Air Pollution and Its Association with the Greenland Ice Sheet Melt

: The Greenland Ice Sheet (GrIS) has been a topic of extensive scientiﬁc research over the past several decades due to the exponential increase in its melting. The relationship between air pollution and GrIS melting was reviewed based on local emission of air pollutants, atmospheric circulation, natural and anthropogenic forcing, and ground/satellite-based measurements. Among multiple factors responsible for accelerated ice melting, greenhouse gases have long been thought to be the main reason. However, it is suggested that air pollution is another piece of the puzzle for this phenomenon. In particular, black carbon (BC) and other aerosols emitted anthropogenically interact with clouds and ice in the Arctic hemisphere to shorten the cloud lifespan and to change the surface albedo through alteration of the radiative balance. The presence of pollution plumes lowers the extent of super cooling required for cloud freezing by about 4 ◦ C, while shortening the lifespan of clouds (e.g., by altering their free-energy barrier to prompt precipitation). Since the low-level clouds in the Arctic are 2–8 times more sensitive to air pollution (in terms of the radiative/microphysical properties) than other regions in the world, the melting of the GrIS can be stimulated by the reduction in cloud stability induced by air pollution. In this study, we reviewed the possible impact of air pollution on the melting of the GrIS in relation to meteorological processes and emission of light-absorbing impurities. Long-term variation of ground-based AERONET aerosol optical depth in Greenland supports the potential signiﬁcance of local emission and long-range transport of air pollutants from Arctic circle and continents in the northern hemisphere in rapid GrIS melting trend.


Introduction
The Greenland Ice Sheet (GrIS) is about the same size as the state of Alaska in the USA and can result in the rise of the global sea level by more than 7.2 m upon complete melting [1]. It has been suggested that 70% of the 269 billion tons of ice lost from the GrIS during 2011-2014 has primarily been due to surface melts rather than the calving-based processes [2]. Although global warming induced by greenhouse gases has long been perceived as the major driver of this ice melt, the possible involvement of other factors and processes has been continuously questioned [3]. For instance, the role of air pollution (e.g., in the form of particulate matter released from the combustion of fossil fuels) is now proposed as one of the potential driving components in addition to all of those identified to date [4][5][6].
The occurrence of atmospheric Arctic currents/circulation such as the negative phase of the Arctic oscillation (AO) and north Atlantic oscillation (NAO) offers a measure for transporting or propagating the pollutants to or within remote areas even to places that are remote from eminent man-made source processes (except volcanic activities) [7,8]. The evolution and transport of pollution plumes from the combustion of fossil fuels and biomass (particularly containing black carbon (BC)), if occurring, lowers the extent of super cooling required for cloud freezing by about 4 • C [7]. Figure 1 elucidates the super cooling freezing temperature (∆T*) based on the space observations for four different cloud types with four liquid-cloud-droplet effective radius (r e ) regimes. The ice fraction (χ Ice ) was calculated based on combined ice cloud-top and liquid temperature distributions [7]. In general, the average ∆T* value was reduced by~1 • C between the upper and lower regimes in r e for all cloud types [7]. Such processes can shorten the lifespan of clouds by altering their free-energy barrier to prompt precipitation. Note that the error bars in Figure 1 represent the uncertainties calculated by Equation (1). It should be noted that a 2 /a 1 is a parameter representing the super cooling temperature for which the ice-cloud fraction becomes equal to the liquid-cloud fraction as detailed in [7].
(1) Figure 1. Super cooling freezing temperature (∆T*) versus the effective radius of the liquid-clouddroplet (r e ) for four cloud category types (Panels (a-d)) differentiated by their top pressure (CTP) and liquid water path (LWP). The mean carbon monoxide (CO) concentration (χ CO ) is represented by the color scale; reproduced with permission from [7]. Detailed description of the utilized terms can be found in [7].
As can be seen in Figure 2, if no precipitation occurs during the transport of air pollutants to the Arctic region, then the net aerosol-cloud interaction parameter (ACI net ) value can be quantified based on the local efficiency of cloud property perturbation due to the increased aerosol quantities [5]. Under such a situation, the expected ACI net values fell in the range of 0 to~0.33 (theoretical maximum) [9]. In cases where precipitation occurs during the transport of pollutants to the Arctic region, aerosol particles may be scavenged via wet deposition only to leave carbon monoxide (CO) behind (utilized as a passive tracer). In such cases, the ACI net value should remain zero even if the aerosols under consideration would have previously acted as effective cloud condensation nuclei (CCN) [10]. Thus, further research endeavors are needed to uncover the driving mechanisms and the resulting consequences of such air pollution-related side effects on the surface mass balance and ice sheet albedo. In this regard, the present review article aims to catalyze scientific discussions towards such meteorological processes for better control of the Arctic ice melt crisis in this study.
