from Mount Saana show no correlation with pH (Table 2). This is indicative of a significant entry of migrated aerosols in the research area, which could predominate in the snow's chemical composition.

140

A

120 100 80 60 40

SDL

20 0 45

B

40 35

4.2. Ion composition Chemical composition varied greatly throughout the research area. The predominant ions were Cl and Naþ (Table 1). The proportion of Cl in the surrounding rocks is very low, forming only 0.2% of rock composition. Its concentration in local surface waters was similar to that of snow (our unpublished data), which is indicative of considerable input of atmospheric Cl in the Kil€rvi areadmost probably from marine aerosols. Generally, the pisja influence of marine aerosols on the chemistry of precipitation decreases with distance from the ocean, and with altitude (e.g., de Caritat et al., 2005; Virkkula et al., 2009). Yli-Tuomi et al. (2003a) estimated the sea salt aerosol factor in the Northern-Finland to be 0.4 to 0.6. However, in cold regions, marine particle concentrations in the air could vary over time because these strongly depend on the existence of ice cover on the surrounding seas (e.g., Heintzenberg and Leck, 1994; Virkkula et al., 1998; Beine et al., 2011). Cl and Naþ as main constituents of seawater can be used as sea salt tracers, because they follow the seawater dilution line (SDL) in marine air masses (e.g., Piccardi et al., 1996; Aristarain and Delmas, 2002). Although Cl and Naþ formed a major part in the chemical composition of the snow samples and their concentrations clearly correlated (R ¼ 0.88), their distribution still deviated from the SDL (Fig. 3A). One of the reasons for the deviation could be Naþ intakes from the local rock. On the other hand, many studies in the Antarctic (e.g., Kerminen et al., 2000; Aristarain and Delmas, 2002; Virkkula et al., 2009) and the Arctic (e.g., Staebler et al., 1999; Toom-Sauntry and Barrie, 2002; de Caritat et al., 2005) show that sea salt aerosols could be modified during transport to inland due to the chemical reaction of sulphuric acid or gaseous SO2 with sea-salt particles according to:

Sea salt þ H2 SO4 ¼ Na2 SO4 or NaHSO4 þ HClðgasÞ

(1) 

HCl is expelled to the gaseous phase and could lead to Cl /Naþ ratios of <1 in the atmosphere and in snow (Kerminen et al., 2000; Virkkula et al., 2009). This process can also add SO4 2 to snow, resulting in SO4 2 enrichment (e.g., Li and Barrie, 1993; Krnavek et al., 2012). Indeed, the SO4 2 content in snow was relatively high and had good correlation with Naþ, but the correlations of SO4 2 with both Ca2þ and Mg2þ were even higher (Tables 1 and 2). While Ca2þ, Naþ and Mg2þ could originate from local rocks (feldspars or dolomite), the sulphur content in local surface rock samples is low and has a negative correlation with the abovementioned elements (our unpublished data). This eliminates the possibility that the sulphate re-emitted from the deposited aerosol originated from the local area. There is only a narrow area of sulphide minerals on the southern slope of Mount Saana (oral

-

2+

Cl /Ca <1.7

30 25 20

-

15 10 5 0

2+

Cl /Ca >1.7

SDL 0

20

40

60

80

100

Cl-, μmol·L-1 Fig. 3. Variations of Naþ (A) and Ca2þ (B) in dependence on Cl in the snow samples of 2009. The dotted line separates the points where Cl/Ca2þ are lower or higher than 1.7.

