Delhi Records Third-Worst Diwali Night Since 2021 As Over Half Of AQI Stations Go Dark

Delhi experienced its third most polluted Diwali night since at least 2021, with particulate pollution remaining close to the 'severe' threshold despite relatively favourable weather conditions. An analysis of air quality data has also raised concerns over the reliability of pollution monitoring in the national Capital, as more than half of the city's air quality monitoring stations stopped reporting readings around midnight, when pollution levels were at their peak.

The available data showed that nearly 80% of the stations that continued functioning recorded 'severe' air quality for most of the period between 10 pm on Diwali night and 6 am the following morning. At 9 pm, the last reading before the number of reporting stations fell sharply, 61% of the 57 stations providing data had recorded an Air Quality Index (AQI) of 400 or above.

Delhi's overall AQI stood at 371 at 10 pm on Sunday, placing it in the 'very poor' category. Of the 59 monitoring stations reporting at that time, 20 recorded 'severe' air quality, while 33 were in the 'very poor' category and six in the 'poor' range. Sonia Vihar recorded the highest AQI at 463, followed by Jahangirpuri at 450.

The city's average AQI for Diwali settled at 376, compared with 369 on Saturday. The Air Quality Early Warning System had also forecast that pollution levels could enter the 'severe' category from November 9 and remain there for the following few days.

Monitoring gaps complicate assessment of Diwali pollution

The sudden disappearance of readings from several monitoring stations has made it difficult to establish a consistent picture of the pollution levels experienced across Delhi during the festival.

More than half of the city's air quality monitoring stations stopped reporting data around midnight. This is particularly significant because the stations that continued functioning were recording pollution levels in the 'severe' category, with AQI readings above 400.

Around 80% of the operational stations remained in the 'severe' category during most of the eight-hour period between 10 pm on Diwali and 6 am the following day. At 9 pm, when 57 stations were still reporting, 61% had already recorded an AQI of at least 400.

The missing readings also make comparisons with previous Diwalis challenging, as the stations that stopped reporting this year were not necessarily the same ones that experienced data gaps in earlier years.

For its analysis, Hindustan Times relied exclusively on the PM2.5 sub-index rather than the overall AQI. The AQI is calculated by determining individual sub-indices for multiple pollutants, with the highest sub-index determining the final reading. Using PM2.5 allowed a wider range of monitoring stations to be compared across different years.

Six monitoring stations reveal how polluted Diwali night was

To overcome the problem of missing data and establish a more reliable comparison with previous years, the analysis examined six monitoring stations that consistently reported readings during most of Diwali's peak pollution hours between 2021 and 2026.

The stations were Alipur, Aya Nagar, Bawana, IHBAS-Dilshad Garden, Narela and Punjabi Bagh. Each selected station had to provide readings for at least nine hours during the period between 6 pm on Diwali and 6 am the following morning.

The average PM2.5 sub-index across these six locations showed that this year's Diwali night was the third most polluted for most of the hours examined since 2021.

The peak hourly PM2.5 sub-index averaged 655, recorded during the hour ending at 1 am on the morning after Diwali.

The corresponding peak readings in 2021 and 2025 were considerably higher, at 812 and 752, respectively. However, the figures for the intervening years were lower, with the peak hourly averages standing at 483 in 2022, 583 in 2023 and 489 in 2024.

There were also two hours when pollution levels this year were the second highest recorded since 2021. These were the hours ending at 4 am and 5 am. However, the analysis cautioned that missing readings from other monitoring stations could have influenced this comparison.

Going further back than 2021 would have further reduced the number of stations available for a consistent year-on-year assessment.

Warmer and windier conditions helped limit pollution peak

The analysis found that Delhi's air quality deteriorated particularly sharply during the initial hours of Diwali, before improving relative to previous years later in the night.

Until the hour beginning at 7 am on November 8, pollution levels were the highest recorded during the corresponding period since 2021. The Capital's air quality remained the second worst even up to the hour beginning at 7 pm.

It was only after 7 pm that this year's readings fell to the third-worst position in the six-year comparison.

The relatively lower peak could have resulted from either fewer firecrackers being burst or more favourable weather conditions that helped disperse pollutants. The analysis suggested that meteorological conditions were the more likely explanation.

Meteorological Aerodrome Report (METAR) data from Palam, compiled by Iowa State University, showed that this year's Diwali night was among the warmest since 2021. Temperatures across different hours placed it as the third or fourth warmest Diwali in the period under consideration.

Wind conditions were also relatively favourable during certain hours, with speeds approaching those recorded during 2022, 2023 and 2024, which were comparatively cleaner Diwali years.

Warmer temperatures and stronger winds would have helped disperse some of the accumulated pollutants, preventing concentrations from rising even further.

Consequently, the analysis suggested that without this meteorological assistance, Delhi could have woken up to even worse pollution levels on the morning after Diwali.

Firecracker restrictions failed to prevent pollution spike

The deterioration in air quality came despite restrictions on the sale and use of firecrackers. The Delhi government had urged residents to follow the Supreme Court's directions permitting only green firecrackers during designated hours.

An order issued by the District Magistrate (East) allowed green firecrackers to be burst on November 7 and 8, between 6 am and 7 am and again between 8 pm and 10 pm.

However, firecracker use was reported across several parts of the Capital, including Pitampura, Burari, Mayur Vihar and areas of Lutyens' Delhi. Residents also reported smoke, reduced visibility and difficulty breathing as celebrations continued.

PM2.5 remained the predominant pollutant, with emissions from firecrackers adding to the existing pollution burden from vehicles, industrial activity, construction dust and crop-residue burning in neighbouring states.

The Decision Support System for Delhi identified the transport sector as the largest contributor to the city's pollution on Sunday, accounting for an estimated 19%. Stubble burning was projected to contribute 3.5%, with its share expected to be around 2.99% on Monday.

The combination of firecracker emissions and seasonal pollution sources, along with weather conditions that can restrict the dispersal of pollutants, continued to pose a challenge for the Capital.

Although relatively warmer and windier conditions helped prevent an even higher pollution peak this year, Delhi's air remained dangerously polluted. The substantial gaps in monitoring data also left an incomplete picture of the extent of pollution during the hours when residents were exposed to the worst air quality.