Ozone+Depletion

=E4: Ozone Depletion= = =

=== Measurements made over the last 40 years show that particularly over the poles there was a thinning of the ozone layer. Satellite measurements of the ozone layer Antartica revealed that there was a hole in the layer, the size of the united states. The ozone layer over Antarctica was depleted by more than 50% in 20 years (1965-1985). A similar hole was discovered above the Arctic, but smaller in size. These holes are seasonal over the poles, being greatest in winter and early spring. Other data from satellites showed that the ozone layer was depleted even more over non-polar regions (about 3%). === ==

As can be seen from the above picture, this hole in the ozone layer over Antarctica is very significant.

The main chemicals responsible for this include:
1. Chlorofluorocarbons (CFC's) found as propellants in spray cans: deodorant, aerosols, coolants refrigerators, air conditioners. CFC's (usually can be easily broke by the UV light in the stratosphere giving highly reactive chlorine free radicals. These can react with more than 100,000 molecules of ozone, thus depleting the ozone layer.

2. Nitrogen oxides from combustion of hydrocarbons. Nitric oxides react with ozone to form oxygen gas (O2), thus interfering with the natural process of formation of ozone. = = =E9: Further Ozone Depletion=

The mechanism for Ozone Depletion through CFCs is shown in the diagram below, and is much easier to understand visually!

===

Due to the nature of this regeneration of free chlorines, it is estimated that a single CFC molecule can destroy approximately 100,000 ozone molecules. **Nitrogen Oxides** have a similar catalysis reaction mechanism with ozone under the influence of UV light.

**Why is depletion worst at the Poles, and during early spring?**
This is thought to be due to the formation of ice-crystals in clouds during the colder months. This ice contains traces of HCl and other chlorine compounds, and so serves as the surface for a reaction that releases additional chlorine radicals into the stratosphere . This effect is increased as the ice melts, hence ozone destruction being worst in early spring, and becomes impossible when most of the ice crystals have melted. This impact can easily be seen in the above diagrams, that compare the amounts of ozone in the ozone layer over the South Pole, between 1st January 1996 and 30th November 1996. The changes over such a relatively short period of time are quite drastic.

So what?
Well, the primary effect upon humans is that more UV light reaches the planet, and so the chances of getting diseases such as skin cancer are increased. Skin cancer is caused by the UV light breaking the bonds in DNA molecules, so leading to the growth of cancerous cells.

Our skin has a natural pigment called melanin that attempts to stop this taking place - this pigment is what determines the darkness of skin, and causes tanninh. However, this is frequently not enough, and so chemical suncreens are employed.

Most of these sunscreens contain substituted benzene molecules (for example, 1,4 amino benzoic acid), which contain delocalised pi electrons. These absorb the UV light, as the delocalised electrons can become excited by the energy contained within the UV without breaking their bonds. Thus, the UV light does not reach the skin. = = Simplified representation of benzene's delocalised pi electrons.

Sunblocks, though often colloquially considered the same as sunscreens, in fact work differently. Substances in them such as zinc oxide //reflect// the UV light instead of absorbing it.

As an interesting aside, it appears that - above a certain point - increasing the SPF value of sunblock does you little good.