The Elbrus (Caucasus, Russia) ice core glaciochemistry to
reconstruct anthropogenic emissions in central Europe: The case of
sulfate

Abstract. This study reports on the glaciochemistry of a deep ice core (182 m long) drilled in 2009 at Mount Elbrus (43°21′ N, 42°26′ E; 5115 m above sea level) in the Caucasus, Russia. Radiocarbon dating of the particulate organic carbon fraction in the ice suggests a basal ice age of ~ 1670 ± 400 cal yr BP. Based on chemical stratigraphy, the upper 168.6 m of the core were dated by counting annual layers. The seasonally resolved chemical records cover the years 1774–2009 (Common Era), thus, being useful to reconstruct many aspects of atmospheric pollution in central Europe from pre-industrial times to present-day. After having examined the extent to which the arrival of large dust plumes originating from Sahara and Middle East modifies the chemical composition of the Elbrus (ELB) snow and ice layers, we focus on the sulfur pollution. The ELB sulfate levels indicate a four- and six-fold increase from 1774–1900 to 1980–1995 in winter and summer, respectively. Remaining close to 116 ± 28 ppb during the nineteen century, the summer sulfate levels started to rise at a mean rate of ~ 6 ppb per year from 1920 to 1950. The summer sulfate increase accelerated between 1950 and 1975 (11 ppb per year), levels reaching a maximum between 1980 and 1990 (730 ± 152 ppb) and subsequently decreasing to 630 ± 130 ppb at the beginning of the twenty first century. Long-term sulfate trends observed in the ELB ice cores are compared with those previously obtained in Alpine ice, the most important difference consists in a more pronounced decrease of the sulfur pollution over the three last decades in western than central Europe.



Introduction
It is now well recognized that the present climate change is not only related to change of long-lived greenhouses gases but also to aerosols, particularly at regional scales. In this way, it has been suggested that aerosols may have weakened the rate of the global warming during the second part of the last century (Andreae et al., 2005). However, uncertainties still exist in quantifying the climatic impact of aerosols, because the spatial distribution of aerosols is very heterogeneous and requires 5 therefore numerous observations to make these parameters useful as inputs and constraints for climate models. An important gap is also related to the fact that direct atmospheric observations are available only from far later (starting with the appearance of the acid rain phenomena in the late 1960s) than man-made activities started to disturb the pre-industrial atmosphere. However, to predict future climate the knowledge of atmospheric changes in aerosol load and composition from present-day polluted atmosphere back to preindustrial times is required. Chemical records of species trapped in snow 10 deposited on cold glaciers provide a unique and powerful way to reconstruct past atmospheric chemistry changes including aerosol load and composition (see Legrand and Mayewski (1997) for a review).
In Europe, a largely industrialized continent, ice cores were extracted from high-elevation glaciers located at various places, including the Alps Schwikowski et al., 2004), the continental Siberian Altai (Eichler et al., 2011;Olivier et al., 2006), and Kamchatka (Kawamura et al., 2012). In the Alps, intimately connected to western European 15 emissions, ice cores have been performed at Col du Dôme (CDD, Mont Blanc, Preunkert, 2001), Fiescherhorn (Bernese Alps, Jenk, 2006), and Colle Gnifetti (CG) in the Monte Rosa region (Schwikowski, 2006;Wagenbach et al., 2012) in view to examine various aspects of atmospheric pollution. The exceptionally high net snow accumulation at the CDD site permitted the extraction of seasonally resolved records of various chemical species over the last 100 years . In older ice layers preservation of winter layers at CDD becomes very limited and summer layers become 20 very thin. Conversely, the low and incomplete net snow accumulation rate at CG, which is controlled by wind erosion, highly limits the preservation of winter ice layers (Wagenbach et al., 2012), but is low enough to provide access to an extended time period, at least over the last millennium. Using the EMEP (European Monitoring