Variation with height

The light gases (hydrogen and helium especially) might be expected to become more abundant in the upper atmosphere, but large-scale turbulent mixing of the atmosphere prevents such diffusive separation even at heights of many tens of kilometres above the surface. The height variations that do occur are related to the source-locations of the two major non-permanent gases – water vapour and ozone. Since both absorb some solar and terrestrial radia­tion, the heat budget and vertical temperature struc­ture of the atmosphere are considerably affected by the distribution of these two gases.

Water vapour comprises up to 4 per cent of the atmosphere by volume (about 3 per cent by weight) near the surface, but only 3-6 ppmv (parts, per million by volume) above 10 to 12 km. It is supplied to the atmosphere by evaporation from surface water or by transpiration from plants and is trans­ferred upwards by atmospheric turbulence. Tur­bulence is most effective below about 10 km and as the maximum possible water vapour density of cold air is anyway very low, there is little water vapour in the upper layers of the atmosphere.

Ozone (03) is concentrated mainly between 15 and 35 km. The upper layers of the atmosphere are irradiated by ultraviolet radiation from the sun, which causes the break-up of oxygen molecules at altitudes above 30 km (i.e. O, —> O + O). These separated atoms (O + O) may then combine individually with other oxygen mole­cules to create ozone, as illustrated by the simple photochemical scheme:

02 + 0 + M—>03+M

where M represents the energy and momentum bal­ance provided by collision with a third atom or mol­ecule. Such three-body collisions are rare at 80 to 100 km because of the very low density of the atmosphere, while below about 35 km most of the incoming ultraviolet radiation has already been absorbed at higher levels. Therefore ozone is mainly formed between 30 and 60 km, where collisions between O and O, are more likely. Ozone itself is unstable; its abundance is determined by three dis­tinctly different photochemical interactions. Above 40 km odd oxygen is destroyed primarily by a cycle involving molecular oxygen; between 20 and 40 km NOx cycles are dominant; while below 20 km a hydrogen-oxygen radical (HO2) is responsible. Additional important cycles involve chlorine (CIO) and bromine (BrO) chains at-various altitudes. Collisions with monatomic oxygen may recreate oxygen, but ozone is mainly destroyed through cycles involving catalytic reac­tions, some of which are photochemical associated with longer wavelength ultraviolet radiation (2.3-2.9 |xm). The destruction of ozone involves a recombination with atomic oxygen, causing a net loss of the odd oxygen. This takes place through the catalytic effect of a radical such as OH (hydroxyl):

The odd hydrogen atoms and OH result from the dissociation of water vapour, molecular hydrogen and methane (CH4).

Stratospheric ozone is similarly destroyed in the presence of nitrogen oxides (NOx, i.e. NO, and NO) and chlorine radicals (CI, CIO). The source gas of the NOT is nitrous oxide (N20), which is produced by combustion and fertilizer use, while chlorofluoro-carbons (CFCs), manufactured for 'freon', give rise to the chlorines. These source gases are transported into the stratosphere from the surface and are con­verted by oxidation into NOх, and by UV photode- composition into chlorine radicals, respectively.

The chlorine chain involves:

2 (CI + 03 CIO + O2)

CIO + CIO C1202

and

CI + 03 CIO + O2

OH + 03 HO, + 202

Both reactions result in a conversion of O3 to 02 and the removal of all odd oxygens. Another cycle may involve an interaction of the oxides of chlorine and bromine. It appears that the increases of CR and Br species during the decades 1970-90 are sufficient to explain the observed decrease of stratospheric ozone over Antarctica (see pp. 10-11). A mechanism that may enhance the catalytic process involves polar stratospheric clouds. These can form readily during the austral spring (October), when temperatures decrease to 185-195 K, permitting the formation of particles of nitric acid (HNO3) ice and water ice. It is apparent, however, that anthropogenic sources of the trace gases are
a primary factor in the ozone decline. Conditions in the Arctic are somewhat dif­ferent as the stratosphere is warmer and there is more mixing of air from lower latitudes.

The constant metamorphosis of oxygen to ozone and from ozone back to oxygen involves a very complex set of photochemical processes, which tend to maintain an approximate equilibrium above about 40 km. However, the ozone mixing ratio is at its maximum at about 35 km, whereas maximum ozone concentration (see Note 1) occurs lower down, between 20 and 25 km in low latitudes and between 10 and 20 km in high latitudes. This is the result of some circulation mechanism transporting ozone downwards to levels where its destruction is less likely, allowing an accumulation of the gas to occur. Despite the importance of the ozone layer, it is essential to realize that if the atmosphere were compressed to sea level (at normal sea-level temper­ature and pressure) ozone would contribute only about 3 mm to the total atmospheric thickness of 8 km.

Упражнение 8.

Выберите из текста Variation with height 10–15 основных,
с точки зрения смысловой нагрузки, слов (ключевые слова). Определите, к каким частям речи они относятся.

Упражнение 10.

Переведите текст письменно. (Контрольное время – 30 минут)

Дата добавления: 2014-12-27