Sustainability 2021, 13, x FOR PEER REVIEW Figure 2. The relationship between aerosol concentrations (along various pathways for lon transport) and a passive tracer (CO) in a tracer-based transport model. The correlation bet aerosols and CO is established at the source region. Based on the occurrence of precipitatio ing transport) and the aerosol efficiency to act as cloud condensation nuclei (CCN), the eff pollution plumes (from distant regions) on clouds in the Arctic region specified by the net cloud interaction parameter (ACI net ) varied between 0 and 1/3; reproduced with permissio [5].

Data Collection and Literature Survey Methodology
The dataset for the Greenland mass variation analysis (since 2002) was obtain ice mass measurements by GRACE satellites (NASA) available from the https://climate.nasa.gov/vital-signs/ice-sheets/. A literature survey for the discu the relationship between air pollutants, such as BC/aerosol, and ice melting in th region was carried out using the Google Scholar search engine with key words melting", "BC", "ship", "biomass burning", "wildfire", and "absorption coefficie

Radiation in the Arctic and Mass Loss of the GrIS
The radiative properties of mixed-phase clouds in the Arctic region play a cru in governing the ice sheet warming rate. Air pollutants can be transported into mote areas due to the meteorological atmospheric currents/circulation patterns (e ative phase of NAO) [7,8,11]. Thus, the reduction of cloud lifespans induced by a tion may result in abnormal regulation of the Arctic surface temperature that can ate the ice-melting rate depending upon the season, sea-ice coverage, and cloud depth [5,7,12]. In addition, the presence of a warmer layer of air on top of colder the Arctic surface traps the particulate air pollutants for months and restricts the sition onto the ice surface [13,14]. Furthermore, the stability of low-level clouds in tic is 2-8 times more sensitive to air pollution (in terms of the radiative and micro properties) than in other regions of the world [5]. Such amplified sensitivity of c the Arctic region may be accounted for by the diminished droplet evaporation rat an increase in the stability of lower troposphere and a decrease in the vertical clo ing possibility with the sub-saturated air [5]. As predicted through a numerica transport model, the aerosol-cloud interactions over the Arctic Circle could efficie ert pronounced negative impacts on the melting of GrIS due to air pollution, such osols liberated from man-made consumption of fossil fuels relative to (natural) Figure 2. The relationship between aerosol concentrations (along various pathways for long-range transport) and a passive tracer (CO) in a tracer-based transport model. The correlation between aerosols and CO is established at the source region. Based on the occurrence of precipitation (during transport) and the aerosol efficiency to act as cloud condensation nuclei (CCN), the effect of pollution plumes (from distant regions) on clouds in the Arctic region specified by the net aerosol-cloud interaction parameter (ACI net ) varied between 0 and 1/3; reproduced with permission from [5].

Data Collection and Literature Survey Methodology
The dataset for the Greenland mass variation analysis (since 2002) was obtained from ice mass measurements by GRACE satellites (NASA) available from the website: https://climate.nasa.gov/vital-signs/ice-sheets/. A literature survey for the discussion of the relationship between air pollutants, such as BC/aerosol, and ice melting in the Arctic region was carried out using the Google Scholar search engine with key words of "ice melting", "BC", "ship", "biomass burning", "wildfire", and "absorption coefficient."