and Evaluation Programme) regional chemistry-transport model and past emission inventories of SO2 in Europe, observed CDD long-term trends of sulfate were fairly well reproduced, leading Fagerli et al., (2007) to conclude that the seasonal changes seen at the CDD 25 alpine site are associated with geographical changes in source regions impacting the site. This is a strong argument for a separate examination of summer and winter data, extracted from alpine ice cores. However, until now, Alpine ice cores document only the last hundred years (at the best back to 1890, Legrand et al., 2018) on a seasonal basis, whereas the early stage of the industrialization time period, which is generally considered to have started around 1850, is missed. An ice core recently extracted from the Elbrus (the highest summit of the Caucasus) indicated excellent preservation of summer and 30 winter layers at least back to 1820 (Mikhalenko et al., 2015). Thus, the ELB ice may contain very valuable information on past atmospheric pollution in central Europe since the beginning of the industrialization. Here we report on the glaciochemistry of a deep ice core (182 m long) drilled in 2009 at Mount Elbrus in the Caucasus, Russia. The seasonally resolved chemical records were obtained back to 1774 (i.e., well prior to the onset of the industrial period). Data are discussed in two companion papers of which this one. The present paper examines first of all the impact of dust plumes, which arrive sporadically from Sahara and Middle East, on the chemical composition of the Elbrus (ELB) snow and ice layers. It then focuses on long-term sulfate trends in relation to growing sulfur pollution. The long-term summer and 5 winter trends of sulfate are discussed with respect to past SO 2 emissions in central Europe and compared to those extracted at the Alpine site of CDD in relation to SO 2 emissions from western Europe. The second paper focuses on calcium (a dust tracer) long-term trend (Kutuzov et al., this issue), discussing its past changes in relation with natural variability, as well as climatic and land use changes in the dust source regions Middle East and North Africa.

Discrete Subsampling of firn and ice, and Chemical Analysis
Pieces of cores were cleaned under a clean air bench located in a cold room (-15°C) using an electric plane tool previously developed to process Alpine firn and ice samples. A total of 3724 subsamples were obtained along the upper 168.6 m of the Elbrus core. The depth resolution decreased from 10 cm at the top to 5 cm at 70 m, and 2 cm at 157 m depth and below.
Given the decrease of the net annual snow accumulation from 1.5 mwe (0.8 mwe in summer and 0.7 mwe in winter) near the 15 surface to 0.18 mwe (0.15 mwe in summer and 0.03 mwe in winter) at 157 m depth, as seen in Figure 1, this sampling permitted to minimize the lost of temporal resolution with depth along the core, particularly in summer. In this way, an average of 9 summer samples per year were sampled at 157 m depth (compared to 15 summer samples per year near the surface). The larger decrease of the net snow accumulation in winter than in summer leads to a more pronounced loss of resolution in winter layers (12 samples per winter near the surface and 1-2 samples per winter at 157 m depth) (Figure 1)
Details on working conditions are reported in Legrand et al. (2013). For all investigated ions, blanks of the ice decontamination procedure were found to be insignificant with respect to respective levels found in the ice cores.
During the drill operations, an incident occurred at the depth of 31 m and a fluid was poured in the hole to liberate the drill device. This has led contamination of the firn at 31 m down to the firn-ice transition located at 55.7 m depth. Samples 30 covering the 1983-1997 years were contaminated for sodium (124 ± 87 ppb compared to 26 ± 28 ppb over the 16 preceding years) and potassium (35 ± 25 ppb compared to 16 ± 15 ppb over the 1966-1982 years). One core section  https://doi.org /10.5194/acp-2019-402 Preprint. Discussion started: 3 June 2019 c Author(s) 2019. CC BY 4.0 License. that covers winter 1875/76, summer 1876, and winter 1876/1877 was not analysed. Finally, a part of the ELB-138 ice core section that covers winter 1877/1878 was of poor quality (splitted ice).

