Written by Sophiya MathewNew Delhi | December 25, 2025 06:06 AM IST 3 min readHigh secondary PM2.5 contributions were also observed in Bihar (39%), Maharashtra, Andhra Pradesh and West Bengal (38% each), as well as Uttar Pradesh and Madhya Pradesh (37% each).Nearly one-third of Delhi’s annual Particulate Matter 2.5 (PM2.5) pollution is made up of secondary ammonium sulfate, resulting from coal plants, industries and agriculture, with the chemically-formed pollutant dominating the city’s air during its worst pollution periods, according to a new analysis conducted by the Centre for Research on Energy and Clean Air (CREA), a Finland-headquartered think-tank.Ammonium sulfate is a secondary inorganic aerosol formed through the oxidation of sulphur dioxide (SO2) into sulfate. This reacts with ammonia emissions from agriculture and other sources. Its formation is enhanced under humid conditions, linking coal-based power generation, industrial activity and agricultural ammonia to elevated PM2.5 levels in Delhi and across the National Capital Region (NCR).This is particularly alarming, as at 11.2 million tonnes, India is the biggest emitter of SO2 globally, and ranks second only to China, an analysis by CREA published earlier this year had flagged.Its latest analysis found that ammonium sulfate accounts for 49% of PM2.5 pollution in Delhi during the post-monsoon period and 41% in winter, compared with just 21% in summer and monsoon.This indicates that the Capital’s most severe pollution episodes are driven largely by region-wide SO2 emissions and secondary chemical formation in the atmosphere, rather than only local primary sources.Secondary particulate matter is a core driver ofPM2.5 pollution in India, with up to 42% of the country’s PM2.5 burden being chemically formed in the atmosphere from precursor gases such as sulphur dioxide and ammonia, CREA said.At the national level, the highest annual contribution of ammonium sulfate to PM2.5 mass was observed in coal-fired power plant-dominated states. Chhattisgarh recorded the highest share at 42%, followed by Odisha (41%), as well as Jharkhand and Telangana (40% each).Story continues below this adHigh secondary PM2.5 contributions were also observed in Bihar (39%), Maharashtra, Andhra Pradesh and WestBengal (38% each), as well as Uttar Pradesh and Madhya Pradesh (37% each).“The current regulatory framework has exempted approximately 78% of coal-fired thermal power plants from installing flue gas desulfurisation systems, weakening sulfur dioxide control at source. Reinstating 2015 emission norms notification is therefore critical for mandatory installation of flue gas desulfurisation to reduce secondary ammonium sulfate formation and PM2.5,” the analysis said.According to Dr Manoj Kumar, one of the authors, the study has implications as the National Clean Air Programme (NCAP) moves towards revision. “NCAP remains largely focused on PM10 and a significant share of PM2.5 is formed from precursor gases,” he said.Sophiya Mathew is a Correspondent at The Indian Express, based in New Delhi. She joined the Delhi bureau in 2024, and has specialization in Integrated Multimedia Journalism from the Asian College of Journalism (ACJ), Chennai. Professional Background Core Beats: Her reporting is primarily focused on the Environment and Education. Specialization: She has gained recognition for her ground-level reporting on the Yamuna floodplains and the socio-economic challenges faced by those living on its banks. She also focuses on the disparities in Delhi's education system, ranging from elite private schools to government institutions and refugee education. Recent Notable Articles (December 2025) Her recent work has been heavily centered on Delhi's severe winter pollution crisis and the government's regulatory responses: 1. The Air Pollution Crisis "A tale of two cities: Delhi govt schools choke in bad air, private classrooms set up air filters" (Dec 20, 2025): A high-impact feature contrasting the "Clean Air Bubbles" in elite schools with the reality of government school students who are exposed to an equivalent of 17 cigarettes a day due to outdoor exposure. "Delhi sees season's worst air day, second worst December AQI in nearly a decade" (Dec 15, 2025): An analytical report on the meteorological patterns trapping pollutants in the NCR. "Delhi bans non-BS VI vehicles from outside: Why curbing vehicular pollution is key" (Dec 17, 2025): Explaining the science behind targeting specific vehicle vintages to lower particulate matter. 2. Enforcement & Regulations "No fuel at pumps in Delhi without valid PUC certificate from December 18" (Dec 17, 2025): Breaking the news on the environment ministry's strict "No PUC, No Fuel" policy. 3. Education Policy "Law to regulate school fee in Delhi risks becoming procedural, say parents" (Dec 13, 2025): Investigating the loopholes in the new Delhi School Education (Transparency in Fixation and Regulation of Fees) Bill, 2025. "Monsoon Session: Private school fee regulation Bill cleared after four-hour debate" (Aug 9, 2025): Covering the legislative passage of the controversial fee hike regulation. Signature Style Sophiya is known for her observational depth. Her reporting often includes vivid details from school corridors, hospital waitlists, or the banks of the Yamuna to illustrate how policy failures affect the city's most vulnerable residents. She is a frequent expert guest on the 3 Things podcast, where she explains the complexities of Delhi’s environmental laws. X (Twitter): @SophiyaMathew1 ... Read MoreStay updated with the latest - Click here to follow us on Instagram© The Indian Express Pvt LtdTags:New Delhi