What Happened
Recent air quality assessments show that nearly one-third of Delhi’s annual PM2.5 pollution load comes from secondary pollutants, with ammonium sulfate emerging as a major contributor. This finding highlights that a large share of Delhi’s air pollution is formed in the atmosphere rather than released directly from local sources.
Context: Why Ammonium Sulfate Has Become a Major Concern
Air pollution discussions often focus on visible sources such as vehicle exhaust, construction dust, or stubble burning. However, a significant portion of fine particulate matter in Delhi is formed through chemical reactions in the air. These pollutants are known as secondary aerosols.
Ammonium sulfate is one such secondary pollutant. It does not come directly from chimneys or vehicles. Instead, it forms when gases released from power plants, agriculture, industries, and transport react in the atmosphere. Because these reactions can occur over long distances, pollution generated hundreds of kilometres away can combine and worsen air quality in Delhi.
This makes ammonium sulfate pollution harder to control and exposes gaps in air-quality policies that focus mainly on local and visible sources.
How Ammonium Sulfate Forms in the Air
Ammonium sulfate forms through a multi-step chemical process.
- Sulphur dioxide (SO₂) is released mainly from coal-based power plants, along with refineries, brick kilns, diesel engines, and shipping.
- In the atmosphere, SO₂ is oxidised into sulphate particles.
- These sulphates react with ammonia (NH₃), which is released from fertiliser use, livestock waste, sewage systems, landfills, biomass burning, and some vehicles and industries.
The resulting ammonium sulfate particles are extremely small. They can remain airborne for several days and travel long distances, contributing to pollution far from their original source.
Why Pollution Peaks in Winter
Weather conditions play a major role in ammonium sulfate formation.
- High humidity and fog speed up chemical reactions.
- Low winter temperatures allow secondary aerosols to form within hours.
- Calm winds and stagnant air trap pollutants near the ground.
As a result, ammonium sulfate accounts for nearly half of PM2.5 pollution during the post-monsoon period and over 40% during winter, compared to much lower levels in summer and monsoon months.
Regional and Policy Factors
Delhi’s ammonium sulfate problem is not limited to emissions within the city.
- Coal-heavy states such as Chhattisgarh, Odisha, Jharkhand, Telangana, and parts of Uttar Pradesh and Bihar contribute significantly to sulphur dioxide emissions.
- India is currently the world’s largest emitter of SO₂ due to its reliance on coal-based power generation.
- In 2025, a large majority of coal-fired power plants were exempted from installing flue gas desulphurisation systems, weakening controls on sulphur emissions at the source.
These regional emissions combine in the atmosphere and intensify pollution episodes in Delhi-NCR.
Health and Air Quality Implications
Ammonium sulfate particles are small enough to penetrate deep into the lungs. Long-term exposure increases the risk of respiratory and cardiovascular diseases.
Delhi already records some of the highest PM2.5 levels among global capitals. In 2024, the city’s annual average PM2.5 concentration was far above safe limits, making secondary pollutants a serious public health concern.
Why This Matters
For citizens, secondary pollution worsens health risks even when visible pollution sources appear controlled.
For governance, it shows that local action alone cannot solve Delhi’s air-quality crisis.
For policy, it underlines the need to regulate precursor gases such as sulphur dioxide and ammonia.
For the future, ignoring secondary aerosols could limit the effectiveness of air-pollution control programmes.
What Readers Should Understand
Delhi’s air pollution is not only about smoke, dust, or traffic. A large share of harmful particles forms invisibly through chemical reactions involving emissions from power plants, agriculture, and industries across multiple states.
Reducing pollution levels will require shifting focus from only controlling particulate matter to also cutting the gases that create secondary pollutants like ammonium sulfate. Without this broader approach, severe winter pollution episodes are likely to continue.