The climate impact of black carbon (BC) is notably amplified in the Arctic by its deposition, which causes albedo decrease and subsequent earlier snow and ice spring melt. To comprehensively assess the climate impact of BC in the Arctic, information on both atmospheric BC concentrations and deposition is essential. Currently, Arctic BC deposition data are very scarce, while atmospheric BC concentrations have been shown to generally decrease since the 1990s. However, a 300-year Svalbard ice core showed a distinct increase in EC (elemental carbon, proxy for BC) deposition from 1970 to 2004 contradicting atmospheric measurements and modelling studies. Here, our objective was to decipher whether this increase has continued in the 21st century and to investigate the drivers of the observed EC deposition trends. For this, a shallow firn core was collected from the same Svalbard glacier, and a regional-to-meso-scale chemical transport model (SILAM) was run from 1980 to 2015. The ice and firn core data indicate peaking EC deposition values at the end of the 1990s and lower values thereafter. The modelled BC deposition results generally support the observed glacier EC variations. However, the ice and firn core results clearly deviate from both measured and modelled atmospheric BC concentration trends, and the modelled BC deposition trend shows variations seemingly independent from BC emission or atmospheric BC concentration trends. Furthermore, according to the model ca. 99 % BC mass is wet-deposited at this Svalbard glacier, indicating that meteorological processes such as precipitation and scavenging efficiency have most likely a stronger influence on the BC deposition trend than BC emission or atmospheric concentration trends. BC emission source sectors contribute differently to the modelled atmospheric BC concentrations and BC deposition, which further supports our conclusion that different processes affect atmospheric BC concentration and deposition trends. Consequently, Arctic BC deposition trends should not directly be inferred based on atmospheric BC measurements, and more observational BC deposition data are required to assess the climate impact of BC in Arctic snow.
Black carbon (BC) is a carbonaceous fine particle with strong light-absorbing ability. It is produced by natural and anthropogenic incomplete combustion of biomass and fossil fuels and may be transported with prevailing winds over thousands of kilometres from its emission sources (e.g. Ramanathan and Carmichael, 2008; Bond et al., 2013). It poses a global environmental threat by warming the atmosphere, but the climate impacts of BC are amplified in the Arctic, where its deposition on snow and ice decreases surface reflectance and hastens snow and ice melt, which further decreases the reflectivity (e.g. Hansen and Nazarenko, 2004). Globally, BC is the second most important climate warming agent after carbon dioxide. However, in the Arctic, due to the snow–albedo feedback, the effect of BC in the observed warming and melting may exceed that of greenhouse gases (e.g. Flanner et al., 2007, 2009; Bond et al., 2013).
Atmospheric BC concentrations have been monitored in the Arctic starting since 1989 at Alert (Canada), Barrow (USA) and later at Zeppelin (Ny-Ålesund, Norway), and the observations show a 40 % decrease from 1990 to 2009 (Sharma et al., 2013). Furthermore, measurements from northern Finland showed a 78 % decrease in atmospheric BC concentrations between 1971 and 2011 (Dutkiewicz et al., 2014). This observed decrease is mostly attributed to the fall of the USSR and a resulting decrease in BC emissions in major source areas of Arctic BC (e.g. Sharma et al., 2013). However, atmospheric observations reflect the effect of BC on Arctic climate only partially, as the climate effect of BC deposited on high-reflectance snow and ice surfaces is notably stronger than of atmospheric BC (Flanner et al., 2007, 2009; Bond et al., 2013). As 85–90 % of BC is suggested to be wet-deposited in the Arctic (Wang et al., 2011), and the BC proportion bound by precipitation is mostly not recorded by atmospheric measurements, the BC emission and atmospheric BC concentration trends may not reliably represent the BC deposition trend. Therefore, to comprehensively assess the effects of BC in Arctic climate change, observations on its deposition rate and trend in the area are also essential.
