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date: 17 May 2021

Saharan Dust Records and Its Impact in the European Alpsfree

  • Marion GreilingerMarion GreilingerZentralanstalt für Meteorologie und Geodynamik
  •  and Anne Kasper-GieblAnne Kasper-GieblVienna University of Technology

Summary

Mineral dust is one of the main natural sources of atmospheric particulate matter, with the Sahara being one of the most important source regions for the occurrence and deposition of mineral dust in Europe. The occurrence of dust events in the European Alps is documented via measurements of airborne dust and its deposits onto the glaciers. Dust events occur mainly in spring, summer, and early autumn.

Dust layers are investigated in ice cores spanning the last millennium as well as in annual snow packs. They strongly affect the overall flux of dust-related compounds (e.g., calcium and magnesium), provide an alkaline input to wet deposition chemistry, and change the microbial abundance and diversity of the snow pack. Still airborne mineral dust particles can act as ice nuclei and cloud condensation nuclei, influencing the formation of cloud droplets and hence cloud formation and precipitation. Dust deposits on the snow lead to a darkening of the surface, referred to as “surface albedo reduction,” which influences the timing of the snowmelt and reduces the annual mass balance of glaciers, showing a direct link to glacier retreat as observed presently in a warming climate.

Mineral Dust in the Atmosphere

Mineral dust primarily emitted from arid and semi-arid regions in the world is a key atmospheric constituent and represents one of the most important natural sources of atmospheric particulate matter (PM). Besides mineral dust, the main constituents of atmospheric PM in general are inorganic species, organic species, other carbonaceous species, and biological particles such as pollen or plant debris. Thereby sea salt, volcanoes, biological debris, and pollen are, besides mineral dust, the main natural PM sources, with sea salt leading the way, followed by mineral dust (Seinfeld & Pandis, 2006).

The dominant global mineral dust source regions are North Africa, the Middle East, and Asia, with the main source being the Saharan-Sahel region followed by the Gobi Desert (Goudie & Middleton, 2001; Prospero et al., 2002; Seinfeld & Pandis, 2006). According to Washington et al. (2009), half of the mineral dust emitted from the Sahara comes from the Bodélé depression in Chad, whereas Salvador et al. (2014) report that Saharan dust mainly originates from the Western Sahara, Morocco, Algeria, and Tunisia.

The review of Goudie and Middleton (2001) lists estimated soil dust emissions from various studies. Reported values show a large range from 500 million tons per year up to 5,000 million tons per year. Also within the same work, values for the source strength of the Sahara only are summarized, again showing a wide range of values from 130 million tons per year to 760 million tons per year. These wide ranges reflect the uncertainties of the emission models related to the different model assumptions regarding erosion, transport, and scavenging of the mineral dust particles as well as differences in the time scales considered (Prospero, 1996).

Mineral dust is introduced into the atmosphere primarily through wind erosion or different saltation-related processes. As a function of wind speed and moisture content of the soil, mineral dust particles of about 100 µm diameter are lifted and “dance” along the surface (the so-called saltation process). These horizontally transported and leaping particles can mobilize additional particles of a wide range of sizes due to the bombardment of the soil surface, which are then also ejected into the air (Kok et al., 2012). Depending on the particle size, the particles stay suspended in the air for short-term or long-term periods. While the mobilization efficiency or emission amount from the source area determine the quantity and the composition of the airborne mineral dust, the subsequent transport process determines the distance of travel and the modification of the chemical and physical properties. Basically, large particles tend to deposit closer to their source region as a result of their greater mass and settling velocities while smaller particles can be transported over longer distances, up to several thousand kilometers (Knippertz & Stuut, 2014; Seinfeld & Pandis, 2006). Thereby the travel distance of airborne mineral dust particles is closely related to their atmospheric lifetime. The average tropospheric lifetime ranges between 1 day and 1 week, depending on the particle size (Boucher et al., 2013). Thus, rain- or washout might shorten the atmospheric lifetime of the dust particles significantly. Generally, particles larger than 100 µm in diameter are found in the source regions, while smaller particles, that is, particles < 10 µm (Seinfeld & Pandis, 2006), are transported over long distances. Thus, Prodi and Fea (1979) investigated a case of Saharan dust transport and deposition over the Italian Peninsula and found a maximum diameter of 15–16 µm. In contrast, Weinzierl et al. (2009), for example, found that particles larger than 10 µm, and in 20% of the measurements even particles larger than 40 µm, were present during the Saharan Mineral Dust experiment (SAMUM) conducted in southern Morocco in May and June 2006 (see also Heintzenberg, 2009). Also, the more recent field campaigns, Chemistry-Aerosol Mediterranean Experiment (ChArMEx) and AERosol Properties–Dust (AER-D), in August 2013 and August 2015, respectively, highlight the presence of large or even giant particles with diameters of up to 80 µm (Marenco et al., 2018; Renard et al., 2018). Potential mechanisms for the long-range transport of these large and giant particles are discussed in van der Does et al. (2018).

As mineral dust can be transported thousands of kilometers (i.e., all over the world) by advection and atmospheric circulation, the European Alps receive a regular flux of mineral dust. This phenomenon and its association with several related impacts is schematically illustrated in figure 1 and is discussed in detail in the next sections.

