Indirect PM2.5 emissions in India largely chemically formed

An analysis by the Centre for Research on Energy and Clean Air (CREA) states that a large share of India’s PM2.5 pollution is not emitted directly but is chemically formed in the atmosphere from precursor gases, as reported by Down To Earth (DTE). Up to 42% of India’s PM2.5 burden is secondary particulate matter, primarily ammonium sulphate formed from sulphur dioxide (SO2). India emits 11.2 million tonnes of SO2, the highest in the globe, and is next only to China when it comes to the emission of nitrogen oxides (NOx) at 9.4 million tonnes and ammonia (NH3) at 10.4 million tonnes.

The DTE report adds that India is the largest SO2 emitter globally, with coal-fired power plants contributing at least 60% of national SO2 emissions, reinforcing the central role of SO2 control in reducing PM2.5 pollution.

A CREA press release states that Indiaʼs clean air action plan demands a decisive shift in focus beyond metropolitan boundaries. According to the new satellite-based assessment, harmful PM2.5 concentrations represent a widespread, year-round crisis, not merely an urban or seasonal issue. Drawing on high-resolution satellite data, the report presents comprehensive annual and seasonal population-weighted PM2.5 concentrations analyses across Indiaʼs districts, states, and airsheds exposing extreme regional disparities and calling for urgent, data-driven interventions. All 33 states and union territories considered in the report exceed the WHO guideline, with 28 states also breaching India’s National Ambient Air Quality Standard (NAAQS). Delhi ranked as the most polluted state/union territory with an annual mean population weighted PM2.5 concentration of 101 µg/m³, 2.5 times the NAAQS and 20 times the WHO guidelines. The top 50 most polluted districts are mostly concentrated in four states (Delhi, Assam, Haryana, and Bihar).

The CREA report finds that in the Indian capital, New Delhi, deadly PM2.5 load is driven less by local dust and more by invisible secondary particles formed as a result of emissions from burning of coal. About one-third of Delhi’s annual PM2.5 is comprised of ammonium sulphate – a fine particulate matter which is formed when sulphur dioxide emitted by coal-based thermal plants reacts with ammonia in the air – rising to almost half during episodes of smog in winter.

According to the CREA report, during the city’s most polluted periods, post-monsoon and winter, ammonium sulphate dominates PM2.5, contributing 49% and 41% respectively, compared with just 21% in summer and the monsoon. Delhi’s worst pollution episodes are driven largely by region-wide SO2 emissions and secondary formation, not only local primary sources. Ammonium sulphate contributed between 17% and 42% of PM2.5 mass across Indian states, with most states clustering between 30% and 40% annually.

Seasonal PM2.5 concentration analysis highlights regional differences in Indiaʼs air quality, the CREA press release says. The Indo-Gangetic airshed remains the most polluted region in the country, consistently non-compliant during winter, summer, and post-monsoon season. The Northeast airshed is an emerging concern, with states such as Assam and Tripura maintaining elevated PM2.5 levels throughout the year. During the monsoon season, all airsheds except Northeast airshed were below the annual National Ambient Air Quality Standard (NAAQS) of 40 µg/m³. However, the rapid return to high PM2.5 levels post-monsoon emphasises that baseline emissions, not meteorology, are the fundamental problem.

The DTE report highlights that according to the CREA study, the highest annual ammonium sulphate contribution was observed in Chhattisgarh (42%), a coal-fired power plant – dominant state, followed closely by Odisha (41%). Aside from Chhattisgarh, several other states across India also recorded high contributions, indicating that secondary sulphate formation is widespread and national in scale, not limited to a few hotspots. Seasonal contributions across Indian states show that ammonium sulphate remains a substantial component of PM2.5 year-round, peaking in winter (31%-52% of PM2.5 mass) and post-monsoon (27%-53%), and remaining significant even in summer (11%-36%) and monsoon season (4%-26%). These patterns demonstrate that secondary particulate matter dominates PM2.5 composition during India’s most polluted months. 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. Without addressing secondary particulate matter, improvements in air quality are likely to remain limited and short-lived.