Ice cores represent a valuable means to study BC deposition as they accumulate direct evidence of contaminant deposition in chronological order, potentially for hundreds to thousands of years. Ice core records are irreplaceable when evaluating e.g. contemporary atmospheric or snow BC concentration variations in the context of past BC variations and when evaluating the role of these variations for the observed climate change in the Arctic and beyond. Despite the importance of ice core records in deciphering the role of BC in Arctic climate change, relatively few records exist at present. Four continuous BC ice core records covering ca. 1750 to 2013 have been published from Greenland (McConnell et al., 2007; McConnell and Edwards, 2008; McConnell, 2010; Keegan et al., 2014) and one 300-year record (1700 to 2004) from Svalbard (Ruppel et al., 2014). The high-elevation Greenland records indicate a BC deposition peak around 1910 followed by rapidly decreasing deposition until 1950 and more or less stable, almost preindustrial values until the present (McConnell, 2010). The Svalbard ice core clearly concurs with the Greenland records for the early 20th century but unexpectedly shows a pronounced increase in BC concentrations and deposition from 1970 to the top of the core in 2004 (Ruppel et al., 2014). The reasons for the observed post-1970 BC deposition increase in Svalbard – while at the same time Greenland ice cores, atmospheric measurements (e.g. Sharma et al., 2013) and model results (e.g. Koch et al., 2011) suggest decreasing BC values – was left partly unresolved (Ruppel et al., 2014). Increasing flaring emissions from northern Russia in the Barents Sea area that do not reach the Greenland ice coring sites due to restricted isentropic uplift in the Arctic, and potentially increasing wet-scavenging efficiency due to increasing temperatures particularly around Svalbard, were the leading hypotheses (Ruppel et al., 2014). A similar rapid increase in BC fluxes between ca. 1970 and 2013 was also observed in two lake sediment records from northern Finland (Ruppel et al., 2015).
The increasing BC deposition on the Svalbard glacier has significant effects on the radiative budget of this site and concurs with substantially increased summer melting of the glacier since the 1980s (Ruppel et al., 2014). The increased melt of the glacier is better explained by the combination of observed increasing summer temperatures and the increasing BC concentrations than by increasing temperatures alone. To estimate the extent of the climatic implications suggested in Ruppel et al. (2014) it is essential to solve whether the observed increasing BC deposition trend in Svalbard since 1970 can be corroborated with other data. Furthermore, it is necessary to thoroughly assess the BC sources responsible and the deposition processes associated to the observed increase because these may affect also other parts of the Arctic.
Here, our objective is to resolve what the BC deposition trend has been
during the last 10 years on the previously studied Svalbard glacier,
Holtedahlfonna. For this, a new 14.7
Map of Svalbard and the location of study sites on the Holtedahlfonna glacier. The inset presents an aerial satellite image of the Holtedahlfonna glacier in summer. The 300-year ice (2005) and firn (2015) core study sites are indicated by red circles.
Svalbard is an archipelago located in the Arctic Ocean (Fig. 1). It has
relatively mild climate despite its location at high latitudes due to an
intrusion of the North Atlantic current bordering western Svalbard and its
location in the pathway of both Arctic and North Atlantic cyclones. Svalbard
is covered up to 60 % by glaciers, of which the majority have retreated during
the last 15–40 years (Nuth et al., 2010), and even the glaciers situated
at highest elevation (ca. 1200
In addition, a 2.53
The firn core was cut in a freezer laboratory (
To ensure comparability, the filtering and EC analysis of the firn core and
snow samples were performed in the same facilities and with same instruments
as in Ruppel et al. (2014), which followed the original procedure of
Forsström et al. (2009). The frozen samples were melted and immediately
filtered through precombusted (at 800
Holtedahlfonna firn core isotope and measured mass balance inferred dating. The red and blue curves present the hydrogen and oxygen isotope profiles for the firn core. The dashed lines indicate the September layers of the firn core against the firn depth in cumulative snow water equivalent centimetres. These data are based on snow accumulation rate (mass balance) measurement on a nearby stake. Please see text for more details.
