1. Conceptual Framework, Definition & pH Thresholds

• Definition: Acid Rain (or more broadly, Acid Deposition) refers to precipitation—including rain, snow, fog, hail—or acidic atmospheric particles that contain elevated concentrations of hydrogen ions (H⁺), mainly due to sulphuric acid (H₂SO₄) and nitric acid (HNO₃).

• Normal Rain pH (approximately 5.6): Unpolluted rainwater is naturally slightly acidic because atmospheric carbon dioxide (CO₂) dissolves in water and forms weak carbonic acid (H₂CO₃).

  CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

• Acid Rain Threshold (pH < 5.6): Precipitation with a pH below 5.6 is commonly referred to as acid rain.

• The actual ecological impact depends on the sensitivity of the ecosystem, its buffering capacity, soil chemistry, and the duration and intensity of acid deposition.

THE pH SCALE & PRECIPITATION ACIDITY SPECTRUM

  0 ────── 2.4 ────── 3.0 ────── 5.1 ────── 5.5 ────── 5.6 ────── 7.0 ────── 14
[Battery  [Recorded   [Severe   [Forest    [Aquatic   [Normal   [Pure      [Alkaline]
  Acid]    Acid Rain]  Smog/Rain] Threshold] Threshold] Rain]    Water]

                    <──── INCREASING ACIDITY ────

2. Precursor Pollutants & Chemical Reaction Mechanism

Atmospheric acidification is primarily associated with two major groups of air pollutants:

1. Sulphur Oxides (SOx) — mainly sulphur dioxide (SO₂)

2. Nitrogen Oxides (NOx) — mainly nitric oxide (NO) and nitrogen dioxide (NO₂)

These pollutants are released mainly from fossil-fuel combustion, industrial processes, vehicles, and other high-temperature combustion sources.

MAJOR ANTHROPOGENIC SOURCES OF PRECURSOR EMISSIONS

                     PRIMARY POLLUTANT EMISSIONS
                                  │
        ┌─────────────────────────┴─────────────────────────┐
        ▼                                                   ▼
SULPHUR DIOXIDE (SO₂) SOURCES                   NITROGEN OXIDES (NOx) SOURCES

• Coal-fired power plants                        • Motor vehicle exhaust
• Petroleum refineries                           • High-temperature boilers
• Metal smelters                                 • Industrial combustion
• Oil and gas processing                          • Biomass combustion
• Chemical industries                             • Forest fires

A. Chemical Formation of Acidic Compounds

Atmospheric oxidation converts SO₂ and NOx into secondary acidic compounds.

PRIMARY POLLUTANTS
      │
      ├── SO₂ ──► Atmospheric oxidation ──► Sulphuric acid (H₂SO₄)
      │
      └── NOx ──► Atmospheric oxidation ──► Nitric acid (HNO₃)
                                      │
                                      ▼
                              ACID DEPOSITION

1. Sulphuric Acid Formation

Sulphur dioxide (SO₂) undergoes atmospheric oxidation through several gas-phase and cloud-water pathways.

A simplified pathway is:

SO₂ → SO₃ → H₂SO₄

Sulphur trioxide reacts rapidly with water:

SO₃ + H₂O → H₂SO₄

A simplified overall representation is:

2SO₂ + O₂ + 2H₂O → 2H₂SO₄

2. Nitric Acid Formation

Nitrogen dioxide undergoes atmospheric oxidation and reactions involving water and atmospheric oxidants.

A simplified reaction is:

2NO₂ + H₂O → HNO₃ + HNO₂

Another important pathway is:

N₂O₅ + H₂O → 2HNO₃

Important Point

Do not treat the exact proportion of sulphuric acid and nitric acid as a universal fixed ratio. Their relative contribution varies according to emission sources and atmospheric conditions.

3. Deposition Modes: Wet vs. Dry Acid Deposition

Acidic substances return from the atmosphere to Earth's surface through two major pathways:

                 ATMOSPHERIC ACID POLLUTANTS
                            │
              ┌─────────────┴─────────────┐
              ▼                           ▼
       WET DEPOSITION                DRY DEPOSITION
              │                           │
       Rain, snow, fog              Gases and particles
              │                           │
              ▼                           ▼
      Direct deposition        Deposition on surfaces
                                      │
                                      ▼
                             Later washed by rain

4. Ecological, Material & Human Health Impacts

                 ACID DEPOSITION
                       │
       ┌───────────────┼───────────────┐
       ▼               ▼               ▼
   MATERIALS       ECOSYSTEMS       HUMAN HEALTH
       │               │               │
   Marble and      Soil + Water     Respiratory
   monuments       + Forests          effects

A. Material Damage, "Marble Cancer" & Taj Mahal

• Acid deposition can damage carbonate-containing building materials such as limestone and marble.

• Marble mainly contains calcium carbonate (CaCO₃).

Chemical Reaction

CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂

• Sulphuric acid can react with calcium carbonate and produce calcium sulphate.

