Definition

Acid rain refers to any form of precipitation (rain, snow, sleet, fog, or dust) that has elevated levels of hydrogen ions (low pH), making it more acidic than normal. It results from atmospheric pollution by sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), which react with water vapor to form sulfuric and nitric acids.

Chemical Reactions

  • Sulfur Dioxide Reaction:

    • SO₂ + H₂O → H₂SO₃ (sulfurous acid)
    • 2SO₂ + O₂ → 2SO₃
    • SO₃ + H₂O → H₂SO₄ (sulfuric acid)

  • Nitrogen Oxides Reaction:

    • 2NO₂ + H₂O → HNO₂ + HNO₃ (nitrous and nitric acid)

Sources of Acid Rain

  • Natural Sources: Volcanic eruptions, lightning strikes, decaying vegetation.
  • Anthropogenic Sources: Burning fossil fuels (coal, oil, gas), industrial processes, vehicle emissions, power plants.

Diagram: Formation of Acid Rain

Acid Rain Formation Diagram

pH Scale Context

  • Normal Rainwater pH: ~5.6 (slightly acidic due to CO₂)
  • Acid Rain pH: <5.0 (can be as low as 4.0 or lower)

Effects of Acid Rain

1. Environmental Impact

  • Aquatic Systems: Lowers pH of lakes and streams, harming fish and aquatic life.
  • Soil: Leaches essential nutrients (calcium, magnesium), releases toxic metals (aluminum).
  • Forests: Damages leaves, weakens trees, increases susceptibility to disease and extreme weather.

2. Built Environment

  • Buildings & Monuments: Corrodes metals, deteriorates limestone and marble, damages historic structures.

3. Human Health

  • Indirect Effects: Acidic particles can aggravate respiratory diseases (asthma, bronchitis).

Surprising Facts

  1. Acid Rain Can Occur Far From Pollution Sources: Pollutants can travel hundreds or even thousands of kilometers before falling as acid rain.
  2. Dry Deposition Is Significant: Over 50% of acidifying pollutants may settle as dry particles, not just as rain.
  3. Plastic Pollution Link: Recent studies have found microplastics in rainwater and in the deepest ocean trenches, suggesting atmospheric pollution is more complex and interconnected than previously thought.

Recent Breakthroughs

1. Recovery of Ecosystems

  • Study (2022, Science): Lakes in North America and Europe are showing signs of recovery due to emission controls, but full biological recovery is slow and incomplete due to persistent soil acidification.

2. Role of Ammonia

  • Finding: Increased agricultural ammonia emissions can neutralize some acid rain, but also contribute to secondary particulate pollution, complicating mitigation strategies.

3. Microplastics & Acid Rain

  • Discovery: Microplastics detected in rainwater indicate that atmospheric transport mechanisms can carry both chemical and physical pollutants simultaneously, potentially altering cloud chemistry and acid formation.

Famous Scientist Highlight

Svante Arrhenius (1859–1927)

  • Swedish scientist who first quantified the effect of atmospheric gases on rain acidity.
  • Developed the Arrhenius equation, foundational for understanding acid-base reactions in the atmosphere.

Latest Discoveries

1. Plastic Pollution in the Deep Ocean

  • Fact: Microplastics have been found in the Mariana Trench, the deepest part of the ocean, carried by atmospheric deposition including acid rain (Zhang et al., 2020, Nature Geoscience).

2. Acid Rain & Biodiversity

  • 2021 Study: Acid rain is linked to declines in soil microbial diversity, which affects nutrient cycling and forest regeneration (Wang et al., 2021, Environmental Science & Technology).

3. Urban Acid Rain

  • 2023 News: Urban areas in Asia are experiencing increased acid rain events due to rapid industrialization and vehicle emissions, with pH values as low as 3.5 recorded in some cities (The Guardian, 2023).

Diagram: Effects of Acid Rain

Effects of Acid Rain

Prevention and Control

  • Emission Reductions: Scrubbers in power plants, catalytic converters in vehicles, switching to low-sulfur fuels.
  • International Agreements: 1999 Gothenburg Protocol, US Clean Air Act Amendments (1990).
  • Restoration Efforts: Liming of lakes and soils to neutralize acidity.

Citation

  • Wang, Y. et al. (2021). “Acid rain reduces soil microbial diversity and alters ecosystem function.” Environmental Science & Technology, 55(10), 6789–6798.
  • Zhang, Y. et al. (2020). “Microplastics in the Mariana Trench.” Nature Geoscience, 13, 123–128.
  • The Guardian (2023). “Asian cities hit by record acid rain amid industrial boom.”

Revision Checklist

  • [ ] Understand acid rain formation and chemical reactions.
  • [ ] Identify sources and effects on environment, infrastructure, and health.
  • [ ] Recall surprising facts and recent discoveries.
  • [ ] Know major breakthroughs and ongoing challenges.
  • [ ] Remember key scientists and international efforts.

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