In aerosol science, dust refers to airborne mineral particles, or soil dust (in contrast to household dust, which contains a high percentage of synthetic particles, such as lint). Dust particles enter the atmosphere directly through the action of wind, assisted in many cases by human activities (for example plowing). Dust source regions are mainly deserts, dry lake beds, and semi-arid desert fringes, but include drier regions where vegetation has been reduced or soil surfaces have been disturbed. It has been estimated that up to 50% of the current atmospheric dust load originates from disturbed soil surfaces, and should therefore be considered anthropogenic in origin. The Northern Hemisphere is a much greater source of dust than the Southern Hemisphere. The mass concentration of dust particles under background continental conditions generally lies between 10's to 100's µg m-3 air. Infrequently, very large quantities of mineral aerosol are injected into the atmosphere during volcanic eruptions. Recent significant eruptions, such as that of Mount Pinatubo in 1991 released enough ash to affect global temperature. The total global dust emission is estimated to be between 1000 to 5000 Tg yr-1. The atmospheric lifetime of dust depends on particle size; large particles are quickly removed from the atmosphere by gravitational settling, while submicron size particles can have atmospheric lifetimes of several weeks. Although dust particles are generally large (> 1 µm), they can be transported long distances from their sources. Sand dunes are negligible sources of aerosols: only 4% of the mass of sand dunes consists of particles that are small enough to undergo long range transport in the atmosphere. In contrast, in arid valleys over 50% of the soil particles may be small enough for long range transport. Typical volume median diameters of dust particles (background conditions) are of the order of 2 - 4 µm. Study of dust storms reaching mass concentrations of 1 mg m-3 air suggest that the mass median diameter of particles produced by such storms falls between 3.5 and 6 µm. Large dust plumes originating in the Sahara are transported across the Atlantic Ocean to the Carribean Islands and the southeastern coast of the U.S. Saharan dust is suspected to influence nutrient cycles in the Amazon Basin
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Particles from dust storms in northern Asia influence biogeochemical cycles in Hawaii and have been detected as far away as North America. For example, dust originating in China's Gobi desert has been transported across the Pacific in sufficient quantities to cause brown skies in the Western U.S. (e.g., Seattle, Washington). Smaller dust events are observed in Australia. Clearly, prevailing winds and seasonal weather patterns have a large influence on the transport of mineral aerosols. Dust particles are composed mostly of silicon, aluminum and iron. Silicon occurs mostly as SiO2, typical of quartz sand. Calcium occurs mostly as calcite. The aluminum content of mineral aerosols results from a wide variety of aluminum silicates (clay minerals), oxides (such as MgAl2O4) and hydroxides (such as gibbsite). Crystalline halite (NaCl) also contributes to dust. The majority of mineral aerosols are not water soluble and, therefore, do not serve as cloud condensation nuclei. Dust particles are, however, not chemically inert. Gaseous SO2, N2O5, O3, and the radical HO2, may be removed from air by dust particles. The reaction of sulfur dioxide on the surface of calcium rich aerosol particles may be an important mechanism for aerosol removal downwind of arid regions. Dust particles can also react with gaseous nitric acid. The evidence for this process lies in the detection of nitrate compounds containing elements of soil origin and bimodal nitrate ion size distributions (suggesting a process of nitrate particle creation distinct from those usually producing nitrate particles in the accumulation mode). Transportation, coal combustion, cement manufacturing, metallurgy, and waste incineration are among the industrial activities that produce primary dust particles. Recent estimates for the current emission of these aerosols are on the order of about 100 Tg/yr. Considering the source strength and the fact that much industrial dust is present in a size fraction that is not optically very active (> 1 mm), it is probably not of climatic importance. On the other hand, growing industrialization without stringent emission controls, especially in Asia, may lead to increases in this source in the next few decades.