Secondary Pollutants, Especially Ammonium Sulphate, Constitute a Third of PM2.5 Pollution (29/6/25)

Context

• A recent study by the Centre for Research on Energy and Clean Air (CREA) reveals that nearly one-third of India’s PM2.5 air pollution originates from secondary pollutants, with ammonium sulphate being a major contributor. 

Key Highlights:

• Secondary pollutants contribute nearly 34% to PM2.5 levels in India.
• Ammonium sulphate is formed from atmospheric reactions between sulphur dioxide (SO₂) and ammonia (NH₃).
• Over 60% of SO₂ emissions in India originate from coal-fired thermal power plants.
• Concentrations of ammonium sulphate are 2.5 times higher within 10 km of coal plants.
• Only 8% of coal plants have installed Flue Gas Desulphurisation (FGD) units despite legal requirements.
• The government is reportedly considering scrapping the FGD mandate altogether.
• In 130 NCAP cities, ammonium sulphate contributes between 20% to 43% of PM2.5.
• Ammonium nitrate and other secondary pollutants also significantly impact air quality.

Detailed Insights:

• Primary pollutants are directly emitted from sources like vehicles, industries, and biomass burning.
• Secondary pollutants are formed in the atmosphere through chemical reactions involving primary pollutants.
• The presence of ammonium sulphate is widespread and not restricted to coal plant regions.
• Trans-boundary pollution leads to its spread across far-off cities and rural areas.
• Nationwide, the average concentration of ammonium sulphate is 11.9 µg/m³.
• Within 10 km of coal plants, the level is 15 µg/m³, contributing 36% to PM2.5.
• 114 out of 130 NCAP cities have more than 30% PM2.5 composed of ammonium sulphate.
• The study uses satellite data and modelling analysis for its findings.
• Mandatory installation of FGD systems remains largely unfulfilled by the power sector.
• Agricultural ammonia emissions also play a role in ammonium-based particulate formation.
• Lack of action exacerbates public health risks from prolonged exposure to fine particulates.
• Air quality policies currently under-address the secondary pollution dimension.
• Regulatory delays and rollbacks could severely hamper pollution reduction targets.

Way Forward

• Enforce mandatory installation of FGD units in all thermal power plants.
• Implement policies for efficient fertiliser management to reduce ammonia emissions.
• Impose strict penalties for violation of emission norms.
• Foster regional cooperation to address pollution that crosses state boundaries.
• Invest in real-time monitoring of PM2.5 components and emission sources.

Scientific/Technical Concepts Involved

• PM2.5: Particulate Matter smaller than 2.5 micrometres; penetrates deep into lungs, causing health issues.
• Ammonium Sulphate [(NH₄)₂SO₄]: Formed from SO₂ and NH₃ in the atmosphere; major secondary pollutant.
• Flue Gas Desulphurisation (FGD): Pollution control device that removes SO₂ from industrial emissions.
• Trans-boundary Pollution: Pollution that spreads beyond its source region due to wind and weather patterns.