Recent data show that India is one of the top global contributors to precursor gases responsible for secondary particulate matter. Precursor gases are estimated to contribute over 50% to overall PM2.5 levels in several regions in India.
India emits the highest amount of SO2 globally at 11.2 million tonnes, and ranks second only to China in emissions of both NOX at 9.4 million tonnes and ammonia (NH3) at 10.4 million tonnes. These elevated levels create ideal conditions for the widespread formation of secondary pollutants such as sulfate, nitrate, and ozone. Once formed, these particles can travel long distances, spreading across cities, states, and regions, making air pollution a transboundary issue that affects both urban and rural populations.
This analysis from the Centre for Research on Energy and Clean Air (CREA) reveals that secondary pollutants, particularly ammonium sulfate formed from the reaction between sulfur dioxide (SO2) and ammonia (NH3) in the atmosphere, are responsible for nearly one-third of India’s fine particulate matter (PM2.5) pollution. This widespread presence of ammonium sulfate has been observed across nearly all National Clean Air Programme (NCAP) cities, irrespective of their proximity to major emission sources, due to the transboundary nature of pollution.
CREA’s study shows that the nationwide average concentration of ammonium sulfate accounts for approximately 34% of the PM2.5 mass. SO2 is the main driver of ammonium sulfate formation and more than 60% of SO2 emissions in India originate from coal-fired thermal power plants. This makes them a critical target for reducing secondary PM2.5 pollution through the implementation of flue gas desulfurization (FGD) systems.
Ammonium sulfate concentration is also 2.5 times higher within 10 km of coal-fired power plants (15 µg/m³) compared to areas beyond 10 km (6 µg/m³). While the contribution to PM2.5 is 36% near coal-fired thermal power plants, it remains significant at 23% in other areas, indicating both local and transboundary impacts of SO₂ emissions.
Across the 130 NCAP cities, concentrations ranged from 3.9 to 22.5 µg/m³, with the share of PM2.5 from ammonium sulfate ranging between 20% and 43%. Notably, ammonium sulfate made up more than 30% of the total PM2.5 levels in 114 out of 130 NCAP cities.
India’s air pollution challenge cannot be effectively tackled without addressing the sources of the growing burden of secondary particulate matter. What makes secondary pollution especially challenging is its ability to travel across city and state boundaries, affecting air quality in regions far away from the original source of emissions. This transboundary nature means that even cities without large local emission sources may experience high levels of PM2.5 due to the drift of secondary particles.
