CAQM Report on Delhi Winter Air Pollution – Explained

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• A recent synthesis report by the Commission for Air Quality Management has identified secondary particulate matter as the single largest contributor to Delhi’s winter air pollution. 

Background: Understanding Delhi’s Air Pollution Challenge

• Delhi and the National Capital Region (NCR) experience severe air pollution every winter due to a combination of meteorological, geographical, and emission-related factors. 
• Low wind speeds, temperature inversion, and high moisture levels trap pollutants close to the ground, leading to persistent smog episodes. 
• Over the years, multiple studies have attempted to identify dominant pollution sources, but variations in methodology often led to conflicting conclusions, complicating policy responses.
• To address this gap, the Commission for Air Quality Management (CAQM) undertook a meta-analysis of existing scientific studies to present a consolidated and evidence-based assessment of pollution sources affecting Delhi during the winter months.

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Key Findings of the CAQM Meta-Analysis

• The CAQM synthesis report presents a clear source-wise contribution to winter air pollution in Delhi:
  • Secondary particulate matter: 27%
  • Transport emissions: 23%
  • Biomass burning (including crop residue and municipal waste): 20%
  • Dust (road and construction): 15%
  • Industrial emissions: 9%
• The report clarifies that no new pollution sources have been identified. 
• Instead, it harmonises findings from earlier studies that used differing analytical techniques, thereby offering a unified scientific basis for policymaking. 

Secondary Particulate Matter as the Dominant Pollutant

• Secondary particulate matter is not emitted directly from sources such as vehicles or factories. 
• Instead, it forms in the atmosphere through chemical reactions involving primary pollutants like sulphur dioxide (SO₂), nitrogen oxides (NOx), and ammonia (NH₃).
• These reactions lead to the formation of sulphates and nitrates, which together constitute nearly 25-60% of PM2.5 concentrations during winter. 
• Fine particles of this nature are especially harmful as they penetrate deep into the lungs and bloodstream, aggravating respiratory, cardiovascular, and ophthalmic diseases.
• The CAQM report emphasises that focusing solely on visible emission sources may be insufficient without addressing these secondary chemical processes.

Role of Ammonia in Pollution Formation

• Ammonia plays a critical enabling role in the formation of secondary particulate matter. 
• Studies cited in the report indicate that nearly 80% of ammonia emissions in India originate from agricultural activities, particularly fertiliser use and livestock excreta.
• In the atmosphere, ammonia reacts with sulphuric and nitric acids, formed from SO₂ and NOx emissions, to produce ammonium sulphate and ammonium nitrate aerosols. 
• These compounds significantly increase PM2.5 levels during winter, underscoring the rural-urban linkage in Delhi’s air pollution problem.

Transport, Biomass Burning, and Dust Contributions

• Transport emissions remain the second-largest contributor, reflecting the high vehicle density, fossil fuel use, and congestion in Delhi-NCR. 
• Biomass burning, including crop residue burning in neighbouring States and municipal waste burning, contributes one-fifth of winter pollution, particularly during peak burning periods.
• Dust pollution arises from unpaved roads, construction activities, and resuspension of settled particles. 
• Although often overlooked, dust contributes substantially to particulate load, especially under stagnant winter conditions.

Policy Implications and Future Studies

• The CAQM report has important implications for air quality governance:
  • It highlights the need for multi-sectoral interventions, including agriculture, transport, energy, and waste management.
  • It calls for integrated emission inventories and updated source apportionment studies, with 2026 as the proposed base year.
  • The findings will support improved Air Quality Early Warning Systems and Decision Support Systems, which had earlier faced limitations in forecasting pollution spikes accurately.
• A more science-driven approach is expected to improve both preventive and responsive policy measures.

Source: TH