DUST DEPOSITED ON COLORADO MOUNTAIN SNOW COVER: EFFECTS ON SNOW ALBEDO AND LINKAGES TO DUST-SOURCE AREAS

Dust deposited on mountain snow cover (DOS) changes snow albedo and enhances absorption of solar radiation, effectively increasing rates of snow melt and often leading to earlier-than-normal spring runoff and overall smaller late-season water supplies. These effects are largely driven by differences in the capacity to absorb solar radiation, particularly in the visible spectrum, between dusty snow and clean snow. Visible-spectrum reflectance values of concentrated DOS samples are on the order of 0.2 (80% absorption). This is in stark contrast to highly reflective pure snow, where visible-spectrum reflectance values approach 1.0.

Samples of DOS were collected from 11 high-elevation mountain sites in Colorado near the end of spring in 2013 and 2014 prior to complete snow melt, when most dust layers deposited during discrete dust events had merged into one layer. DOS samples were analyzed to determine the properties of dust that affect snow albedo and to link DOS to dust-source areas. Total dust mass loadings to snow were grossly determined for 2014 and varied across the region from about 5 to 30 g/m². Median particle sizes for 2013 DOS ranged from 18-26 micrometers with about 90% of the total particles less than 63 micrometers. Dark minerals, “black carbon”, and iron oxide minerals, including nano-sized hematite and goethite, which accelerates snow melt, are present in variable amounts in the DOS samples as determined by optical and electron spectroscopy. Documenting variations in dust-particle masses, sizes, and compositions will help us determine their influences on snow-melt rates and will be useful for modeling the future range of dust snow-melt effects. Furthermore, variations in dust components and particle sizes lead to new ways to recognize sources of dust by comparing the compositions of DOS with different compositions of fine-grained sediments and soils in proposed dust-source areas. Most DOS in the San Juan Mountains of southwest Colorado can be generally linked to southern Colorado Plateau source areas. Understanding dust properties that affect snow albedo and developing the ability to link deposited dust to dust-source areas may guide mitigation of dust emission that affects water resources of the Colorado River basin.