Freak rains may have turned drought-prone areas like Barmer into lakes overnight and flooded parts of Kashmir. But a more freakish downpour drenches one-third of China: acid rain. China’s metal foundries and coal-fired power and coking plants belch millions of tonnes of sulphur dioxide (SO2) and nitrogen dioxide (NO2) into the skies, which come down as acidic rain. This threatens soil quality, increases levels of heavy metals in drinking water, damages crops, corrodes bridges and buildings and causes premature human deaths.
British chemist Robert Angus Smith coined the term ‘acid rain’ in 1870 when he noticed that smoke and steam of early industrialisation contained substances that changed the chemical composition of rain. He also discovered some of its harmful effects, like discolouring of fabrics, corrosion of metal surfaces, deterioration of building materials and withering of plants. Smith’s warning, alas, went unheeded and the ‘developed’ world only began to take acid showers seriously in the Fifties.
Rain turns acidic when SO2 and NO2 mix with atmospheric moisture and are transformed into sulphuric and nitric acid deposits. Acidity is measured using the percentage hydrogen, or pH, scale. Distilled water has a pH of 7, which is considered neutral. A substance with a pH less than 7 is acidic, while those with a pH over 7 are alkaline. Emissions from volcanoes, sea spray, rotting vegetation and plankton make even normal rain mildly acidic, with a pH of 5.7. When Earth was young, chemicals in the atmosphere produced downpours similar to the sulphuric acid showers on Venus today.
But the real threat is human activity. Power plants that burn coal with high sulphur content, and vehicles and heating systems that emit nitrogen oxides are the largest sources of acid deposition. The deadly drizzle they produce has forced ancient buildings and monuments in many cities to weather more during the last 20 years than in the preceding 2,000, as limestone and marble turn to gypsum (a crumbling substance) upon contact with acid.
Paradoxically, reducing acid rain could cause more environmental damage. For, low-sulphur coal has more mercury and trace metals that worsen toxic air pollution. And its lower energy value means power plants must burn more of it to generate the same amount of electricity. This produces more carbon dioxide, contributing to global warming. Tricky, isn’t it — learning how to solve one environmental problem without making another worse?