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News: A significant portion of Delhi’s fine particulate matter, about one-third of the annual PM2.5 load, is made up of secondary pollutants, including ammonium sulphate.
About Ammonium Sulfate as a Pollutant in Delhi
- Ammonium sulfate is a secondary inorganic aerosol, meaning it is not emitted directly but forms in the atmosphere through chemical reactions.
- Formation Process: Ammonium sulfate forms when precursor gases such as sulphur dioxide (SO₂) and ammonia (NH₃) react in the atmosphere.
- Sulphur dioxide (SO₂) primarily comes from coal-fired power plants, with additional sources including oil refineries, heavy industries, brick kilns, diesel combustion, and shipping.
- SO₂ gets oxidised into sulfate, which then reacts with ammonia.
- Ammonia sources include agricultural activities, fertilizer use, livestock waste, sewage systems, landfills, biomass burning, diesel vehicles with catalytic converters, and certain industrial processes.
- The resulting ammonium sulfate particles are microscopic, airborne for days, and can travel long distances, contributing to transboundary pollution.
- Seasonal Impacts: Humidity, fog, and low winter temperatures accelerate chemical reactions, allowing secondary aerosols to form within hours.
- In India, ammonium sulfate contributes 49% of PM2.5 during post-monsoon, 41% in winter, and only about 21% during summer and monsoon.
- It accounts for nearly one-third of Delhi’s annual PM2.5 load, rising sharply during post-monsoon and winter months when pollution episodes are severe.
- Regional and National Implications: Delhi’s ammonium sulfate problem is influenced by regional emissions from coal-heavy states such as Chhattisgarh (42%), Odisha (41%), Jharkhand (40%), Telangana (40%), as well as Bihar, Uttar Pradesh, Maharashtra, Andhra Pradesh, and West Bengal.
- Secondary aerosols formed hundreds of kilometres away combine in the atmosphere to affect Delhi-NCR.
- Key Drivers in Delhi: Coal power plants (SO₂ source), Agricultural and industrial ammonia emissions, Vehicle emissions (NOx), High humidity and low winter temperatures and stagnant atmospheric conditions that facilitate long-range transport and chemical reactions
- India is currently the world’s largest SO₂ emitter, mainly due to coal-based power generation.
- In 2025, the government exempted nearly 78% of coal-fired thermal power plants from installing flue gas desulphurisation (FGD) systems, weakening SO₂ controls at the source.
- Health and Air Quality Concerns:
- Ammonium sulfate penetrates deep into the lungs, increasing risks of respiratory and cardiovascular diseases.
- Delhi consistently records some of the highest PM2.5 levels globally, with an annual average of 91.6 µg/m³ in 2024, making it the most polluted national capital.
- Experts recommend that secondary aerosol formation should be a key focus of the National Clean Air Programme (NCAP), rather than only controlling PM10.
