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.

The seasonal mountain snowpack of the Western US (WUS) is a key water resource to millions of people and an important component of the regional climate system. Impurities at the snow surface can affect snowmelt timing and rate through snow radiative forcing (RF), resulting in earlier streamflow, snow disappearance, and less water availability in dry months. Predicting the locations, timing, and intensity of impurities is challenging, and little is known concerning whether snow RF has changed over recent decades. Here we analyzed the relative magnitude and spatio-temporal variability of snow RF across the WUS at three spatial scales (pixel, watershed, regional) using remotely sensed RF from spatially and temporally complete (STC) MODIS data sets (STC-MODIS Snow Covered Area and Grain Size/MODIS Dust Radiative Forcing on Snow) from 2001 to 2022. To quantify snow RF impacts, we calculated a pixel-integrated metric over each snowmelt season (1st March-30th June) in all 22 years. We tested for long-term trend significance with the Mann-Kendall test and trend magnitude with Theil-Sen's slope. Mean snow RF was highest in the Upper Colorado region, but notable in less-studied regions, including the Great Basin and Pacific Northwest. Watersheds with high snow RF also tended to have high spatial and temporal variability in RF, and these tended to be near arid regions. Snow RF trends were largely absent; only a small percent of mountain ecoregions (0.03%-8%) had significant trends, and these were typically decreasing trends. All mountain ecoregions exhibited a net decline in snow RF. While the spatial extent of significant RF trends was minimal, we found declining trends most frequently in the Sierra Nevada, North Cascades, and Canadian Rockies, and increasing trends in the Idaho Batholith. This study establishes a two-decade chronology of snow impurities in the WUS, helping inform where and when RF impacts on snowmelt may need to be considered in hydrologic models and regional hydroclimate studies.

Tropical glaciers are extremely sensitive to changes of climate variables. Their response to climate change is complex and depends on multiple mechanisms affecting their mass and energy balance, including deposition of light absorbing particles (LAPs) from the atmosphere on snow and ice. Such particles can reduce glaciers surface albedo, thus enhancing the melting process. LAPs include carbonaceous particles (black carbon -BC and organic carbon -OC) and mineral dust aerosol (MDA). Although their relevance in global cryosphere, LAPs observations in the Andes tropical glacier areas are limited and sparse. This review aims at providing a critical evaluation of available data on LAPs in South America tropical glaciers, and highlights research gaps that will help to improve our understanding of natural processes and anthropogenic emissions impacts on the cryosphere of the region.In South American tropical glaciers, LAPs measurements in surface snow are mainly focused in the Cordillera Blanca and limited information are available about their chemical composition (carbonaceous or mineral components), while dust ice core records have been investigated in several sectors of the Andes, including in the Cordillera Blanca, Cordillera Oriental, and Cordillera Real. Remote and field observations in South American tropical glaciers indicate that LAPs might explain a significant fraction of snow albedo variability, however snow albedo reduction from modelling studies varies significantly depending on LAPs concentration and composition. Carbonaceous LAPs sources in South America are dominated by BC emissions from open fires, linked to agricultural and land clearing activities, peaking in the southern hemisphere dry season (August-October). Natural and anthropogenic dust emissions are potentially relevant contributors of LAPs on the Andes glaciers, as well. Satellite and in-situ measurements were deployed to investigate transport episodes of carbonaceous and mineral particles from lower altitudes towards the Andean glaciers. Nevertheless, the small number of atmospheric records of BC, OC, and MDA does not allow a systematic understanding of transport and deposition processes of such species in the region.

The vast majority of surface water resources in the semi-arid western United States start as winter snowpack. Solar radiation is a primary driver of snowmelt, making snowpack water resources especially sensitive to even small increases in concentrations of light absorbing particles such as mineral dust and combustion-related black carbon (BC). Here we show, using fresh snow measurements and snowpack modeling at 51 widely distributed sites in the Rocky Mountain region, that BC dominated impurity-driven radiative forcing in 2018. BC contributed three times more radiative forcing on average than dust, and up to 17 times more at individual locations. Evaluation of 2015-2018 archived samples from most of the same sites yielded similar results. These findings, together with long-term observations of atmospheric concentrations and model studies, indicate that BC rather than dust has dominated radiative forcing by light absorbing impurities on snow for decades, indicating that mitigation strategies to reduce radiative forcing on headwater snow-water resources would need to focus on reducing winter and spring BC emissions.

Light absorbing particles (LAPs) include black carbon (BC) and mineral dust and are of interest due to their positive radiative forcing and contribution to albedo reductions and snow and glacier melt. This study documents historic BC and dust deposition as well as their effect on albedo on South Cascade Glacier (SCG) in Washington State (USA) through the analysis of a 158-m (139.5-m water equivalent [w.e.]) ice core extracted in 1994 and spanning the period 1840-1991. Peak BC deposition occurred between 1940 and 1960, when median BC concentrations were 16 times higher than background, likely dominated by domestic coal and forest fire emissions. Post 1960 BC concentrations decrease, followed by an increase from 1977 to 1991 due to melt consolidation and higher emissions. Differences between the SCG record and BC emission inventories, as well as ice core records from other regions, highlight regional differences in the timing of anthropogenic and biomass BC emissions. Dust deposition on SCG is dominated by local sources and is variable throughout the record. Albedo reductions from LAP are dominated by dust deposition, except during high BC deposition events from forest fires and during 1940-1960 when BC and dust similarly contribute to albedo reductions. This study furthers understanding of the factors contributing to historical snowmelt and glacier retreat in the Cascades and demonstrates that ice cores retrieved from temperate glaciers have the potential to provide valuable records of LAP deposition. Plain Language Summary Light absorbing particles (LAPs) include black carbon (BC, i.e., soot) produced by the incomplete combustion of fossil and biofuels and mineral dust. In the atmosphere, LAP can lead to atmospheric warming, while LAP deposited on snow and glaciers causes darkening, leading to increased solar energy absorption, warming, and faster melt. The role of LAP in climate change is a large source of uncertainty because LAP emissions and deposition are spatially and temporally heterogeneous. We used an ice core retrieved from South Cascade Glacier in Washington State (USA) to reconstruct BC and dust deposition. BC deposition between 1940-1960 is 16 times higher than during the preindustrial period, likely dominated by domestic coal and forest fire emissions. Differences between the SCG ice core and BC emission inventories, as well as ice cores from other regions, highlight regional differences in BC emissions from humans and forest fires. Dust is a larger contributor to snow darkening than BC, except during high BC deposition events from forest fires and during the 1940-1960 period when BC and dust contribute comparably. This study furthers understanding of the role of BC and dust in snow and glacier melt in the Washington Cascades.