The filters were analysed for EC using a thermal–optical method (Sunset
Laboratory Inc., Forest Grove, USA; Birch and Cary, 1996) with the latest
recommended thermal sequence EUSAAR_2 (Cavalli et al., 2010) at Stockholm
University. In the first stage, the method separates organic and carbonate
carbon from the filters under increasing
temperature steps in a helium atmosphere, while EC evolves from the filters
in the second stage under a helium–oxygen atmosphere at temperatures reaching
850
The used methodology includes uncertainties that are described in more detail
in Ruppel et al. (2014). In short, in liquid samples (i.e. melted snow and
ice) smallest EC particles may go through the filter (e.g. Torres et al.,
2014), leading to a quantified undercatch of ca. 22 % for the used
filtering set-up (Forsström et al., 2013). In addition, from each filter
sample (11.34
Temporal trend of BC emissions by sector from 1980 to 2015:
The 14.7
There are very pronounced variations with large seasonal amplitudes in the water isotope records in the uppermost 2 m of the core assumed to be due to different atmospheric sources of precipitate. These annual variations gradually get smoothed out due to diffusion during the firnification process (Fig. 2), rendering the distinction between years more difficult with increasing depth. Therefore, supporting data from the mass-balance stake are useful for dating. Stake 10 has been visited and maintained regularly since 2002 and thus an annual mass balance of the study site is available from 2003. By combining the density and depth data from the firn core, the snow water equivalent along the core profile could be obtained and was compared with the mass-balance (snow water equivalent accumulation) data measured at the stake since 2003. Thereby, the limit of years (September measurement points at the stake) could be determined as a function of depth, and subsequently the core could be dated. The density and water isotope inferred dating of the core match well with the mass-balance inferred limits of years (Fig. 2).
SILAM
(Sofiev et al., 2008), a model developed by the Finnish Meteorological
Institute, was run for a simulation between 1980 and 2015 to study BC
deposition variations and the contribution of different sources of BC
deposited at Holtedahlfonna. SILAM is a meso-to-global-scale chemical
transport model. For this study, the model was driven by ERA-Interim (Dee
et al., 2011) meteorology and by global MACCity anthropogenic BC emissions
(Granier et al., 2011) updated with ECLIPSE emission data set for flaring
(Klimont et al., 2013) and natural fire (open biomass burning) emissions
(Lamarque et al., 2010) shown in Fig. 3. Generally, BC emissions north of
40
EC concentrations (
Annual mean firn core EC concentration and deposition at Holtedahlfonna, stake 10, from 2006 to 2014. The values present total EC concentrations and deposition averaged over the respective year, assuming that the EC deposition rate has stayed constant throughout the respective year. Dating uncertainties of the firn core increase the uncertainties of these values.
The model was run through the period between 1980 and 2015 with 1 h temporal
resolution,
Generally, like many models, SILAM agrees better with observations closer to sources than in the Arctic. In the Arctic the modelled BC concentrations and deposition are systematically low, but the seasonality in atmospheric BC concentrations is captured, specifically capturing the Arctic haze period (see Fig. 5 and discussion below).
Observed atmospheric BC concentrations compared to modelled
atmospheric BC concentrations at Zeppelin monitoring station
Ny-Ålesund, Svalbard.
The EC variations in the snow pit covering the end of summer 2014 to
April 2015 are shown in Fig. 4a. The EC concentrations ranged between 4.7 and
20.3
The EC concentrations of the shallow firn core are between 3.5 and
24.6
Due to the comparably low temporal resolution of the EC samples no annual variation can be detected in the firn core, although the observed EC variation may be partly caused by some samples covering more of the high BC-laden spring to summer snow (e.g. two vs. zero spring layers) compared to cleaner winter snow (see Ruppel et al., 2014). The firn core is too short to indicate any clear temporal trend but, in general, the EC concentrations and deposition seem to be on a lower level from 2005 to 2011 and to increase to higher levels from 2012 to 2015 (Fig. 4c and d, Table 1). The temporal trend of EC deposition is similar to the EC concentration trend observed in the core (Fig. 4c and d).