Figure 1. Scheme of the long-range transport of mineral dust toward the European Alps associated with several impacts, as discussed in the section “Impact of Saharan Dust in the European Alps.”.

Saharan Dust Transport Toward Europe and the European Alps

Mineral dust deposition in Europe is a well-known phenomenon. The Saharan and Sahel region of Algeria, Tunisia, Libya, and Morocco in North Africa can be identified as the main source (compare Goudie & Middleton, 2001; Stuut et al., 2009 and references therein). An estimate of D’Almeida (1986) given more than 30 years ago shows that the Saharan source strength for mineral dust transport toward Europe, based on Sun photometer measurements, ranges from 80 to 120 million tons per year. As many studies investigating mineral dust events in Europe focus on the Sahara, the term “Saharan dust” is used here instead of the more general terms “desert dust” or “mineral dust”. However, contributions from the Arabian Peninsula should not be neglected (Tanaka & Chiba, 2006).

According to Israelevich et al. (2012), and references therein, the Saharan dust transport exhibits annual changes, primarily determined by two independent factors: (a) the seasonal dependence of the dust source strength in Africa; and (b) seasonal changes in the atmospheric circulation. They propose that each Saharan dust event in Europe is associated with a certain trajectory which differs significantly from event to event, but predominant transport paths as well as seasonal variations of these paths were observed. Based on monthly averaged aerosol optical thickness (AOT) satellite measurements from 2001 to 2010, two maxima in aerosol activity associated with Saharan dust intrusion for Central Europe were observed during spring (March to May) and summer (June to September). Also, Mattis et al. (2008) and Marinou et al. (2017) found a similar seasonal cycle of the Saharan transport paths toward Europe based on satellite and light detection and ranging (LIDAR) measurements, with the lowest values in winter and the highest values in spring and summer, whereas Papayannis et al. (2008) found a maximum in early autumn. A study of Barkan et al. (2005) defined the mean synoptic situation during Saharan dust transportation over the Mediterranean toward Europe and identified the troughs that emanate from the Icelandic low southward as well as the subtropical highs as the two main features. Based on a 40-year-long time series (1979–2018) of Saharan dust events in the Carpathian basin, Varga (2020) showed that such events occur mostly in spring and summer and can be classified in three main synoptic meteorological types. The majority of the events (67.4%) were associated with deep troughs over Western Europe and northwestern Africa and subtropical highs, underlining the results of Barkan et al. (2005). The remaining events were associated with Central Mediterranean cyclones (24.8%) and dust-loaded air masses approaching from northwestern directions (7.8%).

Although it is well known that Saharan dust impact is highest for Southern Europe and the Mediterranean region due to the closer vicinity to the source region (Goudie & Middleton, 2001; Pey et al., 2013), significant impacts, sometimes reaching as far north as Scandinavia (Franzén et al., 1995) or even the Arctic (Barkan & Alpert, 2010; Francis et al., 2018), can be observed as well. Thus, the Alpine region might be affected on a regular basis. Another frequent transport path of Saharan dust is towards West over the Atlantic to the Caribbean, North America, and the Brazilian rain forest (Arimoto et al., 1995; Prospero, 1996; van der Does et al., 2018; Yu et al., 2015).

In situ case studies offer the possibility of investigating methodologies to detect and identify Saharan dust events based on ambient air sampling and measurements of aerosol composition and aerosol properties. Such observations provide a starting point essential for climatological studies or for the evaluation of increased particulate matter (PM) concentrations. In this respect, contributions from natural events can be subtracted from observed PM loads and thus do not account for the air quality limit values as set, for example, by the European Commission. Also, for process-oriented studies regarding, for example, the impact of dust events on cloud formation processes, deposition chemistry, or the radiation budget, regular observations are needed. In situ case studies from the Alps are rare and are almost exclusively limited to the high alpine region. This is due to the fact that the Saharan dust “fingerprint” is less pronounced in the lower mixing layer but is more significant at the high mountain sites or even in the free troposphere due to a lower background, although dust events occur with equal frequency at lower altitudes as well (Flentje et al., 2015).

Schwikowski et al. (1995) studied an outstanding Saharan dust event in March 1990 at the high alpine site of Jungfraujoch, Switzerland. A meteorological analysis revealed the transport of Saharan dust-loaded air masses from the Eastern Atlantic to Central Europe. The aerosol showed a 30 times higher coarse particle number than the annual average, accompanied with a depletion in ultrafine particles. The number of fine particles was similar to numbers observed during the whole month. The unusual depletion in ultrafine particles is explained by coagulation processes in the Saharan dust air mass and heterogeneous reactions on the surface of the dust particles. Scavenging ratios were derived for a snowfall event within the time period of the Saharan dust event and showed that the scavenging ratios were enhanced for most of the components during this event, particularly for ammonium and sulfate, most probably due to heavy riming.

