Aerosols are minute suspended particles in the atmosphere. When these particles are sufficiently large, we notice their presence as they scatter and absorb sunlight. Their scattering of sunlight can reduce visibility (haze) and redden a sunrise or sunset. Aerosols interact both directly and indirectly with the Earth’s radiation and climate. As a direct effect, the aerosols scatter sunlight back into space. As an indirect effect, aerosols in the lower atmosphere can modify the size of cloud particles, changing how the clouds reflect and absorb sunlight, thereby affecting the Earth’s energy budget.
Aerosols can also act as sites for chemical reactions to take place (heterogeneous chemistry). The most significant of these reactions are those that lead to the destruction of stratospheric ozone. During winter in the polar regions, aerosols grow to form polar stratospheric clouds. The large surface areas of these cloud particles provide sites for chemical reactions . These reactions lead to the formation of large amounts of reactive chlorine and, ultimately, to the destruction of ozone in the stratosphere. Evidence now exists to show that similar changes in stratospheric ozone concentrations occur after major volcanic eruptions, such as that on Mt. Pinatubo in 1991, when tons of volcanic aerosols were blown into the atmosphere.
THERE ARE THREE TYPES OF AEROSOLS WHICH SIGNIFICANTLY AFFECT OUR ENVIRONMENT
The volcanic aerosol layer forms in the stratosphere after a major volcanic eruption like the one on Mt. Pinatubo. The dominant aerosol layer is actually formed by sulphur dioxide gas which is converted to droplets of sulphuric acid in the stratosphere over the course of a week to several months after the eruption. Winds in the stratosphere spread the aerosols until they practically cover the globe. Once formed, these aerosols stay in the stratosphere for about two years. They reflect sunlight, reducing the amount of energy reaching the lower atmosphere and the Earth’s surface, cooling them. The relative coolness of 1993 is thought to have been a response to the stratospheric aerosol layer that was produced by the Mt. Pinatubo eruption.
In 1995, several years since the Mt. Pinatubo eruption, remnants of the layer continued to remain in the atmosphere. Data from satellites such as the NASA Langley Stratospheric Aerosol and Gas Experiment II (SAGE II) have enabled scientists to better understand the effects of volcanic aerosols on our atmosphere.
Pictures from weather satellites often reveal dust veils streaming out over the Atlantic Ocean from the deserts of North Africa. Fallout from these layers has been observed at various locations on the American continent. Similar veils of dust stream off deserts on the Asian continent. The September 1994 Lidar In space Technology Experiment (LITE), aboard the space shuttle Discovery (STS-64), measured large quantities of desert dust in the lower atmosphere over Africa . The particles in these dust plumes are minute grains of dirt blown from the desert surface. They are relatively large for atmospheric aerosols and would normally fall out of the atmosphere after a short flight if they were not blown to relatively high altitudes (15,000 ft. and higher) by intense dust storms. Because the dust is composed of minerals, the particles absorb sunlight as well as scatter it. Through absorption of sunlight, the dust particles warm the layer of the atmosphere where they reside. This warmer air is believed to inhibit the formation of storm clouds. Through the suppression of storm clouds and their consequent rain, the dust veil is believed to further desert expansion.
Recent observations of some clouds indicate that they may be absorbing more sunlight than was thought possible. Because of their ability to absorb sunlight, and their transport over large distances, desert aerosols are thought to be responsible for such absorption of sunlight by these clouds.
While a large fraction of human-made aerosols come in the form of smoke from burning tropical forests, the major component comes in the form of sulphate aerosols created by the burning of coal and oil. The concentration of human-made sulphate aerosols in the atmosphere has grown rapidly since the start of the industrial revolution. At current production levels, humanmade sulphate aerosols are thought to outweigh the naturally produced sulphate aerosols. The concentration of aerosols is highest in the northern hemisphere where industrial activity is centred.
Sulphate aerosols absorb no sunlight but reflect it, thereby reducing the amount of sunlight reaching the Earth’s surface. Sulphate aerosols are believed to survive in the atmosphere for about 3-5 days.
The sulphate aerosols also enter clouds where they cause the number of cloud droplets to increase but make the droplet sizes smaller. The net effect is to make the clouds reflect more sunlight than they would without the presence of the sulphate aerosols.
Pollution from ships at sea has been seen to modify the low-lying clouds above them. These changes in the cloud droplets, due to the sulphate aerosols from the ships, have been seen in pictures from weather satellites as a track through a layer of clouds. In addition to making the clouds more reflective, it is also believed that these aerosols cause polluted clouds to last longer and reflect more sunlight than non-polluted clouds.
CLIMATE EFFECTS OF AEROSOLS
The additional reflection caused by pollution aerosols has an effect on the climate comparable in magnitude to that of increasing concentrations of atmospheric gases. The effect of the aerosols, however, is opposite to the effect of increasing atmospheric trace gases – cooling instead of warming the atmosphere. The warming effect of the greenhouse gases is expected everywhere, but the cooling effect of the pollution aerosols is
regionally dependent, near and downwind of industrial areas. No one knows what the outcome will be of atmospheric warming in some regions and cooling in others. Climate models are still too primitive to provide reliable insight into the possible outcome.
Current observations of the build-up are available only for a few locations around the globe and even then, are fragmentary. Understanding how much sulphur-based pollution is present in the atmosphere is important to understanding the effectiveness of current sulphur dioxide pollution control strategies.
THE REMOVAL OF AEROSOLS
It is believed that much of the removal of atmospheric aerosols occurs in the vicinity of large weather systems and high altitude jet streams, where the stratosphere and the lower atmosphere become intertwined and exchange air with each other. In such regions, many pollutant gases in the troposphere can be injected in the stratosphere, affecting the chemistry of the stratosphere. Likewise, in such regions, the ozone in the stratosphere is brought down to the lower atmosphere where it reacts with the pollutant rich air, possibly forming new types of pollution aerosols.