Annual Layer Counting
As discussed by Mikhalenko et al. (2015), dating of the Elbrus ice can be done by annual layer counting on the basis of the stratigraphic ammonium and succinic acid records, both exhibiting well-marked winter minima. As previously seen in alpine 5 ice cores, ammonium reveals a well-marked maximum in summer due to a maximum of NH 3 emission together with an efficient upward transport in summer (Fagerli et al., 2007). Succinic acid is a light dicarboxylic acid for which a strong summer maximum and a quasi-null winter level can be observed in the present-day atmosphere in Europe . The very low winter levels of this organic compound are related to the absence of a winter source of this species, which is mainly photo-chemically produced from biogenic precursors. Mikhalenko et al. (2015) assumed a concentration 10 limit of 100 ppb ammonium and 5 ppb of succinate to separate winter and summer in the upper layers down to 75.6 m depth (i.e., 1963). To account for an observed decreasing trend of ammonium concentrations with depth (i.e., due to a post 1950 increase as also seen for species like nitrate and sulfate, see below), the ammonium winter criterion was adjusted to 50 ppb between 75.6 and 86.8 m depth (i.e., 1950-1963) and 30 ppb below. Since no systematic change of succinate with depth is observed, the succinate concentration limit of 5 ppb was also applied in deeper layers. In this way, the annual counting was 15 found to be very accurate dating (a 1-year uncertainty) over the last hundred years when anchored with the stratigraphy with the Katmai 1912 horizon (Mikhalenko et al., 2015). Though the annual counting becomes less evident prior to 1860, Mikhalenko et al. (2015) reported an ice age of 1825 at 156.6 m depth, what is still consistent with the presence of volcanic horizon at around 1833-1940 such as Coseguina (1835).
We here extended the annual counting down to 168.5 m depth (i.e., 131.6 mwe) by considering a concentration limit of 30 20 ppb for ammonium and 5 ppb for succinate. With that, ice dates to 1774 CE at the depth of 168.5 m. In the following we will examine individual half-year summer and winter values as well as monthly means. To calculate monthly means, a uniform snowfall rate is assumed within each half-year. Winter samples were attributed to the last 3 months of the year and the three first months of the following year (i.e., winter 1850/1851 is from 1850.75 to 1851.25). Summer samples are from the fourth to the ninth month (i.e., summer 1850 is from 1850. 25 to 1850.75) in each year. In Figure 2 we report the obtained 25 chronology for three different sequences including the deepest one (1774-1784). It could be seen that in the years prior to 1850, quite often a winter layer is made only of one or two samples, whereas summer layers are made still of more than 6 samples. Below 168.5 m depth, the ice core quality becomes rather bad (numerous small pieces of broken ice) rendering subsampling and ice decontamination not evident. Furthermore, as seen in Figure

Basal Ice Dating
Four ice samples located along the deepest 6 m of the core (bottom at 182.65 m, 142.1 mwe) were analysed for radiocarbon in particulate organic carbon (PO 14 C). The lowest 0.5 m of the core were not analyzed due to a large presence of macro-size inorganic particles. To minimize the time interval covered by each sample, sample lengths were kept as short as possible with respect to the detection limit related to working conditions during sampling and analysis. A typical sample length of 26 5 to 40 cm (available ice core section of 14-21 cm 2 ) was used leading to an initial ice sample mass of 430-580 g. After having decontaminated ice sample (using a DOC decontamination method according to Preunkert et al., 2011), melted ice was filtered and combusted at 340°C at the Institut de Géophysique Externe (Grenoble) using the inline filtration-oxidation-unit REFILOX (Reinigungs-Filtrations-Oxidationssystem, Hoffmann et al., 2018). Hereby, the resulting ice mass was reduced to 260-320 g containing 4.4 to 6.5 µgC of POC (Table 1). After cryogenic extraction of the CO 2 content, radiocarbon analyses 10 were done at the accelerator mass spectrometer facility at the Curt-Engelhorn-Center Archaeometry (CEZA) in Mannheim (Hoffmann et al., 2017). Calibration of the retrieved 14C ages was done using OxCal version 4.3 (Bronk Ramsey, 1995).