Coen et al. (2004) reveal a regular Saharan dust input at the high alpine stations of Jungfraujoch (Switzerland). Therein they present a methodology to identify mineral dust events based on the optical aerosol properties such as the wavelength dependence of the scattering and absorption coefficients as well as of the single scattering albedo. Such an identification method is especially useful for investigations regarding the frequency of the occurrence of Saharan dust episodes and interpretations of other aerosol parameters such as particle mass, particle numbers, or particle size distributions, especially at background sites. The conclusion that the identified mineral dust events originate from the Sahara is drawn based on meteorological evaluations and trajectory analysis. The investigation of Coen et al. (2004) analyzing the period from March 2001 to December 2002 showed that Saharan dust events are more frequent from March to June as well as from October to November compared to the rest of the year. The approach was also used by Schauer et al. (2016) for the identification of Saharan dust events at the Global Atmosphere Watch (GAW) station Hoher Sonnblick (Austria) and a differentiation of dust events and emissions from wildfires.

The study of Baumann-Stanzer et al. (2018) presents a stepwise approach for the investigation of Saharan dust episodes based on aerosol measurements at three mountain top sites in the Eastern Alps combined with aerosol profiles obtained from ceilometer measurements, source region analysis, and coupled WRF-Chem (Weather Research and Forecasting-Chemistry) meteorological and chemical modeling. They used the outstanding Saharan dust event in April 2016 as a case study, featuring three sub-episodes. The source region analysis showed that the local Saharan dust source region varied significantly throughout the sub-episodes, although no clear difference was detected in the physical aerosol properties. While the onset of the Saharan dust event and the maximum aerosol mass peak as well as the end of the event was accurately forecasted by the WRF-Chem model for two sub-episodes, for one sub-episode, the model and observations for the aerosol mass and the temporal timing differ significantly.

Studies from other mountain stations in Southern and Central Europe report on Saharan dust events as well, with similar results. Flentje et al. (2015) present an inventory of Saharan dust events from 1997 to 2013 at the GAW station Hohenpeißenberg, a pre-Alpine hill (300 m a.s.l.) in Germany, representative for the rural central European conditions. They used daily in situ aerosol measurements for the identification of days influenced by Saharan dust. They also found a seasonality with spring and early autumn maxima, and that the presence of Saharan dust typically coincides with a marked concentration jump of particles in the size range of 1–2 µm of the optical particle volume distribution, with mostly negligible contributions from other coarse particles. They compared their results with Saharan dust events detected at the Schneefernerhaus, a high alpine station on the Zugspitze at the northern ridge of the German Alps, and found that about 90% of the Saharan dust events identified between 2011 and 2013 at Hohenpeißenberg were detected within ±1 day at the Schneefernerhaus as well, while 96% of Saharan dust events at the Schneefernerhaus were accompanied within ±1 day by an event at Hohenpeißenberg. Duchi et al. (2016) performed a long-term investigation of Saharan dust transport events at the GAW station of Mount Cimone (2165 m a.s.l.) in the Apennines in Italy over an 11-year period from 2002 to 2012. They identified Saharan dust days based on coupling the measured in situ coarse aerosol particle number concentration with an analysis of modeled back trajectories tracing the origin of air masses from North Africa. Again, a seasonal cycle with highest frequency from spring to autumn was observed, as well as that the Saharan dust events strongly affected the variability of coarse aerosol particles, with an optical particle diameter larger than 1 µm. Brattich et al. (2015) investigated a single outstanding Saharan dust event at Mount Cimone occurring between March 13 and March 15, 2004. During the event, PM10 reached a maximum concentration of 80 µg/m³ compared to the mean and median values of 8.0 µg/m³ and 6.5 µg/m³ between 1998 and 2011. In addition, a clear increase of particles in all the optical particle diameter ranges from 0.3 µm up to 5.0 µm was observed.

Saharan Dust Deposition in the European Alps

Saharan dust is removed from the atmosphere by either dry or wet deposition. Kok et al. (2012) summarized the physics of wind-blown sand and dust and stated that the dry deposition process via gravitational settling is the main process for removing large particles with more than 5 µm in diameter and mostly dominates close to the source region, whereas the removal of the Saharan dust particles from the atmosphere via wet deposition, including both in-cloud scavenging in which dust aerosols serve as cloud condensation or ice nuclei and subsequently precipitate, and below-cloud scavenging in which precipitating raindrops collect dust aerosols, generally dominates for particles smaller than 5 µm in diameter. Studies on Saharan dust deposition in the Alps conclude that dry deposition is negligible, even for coarse particles, and that wet deposition or precipitation scavenging is the main deposition process (Schwikowski et al., 1995; Wagenbach et al., 1996).

Saharan dust deposition in the Alps is a well-known phenomenon (see figure 2) and is studied using, for example, ice cores, which serve as an archive for such events. In addition, marked Saharan dust layers can be used for the dating of the ice cores, such as the outstanding Saharan dust event between 1780 and 1800, as reported in Thevenon et al. (2009), especially when other reference layers are missing. Wagenbach and Geis (1989) investigated ice cores from the Colle Gnifetti glacier, spanning the period from 1936 until 1982, together with snow pit samples for the total mineral dust record and the particle size distribution. They state that Saharan dust accounts for two thirds of the mean annual mineral dust flux.

