Introduction

Over 2015-2025, no northern Indian city recorded “safe” air quality even once, with Delhi emerging as the most polluted city. In contrast, cities in the south and west maintained comparatively better AQI levels. This consistent divergence reflects entrenched geographical, meteorological, and structural constraints that trap pollutants in the Indo-Gangetic Plain while aiding dispersion along the coasts.

Why in the news

A new assessment titled Air Quality Assessment of Major Indian Cities (2015-2025) reported that Delhi continues to be the most polluted city, with AQI stagnating at unhealthy levels. The study shows sharp regional contrasts, revealing that only southern and western cities showed sustained air quality improvements, making this a significant environmental governance concern.

Persistent Regional Air Quality Divide

Why northern cities remain severely polluted

1. Consistent high pollution: Northern cities experienced prolonged severe pollution episodes across the decade.
2. Limited “healthy days”: None recorded AQI within safe thresholds in 2025.
3. Stagnant improvement: Even when AQI dipped (e.g., 2019), levels remained far above healthy limits.

How southern and western cities compare

1. Cleaner AQI bands: Chennai, Chandigarh, Visakhapatnam, and Mumbai maintained AQI between 80-140.
2. Steady progress: These cities displayed clear improvements between 2015-2025.
3. Best performer: Bengaluru recorded the best AQI among all 11 cities.

Why Delhi Emerges as the Worst Performer

Data trends

1. Peak AQI: Delhi saw its worst AQI in 2016 (over 250).
2. Temporary dips: AQI improved in 2019 but did not meet healthy standards.
3. Current status: AQI stagnated at 180.5 in 2025, indicating persistent failure to achieve safe limits.

Structural challenges

1. Urban surface roughness: Dense built-up surfaces inhibit wind flows and pollutant dispersion.
2. Trapping effect: Reduced ventilation leads to prolonged retention of pollutants.

Why Secondary Northern Cities Remain Highly Polluted

Cities in focus: Lucknow, Varanasi, Ahmedabad, and Pune showed:

1. Prolonged elevated AQI: Frequent high pollution days with slow improvement.
2. Mixed progress: Improvements after 2019, but still above healthy limits.
3. Heavy pollutant load: Emissions + weak dispersion exacerbate poor quality.

Why Southern & Western Cities Perform Better

1. Favourable winds: Sea breezes in coastal cities aid pollutant dispersal.
2. Better atmospheric ventilation: Stronger monsoon winds and less winter stagnation.
3. Urban characteristics: Less surface roughness compared to Delhi’s dense built-up terrain.

Outcome

1. Improved AQI stability
2. Lower incidence of sharp pollution spikes

Geography and Winter Inversion: The Deciding Factors

Geographical lock-in

1. Indo-Gangetic Basin: Landlocked region bounded by the Himalayas prevents outflow of pollutants.
2. Pollutant entrapment: Cold northern boundary and flat terrain acts like a “pollution bowl”.

Winter inversion

1. Temperature inversion effect: Warm air traps cold, dense air near the surface and this leads to pollutants settling close to ground level.
2. Seasonal peak: December-February shows intensified pollution due to reduced boundary layer height.

Built environment factor

1. Surface roughness: Urban canyons in Delhi slow wind speed, increasing stagnation.

Seasonal Wind Patterns and Air Dispersion

Why southern/western cities improve during monsoon

1. Strong monsoon flows disperse pollutants effectively.
2. Regular ventilation cycles prevent accumulation.

Why northern cities worsen in winter

1. Weak westerly winds
2. Lower atmospheric mixing height
3. Persistent fog, cold air trapping, and stagnation

Conclusion

The decade-long air quality analysis underscores a structural, region-specific pollution challenge rooted in geography, climate, and urban form. Northern cities, especially those in the Indo-Gangetic Basin, remain trapped in severe winter pollution cycles, while southern and western cities benefit from favourable winds and dispersion conditions. Any meaningful pollution mitigation strategy must therefore be region-sensitive and climatologically informed.

PYQ Relevance

[UPSC 2021] Describe the key points of the revised Global Air Quality Guidelines (AQGs) released by the World Health Organisation (WHO). How are these different from its last update in 2005? What changes in India’s National Clean Air Programme are required to achieve these revised standards?

Linkage: This topic is important for UPSC as it highlights India’s deep regional air-quality disparities and the structural limits of current pollution-control policies. It links directly to GS-3 themes of air pollution, WHO AQGs, NCAP reforms, and the recurring winter inversion-driven smog episodes in north Indian cities.

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