Secondary PM2.5 a Major Contributor to India’s Air Pollution: CREA

A new analysis by the Centre for Research on Energy and Clean Air (CREA) has found that up to 42% of India’s PM2.5 pollution is secondary particulate matter, formed through chemical reactions in the atmosphere rather than being directly emitted. The dominant component of this secondary PM2.5 is ammonium sulphate, primarily generated from sulphur dioxide (SO₂).

Coal Power Plants at the Core
India remains the world’s largest emitter of SO₂, with coal-fired thermal power plants accounting for at least 60% of national SO₂ emissions. The analysis highlights the central role of coal-based electricity generation in driving secondary air pollution.

State-wise Impact
The highest contribution of ammonium sulphate to PM2.5 was observed in Chhattisgarh (42%), followed closely by Odisha (41%)—both states with a high concentration of coal-fired power plants. This underscores the regional concentration of secondary pollution linked to power generation.

Need for Flue Gas Desulphurisation (FGD)
CREA stressed that reinstating and strictly enforcing mandatory flue gas desulphurisation (FGD) across all coal-fired power plants is critical. Effective SO₂ control would significantly reduce ammonium sulphate formation and help meet targets under the National Clean Air Programme (NCAP).

Why Secondary Pollutants Matter
Secondary pollutants like ammonium sulphate are formed in the atmosphere through reactions between SO₂ and ammonia (NH₃). CREA warned that without addressing such secondary particulate matter, air quality improvements are likely to be limited and short-lived, even if direct emissions are controlled.

About Ammonium Sulphate

• Formed from atmospheric reactions between SO₂ and NH₃
• Used industrially as a fertilizer and in biochemical processes due to its high solubility and low toxicity
• In the atmosphere, it contributes significantly to fine particulate pollution (PM2.5)

Major Source of Sulphur dioxide (SO₂)

• Burning of fossil fuels in:
  • Coal-based power plants
  • Oil refineries
  • Heavy industries

Minor Sources

• Industrial processes (e.g., metal extraction from ores)
• Natural sources such as volcanic eruptions
• Transport sector:
  • Ships
  • Locomotives
  • Heavy machinery using high-sulphur fuels

Primary Pollutants:

Definition:
Pollutants that are directly emitted into the atmosphere from identifiable sources.

Examples:

• Particulate Matter (PM₁₀, PM₂.₅)
• Carbon Monoxide (CO)
• Nitrogen Oxides (NOx)
• Sulphur Oxides (SOx)

Sources:
Vehicles, power plants, industries, biomass burning

Secondary Pollutants

Definition:
Pollutants that are not emitted directly, but are formed in the atmosphere through chemical reactions involving primary pollutants.

Examples:

• Ground-level (tropospheric) ozone
• Secondary organic aerosols (haze)
• Ammonium sulphate & ammonium nitrate particles

Formation of Ground-Level (Tropospheric) Ozone

• Not emitted directly
• Formed when:
  • Nitrogen oxides (NOx) react with
  • Volatile Organic Compounds (VOCs)
  • In the presence of sunlight