Published: 8 July 2026 | Author: Team JanVayu | Reading time: 4 min
When we picture air pollution, we picture a source: a truck's exhaust, a brick kiln's chimney, a field of burning stubble, a cloud of construction dust. Something is emitted and we breathe it. Yet a large share of the fine particulate matter that shortens Indian lives was never emitted as particles. It forms in the atmosphere, hours and hundreds of kilometres downwind, out of invisible gases. A December 2025 analysis by the Centre for Research on Energy and Clean Air (CREA) puts a number on it: up to 42% of India's PM2.5 burden is secondary, meaning chemically formed in the air rather than directly released.
That changes what "cleaning the air" has to mean.
Primary and secondary particles
Primary PM2.5 is emitted as particles: soot from diesel, fly ash, road and construction dust. A filter, a cover or a water spray can be pointed at it.
Secondary PM2.5 starts as a gas. Sulphur dioxide (SO₂), nitrogen oxides (NOₓ) and ammonia (NH₃) leave smokestacks, tailpipes and farms, react with sunlight and with each other, and condense into fine particles such as ammonium sulphate and ammonium nitrate. By the time they count as PM2.5, the "source" is a plume of gas that left a chimney a long way away. Using NASA's MERRA-2 reanalysis for 2024, CREA finds that ammonium sulphate makes up 17% to 42% of PM2.5 mass across Indian states (most states 30-40%), with 42% in Chhattisgarh. CREA's headline summary is that up to 42% of India's PM2.5 burden is chemically formed in the air. CREA's analysis quantifies ammonium sulphate.
Coal, sulphur and the missing scrubbers
The main precursor in India is SO₂, and India is the world's largest SO₂ emitter. The biggest single contributor is coal-fired power, at least 60% of national SO₂ emissions (as reported by Down To Earth). That SO₂ becomes ammonium sulphate, and the pattern is regional. CREA attributes 42% of Chhattisgarh's and 41% of Odisha's PM2.5 to ammonium sulphate, and both states depend heavily on coal power. Even in Delhi, roughly one-third of PM2.5 is secondary ammonium sulphate, rising to 49% in the post-monsoon window.
A central way to cut SO₂ is flue-gas desulphurisation (FGD), which means scrubbers on power-plant stacks. CREA says the regulatory framework now exempts about 78% of coal-fired power plants from installing FGD. In effect we are choosing to keep making a large fraction of our own particulate matter.
Why "dust-first" policy falls short
Much of India's clean-air spending targets what can be seen: paving roads, spraying water, sweeping dust, greening verges. These measures are worth doing, but they act mostly on primary coarse particles. They do almost nothing about a sulphate particle formed in the sky from a power-plant gas. In CREA's words, "without addressing secondary particulate matter, improvements in air quality are likely to remain limited and short-lived."
It also explains why two honest monitors in the same airshed can tell different stories, and why a city can pave every road and still miss its PM2.5 target. Fight only the pollution you can see being emitted, and in the most affected states up to about two-fifths of the problem stays untouched.
What it would take
Cutting secondary PM2.5 means regulating gases as well as dust. Scrubbers on coal plants would be the largest single step on SO₂, and the mandate needs enforcing rather than extending. NOₓ controls on power and transport would reduce the ammonium nitrate that forms from them. Ammonia from agriculture is an overlooked precursor, and it is the one that turns SO₂ and NOₓ into particles. And monitoring should record what the PM2.5 is made of, not only how much there is, so that control money follows the chemistry.
What JanVayu takes from this
We have added CREA's December 2025 secondary-PM2.5 analysis to the Reading List. It fits a theme that runs through this platform: the picture of India's air that holds up is rarely the intuitive one. A ward is not one number, and a city's PM2.5 is not one source. A great deal of what you breathe was assembled overhead, out of gases that left a chimney nobody photographed.