Radiation in the Arctic and Mass Loss of the GrIS
The radiative properties of mixed-phase clouds in the Arctic region play a crucial role in governing the ice sheet warming rate. Air pollutants can be transported into such remote areas due to the meteorological atmospheric currents/circulation patterns (e.g., negative phase of NAO) [7,8,11]. Thus, the reduction of cloud lifespans induced by air pollution may result in abnormal regulation of the Arctic surface temperature that can accelerate the ice-melting rate depending upon the season, sea-ice coverage, and cloud optical depth [5,7,12]. In addition, the presence of a warmer layer of air on top of colder air near the Arctic surface traps the particulate air pollutants for months and restricts their deposition onto the ice surface [13,14]. Furthermore, the stability of low-level clouds in the Arctic is 2-8 times more sensitive to air pollution (in terms of the radiative and microphysical properties) than in other regions of the world [5]. Such amplified sensitivity of clouds in the Arctic region may be accounted for by the diminished droplet evaporation rate due to an increase in the stability of lower troposphere and a decrease in the vertical cloud mixing possibility with the sub-saturated air [5]. As predicted through a numerical tracer transport model, the aerosol-cloud interactions over the Arctic Circle could efficiently exert pronounced negative impacts on the melting of GrIS due to air pollution, such as aerosols liberated from man-made consumption of fossil fuels relative to (natural) biomass burning [5,7,15]. A clear distinction as to why the stability of low-level clouds in the Arctic region is affected more detrimentally by aerosols derived from fossil fuel combustion than by those from biomass burning remains to be resolved via further research.
In recent years, GrIS has been losing about 280 Gt yr −1 on average [16,17]. According to data from NASA's GRACE and GRACE Follow-On satellites [18], the ice sheet in November 2019 had lost about 4700 Gt yr −1 of its mass since 2002 (e.g., rate of change in Greenland mass, −283 Gt yr −1 ) due to an increase in the Arctic sea-surface temperature (e.g., Chukchi Sea: 0.07 ± 0.03 • C yr −1 for August of each year, 1982-2017, [16]). Furthermore, the recent rate of change in the Greenland mass (for 2010-2019) has slowed down by −218 Gt yr −1 (Figure 3).

Atmospheric Circulation for the Transport of air Pollutants to GrIS
The most considerable mass loss in GrIS is from NW, SE, and CW regions of Greenland [19]. Anomalous GrIS melt episodes during the warm season often occur under slowmoving high-pressure regimes known as "Greenland blocks," with these blocking anticyclones favored during negative NAO conditions [20][21][22][23]. The NAO is a weather phenomenon in the North Atlantic Ocean of fluctuations in the sea level atmospheric pressure difference between the Icelandic Low and the Azores High. Through the fluctuations in the strength of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and location of storm tracks across the North Atlantic [24]. It is part of the Arctic oscillation that varies over time without particular periodicity. Negative NAO arises from Greenland blocking, while positive NAO mainly represents the absence of blocking [25]. Surface melt anomalies were concentrated across the western and southern GrIS during most of the high-melt seasons from the mid-2000s through the early 2010s because Greenland blocking, which is an essential contributor to recently enhanced GrIS melt rates, exerted a warming effect on West Greenland and Baffin Bay [26]. Greenland blocking has increased significantly in summer (June, July, and August (JJA)) over the past few decades [25,27].
Another form of synoptic atmospheric circulation feature that may exert an essential influence on GrIS surface mass balance (SMB) is the transport of water vapor by atmospheric rivers (ARs). Glacier SMB is the difference between accumulation and ablation (sublimation and melting) (e.g., negative SMB as retreat versus positive SMB as advance). As climate change may affect both temperature and snowfall in GrIS, changes in the SMB are to be accompanied. ARs are narrow corridors of strong horizontal water vapor transport through which most of the annual moisture transport into the high latitudes of the Northern Hemisphere are accomplished during a relatively small number of transient events [28][29][30]. GrIS mass loss arises directly from intense moisture transport over Greenland by ARs [31]. The occurrences of both extreme GrIS melting event (during July 2012) and the less extensive event (during early April 2016) should be ascribable to strong ARs. The strong ARs should have enhanced water vapor greenhouse effect, formation of clouds retaining additional longwave radiation, condensational latent heat release in the advected air mass, and surface melt energy provided by liquid precipitation [32,33]. However,

Atmospheric Circulation for the Transport of air Pollutants to GrIS
The most considerable mass loss in GrIS is from NW, SE, and CW regions of Greenland [19]. Anomalous GrIS melt episodes during the warm season often occur under slow-moving high-pressure regimes known as "Greenland blocks," with these blocking anticyclones favored during negative NAO conditions [20][21][22][23]. The NAO is a weather phenomenon in the North Atlantic Ocean of fluctuations in the sea level atmospheric pressure difference between the Icelandic Low and the Azores High. Through the fluctuations in the strength of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and location of storm tracks across the North Atlantic [24]. It is part of the Arctic oscillation that varies over time without particular periodicity. Negative NAO arises from Greenland blocking, while positive NAO mainly represents the absence of blocking [25]. Surface melt anomalies were concentrated across the western and southern GrIS during most of the high-melt seasons from the mid-2000s through the early 2010s because Greenland blocking, which is an essential contributor to recently enhanced GrIS melt rates, exerted a warming effect on West Greenland and Baffin Bay [26]. Greenland blocking has increased significantly in summer (June, July, and August (JJA)) over the past few decades [25,27].
