A new book suggests nitrogen, essential to agriculture, has created a pollution crisis in India

Nitrogen pollution is the buildup of reactive nitrogen compounds in air. Some come from engines and industry as nitrogen oxides. Much of India’s atmospheric ammonia comes from agriculture. The problem matters because these gases do not remain harmlessly where they are released.

Fertilizer used on fields can release ammonia into the air. Ammonia is a base. It reacts with acidic gases, especially nitrogen oxides and sulphur dioxide. Those reactions form salts, including fine particles such as ammonium nitrate and ammonium sulphate. The atmosphere effectively assembles a new pollutant after the original emission.

The article estimates that agriculture supplies 80 to 85% of India’s ammonia. Ammonia acting alone may account for about a quarter of India’s PM2.5. Controlling nitrogen pollution therefore requires attention to farm emissions as well as visible smoke, vehicle exhaust, and coal-plant pollution.

The Indo-Gangetic Plain stretches across northern India in a 2,500-kilometre arc from Pakistan to Bangladesh. Nearly half a billion people live there. It is also one of the world’s most intensively farmed landscapes. That combination creates a vast population exposed to pollution from farming, combustion, and atmospheric chemistry.

A 2025 CREA study reported that secondary pollutants, especially ammonium sulphate, caused nearly one-third of India’s PM2.5 pollution burden. In Delhi’s most polluted winter months, secondary pollutants made up nearly 41% of all fine particles. They form when gases react, rather than being emitted as finished particles.

Professor Srinidhi Balasubramanian’s modelling gives a broader estimate. Inorganic secondary particles account for roughly 40% when ammonia, nitrogen oxides, and coal-derived sulphur are considered together. Adding organic secondary particles produces an estimated national balance of about 55% secondary pollution and 45% primary pollution.

Primary pollutants come directly from sources such as vehicles, coal plants, and burning organic matter. Secondary pollutants are assembled later in the atmosphere through chemical reactions. This distinction matters because the most important pollution may be invisible at the tailpipe or chimney.

The article explains the chemistry simply: acid plus base forms a salt. Nitrogen oxides from combustion and sulphur dioxide from coal provide the acids. Ammonia, overwhelmingly linked to agriculture in India, provides the dominant base. Their reactions produce inorganic particles, including ammonium sulphate and ammonium nitrate.

The result is a pollution burden that can exceed what visible emissions suggest. CREA linked ammonium sulphate to sulphur dioxide from coal plants meeting ammonia from fertilised fields. Balasubramanian estimates inorganic secondary particles at roughly 40% of India’s PM2.5, with organic secondary particles bringing the secondary share to about 55%.

Agriculture dominates India’s ammonia emissions because fertilised fields release ammonia into the atmosphere. The article cites Professor Srinidhi Balasubramanian’s estimate that farming contributes about 80 to 85% of India’s ammonia. Fertiliser plants and industry account for most of the remainder.

After leaving fields, ammonia acts as a base. It meets nitrogen oxides from engines and industry and sulphur dioxide from coal combustion. The gases react to form salts. These salts become fine particles, including ammonium nitrate and ammonium sulphate. Pollution is therefore created partly after the agricultural emission has travelled away from its source.

That chemistry makes agricultural emissions central to India’s air-quality problem. Balasubramanian estimates ammonia alone accounts for about a quarter of India’s PM2.5. Together with the acids it reacts with, inorganic secondary particles contribute roughly 40%, while secondary pollution overall may reach about 55%.

When ammonia, nitrogen oxides, and sulphur dioxide react, they form fine particles that can enter the air people breathe. The article identifies ammonium sulphate and ammonium nitrate as important examples. These are secondary pollutants because they are assembled in the atmosphere rather than emitted as completed particles.

The chemistry becomes especially powerful when coal-plant sulphur dioxide meets agricultural ammonia. Nitrogen oxides also react with ammonia to form ammonium nitrate. During Delhi’s most polluted winter months, CREA found that secondary pollutants constituted nearly 41% of all fine particles. The particles may be invisible, even though their ingredients came from identifiable sources.

The health consequence described is severe: particles can choke lungs and kill quietly. Fine-particle pollution therefore remains dangerous even when smoke from a particular source is not obvious. Reducing exposure requires cutting both the gases that form particles and the primary particles emitted directly.

Winter pollution becomes severe because the atmosphere both traps precursor gases and favours their conversion into particles. In the Indo-Gangetic Plain, cold temperatures push chemical equilibrium toward the particle phase. The planetary boundary layer, the air lid above the city, also becomes unusually shallow.

Kanpur measurements documented the mechanism. In winter, the boundary layer often compresses below 400 metres, trapping gases near the ground until they react. Nitrogen oxides from vehicles and industry peak at the same time. Those conditions are especially favourable for converting the gases into ammonium nitrate particles.

This seasonal combination explains why IGP winters are chemically different from other seasons. Pollution is not simply a visible plume from one source. Emissions, stagnant air, low temperatures, and atmospheric reactions reinforce one another. Winter controls therefore need to address precursor gases before the trapped atmosphere turns them into fine particles.

Stubble fires and vehicle exhaust release primary pollutants, so reducing them remains relevant. But they do not explain the entire pollution burden. The largest problem may be what happens afterward, when gases from different sources react in the atmosphere and create new particles.

For example, coal plants release sulphur dioxide, vehicles and industry release nitrogen oxides, and fertilised fields release ammonia. These ingredients can combine to form ammonium sulphate and ammonium nitrate. CREA found secondary pollutants made up nearly 41% of Delhi’s winter fine particles, while Balasubramanian estimates secondary pollution at about 55% nationally.

The solution must therefore go beyond visible sources. Cutting tailpipe smoke or stubble burning alone leaves agricultural ammonia and coal-related acids available for atmospheric chemistry. The article’s evidence points to coordinated control of farm ammonia, combustion gases, and the winter conditions that allow secondary particles to accumulate.