14 The evolution, composition, and structure of the
The equation of state
Boyle’s law relates the volume of a gas to its pressure (P=k1/V , where P is pressure, V is volume, and k1 is a constant) and Charles’s law relates pressure and temperature (V=k2T , where T is absolute temperature and k2 is another constant).
Combined, these produce the ‘equation of state’: P=R
ρ
T , where P is pressure,ρ
is density, T is absolute temperature, and R is the gas constant: 8.314 J K-1 mol-1).
Figure 2.15 Structure of the atmosphere
The lowest layer, the troposphere, extends from the surface to an upper boundary, the tropopause, the height of which varies, but averages about 16 km at the equator and 8 km at the poles.
Within the troposphere temperature decreases with height by an average of about 6.5°C km-1 (called the ‘lapse rate’). Above the tropopause temperature remains constant with height to about 20 km. Rising tropospheric air is trapped below the region of constant temperature, which forms a permanent temperature inversion, a layer in which temperature remains constant or increases with altitude, rather than falling. It is this inversion which confines meteorological phenomena to the troposphere.
Temperature then increases from a minimum of about -80°C at the equator in summer, when the tropopause is at its highest, to 0°C or even higher at about 50 km. This region is the stratosphere and its upper boundary is the stratopause. In the mesosphere, above the stratopause, temperature once more decreases with height, to about -90°C at the mesopause, about 80 km, then rises again through the thermosphere. At about 350 km the temperature may exceed 900 °C, probably because of the energy imparted by absorption of ultraviolet radiation by atomic oxygen, but the air is so rarefied that objects such as satellites are not warmed by it, although it still exerts measurable drag on spacecraft moving through it.
Between about 30 and 60 km the density of oxygen molecules is high enough to intercept most of the incoming solar ultraviolet radiation at wavelengths below 0.29 µm. The energy imparted to them separates the molecules (O2→ O + O ). Some of the oxygen atoms then combine with oxygen molecules to form ozone (O + O2→ O3 ). Ozone is unstable and may decompose either by encountering more oxygen atoms (O3 + O → 2O2 ) or by absorbing more ultraviolet radiation.
Ozone is, therefore, constantly forming, decomposing, and re-forming, and the process is in equilibrium above about 40 km. There is also some transport of ozone from low to high latitudes.
There is some mixing of stratospheric air, however, as a result of which a small amount of ozone is transported downward, to accumulate between about 20 and 25 km. This is the ‘ozone layer’.
Its density varies, being lowest over the equator and high over latitudes above 50°. Ordinarily, ozone levels are also high over polar regions in early spring. This is because ozone is neither formed nor destroyed during the polar night, when there is no radiation to drive the reactions, and ozone transported from lower latitudes is stored (BARRY AND CHORLEY, 1982, pp. 2–3).
Despite being known as the ‘ozone layer’, if the air at that altitude were compressed to sea-level pressure the ozone would contribute only about 3 mm to it. In itself, the ozone layer does not shield the surface from ultraviolet radiation, but indicates that radiation is being absorbed at a greater height, shielding both the surface and the ozone layer.
The thickness of the ozone layer is often reported in Dobson units (DU). This unit was devised by G.M.B.Dobson, a British physicist who studied stratospheric ozone in the 1920s. It refers to the thickness of the layer that a gas would form if all the other atmospheric gases were removed and the gas in question were subjected to standard sea-level pressure. In the case of ozone, 1 Dobson unit corresponds to a thickness of 0.01 mm and the amount of ozone in the ozone layer is typically 220–
460 DU, corresponding to a layer 2.2–4.6 mm thick.
The depletion of the ozone layer over Antarctica, first observed in 1986 (ALLABY, 1992, pp.
159–161) by a British scientist, occurs just as spring is commencing. During the polar night, a vortex of very still air forms over Antarctica within which the temperature may be as low as -84°C. Clouds of ice crystals, called polar stratospheric clouds (PSC), form inside the vortex and a series of chemical reactions on the surface of the ice crystals results in chlorine monoxide (ClO) combining to form Cl2O2. These molecules break down when exposed to sunlight (Cl2O2
→ 2Cl + O2 ) and the chlorine atoms combine with ozone in two steps which release free chlorine once more to repeat the process so that a single chlorine atom can destroy many
thousands of ozone mol-ecules (Cl + O3→ ClO + O2; ClO + O → Cl + O2 ) (HIDORE AND OLIVER, 1993, pp. 74–77). CFCs are believed to be the principal source of stratospheric chlorine. Although very stable, they are decomposed by ultraviolet radiation at wavelengths below 0.23 µm, releasing free chlorine. As spring advances, the vortex disappears, ozone moves poleward from lower latitudes, and the ozone layer recovers. Seasonal depletion over the Arctic has also been reported, but it is less severe and of shorter duration, because Arctic winter stratospheric temperatures are higher than those of the Antarctic and a polar vortex rarely forms.
Ozone depletion may lead to increased exposure to ultraviolet radiation at the surface, the biological significance of which is uncertain. Ultraviolet radiation causes cataracts and non-melanoma skin cancer in fair-skinned humans (recent increases being due to the popularity of sunbathing in hot climates to which people are not acclimatized, and not to ozone depletion). It might have an adverse effect on land plants especially susceptible to it and may also affect organisms living in the uppermost few millimetres of the ocean surface; below that depth ultraviolet radiation is absorbed by sea water.
Ozone is a very minor constituent of the atmosphere. In the troposphere it occurs locally, some naturally but more commonly as a pollutant which causes respiratory irritation in humans and can damage plants, produced by photochemical reactions involving vehicle exhaust fumes. It is a constituent of photochemical smog and responsible for some of the damage attributed to acid rain.
The principal ingredients of the atmosphere are listed in Table 2.3. Water vapour comprises up to 4 per cent in the lower atmosphere, but above
about 12 km it is virtually absent. The source of the water vapour from which PSCs form is unknown; it may have entered the stratosphere as water vapour and accumulated in the polar vortex or may result from the oxidation of methane (CH4 + 2O2
→ CO2 + 2H2O ). Water vapour apart, the composition of the atmosphere remains constant to a considerable height, because of mixing caused by turbulence. Beyond the mesosphere, however, the proportions of its ingredients change. Figure 2.16 illustrates how the chemical composition changes with height.