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Nearly one-third of Delhi’s annual PM2.5 pollution is made up of secondary ammonium sulfate, according to analysis by the Centre for Research on Energy and Clean Air (CREA), a Finland-headquartered think-tank.

• Ammonium sulfate – It is a secondary inorganic aerosol formed through the oxidation of sulphur dioxide (SO2) into sulfate.
• Sources – Coal-based power plants (major SO₂ emitters), industrial activity (metallurgy, refineries), agriculture (ammonia emissions from fertilisers and livestock).
• Humidity factor – Its formation is enhanced under humid conditions, worsening Delhi’s smog.
• India’s SO₂ Emissions – India emits 11.2 million tonnes of SO₂ annually, the highest globally, second only to China, making India especially vulnerable to secondary PM2.5 formation.
• Seasonal Contribution in Delhi – Ammonium sulfate accounts for 49% of PM2.5 pollution during the post-monsoon period, 41% in winter, 21% in summer, and 9% in the monsoon.
• Drivers of Delhi’s Severe Pollution – The severe smog episodes are driven by regional SO₂ emissions & secondary chemical reactions in the atmosphere, rather than only local primary sources.
• Secondary PM2.5 in India – Secondary PM is a major contributor to India’s PM2.5 pollution, with up to 42% of India’s PM2.5 burden formed from gases like SO₂ and ammonia.
• Highest contributors – States dominated by coal-fired power plants - Chhattisgarh (42%), Odisha (41%), and Jharkhand and Telangana (40% each).              

Secondary particulate matter – Tiny particles formed in the atmosphere from chemical reactions between primary gaseous pollutants like sulfur dioxide, nitrogen oxides (NOx), ammonia and volatile organic compounds (VOCs).  SO₂, NOx, ammonia, VOCs.

• Other High-Contribution States – Bihar (39%), Maharashtra, Andhra Pradesh and West Bengal (38% each), as well as Uttar Pradesh and Madhya Pradesh (37% each).
• Health Risks – Sulphur dioxide (SO2) primarily harms the respiratory system, causing irritation, coughing, wheezing, and shortness of breath, as well as exacerbating asthma and lung diseases.
• Policy Gap – India’s National Clean Air Programme (NCAP) focuses heavily on PM10 (coarse dust), but this study warns that secondary PM2.5 from SO₂ emissions is being overlooked.
• Urgency – Controlling SO₂ emissions from coal plants and ammonia from agriculture is critical to reducing Delhi’s deadly PM2.5 burden.

Reference

Indian Express | One-third of Delhi’s annual PM2.5 pollution is secondary ammonium sulfate