📚 Acid Deposition: Causes and Effects | 酸沉降的成因与影响
Acid deposition refers to the process by which acidic compounds, primarily sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), are released into the atmosphere and subsequently returned to the Earth’s surface in wet or dry forms. In IB Chemistry, this topic integrates concepts of chemical equilibria, acid–base reactions, and environmental chemistry.
酸沉降是指二氧化硫（SO₂）和氮氧化物（NOₓ）等酸性化合物释放到大气中，随后以湿沉降或干沉降的形式返回地球表面的过程。在IB化学中，这一主题将化学平衡、酸碱反应与环境化学的概念融为一体。
1. Defining Acid Deposition | 酸沉降的定义
Acid deposition includes both wet and dry processes. Wet deposition refers to acidic rain, fog, snow, and dew, while dry deposition includes acidic gases and particles that settle onto surfaces directly. The term “acid rain” is commonly used, but acid deposition is a broader concept.
酸沉降包括湿沉降和干沉降两种过程。湿沉降指酸性雨、雾、雪和露水，而干沉降则指酸性气体和颗粒物直接沉降到地表。通常所说的”酸雨”只是酸沉降的一个狭义类型，酸沉降是更广的概念。
The pH of normal rainwater is about 5.6 because dissolved CO₂ forms carbonic acid (H₂CO₃). Rainwater with a pH below 5.6 is generally considered acid rain.
正常雨水的pH约为5.6，因为溶解的CO₂形成了碳酸（H₂CO₃）。pH低于5.6的雨水通常被认为是酸雨。
CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq)
2. Major Acidifying Pollutants and Their Sources | 主要致酸污染物及其来源
The two principal groups of acidifying pollutants are sulfur oxides (SO₂, SO₃) and nitrogen oxides (NO, NO₂). These originate from both natural and anthropogenic sources.
主要的两类致酸污染物是硫氧化物（SO₂、SO₃）和氮氧化物（NO、NO₂）。它们既有天然来源，也有人为来源。
| Pollutant | Natural source | Anthropogenic source |
| SO₂ | Volcanoes, biological decay | Burning of coal and oil, metal smelting |
| NOₓ | Lightning, microbial activity | Vehicle engines, power plants, fertiliser manufacture |
3. Chemical Reactions in the Atmosphere | 大气中的化学反应
Once released, SO₂ and NOₓ are oxidised in the atmosphere to form stronger acids. SO₂ dissolves in water to form sulfurous acid, which is further oxidised to sulfuric acid.
SO₂和NOₓ释放到大气后会被氧化成更强的酸。SO₂溶于水生成亚硫酸，进一步氧化为硫酸。
SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq)
2H₂SO₃(aq) + O₂(g) → 2H₂SO₄(aq)
Nitrogen monoxide is oxidised to nitrogen dioxide, which then reacts with water to produce both nitric acid and nitrous acid.
一氧化氮被氧化为二氧化氮，然后与水反应生成硝酸和亚硝酸。
2NO(g) + O₂(g) → 2NO₂(g)
2NO₂(g) + H₂O(l) → HNO₃(aq) + HNO₂(aq)
These acids dissociate in water to release H⁺ ions, lowering the pH of precipitation.
这些酸在水中电离释放H⁺离子，从而降低降水的pH值。
4. Dry Deposition and Wet Deposition | 干沉降与湿沉降
Dry deposition occurs when acidic gases and aerosols are deposited directly onto surfaces such as soil, vegetation, and buildings. This process is dominant near emission sources and is strongly influenced by wind speed and surface roughness.
干沉降指酸性气体和气溶胶直接沉降到土壤、植被和建筑物等表面上的过程。这一过程在排放源附近占主导，受风速和地表粗糙度影响显著。
Wet deposition occurs when pollutants are washed out of the atmosphere by precipitation. This includes rain-out (within-cloud scavenging) and wash-out (below-cloud scavenging). Wet deposition often affects areas far from the original emission source.