Figure 2. Left: The high alpine meteorological observatory at Mount Sonnblick, Hohe Tauern, Austria, covered in hoarfrost with extraordinary Sahara dust deposits in November 2002 (© ZAMG/Lug Raser). Right: Snow profile from the Goldbergkees glacier in the vicinity of the Sonnblick observatory from May 2014, featuring two clearly visible Saharan dust layers (© ZAMG).

De Angelis and Gaudichet (1991) studied the Saharan dust deposition over Mont Blanc in the French Alps from 1955 until 1985 based on aluminum and calcium records along a continuous ice core at Col du Dôme (4270 m a.s.l.). They estimated a total input of 885 µg/cm² and 735 µg/cm² for aluminum and calcium, respectively, of which the Saharan dust input was 454 µg/cm² and 161 µg/cm². The long-term evaluation reveals that Saharan dust inputs seem to have significantly increased since the early 1970s, with very high inputs after 1980. Thevenon et al. (2009) analyzed black carbon and mineral dust aerosols deposited in an 80 m-long ice core from the Colle Gnifetti glacier in the Monta Rosa Massif in the Swiss Alps (4455 m a.s.l.) spanning the last millennium. They found an enhanced Saharan dust deposition around the years 1200–1300, 1430–1520, 1570–1690, 1780–1800, and after 1870, suggesting a connection between the occurrence of such events in the Alps with changes in the North Atlantic oscillation (NAO). The observed long-lasting increase of Saharan dust deposition after 1870 might be related to recent human-induced land degradation in the Sahel or to climate response to the anthropogenic forced unusual phases of the NAO during winter and revealed that the Saharan dust transport to the Southern Alps is primarily controlled by large-scale climatic patterns. Sodemann et al. (2006) conclude from two Saharan dust events in March and October 2000, detected in an ice core from the Piz Zupo, Swiss Alps, that different transport patterns rather than differences in the source region lead to major differences in the chemical signature of the deposited dust.

Saharan dust deposition estimates from wet deposition measurements are provided within a long-term study from 1987 to 2017 by Greilinger et al. (2018) using high alpine snow packs sampled in the Austrian Alps. Based on an approach of Rogora et al. (2004), a pH above 5.6 and calcium concentrations higher than 10 µEq/L were used as a “chemical footprint” to identify Saharan dust-affected snow samples. They found a contribution of 14% of Saharan dust affected samples to the mean overall annual ion deposition averaged over all years. In contrast, the interannual variability of the overall annual ion deposition that can be assigned to Saharan dust-affected samples was found to be rather high, with contributions up to 55%, depending on the strength and frequency of the Saharan dust events as well as on the sampling size of the snow samples within the respective year. Especially magnesium and calcium depositions were strongly affected by the dust input and the pH of the samples was markedly increased, with values up to 7.17 in desert dust-affected layers, while the median pH value of all samples was 5.40. This finding underlines the results from other studies that Saharan dust serves as an alkaline input on wet deposition chemistry (De Angelis & Gaudichet, 1991; Rogora et al., 2004). Maupetit and Delmas (1994) and Nickus et al. (1997) investigated the ionic composition of high alpine snow packs from 1989 to 1991 and 1991 to 1993, respectively. While the former study focused on four glaciers in the French Alps only, the latter study used 17 different high alpine sampling sites reaching from France to Austria to study the regional distribution. Both studies found that the high alpine snow was slightly acidic but was episodically affected by Saharan dust events serving an alkaline input, thereby partially or even totally neutralizing the accumulated acidity. They also agreed on the episodic character of Saharan dust deposition leading to a high variability of the soil dust-related ions, with calcium leading the way between the sites and years. Both studies stated that Saharan dust-affected snow can also feature an increased input of sulfate, nitrate, or ammonium deposition, likely to be associated with the influence of Saharan air masses with polluted air from lower altitudes during transport. The regional study of Nickus et al. (1997) also showed that ion concentrations were generally higher in the eastern part of the Alps, whereas the pattern was reversed for calcium, with higher values in the west. The introduction of sulfate via the Saharan dust deposition was found to be rather small by Greilinger et al. (2018) because anthropogenic sources clearly dominate, a finding contradictory to those of Maupetit and Delmas (1994) and Nickus et al. (1997).

Although various ice core studies and snow chemistry studies in the European Alps exist, a detailed investigation on the amount of Saharan dust related to wet or dry deposition is still lacking but might be of interest regarding its impact, especially in remote high alpine areas.

Impact of Saharan Dust in the European Alps

Airborne mineral dust in general can cause several environmental and climatic impacts. It affects the radiative properties of the atmosphere due to scattering and absorption of solar and terrestrial radiation on the particle itself. Thereby the dust concentration, the particle size distribution, the shape and chemical composition, as well as the height and structure of the dust layer are the crucial factors. Additionally, airborne mineral dust particles modify the microphysical and hence the radiative properties, the amount and the lifetime of clouds, and act as cloud condensation nuclei (CCN) and ice nuclei (IN), thus playing a complex role in cloud and precipitation formation (Andreae et al., 2001; Boucher et al., 2013; Seinfeld & Pandis, 2006).

