Littérature scientifique sur le sujet « LAI, snow, glaciers, mineral dust »

Supra-glacial material, including light-absorbing impurities (LAI) such as mineral dust of crustal and soil origin, black carbon, algae and cryoconite, reduce the reflectance of snow and glacier ice. The reduction depends on the amount of LAI and their physical and chemical properties, which vary spatially and temporally. Spectral reflectance data and snow and ice samples, containing LAI, were collected in the ablation zone of the Djankuat Glacier, Central Caucasus, Russia. The spectra of the samples containing mineral dust transported from deserts were characterized by negative visible near-infrared gradients and were different from the spectra of clean aged snow and exposed glacier ice and from the samples containing mineral dust produced locally. Geochemical and mineralogical analysis using X-ray diffraction and X-ray fluorescence spectrometry showed that samples containing desert dust were characterised by a high proportion of clay materials and such minerals as smectites, illite–smectites and palygorskite and by a smaller size of mineral particles. They were enriched in chromium, zinc and vanadium. The latter served as an indicator of dust transport over or origin from the oil-producing regions of the Middle East. There was a strong negative correlation between the amount of organic matter and mineral dust in the collected samples and the albedo of surfaces from which the samples were collected. The results suggested that organic matter reduced albedo more efficiently than mineral dust. The study highlighted the importance of supra-glacial material in changing the surface reflectivity of snow and glaciers in the Caucasus region.

ABSTRACTLight-absorbing impurities (LAI) can darken snow and ice surfaces, reduce snow/ice albedo and accelerate melt. Efforts to allocate the relative contribution of different LAI to snow/ice albedo reductions have been limited by uncertainties in the optical properties of LAI. We developed a new method to measure LAI spectral reflectance at the submicron scale by modifying a Hyperspectral Imaging Microscope Spectrometer (HIMS). We present the instrument's internal calibration, and the overall small influence of a particle's orientation on its measured reflectance spectrum. We validated this new method through the comparison with a field spectroradiometer by measuring different standard materials. Measurements with HIMS at the submicron scale and the bulk measurements of the same standard materials with the field spectroradiometer are in good agreement with an average deviation between the spectra of 3.2% for the 400–1000 nm wavelength range. The new method was used (1) to identify BC (black carbon), mineral dust including hematite and the humic substances present in an environmental sample from Plaine Morte glacier and (2) to collect the individual reflectance spectra of each of these types of impurity. The results indicate that this method is applicable to heterogeneous samples such as the LAI found in snow and ice.

Abstract. Light-absorbing impurities (LAIs) deposited in snow have the potential to substantially affect the snow radiation budget, with subsequent implications for snow melt. To more accurately quantify the snow albedo, the contribution from different LAIs needs to be assessed. Here we estimate the main LAI components, elemental carbon (EC) (as a proxy for black carbon) and mineral dust in snow from the Indian Himalayas and paired the results with snow samples from Arctic Finland. The impurities are collected onto quartz filters and are analyzed thermal–optically for EC, as well as with an additional optical measurement to estimate the light-absorption of dust separately on the filters. Laboratory tests were conducted using substrates containing soot and mineral particles, especially prepared to test the experimental setup. Analyzed ambient snow samples show EC concentrations that are in the same range as presented by previous research, for each respective region. In terms of the mass absorption cross section (MAC) our ambient EC surprisingly had about half of the MAC value compared to our laboratory standard EC (chimney soot), suggesting a less light absorptive EC in the snow, which has consequences for the snow albedo reduction caused by EC. In the Himalayan samples, larger contributions by dust (in the range of 50 % or greater for the light absorption caused by the LAI) highlighted the importance of dust acting as a light absorber in the snow. Moreover, EC concentrations in the Indian samples, acquired from a 120 cm deep snow pit (possibly covering the last five years of snow fall), suggest an increase in both EC and dust deposition. This work emphasizes the complexity in determining the snow albedo, showing that LAI concentrations alone might not be sufficient, but additional transient effects on the light-absorbing properties of the EC need to be considered and studied in the snow. Equally as imperative is the confirmation of the spatial and temporal representativeness of these data by comparing data from several and deeper pits explored at the same time.