湿沉降指污染物被降水从大气中清除的过程，包括云内清除（rain-out）和云下冲刷（wash-out）。湿沉降常影响远离最初排放源的区域。
5. Factors Affecting Acidity and Transport | 影响酸度和迁移的因素
The final pH of acid rain depends on the concentration of H⁺ from acids and the presence of neutralising agents such as ammonia (NH₃) and alkaline dust. In industrial regions, ammonia from agriculture can partially neutralise acid rain to form ammonium salts.
酸雨的最终pH取决于酸释放的H⁺浓度以及氨（NH₃）和碱性粉尘等中和剂的存在。在工业地区，农业来源的氨可以部分中和酸雨，形成铵盐。
NH₃(g) + H₂SO₄(aq) ⇌ NH₄HSO₄(aq)
Prevailing winds can transport SO₂ and NOₓ over hundreds of kilometres before they are deposited. As a result, the effects of acid deposition are often observed in regions far from industrial centres.
盛行风可以将SO₂和NOₓ输送到数百公里之外才沉降。因此，酸沉降的影响常常出现在远离工业中心的地区。
6. Effects on Aquatic Ecosystems | 对水生生态系统的影响
Acid deposition lowers the pH of lakes and rivers. Many aquatic organisms, such as fish eggs and certain algae, are sensitive to low pH. A decrease in pH can disrupt enzyme activity and ion balance in fish, leading to reproductive failure and death.
酸沉降会降低湖泊和河流的pH值。许多水生生物，如鱼卵和某些藻类，对低pH敏感。pH降低会干扰鱼类体内的酶活性和离子平衡，导致繁殖失败和死亡。
Aluminium ions (Al³⁺) are leached from soil under acidic conditions and enter water bodies. Al³⁺ is toxic to fish because it damages gill membranes and reduces the ability of fish to absorb oxygen.
在酸性条件下，铝离子（Al³⁺）从土壤中淋溶进入水体。Al³⁺对鱼类有毒，因为它会损伤鳃膜，降低鱼类吸收氧气的能力。
7. Effects on Soil and Forests | 对土壤和森林的影响
Acid deposition accelerates the leaching of essential nutrients such as calcium (Ca²⁺), magnesium (Mg²⁺), and potassium (K⁺) from the soil. These nutrients are replaced by H⁺ and Al³⁺, reducing soil fertility.
酸沉降加速了钙（Ca²⁺）、镁（Mg²⁺）和钾（K⁺）等必需营养元素从土壤中淋失。这些营养元素被H⁺和Al³⁺替代，导致土壤肥力下降。
In forests, acid rain damages waxy leaf cuticles and disrupts photosynthesis. It also weakens trees, making them more vulnerable to pests, drought, and frost. High-altitude forests are particularly affected because they are often enveloped in acidic clouds.
在森林中，酸雨会破坏叶片表面的蜡质角质层，干扰光合作用。它还会削弱树木，使其更容易遭受病虫害、干旱和霜冻。高海拔森林尤其易受影响，因为它们常被酸性云雾笼罩。
8. Effects on Buildings and Human Health | 对建筑和人类健康的影响
Acidic precipitation reacts with carbonate-based building materials such as limestone (CaCO₃) and marble, causing surface erosion and structural damage.
酸性降水会与石灰岩（CaCO₃）和大理石等含碳酸盐的建筑材料反应，导致表面侵蚀和结构损坏。
CaCO₃(s) + H₂SO₄(aq) → CaSO₄(aq) + CO₂(g) + H₂O(l)
The resulting calcium sulfate is slightly soluble and is washed away, exposing fresh surfaces to further attack. Similarly, acid rain corrodes metals such as iron and steel, accelerating rust formation.