To test the reliability of the DOC decontamination method (Preunkert et al., 2011) for POC analysis, a 340 °C REFILOX mass combustion comparison was made between ultrapure water and decontaminated blank ice (3 samples of 200 to 500 mL). To achieve an impurity-free solid ice the ultra-pure water was slowly frozen in polyethylene (PE) foil (Hoffmann et al., 15 2017 and references in there). The comparison showed that whatever the blank ice volume, the blank values were in the same range than the ultrapure water POC blanks (~0.4 ± 0.25 µgC) which were determined during the course of the ELB radiocarbon sample measurements. Thus, the ice decontamination procedure used for DOC ice measurement is also valid for POC ice decontamination.
Since the CO 2 collection line was recently extended to allow sample pooling, we were now able to directly determine the 20 fraction modern carbon (F 14 C) in blanks done with ultrapure water. A F 14 C value of 0.71 ± 0.07 was found, measured on three blank samples in total (each consisting of four pooled samples). This value is in agreement with F 14 C blank values found in previous studies (as reviewed and adopted in Hoffmann et al., 2018). In Table 1, we report ice sample data after blank correction, including correction of the F 14 C value as well as correction of the extracted POC mass with the respective ultrapure water blank determined before each ice sample extraction. 25 Using a mean mass related combustion efficiency of the device of 0.7 (Hoffmann et al., 2018), the mean POC concentrations of the four samples obtained by combustion at 340°C is of 25.4 ± 3.1 ngC g -1 with highest concentration of 29.0 ngC g -1 for the lowest sample analysed at 142.6 mwe (182.0 m). These values are in good agreement with those observed by Hoffmann et al. (2017) in the 340°C POC fraction of the lowest 8 mwe of a CG ice core (37 ± 16 ngC g -1 ). Since a similarity between CG and ELB was also observed for their preindustrial black carbon content (Lim et al., 2017), we thus exclude significant 30 age errors due to a POC contamination during 14 C sample preparation and analysis.
As seen in Table 1, the mean age of the ELB-178-03 sample (1530 yr cal BP) is older than the mean age of sample ELB-181-01 located 2.3 m below the ELB-178-03 sample. However, given age uncertainties, it is difficult to conclude that, as https://doi.org/10.5194/acp-2019-402 Preprint. Discussion started: 3 June 2019 c Author(s) 2019. CC BY 4.0 License. observed previously at other mid-latitude glacier sites such as CG (Hoffmann et al., 2018), the ELB radiocarbon ages do not increase monotonically with depth as would be expected from well-behaved ice flow. PO 14 C measurements suggest that the ELB ice core extends to ~ 1670 ± 400 yr cal BP (Table 1). This is younger than basal ice ages found at Alpine sites, i.e. ~5000 yr BP for Col du Dome (Preunkert et al., in press), ~ 4000 yr BP (Hoffmann et al., 2018) and >10,000 yr BP (Jenk et al., 2009) for two CG ice cores, and ~7000 yr BP for Mt. Ortles (3905 m asl) (Gabrielli et al., 2016). From the observed 5 temperature gradient in the borehole ELB site, Mikhalenko et al. (2015) calculated a heat flux at the bottom glacier that is 4-5 times larger than the mean value for the Earth's surface, possibly due to a heat magma chamber of the Elbrus volcano. That may lead to basal ice melting and removal of the oldest basal layers. If so, that may explain the young age of basal ice at the volcanic crater site compared to other non-volcanic mountain glaciers. The age of the basal ELB ice is nevertheless largely greater than expected by ice flow model calculations, estimating a basal ice age of less than 400 years at the drill site 10 (Mikhalenko et al. 2015).

The Effect of large Dust events on the Chemistry of ELB Ice
Large dust plumes originating from Middle East and less frequently from Sahara reach the Caucasus (Kutuzov et al., 2013).
