Long-range transport of mineral dust in the global atmosphere: impact of African dust on the environment of the southeastern United States

Abstract

Soil dust is a major constituent of airborne particles in the global atmosphere. Dust plumes frequently cover huge areas of the earth; they are one of the most prominent and commonly visible features in satellite imagery. Dust is believed to play a role in many biogeochemical processes, but the importance of dust in these processes is not well understood because of the dearth of information about the global distribution of dust and its physical, chemical, and mineralogical properties. This paper describes some features of the large-scale distribution of dust and identifies some of the geological characteristics of important source areas. The transport of dust from North Africa is presented as an example of possible long-range dust effects, and the impact of African dust on environmental processes in the western North Atlantic and the southeastern United States is assessed. Dust transported over long distances usually has a mass median diameter <10 microm. Small wind-borne soil particles show signs of extensive weathering; consequently, the physical and chemical properties of the particles will greatly depend on the weathering history in the source region and on the subsequent modifications that occur during transit in the atmosphere (typically a period of a week or more). To fully understand the role of dust in the environment and in human health, mineralogists will have to work closely with scientists in other disciplines to characterize the properties of mineral particles as an ensemble and as individual particles especially with regard to surface characteristics.

Figure 1

AVHRR aerosol optical depth, mean for July, 1988 through July, 1989. (R. Husar, personal communication; see also ref. .)

Figure 2

Statistics on the occurrence of high values of TOMS aerosol index. The shaded area shows the number of days (lightest shading, 5–10 days; heaviest shading, 25–30 days) when moderate-to-high concentrations of absorbing aerosol were detected by the TOMS/NIMBUS-7 satellite. (a) January, 1984. (b) July, 1984. (Figures adapted from ref. , figure 4.)

Figure 3

Daily mineral dust concentrations in Miami, 1989 to 1996. Measurements are made at a coastal site during on-shore wind conditions.

Figure 4

Monthly mean mineral dust concentrations at Barbados and Miami for the period 1989 to 1996. (Note the difference in the scales for Barbados and Miami.)

Figure 5

Monthly mean dust concentrations measured in Miami, Florida, for the period 1974–1996.