Acid rain

In short: Acid rain is precipitation that is noticeably more acidic than normal. It forms mainly when sulphur dioxide and nitrogen oxides from exhaust gases react in the air to form acids.

In more detail: Even clean rain is slightly acidic because carbon dioxide dissolves in it; its pH is around 5.6. When sulphur and nitrogen compounds enter the atmosphere, sulphuric acid and nitric acid form and the pH drops further.

In detail

• Sources: Sulphur dioxide comes mainly from burning sulphur-containing coal and oil. Nitrogen oxides form during hot combustion in engines, power stations and heating systems. Volcanoes also release sulphur gases.
• Long distances: The pollutants can be carried hundreds of kilometres by the wind. Acid rain therefore often falls far from its sources.
• Not just rain: Snow, fog and dry dust can also deposit acidic substances on plants and soils.
• Forests: Acid precipitation washes nutrients out of the soil and releases harmful aluminium compounds. Trees are weakened and become more vulnerable to frost and pests.
• Lakes: In areas with lime-poor bedrock, such as Scandinavia, many lakes became so acidic in the 20th century that fish disappeared.
• Buildings: Acid attacks limestone and marble. Many old statues and facades show clear damage as a result.
• Countermeasures: Flue-gas desulphurisation in power stations, low-sulphur fuels and catalytic converters in cars have greatly reduced emissions in Europe since the 1980s.
• Liming: To relieve acidified soils and waters, lime is spread in some places to neutralise the acid.
• Background: How acidic something is is given by its pH value (ph-wert-saeuren-und-laugen, wald-aufbau-und-bedeutung, feinstaub-grundlagen).