Aerosols are small particles suspended in the atmosphere. They are often not or barely visible to the human eye, yet their impact on climate, weather, health, and ecology are significant. This page introduces the various major types of aerosols, and explains why researching them is important. Aerosols range in size from a few tens of nanometers—less than the width of the smallest viruses—to several tens of micrometers—about the diameter of human hair. The size and composition of aerosol particles affects how far they can travel around the world, their interactions with solar and thermal radiation, and their potential effects on health. Aerosols injected into the atmosphere directly are known as 'primary aerosols'. Sea spray, mineral dust, smoke, and volcanic ash are all primary aerosols. Secondary aerosols are aerosols which were emitted in another form (e.g. gases), then become aerosol particles after going through chemical reactions in the atmosphere, such as sulfate aerosols from volcanoes or industrial emissions. All aerosols can also undergo further chemical changes, referred to as ‘aging effects’. Some more information about these various aerosol types is given below.

Sand dune field over Northern Mali, shrouded by a deep dust haze, fairly typical of the Sahara in summer. Photo taken from an aircraft on 17th June 2012 by Dr. J. R. Banks, Imperial College London, used with permission. Mineral dust is emitted when wind blows over deserts or otherwise dry soils, lifting the particles get carried off into the atmosphere. Mineral dust is one of the most abundant aerosol types, and dust particles are also very large compared to other aerosols, often a size of several micrometers in diameter. One micrometer is 0.000001 meters. Approximately two billion metric tons of mineral dust are emitted per year, with wide-ranging effects. For example, mineral dust from the Sahara can affect the formation of hurricanes in the Atlantic, and fertilize the Amazon basin. Some desert source regions of mineral dust include the Sahara, Sahel, Gobi, Taklamakan, Namib, Salar de Uyun, central Australia, and the United States’ Great Basin. Dust can also be emitted from dry, barren soils, which can be exacerbated by deforestation and overgrazing. North African dust storms occur year-round, while Asian dust storms are most frequent during the spring. The satellite images below show two dust storms on quite difference scales.