📚 Acid Deposition | 酸沉降
Acid deposition, commonly referred to as acid rain, is a serious transboundary environmental problem driven by anthropogenic emissions of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ). These gases travel long distances in the atmosphere, undergoing chemical transformations into sulfuric and nitric acids before returning to the Earth's surface via wet or dry processes. This article examines the chemistry underpinning acid deposition, its principal sources and formation pathways, ecological and structural impacts, and the mitigation strategies adopted to curb its effects.
酸沉降，常被称为酸雨，是由人为排放的二氧化硫（SO₂）和氮氧化物（NOₓ）引发的一个严重的跨境环境问题。这些气体在大气中远距离迁移，经过化学转化成为硫酸和硝酸，然后通过湿沉降或干沉降过程返回地表。本文探讨酸沉降背后的化学原理、其主要来源与形成途径、生态和结构影响，以及为遏制其影响所采取的减缓策略。
1. What Is Acid Deposition? | 什么是酸沉降？
Acid deposition describes the removal of acidic substances from the atmosphere and their deposition onto terrestrial and aquatic surfaces. It encompasses both wet deposition—rain, snow, sleet or fog with an abnormally low pH—and dry deposition, where acidic gases and fine particles settle directly onto surfaces without the involvement of precipitation.
酸沉降是指大气中的酸性物质被移除并沉积到陆地和水体表面的过程。它既包括湿沉降（pH 异常偏低的雨、雪、雨夹雪或雾），也包括干沉降，即酸性气体和细微颗粒在没有降水参与的情况下直接沉降到物体表面。
Natural rainwater is slightly acidic with a pH around 5.6, owing to dissolved carbon dioxide forming weak carbonic acid (CO₂ + H₂O ⇌ H₂CO₃). Acid deposition is generally defined as precipitation with a pH below 5.0, and in heavily industrialised regions values can drop to 4.0 or even below 3.0—making it more acidic than vinegar.
天然雨水因溶解的二氧化碳形成弱碳酸（CO₂ + H₂O ⇌ H₂CO₃）而略呈酸性，pH 值约 5.6。酸沉降通常被定义为 pH 低于 5.0 的降水，在高度工业化地区，pH 值可降至 4.0 甚至低于 3.0，其酸性比食醋更强。
2. Primary Pollutants: SO₂ and NOₓ | 主要污染物：SO₂ 与 NOₓ
The two principal precursors of acid deposition are sulfur dioxide (SO₂) and nitrogen oxides (NOₓ, comprising mainly NO and NO₂). SO₂ originates predominantly from the combustion of sulfur-containing fossil fuels, especially coal and heavy oil, in power stations and industrial boilers. Volcanic eruptions also contribute natural SO₂, but the anthropogenic share has historically dominated regional acid deposition events.
酸沉降的两种主要前体物是二氧化硫（SO₂）和氮氧化物（NOₓ，主要包括 NO 和 NO₂）。SO₂ 主要来自发电厂和工业锅炉中含硫化石燃料（尤其是煤炭和重油）的燃烧。火山喷发也会释放自然来源的 SO₂，但历史上人为排放在区域酸沉降事件中占主导地位。
al NO₂ forms when NO is oxidised in the atmosphere. Together, these primary pollutants set the stage for secondary acid formation.
氮氧化物产生于车辆发动机和工业炉窑中的高温燃烧过程，此时空气中的氮气和氧气发生反应：N₂ + O₂ → 2NO。当 NO 在大气中被氧化时会进一步生成 NO₂。这些一次污染物共同为二次酸的形成埋下伏笔。
3. Formation of Sulfuric Acid (H₂SO₄) | 硫酸（H₂SO₄）的形成
Once emitted, SO₂ is oxidised in the atmosphere through gas-phase and aqueous-phase pathways. In the gas phase, SO₂ reacts with hydroxyl radicals (•OH) to form sulfur trioxide (SO₃), which then reacts instantly with water vapour to yield sulfuric acid:
排放后，SO₂ 通过气相和液相途径在大气中被氧化。在气相中，SO₂ 与羟基自由基（•OH）反应生成三氧化硫（SO₃），后者随即与水蒸气反应生成硫酸：
SO₂ + •OH → HOSO₂
HOSO₂ + O₂ → SO₃ + HO₂
SO₃ + H₂O → H₂SO₄
→ 2H₂SO₄, illustrating the conversion of a gaseous pollutant into a strong diprotic acid.
在云滴中的液相氧化过程受 Fe³⁺ 或 Mn²⁺ 等过渡金属离子催化，进一步将溶解的 SO₂ 转化为硫酸盐。总净反应可简化为 2SO₂ + O₂ + 2H₂O → 2H₂SO₄，表明气态污染物转化为一种强二元酸。
4. Formation of Nitric Acid (HNO₃) | 硝酸（HNO₃）的形成
Nitrogen monoxide (NO) is rapidly oxidised by ozone or peroxy radicals to nitrogen dioxide (NO₂). During daytime, NO₂ reacts with hydroxyl radicals to produce nitric acid directly:
一氧化氮（NO）能迅速被臭氧或过氧自由基氧化为二氧化氮（NO₂）。在白天，NO₂ 与羟基自由基反应直接生成硝酸：
NO₂ + •OH → HNO₃
his masks the true radical-mediated pathways. The resulting nitric acid contributes significantly to the acidity of precipitation and also acts as a source of nutrient nitrogen in ecosystems.
