Kolkata: A new study by Centre for Research on Energy and Clean Air (CREA) revealed that nearly one-third of Kolkata’s PM2.5 pollution is caused by secondary pollutants — invisible particles formed through chemical reactions between gases like sulfur dioxide (SO₂) and ammonia (NH₃). Yet, these critical contributors remain largely overlooked by India’s National Clean Air Programme (NCAP), which continues to focus on direct particulate emissions. Secondary pollutants pose serious health risks, penetrating deep into lungs and entering the bloodstream. Prolonged exposure to PM2.5 is linked to heart disease, respiratory illnesses, and premature death.The CREA analysis shows that ammonium sulfate, a secondary pollutant formed in the atmosphere from SO₂ and NH₃, makes up 34% of the PM2.5 mass on an average across Indian cities. In Kolkata, ammonium sulfate concentrations reach 21.7 µg/m³, placing it among the top five NCAP cities most affected by this pollutant, alongside Patna, Muzaffarpur, Howrah, and Varanasi. “We are ignoring one-third of the problem. Unless we address the gases forming PM2.5, real air quality improvement will remain out of reach,” said Manoj Kumar, analyst at CREA.Secondary pollutants differ from primary pollutants in that they form in the atmosphere through chemical reactions, making them harder to trace and regulate. Because they are transboundary in nature, their impact is felt far beyond state and city borders.CREA’s study finds that 114 out of 130 NCAP cities had ammonium sulfate contributing more than 30% to PM2.5 levels. Cities like Jodhpur, Rajkot, and Srinagar were among the few where the contribution was below 25%. The nationwide average ammonium sulfate concentration is 11.9 µg/m³ and the maximum recorded is 22.5 µg/m³ in Patna.In areas near coal-fired thermal power plants, concentrations of ammonium sulfate are 2.5 times higher than in distant regions. CREA found that SO₂ emissions from these plants are the single largest driver, accounting for over 60% of India’s SO₂ output.Despite the overwhelming evidence, the study finds, NCAP continues to focus largely on PM10, while PM2.5 — finer, more toxic, and deadlier — gets inadequate attention. Secondary particles like ammonium nitrate also add significantly to the burden, with secondary pollutants forming up to 50% of PM2.5 mass in some areas. CREA recommends NCAP and city-level clean air plans expand focus to PM2.5 and its precursor gases, mandate flue gas desulfurisation in thermal power plants to cut SO₂, regulate ammonia sources, and ensure compliance with emission standards. “Without this, tackling secondary PM2.5 will stay a challenge,” Kumar said.
