A third of Delhi’s annual PM2.5 load is made up of secondary pollutants. This is how they are formed

A significant portion of the fine particulate matter that Delhi residents breathe is not emitted directly, but created in the atmosphere itself.

Secondary aerosols now contribute at least one-third of the city’s annual PM2.5 pollution, intensifying exposure during prolonged smog spells and helping explain why air quality can deteriorate sharply even when local sources appear under control.

Primary pollutants emerge directly from factors such as road dust resuspension, construction activity, open burning, vehicle exhaust and industries. Secondary particulate matter, on the other hand, forms after gases are released into the air.

These gases, known as precursor pollutants, undergo chemical reactions influenced by humidity, temperature and sunlight. They eventually form microscopic particles that penetrate deep into the lungs.

Among the most dominant of these pollutants in Delhi is ammonium sulfate, a secondary inorganic aerosol. According to an analysis released earlier this month by the Centre for Research on Energy and Clean Air (CREA), ammonium sulfate alone accounts for nearly one-third of Delhi’s annual PM2.5 load, rising sharply during the post-monsoon winter months when pollution episodes are at their worst.

How does ammonium sulfate form?

Ammonium sulfate is formed from a gas called sulphur dioxide (SO₂), which is largely released by coal-fired power plants. Other SO₂ sources include oil refineries, heavy industries, brick kilns, diesel combustion and shipping.

The SO₂ gets oxidised in the atmosphere to form sulfate. This sulfate then reacts with ammonia, which is released mainly from agricultural activities, such as fertilizer use, livestock waste, sewage systems, landfills, biomass burning, diesel vehicles equipped with catalytic converters and certain industrial processes. The resulting compound becomes suspended in the air as fine particulate matter. It remains airborne for days, travelling long distances and contributing to transboundary pollution.

India is currently the world’s largest emitter of SO₂, largely due to coal-based power generation. In July 2025, the government exempted nearly 78% of coal-fired thermal power plants from installing flue gas desulphurisation (FGD) systems, weakening SO₂ control at the source. The government cited three studies that said SO₂ levels around plants are well within norms. Experts, however, say this is inaccurate.

According to CREA’s satellite-based assessment in 2024, the highest annual contribution of ammonium sulfate to PM2.5 is in coal-dominated states such as Chhattisgarh (42%), Odisha (41%), Jharkhand and Telangana (40% each). The problem was not confined to a single airshed (the typical circulatory region for a body of air). High secondary PM 2.5 shares were also observed in Bihar, Uttar Pradesh, Maharashtra, Andhra Pradesh and West Bengal.

For Delhi-NCR, this has direct implications. Secondary aerosols formed from emissions hundreds of kilometres away can combine in the atmosphere and significantly affect the capital’s air.

This has been flagged as a concern by experts, as precursor pollutants significantly add to the PM2.5 burden. Delhi has consistently recorded some of the highest PM2.5 levels in the world, and was ranked the most polluted national capital globally, according to the 2024 World Air Quality Report by IQAir, with an annual PM2.5 average of 91.6 µg/m3.

As the National Clean Air Programme (NCAP) moves towards revision, experts have argued that secondary aerosol formation must be given focus, rather than PM10.

What are the key drivers of ammonium sulfate?

Humidity plays a critical role in this process. Moist air, fog and low winter temperatures accelerate chemical reactions, allowing gases to transform into particles within hours. This explains why Delhi’s pollution often worsens during stagnant winter conditions, even without a proportional rise in visible emissions.

In India, according to CREA, ammonium sulfate contributes around 49% of PM2.5 during the post-monsoon period and 41% in winter, compared with just about 21% during summer and monsoon months. The findings suggest that Delhi’s most severe smog episodes are driven not only by local sources but also by regional emissions and atmospheric chemistry.

Has such secondary aerosol-driven pollution been seen elsewhere before?

Yes. One example is the deadly 1948 Donora smog disaster in the US. Emissions from zinc and steel plants in the small town accumulated under stagnant weather conditions, forming sulfate aerosols.

Twenty people were killed and thousands faced respiratory problems.

Atmospheric studies from Beijing have shown how concentrations of sulfate, nitrate and ammonium can multiply several-fold during heavy pollution episodes. Research conducted by the Chinese Academy of Sciences found that during severely polluted periods, these secondary components increased between four and eight times, driven by high humidity, low wind speeds and intense chemical transformation in the air.

In Beijing, researchers identified secondary aerosol formation as a major driver of air pollution as early as the late 1990s and early 2000s. At the time, the city consumed about 42 million tonnes of coal annually, much of it sulphur-rich, making sulphur dioxide the dominant air pollutant.

Alongside this, rapid motorisation sharply increased nitrogen oxides: Beijing’s vehicle population rose from about 1.7 million in 2001 to nearly 2.6 million by 2005, with the transport sector accounting for around 40% of NOx emissions by 2000. Studies showed that SO₂ and NOx were readily oxidised in the atmosphere to form sulfate and nitrate, which then combined with ammonia to produce ammonium sulfate and ammonium nitrate — the most dominant inorganic aerosols in Beijing’s air.

Similarly, long-term studies from the UK have shown that ammonia can rapidly convert into ammonium sulfate within hours, through reactions occurring in mist, cloud droplets and humid air even without sunlight.