Techniques + methods used to investigate the chemistry of the stratosphere And links to CFCs

Open Book Paper 1997

It is now an internationally accepted scientific fact that the reactions involving man-made CFCs emitted are responsible for some ozone depletion in the stratosphere. This can be proved in numerous different ways, one early way was to send up helium balloons or planes, With this we are can find out what molecules are present in the stratosphere by the use of infra-red and ultra-violet absorption spectra [1]. All chemicals have different characteristic absorption spectrums. Ozone has a specific absorption range at Ultraviolet radiation of wavelengths below 360nm. By analysing the strength of absorption at these wavelengths, we can work out the concentration of the Ozone present. Measurements are taken in a wide variety of conditions to give us an insight into the normal composition of the stratosphere, but to measure a rate of change we require continuous measurements. Because of the random nature of balloon and plane flights, ground-based stations were setup to take measurements continuously by looking directly upwards into sky, using the sun's rays as the light source. These measurements were also verified by NASA's ozone monitor aboard one of its satellites, which works by measuring UV reflection ratio of the stratosphere directly from space.

In addition to doing this we need to also look at the reactivity of the molecules in the stratosphere which react with ozone, to do this we would need to know the rate of reaction, what wavelength of radiation ozone absorbs and the types of reactions that take place. This is done by using flash photolysis (used because reactions are very quick and hard to observe). Using flash photolysis, we are able to find the rate constant of the reaction. The rate constant depends on the temperature and pressure of which the reaction is happening and determines how fast ...

This is a preview of the whole essay

The alarm of Ozone depletion by CFC was raised in 1972. James Loverlock developed a method to detect CFCs in the troposphere and found that they are so stable that they will accumulate in the atmosphere. Then, Sherwood Rowland found that CFCs will absorb the high energy ultra-violet radiation and release Cl in the stratosphere, which can react with 100,000 molecules of ozone. Ten years later, in 1987, Farman discovered the ozone hole by measurement. Scientist also flew over the hole and found that there is a drop in Ozone where ClO concentration is highest. This is shown below in the diagram which also shows the corresponding ClO concentrations.

A satellite image showing a hole in the ozone layer over Antarctica (right) & Arctic (left),

http://www.jpl.nasa.gov/archive/mpe.html

In the South Pole, the sun disappears in winter and a vortex of cold air forms. This cause clouds to be formed in the stratosphere and they are usually frozen sulphuric acid with nitric acid trihydrate (HNO3•3H2O) coated around them. At 190K, water vapour will condense as ice on these particles as well. This Solid particle provides a surface on which this occurs:

HCl + ClONO2 → Cl2 + HNO3

Chlorine will be released. As the sun comes back in spring, the radiation will cause this to occur:

Cl2 + hv → 2Cl•

This will cause an even more severe Ozone depletion due to more Cl• being around in the stratosphere.

CFCs and their replacements

CFC were considered better than ammonia, CO2, water, propane or SO2, which are some of the early refrigerants available. They either had an unsuitable boiling point which increased the cost and size of the equipment dramatically, or were too inflammable for safe use. Some were even toxic and required special handling procedures. So therefore this is the reason why CFCs where widely used, and because of the thermal insulation properties of some CFCs and their chemical inertness, they also found use in areas as diverse as blowing insulating-foam, cleaning agent for printed circuit-boards, dry-cleaning and as propellant for aerosol cans. It was almost possible to find a CFC for every application.

When CFCs' destructive effects were discovered, there were no known replacements, but it was soon found that HFCs (hydro fluorocarbons), a closely related series of compounds, had most of the wonderful qualities. HFCs don't destroy ozone because fluorine is strongly bound to the carbon and even the energy given by ultraviolet rays is not enough to liberate the fluorine.

Some of these replacements have less energy efficiency than the original CFCs. HFCs are also used as a replacement in the short term - they again have similar properties to CFC but they can still cause small amounts of ozone depletion. The cost for manufacturing HFCs are higher then CFCs as was equipment is needed to suit the properties of HFCs (equipment needed to work at slightly higher pressures). In some cases, a replacement is not even needed at all - many aerosols are being replaced by air pumps or electrostatic charged sprays. This reduces the amount of chemical released into the atmosphere. But they are expensive to implement and are inconvenient.

Abstract

Scientists have linked that the use of CFCs the release of chlorine radicals which destroy the steady state of ozone and oxygen, this was done though such methods as spectroscopy and flash photolysis. CFCs were used in refrigeration, aerosols and cleaning products. HFCs where introduced but there were some difficulties in doing this.