夜间，NO₂ 可与臭氧反应生成硝酸根自由基，进而产生五氧化二氮（N₂O₅）。N₂O₅ 在气溶胶表面水解生成硝酸。教科书上常使用简化的总方程式 4NO₂ + O₂ + 2H₂O → 4HNO₃，但这掩盖了真实的自由基介导途径。生成的硝酸不仅显著增加了降水的酸度，还充当了生态系统中氮的营养源。
5. Wet and Dry Deposition | 湿沉降与干沉降
Wet deposition occurs when sulfuric and nitric acids dissolve in cloud droplets or raindrops and are transported to the ground as acid rain, acid snow or acid fog. The efficiency of removal depends on droplet size, altitude and atmospheric residence time. Regions downwind of major emission sources often receive the highest acid loadings through wet deposition.
湿沉降发生在硫酸和硝酸溶解于云滴或雨滴中，并以酸雨、酸雪或酸雾的形式输送到地面时。去除效率取决于液滴大小、海拔高度和大气停留时间。主要排放源的下风向地区往往通过湿沉降承受最高的酸负荷。
Dry deposition, by contrast, involves the direct settling of acidic gases (SO₂, NO₂, HNO₃ vapour) and sulfate- or nitrate-containing particles onto vegetation, soil and buildings without the mediation of precipitation. Dry deposition can be a dominant input near emission sources and is particularly damaging because it provides a continuous acid supply to surfaces, even in the absence of rain.
与之相反，干沉降是指酸性气体（SO₂、NO₂、HNO₃ 蒸气）以及含硫酸盐或硝酸盐的颗粒物在没有降水参与的情况下直接沉降到植被、土壤和建筑物表面。在排放源附近，干沉降可能是主要的输入途径，并且由于它即便在没有降雨时也会持续向表面供应酸性物质，因此破坏性尤为突出。
6. Impact on Aquatic Ecosystems | 对水生生态系统的影响
Acid deposition lowers the pH of freshwater lakes and streams, often pushing poorly buffered water bodies below the critical pH 5.0–5.5 threshold. Chronic acidification disrupts entire food webs: phytoplankton and zooplankton populations decline, insect larvae die off, and fish, especially salmonids, fail to reproduce or succumb to toxic conditions.
酸沉降降低淡水湖泊和溪流的 pH 值，经常将缓冲能力差的水体推至 pH 5.0–5.5 的关键阈值以下。长期酸化会破坏整个食物网：浮游植物和浮游动物数量下降，昆虫幼虫死亡，鱼类（尤其是鲑科鱼类）无法繁殖或死于毒性环境。
A key mechanism of toxicity is the mobilisation of aluminium from surrounding soils. As pH falls below 5.0, insoluble aluminium compounds dissolve to release Al³⁺ ions, which damage fish gills, cause excessive mucus secretion and disrupt osmoregulation. Furthermore, acid pulses during snowmelt can deliver a sudden influx of acidity and aluminium, triggering massive fish kills.
毒性的一个关键机制是铝从周围土壤中迁移出来。当 pH 降至 5.0 以下时，不溶性的铝化合物溶解并释放出 Al³⁺ 离子，这些离子会损害鱼鳃、引起过多黏液分泌并破坏渗透调节。此外，融雪期间的酸性脉冲会突然输入大量酸和铝，引发大规模鱼类死亡。
7. Impact on Soils and Forests | 对土壤和森林的影响
Acid deposition accelerates the leaching of essential base cations—Ca²⁺, Mg²⁺ and K⁺—from soil exchange sites, reducing soil fertility. Simultaneously, it solubilises potentially toxic metals such as aluminium and manganese, which can be taken up by plant roots and damage fine root systems. Coniferous forests at high elevations are particularly vulnerable because they are frequently bathed in acidic cloud water and are rooted in thin, poorly buffered soils.
酸沉降加速了土壤交换位上必需盐基阳离子（Ca²⁺、Mg²⁺ 和 K⁺）的淋失，从而降低土壤肥力。同时，它使铝和锰等潜在有毒金属溶解，这些金属可被植物根系吸收并损害细根系统。高海拔针叶林尤其脆弱，因为它们经常浸泡在酸性云雾中，并且扎根于浅薄且缓冲能力差的土壤。
Visible symptoms of forest decline include needle yellowing, crown thinning and increased susceptibility to frost, drought and pests. Widespread dieback observed in central European and North American forests during the late twentieth century was closely correlated with chronic acid and nitrogen deposition, though other stressors also played a role.
