In the mountainous headwaters of the Colorado River episodic dust deposition from adjacent arid and disturbed landscapes darkens snow and accelerates snowmelt, impacting basin hydrology. Patterns and impacts across the heterogenous landscape cannot be inferred from current in situ observations. To fill this gap daily remotely sensed retrievals of radiative forcing and contribution to melt were analyzed over the MODIS period of record (2001-2023) to quantify spatiotemporal impacts of snow darkening. Each season radiative forcing magnitudes were lowest in early spring and intensified as snowmelt progressed, with interannual variability in timing and magnitude of peak impact. Over the full record, radiative forcing was elevated in the first decade relative to the last decade. Snowmelt was accelerated in all years and impacts were most intense in the central to southern headwaters. The spatiotemporal patterns motivate further study to understand controls on variability and related perturbations to snow water resources.

We present a multi-year study of Saharan dust intrusions on a mountainous site located in the central Mediterranean Basin regarding their aerosol optical and geometrical properties. The observations were carried out at the Consiglio Nazionale delle Ricerche-Istituto di Metodologie per l'Analisi Ambientale (CNR-IMAA) located in Potenza (40,360N, 15,440E), Italy, from March 2010 to October 2022, using ACTRIS (Aerosol Clouds and Trace Gases Research InfraStructure). A total of 101 night-time lidar measurements of dust intrusions were identified. The following properties were calculated for the periods December, January, February (DJF), March, April, May (MAM), June, July, August (JJA) and September, October, November (SON): aerosol layer center of mass altitude, particle lidar ratio at 355 and 532 nm, particle depolarization ratio at 532 nm and backscattering & Aring;ngstr & ouml;m exponent at 532-1064 nm. Both geometrical and optical aerosol properties vary considerably with the seasons. During SON and DJF, air masses transporting dust travel at lower altitudes, and become contaminated with local continental particles. In MAM and JJA, dust is also likely to travel at higher altitudes and rarely mix with other aerosol types. As a result, aerosols are larger in size and irregular in shape during the warm months. The ratio of the lidar ratios at 355 and 532 nm is 1.11 +/- 0.15 in DJF, 1.12 +/- 0.07 in SON, 0.94 +/- 0.12 in MAM, and 0.92 +/- 0.08 in JJA. The seasonal radiative effect estimated using the Fu-Liou-Gu (FLG) radiative transfer model indicates the most significant impact during the JJA period. A negative dust radiative effect is observed both at the surface (SRF) and at the top of the atmosphere (TOA) in all the seasons, and this could be related to a minimal contribution from black carbon. Specifically, the SRF radiative effect estimation is -14.48 +/- 1.32 W/m2 in DJF, -18.00 +/- 0.89 W/m2 in MAM, -22.08 +/- 1.36 W/m2 in JJA, and -13.47 +/- 1.12 W/m2 in SON. Instead, radiative effect estimation at the TOA is -22.23 +/- 2.06 W/m2 in DJF, -38.23 +/- 2.16 W/m2 in MAM, -51.36 +/- 3.53 W/m2 in JJA, and -22.57 +/- 2.11 W/m2 in SON. The results highlight how the radiative effects of the particles depend on the complex relationship between the dust load, their altitude in the troposphere, and their optical properties. Accordingly, the knowledge of aerosols optical property profiles is of primary importance to understand the radiative impact of dust.

In this study, we used satellite observations to identify 10 typical dust-loading events over the Indian Himalayas. Next, the aerosol microphysical and optical properties during these identified dust storms are characterized using cotemporal in situ measurements over Mukteshwar, a representative site in Indian Himalayas. Relative to the background values, the mass of coarse particles (size range between 2.5 and 10 mu m) and the extinction coefficient were found to be enhanced by 400% (from 24 +/- 15 to 98 +/- 40 mu g/m3) and 175% (from 89 +/- 57 Mm-1 to 156 +/- 79 Mm-1), respectively, during these premonsoonal dust-loading events. Moreover, based on the air mass trajectory, these dust storms can be categorized into two categories: (a) mineral dust events (MDEs), which involve long-range transported dust plumes traversing through the lower troposphere to reach the Himalayas and (b) polluted dust events (PDEs), which involve short-range transported dust plumes originating from the arid western regions of the Indian subcontinent and traveling within the heavily polluted boundary layer of the Gangetic plains before reaching the Himalayas. Interestingly, compared to the background, the SSA and AAE decrease during PDEs but increase during MDEs. More importantly, we observe a twofold increase in black carbon concentrations and the aerosol absorption coefficient (relative to the background values) during the PDEs with negligible changes during MDEs. Consequently, the aerosol-induced snow albedo reduction (SAR) also doubles during MDEs and PDEs relative to background conditions. Thus, our findings provide robust observational evidence of substantial dust-induced snow and glacier melting over the Himalayas.

