Cause 02 · Mostly winter
Secondary particles: pollution made in the sky
Direct answer: secondary particles are not released directly. They form in the air when gases react together. SO₂ from coal, NOx from vehicles and ammonia from farm fertiliser combine into sulphate, nitrate and ammonium particles. In winter they are often the single biggest part of Delhi's PM2.5.
At a glance
- A real-time study for the Delhi Pollution Control Committee found secondary inorganic particles were the largest part of winter PM2.5, about 32% on average.30
- An expert report submitted by CAQM in January 2026 also named secondary particulates as the top cause of Delhi's winter pollution.49
- Only about 36% of Delhi's winter PM2.5 comes from Delhi's own emissions.36
- Our model analysis: the PM2.5 level 100–300 km away was about 57% of Delhi's own level.1
How secondary particles form
Three gases do most of the work. SO₂, mainly from coal power plants and industry, turns into sulphate. NOx, from vehicles, industry and power plants, turns into nitrate. Ammonia, mostly from urea and other fertiliser and from animal waste, joins them to make ammonium sulphate and ammonium nitrate. These reactions take hours to days, so the particles are spread over the whole plain, not just near the source. Cold, humid winter nights help nitrate form and keep it in the air.33
What research says
Show the numbers
| Source | Share of PM2.5 |
|---|---|
| Secondary inorganic | 32% |
| Biomass burning | 24% |
| Vehicles | 17% |
- Secondary particles are a core driver of dangerous PM2.5 across India, and CREA argues that SO₂ and ammonia need more attention in the National Clean Air Programme.33
- Urea use is the largest source of farm ammonia, and India has no rules for farm ammonia comparable to those for other pollutants.17
- Fertiliser ammonia is estimated to add about 3.5–6 µg/m³ to yearly PM2.5 in the region.31
Show the numbers
| Season | From Delhi's own emissions |
|---|---|
| Winter | 36% |
| Summer | 26% |
What our data shows
The regional PM2.5 map shows one long band of haze from Punjab to eastern Uttar Pradesh. The average level in a ring 100–300 km from Delhi was about 57% of Delhi's own level.1,9 Satellite SO₂ hotspots sit over large coal plants (see cause 05), which feed sulphate particles into this regional haze.3
What is being done
Very little targets these particles directly. In July 2025, the sulphur-control (FGD) rule for coal plants was relaxed, and most plants no longer need to fit FGD.58 There are no rules for farm ammonia.17 Because the particles form across the region, action has to be regional; see One airshed, one plan.
Limits of this data
The 57% figure is the regional share of total PM2.5 in the CAMS model, not a direct measure of secondary particles. Ammonia data in the model is not available for India.