Characterization of Dust Transported to El Paso, Texas

The Chihuahuan Desert surrounding El Paso, Texas is one of the most intense sources of dust in the Western Hemisphere. Ephemeral lakes, dry river beds and agricultural lands ~100 km upwind of El Paso have been identified as the main sources of the dust blown into the city. During the late autumn through spring- the dry, windy season in the Chihuahuan Desert- blowing dust is a common occurrence in the region, borne primarily on strong westerly and southwesterly winds. These dust events are a result of synoptic-scale Pacific cold fronts moving across the desert from west to east, and cyclones developing and intensifying to the northeast. We documented 17 dust events (11/ 8/2012-5/31/2013) over the course of the last dry season. Passive dust collection traps were placed on the roof of the Biological Sciences Building at the University of Texas at El Paso when dust events were forecasted, before blowing dust had begun. Settled dust was collected and retained from the traps after each event. These samples were characterized by a variety of physical and chemical techniques.

Dust deposition rates were calculated by determining the total amount of dust collected for each event over a given time. From these 17 events, we calculated a total dust deposition rate to be 195.5 g/m2/yr, or an average of 29.0 g/m2/mo over the ~7-month dry, windy season. These rates are elevated in comparison to reported data on dust deposition elsewhere in the region, and closer to those reported for other global desert areas. The largest dust storm took place from December 18-20, 2012, transporting approximately 15.4% of all the deposited dust. Grain size of the settled dust, obtained through laser diffraction analysis, indicated the mean particle size for 15 events (two events were not analyzed due to small amounts of dust collected) was 123μm for the 2012-2013 dust season- thus, the majority of the material transported was actually blowing sand, not blowing dust. However, 9.25% of the settled particles in the sediment traps were considered PM10 and 3.94% were PM2.5. A Texas Commission on Environmental Quality (TCEQ) air monitoring station sited 217 meters from the dust traps recorded an average PM10 concentration of 61.3 μg/m3 and an average PM2.5 concentration of 12.4 μg/m3 during the hours the traps were deployed. These TCEQ data indicated the peak hourly PM10 of 1955.2 μg/m3 and the maximum hourly PM2.5 of 288.33 μg/m3 were both recorded on February 20, 2013 at 5pm. Dust from each event was also characterized for mineralogy by X-ray diffraction and for elemental composition by ICP-OES. Mineralogy was dominated by quartz, as expected in this desert region, with the presence of other minerals known from regional erodible soils including plagioclase, gypsum, and calcite. Additional data is presented on the chemistry of the dust from different events and their relationship to different source areas as determined by back trajectories.