• This process can contribute to surface deterioration, crust formation, pitting and loss of stone material.

Marble Cancer / Stone Leprosy

"Marble cancer" or "stone leprosy" are commonly used terms for deterioration and discoloration of stone monuments due to atmospheric pollution and acidic deposition.

Taj Mahal & Taj Trapezium Zone

• The Taj Mahal has been affected by air pollution and atmospheric deposition.

• Industrial and vehicular emissions around Agra have been major environmental concerns.

• The Taj Trapezium Zone (TTZ) was established to regulate polluting activities around the Taj Mahal and other heritage structures.

• The Supreme Court directed polluting industries to adopt pollution-control measures, use cleaner fuels such as natural gas, or relocate where required.

B. Aquatic Ecosystem Damage — "Lake Killer"

1. Aluminium Mobilization

Acidification increases the solubility and mobility of certain metals in soils.

One important example is aluminium (Al³⁺).

Acidic water can therefore increase the amount of dissolved aluminium entering streams and lakes.

2. Fish Gill Damage

High concentrations of dissolved aluminium under acidic conditions can damage fish gills.

This interferes with:

• Respiration

• Ion regulation

• Overall fish survival

3. Loss of Aquatic Biodiversity

As water becomes increasingly acidic:

• Sensitive fish species may disappear.

• Aquatic insects and other organisms may decline.

• Food chains may be disrupted.

• Overall biodiversity can decrease.

Global Examples

Historically affected regions include:

• Norway

• Sweden

• Canada

• Northeastern United States

C. Soil Degradation, Nutrient Leaching & Forest Damage

1. Nutrient Leaching

Acid deposition can increase the leaching of essential plant nutrients from soil.

Important nutrients include:

• Calcium (Ca²⁺)

• Magnesium (Mg²⁺)

• Potassium (K⁺)

These nutrients may be displaced from soil particles and carried deeper into the soil or into drainage water.

2. Soil Acidification

Continuous acid deposition can gradually reduce the soil's pH and weaken its buffering capacity.

3. Plant and Forest Damage

Acid deposition can:

• Damage leaf surfaces.

• Reduce photosynthetic activity.

• Cause nutrient deficiency.

• Reduce plant growth.

• Increase vulnerability to drought, frost and pests.

4. Lichens as Bioindicators

Lichens are highly sensitive to air pollution, especially sulphur dioxide (SO₂).

Therefore:

Lichens → Useful bioindicators of air pollution

A decline in lichen diversity or abundance can indicate deteriorating air quality.

D. Human Health Hazards

Acid rain itself is generally not considered a major direct health hazard because the acidic water is relatively dilute.

The more important human-health pathway is through the air pollutants and fine particles associated with acid-forming emissions.

SO₂ and NOx can participate in atmospheric reactions that form fine sulphate and nitrate particulate matter.

These fine particles can:

• Penetrate deep into the lungs.

• Aggravate asthma.

• Worsen bronchitis.

• Increase cardiovascular and respiratory risks.

Therefore:

SO₂ + NOx → Secondary fine particles → Respiratory and cardiovascular health effects

5. Prevention, Control Technologies & Global Frameworks

                 ACID DEPOSITION CONTROL
                          │
        ┌─────────────────┼─────────────────┐
        ▼                 ▼                 ▼
 PRE-COMBUSTION      POST-COMBUSTION    ECOSYSTEM
    CONTROL             CONTROL        RESTORATION
        │                 │                 │
 Coal washing            FGD             Liming
 Fuel switching          SCR             CaCO₃
 Low-sulphur fuel        NOx control     Dolomite

1. Pre-Combustion Control

Fuel Switching

Replace high-sulphur fuels with:

• Low-sulphur coal

• Low-sulphur oil

• Natural gas

• Other cleaner fuels

Coal Washing / Beneficiation

Coal washing removes mineral matter, including some pyritic sulphur (FeS₂), before combustion.

This reduces the amount of sulphur entering the combustion process.

2. Post-Combustion Control

A. Flue Gas Desulphurization (FGD)

FGD systems remove sulphur dioxide (SO₂) from flue gases of industrial plants, especially coal-fired power plants.

A common process uses limestone (CaCO₃) or lime (Ca(OH)₂).

Simplified limestone-FGD reaction:

SO₂ + CaCO₃ + ½O₂ + 2H₂O → CaSO₄·2H₂O + CO₂

The resulting product is:

Gypsum = CaSO₄·2H₂O

Gypsum can be used in:

• Cement

• Plasterboard

• Construction materials

Memory Trick

SO₂ → FGD → Gypsum

B. Selective Catalytic Reduction (SCR)

SCR is used to reduce nitrogen oxide (NOx) emissions.

It converts NOx mainly into:

Nitrogen (N₂) + Water (H₂O)

Thus:

NOx → SCR → N₂ + H₂O

3. Ecosystem Neutralization — Liming

Acidified soils and lakes can sometimes be treated with alkaline materials.

