Atmospheric aerosols are made up of particles suspended in air which are around one millionth of a metre across. They are formed by the mixing of gases in the atmosphere, or by the upward movement of fine material from the ground. Even though these particles are so small, they have the ability to stop some of the Sun's energy from reaching the Earth's surface.
Where do atmospheric aerosols come from?
Atmospheric aerosol particles may be emitted from primary sources as particles, or they may be formed in the atmosphere from gases (secondary sources). Natural sources of aerosols are usually greater than man-made ones, except in certain areas of the world, such as in industrial regions. Primary sources of aerosols include the burning of plants, especially through forest fires.
Fires may be natural or manmade. Other natural sources include wind-blown dust from rocks, soils, and volcanic eruptions. Volcanoes also produce sulphur dioxide when they erupt. When this mixes with water vapour in the air, sulphuric acid aerosols are produced.
Where do atmospheric aerosols go?
Most aerosols are removed from the atmosphere by rainfall. This is called wet deposition. Aerosol particles may also be removed from the atmosphere directly by falling to the ground. This is known as dry deposition. When a volcano emits gases high into the upper atmosphere (stratosphere) above the level at which rain is usually produced, the aerosols that form from them may remain there for several years, gradually spreading around the world. If the eruption is only into the lower atmosphere (troposphere), the pollution will be removed within days by rain.
What do atmospheric aerosols do?
Atmospheric aerosols influence the global climate in two ways, both resulting in cooling. Firstly they can scatter the sunlight, so less of it reaches the Earth’s surface. This effect is particularly important and long-lasting when the aerosols are high up in the atmosphere as a result of a volcanic eruption. Secondly, aerosol particles help to form clouds, which reflect sunlight back out to space, so making the atmosphere cooler.