Relevance: GS-III (Environment, Pollution) | Indian Express; Source: recent scientific studies, air-quality assessments

Context

Recent studies show that a large share of PM2.5 pollution in India, especially over the Indo-Gangetic Plain, now comes from secondary aerosols, mainly ammonium sulphate, rather than only from direct (primary) emissions.

Core Concept 

PM2.5 refers to fine particles (≤2.5 microns) that can enter the lungs and bloodstream.
Secondary aerosols are not emitted directly; they form in the atmosphere when gases react chemically.

Formation pathway :

• Coal-based power plants and industries → Sulphur dioxide
• Agriculture, fertilisers, waste → Ammonia
• Atmospheric reactions (high humidity, winter inversion) → Ammonium sulphate particles (PM2.5)

Why It Matters for India

• Winter pollution peaks in North India are increasingly driven by atmospheric chemistry, not just visible smoke.
• Even with control on vehicles and stubble burning, PM2.5 remains high due to sulphur dioxide and ammonia interaction.
• Indicates limits of city-centric pollution control.

Policy Implications

• Faster installation of flue gas desulphurisation in thermal power plants.
• Better fertiliser and waste management to reduce ammonia.
• Shift to airshed-based regulation under the National Clean Air Programme.

One-line takeaway: India’s PM2.5 challenge is increasingly driven by invisible chemical reactions in the air, not only by direct emission sources.

UPSC Prelims Question

Q. Secondary particulate matter such as ammonium sulphate in PM2.5 is mainly formed due to reactions between:
A. Carbon monoxide and nitrogen
B. Sulphur dioxide and ammonia
C. Methane and ozone
D. Carbon dioxide and water vapour

Answer: B