Atmospheric particulate matter (PM) as light-absorbing particles (LAPs) deposited to snow cover can result in early onset and rapid snow melting, challenging management of downstream water resources. We identified LAPs in 38 snow samples (water years 2013-2016) from the mountainous Upper Colorado River basin by comparing among laboratory-measured spectral reflectance, chemical, physical, and magnetic properties. Dust sample reflectance, averaged over the wavelength range of 0.35-2.50 mu m, varied by a factor of 1.9 (range, 0.2300-0.4444) and was suppressed mainly by three components: (a) carbonaceous matter measured as total organic carbon (1.6-22.5 wt. %) including inferred black carbon, natural organic matter, and carbon-based synthetic, black road-tire-wear particles, (b) dark rock and mineral particles, indicated by amounts of magnetite (0.11-0.37 wt. %) as their proxy, and (c) ferric oxide minerals identified by reflectance spectroscopy and magnetic properties. Fundamental compositional differences were associated with different iron oxide groups defined by dominant hematite, goethite, or magnetite. These differences in iron oxide mineralogy are attributed to temporally varying source-area contributions implying strong interannual changes in regional source behavior, dust-storm frequency, and (or) transport tracks. Observations of dust-storm activity in the western U.S. and particle-size averages for all samples (median, 25 mu m) indicated that regional dust from deserts dominated mineral-dust masses. Fugitive contaminants, nevertheless, contributed important amounts of LAPs from many types of anthropogenic sources.

Rationale. Glaciers in the Tibetan Plateau (TP), especially in the Himalayas, are retreating rapidly due to rising air temperature and increasing anthropogenic emissions from nearby regions. Traditionally, pollutants deposited on the glaciers have been assumed to originate from long-range transport from its outside. Methodology. This study investigated the concentrations of black carbon (BC) and major ions in snowpit samples collected from two glaciers in the south-eastern TP (Demula and Palongzangbu) and one glacier in the west Himalayas (Jiemayangzong). The radiative forcing of BC was calculated based on BC concentration and glacier characteristics. Results. The results revealed that the BC/Ca2+ concentration ratio in snowpit samples from Palongzangbu, located near residential villages, is similar to 2.05 times higher than that of Demula, which is mainly influenced by long-range transported pollutants. Furthermore, on Jiemayangzong glacier, snowpit samples collected with 100-m vertical resolution exhibited that BC-induced radiative forcings at low altitude are similar to 2.37 +/- 0.16 times greater than those at high altitude. Discussion. These findings demonstrated that in addition to long-range transport, emissions from local residents also make substantial contributions to BC and certain major ions (e.g. SO42-). To accurately assess the sources and radiative forcing of BC and other light-absorbing impurities on glaciers of the TP, it is necessary to consider the impact of local populations and altitude-dependent variations.

Light absorbing particles (LAPs) present high absorbance and contribute to reducing the snow albedo when deposited on snow surfaces. This deposition can be caused by aerosols transported from natural or anthropogenic, either distant or nearby sources. In this study, snow was artificially contaminated with soil samples collected in the Central Andes (near El Yeso dam) to simulate the most common nearby source of Mineral Dust (MD) deposition onto snow surface. Andean soil samples previously conditioned were characterized through Single Particle Optical Sizing (SPOS), X-ray diffraction (XRD) analysis, and Scanning Electron Microscope (SEM) for the determination of optical properties. Spectral snow albedo was measured in situ with a spectroradiometric system. To evaluate the heterogeneity of the particle distribution over the snow surface, aerial photographs were taken with a drone to apply a visual color segmentation of the surface and to determine the equivalent MD concentration. Experimental snow albedo was compared with theoretical values obtained with the OptiPar radiative transfer model. Inputs for the model were: the MD refractive index (calculated from the mineralogical composition and morphology of MD) and particle size, cloudiness, snow density, surface roughness, snow grain size, and LAPs concentration (obtained from the snow samples collected during the experiments and analyzed in the laboratory). Small black carbon concentrations were found in natural snow and considered in the simulations. Spectral albedo measurements showed high albedo reductions in the UV and VIS range (300-800 nm), being less significant in the NIR range (800-1700 nm). A nonlinear behavior was observed in broadband albedo when increasing MD concentration. For lower values of MD concentration (lower than 1500 mg center dot kg (-1) ), a significant albedo reduction rate of 0.1 units per 1000 mg center dot kg (-1) was found, while at higher concentrations (> 3500 mg center dot kg (-1) ), such reduction tends to the minimum. Simulated values with OptiPar are in agreement with measured albedo, but some differences are observed, probably due to the refractive index considered, the snow surface roughness, and the non-uniform MD concentration in snow.

