Delhi, India’s national capital, faces one of the worst air pollution crises in the world — particularly during the winter months. Every year, as temperatures drop, the city witnesses thick layers of smog, reduced visibility, and alarming Air Quality Index (AQI) levels.
The causes behind this seasonal air pollution in Delhi are complex and interconnected — involving emissions, geography, weather conditions, and human activities. Below are ten major reasons that explain why winter pollution in Delhi worsens so drastically every year.
Key Factors Behind Winter Pollution in Delhi
The following points summarise the multiple environmental and human factors contributing to Delhi’s deteriorating winter air quality and rising air pollution in Delhi.
1. Crop-residue (stubble) burning in neighbouring states
One of the most frequently cited seasonal sources. Farmers in Punjab and Haryana burn rice-stubble after harvesting to clear fields for the next crop. Smoke and particulate matter from these fires are transported into the Delhi region. For example, SAFAR reports that stubble-burning’s contribution to air pollution in Delhi is significant (~20–25%). Because burning is concentrated in a short seasonal window (post-monsoon, pre-wheat sowing), the pollution load into Delhi becomes very high during that period.
2. Vehicular and industrial emissions (combustion sources)
Although vehicular and industrial emissions persist year-round, their relative share becomes larger in winter because other sources (dust storms, construction dust) are reduced, and dispersion is lower. In winter, combustion sources (vehicles, industries, biomass burning) contribute 85–94% of PM₂.₅ load in Delhi. Vehicles alone account for approximately 25–28% of PM₂.₅ in some studies. Hence, even if emissions don’t rise sharply, their impact on air pollution in Delhi becomes more severe.
3. Domestic biomass burning and heating
During colder months, increased use of biomass (wood, dung, crop residues) for heating and cooking adds fine particulate matter and gaseous pollutants. A 2015 IIT Kanpur study estimated that 17–26% of particulate matter in Delhi during winters comes from biomass burning. Informal burning of plastics and waste releases HCl, which reacts to form particulate chloride compounds, worsening haze and air pollution in Delhi.
4. Meteorological phenomenon: Temperature inversion
Normally, air near the surface rises and disperses pollutants. But in winter, a layer of warm air traps cooler air (and pollutants) below — a phenomenon known as temperature inversion. This “lid” effect prevents pollutants from escaping, leading to heavy accumulation and smog formation near the surface — a major reason for worsening air pollution in Delhi.
5. Low wind speeds and stagnant air
Winter months in Delhi often feature calm winds and stagnant air. With low wind speeds, pollutants fail to disperse horizontally or vertically. For example, in November 2023, Delhi’s average wind speed was just 9.8 m/s — the lowest in six years — causing prolonged smog episodes. Dry weather and minimal rainfall further limit pollutant “washout,” allowing emissions to build up over time, worsening air pollution in Delhi.
6. Geographic/topographic factors and regional transport
Delhi’s location in the Indo-Gangetic Plain acts like a “bowl” that traps pollutants. Surrounded by the Himalayas to the north and plains to the west, the city’s geography restricts air movement. Winter winds from Punjab and Haryana bring additional pollutants into Delhi, compounding local emissions. Studies indicate that nearly 72% of Delhi’s winter winds originate from the northwest — making regional transport a major factor in air pollution in Delhi.
7. Reduction in natural cleaning (rainfall, dust removal)
During monsoon and summer, rainfall and stronger winds help clean the air through wet deposition and dispersion. In contrast, winter’s dry skies and weak convection suppress these natural cleaning mechanisms, causing pollutants to accumulate instead of being flushed out. This lack of natural cleansing increases air pollution in Delhi.
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8. Elevated secondary aerosol formation and chemical reactions
Beyond direct emissions, air pollution in Delhi worsens due to secondary aerosol formation — chemical reactions between gases like ammonia and HCl that form fine particulate matter (PM₂.₅). Cold, humid, and stagnant winter nights provide ideal conditions for these reactions, increasing smog density.
9. Festival events, firecrackers, and local emissions
Winter coincides with festivals such as Diwali, when fireworks emit large amounts of fine particulate matter, metals, and gases. Although short-lived, these bursts of pollution occur during low-dispersion periods, acting as “tipping points” that trigger severe smog episodes — sharply increasing air pollution in Delhi.
10. Road dust, construction dust, and resuspension
While dust emissions are generally higher in summer, in winter they linger longer due to stagnant air. With dry soils, minimal vegetation, and reduced wind speed, road dust remains suspended, adding to Delhi’s particulate load. In some areas, dust sources contribute up to 30% of winter particulate matter — worsening overall air pollution in Delhi.
Interactions, Amplification & Synergy
These ten reasons do not act in isolation. Emissions, regional transport, and weather conditions work together to worsen air pollution in Delhi:
- Emissions (stubble burning, vehicles, biomass) create pollutants that cannot disperse.
- Regional transport adds pollution from outside Delhi.
- Meteorological factors trap these emissions under inversion layers.
- Episodic events like festivals or dust storms intensify already high levels.
In short, air pollution in Delhi is a systemic interplay between emissions and poor dispersion — not just an increase in emissions.
Quantitative Indicators & Research Findings
- In winter, dust sources drop to 6–15%, while combustion sources rise to 85–94%.
- November 2023 saw Delhi’s worst smog in six years, driven by low wind speeds.
- IIT-Kanpur estimated that biomass burning contributes 17–26% of winter particulate matter.
- Winter pollution levels can be up to 7× higher than in summer months.
Implications and Consequences
- Health: High PM₂.₅ and PM₁₀ levels cause respiratory and cardiovascular illnesses.
- Visibility: Dense fog and haze disrupt transport and daily life.
- Economic: Healthcare costs and lost productivity rise during smog periods.
- Policy: Targeted seasonal interventions are essential for mitigation.
Recommendations for Reducing Winter Pollution
- Promote no-burn practices for stubble management in neighbouring states.
- Encourage clean fuels for heating and cooking in peri-urban homes.
- Enforce stricter vehicle and industrial emission controls in winter months.
- Implement dust management measures at construction sites.
- Use weather forecasts to predict inversion or low-dispersion days.
- Enhance regional coordination among Delhi, Punjab, Haryana, and UP.
- Increase public awareness about pollution peaks and health safety.
Conclusion
The air pollution in Delhi during winter is not merely a case of higher emissions — it results from an intricate mix of human activities, regional factors, and adverse weather conditions. To make lasting progress, strategies must tackle both emission sources and meteorological challenges, focusing on long-term, coordinated action across sectors and states.
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Sumita Pradhan
Sumita is a curious and creative IPM student at IIM Jammu, passionate about design, storytelling, and building smart, impactful ideas. With a love for research, writing, and communication, she thrives at the intersection of creativity and strategy. Reach out at sumita300606@gmail.com
