Processes

The formation of upper-atmosphere ozone involves photo-dissociation of normal oxygen. The molecules are split into pairs of separate single atoms by the impact of UV (Figure 1.4), as explained by Sydney Chapman in 1930. These single atoms can subsequently collide and combine with other normal oxygen molecules O2, to create ozone O3. The interaction of the UV and oxygen happens most around 40 km, because there is too little photo-dissociation at lower levels, where the UV has been attenuated by the gases already traversed, whilst the thin air at higher levels contains insufficient oxygen for the collisions of single atoms and normal molecules needed to form ozone.

The upper ozone is eventually destroyed, either by photo-dissociation back into normal oxygen and a single atom, or by reactions with either particles and droplets in the air, the ground or various pollutant gases. The gases of special importance nowadays include chlorofluorocarbons (i.e. CFCs) from spraycans, refrigerators and from making some insulation foams of plastic. CFC concentrations are very small (Note 1.D), yet the reactions are catalytic, i.e. the same CFC molecule can eventually destroy an infinite number of ozone molecules (Note 1.E). The

electrons nuc|eus of ++i\o

(+ + +}—7d+ A + )

ATOM

normal MOLECULE

°V+ +J V + Jo o-vjy

O electron

IONISED molecule

DISSOCIATED molecule

Figure 1.4 Various arrangements of the smallest units of oxygen, for example, showing the processes of ionisation (separation of electrical charges) and dissociation (separation of atoms). Ionisation requires bombardment by high-energy cosmic radiation (Section 1.7), whereas dissociation is caused by solar-radiation energy of particular wavelengths. In the case of oxygen, each atom includes eight positive protons (shown by the positive signs in the nucleus) balanced by eight negative electrons. Atoms normally combine to form a molecule, e.g. oxygen molecules (O2) consist of two atoms, whereas ozone is represented by O3.

Figure 1.4 Various arrangements of the smallest units of oxygen, for example, showing the processes of ionisation (separation of electrical charges) and dissociation (separation of atoms). Ionisation requires bombardment by high-energy cosmic radiation (Section 1.7), whereas dissociation is caused by solar-radiation energy of particular wavelengths. In the case of oxygen, each atom includes eight positive protons (shown by the positive signs in the nucleus) balanced by eight negative electrons. Atoms normally combine to form a molecule, e.g. oxygen molecules (O2) consist of two atoms, whereas ozone is represented by O3.

concentration of CFCs in the atmosphere rose rapidly after about 1970, and the increase has been matched by a decline of October ozone at the South Pole (Figure 1.5). The increase is now slowing down as a result of cuts in CFC production since an international agreement in Montreal in 1987, and should reverse at the start of the next century. Unfortunately, the CFCs already existing will take decades to disappear (Note 1.D). Also, they are not the only man-made gases in the atmosphere which can attack the ozone layer. There are increasing amounts of methyl bromide (a fumigant) and various nitrogen oxides (especially nitrous oxide, N2O) from agricultural activities, from supersonic transports at an altitude of about 20 km, from atom-bomb clouds and from combustion at the ground. The various gases facilitate ozone destruction by acting as catalysts in the presence of clouds of ice crystals formed by winter's cooling of the upper air (Note 1.E). The ozone layer is also damaged by sulphurous dust from major volcanic eruptions, e.g. the great eruption of Pinatubo in the Philippines in June 1991 and the smaller one of Cerro Hudson in Chile in August were followed in September by 10 per cent less than normal ozone above McMurdo at 78°S in Antarctica.

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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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