Widespread firecracker bursting well beyond the stipulated windows set by the Supreme Court to celebrate Diwali deteriorated Delhi's air quality to the higher end of the "very poor" category on Tuesday. Smoke comprising a toxic cocktail of pollutants and heavy metals pushed hourly PM 2.5 levels in parts of the city to around 29 times the permissible standards for these ultra fine particles. The air quality was likely to inch close to the "severe" category in the second half of Tuesday.
The average air quality index (AQI) was 350 (very poor) at 7am on Tuesday, compared to Monday's 4pm average of 345 (very poor).
The Supreme Court permitted "green firecrackers" this Diwali in Delhi, but fixed a time period, allowing people to burst them from 6am to 7am and from 8pm to 10pm on October 20 and 21. An isolated firecracker bursting could be heard during the day on Monday. The intensity increased around 8pm and continued past midnight in most places in the city.
The Central Pollution Control Board (CPCB) data showed Delhi's AQI was improving through the day on Monday. It was 345 at 4pm, but touched 340 by 7pm. It remained 340 at 8pm, before rising to 341 at 9pm and reaching 349 at midnight as firecracker emissions began to envelop the capital.
The Air Quality Early Warning System for Delhi said the AQI is likely to touch the "severe" category on Tuesday and remain in this range on Wednesday, due to the addition of firecracker emissions. It is forecast to touch "very poor" by Thursday.
The CPCB classifies AQI between 0-50 as "good", 51 and 100 as "satisfactory", 101 and 200 as "moderate", 201 and 300 as "poor", 301 and 400 as "very poor", and over 400 as "severe".
The Delhi Pollution Control Committee data showed hourly PM 2.5 concentration was 1,763 micrograms per cubic metre (µg/m³) at Nehru Nagar, around 29 times the national 24-hour PM 2.5 standard of 60 µg/m³. This was followed by an hourly peak of 1,710 µg/m³ at east Delhi's Anand Vihar, a pollution hot spot.
Most stations in Delhi recorded peak pollution levels between midnight and 1am, with PM 2.5 concentration gradually dipping as the bursting of firecrackers waned. The overall average PM 2.5 concentration was 675µg/m³ at midnight, over 11 times the standard. The same concentration was 91µg/m³ at 4pm on Monday and 223µg/m³ at 8pm, CPCB data showed.
The overall peak is higher than last year, when Delhi's average PM 2.5 level peaked at 609µg/m³ and 570µg/m³. There was a complete ban on all types of firecrackers, including green firecrackers, for the previous two years.
The India Meteorological Department said there was a shallow fog with visibility dipping to 600 metres at Palam between 3:30am and 5am on Tuesday. This, combined with the pollutants in the air, created a smog. The visibility began to improve gradually from 5:30am onwards as easterly winds of 5-7 km per hour began to blow.
The wind speed was expected to be between 5 and 10 km per hour during the day on Tuesday, possibly helping some pollutants disperse once the sun comes out. With an early Diwali this year, the maximum temperature is currently above 32°C, while the minimum is around 20°C. The higher the temperature, the higher the mixing height of the atmosphere, an invisible layer of the atmosphere within which particles move freely. Low temperature brings down this mixing height while also slowing down the movement of such particles and pollutants.
Delhi's maximum temperature on Diwali day was 33.5°C, a degree above normal. The minimum was 20.2°C, two degrees higher than normal. The minimum is expected to hover around 21°C on Tuesday, while the maximum should be around 32°C.
Last year, Delhi's AQI on Diwali day (October 31) was 328. This deteriorated to 339 (very poor) the day after Diwali by 4pm. In 2023, it was 218 (poor) on Diwali day on November 12 and 358 the day after. The worst air quality after Diwali was in 2021, when the AQI touched 462 (severe) on November 5.
The Decision Support System data showed stubble burning has yet to significantly impact Delhi's air. Its overall contribution to Delhi's PM 2.5 levels was only 0.8% on Diwali day.