Why is it in News?
- A new analysis by Climate Trends (2025) covering 15 major Indian cities (2015–2025) finds:
- No city recorded safe air quality (AQI < 50).
- Delhi remains the most polluted city across 10 years.
- Pollution shows a regional pattern: north India worst, south-west relatively better.
- Persistent high PM levels in north; annual best AQI in Chennai & Mumbai.
Relevance
GS 1: Urbanisation
- Urban heat island effect
- Population density and air quality impact
GS 2: Governance
- Air quality governance gaps
- NCR states’ coordination failures
GS 3: Environment
- AQI trends
- Climate–pollution interactions
- Geographic determinants of pollution
- Winter inversion, Indo-Gangetic Plain dynamics
What is AQI & Why It Matters?
- Air Quality Index (AQI) categorises air quality from 0–500:
- 0–50: Good
- 51–100: Satisfactory
- 101–200: Moderate
- 201–300: Poor
- 301–400: Very Poor
- 401–500: Severe
- The study uses annual mean AQI—a more reliable long-term pollution indicator than daily spikes.
Overall Air Quality Performance
- Delhi’s annual mean AQI:
- Highest in 2016 (over 250)
- Slight improvement after 2019
- Still remains in poor–very poor category
- In 2025 (so far):
- Delhi AQI ~180–190
- Lucknow, Varanasi, Ahmedabad, Pune: also experienced prolonged poor AQ levels
North Indian Cities Perform the Worst
- Six cities—Delhi, Lucknow, Varanasi, Kanpur, Noida, Ghaziabad—consistently show high PM2.5 & AQI deterioration, especially winter.
- Annual best AQI never enters “good” or even “satisfactory” range.
South & West Indian Cities Perform Better
- Chennai, Bengaluru, Mumbai, Visakhapatnam show:
- Better mean AQI levels (120–140 range)
- More stable improvement post-2019
- But they still fall short of clean air standards.
City-Level Variations
- Chennai & Mumbai: best annual quality among all 15 cities
- Bengaluru: did not record safe annual AQI even once but still far cleaner than north
- Chandigarh, Visakhapatnam, Mumbai saw AQI improvements from 800 → 140 days of good-moderate air.
Why North Is More Polluted: Geographic & Climatic Factors
1. Indo-Gangetic Plain Topography
- North India is landlocked, unlike coastal south/west.
- Bordered by the Himalayas in the north, preventing dispersion of pollutants.
- Creates a “pollution bowl” where PM2.5 gets trapped.
2. Winter Inversion + Cold, Dry Air
- Winter causes thermal (temperature) inversion:
- The layer of warm air sits above cold air near the surface
- Acts as a lid, trapping pollutants
- Result: Smog, stagnation, prolonged pollution episodes.
3. Dust Load + Biomass Burning
- Indo-Gangetic belt has heavy soil dust, crop residue burning, brick kilns, industrial clusters.
4. Weak Wind Speeds
- North experiences slow winds in winter; lack of sea breezes.
- This reduces pollutant flushing.
Why South & West Perform Better ?
- Coastal cities (Chennai, Mumbai):
- Sea breeze circulation disperses pollutants
- Higher humidity and cleaner marine air reduce dust
- Less temperature inversion
- Fewer winter smog events
- Lesser biomass burning and lower dust aerosol load
Structural Factors Adding to North’s Problem
- Dense urban structure → “surface roughness” that slows wind dispersion
- High vehicle density
- More industrial clusters
- High secondary aerosol formation in winter