communication by Prof. Alvar Soesoo), but for most of winter and spring this area is covered with snow. Measuring the ratio of sulphur isotopes in snow and ice cores in the Arctic America and Greenland has demonstrated that industrial sulphur emissions (d34S in the range of 0 to þ7‰) account for the majority of the airborne flux in winter, while in the summer months contributions from sea salt and marine biogenic sources (d34S in the range of þ14‰ to þ22‰) become increasingly more significant (Nriagu et al., 1991; Wasiuta et al., 2006; Mann et al., 2008). Analogous measurements in Sweden show the value of d34SSO4 in the snow across all sites as between þ6.6 and þ 5.6‰ and no difference between the upper and lower sections of the snow cover was found €rth et al., 2008). (Mo Tuovinen et al. (1993) point out that practically all sulphur emissions north of the Arctic Circle originate from two well-defined industrial regions: the Norilsk area in Siberia and the Kola Peninsula in northwest Russia, where metallurgical processes produce massive emissions of sulphur dioxide. However, based on continuous SO2 monitoring (Fisher et al., 2011) and receptor modelling (Yli-Tuomi et al., 2003b), it was found that industrial sources of sulphur in both Norilsk and Kola Peninsula contribute little to the sulphate budget of the Arctic. In winters of 2007e2009, the air €rvi area came predominantly from masses carried to the Kilpisja North-Atlantic (40%) or High Artic (30%) regions. The rest originate from mid-latitudes between the longitudes of 10 W and 50 E (METEX); this region could therefore be primary source of sulphur €rvi snow. Additionally, many North-Atlantic air contained in Kilpisja €rvi via South-Scandinavia, where they masses arrive to Kilpisja could also absorb industrial SO2. During the industrial period, higher SO2 4 content and heavier d34S values in snow or in Arctic ice cores are mostly accompanied by

V. Raidla et al. / Atmospheric Environment 118 (2015) 211e218

~es and NO3  as a marker of anthropogenic pollution (e.g., Simo Zagorodnov, 2001; Hidy, 2003; Ruuskanen et al., 2007; Gabrielli et al., 2008; Mann et al., 2008; Kuramoto et al., 2011). NO3  and €rvi NH4 þ concentrations in the collected samples from the Kilpisja area were low (Table 1), and no relation was found between NO3  and other major ions (Table 2). However, several studies have shown that NO3  in snow can be modified during photolytical processes and then carried away from the region or re-deposited on  and snow surface, where it eventually gets buried (e.g., Domine Shepson, 2002; Jarvis et al., 2009). Observations of the surface snow at Summit, Greenland, (Dibb et al., 2007) and aerosols measurements in Svålbard (Teinil€ a et al., 2003) show SO4 2 maximum and NO3  and NH4 þ minimum at the end of winter. The early spring maximum of condensed sulphur nucleus is not necessarily directly linked to the long-range transport of aerosols. It can also be the result of sulphur's interaction in the troposphere with photochemically oxidised SO2 emitted by industrial sources, because the oxidation of SO2 is limited due to € lack of light in winter (e.g., Polissar et al., 2001; Hidy, 2003; Teinila et al., 2003; Fisher et al., 2011). In the Arctic areas, the highest concentrations of Ca2þ, Mg2þ and þ Na have also been noted in fresh springtime snow and aerosols (Drab et al., 2002; Teinil€ a et al., 2003; Dibb et al., 2007; Kuramoto et al., 2011). This has been associated with the increased longrange input from middle-latitudes and with the decreasing snow cover on the local landscape, both of which support the production of mineral dust. Observations made for Asian dust samples show that most CaCO3 species completely react to produce mainly Ca(NO3)2 and, to a much lesser extent, CaSO4 (Jordan et al., 2003; € et al., 2003 have also observed that Hwang and Ro, 2006). Teinila in the Arctic, NO3  is more often presented in supermicron-size particles, which have relatively short atmospheric lifetimes, while SO4 2 forms preferably in submicron-size particles. We speculate that on its route to north, carbonate dust from continental air masses has caused depletion of NO3  and afterwards in the Arctic, it has mixed with photochemically produced SO4 2 , forming CaSO4. This may be the reason why the SO4 2 had a very good correlation with Ca2þ and slightly weaker correlation with other long-range €rvi snow (Table 2). If terrestrial ions (Naþ, Mg2þ) in the Kilpisja Ca2þ did not originate solely from carbonate dust, it could explain why we observed no clear correlation between Ca2þ and pH in the snow. 4.3. Spatial distribution of ions

