Acid rain

What do we mean when we talk about acid rain?

In general, the term acid rain is used for precipitation which has a pH of less than about 5. This is more acid than the entirely natural process whereby carbon dioxide in the atmosphere dissolves in riianwater to give a weak solution of carbonic acid. A whole range of substances get into the air as a result of burning coal, petrol and other fossil fuels, forest and other landscape fires, agriculture and also from industrial emissions. All these give off a number of chemicals as among which are oxides of sulphur and nitrogen. There is also a more complex process wherebu ammonia can enter the atmosphere, much of it from agricultural sources, and also be converted to oxides of nitrogen. These oxides react with moisture droplets in the clouds to form a weak solution of sulphuric and nitric acid. The rain thus becomes more acidic than it otherwise would be, and falls as 'acid rain'.

Since the emissions are carried in the air and the acid rain falls far from the origins of the emissions, the damage often occurs far from the source of the problem. For example, pollution in Britain is carried by the south-westerly prevailing winds and is believed to have caused much damage in the Scandinavian countries.

What damage does acid rain do?

Acid rain can damage plants, the soil, and lakes and streams into which it enters (either directly or via the soil). Trees, especially conifers with their dense canopies of needle-like leaves, can trap acid rain clouds very effectively. The acid rain droplets are scavenged by the trees and the surfaces of their leaves can be damaged by the acidity which then makes them, and the tree, susceptible to damage by pests and disease. In addition, the concentration of the acid rain drops on the leaf surface can cause the rain to drip down onto the soil below in very large amounts.. The acid enters the soil, can upset the supply of nutrients to the trees and other plants, can seriously damage their roots (especially the fine ones which the plants depend on greatly for absorbing nutrients), and can also make the soil more acid. This does not happen in soils that are naturally alkaline, but soils that are naturally acid can become more so. This can cause acid-sensitive plants to die and can affect the wildlife which feeds on these plants. Thus the overall variety of plants able to grow in a specific area will alter. Plants and trees weakened by the effects of acid rain in this way become more susceptible to disease and early death.

One particular problem associated with acid rain is where the acidity in the rain (typically pH < 4) causes the element aluminium in soil to be released, Not only is this form of aluminium toxic to plants, but some of the dilute acid, and its associated aluminium will pass through the soil and into the streams, rivers and lakes. The acidity itself may kill fish and other water creatures and in Sweden, for example, has led to completely dead lakes, with no living plants or creatures. but this soluble aluminium is highly toxic to fish. Where the soils have sufficient calcium in them, the toxic effects of the acidity and the soluble aluminium can be buffered to a greater or lesser degree.

Acid rain can also cause the mobilisation of potentially toxic metals from the soil - in particular, metals such as Copper and Lead. These metals can have significant health effects for animals and humans. The nitrogen that is added to the land surface through acid rain can be useful to those planrts that require a good supply for their growth, but in ecosystems that require little nitrogen to support the plant communities then problems can occur. In almost any plant ecosystem, there is competition between the plants. If the supply of nitrogen, for example, increases then those plants that make use of it more effectively can outcompete the neighbouring plants that are less successful in this way. This influx of extra nitrogen can, therefore, disturb ecosystems very greatly. This can have an effect on the composition of plant communities, with those plants able to cope with excess Nitrogen outcompeting those that cannot. Similar problems can also arise with the sulphut from acid rain, although these are usually much less severe than with nitrogen. However, the overall reduction in sulphur deposition to soils as the sources of sulphur emissions have been controlled have resulted in problems for some agricultural crops that received most of their sulphur from acid rain. In some parts of the country, it is now necessary to add sulphur to the soil to make up for the shortfall and achieve reasonable crop yields.There are also some reports that acid rain may be a contributing factor in flushes of 'dark water' - water with a high dissolved organic matter content, although the relationship is far from clear and the subject on ongoping research. Discolouration of water can be a real problem for water companies trying to provide clean drinking water.

Buildings are also affected by acid rain, especially if they are built of limestone, sandstone in which the sand grains are cemented together by lime, or concrete. The materials are alkaline and sas the acid reacts with the surface lime it eats into the surfaces and causes cracks to form, this allows weather erosion to take place more quickly.

What can be done about acid rain?

The worst areas in the world for producing acid rain are North America and Europe and, increasingly, China and India. Efforts are being made to reduce pollution caused by industry and by cars. A reduction in the use of fossil fuels such as coal can help, and many cars today run on diesel fuel which generally emits less nitrogen and sulphur than petrol (biofuels are even better in this respect). However there is still much to be done to find alternatives which cause less damage.

Practical things which we can do include walking and using public transport whenever possible; using less electricity, in ways like turning off unused lights, not leaving TVs and computers on stand-by mode, and by using energy-saving devices when we can - such as long-life light bulbs. Even using less water can help as considerable amounts of energy are used to pump water from place to place and to make it drinkable. There are techniques to improve 'dead' lakes and damaged soil, but these are expensive and will not last for long unless we can reduce the causes of acid rain.

Overall, acid rain is not heard of as much now in the media as it was in the 1980's and 1990's. The press seems to have moved on to other issues such as climate change, but this does not mean acid rain has gone away as an issue!