The influence of Saharan dust transport on the alpine region and its impact on cloud properties and precipitation formation, on snow chemistry, on high alpine ecosystems such as the nutrient input of alpine lakes or the impact on the bacterial community, as well as on the radiative effect by altering the albedo of snow and glacier surfaces is summarized here.

Impact on CCN Formation and IN Properties

Cloud droplets in the atmosphere can form when aerosols are present in supersaturated air. Those aerosol particles that are able to activate and become cloud droplets are referred to as cloud condensation nuclei (CCN) (Seinfeld & Pandis, 2006).

Jurányi et al. (2010) investigated the CCN number concentration at the high alpine site of Jungfraujoch in May 2008. They performed a closure study between measured CCN and CCN predictions based on the dry number size distribution and bulk chemical composition data of the particles using a simplified Köhler theory, thereby neglecting mineral dust in their measurement setup (measurements of particles smaller than 1 µm only and no measurement of refractory particles). They showed that ignoring the mineral dust component in the bulk chemical composition does not impair CCN prediction during an observed Saharan dust event lasting for 3 days. On the contrary, they stated that if mineral dust would be included in the measurement of the bulk chemical composition, the CCN prediction would be biased because coarse mode mineral dust gives a major contribution to total mass, whereas the contribution of mineral dust to particle number concentration at the Aitken and accumulation mode sizes in the CCN cut-off range is negligible. Different from CCN formation where the effect of Saharan dust might be less relevant, dust particles are well known to be good IN, thereby enhancing precipitation formation via the Bergeron–Findeisen process by enhancing the freezing of cloud droplets and subsequently aggregation and riming, a process most relevant in mixed-phase clouds (Ansmann et al., 2005; DeMott et al., 2003).

Zubler et al. (2011) evaluated the microphysical processes in mixed-phase clouds for different thermodynamic conditions in a statistical sense by 270 pairs of two-dimensional simulations with clean and polluted remote continental aerosol configurations typical for the Alpine region and Switzerland. The main focus of this study was the investigation of the importance of the ice phase for precipitation formation in the presence of black carbon and mineral dust, both efficient IN. Under warm and moist air advection from the south, accompanied by strong winds (i.e., during conditions more frequent during late spring and early summer), mineral dust would be inactivated as IN and would preferably act as CCN, increasing the number of cloud droplets and potentially suppressing precipitation. Under cold conditions, more cloud droplets would freeze as a consequence of the lower temperatures, thus the presence of mineral dust reduces the number of cloud droplets by riming and aggregation processes and shows a tendency to increase precipitation. They concluded that the amount of cloud droplets and precipitation is not only determined by the amount of aerosols, but also by the ice phase and particularly the freezing of cloud droplets in the presence of IN, such as Saharan dust, and thus depends on ambient temperature, wind speed, and humidity as well. This reveals that the presence of IN such as Saharan dust influences the formation of cloud droplets and hence cloud formation.

In situ studies of Klein et al. (2010) and Chou et al. (2011) underline the aforementioned model study of Zubler et al. (2011). Klein et al. (2010) measured the number concentration of IN on a daily basis at the Taunus Observatory at Mount Kleiner Feldberg, a low mountain range (825 m a.s.l.) in central Germany, and investigated these observations in detail for May and June 2008 when a strong Saharan dust event occurred. They found that the IN number concentration peaked during the event. The fact that mineral dust serves as the dominant contributor to IN is supported by single particle analysis as well as comparison of the activated fraction of aerosol particles with laboratory data of Saharan dust aerosols analyzed at approximately the same nucleation conditions. Based on their results, they suggested that Saharan dust may be a main constituent of ice nucleating aerosols in Central Europe. Chou et al. (2011) measured the number concentration of IN at the high alpine research station Jungfraujoch (3580 m a.s.l.) in March and June 2009. IN number concentration increased during the presence of Saharan dust and a high correlation was observed, induced by a higher concentration of larger particles.

Impact on Alpine Ecology

The effect of Saharan dust deposition on the biogeochemical cycles in seawater and in the Amazonas has been widely studied. Thus, phosphorus especially, iron, and, to a lesser extent, nitrogen are most important for seawater (Field et al., 2010; Gruber & Sarmiento, 1997; Schulz et al., 2012), stimulating the productivity of oceanic plankton, whereas iron, sodium, calcium, potassium, and magnesium act as fertilizer or micronutrients for the Amazonas basin (Rizzolo et al., 2017; Swap et al., 1992). Although various studies of Saharan dust on microbiology, nutrient supply, acid neutralization, and geochemistry in Europe and the Mediterranean exist (Avila & Peñuelas, 1999; Avila & Roda, 1991; Avila et al., 1998; Gallisai et al., 2014; Guerzoni et al., 1999; Guieu, 2002; Herut et al., 2005; Lequy et al., 2013; Riccio et al., 2009; Roda et al., 1993), the effect on alpine biogeochemistry is still poorly investigated. The aforementioned ice core studies point out that the Saharan dust aerosol is important for alpine biogeochemical cycles influencing the neutralization of acidic compounds (De Angelis & Gaudichet, 1991; Wagenbach et al., 1996; Wagenbach & Geis, 1989).