to polar regions, gradual rainout occurs, which leads to higher isotopic depletion of d18O in meteoric waters at higher latitudes according to the Rayleigh process (latitude effect). The process occurs even without the rainout, because heavy water molecules are preferentially converted to a more condensed phase. Cooler temperatures also support isotopic fractionation in the evaporated water, which leads to isotopic depletion (the temperature effect) (Ingraham, 1998). In the North-Atlantic region, precipitation shows relatively enriched d18O values, probably due to the regional temperature anomaly that is caused by the Gulf Stream (Rozanski et al., 1993). In the Arctic, abrupt isotopic lightening occurs (Clark and Fritz, 1997) and air masses from the Barents Sea, which is ice-free throughout the year, can transport isotopically more negative and Cl-rich pre€rvi area, whereas mid-latitude air masses cipitation to the Kilpisja bring more positive and Ca-rich precipitation. Air masses from the North-Atlantic could carry both terrestrial and sea aerosols, because they rather often move north over South-Scandinavia (METEX). Fig. 4 shows excellent correlations between d18O and Cl/Ca2þ in snow samples of 2008, where Cl was predominating at more negative (from north) and Ca2þ at more positive d18O values (from south). Interestingly, while in 2008 Cl/Ca2þ values remained between 6.6 and 20.3, in 2009 they were only between 0.6 and 3.3. These correspond to d18O values between 11 and 13‰. Such positive d18O values are still unrealistic, because in North-Finland, the annual precipitation of d18O is estimated to remain below 15‰ (Kortelainen and Karhu, 2004). Fig. 3B shows a clear difference between Cl/Ca2þ ratios in 2009. Using these ratios, where Cl presents marine and Ca2þ terrestrial aerosols, we separated the areas where sea aerosols (Clˉ/Ca2þ>1.7) or terrestrial dust (Cl/Ca2þ<1.7) prevailed in the snow cover (Fig. 1). The separation shows that marine aerosols predominate in the southern and northern part of the Lake Saana catchment, while terrestrial aerosols affect the chemical composition of snow in its northeastern part. Although Cl/Ca2þ ratio in the Je€ ahkasnjoaskejavri catchment remained below 1.7, the variability was rather extensive there (0.55e1.66) and terrestrial input, similarly to the Lake Saana catchment, more discernable on northeastern shore of the lake. It is noteworthy that precipitation of terrestrial origin dominated on areas sloping southward, while sea salts prevailed on flat areas. Several studies (e.g., Toom-Sauntry and Barrie, 2002; Beine et al., 2006) in polar areas show that freshly fallen snow always has low ionic concentrations. Somewhat later pressing winds carry the majority of mineral ions in the form of dry sediment into the

Using Cl as a reference element for the sea salt contribution and assuming that all the observed Cl originates from sea salt and is unmodified, the non-sea salt fraction (nssX) has been calculated:

25

i h ½nssX ¼ ½X  a Cl ;

20

(2)

Where X is the fractionated species and a ¼ [X]/[Cl] in sea water (Aristarain and Delmas, 2002). The calculated non-sea salts faction showed that the nssCa2þ percentage was 0.95e0.99 and it was the most characteristic terrestrial aerosol in the research area. The result is in good agreement with earlier observations, which show that while Naþ and Cl are sea salt indicators (e.g., Piccardi et al., 1996; Aristarain and Delmas, 2002), Ca2þ is a marker of terrestrial aerosols (e.g., Kuramoto et al., 2011; Krnavek et al., 2012). During the transport of evaporated water from lower latitudes

215

15 10 5 0 -10

-15

-20

18

δ O, ‰ Fig. 4. d18O and Cl/Ca2þ variations in snow samples of 2008.