森林衰退的可见症状包括针叶发黄、树冠稀疏以及对霜冻、干旱和病虫害的敏感性增强。二十世纪后期在中欧和北美森林中观察到的大面积枯梢现象与长期的酸沉降和氮沉降密切相关，尽管其他胁迫因素也发挥了作用。
8. Damage to Buildings and Cultural Heritage | 对建筑和文化遗产的损害
Limestone (CaCO₃) and marble, widely used in historical monuments and buildings, are particularly susceptible to acid attack. Sulfuric acid reacts with calcium carbonate to form calcium sulfate, which either washes away as a soluble salt or crystallises as gypsum (CaSO₄·2H₂O). The gypsum crust can trap pollutants, cause surface blistering and eventually spall off, leading to irreversible loss of sculptural detail.
广泛用于历史古迹和建筑的石灰石（CaCO₃）和大理石特别容易受到酸的侵蚀。硫酸与碳酸钙反应生成硫酸钙，该物质或以可溶盐形式被冲走，或结晶为石膏（CaSO₄·2H₂O）。石膏结壳会包裹污染物，引起表面鼓泡并最终剥落，造成雕塑细节不可逆转的损失。
CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂
Nitric acid can similarly dissolve carbonate stone, forming calcium nitrate, which is highly soluble and easily washed away. Modern structures constructed from concrete, sandstone and even certain metals also corrode under acidic conditions, raising maintenance costs and shortening service lifetimes.
硝酸同样能溶解碳酸盐石材，生成高度可溶且易被冲走的硝酸钙。由混凝土、砂岩甚至某些金属建造的现代结构在酸性条件下也会发生腐蚀，从而推高维护成本并缩短使用寿命。
9. Mitigation and Control Technologies | 减缓与控制技术
Flue gas desulfurisation (FGD) is the most widely adopted technology for reducing SO₂ emissions from power plants. In wet scrubbing systems, the flue gas is passed through a slurry of limestone or lime, converting SO₂ into calcium sulfite, which is often further oxidised to gypsum and sold for construction use:
烟气脱硫（FGD）是降低发电厂 SO₂ 排放采用最广泛的技术。在湿法洗涤系统中，烟气通过石灰石或石灰浆液，将 SO₂ 转化为亚硫酸钙，后者常被进一步氧化为石膏并出售给建筑行业：
CaCO₃ + SO₂ → CaSO₃ + CO₂
2CaSO₃ + O₂ + 4H₂O → 2CaSO₄·2H₂O
₂. Stricter vehicle emission standards and the shift to electric vehicles further decrease nitrogen oxide loads.
对于移动源，三元催化转化器可同时削减 NOₓ、CO 和未燃烧的碳氢化合物。以 CO 和 H₂ 作还原剂将 NO 还原为 N₂：2NO + 2CO → N₂ + 2CO₂。更严格的车辆排放标准以及向电动车的转型进一步降低了氮氧化物的排放总量。
Other strategies include switching to low-sulfur fuels, fluidised bed combustion, and the use of alternative energy sources such as wind and solar. Liming of acidified lakes and soils—adding crushed limestone or dolomite—provides a temporary remedy by neutralising acidity and replenishing calcium, though it does not address the root cause.
其他策略包括改用低硫燃料、采用流化床燃烧技术，以及使用风能、太阳能等替代能源。对酸化湖泊和土壤施用石灰（添加碎石灰石或白云石）能中和酸性并补充钙质，但仅为治标之策，无法触动根源。
10. Monitoring and International Cooperation | 监测与国际合作
Continuous monitoring of precipitation chemistry, stream pH and soil indicators allows scientists to track trends in acid deposition and ecosystem recovery. Networks such as the European Monitoring and Evaluation Programme (EMEP) and the US National Atmospheric Deposition Program (NADP) provide long-term data that underpin policy decisions.
对降水化学、溪流 pH 和土壤指标的持续监测使科学家得以追踪酸沉降趋势和生态系统的恢复情况。欧洲监测与评估计划（EMEP）和美国国家大气沉降计划（NADP）等网络提供长期数据，支撑着政策决策。
Because acid deposition is a transboundary issue—sulfur and nitrogen compounds can travel hundreds of kilometres—it demands international cooperation. The 1979 Convention on Long-Range Transboundary Air Pollution and its subsequent protocols, including the 1999 Gothenburg Protocol, set binding emission ceilings for SO₂, NOₓ, ammonia and volatile organic compounds across Europe and North America. These agreements have led to dramatic reductions in sulfate deposition and, more slowly, in nitrate deposition, enabling the partial recovery of sensitive ecosystems.
由于酸沉降是一个跨境问题——硫和氮化合物可传输数百公里——因此需要国际合作。1979 年《长距离越境空气污染公约》及其后各项议定书，包括 1999 年《哥德堡议定书》，在欧洲和北美设定了具有约束力的 SO₂、NOₓ、氨和挥发性有机化合物的排放上限。这些协定促使硫酸盐沉降大幅降低，硝酸盐沉降也缓慢下降，从而实现了敏感生态系统的部分恢复。
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