What's the Impact of a Volcano on the Atmosphere?

People and governments in many countries of the world are increasingly concerned about global warming and climate change. Human activity has released a lot of carbon dioxide and other pollutants that act as greenhouse gases during the last 250 years, since the Industrial Revolution.

While we are trying now to do something about this, there are events that also increase pollution and contribute to environmental change, but that we have no ability to control. One of these events is a volcanic eruption. Some eruptions can be predicted as we constantly monitor seismic and lower Earth activity in the earthquake and volcanic zones but these can still sometimes occur without much warning. After the devastating earthquake hit Japan in March 2011, there was a 30-50 minute warning only for people on the coast that a tsunami had been triggered.

What Gases Does a Volcano Produce?

The major component of the eruption from many volcanoes is water vapour. This can act as a greenhouse gas, and can contribute to short term effects, but it is generally regarded as being relatively harmless. Once the eruption is over, this generally disappates in the atmosphere and the environment.

Carbon dioxide is also produced and this is blown into the atmosphere with great force. The carbon emissions from volcanoes do contribute to global warming and climate change by their impact on weather but it is important to keep a perspective on their overall effect compared to the activities that we are responsible for. Human industrial activity pushes about 10 billion tons of carbon emissions into the atmosphere in a 12 month period – volcanic activity produces only about 110 million tons in the same timeframe.

The other gases produced when a volcano erupts have the potential to be much more dangerous. Two of the most worrying are sulphur dioxide, chlorine and fluorine, which dissolve very easily in water forming the strong acids hydrochloric acid and hydrofluoric acid.

The Impact of Sulphur Dioxide

Sulphur dioxide is often produced in a volcanic eruption – it has the characteristic smell of rotten eggs and is often smelled in an area before a volcanic eruption as it seeps through the surface layers of the Earth. Once the eruption has happened, sulphur dioxide gas enters the lower atmosphere, which has a much lower temperature than the surface of the Earth. Light from the Sun stimulates the reaction between sulphur dioxide and the water vapour also produced, forming an aerosol of sulphuric acid.

This, unlike the fine ash particles that are released in the eruption, do not fall to Earth. They are much smaller and stay in a layer about 15km above the Earth‘s surface, diffusing over a wide area and forming an acidic band in the Earth’s atmosphere. This has the opposite effect of carbon dioxide in the atmosphere, because it leads to global cooling. The effects are not great enough to counteract the effects of the greenhouse gases, but large eruptions have been associated with climate change caused by a lowering of average temperatures in the few years that follow.

Gradually, the layer disappears but every time there is a new volcanic eruption, a new layer forms, so there is generally always some sulphuric acid in the atmosphere. Recent research has shown that replenishment occurred after the El Chichon eruption in Mexicco in 1982, and after Mount Pinatubo exploded in 1991.

Chlorine and Fluorine

Both of these gases are released in a volcanic eruption and both dissolve in water to form very corrosive acids. Hydrochloric acid degrades to form free chlorine and chlorine monoxide, both of which are highly poisonous and damaging to plant life and buildings as they can decrease the pH of rain – a phenomenon known as acid rain. The chlorine free radicals that also form react with ozone, and can cause significant damage to the protective ozone layer that exists in the atmosphere.

This effect was exacerbated by human activity when we started using aerosols that contained chlorofluorocarbons (CFCs) but since these have been banned globally, the effect is not as great. Fluorine is very reactive with water, and tends to fall to Earth as highly toxic acid rain; this can lead to local pollution that can wreak havoc on plant and animal life.