Abstract. Mineral dust is a major light-absorbing aerosol, which can significantly reduce snow albedo and accelerate snow/glacier melting via wet and dry deposition on snow. In this study, three scenarios of internal mixing of dust in ice grains were analyzed theoretically by combining asymptotic radiative transfer theory and (core–shell) Mie theory to evaluate the effects on absorption coefficient and albedo of the semi-infinite snowpack consisting of spherical snow grains. In general, snow albedo was substantially reduced at wavelengths of <1.0 µm by internal dust–snow mixing, with stronger reductions at higher dust concentrations and larger snow grain sizes. Moreover, calculations showed that a nonuniform distribution of dust in snow grains can lead to significant differences in the values of the absorption coefficient and albedo of dust-contaminated snowpack at visible wavelengths relative to a uniform dust distribution in snow grains. Finally, using comprehensive in situ measurements across the Northern Hemisphere, we found that broadband snow albedo was further reduced by 5.2 % and 9.1 % due to the effects of internal dust–snow mixing on the Tibetan Plateau and North American mountains. This was higher than the reduction in snow albedo caused by black carbon in snow over most North American and Arctic regions. Our results suggest that significant dust–snow internal mixing is important for the melting and retreat of Tibetan glaciers and North American mountain snowpack.

AbstractWind-blown mineral aerosol dust derived from the crustal surface is an important atmospheric component affecting the Earth’s radiation budget. Deposition of water-insoluble dust was determined in snow deposited on Ürümqi glacier No. 1, Haxilegen glacier No. 51 and Miaoergou glacier, eastern Tien Shan, China. Analysis of the horizontal distribution of snow depth and concentration, and flux of dust particles in the snow cover suggests that dust deposition differs on each of these glaciers as the atmospheric environment changes from west to east. Mean mass concentrations of micro-particles in the size range 0.57–26 μm diameter at the three locations are respectively 969, 1442 and 3690 μg kg−1, with an increasing trend from west to east. Dust layers in the snow cover contain Na- and Ca-rich materials typically found in central Asian dust particles. Volume size distributions of dust particles in the snow showed single-modal structures having volume median diameters of 3–22 μm. Dust profiles in the snow cover over the past 4 years reveal frequent, sporadic high dust concentrations with a large year-to-year variability, implying that dust deposition in the eastern Tien Shan is very sensitive to atmospheric environment change.

AbstractSources and implications of black carbon (BC) and mineral dust (MD) on two glaciers on the central Tibetan Plateau were estimated based on in situ measurements and modeling. The results indicated that BC and MD accounted for ~11 ± 1% and 4 ± 0% of the albedo reduction relative to clean snow, while the radiative forcing varied between 11 and 196 and 1–89 W m−2, respectively. Assessment of BC and MD contributions to the glacier melt can reach up 88 to 434 and 35 to 187 mm w.e., respectively, contributing ~9–23 and 4–10% of the total glacier melt. A footprint analysis indicated that BC and MD deposited on the glaciers originated mainly from the Middle East, Central Asia, North China and South Asia during the study period. Moreover, a potentially large fraction of BC may have originated from local and regional fossil fuel combustion. This study suggests that BC and MD will enhance glacier melt and provides a scientific basis for regional mitigation efforts.

Abstract. Light-absorbing dissolved organic carbon (DOC) constitutes a major part of the organic carbon in glacierized regions, and has important influences on the carbon cycle and radiative forcing of glaciers. However, few DOC data are currently available from the glacierized regions of the Tibetan Plateau (TP). In this study, DOC characteristics of a medium-sized valley glacier (Laohugou Glacier No. 12, LHG) on the northern TP were investigated. Generally, DOC concentrations on LHG were comparable to those in other regions around the world. DOC concentrations in snow pits, surface snow and surface ice (superimposed ice) were 332 ± 132, 229 ± 104 and 426 ± 270 µg L−1, respectively. The average discharge-weighted DOC of proglacial stream water was 238 ± 96 µg L−1, and the annual DOC flux released from this glacier was estimated to be 6949 kg C yr−1, of which 46.2 % of DOC was bioavailable and could be decomposed into CO2 within 1 month of its release. The mass absorption cross section (MAC) of DOC at 365 nm was 1.4 ± 0.4 m2 g−1 in snow and 1.3 ± 0.7 m2 g−1 in ice, similar to the values for dust transported from adjacent deserts. Moreover, there was a significant relationship between DOC and Ca2+; therefore, mineral dust transported from adjacent arid regions likely made important contributions to DOC of the glacierized regions, although contributions from autochthonous carbon and autochthonous/heterotrophic microbial activity cannot be ruled out. The radiative forcing of snow pit DOC was calculated to be 0.43 W m−2, demonstrating that DOC in snow needs to be taken into consideration in accelerating melt of glaciers on the TP.

