Composition of the Atmosphere

The gaseous envelope that covers the earth's surface has no upper limit. It gradually merges into the interplanetary space. It exists today as a mixture of several gases, the composition of which within the first 25 km of the earth's surface is presented in Table 2.1.

As shown in the Table, nitrogen and oxygen are the two main constituents of the earth's atmosphere with their combined proportions approaching almost 99% by mass as well as by volume. Their compositions vary little with time, so they are treated as permanent gases. Other gases exist in small amounts only. The proportions of carbon dioxide and ozone are variable. Another constituent of the atmosphere which finds no place in the Table but is highly important for meteorology is water vapour which also occurs in small and variable proportion. But, as we shall see later, the three gases, viz., water vapour, carbon dioxide and ozone, though they occur in small proportions, play very important roles in atmospheric processes because of their radiative and thermodynamic properties.

The presence of water substance in the atmosphere is especially important, because it can exist in three phases, viz., vapour, liquid and solid. A change of phase involves either liberation or absorption of a large quantity of heat which affects atmospheric properties and behaviour. Evaporation of water from the world's oceans, condensation of water vapour into cloud and rain in the atmosphere, formation of polar ice caps on our planet, are all examples of change of phase of the water substance. Minor constituents at low levels of the atmosphere may include variable quantities of dust, smoke and toxic gases and vapours such as sulphur dioxide, methane, oxides of nitrogen, etc., some of which pollute the atmosphere and are highly injurious to health.

The above-mentioned composition of the earth's atmosphere undergoes changes from about 25 km upward, under the effect of the sun's ultra-violet radiation. The gases most affected by this process are oxygen and nitrogen. Their molecules gradually break up leading to formation of ozone in the middle atmosphere (20-50 km

Table 2.1 Composition of pure dry air

Constituent Gas By Mass (%) By Volume (%) Molecular Wt

Table 2.1 Composition of pure dry air

Constituent Gas By Mass (%) By Volume (%) Molecular Wt

Nitrogen (N2)

75.51

78.09

28.02

Oxygen (O2)

23.14

20.95

32.00

Argon (Ar)

1.3

0.93

39.94

Carbon dioxide (CO2)

0.05

0.03

44.01

Neon (Ne)

1.2 x 10-3

1.8 x

10-

3

20.18

Helium (He)

8.0 x 10-4

5.2 x

10-

-4

4.00

Krypton (Kr)

2.9 x 10-4

1.0 x

10-

-4

83.7

Hydrogen (H2)

0.35 x 10-5

5.0 x

10-

-5

2.02

Xenon(X)

3.6 x 10-5

0.8 x

10-

-5

131.3

Ozone (O3)

0.17 x 10-5

0.1 x

10-

-5

48.0

Radon (Rn)

-

6.0 x

10-

-18

222.0

approximately) and atoms and ions (charged particles) in the upper layers of the atmosphere (> 80km). Further discussion about the changes in the composition of air in the upper layers of the atmosphere will be taken up in Chap. 8. Due to preponderance of diatomic molecules in the atmosphere, the mean gram-molecular mass of dry air is taken as 28.699. For water vapour, which is triatomic (H2O), the mean gram-molecular mass is 18.016.

We may now, perhaps, address the question raised in Chap. 1 regarding the loss of lighter elements, especially hydrogen and helium, from the earth's atmosphere. According to the kinetic theory of gases, the mean square velocity of a molecule of a gas is directly proportional to the absolute temperature, as given by the relation 2.2.1 (see, e.g., Saha and Srivastava, 1931, Fifth corrected edition 1969, reprinted 2003), c2 = 3RT (2.2.1)

where c is the molecular velocity, R is the gas constant and T is the absolute temperature of the gas. Table 2.2 gives the values of the mean molecular velocity of some of the gases in the earth's atmosphere at different temperatures. The values are taken from a paper entitled 'Is life possible in other planets?' by Saha and Saha (1939).

However, it may be assumed that our atmosphere had been quite different in the past from what it is to-day, especially at the time when the earth got separated from the sun and the temperature of the sun, in all probability, might have been much in excess of the present value of about 6,000 °C. At such a high temperature the mean velocity of the hydrogen atoms would be 12.8km s_1, and that for the hydrogen molecules would be about 9km s^1. So, a mass like the earth, just separated from the sun, engulfed in hot gases, would rapidly lose all hydrogen atoms and most of the hydrogen molecules.

But even if the temperature was lower, there would be steady loss of the lighter constituents, for according to Maxwell's law of distribution of velocities, all molecules in a gas do not move with the same velocity; there would be some whose velocities may even at the ordinary temperatures exceed the velocity of escape and such particles would escape. The rate of loss will increase with higher temperature and lower molecular weight. Jeans has in fact calculated the time required for loss

Table 2.2 Values of the mean molecular velocity c (km s 1) at different temperatures (°C)

Gas Temperatures (°C)

Table 2.2 Values of the mean molecular velocity c (km s 1) at different temperatures (°C)

Gas Temperatures (°C)

-100

0

300

Hydrogen

1.47

1.80

2.66

Helium

1.04

1.31

1.90

Water vapour

0.49

0.61

0.88

Nitrogen

0.39

0.49

0.71

Oxygen

0.37

0.46

0.67

Argon

0.33

0.41

0.59

Carbon dioxide

0.31

0.39

0.57

of planetary atmospheres from different planets and for different temperatures. He finds that if the mean molecular velocity of the gas is one-fourth the critical velocity of escape, the atmosphere would be lost in 50,000 years. But if the ratio is one-fifth, 25 million years would be needed for complete loss.

It is probable that the earth's atmosphere lost most of its primordial hydrogen, helium and other lighter gases quite early in the course of its geological history, while the heavier gases were retained.

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