Another form of synoptic atmospheric circulation feature that may exert an essential influence on GrIS surface mass balance (SMB) is the transport of water vapor by atmospheric rivers (ARs). Glacier SMB is the difference between accumulation and ablation (sublimation and melting) (e.g., negative SMB as retreat versus positive SMB as advance). As climate change may affect both temperature and snowfall in GrIS, changes in the SMB are to be accompanied. ARs are narrow corridors of strong horizontal water vapor transport through which most of the annual moisture transport into the high latitudes of the Northern Hemisphere are accomplished during a relatively small number of transient events [28][29][30]. GrIS mass loss arises directly from intense moisture transport over Greenland by ARs [31]. The occurrences of both extreme GrIS melting event (during July 2012) and the less extensive event (during early April 2016) should be ascribable to strong ARs. The strong ARs should have enhanced water vapor greenhouse effect, formation of clouds retaining additional longwave radiation, condensational latent heat release in the advected air mass, and surface melt energy provided by liquid precipitation [32,33]. However, AR events can also provide positive inputs to SMB through snow accumulation. In addition, the AR events can decrease solar radiation over the low-albedo ablation zone [34]. Net AR impacts on SMB are likely to vary according to a number of factors such as season, elevation, latitude, and moisture transport intensity [35,36]. It was found that the observed overall trend in BC and sulfate measurement at the Arctic stations can be explained by the changes in their emissions while the long-term atmospheric circulation can only explain a minor fraction of the overall trend [37]. The positive correlation between NAO and concentrations of air pollutants was also found in Greenland ice cores [38] and Arctic stations [37]. Northern Eurasia is the region of dominant emission source for both BC and sulfate at all Arctic stations. There are indications that the BC emissions from Eurasia in wintertime have increased over the last decade, probably reflecting the emission increases in China and other East Asian countries [37]. Large-scale atmospheric circulation patterns such as negative NAO and Greenland blocking can transport air pollutants, which can enhance GrIS mass loss indirectly through absorbing solar radiation.

Air Pollution and Emissions
BC (and aerosol) in air has been reported to exert influences on climate change directly through radiative forcing and indirectly through cloud properties [16,[39][40][41][42][43][44][45]. BC is typically produced from the incomplete combustion of natural biomass (wildfires) and fossil fuels (anthropogenic activities). Major emission sources of BC in the Arctic area are shipping and biomass burning (e.g., boreal wildfires) [46]. Wildfires in British Columbia and the northwest territories in 2017 were also reported to have contributed greatly to the enhancement of total column concentrations of trace gases, such as NH 3 , CO, HCN, and C 2 H 6 , over a decadal-scale (1999-2017) based on ground-based observations [47]. This observation suggested the importance of wildfires as one of the driving forces affecting the Arctic climate.
Roughly two-thirds of the BC emissions (e.g., 193 tons) made in 2015 over the Arctic could be attributed to ships (e.g., general cargo, oil tankers, and fishing vessels) as the major consumers of heavy oil, which was largely by Russian vessels [48]. In the new millennium, there have been notable increases in Arctic ship traffic and associated BC emissions due to the long record of, on average, shipping of supplies for offshore gas/oil extraction industries, heightened last-chance tourism, and receding ice-sea extent [46,49]. BC emissions are projected to rise dramatically to 282 ton in 2025, which is an increase of 46% from 2015, if ships are diverted from the Suez and Panama canals to benefit from the shorter routes to the Arctic from Europe, Asia, and North America [48]. As such, the rising emissions of BC due to marine traffic in the Arctic could enhance Arctic warming primarily through decreases in the Arctic albedo [48,50,51].
