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Sandstorms and blowing dust are a frequent occurrence in the Chihuahuan Desert
borderlands of Texas, New Mexico, and Chihuahua. However, unlike many of the
world’s other “dusty deserts” such as the Sahara and the Gobi, relatively little is known
about the exact sources, processes, and characteristics of blowing dust in the
Chihuahuan Desert. The Texas Commission on Environmental Quality (TCEQ) has
documented many dust storms originating in this region in its “Air Pollution Events” web
pages on the Internet. These events have caused PM10 concentrations in El Paso,
Texas to be as high as thousands of micrograms per cubic meter, leading to reduced
visibilities, posing a threat to transportation and infrastructure, and potentially creating
negative human health effects. Dust from the Chihuahuan Desert region has been
documented to be transported very long distances, spanning anywhere from the Lower
Rio Grande Valley of Texas to Canada (Doggett et al., 2002; Gill et al., 2006). Dust
storms in this region can be characterized as synoptic scale (affecting large areas), or
mesoscale (typically produced by thunderstorms, and smaller in scale). The synoptic
scale events are of greatest concern.

The TCEQ’s “Air Pollution Events” summaries have consistently described these dust
outbreaks as originating “about 50 to 100 miles to the west-southwest of El Paso in
northern Mexico near a large dry lake bed,” and global-scale remote sensing studies
(Prospero et al., 2002) also suggested major dust source areas as existing west-
southwest of the Paso del Norte metropolitan area. The geographical and geological
characteristics of these dust source areas, like the dust storms themselves, have
received little previous scientific attention. This study aimed to start filling those
knowledge gaps by determining the specific point sources of dust in major Chihuahuan
Desert aerosol outbreaks, characterizing the land surface features of these points,
collecting samples of their wind-erodible surface soils in the field, and characterizing
their particle size and chemistry in the laboratory.
To determine the source points of ten major Chihuahuan Desert synoptic-scale dust
outbreaks from 2002- 2006, remote sensing imagery from the NOAA-GOES, NOAA-
POES, Landsat 7, and/or NASA MODIS Terra and Aqua satellites was examined. A
special focus was made on the December 15-16, 2003 event, one of the largest dust
storms ever documented in the region. An advanced methodology for locating dust
sources from the satellite imagery was developed for this project, consisting of
examining difference “split-window” images for far infrared (IR) channels (Gu et al.,
2003) to highlight the dust plumes, and application of a band math analysis using the
10.7 μm and 12 μm wavelength IR brightness temperatures (Ackerman, 1997). The
Environment for Visualizing Images (ENVI, 2004) software was used to visualize dust
plumes and pinpoint their initiation points.

GeoCover land cover (LC) data products were used to identify the land cover/land use
of the point sources of dust from these ten events. In addition, a Geographical
Information System (GIS) was used to evaluate 50 point sources of the December 15,
2003 dust storm. The land use/land cover of the dust storm initiation points was
determined by utilization of the LC data products, aerial photography, and field work.
Weather maps and Drought Monitor data were used to characterize the meteorological
and climatic conditions occurring at the time of each dust event.

Dozens of sites in Chihuahua, Texas, and New Mexico were visited in the field,
photographed, characterized geomorphically/ geographically, and samples of wind-
erodible surface sediments (soils) were collected from them. Soil samples were
analyzed for particle size with a Malvern 2000 laser diffractometer and for elemental
chemistry by particle-induced X-ray emission spectrometry. An extended field study,
including the collection of aeolian dust samples, was performed at four sites in the Salt
Basin east of El Paso, Texas, at three dust point sources and a national aerosol
monitoring site. Salt Basin samples were also analyzed by ion chromatography and X-
ray diffraction.

Results of the remote sensing analyses showed that the point sources of aerosols in the
studied Chihuahuan Desert dust outbreaks came from a variety of different land cover
types. The predominant land cover types are shrub/scrub land (rangelands),
grasslands, and barren/ minimally vegetated lands (including ephemeral wetlands and
dry lakes or playas). Some persistent dust sources are also located in agricultural lands
of the Rio Grande basin of Texas and New Mexico and the Casas Grandes basin of
Chihuahua. Approximately 53% of the source points identified were in shrub/scrub
lands, 17% in barren/ minimally vegetated lands or ephemeral wetlands, 8% in
grasslands, and 4% in agricultural lands. Other points fell in undetermined land uses or
multiple classifications. For the December 15, 2003 dust event in the Chihuahuan
Desert, source points were 50% on playas, 46% on rangelands, and 4% on agricultural

Dust storms rarely originated in exactly the same points from event to event and year to
year: the point sources occurred in many different areas of the Chihuahuan Desert over
time. In highly dynamic desert landscapes, surface sediments and land cover at a given
point may change rapidly from year to year, as observed over the course of this study at
many dust source areas. For example, in the Paleo Lake Palomas basin of Chihuahua

and extreme south-central New Mexico, the surface sediments were texturally reset
after the heavy monsoon rains of 2005- 2006. In addition, dust plumes originated from
small isolated fractions of the landscape not geomorphologically distinguishable from
their surroundings. This suggests that small-scale variations in soil erodibility and wind
erosivity may control the exact points of dust emission during an individual windstorm.

