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The Great Basin physiographic province is the largest desert region in the USA (Figure 1), and has been described as an important dust source area (Prospero et al., 2002). Details of the ascendant controls on dust dynamics in this part of the Intermountain West are beginning to be resolved (Hahnenberger and Nicoll 2012, 2014). This is important because dust storms that elevate particulate matter (PM) levels adversely affect the air quality in populated areas, contributing to acute and chronic health impacts, as well as traffic accidents, power outages, wild fire runs, impaired visibility due to blowing dust, and other hazards (e.g., Pope, 1991; Pauley et al., 1996; Griffin and Kellogg, 2004; Malek et al, 2006; AZDPS, 2017).
Dust storms are also known to deposit dust particles on montane snowpack (West and Steenburgh, 2010); skiers call the dust adhering to the snow surface “snirt.” Dust-On-Snow (DOS) may affect the major and trace element chemistry of snowpack (Arens, 2010; Carling et al., 2012; Reynolds et al., 2014, 2016). Dust-containing snow in northern Utah may have as much as 5x higher elemental concentrations, compared to dust-free snow (e.g. Carling et al., 2012). The composition of DOS layers in the Wasatch Mountains has been shown to affect surface reflectance (i.e., albedo) and radiative properties of snowpack (Reynolds et al., 2014). Dust layers can also impact slope stability, cause avalanches and may increase montane snowpack melt rates. DOS in the montane areas of the Colorado Plateau causes an earlier melt and “snow-free” date by up to a month, contributing to flashy streamflows (Painter et al., 2007; Painter et al., 2010). Dust-forced changes in hydroclimatology might have a greater impact on regions that rely on seasonal snowpack for their main water supply, such as the Salt Lake Valley along the Wasatch Mountain Front.
Here, we document the “Black Tuesday Storm” as it was called by the press, which occurred 14-15 April 2015, the timeframe when U.S. Americans are filing their taxes. This intense frontal system passed through the eastern Great Basin and produced strong winds and a dust storm, which deposited a discrete dust layer on Wasatch Mountain snowpack near Alta, Utah. This storm caused PM 2.5 levels to reach 8 times the National Ambient Air Quality Standards (NAAQS) in the Salt Lake City metro area and along the Wasatch Front, where most Utahns reside. We analyzed the coincidence of this strong intermountain cyclone with a dust-event day (DED) and an associated Dust-On-Snow (DOS) deposition event. We sampled this single event dust layer at 2824 masl immediately after the storm, enabling direct analysis of the stratigraphy related to the DOS event, and avoiding the effects of time averaging and integration due to freeze-thaw snowpack processes. In this paper, we 1) describe the synoptic and local meteorology as this frontal passage (fropa) caused a dust storm that adversely affected air quality downwind; 2) document dust mobilization observable from specific point sources on MODIS images; 3) explain aeolian transport of dusts from basin “source-to-sink” deposits in a snowpack layer in the Wasatch; and 4) analyze the specific dust properties and composition of the single event DOS layer.
Furthermore, we compare the single “dust event” layer deposited on 14 April 2015 with other studies that analyzed coaslesced dust layers from several dust-fall events over a season (Table S1).Documenting the “Black Tuesday Storm” and the 14 April 2015 DOS event represents the first dust source-to-sink case study for the Eastern Great Basin. We evaluate one specific dust storm that deposited a DOS layer, which we sampled and analyzed as a “dust event layer.” This research provides valuable insights for understanding the dynamics of storm-mediated dust mobilization, aeolian transport and deposition of mineral aerosols as a discrete dust event layer in montane snowpack. Assessing the properties and composition of the dust in a single event DOS layer provides a basis for comparison to other depth-integrated, seasonally-aggregated, coaslesced dust samples analyzed from locations in the Wasatch (Carling et al., 2011; Reynolds et al., 2014).
Within the Great Basin semi-arid region, intermountain cyclonic storms and resultant aeolian dynamics are important to dust entrainment and transport. Since the 1930s, Utah newspapers have reported spring cold front passages associated with dust storms that caused it to “rain mud” (Brough et al., 1987). Various studies on the prevailing winds and synoptic meteorological conditions relate the development of strong intermountain cyclones with elevated wind speeds and dust storm events (Shafer and Steenburgh, 2008; West and Steenburgh, 2010; Jeglum et al., 2010; Jewell and Nicoll, 2011; Hahnenberger and Nicoll, 2012; Steenburgh et al., 2012). In Utah, dust event days (DEDs) typically peak in frequency during the spring months, when southerly ‘Hatu winds’ blow (“Utah” spelled backwards – see Hahnenberger and Nicoll, 2012). The highest frequencies of dust storms in the region occur during March and April, times of enhanced baroclinicity (Hahnenberger and Nicoll, 2012). Key source areas providing dust for transport downwind to the populated regions of Utah were identified by Hahnenberger and Nicoll (2014). Included among the recurrently active “hotspot” areas that are prone to dust emission are: barren and sparsely vegetated land; fallow fields; playa lake (ephemeral lake) surfaces relict from Pleistocene Paleolake Bonneville (Gilbert, 1890).
Additionally, areas that have been disturbed by wildfire, agriculture, vehicular traffic, and military activities (Figure 2). Some “hotspot” source areas experience intensified winds due to a location downstream of mountain gaps, or along fetches with higher wind speeds due to terrain contouring (Hahnenberger and Nicoll, 2014). Dust production is enhanced from source areas affected by severe drought and/or wildfire; in addition, the 2007 Milford Flat Burned Area generated more dust after revegetation techniques such as drilling, chaining, and use of herbicides disturbed the soil (Miller et al., 2012).
Dust storms are known to deposit layers of particulate matter on montane snowpack in the Wasatch Mountains. Various studies published about DOS have presented compositional data from analysis of composite layers, which represent multiple dust events occurring over the duration of a snow season. Dust sample compositions reported in Reynolds et al. (2014) were mostly collected at the end of the 2010 snow season and samples by Carling et al. (2012) were collected before and after the spring dust; as such, these analytical results are aggregated and time-depth integrated. However, in this study, we collected dust from just one dust event, and we have analyzed one single dust layer that was specifically from the 14-15 April 2015 depositional event. The collected samples collected for this study represent the one event exclusively, and so our results are not comparably time-integrated over an entire season (Table S1).
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