生成的硫酸钙微溶，会被雨水冲走，使新的表面继续受到侵蚀。同样，酸雨还会腐蚀铁和钢等金属，加速生锈。
In terms of human health, acid deposition itself is not directly harmful, but the gas precursors SO₂ and NOₓ can aggravate respiratory conditions such as asthma and bronchitis. Fine sulfate and nitrate particles can penetrate deep into the lungs.
在人类健康方面，酸沉降本身不直接有害，但其前体气体SO₂和NOₓ会加重哮喘和支气管炎等呼吸系统疾病。细小的硫酸盐和硝酸盐颗粒可以深入肺部。
9. Case Studies: Europe and China | 案例研究：欧洲与中国
In the 1970s and 1980s, thousands of lakes in Scandinavia became acidified due to sulfur emissions from the United Kingdom and central Europe. Many lakes lost their fish populations, and forests in Germany showed widespread decline.
20世纪70至80年代，由于英国和中欧的硫排放，斯堪的纳维亚半岛成千上万的湖泊酸化。许多湖泊鱼类消失，德国森林出现大面积衰退。
In China, rapid industrialisation has made the country the world’s largest emitter of SO₂. Acid rain is especially severe in the south and southwest, where high-sulfur coal is burned and alkaline dust concentrations are low.
在中国，快速工业化使该国成为全球最大的SO₂排放国。酸雨在南部和西南部尤其严重，因为那里燃烧高硫煤，且碱性粉尘浓度较低。
10. Mitigation Strategies | 减缓措施
Acid deposition can be reduced by lowering emissions of SO₂ and NOₓ. Key strategies include desulfurisation of flue gas, catalytic converters in vehicles, and switching to renewable energy sources.
减少酸沉降可通过降低SO₂和NOₓ排放来实现。关键策略包括烟气脱硫、汽车催化转化器，以及转向使用可再生能源。
- Flue gas desulfurisation (FGD): SO₂ is removed by passing exhaust gases through a slurry of limestone or lime, forming calcium sulfite.
- 烟气脱硫（FGD）：将废气通过石灰石或石灰浆液，除去SO₂并生成亚硫酸钙。
- Catalytic converters: NOₓ is reduced to N₂ and CO is oxidised to CO₂ in vehicle exhaust systems.
- 催化转化器：在汽车排气系统中将NOₓ还原为N₂，将CO氧化为CO₂。
- Liming of lakes: Adding calcium carbonate (CaCO₃) to acidified lakes neutralises acidity and restores pH.
- 湖泊加石灰：向酸化湖泊中加入碳酸钙（CaCO₃），中和酸性，恢复pH值。
11. Key IB Chemistry Connections | IB化学考点连接
In IB Chemistry, acid deposition is often assessed through the interpretation of chemical equations, acid–base equilibria, and the concept of pH. Students should be able to write balanced equations for the formation of H₂SO₄ and HNO₃ and explain how acid rain affects ecosystems.
在IB化学中，酸沉降常通过化学方程式书写、酸碱平衡和pH概念来考查。学生应当能够写出H₂SO₄和HNO₃形成的平衡方程式，并解释酸雨如何影响生态系统。
Another important connection is the use of equilibrium constants (Kₐ) and acid strength. Sulfuric acid is a strong diprotic acid, while sulfurous acid is weak. Understanding these differences helps explain why oxidation of H₂SO₃ to H₂SO₄ significantly increases acidity.
另一个重要考点是平衡常数（Kₐ）和酸强度的应用。硫酸是强二元酸，而亚硫酸是弱酸。理解这些差异有助于解释为何H₂SO₃氧化为H₂SO₄会显著增强酸性。
pH = −log₁₀[H⁺]
A decrease of one pH unit represents a tenfold increase in hydrogen ion concentration. Thus, a change from pH 5.6 to pH 4.6 corresponds to a 10-fold increase in acidity, and from pH 5.6 to pH 3.6 a 100-fold increase.
pH每降低一个单位，表示氢离子浓度增大到10倍。因此，从pH 5.6降到4.6，酸度增加10倍；从5.6降到3.6，酸度增加100倍。
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