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Light Absorbing Particles Deposited to Snow Cover Across the Upper Colorado River Basin, Colorado, 2013–2016: Interannual Variations From Multiple Natural and Anthropogenic Sources
Summary
Scientists studying snow in Colorado's mountains found it's coated with tiny dark particles from both nature (desert dust, wildfire soot) and human activity (including bits worn off car tires), which make snow absorb more sunlight and melt faster. This matters because the Colorado River supplies drinking water to millions of people in the Southwest, and faster snowmelt disrupts the timing and amount of water available downstream. While the human health risks of inhaling or ingesting these particles weren't the focus of this study, the presence of tire-wear particles and pollution in mountain snow highlights how far human-made contaminants can travel into
Abstract 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 μm, 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 μm) indicated that regional dust from deserts dominated mineral‐dust masses. Fugitive contaminants, nevertheless, contributed important amounts of LAPs from many types of anthropogenic sources.