Rain is naturally acidic, even in unpolluted areas, due to carbon dioxide in the atmosphere which dissolves into rain water. As a consequence, rain has a pH close to 5.4. Acid rain has a pH that is lower than normal 5.4.
Rain can become more acidic when man-made pollutants enter into the atmosphere and become dissolved into rainwater. The oxides of sulphur (SO2) and nitrogen (NO2) are the main source of acidifying pollutants, principally derived from the burning of fossil fuels such as coal and oil as well as ammonia which is mainly derived from intensive agriculture. However there are also natural sources, such as from volcanoes, oceans, biological decay and forest fires.
The increasing demand for electricity and the rise in the number of motor vehicles in recent decades has meant that emissions of acidifying pollutants have increased dramatically from human sources, particularly since the 1950s. Emissions of such pollutants are heavily concentrated in the northern hemisphere, especially in Europe and North America. As a result, precipitation is generally acidic in these countries.
Most of the SO2 and NO2 produced in Britain now comes from power stations and large industry. Transport has also contributed to the problem, although, since catalytic converters have been fitted to new cars the amount of nitrogen oxides emitted per car has reduced. However, since the volume of traffic in the UK has increased, levels of NO2 from this source have not fallen significantly.
Acid rain is a problem because it:
- Can be carried over large distances by winds affecting places far removed from its source. As lakes acidify, fish and small invertebrates are killed.
- It dissolves nutrients in the soil, which are then leached out, making the soil infertile and killing large numbers of trees.
- Attacks the stonework of buildings and certain metal structures, costing millions of pounds to treat.
It is a ‘transboundary’ problem and as a result the first international agreement, the Convention on Long Range Transboundary Air Pollution, was agreed in 1979. Since then a range of protocols have been agreed to cut emissions, the most recent of which set emissions targets for sulphur dioxide, nitrogen oxides, ammonia, and volatile organic compounds, to be met from 2010. The UK is committed to annual emissions ceilings of 625 ktonnes for sulphur dioxide, 1,181 ktonnes for nitrogen oxides and 297 ktonnes for ammonia.
