Subject: Environment

Topic: A third of Delhi's PM2.5 load has a secondary source: ammonium sulfate

Context: A recent analysis highlights that a significant portion of Delhi's air pollution does not come from direct emissions but is created in the atmosphere itself. Specifically, Ammonium Sulfate, a secondary inorganic aerosol, accounts for nearly one-third of Delhi's annual PM2.5 load, drastically increasing during post-monsoon winter months.

1. Nature of Pollutants: Primary vs. Secondary

• Primary Pollutants: These emerge directly from sources like road dust, construction activity, open burning, vehicle exhaust, and industries.

• Secondary Pollutants: These form in the atmosphere after "precursor pollutants" (gases) undergo chemical reactions influenced by sunlight, temperature, and humidity.

  • Key Finding: Secondary aerosols now contribute at least one-third of Delhi's annual PM2.5, explaining why air quality deteriorates even when local primary sources are controlled.

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2. The Key Culprit: Ammonium Sulphate

• Composition: It is a secondary inorganic aerosol formed from the reaction of Sulphur Dioxide (SO2 ) and Ammonia (NH3 ).

• Formation Mechanism:

  1. SO2 (mostly from thermal power plants)  oxidizes in the atmosphere to form sulfate.

  2. Sulfate reacts with ammonia (NH3) present in the air.

  3. Persistence: The resulting particles are fine particulate matter that can remain airborne for days and travel long distances (transboundary pollution).

3. Sources of Precursors

• Sulphur Dioxide (SO2 ):

  • Major Source: Coal-fired power plants (India is the world's largest emitter of SO2 ).

  • Other Sources: Oil refineries, heavy industries, brick kilns, diesel combustion, and shipping.

• Ammonia (NH3 ):

  • Released mainly from agricultural activities (fertilizer use, livestock waste), sewage systems, landfills, and biomass burning.

4. Environmental Drivers

• Humidity & Winter Conditions: High humidity, fog, and low winter temperatures accelerate the chemical reactions, converting gases into particles within hours.

• Seasonal Spike: Ammonium sulfate contributes ~49% of PM2.5 in post-monsoon periods and 41% in winter, compared to just 21% in summer.

5. Regional Impact and Policy Concerns

• High Burden States: According to CREA's 2024 assessment, the highest annual contribution of ammonium sulfate to PM2.5 is in coal-dominated states: Chhattisgarh (42%), Odisha (41%), Jharkhand, and Telangana.

• Transboundary Effect: Emissions from these plants can travel hundreds of kilometers, significantly affecting Delhi-NCR's air.

• Regulatory Issues:

  • In July 2025, the government exempted nearly 78% of coal-fired thermal power plants from installing Flue Gas Desulphurisation (FGD) systems, citing studies that claimed local SO2  levels were within norms.

  • Experts argue this ignores the secondary formation of aerosols downwind.

Way Forward: The National Clean Air Programme (NCAP) revision must focus on secondary aerosol formation rather than just PM10.