Aerosols emitted from anthropogenic activities can interact with clouds in the Arctic Circle to potentially shorten their lifespan (as mentioned earlier in Section 1). The anthropogenic aerosol-cloud interaction can increase the droplet concentration, which, in turn, may heighten the cloud albedo [52]. Thus, the aerosol perturbations can impart evaporation-entrainment feedback (particularly for non-precipitating cumulus clouds) to shorten the cloud lifespan [52]. However, it was also reported that anthropogenic aerosols can suppress the precipitation to yield clouds with longer lifespans, higher liquid water content, and larger fractional cloudiness [52,53]. Hence, further studies are needed to clarify the contrasting observations to pinpoint the exact influence of anthropogenic aerosols on cloud lifespan in the Arctic region. The low-level clouds in the Arctic region are estimated to be 2-8 times more susceptible to air pollution than those in the rest of the world. This, in turn, could have severe adverse effects on the radiative balance in the Arctic region. Furthermore, as BC from wildfires has increased markedly in the atmosphere above the Arctic in a warmer climate [54], it can stimulate the deposition of BC on the GrIS to accelerate ice melting over time [55]. Wildfires can be held accountable for much of the documented inter-annual variability in the concentrations of carbonaceous aerosol that induce changes in regional climate and air quality on an inter-annual cycle [56]. They have also been found to be major contributors to inducing an abnormal snowmelt rate in the arctic region [57]. However, long-term observations of BC made at three Arctic stations: Alert, Barrow, and Zeppelin, showed its downward trend (0.3-7.2%) of −2.1 ng m −3 yr −1 for 1998-2008, primarily due to the transport of air pollutants from Northern Eurasia and decreasing emissions [37,58]. A slower downward trend of −1.4 ng m −3 yr −1 was seen at Alert and Zeppelin for 2002-2009, while no such statistically significant trend was seen at Barrow. At Alert and Barrow, 2000-2001 saw a rise in BC concentrations [59]. The general downward trend (1989-2013) at Barrow, Alert, and Zeppelin was weakened after 2000 and a slight upward trend was seen at Barrow and Zeppelin after 2000 [40,60,61]. The overall decreasing trend in BC at those three stations was in contrast to the BC emission inventory estimated for the Svalbard area (including Zeppelin, 74 It should be noted that strongly increasing emissions from 2000 to 2007 were mostly driven by increased shipping emissions [62]. The Global Fire Assimilation System (GFAS) can estimate the deposition of burningderived BC on the GrIS. The daily estimates of BC emission from biomass burning and wildfires is facilitated by the satellite-based fire radiative power (FRP) observations of the GFAS [63][64][65]. The FRP is a quantitative measurement of the amount of energy released by burning and, hence, it is possible to project how much vegetation has been burned. It was found that the smoke plumes from wildfires were often pushed towards the GrIS by westerly winds [66]. Thus, a large fraction of the emissions (30%) was deposited on snow-or ice-covered surfaces. The calculated deposition was small compared to the deposition from global sources, although not entirely negligible. The albedo changes and instantaneous surface radiative forcing in Greenland due to the fire BC emissions were estimated to be relatively small (e.g., 0.006 for wildfires burned area (2345 ha) in western Greenland between 31 July and 21 August 2017) [66]. The large fraction of BC deposited on the GrIS made these fires very efficient climate forcers on a per unit emission basis. Substantial albedo changes and accelerated GrIS melting are expected to occur in the future as the warming of Greenland could produce wildfires at much larger scale. As limited satellite records are often obtainable for fires, the large inter-annual variability of fires around the globe limits our capacity to detect a change in the climate signal from the available global fire emission data [67][68][69].
A decline in the ice sheet albedo has been documented since the mid-1990s [17]. Because the surface melt in GrIS is closely connected to changes in the surface albedo, the assessment of multidecadal changes in the ice sheet albedo offers insight into surface melt and associated changes in its SMB. It was found that the rate of albedo decrease during summer melt has been accelerated during the 2000s (relative to the early 1980s). Furthermore, the surface albedos often decrease to values typical of bare ice at elevations 50-100 m above the ice sheet (analysis of the second edition of the satellite-derived climate data record (1982-2015) CLARA (The CM SAF cloud, Albedo and Surface Radiation dataset from Advanced Very High Resolution Radiometer (AVHRR) data-second edition denoted as CLARA-A2)) [70]. The southern margins exhibited the opposite behavior, probably due to increased snowfall over the area [70].