As seen in the Alps, these dust events disturb the chemistry of snow deposits, in particular with calcium rich alkaline snow layers (Wagenbach et al., 1996). Depositions of these plumes disturb the level of numerous chemical species in Alpine ice 15 because either they are present in dust at the emission stage or, being acidic, they were uptake by the alkaline dust material during transport (Usher et al., 2003). Preunkert (2001) showed that the arrival of dust plumes at CDD enhanced depositions of several cations (sodium, potassium, magnesium, and sodium) as well as acidic anions (sulfate, nitrate, chloride, fluoride, and carboxylates). To identify these layers in the ELB snow and ice we have estimated the acidity (or alkalinity) of samples by checking the ionic balance between anions and cations with concentrations expressed in micro-equivalents per liter, µEq 20 25 In this work, samples which contain more than 120 ppb of calcium and which are below the 25% quartile of a robust spline through the calculated raw acidity profile, were considered as impacted by dust events. In this way, 616 (on a total of 2524) summer and 67 (on a total of 1150) winter samples were considered. Note that the results are quite similar when changing the calcium concentration criteria from 120 ppb to 100 or 140 ppb. Since the frequency of these events has changed over time (Kutuzov et al., this issue) the significance of their impact on the deposition of chemical species is examined. Figure 4 Fig. 4A), the increase of calcium, accompanied by a strong increase of the alkalinity, reaches a factor of 7.4 compared to dust-free samples (Fig. 4C). In addition, this calcium enhancement is accompanied by an increase of a factor close to 8 for chloride, sodium, potassium and magnesium, whereas ammonium, nitrate, sulfate, and carboxylates are, at the best, enhanced by a factor of 2. When comparing dust samples containing weaker calcium contents (i.e. Ca 2+ < 600 ppb Fig.   4B) with dust-free samples (Fig. 4C), in addition to the increase of calcium (factor of 2) the most significant changes are 5 seen for magnesium (x1.5), sodium and chloride (x1.6), and potassium (x1.3), respectively. In brief, all cations (except ammonium) and chloride are present in dust at the emission stage. Furthermore, acidic chloride can be taken up by dust during transport.
For species present in dust at the emissions stage, it is interesting to compare their ratio to calcium in ELB dust layers ( Figure 5) with atmospheric aerosol data obtained at sites impacted by dust events. For magnesium, a species predominantly 10 originating from dust in ELB ice, the mean [Mg 2+ ]/[Ca 2+ ] ratio in ELB dust events is 0.035 ( Figure 5). Koçak et al. (2012) reported dust event related aerosol concentrations of sodium, magnesium, and calcium from two Eastern Mediterranean sites, i.e. from Erdemli (Turkey) with dust arriving from Middle East and from Heraklion (Crete) with dust from Sahara. It is important to emphasize that, as for the ELB ice data, the atmospheric concentrations of these cations correspond to their water-soluble fraction (not the total fraction), which were measured with IC. In the case of Erdemli during Middle East dust 15 events, Koçak et al. (2012) reported atmospheric concentrations of 7085 ng m -3 for Ca 2+ and 423 ng m -3 for Mg 2+ (Table 2).
Since Erdemli is located at 22 m above sea level and 10 m away from the sea, in addition to the leachable fraction of magnesium from dust, a fraction of magnesium would here come from sea-salt. To correct concentration from the sea-salt contribution, we have used the Na + concentration (1148 ng (Table 2). A similar value is obtained for aerosol at Heraklion during a Saharan dust event (0.043, Table 2).
The content of Mg 2+ in ELB samples impacted by dust is therefore very consistent with what is observed in atmospheric aerosol from the Eastern Mediterranean region during dust events. Note that the same is true for the Mg 2+ content of CDD 25 samples impacted by Saharan dust (mean [Mg 2+ ]/[Ca 2+ ] ratio of 0.045, Figure 5).