To evaluate the performance of SILAM for Svalbard and BC, atmospheric BC observations made at the Zeppelin (Ny-Ålesund) monitoring site were compared to model results from the correspondent model grid cell in Fig. 5. Figure 5a shows the model results from 1980 to 2015 while atmospheric observations were available only for 2002 to 2011. Both the observations and model results show large variation in atmospheric BC concentrations from one year to the next, but with an overall decreasing trend (Fig. 5a). However, compared to the observations, the model significantly underestimates the atmospheric BC concentrations (on average by a factor of 5). Such underestimations of atmospheric BC concentrations are particularly common for the Arctic where previous comparisons to observations have shown atmospheric BC concentrations being underestimated in chemistry models by up to a magnitude (e.g. Koch et al., 2009; Lee et al., 2013; Dutkiewicz et al., 2014).
Figure 5b and c present the seasonality of observed and modelled monthly BC concentrations for 2006 and 2007. The model captures the seasonality seen in the observations but fails to reproduce the magnitudes observed especially in spring time. Note that the timing of observed spring peaks (Arctic haze) varies from year to year. This corroborates with several multi-model studies (Shindell et al., 2008; Koch et al., 2009; Eckhardt et al., 2015) showing that atmospheric models are usually not able to simulate the seasonality of BC in the Arctic precisely, typically underestimating the Arctic haze season occurring during the winter and early spring. A more detailed discussion on the uncertainties of the model and input driving the runs is presented in Sect. 4.
Modelled annual BC deposition and atmospheric concentrations at Holtedahlfonna.
The annual source sector contribution to the modelled total BC
deposition and atmospheric BC concentrations at Holtedahlfonna
between 1980 and 2015. The sources for
The results of modelled atmospheric BC concentrations and BC deposition at
Holtedahlfonna are presented in Fig. 6. The modelled annual atmospheric BC
concentrations decrease quite constantly from 1990 onwards after notably
higher values modelled for the 1980s (slope
The model results suggest that the total BC deposition is dominated by wet deposition at Holtedahlfonna (98.7 %). There are notable differences in the source contributions for the modelled BC deposition and atmospheric BC concentrations at Holtedahlfonna (Fig. 7). Over the period of 1980 to 2015 transport and domestic emissions are the most important sources for BC deposited at Holtedahlfonna (Fig. 7a), both with ca. 30 % contribution, while the domestic sector (43 % on average) is the most important emission source for atmospheric BC concentrations at the glacier, followed by the industry and transport sectors (Fig. 7b). For both the modelled atmospheric BC concentrations and deposition the contribution of domestic emissions has decreased during the investigated time period while the contribution of transport, including shipping, and natural fires has increased, and the contribution of industry and other sectors has stayed quite constant.
EC deposition at Holtedahlfonna between 1850 and 2015. EC deposition
(
Previous EC concentrations from surface snow in 2007, 2008 and 2009
(Forsström et al., 2013) and the snow pit and firn core data collected
from the Holtedahlfonna 2015 coring site (stake 10) corroborate each other
(Fig. 4b). However, the firn core EC concentrations measured at stake 10 (an
average of 10.4
EC
In contrast, EC
The annual snow accumulation rate is the sum of snow accumulating (precipitation, wind drift) and reducing (ablation, run-off) processes. The precipitation amount at the sites is considered the same, and therefore wind drift, summer melt and sublimation are the probable causes for the different net snow accumulation at the sites. Summer melt occurs frequently on Holtedahlfonna (Beaudon et al., 2013). However, BC has a low post-depositional scavenging efficiency due to its hydrophobic properties; i.e. it is concentrated in melting snow and not flushed unless the melting is strong (e.g. Doherty et al., 2013). No summer surface run-off or high amounts of refrozen water (signalling strong vertical movement of meltwater) in the snow stratigraphical record have been observed on Holtedahlfonna, indicating that the summer melt on Holtedahlfonna is not strong enough to flush EC laterally or vertically. Therefore, it is unlikely that melting or run-off would cause the different EC deposition amounts at the two coring sites.
