Delhi Winter Smog Explained: CAQM Flags Secondary Particulates as the Biggest Polluter

Why in News?

A recent synthesis report by the Commission for Air Quality Management (CAQM) has identified secondary particulate matter as the single largest contributor to Delhi’s winter air pollution, reshaping the understanding of pollution sources and policy priorities in the National Capital Region (NCR).

Background: Understanding Delhi’s Air Pollution Challenge

Delhi and the NCR experience severe air pollution every winter due to a complex interplay of:

• Meteorological factors: Low wind speeds, temperature inversion, and high moisture levels trap pollutants close to the surface.
• Geographical factors: Landlocked terrain and proximity to major agricultural and industrial belts.
• Emission-related factors: High vehicular density, industrial activities, biomass burning, construction dust, and energy use.

Over the years, numerous scientific studies have attempted to apportion pollution sources. However, differences in data sets, methodologies, and time frames led to conflicting estimates, making it difficult to design targeted interventions.

To bridge this gap, the CAQM undertook a meta-analysis of existing peer-reviewed studies, producing a consolidated and evidence-based assessment of pollution sources affecting Delhi during the winter months.

Key Findings of the CAQM Synthesis Report

The report provides a source-wise contribution to Delhi’s winter air pollution:

• Secondary particulate matter: 27%
• Transport emissions: 23%
• Biomass burning (crop residue + municipal waste): 20%
• Dust (road and construction): 15%
• Industrial emissions: 9%

Importantly, the report notes that:

• No new pollution sources have emerged.
• The findings harmonise earlier studies, offering a unified scientific basis for policymaking and regulatory action.

Secondary Particulate Matter as the Dominant Pollutant

Unlike primary pollutants that are directly emitted, secondary particulate matter forms in the atmosphere through chemical reactions involving:

• Sulphur dioxide (SO₂)
• Nitrogen oxides (NOx)
• Ammonia (NH₃)

These react to form sulphates and nitrates, which together constitute nearly 25–60% of PM2.5 concentrations during winter.

Why this matters:

• These fine particles penetrate deep into the lungs and bloodstream, exacerbating respiratory, cardiovascular, and eye diseases.
• The CAQM report stresses that controlling only visible emission sources (like vehicles or chimneys) is insufficient unless the underlying chemical processes are also addressed.

Role of Ammonia in Pollution Formation

Ammonia plays a critical enabling role in the formation of secondary particulate matter.

• Nearly 80% of ammonia emissions in India originate from:
  • Fertiliser application
  • Livestock excreta
• In the atmosphere, ammonia reacts with sulphuric and nitric acids (from SO₂ and NOx) to form:
  • Ammonium sulphate
  • Ammonium nitrate aerosols

These compounds significantly increase PM2.5 levels during winter, highlighting the rural–urban linkage in Delhi’s air pollution problem and the need to integrate agricultural practices into air quality governance.

Transport, Biomass Burning, and Dust Contributions

1. Transport Emissions (23%)

• High vehicle density, fossil fuel dependence, and traffic congestion make transport the second-largest contributor.
• Emissions include NOx, PM, and hydrocarbons, which also act as precursors to secondary particles.

2. Biomass Burning (20%)

• Includes:
  • Crop residue burning in neighbouring States
  • Municipal waste burning
• Contributions spike during harvest seasons, compounding winter pollution episodes.

3. Dust Pollution (15%)

• Originates from:
  • Unpaved roads
  • Construction activities
  • Resuspension of settled particles
• Often underestimated, but becomes significant under stagnant winter conditions.

Policy Implications and Future Directions

The CAQM report has far-reaching implications for air quality management:

• Multi-sectoral approach: Pollution control must integrate agriculture, transport, energy, industry, and waste management.
• Integrated emission inventories: Calls for updated and harmonised emission databases.
• Updated source apportionment studies: Proposed 2026 as the base year for new scientific assessments.
• Improved forecasting systems: The findings will strengthen Air Quality Early Warning Systems and Decision Support Systems, which previously faced limitations in predicting pollution spikes.
• Science-driven governance: A shift toward evidence-based policymaking is expected to enhance both preventive and responsive interventions.