The albedo of snow (and sea ice) is affected by light-absorbing impurities and grain sizes, such as microbiota and BC [45,[71][72][73]. The observed albedo decreases are closely tied to the increasing mass loss of the ice sheet by enhancing its surface melt [74,75]. The 1982-2015 decadal albedo trends reached a maximum of approximately −0.05 over the ablation region (dark zone area) in the Kangerlussuaq sector in July [70]. The albedo decreases along the west coast mainly occurred between 2000 and 2012 due to a change in atmospheric circulation, which promoted the advection of warm and moist southerly air masses along the west coast. Meanwhile, springtime darkening in Greenland since 2009 was attributed to a widespread increase in the amount of light-absorbing impurities in snow as well as in the atmosphere [76]. The enhancement BC concentrations was suggested to significantly contribute to albedo feedback that triggered the widespread 2012 melt [77].
The concentrations of BC and light-absorbing dust impurity were analyzed along with their impact on snow albedo during the 2012-2014 snowfall season across north-west Greenland [78]. Albedo reductions due to light-absorbing impurities are small with an average of 0.003 (episodic enhancements result in reductions in the 0.01-0.02 range). No significant increase in BC or dust concentrations was found relative to recent decades. Such an observation indicates that the enhanced deposition of light-absorbing impurities does not cause significant reduction in dry snow albedo or increases in melt events. Recently, the effect of pigmented glacier algae on albedo change was suggested as one of the causes for the surface darkening (both within and outside the south-west GrIS dark zone) [79].
In this study, the fine-mode aerosol optical depth (fAOD) and total AOD (Level 2.0, cloud-screened and quality-assured) data, derived from Sun photometer at ground-based Aerosol Robotic Network (AERONET) at four measurement sites during the summer season in Greenland, were analyzed to evaluate the long-term trend of column-integrated anthropogenic particulate pollutants. The fAOD is the product of the total AOD and the fine-mode fraction (FMF). AOD is a measure of the extinction of the solar beam by particles in the atmosphere (dust, smoke, and BC) relative to the aerosol amount in the vertical atmospheric column over the observation location. As shown in Figure 4, the upward trends of fAOD and AOD were observed at all sites in Greenland, except for a western site (Kangerlussuaq). In general, this positive correlation of fAOD with the melting of GrIS implies the significant role of anthropogenic air pollutants transported from Arctic regions in melting GrIS. Measurements of AOD in western Greenland were strongly influenced by forest fires in Canada and wildfires in Greenland [66]. In general, this positive correlation of fAOD with the melting of GrIS indicates the potent role of anthropogenic air pollutants transported from various Arctic regions. Thus, the discrepancy of the role of albedo change by light-absorbing impurities in GrIS melting should therefore be resolved in projections of Greenland mass loss. The concentrations of BC and light-absorbing dust impurity were analyzed along with their impact on snow albedo during the 2012-2014 snowfall season across north-west Greenland [78]. Albedo reductions due to light-absorbing impurities are small with an average of 0.003 (episodic enhancements result in reductions in the 0.01-0.02 range). No significant increase in BC or dust concentrations was found relative to recent decades. Such an observation indicates that the enhanced deposition of light-absorbing impurities does not cause significant reduction in dry snow albedo or increases in melt events. Recently, the effect of pigmented glacier algae on albedo change was suggested as one of the causes for the surface darkening (both within and outside the south-west GrIS dark zone) [79].
In this study, the fine-mode aerosol optical depth (fAOD) and total AOD (Level 2.0, cloud-screened and quality-assured) data, derived from Sun photometer at ground-based Aerosol Robotic Network (AERONET) at four measurement sites during the summer season in Greenland, were analyzed to evaluate the long-term trend of column-integrated anthropogenic particulate pollutants. The fAOD is the product of the total AOD and the fine-mode fraction (FMF). AOD is a measure of the extinction of the solar beam by particles in the atmosphere (dust, smoke, and BC) relative to the aerosol amount in the vertical atmospheric column over the observation location. As shown in Figure 4, the upward trends of fAOD and AOD were observed at all sites in Greenland, except for a western site (Kangerlussuaq). In general, this positive correlation of fAOD with the melting of GrIS implies the significant role of anthropogenic air pollutants transported from Arctic regions in melting GrIS. Measurements of AOD in western Greenland were strongly influenced by forest fires in Canada and wildfires in Greenland [66]. In general, this positive correlation of fAOD with the melting of GrIS indicates the potent role of anthropogenic air pollutants transported from various Arctic regions. Thus, the discrepancy of the role of albedo change by light-absorbing impurities in GrIS melting should therefore be resolved in projections of Greenland mass loss.