Consequently, wind drift and sublimation may be the most plausible post-depositional explanations for the observed differences in snow accumulation rate and EC deposition levels at the two coring sites, as these processes actually have the potential to remove or add snow and EC to the annual snow pack. Redistribution of snow mass by wind drift has significant impacts on the snow accumulation rates on Svalbard (Jaedicke and Gauer, 2005; Beaudon et al., 2011; Sauter et al., 2013). Sauter et al. (2013) showed that on Vestfonna ice cap, eastern Svalbard, up to 20 % of primary accumulated snow is redistributed by wind drift. To explain the higher EC deposition at the 2005 site it should receive more EC-laden snow by wind drift than the 2015 site. Higher wind drift could also explain higher EC concentrations at the site, since part of snow mass is sublimated during its transport (Sauter et al., 2013), which concentrates EC in wind-blown snow. However, if the higher EC deposition at the 2005 site would be solely explained by it receiving more snow by wind drift than the 2015 site, then its snow accumulation rate should also be higher than that of the 2015 site. As the snow accumulation rate is actually lower at the 2005 than the 2015 site, wind drift cannot explain the differences alone.
Sublimation, which is a function of air temperature, humidity and wind speed, may affect the varying net snow accumulation rate at the Holtedahlfonna coring sites, as Arctic winter sublimation commonly reaches values of 10–50 % of total winter precipitation (Liston and Sturm, 2004, and references therein). During sublimation water is lost from the snow pack while EC is left behind and concentrated (e.g. Doherty et al., 2010, 2013). The 2005 site is most likely windier than the 2015 site and may therefore be more prone to sublimation, which would result in the lower net snow accumulation rate observed at this site compared to the 2015 site. However, although significant amounts of water may be lost from snow/glacier surfaces due to sublimation, this process does not affect EC deposition rates.
Thus, to explain simultaneously the differences in snow accumulation rates and EC deposition amounts at the two Holtedahlfonna coring sites by post-depositional processes, a combination of high snow drift and sublimation would need to be considered. However, the differences in snow accumulation rates and EC values between the sites are so large that based on current knowledge on the amount of snow remobilization by wind and sublimation discussed above, it seems improbable that these processes would explain the differences between the sites alone.
Consequently, while the post-depositional processes certainly affect the
measured snow accumulation rate and EC concentration, as well as wind drift the EC
deposition, none of these processes are alone or together likely to
entirely account for the different level of EC deposition observed in the
2005 and 2015 firn/ice cores. It is therefore more plausible that a drop in
EC deposition has occurred between 2003 and 2005 at the Holtedahlfonna
glacier. The magnitude of this drop remains uncertain, since the differences
between the ice and firn core are affected by the above described
post-depositional processes to an unknown extent. Sudden drops are not
unprecedented in the 300-year Holtedahlfonna record, in which EC deposition has
dropped strongly, for instance from peak values of
34
Atmospheric BC deposition at Holtedahlfonna (as at remote Arctic regions in general) is a complex end result of BC emissions within and outside of the Arctic, the prevailing atmospheric transport pathways, meteorological conditions along the way to Svalbard, BC ageing processes and local meteorological processes at the glacier. All these factors contribute to what emission source areas and sectors are the most significant for the EC deposited at Holtedahlfonna, how much of the emitted BC is scavenged and deposited from the atmosphere before reaching Holtedahlfonna, how efficient in-cloud and below-cloud BC scavenging is at a specific time at Holtedahlfonna, and thereby how much atmospheric and in-cloud EC present at Holtedahlfonna is actually deposited. These processes may vary temporally with notable effects on the observed EC deposition trend at Holtedahlfonna. As according to our model results almost 99 % of BC is wet-deposited at Holtedahlfonna, the significance of meteorological processes and their variation in comparison to sole BC emissions for the observed EC deposition trend have to be considered. Moreover, it would be a gross oversimplification to assume that the EC deposition trend at Holtedahlfonna would solely reflect BC emission trends in source areas and/or atmospheric BC concentration trends, since local and regional meteorological processes affect the EC deposition rate notably. As a possible example of the consequences of disregarding temporal meteorological variation, previous modelling results of historical BC deposition in Finland using constant (year 1997) meteorology since 1850 show that the modelled BC deposition trend closely follows the inventory BC emission trend, while the observed BC deposition trend clearly diverged from the modelled trend (Ruppel et al., 2015). One possible explanation for the described discrepancy are variations in meteorological processes affecting BC scavenging efficiencies that were unaccounted for in the model. Thus, to produce generally more plausible modelled data, atmospheric BC deposition at Holtedahlfonna was here modelled only beginning from 1980, the start of ERA-Interim meteorological data.