216

V. Raidla et al. / Atmospheric Environment 118 (2015) 211e218

snow. Wind direction (Fig. 2A and B) and average speed were similar in winter of 2008 (8.5 m s1) and 2009 (9.0 m s1). However, while there were 11 storm events in winter of 2008/2009, only 2 occurred in winter of 2007/2008. Moreover, in 2008/2009, the bulk of precipitations came from northwest (Fig. 5A), although southern winds were stronger (Fig. 5B) (Finnish Meteorological Institute, 2009). It is possible that heavy winds carried terrestrial material to the research area in the form of dry deposits that were then absorbed into windward slopes, and snowfalls transported sea salts. Bulkier dry deposition of Ca2þ in 2008/2009, compared to that of 2007/2008, could explain why Cl/Ca2þ ratios in the two seasons showed such a significant difference. 4.4. Changes in NO3  and NH 4 þ over the years Fig. 6 A and B displaying 2008 data illustrate how NO3  considerably influenced the variability of pH and d18O. In winter of 2008/2009, about half of the accumulated snow fell during eight days, unlike in winter of 2007/2008, when it was snowing steadily throughout the season. Such uniform accumulation ensures snow pack aeration. Under aerobic conditions and in case of low bacterial activity, ammonium is oxidised by nitrification to the most stable form of nitrogen (after the N2 gas)dnitrate. Relatively low NHþ 4 concentrations in the snow cover in 2008, compared to those of 2009, also support this view (Table 1). Nitrification could decrease pH and increase NO3  in the snow cover, decreasing the value of pH/mNO3  (Fig. 6B). Controversially, many samples show simultaneously low pH and NO3  concentrations. The same mechanisms that deposit soluble mineral ions, Fig. 6. Variations of pH (A) and pH/NO3  (B) in dependence on d18O in snow samples of 2008.

day precipitation, mm

25

A

20

probably also deposit organic compounds (Beine et al., 2006, 2011). In the snow pack, microbiological activities such as oxidation of organic material could modify the nutrient cycle (e.g., Felip et al., 1999; Carpenter et al., 2000; Takeuchi et al., 2001). Based on metagenomic comparisons of different ecosystems, Arctic snow has among the highest amounts of bacteria associated with nitrate ammonification pathways (Larose et al., 2013), which automatically increase the pH as well. However, Reddy and Patrick (1984) showed that nitrification and ammonification could occur simultaneously in the same environment which leads to acidification and produces inert and well-migratable molecular nitrogen. Why ammonification predominates in case of more positive d18O values, is not clear. It could depend on the difference between organic and mineral compounds of continental and sea aerosols, or on higher amount of precipitation from the north, which, in turn, can cause deeper burying and therefore also worse aeration conditions. However, this phenomenon is still unclear and deserves further study, based on nitrogen isotopes.

15 10 5 0

wind speed, m·s-1

50

B

40 30 20

5. Conclusion

10 0

0

100

200

300

degree of wind direction Fig. 5. Snowfall (A) and wind (B) variability according to the compass degree from 1st November to 30th April 2008/2009. Wind direction is indicated in terms of degrees from true north (360 ).

The obtained results showed considerable variations in chemi€rvi area of less than a few hundred cal composition in the Kilpisja metres in diameter. Such variability is primarily attributable to the dominant northwestern and southern winds in the area, which carry precipitation, enriched by marine or terrestrial aerosols, respectively. Despite the high proportion of snowfall from the northwest (from the ocean), Naþ and Cl concentrations did not follow the SDL, and the ratio of nssCa2þ in the snowfall was very high (94e98%). Higher Ca2þ concentrations can be attributed to the