Abstract. Alpine glacier samples were collected in four contrasting regions to measure supraglacial dust and debris geochemical composition and quantify regional variability. A total of 70 surface glacier ice, snow and debris samples were collected in Svalbard, southern Norway, Nepal and New Zealand. Trace elemental abundances in snow and ice samples were measured via inductively coupled plasma mass spectrometry (ICP-MS). Supraglacial debris mineral, bulk oxide and trace element composition were determined via X-ray diffraction (XRD) and X-ray fluorescence spectroscopy (XRF). A total of 45 major, trace and rare earth elements and 10 oxide compound abundances are reported. Elemental abundances revealed sea salt aerosol and metal enrichment in Svalbard, low levels of crustal dust and marine influences to southern Norway, high crustal dust and anthropogenic enrichment in the Khumbu Himalayas, and sulfur and metals attributed to quiescent degassing and volcanic activity in northern New Zealand. Rare earth element and Al/Ti elemental ratios demonstrated distinct provenance of particulates in each study region. Ca/S elemental ratio data showed seasonal denudation in Svalbard and southern Norway. Ablation season atmospheric particulate transport trajectories were mapped in each of the study regions and suggest provenance pathways. The in situ data presented provides first-order glacier surface geochemical variability as measured in the four diverse alpine glacier regions. The surface glacier geochemical data set is available from the PANGAEA database at doi:10.1594/PANGAEA.773951. This geochemical surface glacier data is relevant to glaciologic ablation rate understanding as well as satellite atmospheric and land-surface mapping techniques currently in development.

Abstract. Black carbon (BC), water-insoluble organic carbon (OC), and mineral dust are important particles in snow and ice which significantly reduce albedo and accelerate melting. Surface snow and ice samples were collected from the Karakoram–Himalayan region of northern Pakistan during 2015 and 2016 in summer (six glaciers), autumn (two glaciers), and winter (six mountain valleys). The average BC concentration overall was 2130 ± 1560 ng g−1 in summer samples, 2883 ± 3439 ng g−1 in autumn samples, and 992 ± 883 ng g−1 in winter samples. The average water-insoluble OC concentration overall was 1839 ± 1108 ng g−1 in summer samples, 1423 ± 208 ng g−1 in autumn samples, and 1342 ± 672 ng g−1 in winter samples. The overall concentration of BC, OC, and dust in aged snow samples collected during the summer campaign was higher than the concentration in ice samples. The values are relatively high compared to reports by others for the Himalayas and the Tibetan Plateau. This is probably the result of taking more representative samples at lower elevation where deposition is higher and the effects of ageing and enrichment are more marked. A reduction in snow albedo of 0.1–8.3 % for fresh snow and 0.9–32.5 % for aged snow was calculated for selected solar zenith angles during daytime using the Snow, Ice, and Aerosol Radiation (SNICAR) model. The daily mean albedo was reduced by 0.07–12.0 %. The calculated radiative forcing ranged from 0.16 to 43.45 W m−2 depending on snow type, solar zenith angle, and location. The potential source regions of the deposited pollutants were identified using spatial variance in wind vector maps, emission inventories coupled with backward air trajectories, and simple region-tagged chemical transport modeling. Central, south, and west Asia were the major sources of pollutants during the sampling months, with only a small contribution from east Asia. Analysis based on the Weather Research and Forecasting (WRF-STEM) chemical transport model identified a significant contribution (more than 70 %) from south Asia at selected sites. Research into the presence and effect of pollutants in the glaciated areas of Pakistan is economically significant because the surface water resources in the country mainly depend on the rivers (the Indus and its tributaries) that flow from this glaciated area.

Abstract. Anthropogenic activities on the Indo-Gangetic Plain emit vast amounts of light-absorbing particles (LAPs) into the atmosphere, modifying the atmospheric radiation state. With transport to the nearby Himalayas and deposition to its surfaces the particles contribute to glacier melt and snowmelt via darkening of the highly reflective snow. The central Himalayas have been identified as a region where LAPs are especially pronounced in glacier snow but still remain a region where measurements of LAPs in the snow are scarce. Here we study the deposition of LAPs in five snow pits sampled in 2016 (and one from 2015) within 1 km from each other from two glaciers in the Sunderdhunga Valley, in the state of Uttarakhand, India, in the central Himalayas. The snow pits display a distinct enriched LAP layer interleaved by younger snow above and older snow below. The LAPs exhibit a distinct vertical distribution in these different snow layers. For the analyzed elemental carbon (EC), the younger snow layers in the different pits show similarities, which can be characterized by a deposition constant of about 50 µg m−2 mm−1 snow water equivalent (SWE), while the old-snow layers also indicate similar values, described by a deposition constant of roughly 150 µg m−2 mm−1 SWE. The enriched LAP layer, contrarily, displays no similar trends between the pits. Instead, it is characterized by very high amounts of LAPs and differ in orders of magnitude for concentration between the pits. The enriched LAP layer is likely a result of strong melting that took place during the summers of 2015 and 2016, as well as possible lateral transport of LAPs. The mineral dust fractional absorption is slightly below 50 % for the young- and old-snow layers, whereas it is the dominating light-absorbing constituent in the enriched LAP layer, thus, highlighting the importance of dust in the region. Our results indicate the problems with complex topography in the Himalayas but, nonetheless, can be useful in large-scale assessments of LAPs in Himalayan snow.

