42% of India’s PM2.5 comes from secondary particles, mainly ammonium sulfate: Study

A large share of India’s PM2.5 pollution is not emitted directly but is chemically formed in the atmosphere from precursor gases, with 42 per cent of the burden being secondary particulate matter, primarily ammonium sulfate formed from sulfur dioxide (SO2). India is the largest SO2 emitter globally, with coal-fired power plants contributing at least 60 percent of national SO2 emissions, reinforcing the central role of SO2 control in reducing PM2.5 pollution.

The Centre for Research on Energy and Clean Air (CREA) in its study shows that a large share of India’s PM2.5 pollution is not emitted directly, but is chemically formed in the atmosphere from precursor gases. The assessment finds that up to 42 percent of India’s PM2.5 burden is secondary particulate matter, primarily ammonium sulfate formed from sulfur dioxide.

“Despite this evidence, the current regulatory framework has exempted around 78 percent of coal-fired power plants from installing flue gas desulphurisation (FGD) systems, significantly weakening SO2 control at the source,” it said.

During the city’s most polluted periods, post-monsoon and winter, ammonium sulfate dominates PM2.5, contributing 49 pc and 41 pc respectively, compared with just 21 pc in summer and the monsoon.

“This shows that Delhi’s worst pollution episodes are driven largely by region-wide SO2 emissions and secondary formation, not only local primary sources. The assessment, using NASA’s MERRA-2 reanalysis data for 2024, finds that ammonium sulfate contributes between 17 pc and 42 pc of PM2.5 mass across Indian states, with most states clustering between 30 and 40 pc annually,” the statement said.

The findings also highlight major gaps in current air quality strategies, which continue to prioritise PM10, road dust, and other visible pollution sources, while largely overlooking the role of precursor gases such as SO2, nitrogen dioxide and ammonia.

According to the analysis, the highest annual ammonium sulfate contribution is observed in Chhattisgarh (42 pc), a coal-fired power plant-dominant state, followed closely by Odisha (41 pc).  “This shows that reinstating mandatory FGD requirements across all coal-fired thermal power plants is critical to reducing secondary ammonium sulfate formation and PM2.5 under the National Clean Air Programme (NCAP),” it said.

The analysis noted that around one-third of Delhi’s annual PM2.5 is secondary ammonium sulfate. “This establishes secondary particulate matter as a core driver of India’s PM2.5 burden, rather than a marginal or seasonal concern. Aside from Chhattisgarh, several other states across India also recorded high contributions, indicating that secondary sulfate formation is widespread and national in scale, not limited to a few hotspots,” it added.

Seasonal contributions across Indian states show that ammonium sulfate remains a substantial component of PM2.5 year-round, peaking in winter (31-52 pc of PM2.5 mass) and post-monsoon (27-53 pc), and remaining significant even in summer (11-36 pc) and monsoon season (4-26 pc).

“These patterns demonstrate that secondary particulate matter dominates PM2.5 composition during India’s most polluted months,” it said.

CREA has also recommended that, without addressing secondary particulate matter, improvements in air quality are likely to remain limited and short-lived.

“As the National Clean Air Programme (NCAP) is revised, India must focus not only on PM2.5 concentrations but also on what the pollution is made of. With secondary ammonium sulfate accounting for up to 42 percent of PM2.5, largely driven by SO2 from coal-based power plants, precursor controls and composition monitoring are essential for air quality improvement,” said Manoj Kumar, India Analyst at CREA.