PROPERTIES OF ATMOSPHERIC MINERAL DUST DEPOSITED ON MOUNTAIN SNOW COVER IN THE WESTERN UNITED STATES

Accumulated layers of atmospheric dust from regional dust storms can decrease the albedo of downwind mountain snow cover, and can thereby advance the timing and rate of snowpack melting. Understanding of these effects benefits from determining physical and chemical properties of dust in snow cover. Reflectance from high-resolution reflection spectroscopy, particle-size distribution, organic matter concentration, and mineralogy, with emphasis on mineral habitats for iron were evaluated to contribute to the development of radiative transfer models. Our studies examined dust-on-snow in the San Juan Mountains (SJM, southwest Colorado), which has dominant dust sources 200-400 km to the south and southwest, and in the Wasatch Mountains (WM, northern Utah), which has major dust sources within 250 km to the west and southwest.

Physical and chemical properties have been measured on SJM dust layers deposited since 2003. During the most recent water year (WY11, October 2010-June 2011,) 11 dust layers amounted to about 14 g m^-2. Dust layers collected since 2003 consist of aggregates and discrete grains having median sizes of about 22 µm and contain about 3 wt.% organic matter. Average reflectance value for the dust layers range from 21-42% over the reflected solar region (0.35 to 2.5 µm) and 9-19% over the near-infrared region (NIR; 0.35 to 0.7 µm) where iron-oxide minerals have a strong effect. Magnetic properties, reflectance spectroscopy, as well as scanning electron and reflected-light microscopy, identified nanohematite, goethite and magnetite as the common iron oxides.

A similar assemblage of iron-oxide minerals was recognized in probable dust source areas. Laboratory analyses and satellite images identified nanohematite in sediments derived from Lower Mesozoic redbeds, goethite in Upper Mesozoic marine rocks, and magnetite in Quaternary basalt.

Six dust layers from WM snowpack deposited in WY10 have been analyzed. The layers are characterized by median grain sizes of about 15 µm, and about 6.5 wt.% organic matter. Average reflectance values range from 35-51% in the reflected solar region and 17-23% in the NIR region. On the basis of reflectance spectroscopy, goethite appears to be the main ferric oxide mineral in both the WM dust and in sediments of a significant source area—the Milford Flat Fire area to the southwest.