Ozone chemistry speech. Today, I am here to inform you that an ozone hole has been detected over Antarctica and the concentration of ozone will continue to decrease across the globe if the emission of ODCs, such as CFCs, does not reduce.

USA: And who's going to pay the price if we abide to this Montreal Protocol you are suggesting? It would cost the world's economy hundreds of billions of dollars.

With all due respect, Mr. Vice President, the cost of doing nothing could be even higher. Our climate is fragile.

USA: Professor Rowland, our economy is every bit as fragile as the environment. Perhaps you should keep that in mind before making sensationalist claims.

Well, the ozone hole over Antarctica covers an area of approximately 10 million square kilometres. Some people might call that pretty sensational. Please ladies and gentlemen, allow me to first of all explain the equilibrium system of ozone.

(Slide 4) In the stratosphere, where ozone is primarily formed, there is a dynamic equilibrium between the production and destruction of ozone. The forward reaction, that is the formation of ozone, is a combination and photochemical reaction as high-energy UV radiation is necessary to split oxygen molecules into atomic oxygen.

O2 → O + O

The forward reaction can be described as 1 mole of atomic oxygen combined with 1 mole of oxygen molecule to form 1 mole of triatomic ozone.

O + O2 O3

The reverse dissociation and photochemical reaction is when 1 mole of ozone molecule absorbs UV radiation and breaks down into 1 mole of oxygen molecule and 1 mole of atomic oxygen. Harmful UV radiation is converted into heat in the process; therefore the reverse reaction is exothermic. The forward reaction, according to the Law of Conservation of Energy, should be endothermic so the ∆H value of the equilibrium is positive. The equilibrium constant, K, is a very small value (display K<10-4) given that the amount of ozone in the stratosphere equates to a blanket of approximately 3mm thick over the Earth’s surface. The equilibrium lies to the far left, where the reactants are favoured. K has this expression: K=.

(Slide 5) The ozone layer, that is approximately in the middle of the stratosphere (refer to the two graphs), contains the highest concentration of ozone and this can be explained by Le Chatelier’s principle. At the lower and upper stratospheres, the conditions shift the equilibrium to favour the reverse reaction; therefore they have smaller amounts of ozone. The lower stratosphere has a lower temperature to the ozone layer (point to graph), so the equilibrium responds by shifting in the direction of the exothermic reaction and that is to the left, where K has an even smaller value. (Slide 6) At a higher altitude, the pressure drops; therefore the upper stratosphere has a lower pressure, shifting the equilibrium to the left to produce more gas molecules to increase the pressure and attempt to restore the system. Ozone cannot be found higher than the stratopause because the pressure is too low to allow for the forward reaction. In the troposphere, it contains a minimal amount of ozone (point to graph). It is, however, produced differently to the stratosphere and does not form an equilibrium. Since its concentration is just 10% of total ozone and does not relate to the issue we are exploiting today, I will not be discussing ozone in the troposphere.

We should now have an understanding of the equilibrium, so let’s get back onto chlorofluorocarbons. (Slide 7) CFCs do not react on the Earth’s surface and are not even soluble in water so they are free to float to the stratosphere. There, the molecules get broken down by UV radiation, producing chlorine atoms.

(Point to equation) CFCl3 → CFCl2 + Cl
Chlorine atoms readily react with ozone molecules and form chlorine monoxide molecules and oxygen molecules (refer to equation below).

Cl + O3 → ClO + O2
However, chlorine monoxide molecules are not stable- they will react with free oxygen atoms to form oxygen molecules and chlorine atoms.

ClO + O → O2 + Cl

So chlorine atoms are once again produced and are free to destroy more ozone molecules. It has been calculated that 1 chlorine atom can destroy over 100 000 ozone molecules. It acts as a homogeneous catalyst in the equilibrium since an intermediate compound, that is the chlorine monoxide molecule (point to relating equations), is formed that breaks down to give the product and reform the catalyst. The chlorine atom increases the rate of the reverse reaction, significantly reducing the amount of ozone in the stratosphere although it does not shift the position of the equilibrium.

(Slide 8) The ozone hole appears during the winter months of Antarctica, when there is no sunlight, due to its unique stratospheric conditions. Ice clouds called Polar Stratospheric Clouds, PSCs, which contain chlorine nitrate molecules (ClONO2), can be formed and they act as a more powerful catalyst for the reverse reaction in destroying ozone molecules. Chlorine nitrate reacts with hydrogen chloride (HCl), which is a stable form of chlorine gas, to produce molecular chlorine and nitric acid (refer to equation below).

ClONO2 + HCl → Cl2 + HNO3

In early spring when there is sunlight, UV radiation breaks molecular chlorine into chlorine atoms.

Cl2 → Cl + Cl

Since there is an increase amount of reactive chlorine, there will be an abundance of chlorine oxide from chlorine atoms reacting with ozone molecules (Cl + O3 → ClO + O2). This allows a reaction of chlorine oxide with another chlorine oxide that leads to a massive production of reactive chlorine atoms where they are able to destroy many more ozone molecules (point to the equations that show chlorine atoms as products).

ClO + ClO → (ClO)2

(ClO)2 + sunlight →ClOO + Cl

ClOO → Cl + O2

PSCs further increase the rate of the reverse reaction of the equilibrium, destroying ozone very effectively in a short period of time and it is this severe ozone depletion that has led to the development of an ozone hole.

Therefore, I urge all countries to take action because the act of doing nothing will prolong the recovery of the ozone layer and we will be the ones to suffer the consequences at the end. Thank you.

*Reminders: UV~ ultraviolet

ODCs~ ozone-destroying chemicals

CFCs~ chlorofluorocarbons

PSCs~ polar stratospheric clouds

**Many lines used at the beginning of the conference are actual quotes from The Day After Tomorrow.