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India’s air pollution crisis is often described through visible smoke, dust, and traffic fumes. However, new research shows that a large share of the country’s most harmful air pollutant is not released directly into the air at all. According to a recent analysis by the Centre for Research on Energy and Clean Air, a significant portion of fine particulate matter or PM2.5 forms later through chemical reactions in the atmosphere. This secondary pollution remains largely invisible in daily discussions about pollution, even though it plays a significant role in determining air quality across regions and seasons. The findings challenge existing approaches to air pollution control in India, suggesting that focusing solely on visible sources may leave a substantial portion of the problem unaddressed.
PM2.5 That Forms After Emission
The CREA analysis finds that as much as 42% of India’s PM2.5 pollution is secondary in nature. This means the particles are formed in the air after gases such as sulphur dioxide, nitrogen oxides, and ammonia react under suitable atmospheric conditions. Unlike dust or vehicle soot, these particles cannot be traced back to a single visible source, making them harder to regulate and monitor.
Among these secondary particles, ammonium sulphate stands out as the most dominant component. It forms when sulphur dioxide reacts with ammonia and other compounds in the atmosphere. This process intensifies during cooler months, when stagnant air and lower temperatures allow chemical reactions to continue for longer periods. The study shows that secondary PM2.5 is not a marginal contributor but a persistent feature of India’s air pollution throughout the year.
Coal Power and Sulphur Emissions
India currently ranks as the world’s largest emitter of sulphur dioxide, a key precursor for ammonium sulphate formation. Coal-fired thermal power plants are responsible for nearly 60% of these emissions. Despite this, a large majority of plants continue to operate without flue gas desulphurisation systems, which are designed to remove sulphur dioxide before it enters the atmosphere.
The study notes that nearly 78% of India’s coal-based power plants have been granted exemptions from installing these pollution control devices. As a result, sulphur emissions remain high, particularly in coal-heavy states. This regulatory gap allows secondary particulate pollution to build up far from the source, affecting urban and rural regions alike.
Regional Patterns and Seasonal Shifts
The contribution of secondary PM2.5 varies widely across states. Coal-producing regions such as Chhattisgarh and Odisha show some of the highest annual shares, with ammonium sulphate accounting for over 40% of PM2.5 pollution. In contrast, other states record slightly lower but still substantial shares, often between 30 and 40%.
In Delhi, secondary pollution plays a particularly important role during high pollution months. The study estimates that about one-third of the city’s annual PM2.5 load comes from ammonium sulphate. During post monsoon and winter periods, this share rises sharply, reaching nearly half of the total PM2.5 at certain times. These seasonal shifts help explain why pollution levels remain severe even when visible sources appear unchanged.
The Policy Blind Spot in India’s PM2.5 Fight
The findings raise important questions about the effectiveness of current air quality policies, including the National Clean Air Programme. Much of India’s pollution control framework continues to prioritise dust suppression and PM10 reduction, while the chemistry behind PM2.5 formation receives far less attention.
CREA researchers argue that meaningful reductions in fine particulate pollution will require tighter controls on precursor gases rather than only targeting end-stage particles. This includes enforcing sulphur emission standards at power plants and improving monitoring of pollution composition. Without addressing secondary formation, large gains in air quality may remain out of reach, even as visible pollution sources are brought under control.
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