The Ozone Layer (homepage) - still under construction - NB: Headers are currently links

{{Stuff to mention at some point:1) At high altitude, low P, not very many atoms chilling out, collisions relatively rare.2)Ozone layer is naturally thin already - if total ozone column compressed to surface p,T, would be ~ 3mm thick. }}

This series of entries hopes to explain how ozone protects our planet, what we have done to interfere with this protection, and how we are trying to make things better. This first entry is a sort of grand overview, and each main section also has a "satellite" entry with more¹ specific information which can be reached by clicking on the header. Hopefully things will be kept simple, but before going any further, the following should be noted:

Study of the atmosphere is made insanely complicated by the fact that there really is a huge amount of air up there, and it is made up of thousands of different compounds. To make matters worse, the potential chemistry that could happen changes with the sun, and the sun switches on and off every 12 hours at the equator, every 6 months at the poles, and at different times everywhere else. Then there is the bad news: the atmosphere spends all of its time moving around an awful lot in mysterious ways that differ greatly all around the planet.

The upshot of all this that it is not particularly clever to make sweeping generalisations about what is going on in the atmosphere. However, in the interests of simplicity, some short cuts will be taken in these entries.

Natural Stratospheric Ozone

Ozone is the common name for triatomic oxygen (O₃). Along with diatomic oxygen (O₂), ozone plays a very important role in the earths stratosphere, where the two compounds both filter harmful wavelengths of light from the sun.

The so called ozone layer is a region of the stratoshpere about 30 km above the surface of the earth. In this region the relative concentration of ozone compared to all the other compounds present is higher than at any other altitude.

Without the interference of man, the ozone layer would have a relatively constant concentration, as the processes for ozone formation and ozone destruction in nature are roughly in equillibrium.

CFCs

Unfortunately, human kind has upset this equillibrium by releasing nasty chemicals into the atmosphere. CFCs² are very useful compounds, as they make effective refrigerants and aerosol propellants. Unfortunatley, they also make good sources of chlorine radicals, and these in turn make good destroyers of ozone.

Chlorine (Cl) and fluorine (F) are elements from the 7th group of the periodic table. This group also contains bromine (Br) and iodine (I), and collectively these elements are called the halogens.

As well as CFCs, many other halogen containing compounds have been released, and after a while doing nothing very much in the troposphere, they drift up into the stratosphere, react with sunlight, and then produce halogen radicals³ which then destroy the ozone.

The Hole in the Ozone Layer

Ozone depletion is at its worst in the spring over the poles, particularly over the antarctic. This is due to the extreme conditions present during the antarctic winter - the constant darkness for half a year not only makes it intensely cold, but also effects the chemistry that goes on.

During winter a large concentration of photo sensitive chlorine containing species build up, and when the sun eventually rises, these species are broken up, creating a large pulse of chlorine radicals which then go on to destroy the ozone, creating a local "hole" in the ozone layer above the pole, which gradually recovers during the summer.

The Montreal Protocol

In 1987 many major governments agreed that the release of CFCs into the atmosphere should be controlled and eventually stopped altogether. In Montreal, a plan to slowly phase out CFCs, and the other compounds which damage the ozone layer, was created, replacing them with compounds less harmful for the environment.

This is all well and good, but unfortunately many of the CFCs that were released into the troposphere before the Montreal Protocol was signed are still there - mixing between the troposphere and the stratosphere is very slow. Most CFCs and other ozone depleting compounds are very long lived however, so they will eventually make their way up into the stratosphere and become ozone destroying radicals. However, despite this time lag of pre-released ozone depleting compounds, the hole in the ozone layer is already smaller than it was jsut before the Montreal Protocol was signed.

Ozone in the Troposphere

In a final twist to this tale of stratospheric ozone depletion, there is in fact too much ozone in many parts of the troposphere. Produced in the troposphere via the photolysis of NO₂, ozone is one of the contributors to photochemical smog, the dense smog which plagues many major cities such as Los Angeles.

Sadly, as mentioned before, the mixing between the troposphere and the stratosphere is very slow, so the bulk of the polluting ozone in the troposphere will not make it into the stratosphere where it is badly needed to protect this planet from the sun.