Sources 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.

Through a comprehensive investigation into the historical profiles of black carbon derived from ice cores, the spatial distributions of light-absorbing impurities in snowpit samples, and carbon isotopic compositions of black carbon in snowpit samples of the Third Pole, we have identified that due to barriers of the Himalayas and remove of wet deposition, local sources rather than those from seriously the polluted South Asia are main contributors of light-absorbing impurities in the inner part of the Third Pole. Therefore, reducing emissions from residents of the Third Pole themselves is a more effective way of protecting the glaciers of the inner Third Pole in terms of reducing concentrations of light-absorbing particles in the atmosphere and on glaciers.

It is increasingly recognized that light-absorbing impurities (LAI) deposited on snow and ice affect their albedo and facilitate melting processes leading to various feedback loops, such as the ice albedo feedback mechanism. Black carbon (BC) is often considered the most important LAI, but some areas can be more impacted by high dust emissions. Iceland is one of the most important high latitude sources for the Arctic due to high emissions and the volcanic nature of the dust. We studied optical properties of volcanic dust from Iceland and Chile to understand how it interacts with the Sun's radiation and affects areas of deposition as LAI. Optical properties of dust samples were measured at the laboratory of the Finnish Geospatial Research Institute (FGI) using the latest setup of the FGI's goniospectrometer. We found that, depending on the particle size, the albedo of dry volcanic dust on the visible spectrum is as low as 0.03, similar to that of BC, and the albedo decreases with increasing particle size. Wet dust reduces its albedo by 66% compared to dry sample. This supports the comparability of their albedo reducing effects to BC as LAIs, and highlights their significant role in albedo reduction of snow and ice areas. The potential use of the results from our measurements is diverse, including their use as a ground truth reference for Earth Observation and remote sensing studies, estimating climate change over time, as well as measuring other ecological effects caused by changes in atmospheric composition or land cover.

Aerosol optical properties, including absorption and scattering coefficients (B-abs, and B-scat), extinction coefficient (B-ext), single scattering albedo (SSA), and so forth, are critical metrics to estimate the radiative balance of the atmosphere. However, their ground measurements are sparsely distributed in the world, where Central Asia is void in these measurements. We had been performing the measurements of AOPs and BC with a photoacoustic extinctiometer (PAX) in Jimunai, a border town of China neighboring Kazakhstan, Central Asia, from Aug 2016 to Apr 2019. This three-year study first reported statistically significant trends of B-abs, B-scat, B-ext, SSA, and derived concentrations of BC (Mann-Kendall trend test, p-value 0.05) in the Central-Asian area. B-abs and B-scat show increasing trends and SSA was decreasing determined by the greater increasing pace of B-abs than B-scat. Seasonal and diurnal variations of the AOPs were associated with climate shift and residents' commute activity, respectively. The difference in the magnitudes and trends of AOPs between the measurements and satellites' observations advise that more care should be invested when choosing remote-sensing data to represent the AOPs at a specific site. The increasing trend of derived BC concentrations is reflected in the deposition record of BC in a snowpit of the nearby Muz Taw glacier. We suppose that the dramatically increasing BC particles emitted from Jimunai are significant factors triggering the melting of the adjacent mountain glaciers. The outflow of dust from the neighboring Gurbantiinggiit Desert could occasionally invade into Jimunai and deteriorate the local air quality, as evidenced by a probable dust event captured by the PAX on Feb 15, 2018. Finally, we outlook the future perspectives of measurements in Jimunai as a long-standing station.