# Composition and structure  (Page 3/3)

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${O}_{2}\to O+O$
$O+{O}_{2}\to {O}_{3}$

The change from one type of molecule to another produces energy, and this contributes to higher temperatures in the upper part of the stratosphere. An important function of the ozone layer is to absorb UV radiation and reduce the amount of harmful radiation that reaches the Earth's surface.

## Cfcs and the ozone layer

You may have heard people talking about 'the hole in the ozone layer'. What do they mean by this and do we need to be worried about it?

Most of the earth's ozone is found in the stratosphere and this limits the amount of UV radiation that reaches the earth. However, human activities have once again disrupted the chemistry of the atmosphere. Chlorofluorocarbons (CFC's) are compounds found in aerosol cans, fridges and airconditioners. In aerosol cans, it is the CFC's that cause the substance within the can to be sprayed outwards. The negative side of CFC's is that, when they are released into the atmosphere, they break down ozone molecules so that the ozone is no longer able to protect us as much from UV rays. The 'ozone hole' is actually a thinning of the ozone layer approximately above Antarctica. Let's take a closer look at the chemical reactions that are involved in breaking down ozone:

1. When CFC's react with UV radiation, a carbon-chlorine bond in the chlorofluorocarbon breaks and a new compound is formed, with a chlorine atom.
$CFC{l}_{3}+UVlight\to CFC{l}_{2}^{-}+C{l}^{+}$
2. The single chlorine atom reacts with ozone to form a molecule of chlorine monoxide and oxygen gas. In the process, ozone is destroyed.
$C{l}^{-}+{O}_{3}\to ClO+{O}_{2}$
3. The chlorine monoxide then reacts with a free oxygen atom (UV radiation breaks O ${}_{2}$ down into single oxygen atoms) to form oxygen gas and a single chlorine atom.
$ClO+O\to Cl+{O}_{2}$
4. The chlorine atom is then free to attack more ozone molecules, and the process continues. A single CFC molecule can destroy 100 000 ozone molecules.

One observed consequence of ozone depletion is an increase in the incidence of skin cancer in affected areas because there is more UV radiation reaching earth's surface. CFC replacements are now being used to reduce emissions, and scientists are trying to find ways to restore ozone levels in the atmosphere.

## The mesosphere

The mesosphere is located about 50-80 km above Earth's surface. Within this layer, temperature decreases with increasing altitude. Temperatures in the upper mesosphere can fall as low as -100 ${}^{\circ }$ C in some areas. Millions of meteors burn up daily in the mesosphere because of collisions with the gas particles that are present in this layer. This leads to a high concentration of iron and other metal atoms.

## The thermosphere

The thermosphere exists at altitudes above 80 km. In this part of the atmosphere, ultraviolet (UV) and shorter X-Ray radiation from the sun cause neutral gas atoms to be ionised . At these radiation frequencies, photons from the solar radiation are able to dislodge electrons from neutral atoms and molecules during a collision. A plasma is formed, which consists of negative free electrons and positive ions. The part of the atmosphere that is ionized by solar radiation is called the ionosphere . At the same time that ionisation takes place however, an opposing process called recombination also begins. Some of the free electrons are drawn to the positive ions, and combine again with them if they are in close enough contact. Since the gas density increases at lower altitudes, the recombination process occurs more often here because the gas molecules and ions are closer together. The ionisation process produces energy which means that the upper parts of the thermosphere, which are dominated by ionisation, have a higher temperature than the lower layers where recombination takes place. Overall, temperature in the thermosphere increases with an increase in altitude.

## The ionosphere and radio waves

The ionosphere is of practical importance because it allows radio waves to be transmitted. A radio wave is a type of electromagnetic radiation that humans use to transmit information without wires. When using high-frequency bands, the ionosphere is used to reflect the transmitted radio beam. When a radio wave reaches the ionosphere, the electric field in the wave forces the electrons in the ionosphere into oscillation at the same frequency as the radio wave. Some of the radio wave energy is given up to this mechanical oscillation. The oscillating electron will then either recombine with a positive ion, or will re-radiate the original wave energy back downward again. The beam returns to the Earth's surface, and may then be reflected back into the ionosphere for a second bounce.

## Interesting fact

The ionosphere is also home to the auroras . Auroras are caused by the collision of charged particles (e.g. electrons) with atoms in the earth's upper atmosphere. Charged particles are energised and so, when they collide with atoms, the atoms also become energised. Shortly afterwards, the atoms emit the energy they have gained, as light. Often these emissions are from oxygen atoms, resulting in a greenish glow (wavelength 557.7 nm) and, at lower energy levels or higher altitudes, a dark red glow (wavelength 630 nm). Many other colours can also be observed. For example, emissions from atomic nitrogen are blue, and emissions from molecular nitrogen are purple. Auroras emit visible light (as described above), and also infra-red, ultraviolet and x-rays, which can be observed from space.

## The composition of the atmosphere

1. Complete the following summary table by providing the missing information for each layer in the atmosphere.
 Atmospheric layer Height (km) Gas composition General characteristics Troposphere 0-18 Turbulent; part of atmosphere where weather occurs Ozone reduces harmful radiation reaching Earth Mesosphere High concentration of metal atoms more than 80 km
2. Use your knowledge of the atmosphere to explain the following statements:
1. Athletes who live in coastal areas need to acclimatise if they are competing at high altitudes.
2. Higher incidences of skin cancer have been recorded in areas where the ozone layer in the atmosphere is thin.
3. During a flight, turbulence generally decreases above a certain altitude.

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