Secondary Pollutant – Ammonium Sulfate and PM2.5 Pollution

Why in news?

A study by the Centre for Research on Energy and Clean Air (CREA) released on 30 June 2025 found that ammonium sulfate – a secondary pollutant formed in the atmosphere – accounts for roughly one‑third of India’s fine particulate matter (PM_2.5) pollution. The finding urges policymakers to address precursor gases rather than focusing solely on primary emissions.

Primary vs secondary pollutants

• Primary pollutants are directly emitted from sources (e.g., dust from construction, soot from vehicles).
• Secondary pollutants form when primary pollutants undergo chemical reactions in the atmosphere. Ammonium sulfate forms when sulfur dioxide (SO₂) from coal‑burning power plants reacts with ammonia (NH₃) from fertiliser use or livestock waste.

Key findings of the study

• Ammonium sulfate contributes around 34 percent of India’s PM_2.5 mass on average.
• The concentration of ammonium sulfate near coal‑fired power plants is about 15 micrograms per cubic metre, 2.5 times higher than areas beyond 10 km.
• Near coal‑based power plants, ammonium sulfate makes up about 36 percent of PM_2.5; in other urban areas it is around 23 percent.
• The study examined 130 cities under the National Clean Air Programme. Ammonium sulfate concentrations ranged from 3.9 µg/m³ to 22.5 µg/m³, with the share of PM_2.5 from ammonium sulfate varying between 20 and 43 percent.
• More than 60 percent of India’s SO₂ emissions come from coal‑fired thermal power plants, making flue‑gas desulfurisation and cleaner fuels critical.
• Secondary pollutants can drift across city and state boundaries; thus regions without major emission sources may still experience high pollution levels.

Implications and recommendations

• Air‑quality plans should target precursor gases: Implement flue‑gas desulfurisation (FGD) in power plants to cut SO₂ emissions and promote better fertiliser management to curb NH₃ emissions.
• Integrate secondary pollutants in policy: The National Clean Air Programme focuses mainly on primary pollutants; it needs to consider secondary formation pathways.
• Enhance regional cooperation: Since secondary particles travel long distances, air‑quality management must coordinate between states and neighbouring countries.
• Promote clean energy: Shifting from coal to renewables reduces both direct and precursor emissions.
• Strengthen monitoring and research: Expand chemical speciation of PM to monitor secondary components and support evidence‑based policies.