Ozone, O₃, is a naturally occurring component of the stratosphere. It is a very pale bluish with an acrid (pungent-smelling) odour, very active chemical properties (a powerful oxidising agent) and has harmful effects on living matter. It plays a different role in the upper and lower atmospheres. Inhaling a small amount of ozone can be harmful, yet it is essential to life and health by its presence in the ozone layer. The ozone layer is about 15 to 45 km above the earth’s surface and holds much of the air’s ozone.

Very short wavelength ultra violet (uv) light from the sun splits O₂ molecules into oxygen atoms which are extremely reactive free radicals (represented by •)

O_{2 (g )} + uv light → 2•O

These oxygen atoms react with other oxygen molecules, O₂ to form O₃:

O_{2 (g)} + •O _(g) → O_{3 (g)}

The photo-dissociation of molecular oxygen by uv light represents the principle mechanism of ozone’s formation in the upper atmosphere. The reverse reaction takes place when O₃ absorbs longer wavelength uv light.

O_{3 (g)} → O_{2 (g)} + •O _(g)

O_{3 (g)} + •O _(g) → 2O_{2 (g)}

So, O₃ is constantly being formed and broken down. The ozone layer acts as a shield by absorbing 99% of the sun’s harmful uv light of longer wavelength than that absorbed by O₂ and N₂.

Satellite data over a sixteen year period from 1979 to 1995 shows a clear decline in ozone concentration by about 6% in latitudes 60° south to 60°north. Similarly satellite pictures show that greatest destruction of the ozone layer at the South Pole, covering an area almost equal in size to North American continent! According to Environment Canada, The Antarctic showed a 70% decrease on the thickness of the ozone layer during the spring of 1996, while in Arctic, the ozone values where 45% below normal. Satellite pictures over the North Pole also clearly indicate Ozone holes where depletion of ozone has taken place.

(Source: NASA, year 2006) (The blue and purple colors are where there is the least ozone, and the greens, yellows, and reds are where there is more ozone.)

 

Ozone in the ozone layer is being reduced by:

1. CFCs, chlorofluorocarbons (freons) are used in spray cans as propellant and in and refrigerators and air conditioners, in fire extinguishers and as solvents. When released, the CFCs, because they are chemically very inert, do not decompose and float slowly through the atmosphere into stratosphere. When they reach the unfiltered ultraviolet rays of the sun, they are turned into extremely reactive chlorine atoms with an unpaired electron which very readily reacts with ozone.

CCL₂F₂ + uv light → •CClF₂ + •Cl

The average bond enthalpy of C-F bond is 484 kJ mol^-1 and only 33 kJ mol^-1 for the C-Cl bond; the C-Cl bond is weaker and breaks. The Cl atom, also a free radical, is very reactive and readily reacts with O₃ to produce O₂.

•Cl + O₃ → ClO + O₂

2. Nitric Oxide, formed from the high temperature reaction of N₂ and O₂ in supersonic aircraft engines, reacts with ozone reducing its concentration:

NO + O₃ → NO₂ + O₂

A decrease in ozone concentration means more uv light reaches the earth, thus increasing cases of skin cancer and eye cataracts, more sunburn and damage to animals and plants including suppression of plant growth, genetic mutations and changes in the world’s climate. Note as little 0.2 ppm O₃ near the earth’s surface promotes photochemical reactions responsible for smog. Near the earth’s surface, O₃ attacks many products such as tyres, rubber products and tobacco crops and can cause extensive crop damage.

 

Alternatives to CFCs:

1) Use of propane, C₃H₈ and 2-methylpropane hydrocarbons as refrigerant coolants. Although these do not lead to ozone depletion, they are flammable as well as being greenhouse gases (Able to absorb infrared radiation) and would lead to global warming.

2) Fluorocarbons: These are neither toxic nor flammable and very strong C-F bond makes them stable to uv radiation so they cannot catalyze ozone depletion. However, these are greenhouse gases and would eventually lead to an increase in the global temperature.

3) Hydrochlorofluorocarbons, HClFCs contain hydrogen, chlorine, fluorine, and carbon atoms in their molecules. The presence of a hydrogen atom makes the compound decompose less easily since the C-H bond is stronger than the C-Cl bond in the molecule and can only be considered as temporary solution.

4) HFCs (hydrofluorocarbons), without any chlorine atoms, are considered good alternative to CFCs as no chlorine atoms, which are primarily responsible for ozone depletion, are involved. One such example is CF₃CH₂F, 1,1,1,2-tetrafluoroethane.