L’obiettivo del mio dottorato è studiare l’impatto delle light-absorbing impurities (LAI) sulla criosfera tramite l’utilizzo di telerilevamento ottico. Le LAI sono particelle atmosferiche, come polveri minerali e black carbon, che possono depositarsi su neve e ghiaccio riducendone l’albedo e favorendone la fusione. L’impatto delle LAI sulla criosfera è stato studiato a livello globale e locale, ma ancora poca letteratura scientifica è dedicata allo studio del fenomeno nelle Alpi Europee. Durante il primo anno è stata sviluppata un’analisi di sensitività di un modello di trasferimento radiativo (SNow, ICe, and Aerosol Radiative model, SNICAR) con l’obiettivo di studiare le proprietà ottiche di neve e ghiaccio. In particolare, questo modello permette di simulare la riflettanza spettrale della neve in funzione di diverse variabili quali: la dimensione dei cristalli di neve [μm], la concentrazione di polveri minerali [ppm] e la loro distribuzione dimensionale [μm], la concentrazione di black carbon [ppb], l’angolo zenitale solare e la densità della neve, usando differenti profili atmosferici. In seguito a questa analisi di sensitività, sono state organizzate alcune campagne sperimentali, con l’obiettivo di misurare la riflettanza della neve in seguito ad un evento naturale di deposizione di LAI, quindi confrontare i dati osservati con quelli simulati con il modello SNICAR. Durante le campagne di misura, sono stati organizzati dei sorvoli con un Unmanned Aerial Vehicle (UAV) su zone coperte da neve nelle Alpi Europee. I dati acquisiti da terra, da UAV e da satellite (Landsat 8 - Operational Land Imager, OLI) sono stati analizzati con l’obiettivo di stimare l’impatto delle polveri minerali sulle proprietà ottiche della neve. In seguito, è stato sviluppato e testato un nuovo indice spettrale (Snow Darkening Index, SDI), non-linearmente correlato alla concentrazione di polveri minerali nella neve. Durante l’ultimo anno di dottorato, mi sono concentrato sull’impatto delle LAI sul ghiaccio in ambiente Alpino. I ghiacciai alpini rappresentano infatti un’importante fonte di acqua dolce a livello globale. Queste riserve sono seriamente minacciate dai cambiamenti climatici in atto, e in anni recenti è stata osservata una significativa riduzione delle masse glaciali. I processi superficiali che favoriscono la fusione del ghiaccio sono guidati da temperatura, precipitazioni e albedo. Quest’ultima è influenzata principalmente dalla dimensione dei cristalli di neve e dal contenuto di LAI. L’origine di queste impurità può essere prossimale o remota, e spesso queste si possono aggregare sulla lingua glaciale formando le caratteristiche crioconiti, le quali diminuiscono l’albedo del ghiaccio favorendone la fusione. Durante l’estate del 2015 sono state organizzate due campagne di misura sul ghiacciaio del Morteratsch (Alpi Svizzere). L’obiettivo delle campagne era quello di acquisire spettri di riflettanza e campioni di neve e ghiaccio, in modo da caratterizzare radiativamente e geochimicamente le polveri e i materiali depositati sul ghiacciaio. Inoltre, sono stati analizzati dati satellitari dei sensori Hyperion e Landsat, acquisiti a pochi giorni di distanza dalla campagna. I risultati hanno mostrato che le crioconiti possono diminuire la riflettanza del ghiaccio fino a 0.2 nelle lunghezze d’onda del visibile e vicino infrarosso. Questo processo può fortemente alterare i bilanci radiativi del ghiacciaio, siccome provoca la presenza di acqua di scorrimento superficiale, la quale assorbe ulteriore radiazione incidente e favorisce la fusione del ghiaccio sottostante. I dati Hyperion e Landsat hanno mostrato una grande variabilità nelle proprietà spettrali del ghiacciaio, in particolare tra zona di accumulo ed ablazione. Nella zona a cavallo della Equilibrium Line Altitude (ELA), degli strati di polveri sahariane sono inoltre visibili ed evidenziate dalle mappe di SDI.
