Acid Rain

Rainfall is naturally acidic. Carbon dioxide reacts with water vapour in the atmosphere to produce weak carbonic acid. This is enough to give rainfall a typical low acidity of between 5-6 pH (where 7 is neutral). By itself this can account for chemical weathering of carbonate-based rocks (chalk and limestone) over time.

The Causes of Acid Rain

Human activity has resulted in greater acidity of rainfall and more widespread impacts over shorter time periods. The release of large quantities of sulphur dioxide and nitrogen oxide into the atmosphere, largely as a result of burning fossil fuels, has resulted in reactions with water vapour to produce much stronger acidity in rainfall (pH of 3 and 4) and precipitation (rain and/or snow) being the chemical equivalent of sulphuric acid and nitric acid.

The most significant contributors of sulphur dioxide and nitrogen oxide in the past 80 years have been:

• Coal-fired power-stations
• Rapid industrialisation in newly emerging economies burning coal and oil.
• The rise in car production, ownership and use.
• Increasing air traffic.

International Scale of Acid Rain

Whilst the contributing gases to acid rain are generated in urban, industrial centres, the effects of acid rain are often experienced hundreds, if not thousands, of miles away from the pollution source. The action of prevailing winds distributes the atmospheric pollutants downwind so that when the resultant acid rain/snow falls, it is affecting different territory in, often, different countries.

The international scale of this problem makes it difficult to attribute blame to any one particular source area or get source countries to accept responsibility for their emissions. Norway, for example, receives over 90% of its sulphur deposition from other countries, but that may be from Britain, northern France, Germany or Poland depending on the wind direction.

The Effects of Acid Rain

If rain or snow falling onto ground, buildings or into lakes is highly acidic, the chemical impacts can have rapid and widespread consequences:

• Building stone weathers faster – particularly older monuments constructed of limestone and chalk. Cathedrals and minsters, such as York Minster, require more regular, expensive, renovation of weathered stone, and the Taj Mahal in India is experiencing similar problems as a result of India’s rapid emergence as an industrial power.
• Forests can be decimated as roots are highly sensitive to acidity of water. The fine nutrient-absorbing rootlets are damaged, weakening growth and opening the tree up to infection and disease. Over 50% of forests in the Netherlands are reported to have been damaged by acid rain. In countries where the production of timber is a significant export, such as Canada, any damage affects the economy as well as the environment.
• A secondary consequence, as forests decline, is that habitat and the forest ecosystem suffer. Plants and animals that depend on the forest cover reduce in number.
• Rivers and lakes may receive a constant influx of acidic water from the catchment area, or it may be a sudden influx in spring as acid ‘snow’ melts with warmer temperatures, releasing a flush of acidity that has accumulated over the winter. This can be particularly damaging to young immature fish or ‘fry’. Many salmon rivers and lakes of Scotland and Canada have seen significant falls in salmon stock that is attributed to the effects of raised acidity of the water.
• Human health may be affected if acidity in the water increases corrosion of water pipes. Swedish health consultants traced the cause of intestinal discomfort and ‘green hair’ amongst children in rural parts of Sweden in the 1980s to rapid corrosion of copper pipes supplying water to older properties.

Responding to Acid Rain

In affected regions:

• Lime (crushed limestone) has been spread over the surface of lakes to neutralise the acid in Scotland and Canada. Lime is alkali.
• Copper pipes have been replaced by plastic piping delivering water to homes in Scandinavia.

In source regions:

• International agreements since the 1970s have resulted in cuts in emissions of contaminants and polluting emissions, particularly in Europe and North America. The European Union has been a major driver in seeking agreement amongst member countries to reduce sulphur dioxide and nitrogen oxide emissions since 1979. In 1988 an EU directive required power stations to reduce emissions – in the UKs case, by 60% in sulphur dioxide and 30% in nitrogen oxide. This is achieved in part by lining chimney flues with limestone to neutralise the acidic gas emitted (flue-gas desulphurisation), partly by subsidising power-stations to change to low-sulphur coal, and by encouraging the growth of gas-fired power-generation, subsidising sustainable, renewable energy production(particularly wind power) and reducing coal-burning facilities.

International Assessment of Acid Rain

The areas suffering the worst effects of acid rain emerged in the 1970s and ‘80s in NE North America and Central, Eastern and Northern Europe.

International transboundary emission agreements since then have been largely effective in significantly reducing sulphur and nitrogen emissions and reducing the scale and impact of acid rain degradation. The problem has not been solved, but by contributing countries agreeing to strict emission reductions the issue is less severe than in the latter decades of the twentieth century.

The current focus for acid rain action is in Asia where the rapid industrialisation of India, China and neighbouring nations has seen the rapid expansion of coal-fired power stations and direct burning of high-sulphur-content coal by industry. Compounded by the continuing growth of Asian megacities and the rapid rise in car ownership, the challenge for dealing with the consequences of acid rain has shifted firmly to the Newly Industrialising Countries (NICs) of Asia.