Acid rain

What is acid rain?

Acid rain is a harmful and visible side-effect of air pollution caused by contaminating chemicals entering the environment at unsustainable levels.

A primary cause of acid rain is the burning of fossil fuels: the gases released during the combustion process (in particular nitrogen oxides and sulphur dioxide) can in high concentrations react with the water in clouds to form sulphuric and nitric acids. Acid rain can be carried long distances by the wind when in 'wet' form, but it has not always appeared only as rain: it can also take the form of snow, mists and dry dusts.

Acid rain has the potential to be extremely harmful to both the natural and built environments. As well as the obvious consequences of killing plant life and damaging buildings, acid rain increases the acidity of the soil and water, causing extensive and long-lasting agricultural and ecological damage, both at this stage and when it enters the food chain.

Acid pollution in river systems can be reversed to some extent by adding powdered limestone to them in a process called 'liming'.

Experts suggest that a more accurate term for this phenomenon is 'acid deposition'.


Background

The term 'acid rain' was used as long ago as 1858 to mean rain made more acidic by acid gas pollution.

However, the issue of acid rain first came to the attention of the international community in the late 1960s, having been identified in certain areas of southern Scandinavia, where it was damaging forests. The matter quickly became an international issue when it was discovered that the acid deposits in these areas were a result of heavy pollution in the UK and other parts of northern Europe.

Acid rain and air pollution emerged from the industrial boom of the early 1900s onwards and the increasing levels of chemical production associated with these processes. The building of taller industrial chimneys from the 1960s onwards was largely held to be responsible for pollutants generated in the UK blowing as far as Scandinavia.

The Convention on the Long Range Transport of Air Pollution was adopted in 1979. The Convention covers Europe and North America and calls on countries to "endeavour to limit and, as far as possible, gradually reduce and prevent air pollution, including long-range transboundary air pollution". The most recent Protocol to the Convention (the Multi-pollutant- Multi-effect Protocol) was signed in Gothenburg in 1999.

Emissions from industrial sources dropped substantially during the 1990s due to a combination of European Directives forcing the installation of desulphurisation systems and the move away from the utilisation of fuels such as coal. The use of catalytic converters in cars has also helped to reduce nitrogen oxide levels, although there has been a continuing increase in traffic volumes.

Despite the steps taken to combat air pollution and the relative decline of the profile of this issue compared to other environmental concerns, acidification remains a major legacy problem for Europe.


Declining relevance?

In recent years acid rain has been replaced by a much bigger green spectre on the political stage: climate change.

Concerns about the impact of CO2 emissions on the environment established themselves as a major headache for 21st century politicians. With the replacement for the Kyoto protocols now on the agenda expectations are high for real action on the issue.

Acid rain, by contrast, appears to be a declining problem. A study published in November 2007 by researchers from University College London found Britain's streams were becoming darker, reflecting a welcome increase in dissolved organic matter.

Scientists said a "major reduction in acid rain since the 1970s" was causing the change.


Statistics

Unpolluted rain has a pH value of between five and six. Acid rain can increase to a pH value of four. Some rain has even been recorded as being pH2

In Norway, between 1985-95 only four per cent of sulphur deposition was accredited to domestic sources, 96 per cent having come from abroad

In the UK, during the same period, 80 per cent of acidification was attributed to domestic sources

About 20 per cent of annual UK sulphur emissions are deposited within the country. The remainder are exported by the wind to other regions of Europe or the sea

Statistic 1 - (Source: Peoples Trust for the Environment, 2003); Statistics 2 and 3 - (Source: EEA, 2003); Statistic 4 - (Source: Department for Environment, Food & Rural Affairs, Acid Rain: Acidification in the UK, Updated 2001)


Acid rain quotes

"Although some sensitive ecosystems continue to be harmed by acidic pollution and the damage already caused by this pollution will in many places take a long time to be repaired, the widespread phenomenon of acid rain is today on its way to the history books, at least in Western Europe. That didn't happen by chance, we environmentalists made it happen."

Tony Juniper, Executive Director of Friends of the Earth, September 2003