The objective of my Ph.D. is to investigate the impact of light-absorbing impurities on the cryosphere using optical remote sensing data. Light-absorbing impurities (LAI) are particulate matter, such as mineral dust and black carbon, that can be deposited on snow and ice, reducing their albedo and accelerating the melt. The impact of LAI on the cryosphere has been studied at a global and regional scale, but still few scientific literature focuses on the European Alps. In the first year, I conducted a sensitivity analysis of a radiative transfer model, the SNow, ICe, and Aerosol Radiative model (SNICAR) in order to study the optical properties of snow and ice. In particular, this model allows to simulate spectral reflectance of snow, as a function of different variables, such as snow grain size [μm], mineral dust concentration [ppm] and dimension [μm], black carbon concentration [ppb], solar zenith angle and snow density, using different atmospheric profiles. During the second year of Ph.D., different field campaigns were organized in order to measure spectral reflectance of snow after LAI depositional events, and to compare observed with simulated spectra. During field campaigns, we flew an Unmanned Aerial Vehicle (UAV) over a flat snow-covered area in the European Alps. Data collected from ground, UAV and satellite (Landsat 8 - Operational Land Imager, OLI) were analysed to estimate the impact of mineral dust on snow optical properties. A novel spectral index non-linearly correlated with mineral dust concentration was proposed and tested at different scales. During the third year, I focused on the impact of LAI on ice in the Alps. Mountain glaciers represent an important source of fresh water across the globe. Those reservoirs are seriously threatened by global climate change, and a widespread reduction of glacier extension has been observed in recent years. Surface processes that promote ice melting are driven both by temperature/precipitation and by albedo. The latter is mainly influenced by the growth of snow grain size and by the impurities content (such as dust, soot, ash, algae etc.). The origin of these light-absorbing impurities can be local or distal; often they aggregate on the glacier tongue forming characteristic cryoconites, that decrease ice albedo promoting the melting. During summer 2015, two field campaigns were conducted at the Vadret da Morteratsch glacier (Swiss Alps). The aim of the campaings was to collect ground hyperspectral reflectance data and ice/snow samples at the glacier ablation zone. During August 2015, the Earth Observing One (EO-1) satellite was planned to acquire a series of scene over the Morteratsch glacier. Furthermore, a Landsat 8 Operational Land Imager (OLI) was downloaded from the Earth Explorer portal. Results from spectra analysis showed interesting features in albedo distribution at Morteratsch glacier. In particular, the ablation area showed very low albedo values (circa 0.2), and this is probably due to multiple processes such as accumulation of particulate matters, collapsing of lateral moraine and debris covering. In addition, the presence of surface cryoconites strongly lowers ice albedo, ground measurements showed that these objects have an albedo smaller than 0.1 and that creates melt pond and surface run off that further increase the absorption of incident radiation and accelerate the melting. Hyperion and Landsat data showed that the glacier has areas with different spectral characteristics. In the area across the Equilibrium Line Altitude (ELA), outcropping dust from a Saharan event was also visible, this is highlighted by high Snow Darkening Index (SDI) values.

Situated at the southern edge of the Tibetan Plateau (TP), the Hindu-Kush-Himalayas-Gangetic (HKHG) region is under the clear and present danger of climate change. Flash-flood, landslide, and debris flow caused by extreme precipitation, as well as rapidly melting glaciers, threaten the water resources and livelihood of more than 1.2 billion people living in the region. Rapid industrialization and increased populations in recent decades have resulted in severe atmospheric and environmental pollution in the region. Because of its unique topography and dense population, the HKHG is not only a major source of pollution aerosol emissions, but also a major receptor of large quantities of natural dust aerosols transported from the deserts of West Asia and the Middle East during the premonsoon and early monsoon season (April–June). The dust aerosols, combined with local emissions of light-absorbing aerosols, that is, black carbon (BC), organic carbon (OC), and mineral dust, can (a) provide additional powerful heating to the atmosphere and (b) allow more sunlight to penetrate the snow layer by darkening the snow surface. Both effects will lead to accelerated melting of snowpack and glaciers in the HKHG region, amplifying the greenhouse warming effect. In addition, these light-absorbing aerosols can interact with monsoon winds and precipitation, affecting extreme precipitation events in the HKHG, as well as weather variability and climate change over the TP and the greater Asian monsoon region.