Engulfed in a thick and acrid blanket of smog, Delhi continued to struggle to breathe as it witnessed the worst air day of the season on Sunday. The average air quality index (AQI) was recorded at 461, up from 431 a day before. This is the second worst air day in December since 2015 when the Central Pollution Control Board (CPCB) began AQI monitoring.
The only day, as per the CPCB, when the city’s air quality was worse than this in the month of December was on December 21, 2017, when the AQI touched 469. Another time when the Capital AQI spike was higher than this — at 494 — was on November 18, 2024.
According to the Indian Institute of Tropical Meteorology’s (IITM) Decision Support System (DSS), which analyses data, the transport sector remained the single largest contributor to Delhi’s PM2.5 levels on Sunday, accounting for 13.7% of emissions. Transboundary pollution from neighbouring cities and districts also remained high, with Ghaziabad contributing 9.98%, Bulandshahr 8.88%, and Gautam Buddha Nagar 8.2% to the Capital’s PM2.5 load. Industrial emissions within Delhi accounted for 6.28%, underscoring the combined impact of local sources and regional pollution under stagnant winter conditions.
Sunday was the second straight day of the Capital registering AQI in the ‘Severe’ category, which according to the CPCB, impacts healthy people and seriously affects those with existing diseases. Large parts of the National Capital Region (NCR) also continued to choke — Noida recorded an AQI of 466, Ghaziabad 459, and Greater Noida 435.
At the heart of the air pollution crisis in Delhi-NCR is a familiar pattern seen during winters in North India. As temperatures drop, the air closer to the ground becomes colder and dense, preventing pollutants from rising and dispersing.
At the same time, wind speeds remain weak, leaving emissions from vehicles, industries, power plants, construction sites and biomass burning trapped near the surface. This leads to the formation of temperature inversion, a condition in which warmer air sits above colder air near the ground, effectively sealing pollutants within a shallow layer of the atmosphere. As a result, the mixing height — the vertical space available for polluted air to disperse — shrinks sharply, often to less than a kilometre during the day and even lower at night. With limited vertical movement and sluggish winds, the ventilation capacity of the atmosphere drops, allowing pollution to accumulate rapidly.
Experts have stressed that while such meteorological conditions are seasonal, the severity of the episode reflects persistently high baseline emissions across Delhi and the surrounding region.
Meteorological observations further showed unfavourable conditions. According to the India Meteorological Department (IMD), dense to moderate fog was reported across parts of Delhi-NCR on Sunday morning. Safdarjung, the city’s base station, recorded visibility as low as 200 metres at 8 am. A combination of weak surface winds and foggy conditions significantly restricted the dispersion of pollutants.
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The IITM, in its Air Quality and Weather Bulletin for Delhi issued on Sunday, noted that the maximum mixing depth was around 800 metres on Sunday, while the ventilation index was estimated at just 800 m²/s, far below the threshold of 6,000 m²/s considered favourable for pollution dispersion. With average wind speeds below 10 kmph, IITM cautioned that conditions remained unfavourable for clearing accumulated emissions.
Dr Dipankar Saha, former head of the Air Laboratory Division at the CPCB, explained that total emissions across the region remain broadly similar throughout the year, but winter meteorology compresses pollution into a smaller volume of air. “The vertical and horizontal movement of air is restricted due to low mixing height, temperature inversion and weak winds. What we measure in winter is compressed pollution,” he said. He stressed that there have been no effective measures taken so far over the years to tackle traffic congestion, improve public infrastructure, and road enhancement, along with the updation of emissions inventory to tackle emissions at the source.
Sunil Dahiya, founder and lead analyst at think tank Envirocatalysts, said the current spell was the result of high baseline emissions and inadequate action at source. He pointed to growing contributions from industrial clusters and coal-based power plants in Bulandshahr, Gautam Buddh Nagar, Faridabad, Panipat and Jhajjar, particularly when winds blow from the west.
“We are relying on artificial or band-aid solutions. When temperatures drop and winds slow down, emissions within Delhi as and the larger airshed remain very high,” he said.
“In the absence of strict measures to curtail emissions from the power and transport sectors, this situation becomes inevitable. This will happen every time the temperature drops coincide with low wind speeds. Except during the monsoon, we remain highly vulnerable,” Dahiya added. He also flagged gaps in pollution forecasting, saying the DSS relies on outdated emissions data.
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Meanwhile, former DPCC additional director Dr Mohan George said emissions from construction activity, industrial belts such as Ghaziabad and Sahibabad, and biomass burning, including for livestock and domestic purposes, become more pronounced under foggy conditions. “When you have only about 800 m for dispersion during the day, and even less at night, routine emissions accumulate rapidly. Wind speed is low, and ventilation is poor. We have to control our local sources, mainly construction and biomass burning, more aggressively,” he explained.
