Atmospheric Aerosols

In addition to the gaseous components of the atmosphere and the water in its various forms, there are also solid or liquid particles dispersed in the air. These are called aerosols, and include dust, soot, salt crystals, spores, bacteria, viruses and a variety of other microscopic particles. Collectively, they are often regarded as equivalent to air pollution, although many of the materials involved are produced naturally by volcanic activity, forest and grass fires, evaporation, local atmospheric turbulence, and biological processes. The proportion of particulate matter in the atmosphere has increased from time to time in the past, sometimes dramatically, but in most cases the atmosphere's built-in cleansing mechanisms were able to react to the changes, and the overall impact was limited in extent and duration. When the island of Krakatoa exploded in 1883, for example, it threw several cubic kilometres of volcanic dust into the atmosphere. Almost all of it is thought to have returned to the earth's surface in less than five years, as a result of particle coagulation, dry sedimentation and wash-out by precipitation (Ponte 1976). The 'red-rain' which occasionally falls in northern Europe is a manifestation of this cleansing process, being caused when dust from the Sahara is carried up into the atmosphere by turbulence over the desert, and washed out by precipitation in more northerly latitudes (Tullett 1984). Thus, the atmosphere can normally cope with the introduction of aerosols by natural processes. Cleansing is never complete, however. There is always a global background level of atmospheric aerosols which reflects a dynamic balance between the output from natural processes and the efficiency of the cleansing mechanisms. Data collected over the past several decades suggest that the background level is rising, as a result of the increasing volume of aerosols of anthropogenic origin, although the evidence is sometimes contradictory (Bach 1979).

Measurements since the 1930s—in locations as far apart as Mauna Loa in Hawaii, Davos in Switzerland and the Russian Caucasus—show a sharp rise in the atmosphere's aerosol content, or turbidity as it is called. Results from such stations, located at high altitudes, and relatively remote from the world's main industrial areas, are considered representative of global background aerosol levels. Recent observations of increasing cold season atmospheric pollution in high latitudes—the so-called 'Arctic haze'— are also considered indicative of rising global levels (Environment Canada 1987). Volcanic activity may also have provided some natural enhancement in recent years, but the close correspondence between elevated turbidity levels and such indicators of human development as industrialization and energy use suggests that anthropogenic sources are major contributors. Some studies claim, however, that the observations are insufficient to allow the human contribution to increased turbidity to be identified (Bach 1979).

Any increase in the turbidity of the atmosphere should cause global temperatures to decline, as the proportion of solar radiation reaching the earth's surface is reduced by scattering and absorption. In addition, the condensation of water vapour around atmospheric aerosols would lead to increased cloudiness and a further reduction in the transmission of incoming radiation. This approach has been used to explain the decline in global average temperatures which occurred between 1940 and 1960, and in the 1970s it was seen by some as the mechanism by which a new ice age would be initiated (Ponte 1976). Such thinking is also central to the concept of nuclear winter which would be caused by a rapid temperature decline following the injection of large volumes of aerosols into the atmosphere (Bach 1986).

Providing a dissenting opinion are those who claim that an increase in atmospheric aerosols would have less serious results. The reduction in insolation is accepted, but it is also considered that there would be a concomitant reduction in the amount of terrestrial radiation escaping into space, which would offset the cooling, and perhaps result in some warming of the lower atmosphere. The overall effects would depend very much on the altitude and distribution of the aerosols (Mitchell 1975).

Contradictory conclusions, such as these— drawn from the same basic information—are to be expected in climatological studies. They reflect the inadequacy of existing knowledge of the workings of the earth/atmosphere system, and, although research and technological development is changing that situation, it remains a major element in restricting society's response to many global issues.

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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