Paper: GS – III, Subject: Environment and Ecology, Topic: Pollution, Issue: Ammonium sulfate.
Context:
A recent scientific analysis reveals that nearly one-third of Delhi’s PM2.5 pollution comes from a secondary pollutant ammonium sulphate, formed in the atmosphere rather than emitted directly.
Key Takeaways:
Ammonium sulfate: A secondary inorganic aerosol that is not directly emitted but formed in the atmosphere.
- Formation Process: Sulphur dioxide from coal-based power plants and industries oxidises into sulfate, which then reacts with ammonia released mainly from agriculture.
What is the Issue?
Unlike primary pollutants (vehicular exhaust, dust, biomass burning), ammonium sulphate forms through atmospheric chemical reactions involving:
- Sulphur dioxide (SO₂) (mainly from coal combustion)
- Ammonia (NH₃) (from agriculture, livestock, waste)
These react under favourable meteorological conditions (high humidity, low temperature) to form secondary inorganic aerosols, worsening smog episodes.
| Key Findings & Data: 33% of Delhi’s PM2.5 is linked to secondary aerosols, especially ammonium sulphate. According to Centre for Research on Energy and Clean Air (CREA) satellite-based assessment (2024):PM2.5 is strongly linked to SO₂ emissions from coal-fired thermal power plants. India is the world’s largest SO₂ emitter, largely due to coal-based electricity. Delhi recorded 91.6 µg/m³ annual PM2.5 (2023) over 6 times WHO limits. |
Sources of Ammonium Sulphate:
- Sulphur dioxide (SO₂): Coal-fired power plants (largest contributor), Oil refineries, Heavy industries and Shipping.
- Ammonia (NH₃): Fertiliser use, Livestock waste and Open garbage and sewage
Once formed, ammonium sulphate can travel hundreds of kilometres, adding to regional and transboundary pollution.
Why is this a Serious Concern?
- Invisible source: Not easily traceable at city level
- Long atmospheric life: Remains airborne for days
- Winter amplification: Low temperatures and high humidity accelerate formation
- Health impacts: Respiratory and cardiovascular diseases
- Reduced lung function
- Premature deaths
Delhi’s pollution crisis is therefore not just local, but regional and chemical in nature.
Why India Struggles:
- Delayed implementation of FGD units in thermal power plants (to control SO₂)
- Weak enforcement of emission norms
- Focus on local sources (vehicles, dust) while ignoring secondary pollutants
- Fragmented air-quality governance across states
Way Forward:
Policy & Technology:
- Fast-track Flue Gas Desulphurization (FGD) in coal plants
- Strengthen National Clean Air Programme (NCAP) with regional coordination
Agriculture & Waste:
- Rational fertiliser uses to cut ammonia emissions
- Scientific management of livestock and urban waste
Scientific Approach:
- Shift from PM10-centric control to PM2.5 and precursor gases (SO₂, NOx, NH₃)
- Use satellite data and chemical transport models for policy decisions.
Delhi’s air pollution is increasingly driven by secondary PM2.5 from coal-based SO₂ and ammonia, demanding regional coordination, precursor control, and science-based policies beyond city-focused interventions.
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