NEW DELHI: Around one-third of Delhi’s annual PM2.5 pollution is made up of secondary ammonium sulfate, with its share rising sharply during the city’s worst pollution months, according to a new analysis by the Centre for Research on Energy and Clean Air (CREA).
During the post-monsoon period, ammonium sulfate accounts for 49 per cent of PM2.5 in the capital, while in winter it contributes 41 per cent, far higher than in summer or the monsoon season. The findings suggest that Delhi’s most severe pollution episodes are driven not only by local sources such as traffic and dust, but also by region-wide emissions of precursor gases, particularly sulphur dioxide (SO2), that undergo chemical reactions in the atmosphere to form fine particulate matter.
This challenges the dominant policy focus on visible pollutants like road dust and PM10, the report said.
CREA’s analysis, based on NASA’s MERRA-2 reanalysis data for 2024, shows that secondary particulate matter contributes between 17 per cent and 42 per cent of PM2.5 mass across Indian states, with most states clustering between 30 and 40 per cent annually.
At the national level, up to 42 per cent of PM2.5 pollution is formed secondarily in the atmosphere, primarily as ammonium sulfate derived from SO2 emissions. India is currently the world’s largest emitter of SO2, with coal-fired power plants responsible for at least 60 per cent of national emissions.
Despite this, around 78 per cent of coal-based thermal power plants have been exempted from installing flue gas desulphurisation (FGD) systems, which are designed to curb SO2 at source, significantly weakening regulatory control, the analysis noted.
State-level data underline the link between coal power and secondary pollution. Chhattisgarh recorded the highest annual ammonium sulfate contribution at 42 per cent, followed by Odisha at 41 per cent, both states with a heavy concentration of coal-fired plants.
However, CREA said elevated levels across many other states show that secondary sulfate formation is a national problem rather than one confined to a few hotspots.
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