The Science of the Ozone Hole

Evidence that human activities affect the ozone layer has been building up over the last 20 years, ever since scientists first suggested that the release of chlorofluorocarbons (CFCs) into the atmosphere could reduce the amount of ozone over our heads.

The breakdown products (chlorine compounds) of these gases were detected in the stratosphere. When the ozone hole was detected, it was soon linked to this increase in these chlorine compounds. The loss of ozone was not restricted to the Antarctic - at around the same time the first firm evidence was produced that there had been an ozone decrease over the heavily populated northern mid-latitudes (30-60N). However, unlike the sudden and near total loss of ozone over Antarctica at certain altitudes, the loss of ozone in mid-latitudes is much less and much slower - only a few percentage per year. However, it is a very worrying trend and one which is the subject of intense scientific research at present. More on this in Part IV of the tour.

Many of these findings have since been reinforced by a variety of internationally supported scientific investigations involving satellites, aircraft, balloons and ground stations, and the implications are still being quantified and assessed. In trying to understand how the ozone loss occurs and the things that need to happen to destroy so much ozone, it helps to think of it as a 'recipe'. We need several ingredients to make the ozone loss occur. We'll now look at these 'ingredients' one at a time.

The Special Features of Polar Meteorology

We start by looking at the way the atmosphere behaves over the poles - the features of the meteorology in the stratosphere.

The figure to the right shows schematically what happens over Antarctica during winter. During the winter polar night, sunlight does not reach the south pole. A strong circumpolar wind develops in the middle to lower stratosphere. These strong winds are known as the 'polar vortex'. This has the effect of isolating the air over the polar region.

Since there is no sunlight, the air within the polar vortex can get very cold. So cold that special clouds can form once the air temperature gets to below about -80C. These clouds are called Polar Stratospheric Clouds (or PSCs for short) but they are not the clouds that you are used to seeing in the sky which are composed of water droplets. PSCs first form as nitric acid trihydrate. As the temperature gets colder however, larger droplets of water-ice with nitric acid dissolved in them can form. However, their exact composition is still the subject of intense scientific scrutiny. These PSCs are crucial for ozone loss to occur.

So, we have the first few ingredients for our 'ozone loss recipe'. We must have:

1. Polar winter leading to the formation of the polar vortex which isolates the air within it.

2. Cold temperatures; cold enough for the formation of Polar Stratospheric Clouds. As the vortex air is isolated, the cold temperatures persist.

Chemical Processes Leading to Polar Ozone Depletion

It is now accepted that chlorine and bromine compounds in the atmosphere cause the ozone depletion observed in the `ozone hole' over Antarctica and over the North Pole. However, the relative importance of chlorine and bromine for ozone destruction in different regions of the atmosphere has not yet been clearly explained. Nearly all of the chlorine, and half of the bromine in the stratosphere, where most of the depletion has been observed, comes from human activities.

The figure above shows a schematic illustrating the life cycle of the CFCs; how they are transported up into the upper stratosphere/lower mesosphere, how sunlight breaks down the compounds and then how their breakdown products descend into the polar vortex.

The main long-lived inorganic carriers (reservoirs) of chlorine are hydrochloric acid (HCl) and chlorine nitrate (ClONO₂). These form from the breakdown products of the CFCs. Dinitrogen pentoxide (N₂O₅) is a reservoir of oxides of nitrogen and also plays an important role in the chemistry. Nitric acid (HNO₃) is significant in that it sustains high levels of active chlorine (as explained soon).