Atmospheric BC concentration trends, in contrast, have been generally
observed to follow BC emission trends in the Arctic (e.g. Sharma et al.,
2013). In our results the modelled atmospheric BC concentration decreases
between 1980 and 2015 (Fig. 6), as has also been observed between 1990 and
2009 at the three long-term Arctic BC monitoring stations in Alert, Barrow
and Ny-Ålesund (Sharma et al., 2013), and in a 47-year weekly measurement
record from northern Finland (Dutkiewicz et al., 2014). The modelled
atmospheric BC concentrations at Holtedahlfonna underestimate the values
measured at the closest measurement station, Ny-Ålesund (Zeppelin), from
2001 to 2015 by an order of magnitude (see Sharma et al., 2013). However, the
comparison between these sites should be done carefully, since the sites are
located in different grid cells in the model and at different altitudes
(Holtedahlfonna at 1150
Notably, however, the modelled annual BC deposition does not clearly follow
(or correlate to) the declining north of 40
Modelled BC deposition compared to ice and firn core EC deposition at Holtedahlfonna from 1980 to 2015; 5-year running averages are included.
The modelled BC deposition at Holtedahlfonna is about a magnitude lower than
the measured EC deposition in the ice and firn cores (Fig. 9). Similar
notable underestimations in modelled BC values compared to observations have
been previously reported in the Arctic for both snow BC concentrations (e.g.
Forsström et al., 2013) and BC deposition (e.g. Ruppel et al., 2013,
2015). The modelled BC deposition trend at Holtedahlfonna does not show clear
consistency with the observed EC deposition in the ice and firn cores,
although some similarities can be observed. The notable variation in the
measured ice/firn core EC deposition from one data point to the next in
addition to the year-to-year variation in the modelled BC deposition
highlights the significance of wet deposition patterns and underlying varying
meteorological processes to the surface deposition trends. In addition, the
modelled BC deposition trend seems to support a notable drop in BC deposition
observed between the ice and firn core. The 300-year ice core recorded an
average EC deposition of 18.5
Consequently, the model results support some features of the ice and firn
core observations, such as higher EC deposition in the 1980s and 1990s and
a drop in deposition thereafter, but these variations are smoothed and
lowered by the model in comparison to the ice and firn core values (Fig. 9).