Common materials include:

• Limestone (CaCO₃)

• Dolomite

Liming:

• Neutralizes acidity.

• Raises pH.

• Increases buffering capacity.

• Helps restore acidified lakes and soils.

4. International Agreements

1985 Helsinki Protocol

• Adopted under the UNECE Convention on Long-Range Transboundary Air Pollution (CLRTAP).

• Focused on reducing sulphur emissions.

• The protocol included a commitment to reduce sulphur emissions by at least 30% relative to the specified baseline.

Memory

Helsinki → Sulphur

Gothenburg Protocol

• Adopted in 1999 under CLRTAP.

• Addresses pollutants responsible for:

  • Acidification

  • Eutrophication

  • Ground-level ozone

Major pollutants include:

• SO₂

• NOx

• NH₃

• VOCs

Memory

Gothenburg → SO₂ + NOx + NH₃ + VOCs

6. Comprehensive Summary Matrix: Acid Rain

Quick Revision

• Definition: Acid rain = precipitation with pH below 5.6.

• Normal rain: pH approximately 5.6.

• Why normal rain is acidic: Dissolved CO₂ forms carbonic acid.

• Main precursor pollutants: SO₂ + NOx.

• Main acids: H₂SO₄ + HNO₃.

• Deposition: Wet + Dry.

• Monument damage: Acidic deposition damages carbonate stone.

• Marble: Mainly CaCO₃.

• Taj Mahal: Protected through the Taj Trapezium Zone (TTZ).

• Aquatic impact: Acidification → Al³⁺ mobilization → fish and ecosystem damage.

• Soil impact: Acid deposition → nutrient leaching.

• Bioindicator: Lichens.

• SO₂ control: FGD.

• NOx control: SCR.

• Ecosystem restoration: Liming.

• Helsinki Protocol: Sulphur reduction.

• Gothenburg Protocol: SO₂ + NOx + NH₃ + VOCs.

Memory Map

SO₂ + NOx
    ↓
Atmospheric Oxidation
    ↓
H₂SO₄ + HNO₃
    ↓
Acid Deposition
    ↓
┌──────────────┬───────────────┬─────────────────┐
↓              ↓               ↓
WET            DRY             ENVIRONMENTAL
DEPOSITION     DEPOSITION      IMPACTS
                                 ↓
              ┌──────────────────┼─────────────────┐
              ↓                  ↓                 ↓
          MONUMENTS            SOIL             WATER
              ↓                  ↓                 ↓
           CaCO₃              Nutrient         Al³⁺
           damage             leaching         mobilization
              ↓                  ↓                 ↓
          Marble             Forest          Aquatic
           damage             stress           damage

Must Remember

1. Normal rain → pH ≈ 5.6

2. Acid rain → pH < 5.6

3. Main precursors → SO₂ + NOx

4. Main acids → H₂SO₄ + HNO₃

5. Wet + Dry → Acid deposition

6. Marble → CaCO₃

7. Taj Mahal → TTZ

8. Aquatic damage → Al³⁺

9. Soil damage → Ca²⁺ + Mg²⁺ leaching

10. Bioindicator → Lichens

11. SO₂ control → FGD

12. NOx control → SCR

13. Acidified ecosystem → Liming

14. Helsinki → Sulphur

15. Gothenburg → SO₂ + NOx + NH₃ + VOCs

Key Terms

• Acid Deposition: Transfer of acidic substances from the atmosphere to Earth's surface through wet or dry pathways.

• Acid Rain: Acidic precipitation, conventionally defined as precipitation with pH below 5.6.

• Marble Cancer / Stone Leprosy: Common terms used for deterioration and discoloration of stone monuments due to atmospheric pollution and acidic deposition.

• Flue Gas Desulphurization (FGD): Technology used to remove SO₂ from industrial flue gases.

• Liming: Application of limestone or dolomite to neutralize acidity in soils or water bodies.

• Taj Trapezium Zone (TTZ): Regulated area around the Taj Mahal intended to control industrial pollution and protect heritage structures.

1-Line Bits / Short Points

• Normal rain has a pH of approximately 5.6.

• Acid rain is generally defined as precipitation with a pH below 5.6.

• SO₂ and NOx are the major precursor pollutants.

• SO₂ can ultimately form H₂SO₄ in the atmosphere.

• NOx can ultimately form HNO₃.

• Acid deposition occurs through wet and dry pathways.

• Acid deposition can cause soil nutrient leaching.

• Acidification can increase the mobility of Al³⁺.

• Acid deposition can damage marble and limestone.

• Lichens are useful bioindicators of air pollution.

• FGD controls SO₂ emissions.

• SCR controls NOx emissions.

• Liming can help neutralize acidified soils and lakes.

• Helsinki Protocol (1985) → sulphur reduction.

• Gothenburg Protocol (1999) → acidification, eutrophication and ozone-related pollutants.