Figures 5 compares the chemical content of dust deposited in the Caucasus (ELB) and in the Alps (CDD). In the Alps, most of dust events are related to sporadic arrivals of Saharan dust plumes (Wagenbach et al., 1996). As discussed above, whereas Saharan dust events also sporadically reach the Elbrus site and are characterized by very large amount of calcium (see Fig.   4A) more frequent are dust events from Middle East that contain less calcium (Fig. 4B). As seen in Figure 5 ] ratio of 0.90). Such a lower neutralisation of alkaline material by acidic species in dust plumes reaching 5 the ELB site compared to the CDD site is probably related to a reduced availability of acidic species along the transport of the dust plumes towards the site To evaluate the effect of dust on the deposition of chemical species, we compare in Table 5 averaged chemical concentrations of all samples with those not impacted by dust. Averages were obtained on the base of half-year summers over the half-decades 1996-2000 and 1974-1978 characterized by high and low dust content, respectively (Kutuzov et al.,10 this issue). Though the main impact of dust is as expected, on cations (except ammonium) and chloride, i.e. the constituents of dust particles, the impact is also significant for acidic species like nitrate and sulfate. For instance the increase of nitrate from 1974from -1978from to 1996from -2000 in the whole dataset is largely (three quarters) related to the increase of dust as indicated by the smaller increase after removal of dust samples (increase of 31 ppb for the NO 3 red. value). Same is true for sulfate, for which the apparent increase between the two periods (90 ppb) is not due to an increase of pollution but of dust, as 15 indicated by the drop of values when the dust free data set is considered (decrease of 48 ppb for the SO 4 2red. value). The effect of changing dust inputs over time has to be therefore considered when discussing long-term trends in view to relate them to growing anthropogenic emissions (see Sect. 5 for sulfate in relation with SO 2 emissions). Finally, the large effect of dust seen for formate (HCOO -) and not for acetate (CH 3 COO -) is in agreement with previous observations made by Legrand et al. (2003) in Alpine ice and by (Legrand and De Angelis, 1995) in Greenland ice. These studies showed that the presence 20 of formate and acetate in ice follows the uptake of formic and acetic acid from the atmospheric gas phase, and that the incorporation of these weak acids into hydrometeors is pH dependent with a stronger dependence for formic acid, which is a stronger acid than acetic acid.

Long-term summer and winter trends of sulfate in the Elbrus ice
From the winter/summer dissection made on the basis of the ammonium and succinate stratigraphy (Sect. 2), monthly means 25 as well as half-year summer and winter means were calculated over the 1774 to 2010 period. In Figure 6, we report the seasonal cycle of sulfate, ammonium, and succinate averaged across a pre-industrial period (1775-1825 AD) and two different periods of the industrial period (1940-1960 and 1980-2000

The Alpine CDD ice core Sulfate Records 30
The ELB sulfate long-term trend is compared with those previously extracted from the Alpine CDD site (ice cores denoted C10 and CDK in Figure 9). C10 sulfate data were presented in Preunkert et al. (2001), and those form CDK in (Legrand et https://doi.org/10.5194/acp-2019-402 Preprint. Discussion started: 3 June 2019 c Author(s) 2019. CC BY 4.0 License. al., 2013). Since winter data from CDD are more limited (only a few pure winter layers are available between 1890 and 1930, Legrand et al., (2018) we here focus on the comparison of summer levels. The two CDD cores were dated by annual layer counting using the pronounced seasonal variations of ammonium. The two chronologies were in excellent agreement over their overlapping period from 1925Preunkert et al., 2000). A re-evaluation of the C10 chronology based on very recently made continuous measurements of heavy metals, as well as a comparison to a well-dated 5 Greenland ice core record (McConnell and Edwards, 2008), resulted in a revised C10 chronology (Legrand et al., 2018). As for C10, continuous measurements of heavy metals are also available in the lowest part of CDK (Preunkert et al., in press). It was thus possible to identify the distinct Greenland increases of thallium, lead, and cadmium associated with the widespread start of coal burning at the beginning of the Industrial Revolution in 1890 CE also in the CDK core (at 117.8 m (90.5 mwe)).
This time marker was then used to constrain a revised annual layer counting in the early 20 th -century part of the CDK record. 10 In the following we compare summer trends of both Alpine CDD cores with the ELB ice core record, considering long term sulfate trends regarded to be free of dust influence (i.e., the SO 4 2red. values).

