酸雨Acid rain

清洁雨水因溶解 CO₂ 而 pH 约为 5.6，所以这一数值被作为酸雨的界限。SO₂ 在气相中被 OH 自由基氧化，或在云滴中被 H₂O₂、O₃ 氧化为 H₂SO₄；NO 和 NO₂ 被氧化后与水生成 HNO₃。中国的酸雨过去以硫酸型为主，近年随着脱硫推进，硝酸型比例上升。

Clean rain has a pH of about 5.6 because of dissolved CO₂, so that value marks the boundary of acid rain. SO₂ is oxidised by OH radicals in the gas phase or by H₂O₂ and O₃ in cloud droplets to H₂SO₄, while NO and NO₂ are oxidised and combine with water to give HNO₃. China's acid rain was historically mostly sulfate-type, but as desulfurisation has spread the nitrate share has grown.

1852 年苏格兰化学家史密斯在曼彻斯特发现城市降水呈酸性，1872 年他在《空气与雨》一书中首次使用 acid rain 一词。1960–1970 年代斯堪的纳维亚湖泊鱼类大量死亡、欧洲中部和北美森林衰退，使酸雨成为跨国环境问题。

In 1852 the Scottish chemist Robert Angus Smith found that rain in Manchester was acidic, and in his 1872 book Air and Rain he first used the term acid rain. Fish kills in Scandinavian lakes and forest decline in central Europe and North America in the 1960s and 1970s made it a transboundary environmental issue.

酸雨使土壤中的铝离子溶出毒害植物根系，使湖泊鱼类死亡，并腐蚀大理石和金属：CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂。治理措施包括燃煤电厂烟气石灰石-石膏法脱硫、选择性催化还原脱硝、使用低硫燃料和机动车尾气催化转化器。1979 年《远距离越境空气污染公约》和美国 1990 年《清洁空气法》修正案使欧美酸雨显著减轻。

Acid rain mobilises aluminium in soil, poisoning plant roots, kills fish in lakes and corrodes marble and metals: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. Controls include limestone–gypsum flue-gas desulfurisation, selective catalytic reduction of NOₓ, low-sulfur fuels and catalytic converters. The 1979 Convention on Long-range Transboundary Air Pollution and the 1990 US Clean Air Act Amendments greatly reduced acid rain in Europe and North America.