A third of Delhi's PM2.5 load has a secondary source: ammonium sulfate, Pg18

Key Highlights:

• Secondary aerosols, particularly ammonium sulfate, contribute to at least one-third of Delhi's annual PM2.5 pollution.
• Ammonium sulfate forms from the reaction of sulfur dioxide (SO2), largely from coal-fired power plants, and ammonia, mainly from agricultural activities.
• The government exempted nearly 78% of coal-fired thermal power plants from installing flue gas desulphurisation (FGD) systems in July 2025.
• Delhi recorded an annual PM2.5 average of 91.6 µg/m3 in the 2024 World Air Quality Report by IQAir, ranking it the most polluted national capital globally.

Detailed Insights:

• Primary pollutants directly emitted from sources like road dust and vehicle exhaust, while secondary particulate matter forms from gases undergoing chemical reactions in the atmosphere.
• Sulfur dioxide (SO2), a precursor pollutant, oxidizes to form sulfate, which then reacts with ammonia from agriculture, livestock, and other sources to create ammonium sulfate.
• High humidity and low winter temperatures accelerate the chemical reactions that transform gases into particles, worsening Delhi's pollution during stagnant winter conditions.
• States with high coal consumption like Chhattisgarh, Odisha, Jharkhand, and Telangana have the highest annual contribution of ammonium sulfate to PM2.5 levels.
• Experts argue that the National Clean Air Programme (NCAP) revision should focus on secondary aerosol formation, rather than just PM10, to effectively combat air pollution.

Key Concepts Involved:

• PM2.5: Fine particulate matter with a diameter of 2.5 micrometers or less, posing significant health risks due to its ability to penetrate deep into the lungs.
• Secondary Aerosols: Particulate matter formed in the atmosphere through chemical reactions of precursor gases, as opposed to being directly emitted from a source.
• Flue Gas Desulphurisation (FGD): Technology used to remove sulfur dioxide (SO2) from the exhaust flue gases of coal-fired power plants.