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What is ozone?

Ozone is a gas present in trace quantities in the atmosphere. It is a form of oxygen with three oxygen molecules instead of the normal two. More than 90 per cent of ozone is found in the stratosphere, 20–25 kilometres above the earth, in what is known as the ozone layer.

Ozone plays an important role in protecting the earth from the sun’s harmful effects. Ozone is produced when ultraviolet (UV) radiation from the sun meets oxygen molecules. Once formed, ozone molecules absorb UV radiation before it reaches the surface of the earth. Without this process, life as we know it could not exist.

Although ozone is constantly created and destroyed in the stratosphere through natural processes, some chemicals increase the amount of ozone destroyed. This is particularly true of chlorofluorocarbons (CFCs) – chemicals that were used widely in the past as refrigerants and in some industrial processes.

When gases containing chlorine and bromine reach the stratosphere, they break down to release reactive molecules of chlorine or bromine, which alter the natural balance of ozone creation and destruction. As the concentrations of chlorine and bromine in the stratosphere rise, there is a consequential increase in the depletion of the ozone layer, and in the amount of UV radiation reaching the ground.

Atmospheric ozone and ground-level ozone

While ozone depletion in the upper atmosphere can cause harm to both the environment and human health, an increasing quantity of ozone at ground level can be a concern for air quality.

What is the ‘ozone hole’?

Over the past 30 years, ozone levels over Antarctica have dropped by almost 60 per cent during the spring of each year, and a ‘hole’ in the ozone layer is clearly visible in satellite observations. This hole does not extend over New Zealand. In fact, New Zealand experiences its highest ozone levels in October, at the same time as the ozone hole occurs over Antarctica.

Nonetheless, summertime ozone levels over New Zealand continue to be strongly influenced by Antarctic ozone depletion. When the ozone hole over Antarctica breaks up in November or December, ozone-depleted air moves into surrounding areas in the southern hemisphere, including New Zealand. The later the ozone hole breaks up, the higher the sun is in the sky over New Zealand and so the larger the effect on UV levels. If New Zealand experiences a combination of lower ozone with high sun and few clouds, then skin-damaging UV levels can be extreme.

Increased UV levels

New Zealand’s location in the remote southern Pacific and Southern oceans and its low population density mean it has an exceptionally ‘clean’ atmosphere over most of the country. For example, New Zealand has little of the particulates and shorter-lived industrial gases that affect heavily industrialised countries in the northern hemisphere.

Our latitude and clarity of atmosphere mean that high levels of radiation from the sun reach the ground relatively unhindered. This results in higher levels of UV radiation at ground level in New Zealand than in other developed countries. Ozone depletion further exacerbates this effect, resulting in increased intensity of the UV radiation that causes sunburn.

The environmental and health impacts of raised levels of UV can be severe, particularly for New Zealand where high levels of UV are already experienced. The impacts include increases in the rates of skin cancers and eye cataracts. Raised levels of UV also suppress human and animal immune systems. Plants can also be affected. For example, high UV levels reduce the growth of plankton, a critical building block of the marine food chain. Increased exposure to UV damages some man-made materials such as paints, plastics and construction materials.

Did you know?

In New Zealand there is high seasonal variability in UV levels, especially in the south of the country, where:

  • winter levels of sunburn-causing UV are less than 10 per cent of those in summer
  • winter intensities of UV that produce vitamin D are probably less than 5 per cent of their summertime peaks.

This variability means there are two concerns relating to UV in New Zealand. In the summer months, high UV intensities increase our risk of skin damage compared with corresponding northern hemisphere countries. But during the winter months, there may not actually be enough UV radiation to produce sufficient vitamin D in our bodies.


Links between climate change and ozone depletion

Climate change and ozone depletion are accelerated by human activity. Many ozone-depleting gases are also greenhouse gases. By reducing the use of ozone depleting gases, we can both protect the ozone layer and reduce climate change.

 

This information has come from the latest national state of the environment report Environment New Zealand 2007.

 

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Last updated: April 2009