V. Raidla et al. / Atmospheric Environment 118 (2015) 211e218

local rock dust from the slopes and result in higher Mg2þ and Naþ concentrations. However, there was a strong correlation between Ca2þ and SO4 2 , despite low sulphur content in the local rocks. Therefore, the occurrence of Ca2þ and SO4 2 alongside other associated ions (Naþ and Mg2þ) can also be linked to the long-range transport of aerosols. Ca2þ enrichment relative to Cl in marine aerosols was particularly strong in northeastern parts of the lake valleys. This could be explained by dry deposition of terrestrial aerosols brought by southern storms and spread according to peculiarities of local topography. Both carbonate dust of local origin and dust carried from the south seem to control the pH of snow in the Kilpisj€ arvi region, but pH seems to be acidified in some places, possibly due to nitrification. Acknowledgements ~nu Martma (IG TUT) for d18O analyses and Mrs. We thank To Helle Pohl-Raidla for correcting the English. The activities of the present study were supported by the LAPBIAT (The research Infrastructure Lapland Atmosphere-Biosphere Facility) grant to EK and by the contract No 10.1/8.1/11/488-6 between the Ministry of Education and Research of Estonia and Tallinn University of Technology. We are indebted to the Finnish Meteorological Institute for providing the weather data and to the Institute of Ecology and Earth Sciences in the Department of Geology at University of Tartu for their technical support. The manuscript was improved by constructive comments from two anonymous reviewers. References , F., Shepson, P.B., 2002. Air-snow interactions and atmospheric chemistry. Domine Science 297, 1506e1510. http://dx.doi.org/10.1126/science.1074610. Appelo, C., Postma, D., 1999. Geochemistry, Groundwater and Pollution. Balkema, Rotterdam, Netherlands. Aristarain, A.J., Delmas, R.J., 2002. Snow chemistry measurements on James Ross Island (Antarctic Peninsula) showing sea-salt aerosol modifications. Atmos. Environ. 36, 765e772. http://dx.doi.org/10.1016/S1352-2310(01)00362-4. Atlas of Finland, 1986. Geology, Folio 123e126. National Board of Survey, Geographical Society of Finland. , F., King, M.D., Nardino, M., Ianniello, A., France, J.L., Beine, H.J., Amoroso, A., Domine 2006. Surprisingly small HONO emissions from snow surfaces at Browning Pass, Antarctica. Atmos. Chem. Phys. 6, 2569e2580. http://dx.doi.org/10.5194/acp-62569-2006. , F., Voisin, D., Beine, H.J., Anastasio, C., Esposito, G., Patten, K., Wilkening, E., Domine Barret, M., Houdier, S., Hall, S., 2011. Soluble, light-absorbing species in snow at Barrow, Alaska. J. Geophys. Res. Atmos.116 http://dx.doi.org/10.1029/2011JD016181. D00R05. Bishop, K., Pettersson, C., 1996. Organic carbon in the boreal spring flood from adjacent subcatchments. Environ. Int. 22, 535e540. http://dx.doi.org/10.1016/ 0160-4120(96)00036-0. € m, J., Laing, T., Snyder, J., MacDonald, G.M., Smol, J.P., Blom, T., Korhola, A., Weckstro 2000. Physical and chemical characterisation of small subarctic headwater lakes in Finnish Lapland and the Kola Peninsula. Verheissungen Int. Ver. gesamten Limnol. 27, 316e320. Carpenter, E.J., Lin, S.J., Capone, D.G., 2000. Bacterial activity in South Pole snow. Appl. Environ. Microbiol. 66, 4514e4517. http://dx.doi.org/10.1128/AEM.66.10.45144517.2000. Clark, I., Fritz, P., 1997. Environmental Isotopes in Hydrogeology. Lewis Publishers, New York. de Caritat, P., Hall, G., Gislason, S., Belsey, W., Braun, M., Goloubeva, N.I., Olsen, H.K., Scheie, J.O., Vaive, J.E., 2005. Chemical composition of arctic snow: concentration levels and regional distribution of major elements. Sci. Total Environ. 336, 183e199. http://dx.doi.org/10.1016/j.scitotenv.2004.05.031. Dibb, J.E., Whitlow, S.I., Arsenault, M., 2007. Seasonal variations in the soluble ion content of snow at Summit. Greenland: constraints from three years of daily surface snow samples. Atmos. Environ. 41, 5007e5019. http://dx.doi.org/ 10.1016/j.atmosenv.2006.12.010. Douglas, T.A., Sturm, M., 2004. Arctic haze, mercury and the chemical composition of snow across northwestern Alaska. Atmos. Environ. 38, 805e820. http:// dx.doi.org/10.1016/j.atmosenv.2003.10.042. Drab, E., Gaudichet, A., Jaffrezo, J.L., Colin, J.L., 2002. Mineral particles content in recent snow at Summit (Greenland). Atmos. Environ. 36, 5365e5376. http:// dx.doi.org/10.1016/S1352-2310(02)00470-3. Felip, M., Camarero, L., Catalan, J., 1999. Temporal changes of microbial assemblages in the ice and snow cover of a high mountain lake. Limnol. Oceanogr. 44, 973e987.

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