For the Alps especially, the ionic composition and deposition load of snow is of high biogeochemical interest because snow serves as an interface where water and nutrient cycles interact (De Angelis & Gaudichet, 1991; Kuhn, 2001). If the chemical composition of high alpine snow is investigated, the main ions analyzed generally are chloride, sulfate, nitrate, ammonium, sodium, potassium, magnesium, and calcium, as well as pH. The origin can be assigned to different sources such as anthropogenic sources, sea salt, or mineral dust (Greilinger et al., 2016; Maupetit & Delmas, 1994).

In the Alps, nutrients are preserved in the snow, which accumulates during the winter, and are released in a rather short time period when significant melt off in spring begins. Thereby an acidifying ionic pulse for downstream ecosystems can be caused by melt water, enriched with inorganic nutrients. Especially high mountain lakes are known to be sensitive to changes in atmospheric deposition because of their rather small size, low temperature, and low weathering of their bedrock. Studies from lakes in the high Alps show how water chemistry in areas of low weathering rocks are highly dependent on atmospheric input, especially changes in acidification, which are of major concern (Marchetto et al., 1995; Psenner, 1989). Thereby the effect of Saharan dust deposition on alpine lakes, especially its contribution to alkalinity and nutrient levels, is still not well understood (Psenner, 1999). Research focusing on the acidity of high alpine catchments is available only before the year 2000, followed by studies focusing more on microbiological impacts.

Beside the potential impact of aeolian dust deposition and, as such, Saharan dust deposition on snow and lake water chemistry and related nutrient input, many microorganisms use aeolian dispersals as transport vehicles to colonize new habitats. It is assumed that microorganisms which are able to survive in the harsh conditions of the Sahara and the atmosphere during transport may also be able to colonize other sites with comparably challenging conditions, such as the high Alps. Chuvochina, Alekhina, et al. (2011) and Chuvochina, Marie, et al. (2011) assessed the bacterial abundance and diversity in snow packs of the Mont Blanc glacier containing Saharan dust deposits during the period 2006–2009. They found higher diversity indices for microbial communities derived from snow samples containing Saharan dust compared to those detected in clean snow samples. Thus, the structure and the microbial community composition associated with four distinct Saharan dust events varied considerably. This phenomenon does not correlate with the time the dust stays in the snow nor with the dust source itself, but was rather associated with differences in the transport history or different conditions of dust mobilization in the epicenter of the respective dust storm. Also, other studies (Hervàs et al., 2009; Meola et al., 2015; Peter et al., 2014) characterized Saharan dust-associated bacteria transported to the European Alps and found distinct differences in the bacterial community composition and structure if Saharan dust-affected samples were compared to nonaffected samples. Interestingly, Meola et al. (2015) found that sporulating bacteria were not enriched in Saharan dust samples, although they featured adaptive strategies to survive the harsh conditions of long-range airborne transport and the cold high alpine environment. Instead, they found a higher species enrichment of pigment-producing bacteria that were adapted to cope with UV radiation and desiccation stress. Weil et al. (2017) showed that the immigrated microbial passengers introduced to high alpine snow packs via Saharan dust can favor a rapid microbial contamination of sensitive habitats, especially after snowmelt. During the warm season, the number of microorganisms introduced to the Alps via Saharan dust deposition is likely to be diluted and dispersed by surface runoff and infiltration. But during winter, microbes accumulate in the snow pack and then, during melt off, are released in masses in a rather short time period and on a limited surface.

Due to the potential to provide nutrient input and to serve as vehicles for microbes, the occurrence of cryoconites, a hotspot for biodiversity and biological activities on glaciers, needs to be mentioned. Cryoconite is a characteristic sediment on glaciers, constituted of mineral dust and organic matter which can accumulate in typical holes in the ablation areas of glaciers (i.e., cryoconite holes) or can be dispersed on their surface. While most of the literature is focused on Arctic, Antarctic, and Asian glaciers (Ambrosini et al., 2017; Cameron et al., 2012; Gokul et al., 2016; Takeuchi et al., 2001; Uetake et al., 2016), only a few studies are available for the European Alps, highlighting the sources and temporal variability of the bacterial communities in cryoconites (Edwards et al., 2013; Franzetti et al., 2017b, 2017a; Pittino et al., 2018), as well as the impact on snow and ice reflectance (Di Mauro et al., 2017), an effect thoroughly discussed in the section “Impact on Radiative Properties of Snow and Ice.”

The overall conclusion of the studies presented here is that for efficient research on the long-distance dispersal of microbes by large-scale desert dust events and their impact on distant ecosystems, the impact of Saharan dust on providing nutrients, and its impact on high alpine snow and lake water chemistry, interdisciplinary research and a collaborative effort on the part of geologists, atmospheric chemists, and microbiologists is strongly recommended.