Light Absorption
The light absorbing fraction of carbonaceous aerosols in the Arctic air has also been measured in the form of an aerosol light absorption coefficient (AAC) by several light absorption methods (filter absorption photometry and photoacoustic photometry). Long-term observations of BC optical properties were established after the discovery of Arctic haze, which includes a core monitoring site at Summit in Greenland (since 2003) [80,81]. The AACs in Svalbard showed a clear seasonality with the largest values being documented in the spring, and the smallest in the fall/summer seasons [82] due to the enhanced transportation of pollution from the mid-latitudes in the winter and spring [83], and the higher efficiency of wet removal in the summer [84]. In contrast to the direct measurement of GrIS melting with time, there are no generalized or clear trends for absorption and scattering coefficients for aerosol radiative properties for the past 10 years (2006-2018) in the Arctic region. Such limitations can be explained by the possibly large measurement uncertainties, which make it difficult to interpret the trend of evolving radiative properties in polar regions [85].
According to climate modeling, greenhouse gas warming of the Arctic climate should have been accelerated by global emissions of BC that absorbed solar energy in the atmosphere and snowpack [57,[86][87][88][89][90][91]. In contrast, one climate-model study by [92] indicated the slight cooling of Arctic surface due to the presence of atmospheric BC in the Arctic region. Such a phenomenon was caused by the lowering of surface isolation and poleward energy flux. [44] found that the simulated distribution of Arctic atmospheric BC cooled the surface slightly, while the BC deposited by the local cryospheric atmosphere warmed the Arctic in line with the findings of [92]. Due to the model limitations in terms of indirect aerosol effect exclusion, ocean heat transport-mechanism changes, and radiative forcing, study of the Arctic climate response to BC is being explored further.
In addition to the deposition of BC on the GrIS, the role of actively photosynthesizing and heavily pigmented cyanobacteria and microalgae found on the bare ice is also important. These microbes darken the GrIS surface during the summer months to reduce albedo and speed up melting [71]. The potential of black-brown ice-algae and red/green snow algae in reducing ice and snow albedos was estimated to be high enough (between 30-40%) relative to clean ice and snow, respectively [71,73]. Thus, it is imperative to accurately assess the role of Arctic microbiota in the context of designing better climate change models and establishing proper preventive measures. The discussion on the role of such bacterial colonies in controlling the GrIS ice melting is not extended further, as it is beyond the scope of this review [93][94][95].
The Intergovernmental Panel on Climate Change (IPCC) announced on 25 September 2019, that a reduction in global air pollution is imperative to preserve the ecology of the cryosphere [3]. In addition, the "Ocean and Cryosphere in a Changing Climate" (IPCC special report) was accepted by 195 member governments to control the air pollutant emissions in accordance with the goals set in the 2015 Paris Agreement [3]. If the present usage of fossil fuels and associated resources to reduce aerosol emissions is not controlled properly, we may soon reach a tipping point [96,97] from which the meltdown of Greenland may proceed irreversibly with a catastrophic rise in sea-level over the next 1000 years. According to the same IPCC report, the tipping point may be reached even if anthropogenic CO 2 emissions are reduced to zero overnight due to other factors (e.g., afforestation/reforestation).

Conclusions
Review on relationship between air pollution and GrIS melting was reviewed based on local emission of air pollutants, atmospheric circulation, natural and anthropogenic forcing, and ground-based and satellite-based measurements of parameters. BC from wildfires increased markedly over the Northern Hemisphere in a warmer climate, enhancing BC deposition on the GrIS. Recent BC emissions in the Arctic were also attributed to shipping activities. The BC emissions from Arctic ship traffic have increased in recent years due to the decreased ice-sea extent, heightened last-chance tourism, and supply shipping to the offshore gas/oil extraction industries. During the most recent two decades, ice sheet melting for the last decade has been slowing down with the steady loss of GrIS. This result might be reflected in parallel with the trend of emissions from biomass burning and ship traffic. However, observation (ground and satellite) model studies did show clear evidence of a relationship between ice melting rate and BC emissions. If the expected warming of Greenland is extended further to cause much larger wildfires in the future, this could indeed include substantial albedo changes with the accelerated melting of the GrIS. The study on the Arctic climate response to light-absorbing impurities should thus be explored further to properly project how this system interacts with the global climate system. Additionally, the predictions on the future GrIS mass loss should be made to help clarify the exact role of albedo change (due to light-absorbing impurities) in controlling the melting trend of GrIS.