A significant share of India’s deadly PM2.5 pollution is not released directly from chimneys or vehicle exhausts but is chemically formed in the atmosphere from precursor gases, a new analysis by the Centre for Research on Energy and Clean Air has revealed. The findings challenge the country’s current air pollution control strategies, which largely focus on visible and primary emission sources.
According to the assessment, up to 42 per cent of India’s PM2.5 burden consists of secondary particulate matter, with ammonium sulphate emerging as the dominant component. This pollutant forms in the atmosphere when sulphur dioxide reacts with ammonia and other compounds, highlighting the crucial role of gaseous emissions rather than direct particle release.
India currently ranks as the world’s largest emitter of sulphur dioxide, with coal-fired power plants responsible for at least 60 per cent of national SO2 emissions. Despite this, nearly 78 per cent of coal-based thermal power plants have been exempted from installing flue gas desulphurisation systems, a regulatory dilution that has significantly weakened efforts to control SO2 at the source.
The study found stark regional patterns. Chhattisgarh recorded the highest annual contribution of ammonium sulphate to PM2.5 at 42 per cent, followed closely by Odisha at 41 per cent. Both states have a high concentration of coal-fired power plants, underscoring the link between coal combustion and secondary particulate pollution.
Delhi’s pollution profile further illustrates the problem. Around one-third of the capital’s annual PM2.5 load is made up of secondary ammonium sulphate. During the most polluted months after the monsoon and in winter, this share rises sharply to 49 per cent and 41 per cent respectively, compared to just 21 per cent during summer and the monsoon season. CREA noted that Delhi’s worst pollution episodes are driven largely by regional SO2 emissions and atmospheric chemical reactions, rather than only local sources.
Across India, ammonium sulphate contributed between 17 and 42 per cent of PM2.5 mass, with most states falling in the 30–40 per cent range on an annual basis. The analysis, based on NASA’s MERRA-2 reanalysis data for 2024, establishes secondary particulate matter as a central driver of India’s PM2.5 pollution, rather than a marginal or seasonal factor.
Seasonal trends show that secondary sulphates dominate PM2.5 composition during the country’s most polluted periods. In winter, ammonium sulphate accounted for 31–52 per cent of PM2.5 mass, while in the post-monsoon season the range was 27–53 per cent. Even during summer and the monsoon, when pollution levels are relatively lower, secondary particulates remained significant.
The findings expose major gaps in India’s air quality management approach. Current strategies continue to prioritise PM10, road dust and other visible sources, while paying insufficient attention to precursor gases such as sulphur dioxide, nitrogen dioxide and ammonia that drive secondary particle formation.
Without targeting secondary particulate matter, CREA warned, air quality improvements are likely to be limited and short-lived. The study stressed that reinstating mandatory flue gas desulphurisation requirements for all coal-fired power plants is essential to reducing ammonium sulphate formation under the National Clean Air Programme.
“As the NCAP is revised, India must focus not only on PM2.5 concentrations but also on what the pollution is made of,” said Manoj Kumar, India analyst at CREA. “With secondary ammonium sulphate accounting for up to 42 per cent of PM2.5 and largely driven by SO2 from coal-based power plants, controlling precursor emissions and monitoring pollution composition are critical for lasting air quality improvement.”