While the city's AQI stood at 327 at 11 p.m.. It dipped to 323 at 3.30 a.m., just about when it was expected to enter the "severe" category.
It, however, increased to 340 at 8:30 a.m., as the weather department reported occurrence of shallow fog.
An AQI between 0-50 is considered "good", 51-100 "satisfactory", 101-200 "moderate", 201-300 "poor", 301-400 "very poor", and 401-500 "severe". Above 500 is "severe-plus emergency" category.
Thirty-six of the 37 air quality monitoring stations in the capital recorded their AQI in "very poor" category.
Delhi's air quality was, however, better than satellite towns of Ghaziabad (375), Greater Noida (356), Gurgaon (352) and Noida (375), according to Central Pollution Control Board's data.
Last night, people reported violation of the two-hour window in Malviya Nagar, Lajpat Nagar, Kailash Hills, Burari, Jangpura, Shahdara, Laxmi Nagar, Mayur Vihar, Sarita Vihar, Hari Nagar, New Friends Colony, Hauz Khas, Gautam Nagar, Dwarka among others places.
Residents in Noida, Gurgaon, Ghaziabad also reported extensive fireworks much beyond the timeframe.
People could also be seen bursting crackers before 8 PM, however, the intensity remained low.
The levels of PM10 tiny particulate matter of diameter 10 or less than 10 microns that can enter deep into the lungs had on Sunday reached as high as 515 micrograms per cubic metre in Anand Vihar.
In Wazirpur and Bawana, PM 2.5 levels had crossed the 400 mark.
After last year's Diwali, Delhi's AQI had crossed the 600-mark, which is 12 times the safe limit. The AQI post-Diwali was 367 in 2017 and 425 in 2016.
With Delhi's air quality plummeting to dangerous levels around Diwali every year, the Supreme Court in 2018 banned polluting firecrackers and ordered that only green firecrackers, which is said to cause 30 per cent less pollution, can be manufactured and sold.
But the green pyrotechnics have failed to draw good response both from sellers and buyers, primarily due to lack of variety, limited stock and high prices.
Last year also, people continued to buy the conventional firecrackers and use them.
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