Evidence of wind erosion was found at the sites visited in the field. Many point sources
are associated with Paleo Lake Palomas (Reeves, 1969), a huge, generally dry lake
bed (playa) in northern Chihuahua, the dust source described by the TCEQ as “about
50 to 100 miles to the west-southwest of El Paso in northern Mexico near a large dry
lake bed.” Paleo Lake Palomas has numerous sub-basins each acting as individual
source areas, as well as an “aeolian corridor” of sands leading from the playa to the
Samalayuca Dunes. In some sub-basins, the playa sediments desiccate during drought
periods to form desiccation polygon (ped) structures. These peds usually have a thin
crust of ultra-fine sediments or loose fine curls of flaked clay crusts which when eroded
are prone to produce large amounts of mineral aerosols. Areas along the edge of the
playas, where fine lacustrine sediments mingle with sands, are also potentially
important dust sources, as are the boundaries between alluvial deposits (gobi-type
surfaces) and lacustrine sediments. Some sites were visibly effected by anthropogenic
disturbance to the landscape from agricultural and vehicular activities, which could
increase their potential to emit dust aerosols.

Particle size analysis of surface sediment samples collected from dust source areas
revealed that many of these materials have granulometries that create a “perfect storm”
for aerosol emission. They include two or three distinct grain size populations: very fine
clay particles in the PM2.5 range (including grains as small as 0.2 micron in some sites)
and fine sands (50- 200 µm), augmented in many locations by silts (10- 50 µm particle
size). This is consistent with current state-of-the-science knowledge of desert sediments
likely to be erodible by the wind. Fine sand grains saltate (bounce) across the land
surface in wind storms, breaking apart and “sandblasting” silt and clay aggregates and
releasing their individual constituent grains, as small as a fraction of a micron, into the
atmosphere as dust. Sand grains near the edges of playas and in agricultural lands
were shown to possess a “tail” of fine (PM10 or smaller) particles associated with the
sands. As sand moves downwind from playas along the “aeolian corridor” towards
dunes, the fine particles disappear, having been “winnowed out” by abrasion and
emission into the air as dust. The Samalayuca dune field sand is devoid of fines, likely
eliminating these dunes as a dust source.

Elemental analysis of dozens of the sediment samples was performed to better
understand their chemical composition and to try and build geochemical “signatures” of
individual dust source locations, as well as to assess potential health impacts. In
addition to the expected silicon and lithogenic elements, many samples were enriched
in elements associated with salts such as sodium, chlorine, and sulfur. The potential
human health effects of inhaling salt aerosols are unclear, so some attention should be
paid to this finding. Small concentrations of lead, arsenic and other transition metals
were detected in some samples, and in some cases enriched above typical crustal
values. Data from dust samples collected in the Salt Basin suggested that the chemistry
of aerosols from a specific site may vary from season to season.

This project has provided initial data which may be used in source apportionment of the
most intense high-PM episodes in the Paso Del Norte region. The results of this project
have led to a better understanding of the source areas and source sediment
characteristics of Chihuahuan Desert dust storms, as well as proof of improved
techniques for detecting airborne dust on remote sensing and satellite images. These
findings may lead to better forecasting, warning, and modeling of future dust events by
agencies such as the National Weather Service, United States Geological Survey,
TCEQ, and state transportation departments. This study has also improved the scientific
understanding of the spatial dimensions, variability, and geographic scaling of dust
emissions, and the specific types of landforms, land cover, land uses and sediments
that give rise to dust plumes in the Chihuahuan Desert.

The SCERP grant has provided leveraging for five other proposals submitted to date,
and will provide crucial data and preliminary findings for other future proposals. The
project to date has resulted in a journal article accepted for publication (Lee et al., in
press), has provided incentive for a new project be funded, has led to an award-winning
Master’s thesis, four papers published in conference proceedings, and ten additional
international conference presentations. Eight students participated in the project.

This project will spur much new research on the causes, characteristics, impacts, and
assessment of Chihuahuan Desert dust outbreaks.

Ackerman, S. A., 1997. “Remote sensing aerosols using satellite infrared observations.”
Journal of Geophysical Research 102 (D14): 17069- 17079.

Doggett, A.L. IV, T.E. Gill, R.E. Peterson, A.J.M. Bory, and P.E. Biscaye, 2002.
“Meteorological characteristics of a severe wind and dust emission event; southwestern
USA, 6-7 April 2001.” Proceedings of the 21st Conference on Severe Local Storms,
American Meteorological Society, San Antonio, TX, August 2002, pp. 78- 80.

ENVI, 2004. The Environment for Visualizing Images; User’s Guide. RSI, Boulder, CO.

Gill, T.E., R.J. Vet, P.E. Biscaye, M.P. Bleiweiss, and M. Shaw, 2006. “Recurrent
transcontinental dust transport from southwestern North America to Canada.” P. 150 in
Program and Abstracts of the Sixth International Conference on Aeolian Research,
Guelph, Canada, July 2006, W.G. Nickling, , S. Turner, J.A. Gillies and M. Puddister,
editors. Guelph, Canada: Department of Geography, University of Guelph.

Gu Yingxin, W.I. Rose, and G.J.S. Bluth, 2003. “Retrieval of mass and sizes of particles
in sandstorms using two MODIS IR bands: A case study of April 7, 2001 sandstorm in
China.” Geophysical Research Letters 30(15), 1805, doi:10.1029/2003GL017405.

Lee, J. A., T.E. Gill, M.R. Mulligan, M. Domínguez Acosta, and A.E. Perez, in press.
“Land use/ land cover and point sources of the December 15, 2003 dust storm in
southwestern North America.” Geomorphology, in press.

Prospero, J. M., P. Ginoux, O. Torres, S.E. Nicholson, and T.E. Gill, 2002.
“Environmental characterization of global sources of atmospheric soil dust identified
with the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) Absorbing Aerosol
Product.” Reviews of Geophysics 40 (1), 1002, doi:10.1029/2000RG000095.

Reeves, C. C., 1969. “Pluvial Lake Palomas, northwestern Chihuahua, Mexico.” New
Mexico Geological Society Guidebook 20: 143 – 154.