ELB versus CDD sulfate trends
For summer (see Figure 9 a and b), the pre-industrial sulfate ELB value (SO 4 2red. = 113 ppb) thus exceeded the CDD one (66 ppb) (Preunkert et al., 2000). A similar difference is observed for winter with SO 4 2red. close to 68 ppb at ELB (Figure 7) 15 compared to 20 ppb observed by Preunkert et al. (2000) at CDD. It is out of the scope of this work to discuss the cause of this difference between the two ice cores but we can first mention the existence of local volcanic sulfur emissions (as evidenced by direct on site observations of a sulfur smell nearby the ELB drill site). The pre-industrial summer level of dust free calcium samples at ELB (74 ppb, Kutuzov et al., this issue) is higher than the one at CDD (45 ppb, Legrand, 2002). That may also contribute to the ELB/CDD difference of the sulfate pre-industrial level. Clearly, more work, including simulations 20 with transport and chemistry models considering also oceanic emissions of DMS may help here. Figure 9 compares the increasing summer sulfate trends of the ELB and CDD sites. Three major differences between the two sites are revealed: (1) an impact of anthropogenic emissions already significant in 1910 at CDD and not at ELB, (2) a maximum of the anthropogenic perturbation from 1970 to 1980 at CDD and 10 years after (1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990) at ELB, and (3) a far less pronounced re-decrease at the beginning of the 21 th century at ELB compared to CDD. 25 As discussed by Kutuzov et al. (this issue), 10 day backward air mass trajectories calculated for the ELB site using the NOAA HYSPLIT-4 model suggest that, in summer, air masses arriving at ELB mainly originate from the nearby Georgia, Azerbaijan, Syria, Irak, and from Turkey, South Russian, and North of Iran. We report in Figure 10 emissions of SO 2 from these countries and from a few others located further north (Ukraine) and west (Bulgaria). In these countries SO 2 emissions reached maximum in the late 80's or later (for Turkey and Iran). This feature clearly differs from the situation at CDD where 30 countries around the site (France, Italy, Spain, Switzerland and Germany), thought to be the main contributors for sulfate in CDD ice (Fagerli et al., 2007), exhibit a maximum between the early 70's and the early 80's ( Figure 10 On this basis and as a first attempt, we compare the ELB and CDD summer sulfate trends with SO 2 emissions from surrounding countries. It can be seen that the impact of growing anthropogenic SO 2 emissions started later at ELB (after 1920) compared to CDD (after 1900). The 10-year delay of the sulfate maximum at ELB compared to CDD is also well seen in the enhancement course of SO 2 emissions. Note also that as indicated by the emissions, the maximum enhancement at ELB (550 ppb between 1980 and is slightly weaker that the one at CDD (665 ppb between 1974 and1984) (Table 3). 5 Finally, consistently with SO 2 emission changes, the recent sulfate decrease is more pronounced at CDD than ELB with a Still moderate at the beginning of the 20 th century, the sulfate increase accelerated after 1950, levels reaching a maximum in 1980-1990 (730 ± 152 ppb in summer) and subsequently decreasing to 630 ± 130 ppb in summer at the beginning of the 21 th century. These long-term sulfate changes observed in the ELB ice cores are compared with those previously obtained in 20 Alpine ice. Consistently with past SO 2 emission inventories, a more pronounced decrease of the sulfur pollution over the three last decades is observed in western than central Europe.

Data availability
Sulfate and calcium data can be made available for scientific purposes upon request to the authors (contact: suzanne.Preunkert@univ-grenoble-alpes.fr or michel.legrand@univ-grenoble-alpes).  Table 1. Overview of masses (corrected for blanks but not for combustion efficiency) and conventional 14 C ages of the Elbrus ice core samples combusted in the REFILOX system. Calibrated date ranges are shown at 68.2% confidence level and are rounded ac cording to (Millard, 2014 Table 3. Mean chemical composition of snow layers deposited in periods characterized by low and high dust inputs (1974-1978 and 1996-2000, respectively