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Dust transport

NAME: Dust transport

(With contributions from A.S. Goudie, N. Lancaster, R.E. Vance, N. Brooks, G. Taylor. Revised March 2004)

BRIEF DESCRIPTION: The frequency, duration and magnitude of dust storms are gauges of the transport of fine sediment and particulate matter from arid and semi-arid regions and those undergoing prolonged drought [see wind erosion]. Desert winds transport more fine sediment than any other geological agent. The global annual input of dust and mineral aerosols to the atmosphere is estimated at up to 3000 million tonnes/year, with as much as 1/3 coming from the Sahara alone. An increased flux of dust has been correlated with periods of drier and/or windier climates in arid regions, historically and from proxy records in ocean and ice cores. Dust from the central Asian deserts has been detected as far as away as 5000 km, and from the Sahara is regularly seen over the Mediterranean and on the western side of the tropical Atlantic. Although not spatially associated with drylands, volcanic eruptions can also produce huge volumes of particulate matter, which can influence climate patterns for several years following an eruption [see volcanic unrest].

SIGNIFICANCE: Dust storms can affect air temperatures through the absorption and scattering of longwave and solar radiation. They can also affect climate through their influence on marine primary productivity and by their impact on potential cloud condensation nuclei and convectional activity. Dust probably influences physical and biogeochemical exchanges between the atmosphere, land and oceans. In addition, dust provides considerable quantities of nutrients (e.g. iron and other micronutrients) to ocean surface waters and to marine and terrestrial ecosystems far distant from their source. Dust storms can reflect extensive soil erosion, removing large quantities of surface sediments and topsoil. They are a facet of desertification. In the 1930s, drought and dust storms created the "Dust Bowl", greatly reducing agricultural production on the North American prairies. Dust storms can contribute nutrients to desert margins, and build up deposits of loess.

Wind-blown dust derived from dry lake basins can be highly saline, and the finest aerosols (particles < 10 mm) can be injurious to health. Silicosis and pulmonary diseases in China have been linked to dust wind-blown from loess, an ancient aeolian deposit. Dust storms can transport pathogens and agricultural pesticides. The high incidence of a range of respiratory diseases in the region of the recently exposed bed of the Aral Sea [see lake levels and salinity] has been linked to dust transport, and coral diseases in the Caribbean have also been attributed to far-traveled dust. Aerosols and noxious gases of volcanic origin ("volcanic fog") can harm human health for considerable distances down-wind.

HUMAN OR NATURAL CAUSE: Dust clouds of desert and volcanic origin are natural events, but the amount of particulate matter available for transport can be strongly influenced by industrial activity and by surface disturbance such as overgrazing, plowing, and the removal of vegetation. Likewise, good land practices can reduce the availability of fine material for wind transport. Dessication of lake beds, whether due to drought or to water diversion schemes, as in the Aral Sea, can also lead to increased dust storm activity.

ENVIRONMENT WHERE APPLICABLE: Global, but especially in arid and semi-arid regions, temperate, tropical and sub-tropical latitudes.

TYPES OF MONITORING SITES: Downwind of source areas, near urban and/or agricultural areas, and away from local wind barriers.

SPATIAL SCALE: Landscape to mesoscale / regional to continental

METHOD OF MEASUREMENT: Determine frequency, length of storm season, volume of transported material, with visibility observations at first-order meteorological stations. Reduction of visibility to WMO-specified limits gives an index of event intensity: the duration gives an approximation of magnitude. Satellite measurements of dust storms are now routinely used for regional monitoring and tracing pathways. Two particular methods are now being widely used: the Infra-red Difference Dust Index derived from METEOSAT, and the Total Ozone Mapping Spectrometer carried on the Earth Probe Satellite.

FREQUENCY OF MEASUREMENT: Each event should be recorded. The best statistic is the annual frequency of occurrence. Estimate long-range sediment transport at least every 10 years.

LIMITATIONS OF DATA AND MONITORING: Restricted geographical distribution of monitoring sites.

APPLICATIONS TO PAST AND FUTURE: Good index of aridity and/or wind speeds. Paleo-records may be obtained from ancient dust storm deposits found in ice cores, ocean sediments, soils and loess.

POSSIBLE THRESHOLDS: Wind speeds of more than 5-10 m/sec are required for entrainment. Thresholds are strongly affected by the character of the ground surface and the vegetation cover, though the interactions are not well understood.

Brooks, N. & Le Grand, M. 2000. Dust variability over northern Africa and rainfall in the Sahel. In S.J. McLaren & D. Kniverton (eds.) Linking Land Surface Change to Climate Change. Dordrecht: Kluwer, 1-20.
Chiapello, I., J.M. Prospero, J.R. Herman & N.C. Hsu. 1999. Detection of aeolian dust over the North Atlantic Ocean and Africa with the Nimbus 7 TOMS. Journal of Geophysical Research 104 (D8), 9277-9271
Griffin, D.W. 2002. The global transport of dust. American Scientist, 228-235
Kohfeld, K.E., and S.P. Harrison 2001. DIRTMAP: The geologic record of dust. Earth Science Reviews, 54 (1-3), 81-114
Middleton, N.J. & A.S. Goudie 2001. Saharan dust: sources and trajectories. Transactions of the Institute of British Geographers.
Yang, Y.L., V. Squires, & Qi, L. 2002. Global Alarm: Dust and sandstorms from the world's drylands. United Nations Convention to Combat Desertification, 345p.

OTHER SOURCES OF INFORMATION: Geological surveys, environment agencies, desert research institutes, World Meteorological Orgnaization, Wind Erosion Research Unit of the US Department of Agriculture, DIRTMAP database of the Max-Planck-Institute for Biogeochemistry. Environmental and meteorological agencies may have some useful data from air quality measurements.

RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Wind erosion, changes in hydrological systems. aerosols.

OVERALL ASSESSMENT: Dust storms can affect climate patterns, contribute to reduced air quality, and be hazardous to human and ecosystem health. Their magnitude, duration and frequency are valuable indicators.

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