Explanations for the observed variations being smoothed out in the model
results could relate to the spatial and time resolution of meteorology and
emissions. The spatial horizontal resolution of the ERA-Interim meteorology
is
Nonetheless, the modelled BC deposition variation suggests that the BC
deposition trend may diverge from the atmospheric BC concentration trend on
an annual scale (Fig. 6), which is likely explained by meteorological
processes affecting for instance BC scavenging. Meanwhile, the model could be
improved by including a temperature dependency to the scavenging efficiency
of BC, as Cozic et al. (2007) showed that the scavenging efficiency of BC
increases significantly from temperatures of
In Ruppel et al. (2014) it was hypothesized that the observed increase in the Holtedahlfonna ice core EC deposition from 1970 to 2004 could have been partly caused by simultaneously increasing flaring emissions from north-western Russia. That area is a major source for BC in Svalbard (e.g. Hirdman et al., 2010; Stohl et al., 2013), and according to Stohl et al. (2013) flaring may contribute to 20–40 % of annual mean surface BC concentrations in Svalbard, but these emissions have been strongly underestimated or even disregarded in emission inventories (Stohl et al., 2013; Huang et al., 2015). However, our current model results suggest a significantly lower contribution of flaring to the BC values on Holtedahlfonna between 1980 and 2015: ca. 7 % for the atmospheric concentrations and 2 % for the deposited BC (Fig. 7). Only in sporadic years, such as 1982 and 2010, is the flaring contribution suggested to have increased to over 10 % of the total BC deposited. Interestingly, this modelled contribution of flaring matches well with state of the art dual-carbon isotope source apportionment measurements of atmospheric EC from Tiksi, north-eastern Russia, which suggested flaring to contribute only to 6 % of annual atmospheric EC concentrations at the site (Winiger et al., 2017). No increase in the contribution of flaring to total BC deposition is evident in our modelling data from 1980 to 2015 and, even in the case of possible continued underestimations of flaring in current emission inventories, the hypothesis of Ruppel et al. (2014) that flaring partly caused the increase observed in 1970–2004 in the Holtedahlfonna ice core can be rejected by the current modelling data.
As seen in Fig. 7, there are notable differences in the source contributions for the modelled BC deposition and atmospheric concentrations. While transport and domestic emissions appear to be the most important sources for BC deposited at Holtedahlfonna, the domestic sector seems to be the most important emission source for atmospheric BC concentrations at the glacier. This difference in the source contribution to the modelled BC deposition vs. atmospheric concentration can be explained by the difference in emission location, injection height, transport pathways and removal of BC from the atmosphere. In the current setting of chemical transport models, such as SILAM, the physical properties of the emitted particles (type, size, hygroscopic properties) are characterized on a low description level, e.g. no aerosol dynamics, and thus no substantial difference in physical properties between the different emission sectors is present. Nevertheless, in long-term assessments of BC, the meteorology is key to determining transport pathways and scavenging of the particles from the atmosphere and may thereby affect the differences between source contributions of modelled atmospheric and deposited BC.
For the modelled BC deposition, the contribution of domestic emissions has
decreased while transport emissions have generally increased from 1980 to
2015, particularly when including shipping (Fig. 7a). The BC emissions north of
40
In summary, emissions from the domestic and transport sector, followed by industry, seem to affect the BC values at Holtedahlfonna the most. None of the anthropogenic or natural fire emissions have varied independently or together in a manner that could solely explain the observed EC variation in the Holtedahlfonna ice and firn cores. Furthermore, the amount of BC emissions from individual sectors (Fig. 3) does not equal the modelled contribution of these emission sectors to the atmospheric BC concentrations or especially BC deposition at Holtedahlfonna (Fig. 7). Consequently, it seems most likely that meteorological processes affecting wet deposition patterns at the glacier (and during transport) have had a stronger influence on the EC deposition trends at Holtedahlfonna than the BC emission trends.
According to a shallow firn core collected in spring 2015 from Holtedahlfonna glacier, Svalbard, EC concentrations and deposition have dropped to lower values in the 21st century after rapidly increasing values recorded from 1970 to 2004 at the glacier in a 300-year ice core (Ruppel et al., 2014). Neither the increasing trend from 1970 nor the rapid drop in EC deposition from 2003 to 2005 is supported by the Arctic atmospheric BC concentration measurement or BC emission inventory trends. A meso-to-global-scale chemical transport model (SILAM) was run to investigate the difference in atmospheric BC concentration and BC deposition trends, and to evaluate BC emission sources affecting the Holtedahlfonna glacier between 1980 and 2015.
