Dust Deposited on Snow Cover in the San Juan Mountains, Colorado, 2011–2016: Compositional Variability Bearing on Snow‐Melt Effects

Abstract

Light‐absorbing particles in atmospheric dust deposited on snow cover (dust‐on‐snow, DOS) diminish albedo and accelerate the timing and rate of snow melt. Identification of these particles and their effects is relevant to snow‐radiation modeling and water‐resource management. Laboratory‐measured reflectance of DOS samples from the San Juan Mountains (USA) were compared with DOS mass loading, particle sizes, iron mineralogy, carbonaceous matter type and content, and chemical compositions. Samples were collected each spring for water years 2011–2016, when individual dust layers had merged into one (all layers merged) at the snow surface. Average reflectance values of the six samples were 0.2153 (sd, 0.0331) across the visible wavelength region (0.4–0.7 μm) and 0.3570 (sd, 0.0498) over the full‐measurement range (0.4–2.50 μm). Reflectance values correlated inversely to concentrations of ferric oxide, organic carbon (1.4–10 wt.%), magnetite (0.05–0.13 wt.%), and silt (PM63‐3.9; median grain sizes averaged 21.4 μm) but lacked correspondence to total iron and PM10 contents. Measurements of reflectance and Mössbauer spectra and magnetic properties indicated that microcrystalline hematite and nano‐size goethite were primarily responsible for diminished visible reflectance. Positive correlations between organic carbon and metals attributed to fossil‐fuel combustion, with observations from electron microscopy, indicated that some carbonaceous matter occurred as black carbon. Magnetite was a surrogate for related light‐absorbing minerals, dark rock particles, and contaminants. Similar analyses of DOS from other areas would help evaluate the influences of varied dust sources, wind‐storm patterns, and anthropogenic inputs on snow melt and water resources in and beyond the Colorado River Basin.

Spectral Characterization of Parent Soils From Globally Important Dust Aerosol Entrainment Regions

“…In addition to these iron‐bearing minerals with marked absorption features, global surface soils contain dark iron oxides such as magnetite and ilmenite (e.g., S2001 in https://doi.org/10.5281/zenodo.6841789, Appendix , Table A1). These dark iron oxides do not have features in VSWIR, but contribute to lowering the reflectance over the whole VSWIR spectrum (Nash & Conel, 1974; Reynolds et al., 2020; Singer, 1981). As we observed, sample S2001 that contains both magnetite and ilmenite display one of lowest values for absolute reflectance (less than 52%).…”

Drivers of Dust-Enhanced Snowpack Melt-Out and Streamflow Timing

“…Total dust concentrations were determined from the ALM samples collected towards the end of each ablation season. Bulk samples were processed by the U.S. Geological Survey of Colorado to yield dust loading and geochemical properties [48]. Dust loading (in mg/g) was transformed to end-of-year dust concentration using the 3 cm × 0.5 m 2 sample volume [2] and the bulk snow density observed on the collection date.…”

Enhanced light absorption and reduced snow albedo due to internally mixed mineral dust in grains of snow

“…The SBDART model can be used to compute radiative transfer at different heights and directions under both clear-and cloudy-sky conditions. Details on the SBDART model can be found in Ricchiazzi et al (1998).…”