The Atmospheric Gases

The constituents of the atmosphere are collectively referred to as air, although air itself is not a specific gaseous element, rather it is a mixture of individual gases each of which retains its own particular properties. Although traces of atmospheric gases have been detected well out into space, 99 per cent of the mass of the atmosphere lies within 30 km of the earth's surface, and 50 per cent is concentrated in the lowest 5 km. Most of the world's weather develops in these lower layers, but certain elements in the upper reaches of the atmosphere are also involved, and some appear as important components in the global environmental issues to be examined here.

Oxygen and nitrogen

Ignoring for the moment the liquids and solids always present, the gaseous mixture which makes up the atmosphere has a remarkably uniform composition in the troposphere where most of the air is located. Two gases, oxygen and nitrogen, account for 99 per cent of the total by volume (see Table 2.1). Oxygen (21 per cent by volume) participates readily in chemical reactions, and provides one of the necessities of life. It is also capable of absorbing solar radiation. In contrast, nitrogen (78 per cent by volume) is basically inert, seldom becoming directly involved in atmospheric chemical or biological processes except under extraordinary circumstances. During thunderstorms, for example, the enormous energy flow in a lightning stroke may cause nitrogen to combine with oxygen to produce oxides of nitrogen. On a less spectacular, but ultimately more important level, certain soil bacteria—such as Clostridium and Azobacter— along with those found in the root nodules of leguminous plants, are capable of fixing the atmospheric nitrogen essential for the creation of the complex nitrogen compounds found in all forms of life on earth (Steila 1976).

Efficient recycling processes maintain the volume of both gases, and turbulent mixing

Table 2.1 Average gaseous composition of dry air in the troposphere

Per cent by

Gas volume Parts per million

Per cent by

Gas volume Parts per million










Carbon dioxide
























ensures that they are evenly distributed. There is no evidence that the relative levels of oxygen and nitrogen are changing significantly, although there have been measurable changes in the proportions of other gases in the atmosphere. Changes in the nature of oxygen, however, are involved in the depletion of the ozone layer, now recognised as one of the world's major environmental issues. Oxygen can exist in the atmosphere as atomic, diatomic or triatomic oxygen, depending upon the number of atoms in a molecule. The most common form is diatomic oxygen (O2), but the triatomic form called ozone (O3), created through the combination of the other two types, is present in the upper atmosphere. Ozone is also found close to the

Figure 2.1 S pectral distribution of solar and terrestrial radiation. Solar radiation is represented by a curve for a black body at 6000°K and terrestrial by a black body at 300°K. A black body is a perfect radiator or absorber of energy

Figure 2.1 S pectral distribution of solar and terrestrial radiation. Solar radiation is represented by a curve for a black body at 6000°K and terrestrial by a black body at 300°K. A black body is a perfect radiator or absorber of energy

Atmospheric Environmental Issues

surface, on occasion—usually as a product of air pollution—but its main location is in the upper atmosphere, where it effectively filters out shortwave solar radiation at the ultraviolet end of the spectrum. Any change in ozone levels, allowing an increase or decrease in the transmission of radiation, would therefore cause disruption of the earth's energy budget, and lead to alterations in temperature levels and distribution patterns. It is also estimated that a reduction in ozone in the upper atmosphere would allow an increase in the incidence of skin cancer in humans, as well as genetic mutation in lower level organisms as a result of the increase in the proportion of ultraviolet radiation reaching the earth's surface (Dotto and Schiff 1978).

The minor gases and the greenhouse effect

Although gases other than oxygen or nitrogen account for only about 1 per cent of the atmospheric total, they have an influence quite out of proportion to their volume. The most abundant of these is argon at 0.93 per cent by volume, but it is inert. Another of these minor gases, carbon dioxide has a much more active role in environmental processes. It comprises only 0.03 per cent by volume, yet it makes a significant contribution to the heating of the atmosphere, and is a major participant in the process of photosynthesis by which sugars, starches and other complex organic compounds are produced in plants.

The atmosphere is quite selective in its response to solar radiation. It is transparent to high energy, short-wave radiation, such as that from the sun, but partially opaque to the lower energy, long-wave radiation emanating from the earth's surface. For example, a major proportion of the radiation in the visible range of the spectrum, between 0.3 and 0.7 micrometres (pm), is transmitted through to the surface without losing its high energy content (see Figure 2.1). Once it arrives it is absorbed, the surface heats up, and begins to emit terrestrial long-wave radiation back into the atmosphere. This radiation, from the infrared end of the spectrum—with wavelengths between 1-30 pm—is captured, and the temperature of the atmosphere rises. The capture of the outgoing terrestrial radiation is effected largely by water vapour and carbon dioxide, along with methane and traces of about twenty other gases, which together are called the greenhouse gases. The whole process was labelled the greenhouse effect since the gases, by trapping the heat, appeared to work in much the same way as the glass in a greenhouse. The name remains in common use, although it is now generally accepted that the processes involved are not exactly the same. For example, the glass in the greenhouse acts as a physical barrier to the transfer of energy. There is no such barrier in the atmosphere. Whatever the accuracy of the analogy, the selective nature of the atmosphere in its response to radiation is of supreme importance to the earth's energy budget.

Since the greenhouse effect depends upon carbon dioxide and the other gases in the atmosphere, it follows that any change in these gases, including their relative concentration, will have an effect on the intensity of the greenhouse effect. Changes in greenhouse gas levels in the past were brought about by natural processes, but, since the middle of the nineteenth century, human activities have had a major role in increasing the intensity of the greenhouse effect through the production of higher volumes of carbon dioxide, methane and a number of other greenhouse gases. Concern over the impact of such changes has promoted the intensification of the greenhouse effect to its present position as a significant environmental issue.

Oxides of sulphur and nitrogen

There are many other gases which from time to time become constituents of the atmosphere. These include sulphur dioxide, oxides of nitrogen, hydrogen sulphide and carbon monoxide, along with a variety of more exotic hydrocarbons, which even in small quantities can be harmful to the environment. All of these gases are natural constituents of the atmosphere, released as a result of biological activity, created during volcanic eruptions or produced by natural wood and grass fires. Increasingly, however, their presence is associated with pollution from industrial or vehicular sources. In recent years, concern has centred on the widespread dissemination and detrimental environmental impact of some of these gases. Increasing industrial activity, and the continued reliance on fossil fuels as energy sources, has caused a gradual, but steady, growth in the proportion of sulphur and nitrogen oxides in the atmosphere over the past 2-3 decades. In combination with atmospheric water, these gases—whether natural or anthropogenic in origin—are the main ingredients of acid rain. Anthropogenically produced acid rain is commonly many times more acid than its natural conterpart, however, and has been identified as a major cause of damage to aquatic and terrestrial ecosystems in North America and Europe. Although it has received less attention in recent years, it remains a significant environmental problem in the industrialized nations of the world, and there is growing evidence that areas currently little affected—the southern hemisphere, for example—may not always be immune.

Continue reading here: Water In The Atmosphere

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