Modelling the long-term atmospheric concentrations and deposition of BC at Holtedahlfonna allowed us to discern the annual variation and decadal trends for BC. As expected, the modelled atmospheric BC concentration trend corresponds to the declining BC emission trend. However, although the modelled BC deposition decreases weakly throughout the study period (1980–2015), the trend does not clearly follow BC emission or atmospheric concentration trends. Our results show that almost 99 % of BC mass is wet-deposited at Holtedahlfonna. This number is probably exacerbated by the lack of aerosol ageing processes in the model which results, for instance, in the transported particles being too small for dry deposition in the Arctic and consequently wet scavenging overly dominating the deposition. Nonetheless, the results based on the current settings of SILAM corroborate with the 85 to 90 % of BC wet deposition generally suggested for the Arctic by Wang et al. (2011). Thus, precipitation and other meteorological factors (such as temperature and cloud phase (liquid, mixed or ice)) are crucial parameters as they drive the scavenging of BC, both on site and during the transport of BC to the Arctic. Consequently, it seems oversimplified to assume that the BC deposition trend would strictly follow its emission and/or atmospheric concentration trends.
The modelled BC deposition trend shows similarities with the observed ice and
firn core EC trends, with highest deposition values reached in the 1980s and
1990s and a subsequent decrease. The ice and firn core data show stronger
variation and steeper fluctuations in EC deposition trends than the model.
This is likely caused by key input data of the model, as the emission
inventory data are based on emission scenarios that are only available for
every 5 or 10 years, and the model is run with the same grid size as the
meteorology (
Notably, the recorded firn EC concentrations (from 2005 to 2015) are lower than the EC concentrations recorded in the first half of the 1980s in the 300-year ice core. Similarly decreasing BC concentrations were reported by Doherty et al. (2010) comparing ca. 1200 surface snow samples collected mainly between 2005 and 2009 to snow collected by Clarke and Noone (1985) in 1983 and 1984 from Arctic snow packs, including Svalbard. While Doherty et al. (2010) concluded that it was doubtful that BC in Arctic snow would have contributed to the rapid decline of Arctic sea ice observed since 1979 (e.g. AMAP, 2011), our ice and firn core results highlight that such snow measurements provide only temporal snap shots in a decadal perspective, and significant BC variation relevant for climate impact assessment may be overlooked without continuous records. In other words, only continuous long-term records can reliably show decadal trends upon which the significance of year-to-year variability can be assessed.
Observational data on Arctic EC or BC deposition are currently quite scarce and geographically restricted (mostly to Greenland and the European Arctic). Moreover, several firn/ice cores should be retrieved from same glaciers to assess the effect of local post-depositional processes and micrometeorology on the BC concentrations and deposition. The present data indicate that EC deposition at a Svalbard glacier is not solely driven by BC emission or atmospheric concentration trends, as basically all EC is wet-deposited and thereby mostly affected by precipitation and EC scavenging efficiency variations. However, it is currently unknown how widespread or pronounced such discrepancies between atmospheric BC and deposition trends generally are in the Arctic. Much further BC deposition data are required before general conclusions on the climatic implications on BC in the Arctic should be attempted, since it is specifically BC deposition on reflecting surfaces that amplifies the climate impact of BC in the Arctic compared to atmospheric BC. Furthermore, the current data suggest that Arctic BC deposition trends cannot straightforwardly be reconstructed based on atmospheric BC concentration trends, or vice versa.
The data are available from the authors (meri.ruppel@helsinki.fi) upon request.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Interactions between climate change and the Cryosphere: SVALI, DEFROST, CRAICC (2012–2016) (TC/ACP/BG inter-journal SI)”. It is not associated with a conference.
We are deeply grateful for the support and funding received from the NordForsk Top-level Research Initiative Nordic Centre of Excellence, CRAICC (Cryosphere–Atmosphere Interactions in a Changing Arctic Climate), and the Academy of Finland projects 257903 and 296646. The field support was provided by Norwegian Polar Institute. Support for atmospheric aerosol observations at Zeppelin (Ny-Ålesund) by the Swedish EPA is greatly acknowledged. Edited by: Michael Boy Reviewed by: two anonymous referees