Aerosols And Radiation

Atmospheric aerosols comprise a very heterogeneous group of particles, and the mix within the group changes with time and place. Following volcanic activity, for example, the proportion of dust particles in the atmosphere may be particularly high; in urban areas, such as

Los Angeles, photochemical action on vehicle emissions causes major increases in secondary particulate matter; over the oceans, 95 per cent of the aerosols may consist of coarse sea-salt particles. Such variability makes it difficult to establish the nature of the relationship between atmospheric aerosols and climate. It is clear, however, that the aerosols exert their influence on climate by disrupting the flow of radiation within the earth/atmosphere system, and there are certain elements which are central to the relationship. The overall concentration of particulate matter in the atmosphere controls the amount of radiation intercepted, while the optical properties associated with the size, shape and transparency of the aerosols determines whether the radiation is scattered, transmitted or absorbed (Toon and Pollack 1981). The attenuation of solar radiation caused by the presence of aerosols provides a measure of atmospheric turbidity, a property which, for most purposes, can be considered as an indication of the dustiness or dirtiness of the atmosphere.

Several things may happen when radiation strikes an aerosol in the atmosphere. If the particle is optically transparent, the radiant energy passes through unaltered, and no change takes place in the atmospheric energy balance. More commonly, the radiation is reflected, scattered or absorbed—in proportions which depend upon the size, colour and concentration of particles in the atmosphere, and upon the nature of the radiation itself (see Figure 5.3). Aerosols which scatter or reflect radiation increase the albedo of the atmosphere and reduce the amount of insolation arriving at the earth's surface. Absorbent aerosols will have the opposite effect. Each process, through its ability to change the path of the radiation through the atmosphere, has the potential to alter the earth's energy budget. The water droplets in clouds, for example, are very effective in reflecting solar energy back into space, before it can become involved in earth/atmosphere processes. Some of the energy scattered by aerosols will also be lost to the system, but a proportion will be scattered forward towards its original destination. Most aerosols, particularly sulphates and fine rock particles scatter solar radiation very effectively.

The most obvious effects of scattering are found in the visible light sector of the radiation spectrum. Particles in the 0.1 to 1.0 pm size range scatter light in the wavelengths at the blue end of the spectrum, while the red wavelengths continue through. As a result, when the aerosol content of the atmosphere is high, the sky becomes red (Fennelly 1981). This is common in polluted urban areas towards sunset when the path taken by the light through the atmosphere is lengthened, and interception by aerosols is increased. Natural aerosols released during volcanic eruptions produce similar results. The optical effects which followed the eruption of Krakatoa in 1883, for example, included not only magnificent red and yellow sunsets, but also a salmon pink afterglow, and a green colouration when the sun was about 10° above the horizon (Lamb 1970). As well as being aesthetically pleasing, the sequence and development of these colours allowed observers to calculate the size of particles responsible for such optical phenomena (Austin 1983).

Among the atmospheric aerosols, desert dust and soot particles readily absorb the shorter solar wave lengths (Toon and Pollack 1981; Lacis et al. 1992) with soot a particularly strong absorber across the entire solar spectrum (Turco et al. 1990). The degree to which a substance is capable of absorbing radiation is indicated by its specific absorption coefficient. For soot, this value is 810 m2g-1, which means that 1 g of soot can block out about two-thirds of the light falling on an area of 8-10 m2 (Appleby and Harrison 1989). Individual soot particles in the atmosphere are approximately 0.1 pm in diameter and tend to link together in branching chains or loose aggregates. With time these clusters become more spherical and their absorption coefficient declines, but even when the aggregate diameters exceed 0.4 pm, the specific absorptivity may remain as high as 6 m2g-1 (Turco et al. 1990). Thus, the injection of large amounts of soot into the atmosphere has major implications for the earth's energy budget.

In addition to disrupting the flow of incoming solar radiation, the presence of aerosols also has an effect on terrestrial radiation. Being at a lower energy level, the earth's surface radiates energy at the infrared end of the spectrum. Aerosols— such as soot, soil and dust particles—released into the boundary layer absorb infrared energy quite readily, particularly if they are larger than 1.0 pm in diameter (Toon and Pollack 1981), and as a result will tend to raise the temperature of the troposphere. However, the absorption efficiency of specific particles varies with the wavelength of the radiation being intercepted. The absorption coefficient of soot at infrared wavelengths, for example, is only about one-tenth of its value at the shorter wavelengths of solar radiation. In addition, since they are almost as warm as the earth's surface, tropospheric aerosols are less efficient at blocking the escape of infrared radiation than colder particles, such as those in the stratosphere (Bolle et al. 1986). Thus, longer-wave terrestrial radiation can be absorbed by particles in the stratosphere, and re-radiated back towards the lower atmosphere where it has a warming effect. Much depends upon the size of the stratospheric aerosols. If they are smaller in diameter than the wavelengths of the outgoing terrestrial radiation, as is often the case, they tend to encourage scattering and allow less absorption (Lamb 1970). The net radiative effects of particulate matter in the atmosphere are difficult to measure or even estimate. They include a complexity which depends upon the size, shape and optical properties of the aerosols involved, and upon their distribution in the stratosphere and troposphere.

Any disruption of energy fluxes in the earth/ atmosphere system will be reflected ultimately in changing values of such climatological parameters as cloudiness, temperature or hours of bright sunshine. Although atmospheric aerosols produce changes in the earth's energy budget, it is no easy task to assess their climatological significance. Many attempts at that type of assessment have concentrated on volcanic dust, which for a number of reasons is particularly suitable for such studies. For example, the source of the aerosols can be easily pin-pointed and the volume of material injected into the atmosphere can often be calculated; the dust includes particles from a broad size-range and it is found in both the troposphere and the stratosphere. Recent studies, however, have suggested that the sulphate particles produced during volcanic eruptions have a greater impact on the energy budget than volcanic dust and ash.

The Basic Survival Guide

The Basic Survival Guide

Disasters: Why No ones Really 100 Safe. This is common knowledgethat disaster is everywhere. Its in the streets, its inside your campuses, and it can even be found inside your home. The question is not whether we are safe because no one is really THAT secure anymore but whether we can do something to lessen the odds of ever becoming a victim.

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