Composition Of

Air is a mixture of various gases added together. It also contains water vapour, dust and droplets, in quantities which vary with time, location and altitude.

Samples collected by balloon as early as 1784 showed the uniformity of air's composition up to 3 km. Later measurements have confirmed that the air up to 80 km or so consists chiefly of nitrogen and oxygen in almost constant proportions, forming a well-mixed layer called the homosphere, within which only the amount of water vapour and ozone vary. But the ratio of oxygen to nitrogen at 300 km is twelve times what it is at sea-level. Beyond about 600 km there is a preponderance of helium and then hydrogen, the lightest gas. Gases at those heights exist mostly as isolated atoms, rather than molecules of linked atoms.

If the components of a litre of surface air were separated, 0.781 litres (i.e. 781 millilitres, 781 mL or 78.1 per cent by volume) would be occupied by nitrogen, a colourless, tasteless, odourless gas. The proportion is slightly lower if masses are considered instead of volumes,

Table 1.3 The proportions of dry air within the homosphere taken up by component gases, apart from traces of other gases such as neon, helium, krypton, hydrogen and xenon, which are mostly inactive chemically

Gas

Proportion by volume (%)

Proportion by mass (%)

Nitrogen

78.1

75.5

Oxygen

21.0

23.1

Argon

0.93

1.28

Carbon dioxide

0.035, approx

0.053, approx

because the densities of nitrogen and oxygen differ (Table 1.3). Volume proportions are usually of more importance, in practice.

In addition to the gases shown in Table 1.3, air near the ground holds substantial amounts of water vapour according to location and season. At the equator there may be 2.6 per cent by volume of water vapour, while colder air (e.g. at 70 degrees latitude or on high mountains) might have less than 0.2 per cent. (This is explained in Chapter 4 and discussed further in Chapter 6.) Incidentally, the density of water vapour is less than that of nitrogen, and therefore less than that of air as a whole (Note 1.C).

Carbon Dioxide

Only about one molecule out of each 3,000 or so molecules of air consists of carbon dioxide. That is much less than on nearby planets (Table 1.2). Despite its small concentration, it is important because of the effects of carbon dioxide on photosynthesis and on global temperatures (Chapter 2), and the fact that the concentration is increasing. Analysis of bubbles of air trapped in Antarctic ice down to 2 km depth suggests that there were about 200 parts per million by volume (ppmv), 160,000 years ago. This increased to 270 ppm by 130,000 BP, falling to 210 ppm again by 18,000 BP, when the last Ice Age was at its most extreme (Chapter 15). There was a subsequent global warming and around 10,000 BP the concentration had risen to 275 ppm, where it remained until the middle of the nineteenth century, prior to industrialisation. It is now over 350 ppm and increasing at an accelerating rate (Figure 1.2), so that 600 ppm may well be exceeded in the next century (Chapter 15). The atmosphere already holds about 2.7 million million tonnes (i.e. 2,700 gigatonnes, written as 2,700 Gt) of CO2, containing 740 gigatonnes of carbon, written as 740 GtC. There is almost 60 times as much carbon dioxide in the oceans, which readily dissolve it, especially in the cold water near the poles (Figure 1.3).

Even the world's human population adds about 0.4 GtC to the atmosphere annually by exhaling; about 5 per cent of what we breathe out is carbon dioxide. However, the main sources of carbon dioxide in human society are industry, power generation, heating and transport, due to the combustion of the carbon in wood, oil, coal and natural gas. Emissions are chiefly in the northern hemisphere, but concentrations are uniform around the world because the time taken to circulate the gas in global winds (Chapter 12) is much less than the time it remains in the atmosphere (Note 1.D).

One way of reducing the amount of CO2 is repeatedly to grow trees for felling, and then to lock up the carbon by using the wood in permanent structures like housing. A hectare of mature Pinus radiata in New South Wales (NSW) takes about 11 tonnes of carbon from the atmosphere annually. That means that continually planting and harvesting 10 million hectares (i.e. 1.3 per cent of Australia's area)

Figure 1.2 Changes of concentrations of (a) carbon dioxide in the atmosphere, measured on a mountain in Hawaii, (b) carbon dioxide measured at the South Pole, (c) nitrous oxide measured in north-west Tasmania and (d) methane in Tasmania.
Weather Cycle Diagram

Figure 1.3 Estimates concerning the global carbon cycle. The diagram shows the approximate amounts stored at each stage in the cycle (in gigatonnes of carbon, i.e. GtC), and the annual transfer of carbon in the carbon dioxide moving between stages (GtC/a), and the resulting annual change of storage, in brackets. Inequality in some stages between (a) the change of storage, and (b) the difference between inputs and outputs, is due to the still approximate nature of the data.

Figure 1.3 Estimates concerning the global carbon cycle. The diagram shows the approximate amounts stored at each stage in the cycle (in gigatonnes of carbon, i.e. GtC), and the annual transfer of carbon in the carbon dioxide moving between stages (GtC/a), and the resulting annual change of storage, in brackets. Inequality in some stages between (a) the change of storage, and (b) the difference between inputs and outputs, is due to the still approximate nature of the data.

would extract 0.11 Gt of carbon annually, which is only about half as much as the country's mining of carbon as coal in 1992-93. Old-growth forests, on the other hand, make little contribution to removing carbon from the air, being in equilibrium, giving off as much carbon dioxide in decay as is absorbed in growth. So the huge forest of the Amazon basin, for instance, does not help, except in a negative sense; its continuation prevents the liberation of considerable carbon dioxide which occurs in destroying a forest.

The carbon dioxide concentration in the air varies in time. It may rise during the early morning to 40 ppm above the daily mean at 10 metres height within a forest, and fall to 15 ppm below in the afternoon, because of photosynthesis locally. There is also an annual swing, with a decrease at midlatitudes of about 5 ppm at midyear, due to plant growth in the northern hemisphere summer, and a similar increase at year end due to the decay of foliage from deciduous plants there (Figure 1.2). The seasonal variation is only about 2 ppm at the South Pole (Figure 1.2), on account of much less vegetation in the southern hemisphere, and considerable mixing of the atmosphere across the equator.

Carbon dioxide is one of the 'greenhouse gases' affecting world temperatures, discussed in the next chapter. Other such gases include methane, denoted by CH4 in Figure 1.2. It comes from rice fields (for the swelling human population), swamps, the flatulence of cows, and leakage from natural-gas pipelines and coalmines, for instance. Present emissions amount to about half a gigatonne of methane annually, of which 6 per cent accumulates in the air. The methane concentration was less than 700 parts per billion until three centuries ago, but has now more than doubled (Figure 1.2). There is much less methane than carbon dioxide, but it is about fifty times as effective in causing global warming, so that the annual augmentation is equivalent to about 0.8 Gt/a (i.e. 0.5x0.06x26) of CO2, or 0.2 Gt/a of carbon. Similarly, there are rising concentrations of nitrous oxide (N2O) and chlorofluorocarbons (CFCs), but reduced ozone (Section 1.4). Emissions of N2O are increasing by 0.3 per cent annually, partly from the increased use of certain agricultural fertilisers, and the burning of timber. The combined warming potential of these various gases has become comparable with that of the carbon dioxide.

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