Ammonia and agricultural emissions drive invisible secondary air pollution across northern India

The Indo-Gangetic Plain stretches 2,500 kilometres across northern India from Pakistan to Bangladesh, supporting nearly half a billion people in one of the most chemically burdened farming landscapes on earth. A book titled The Nitrogen Bomb by Jacob Koshy explores how invisible secondary pollutants, rather than just visible smoke, worsen winter air quality across this densely populated region.

The Role of Ammonia

A 2025 study by the Centre for Research on Energy and Clean Air found that secondary pollutants like ammonium sulphate caused nearly one-third of India's PM2.5 pollution burden. During Delhi's heavily polluted winter months, these secondary particles made up nearly 41 per cent of all fine particles in the air. Professor Srinidhi Balasubramanian of IIT Bombay states that farming accounts for 80 to 85 per cent of India's ammonia emissions, while the rest leaks from fertiliser plants and industry. Her modelling estimates that ammonia alone accounts for about a quarter of India's PM2.5, and when combined with nitrogen oxides and coal-derived sulphur, the inorganic secondary fraction rises to roughly 40 per cent.

Atmospheric Chemistry

Foundational work on tracing these chemical patterns began at IIT Kanpur, where Sachchida Nand Tripathi established the National Aerosol Facility in 2003. A 2014 study from his laboratory documented four years of continuous trace gas and aerosol measurements, showing how cold winter temperatures push chemical equilibrium toward the particle phase. At the same time, the planetary boundary layer often compresses below 400 metres, trapping precursor gases and creating winter loading patterns unique to the region.

Context

Air pollution in northern India is driven by a complex mix of local emissions and atmospheric reactions that change with the seasons. Understanding these chemical processes helps researchers look beyond visible smoke sources like stubble burning to address the invisible particles forming in the atmosphere.