Context:
A recent meta-analysis commissioned by the Commission for Air Quality Management (CAQM) in the National Capital Region has identified secondary particulate matter as the single largest contributor to Delhi’s winter air pollution. According to the report, secondary particulates account for 27% of winter PM2.5 pollution, surpassing traditionally blamed sources such as vehicular emissions and crop residue burning.
Key Findings of the Meta-Analysis:
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- The CAQM report synthesised multiple source-apportionment studies on Delhi’s air pollution. The major contributors to winter pollution were identified as:
- Secondary particulate matter: 27%
- Transport emissions: 23%
- Biomass burning: 20% (including crop residue and municipal solid waste burning)
- Dust: 15%
- Industry: 9%
- Secondary particulate matter: 27%
- The report does not identify new pollution sources but aims to harmonise varying methodologies used in earlier studies to arrive at a more unified assessment for policy action.
- The CAQM report synthesised multiple source-apportionment studies on Delhi’s air pollution. The major contributors to winter pollution were identified as:
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What is Secondary Particulate Matter?
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- Secondary particulate matter is not emitted directly. It forms in the atmosphere through chemical reactions involving primary pollutants such as nitrogen oxides (NOx), sulphur dioxide (SO₂), volatile organic compounds (VOCs), and ammonia (NH₃).
- In Delhi’s context:
- SO₂ from coal combustion and brick kilns forms sulphuric acid (H₂SO₄).
- NOx from vehicles and power plants forms nitric acid (HNO₃).
- SO₂ from coal combustion and brick kilns forms sulphuric acid (H₂SO₄).
- These acids react with ammonia, largely originating from fertiliser use and livestock excreta to form ammonium sulphate and ammonium nitrate aerosols, which constitute a large fraction of PM2.5.
- Studies indicate that 25–60% of PM2.5 consists of such sulphates and nitrates, making secondary particulates a dominant pollutant.
- Secondary particulate matter is not emitted directly. It forms in the atmosphere through chemical reactions involving primary pollutants such as nitrogen oxides (NOx), sulphur dioxide (SO₂), volatile organic compounds (VOCs), and ammonia (NH₃).
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Health and Environmental Impacts:
Secondary particulates are especially dangerous due to their fine size (PM2.5), allowing them to penetrate deep into the lungs and bloodstream. They are linked to asthma, chronic obstructive pulmonary disease (COPD), lung cancer, cardiovascular disorders, ophthalmic diseases, and acute respiratory infections. Their formation is influenced by meteorological conditions, making them difficult to control through source-specific bans alone.
Policy Implications and Way Forward:
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- The findings highlight the limitations of focusing only on visible sources like stubble burning or traffic curbs. Effective mitigation requires:
- Reducing precursor gases (NOx, SO₂, NH₃) through cleaner fuels, industrial controls, and sustainable agricultural practices.
- Developing robust emissions inventories and source apportionment studies, which CAQM plans to undertake with 2026 as the base year.
- Strengthening air quality forecasting through improved Early Warning and Decision Support Systems.
- Reducing precursor gases (NOx, SO₂, NH₃) through cleaner fuels, industrial controls, and sustainable agricultural practices.
- The findings highlight the limitations of focusing only on visible sources like stubble burning or traffic curbs. Effective mitigation requires:
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Conclusion:
The CAQM report underscores that Delhi’s air pollution problem is as much a chemical and systemic challenge as a local emissions issue. Addressing secondary particulate formation demands coordinated action across energy, transport, agriculture, and urban governance to achieve durable improvements in air quality.
