High concentrations of regional dust from deserts to plains across the central Rocky Mountains, USA

Abstract:

Regional mineral dust in the American Southwest affects snow-melt rates, biogeochemical cycling, visibility, and public health. We measured total suspended particulates (TSP) across a 500-km-long sampling network of five remote sites in Utah and Colorado, USA, forming a gradient in distance from major dust emitting areas. The two westernmost sites on the Colorado Plateau desert had similar TSP concentrations (2008-2012, daily average=126 µg m^-3; max. daily average over a two-week period=700 µg m^-3 at Canyonlands National Park, Utah), while the easternmost High Plains site, close to cropped and grazed areas in northeastern Colorado, had an average concentration of 143 µg m^-3 in 2011-2012 (max. daily average=656 µg m^-3). Such concentrations rank comparably with those of TSP in several African and Asian cities in the paths of frequent dust storms. Dust loadings at the two intervening montane sites decreased from the western slope of the Rocky Mountains (Telluride, daily average=68 µg m^-3) to an eastern site (Niwot Ridge, daily average=58 µg m^-3). Back-trajectory analyses and satellite retrievals indicated that the three westernmost sites received most dust from large desert-source regions as far as 300 km to their southwest. These sources also sometimes sent dust to the two easternmost sites, which additionally captured dust from sources north and northwest of the central Rocky Mountains as well as locally at the Plains site. The PM₁₀ fraction accounted for <15% of TSP, but most TSP is only slightly larger (typical median size, 15-20 µm) after about 100-800 km transport distances. Correlations between TSP and PM₁₀ values indicate increases in both fractions during regional wind storms, especially related to Pacific frontal systems during late winter to late spring. These measurements and observations indicate that most dust deposition and associated air-quality problems in the interior American West are connected to regional dust sources and not to those in Asia.