Impact on Radiative Properties of Snow and Ice

Atmospheric impurities deposited on the surface of glaciers or snow-covered areas have a fundamental impact on respective surface energy balances by increasing the absorption of solar radiation due to a darkening of the surface, often referred to as “surface albedo reduction.” The increased absorption of solar radiation induces a warming of the surface, which subsequently accelerates melt off. This results in a positive feedback mechanism, the “snow or ice albedo feedback,” a self-enhancing acceleration of the melting process feedback (Warren & Wiscombe, 1980). This feedback process impacts not only the radiative properties and the timing of snowmelt, but may also impact the chemical properties of the snow, snow hydrology, glacier mass balance and hence glacier retreat, as well as the vegetation phenology linked to the migration and breeding of interacting animal species (see Di Mauro et al., 2015, 2019, and referenced therein).

The most relevant impurities influencing snow optical properties are the “light absorbing particles (LAPs)” or “light absorbing impurities (LAIs),” with carbonaceous particles, volcanic ash, and mineral dust leading the way, but may also include soil organics, algae, and other biological organisms and constituents (e.g., Di Mauro et al., 2015; Tuzet et al., 2017, and references therein). Besides deposited impurities, the physical properties of the snow microstructure influence snow albedo. Enhanced melting due to the darkening of the snow caused by the deposition of impurities also accelerates the growth and metamorphism of snow grains. The size and shape of the snow crystal are particularly crucial parameters leading to a further reduction in the albedo, mainly in the near infrared, by decreasing reflected radiance. In contrast, light-absorbing particles mainly reduce the albedo for the visible wavelength (Gabbi et al., 2015). Thus, addressing the impact of light-absorbing particles on the optical properties of ice or snow-covered areas must take the snow’s physical properties into account.

The impact of light-absorbing particles on the snow albedo and melting has already been studied in the Colorado river basin, the Himalayas, Greenland, Iceland, and the Caucasus (see Gabbi et al., 2015, and references therein). Also snow radiative transfer models accounting for light-absorbing particles are available, estimating snow albedo and light penetration into snow for given physical snow properties and a given impurity content. Still, for the simulation of the radiative properties of an evolving snow pack and to account for snow albedo feedback, the radiative transfer model needs to be coupled with a detailed snow evolution model. Tuzet et al. (2017) performed a modeling study simulating the radiative impacts of LAIs using the Crocus model and evaluated the model results with field measurements at the Col de Porte experimental site in the French Alps during the 2013 to 2014 accumulation period. Depending on the configuration chosen for the LAI parameters in the model, the complete snow melt out date advanced 6-9 days in comparison with pure snow simulation, which is of crucial importance for hydrological concerns. Estimates of the direct and indirect proportion of LAI radiative forcing shows an 85% direct impact due to the darkening of the snow and a 15% indirect impact related to an enhanced snow metamorphism. In contrast to the study by Tuzet et al. (2017), their latest study (Tuzet et al., 2020) shows that the indirect effect of LAPs on radiative forcing is negligible for the two snow seasons (2016–2017 and 2017–2018) considered and is mainly attributed to the meteorological condition and on the timing of LAP deposition. To evaluate model performances, field studies on the impact of LAPs and mineral dust on the optical snow and ice properties are essential and needed by the model community. It is also necessary to further investigate the coupling between LAP deposition and snowpack evolution to better understand the spatiotemporal variability of their indirect impact on radiative forcing.

The first investigations of the impact of mineral dust on snow date back to the last century when Jones (1913) estimated a shift in snowmelt toward an earlier onset and melt off due to dust deposition in the United States. Since then, several studies on the impact of dust on snow melting were initiated, mostly in the United States, using aerial satellites and AWS data (e.g., Painter et al., 2012, 2013) due to the proximity of arid regions to the Rocky Mountains. Studies from Painter et al. (2007), Reynolds et al. (2014), and Skiles et al. (2012) found that dust deposition caused an earlier snowmelt of up to 51 days, at the most, strongly impacting water supplies in the surrounding area. Despite increasing interest regarding the impact of mineral dust deposition on snowmelt, on snow optical properties, and on snow dynamics, studies that evaluate this impact for the European Alps are rare. Still, the importance of mineral dust deposition for the European Alps can be stressed by the fact that regular mineral dust transport from the Sahara occurs.

The work of Di Mauro et al. (2015) was the first study investigating the radiative impact of Saharan dust in the Alps using ground-hyperspectral measurements, unmanned aerial vehicle (UAV) measurements, and satellite data in March 2014 at the Artavaggio plains in Lecco, Italy after a strong Saharan dust event a few weeks earlier. Based on their findings that the dust reduced the spectral albedo, especially from 350 nm to 800 nm, they defined a novel spectral index, the snow darkening index (SDI), which was found to be highly correlated with mineral dust concentrations. Based on their results, they were able to estimate a radiative forcing of up to 153 W/m² for the most concentrated sampling area. In a subsequent contribution, Di Mauro et al. (2019) evaluated the impact of Saharan dust depositions on the melt dynamics of high alpine snow packs in the Aosta Valley, Italy during three accumulation periods from 2013 to 2016. They compared observed and simulated snow depth using the Crocus snow model and found that the disappearance of snow was anticipated up to 38 days, out of 7 months of typical snow cover duration, for the strongest dust deposition event in the investigated time period, which was the accumulation period 2015–2016. During the other accumulation periods, the snow melt out date was found to be 18 and 11 days earlier. Another study by Gabbi et al. (2015) investigated the long-term effect of Saharan dust and black carbon on the surface albedo and on the glacier mass balance over the period from 1914 to 2014 based on firn and ice core measurements of two sites in the Swiss Alps (Colle Gnifetti, Monte Rosa and Fiescherhorn, Bernese Alps). They employed a combined mass balance and snow–firn layer model to study the effects of melt and accumulation processes due to the impurity concentration on the surface and thus on the albedo and mass balance. They found that the albedo was lowered by the impurities and consequently the annual melt was increased by 15–19%; the mean annual mass balance was reduced by 280 to 490 mm water equivalent. An additional study by Oerlemans et al. (2009) on the impact of mineral dust on the radiative properties of snow and ice exists, although the authors focused on mineral dust introduced to the Morteratsch glacier in Switzerland from exposed side moraines rather than Saharan dust input, but they mentioned that occasionally introduced Saharan dust might be relevant as well.

