Geoscience Reference
In-Depth Information
commonly considered to consist of particles with diame-
ters
particularly injurious if contaminated by trace chemicals
(e.g. arsenic, cadmium, lead; see Galloway
et al.
, 1982).
Such contaminants are commonly found in dust derived
from anthropogenic sources such as Owens Lake in Cali-
fornia (Reheis
et al.
, 2009) and the dried shoreline of the
Aral Sea (O'Hara
et al.
, 2000).
0.08 mm (80 µm) (Bagnold, 1941), although a more
fashionable limit is now taken to be at the silt/sand bound-
ary of 62.5 µm (Goudie and Middleton, 2006). While
particles in this size range are difficult to entrain (see
Chapter 18), once they are airborne they can be held aloft
by wind turbulence for a considerable amount of time
(hours-days) and are therefore capable of travelling very
large distances (thousands of kilometres). Saharan dust
can be found deposited in the Amazon basin (Swap
et al.
,
1992), the Caribbean (Colarco, Toon and Holben, 2003)
and Europe (Ansmann
et al.
, 2003), Australian dust has
been found in the highlands of New Zealand (McGowan
et al.
, 2005) and Chinese dust can be carried far out into
the Pacific (Zhao
et al.
, 2003). Such long residence times
in the atmosphere allow dust to interact with regional and
global climatology and it is the distance travelled that pro-
motes its impact with regard to redistribution of important
minerals and nutrients at a global scale, unrestricted by
topography.
Further, it is these fine-textured particles in agricultural
soils that are capable of retaining moisture and nutrients
and when wind erosion removes these particles there can
be serious deleterious impacts on agricultural productivity
(Figure 20.1; see Zobeck and Van Pelt, 2006) with ensu-
ing off-site contamination of downwind soils and habitats
(Plumlee and Ziegler, 2003). The fine texture of dust parti-
cles also increases its potential significant health impacts.
Dust with aerodynamic diameters of
<
20.1.1.2
Dust sources
There are many mechanisms in arid regions that may con-
tribute to the production of dust-sized particles (Goudie
and Middleton, 2006). These include abrasion of sands
by both wind (Bullard, McTainsh and Pudmenky, 2004)
and water, the physical weathering of in situ rock (Smith,
Wright and Whalley, 2002, see Chapter 6) or the pro-
duction of 'fluffy' evaporate minerals on the surfaces of
'wet' playas (Figure 20.2; see Reynolds
et al.
, 2007, and
Chapter 15). The numerous mechanisms for dust produc-
tion result in dust sources being found in a wide variety
of geomorphological contexts, although the emission pro-
cesses operating in each are inadequately known. Signif-
icant dust sources are commonly found where silt-sized
material has been concentrated by the action of water. Im-
portant sources are therefore found in closed topographic
depressions in dryland areas such as ephemeral lakes and
pans (Prospero
et al.
, 2002; Engelstaedter
et al.
, 2003;
Bryant
et al.
, 2007) and alluvial surfaces, floodplains and
dry river valleys (Reheis and Kihl, 1995). However, stone
pavements have also been shown to be a significant source
in China (Wang, Zhou and Dong, 2006), while aeolian
sand can also produce large quantities of dust. Recent re-
search suggests that such sand sources (e.g. dunefields)
have been largely overlooked to date, with Bullard
et al.
10, 5 and 2.5 µm
(termed PM10, PM5 and PM2.5 respectively) are of a
size that is capable of reaching into the recesses of human
lungs. While the fine texture of common SiO
2
(quartz)-
based dust may be deleterious to health, such dust may be
<
