Mineral dust is an atmospheric aerosol originating from the suspension of minerals that constitute the soil, composed of various oxides and carbonates. It is one of the most abundant natural aerosols in the atmosphere and affects climate, biogeochemical cycles, human health and ocean productivity.
Human activities lead to about 25% of the airborne dust, or particulate, load in the atmosphere. This fraction comes from land clearance, desertification, overgrazing, water use and industrial activities that destabilise the land surface.
The Sahara Desert is the major source of mineral dust. Dust subsequently spreads across the Mediterranean, where it is the origin of rain dust, and across the Caribbean Sea into northern South America, Central America, eastern North America and Europe. It also plays a significant role in the nutrient inflow to the Amazon rainforest.
The Gobi Desert is another source of atmospheric dust, affecting eastern Asia and western North America. In spring, dust from the Gobi and southern Mongolia is lifted by prevailing winds and forms Asian dust, which travels eastward over China, Korea and Japan and sometimes reaches the western United States.
Mineral dust is mainly composed of the oxides and carbonates that make up the Earth's crust, including silicon dioxide, aluminium oxide, iron oxide, calcium oxide, calcium carbonate and magnesium carbonate. The composition of mineral dust, usually called its mineralogical composition in atmospheric science, affects many physical and chemical processes.
Iron-bearing oxides, for example, affect the optical properties of dust and therefore climate, while minor components can modify the role of dust as cloud condensation nuclei. Clay minerals, quartz, calcite, feldspar, mica and iron oxides differ in hygroscopicity, refractive index and chemical reactivity, so the same mass concentration of dust from different sources and under different atmospheric conditions can produce different radiative forcing.
Global mineral dust emissions are estimated at 1,000 to 5,000 million tons per year, of which the largest part is attributed to deserts. Although this aerosol class is usually considered natural, about a quarter of mineral dust emissions may be ascribed to human activities through desertification and land-use change. Recent studies also suggest that the fraction of large particles, called coarse dust, may be higher than previously thought, so total emissions may also be higher.
Large dust concentrations may cause problems for people with respiratory disease. Dust clouds also produce more colourful sunsets because fine particles scatter blue light and transmit red and orange light.
The Sahara is the major source of mineral dust on Earth, at about 60 to 200 million tons per year. Saharan dust can be lifted by convection over hot desert areas to very high altitudes and then transported worldwide by winds over thousands of kilometres.
The dust, combined with the extremely hot, dry air of the Sahara, often forms an atmospheric layer called the Saharan Air Layer, which has significant effects on tropical weather, especially by interfering with hurricane development. The dry air and dust together suppress cloud growth and tropical-cyclone intensification.
Dust transport across the Atlantic into the Caribbean and Florida varies greatly from year to year. In some years African dust is observed along much of the east coast of the United States and is visible in the sky. Because of the trade winds, very large concentrations of mineral dust can occur in the tropical Atlantic and reach the Caribbean, with episodic transport to the Mediterranean.
Saharan plumes can form iberulites, a particular tropospheric aggregation of aerosols, when they travel through North Africa and the eastern North Atlantic and reach the circum-Mediterranean areas of western Europe. In the Mediterranean, Saharan dust is a major source of nutrients for phytoplankton and other aquatic organisms.
Saharan dust carries the fungus Aspergillus sydowii and other microorganisms. Aspergillus carried by dust falling into the Caribbean Sea may infect coral reefs with sea-fan disease, or aspergillosis. Dust events have also been linked to increased paediatric asthma attacks in the Caribbean. Since 1970, dust outbreaks have worsened because of drought periods in Africa, and dust events have been linked to declines in coral-reef health across the Caribbean and Florida, primarily since the 1970s.
According to a NASA article, NASA satellites showed that the chilling effect of dust was responsible for one-third of the drop in North Atlantic sea-surface temperatures between June 2005 and 2006, possibly contributing to the difference in hurricane activity between the two seasons. There were only five hurricanes in 2006 compared with fifteen in 2005.
Warm ocean surface temperatures are one of the major factors that create hurricanes. Evidence shows that dust from the Sahara caused surface temperatures to be cooler in 2006 than in 2005, suppressing hurricane activity. Dust cools the sea surface by shading sunlight and reduces convective energy in the atmosphere, thereby affecting tropical-cyclone formation and intensity.
In eastern Asia, mineral dust events originating in the Gobi Desert of southern Mongolia and northern China during spring give rise to the phenomenon called Asian dust. The aerosols are carried eastward by prevailing winds and pass over China, Korea and Japan, and sometimes significant concentrations reach the western United States.
Areas affected by Asian dust experience decreased visibility and health problems such as sore throat and respiratory difficulty. The effects are not strictly negative, however, because deposition is thought to enrich soils with important trace minerals.
An American study analysing the composition of Asian dust events reaching Colorado associated them with carbon monoxide, possibly incorporated as the air mass passed over industrialised regions of Asia. Although dust storms in the Gobi have occurred throughout history, they became a pronounced problem in the second half of the 20th century because of intensified agricultural pressure and desertification.
Mineral dust originates from several sources on the North American continent, including the Southwest, the Great Plains and Alaska. In the Southwest, dust affects human health, visibility, lake productivity and the rate of snowmelt in the Rocky Mountains. Dust deposition has increased dramatically since the early 1800s compared with the natural background because of intensified human activity.
Arid and semi-arid regions are naturally prone to dust emissions. Soil moisture is an important variable controlling emissions, together with vegetation cover, wind speed and soil type. Several studies based on modern observations show positive relationships between dust and drought in each phase of the dust cycle, from emission to atmospheric burden to deposition.
However, studies based on palaeo records of dust deposition, such as lake sediments, that specifically examined megadroughts show both increases and no change. Routson reported increased deposition during megadroughts but used a dust-concentration measure affected by sedimentation rate. When dust accumulation rates were used, no difference was found between drought years, megadroughts and normal hydroclimate conditions. Dust deposition appears more likely controlled by transport mechanisms and sediment supply than by hydroclimate. Arcusa likewise found no evidence for higher dust deposition during drought at multi-decadal and centennial scales, and found that sediment supply played a key role, as shown by a 60% increase in deposition in the 1800s caused by accelerating land disturbance.
Mineral dust affects climate directly and indirectly by scattering and absorbing solar radiation, modifying cloud microphysics and influencing sea-surface temperature. Iron-bearing dust deposited in the ocean can stimulate phytoplankton growth and thereby affect the ocean carbon cycle, a process called iron fertilisation. Dust also supplies phosphorus, iron and calcium to tropical rainforests and marine ecosystems, forming an important flux linking land, atmosphere and ocean.
The environmental effects of dust are twofold. On one hand, dust carries allergens, pathogens and pollutants and harms respiratory health. On the other, it is an important part of global biogeochemical cycles and sustains productivity in ecosystems far from land. Understanding dust sources, transport paths and deposition fluxes is therefore important for climate prediction, public health and ecosystem management.