Motivation and Outlook

Over centuries, scientists recognized the transport and deposition of windblown dust particles, or “aeolian dust.” In 1846, Charles Darwin wrote an article on “fine dust which often falls on vessels in the Atlantic Ocean,” speculating that the dust originated from Africa (Darwin, 1846). In the beginning of the 20th century, the impact of dust deposition on an earlier onset of snowmelt was recognized (Jones, 1913). Thus, research on dust deposition poses a direct link to the increasing glacier retreat as it is presently observed in a warming climate.

The importance of mineral dust began to raise scientific interest in the early 1980s. During that time, the investigation of the deposition of atmospheric pollutants gained more and more importance due to the prominent issue of acid rain. Studies investigating wet deposition measurements in Europe reported on the neutralizing capacity of mineral dust compared to acid components. Ice core studies from the Alps were increasingly used during this time to underline a regular occurrence of mineral dust inputs.

Since then, an increasing number of studies on the impact of mineral or Saharan dust in Europe is highlighting the frequent deposition of Saharan dust and investigating related environmental and climate aspects. While long-term studies mainly focus on the long-term impact of Saharan dust, case studies are used to develop and improve methodologies to detect and identify Saharan dust events or to investigate processes such as the impact of dust particles on ice and cloud formation and precipitation, on snow chemistry, on the high alpine ecology, and on radiative effects.

Previous work on the occurrence and deposition of Saharan dust in the European Alps prepared the way for ongoing and future research. Core questions will be related to model projections of Saharan dust emission, transport, and deposition in a changing climate. This is reflected in the evolving real-time dust forecasts for Europe, including the alpine region. Respective measurements at the receptor sites will allow verification of possible changes in frequency and intensity of dust events. Changes in snowmelt and the mass balance of glaciers as well as the observed glacier retreat set by the warming climate will influence investigations related to snow and ice albedo feedback. The contribution of Saharan dust to the formation of precipitation and its impact on alkalinity and nutrient levels of high alpine ecosystems compared to local sources needs further investigation as well as continuing studies on the dispersal of microbes via dust events.

Overall, a broad interdisciplinary and international research approach is needed to further understand the phenomenon of long-range transported mineral dust and its impacts in the European Alps.

Further Reading

  • Andreae, M. O., Annegarn, H., Barrie, L., Feichter, J., Hegg, D., Jayaraman, A., Leaitch, R., Murphy, D., Nganga, J., & Pitari, G. (2001). Aerosols, their direct and indirect effects. In J. T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell, & C. A. Johnson (Eds.), Climate change 2001: The scientific basis. Cambridge University Press.
  • Boucher, O., Randall, D., Artaxo, P., Bretherton, C., Feingold, G., Forster, P., Kerminen, V.-M., Kondo, Y., Liao, H., Lohmann, U., Rasch, P., Satheesh, S. K., Sherwood, S., Stevens, B., & Zhang, X. Y. (2013). Clouds and aerosols. In T. F. Stocker, D. Qin, G.-K. Plattner, M. M. B. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, & P. M. Midgley (Eds.), Climate change 2013: The physical science basis. Cambridge University Press.
  • Israelevich, P., Ganor, E., Alpert, P., Kishcha, P., & Stupp, A. (2012). Predominant transport paths of Saharan dust over the Mediterranean Sea to Europe. Journal of Geophysical Research: Atmospheres, 117, D02205.
  • Knippertz, P., & Stuut, J.-B. W. (Eds.). (2014). Mineral dust: A key player in the Earth system. Springer Nature.
  • Kok, J. F., Parteli, E. J. R., Michaels, T. I., & Karam, D. B. (2012). The physics of wind-blown sand and dust. Reports on Progress in Physics, 75(10), 106901.
  • Prospero, J. M. (1996). Saharan dust transport over the North Atlantic Ocean and Mediterranean: An overview. In S. Guerzoni & R. Chester (Eds.), The impact of desert dust across the Mediterranean (pp. 133–151). Springer Nature.
  • Seinfeld, J. H., & Pandis, S. N. (2006). Atmospheric chemistry and physics: From air pollution to climate change (2nd ed.). Wiley.

References