Composition of the atmosphere

Bulk composition of the atmosphere is quite similar all over the Earth because of the high degree of mixing within the atmosphere. This mixing is driven in a horizontal sense by the rotation of the Earth. Vertical mixing is largely the product of heating of the surface of the Earth by incoming solar radiation. The oceans have a much slower mixing rate, but even this is sufficient to ensure a relatively constant bulk composition in much the same way as the atmosphere. However, some parts of the atmosphere are not so well mixed and here quite profound changes in bulk composition are found.

The lower atmosphere, which is termed the troposphere (Fig. 3.2), is well mixed by convection. Thunderstorms are the most apparent of the convective driving forces. Temperature declines with height in the troposphere (Fig. 3.2); solar energy heats the surface of the Earth and this in turn heats the directly overlying air, causing the convective mixing. This is because the warmer air that is in contact with the surface of the Earth is lighter and tends to rise. However, at a height of some 15-25 km, the atmosphere is heated by the absorption of ultra-

200 180 160 140 120

Pressure (Pa) 10-2 100

Mesosphere

Stratosphere

Tropopause

Troposphere

2000

-

1 Heterosphere

01 ■o

-

1 Helium

<

-

1000

-

\ ■ ^— Atomic Atomic oxygen hydrogen

600

- **

•v \

200

Molecular^^^^^ oxygen

104 106 108 Concentration (cm-3)

300 400 Temperature (K)

Fig. 3.2 The vertical structure of the atmosphere and associated temperature and pressure variation. Note the logarithmic scale for pressure. The inset shows gas concentration as a function of height in the heterosphere and illustrates the presence of lighter gases (hydrogen and helium) at greater heights.

violet radiation by oxygen (O2) and O3. The rise in temperature with height has the effect of giving the upper part of the atmosphere great stability against vertical mixing. This is because the heavy cold air at the bottom has no tendency to rise. This region of the atmosphere has air in distinct layers or strata and is thus called the stratosphere. The well-known O3 layer forms at these altitudes. Despite this stability, even the stratosphere is well mixed compared with the atmosphere even higher up. Above about 120 km, turbulent mixing is so weak that individual gas molecules can separate under gravitational settling. Thus the relative concentrations (see Section 2.6) of atomic oxygen (O) and nitrogen (N) are greatest lower down, while the lighter hydrogen (H) and helium (He) are found to predominate higher up.

Figure 3.2 shows the various layers of the atmosphere. The part where gravitational settling occurs is usually termed the heterosphere, because of the varying composition. The better-mixed part of the atmosphere below is called the homo-sphere. Turbopause is the term given to the boundary that separates these two parts. The heterosphere is so high (hundreds of kilometres) that pressure is extremely low, as emphasized by the logarithmic scale in the figure.

In a mixture of gases like the troposphere, Dalton's law of partial pressure (Box 3.1) is obeyed. This means that individual gases in the atmosphere will decline in pressure at the same rate as the total pressure. This can all be conveniently represented by the barometric equation:

Box 3.1 Partial pressure

The total pressure of a mixture of gases is equal to the sum of the pressures of the individual components. The pressure-volume relationship of an ideal gas (i.e. a gas composed of atoms with negligible volume and which undergo perfectly elastic collisions with one another) is defined as:

where p is the partial pressure, Vthe volume, n the number of moles of gas, R the gas constant and T the absolute temperature. Real gases behave like ideal gases at low pressure and we denote a mixture of gases (1, 2, 3) through the use of subscripts:

Hence:

where pT is the total pressure of the mixture. The implication that partial pressure p, is a function of n, means that the barometric law (Section 3.2):

can be rewritten:

or even:

where c is some unit of the amount of material per unit volume (gm-3 or molecules cm-3). The barometric law predicts that pressure and concentration of gases in the atmosphere decline at the same rate with height.

The relationship between partial pressure and gas-phase concentration explains why concentrations in the atmosphere are frequently expressed in parts per million (ppm) or parts per billion (ppb) (see Table 3.1). This is done on a volume basis so that 1 ppm means 1 cm3 of a substance is present in 106cm3 of air. It also requires that there is one molecule of the substance present for every million molecules of air, or one mole of the substance present for every million moles of air. This ppm unit is thus a kind of mole ratio. It can be directly related to pressure through the law of partial pressure, so at one atmosphere (1 atm) pressure a gas present at a concentration of 1 ppm will have a pressure of 10-6atm.

Table 3.1 Bulk composition of unpolluted air. These are the components that provide the background medium in which atmospheric chemistry takes place. From Brimblecombe (1986).

Gas Concentration

Table 3.1 Bulk composition of unpolluted air. These are the components that provide the background medium in which atmospheric chemistry takes place. From Brimblecombe (1986).

Gas Concentration

Nitrogen

78.084%

Oxygen

20.946%

Argon

0.934%

Water

0.5-4%

Carbon dioxide

360 ppm

Neon

18.18ppm

Helium

5.24ppm

Methane

1.7ppm

Krypton

1.14ppm

Hydrogen

0.5ppm

Xenon

0.087 ppm

where pz is the pressure at altitude z, p0 the pressure at ground level and H, the scale height (about 8.4km in the lower troposphere and a measure of the rate at which pressure falls with height). We can solve this equation and show that the pressure declines so rapidly in the lower atmosphere that it reaches 50% of its ground level value by 5.8 km. This is painfully obvious to people who have found themselves exhausted when trying to climb high mountains. We should note that if equation 3.1 is integrated over the troposphere it accounts for about 90% of all atmospheric gases. The rest are largely in the stratosphere and the low mass of the upper atmosphere reminds us that it will be sensitive to pollutants (Section 3.10). There is so little gas in the stratosphere that relatively small amounts of trace pollutants can have a big impact. Furthermore, pollutants will be held in relatively well-defined layers because of the restricted vertical mixing and this will prevent dispersal and dilution.

It is well known that the atmosphere consists mostly of nitrogen (N2) and O2, with a small percentage of argon (Ar). The concentrations of the major gases are listed in Table 3.1. Water (H2O) is also an important gas, but its abundance varies a great deal. In the atmosphere as a whole, the concentration of water is dependent on temperature. Carbon dioxide (CO2) has a much lower concentration, as do many other relatively inert (i.e. unreactive) trace gases. Apart from water, and to a lesser extent CO2, most of these gases remain at fairly constant concentrations in the atmosphere.

Although the non-variant gases can hardly be said to be unimportant, the attention of atmospheric chemists usually focuses on the reactive trace gases. In the same way, much interest in the chemistry of seawater revolves around its trace components and not water itself or sodium chloride (NaCl), its main dissolved salt (see Chapter 6).

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