Agencies | New Delhi:
A day after Diwali, Delhi recorded an improvement in air quality, raising questions over how pollution levels declined despite firecracker emissions during the festival and an increase in stubble-burning incidents in neighbouring Punjab.
Experts have attributed the improvement to two possible factors: favourable wind conditions that helped disperse pollutants and missing data from a substantial number of air quality monitoring stations during a period of high pollution.
Dr Manoj Kumar, an air quality and energy analyst at the Centre for Research on Energy and Clean Air (CREA), raised concerns over the absence of PM2.5 readings from 36 of Delhi’s 59 monitoring stations during the early hours of November 9.
He said the missing data from such a large number of stations could affect the city’s overall pollution averages and potentially present an incomplete picture of air quality.
“For a particular period, data from nearly 36 out of Delhi’s 59 monitoring stations went missing. That’s a substantial number. When data from so many stations is unavailable, the city’s overall average PM2.5 concentration can come down, and we may not get an accurate picture of pollution levels at that particular time,” Kumar said.
He added that excluding unavailable readings could also lower the daily average PM2.5 concentration and consequently affect the Air Quality Index (AQI) calculation, making pollution levels appear lower than they actually were.
According to Kumar, 36 monitoring stations had missing hourly PM2.5 data at 1 am on November 9, raising questions about the accuracy and representativeness of pollution measurements during a critical period.
Data from the Central Pollution Control Board (CPCB) showed that Delhi’s AQI stood at 344 at 4:53 pm on November 9, compared with 376 on November 8, 369 on November 7 and 382 on November 6.
However, Kumar said the improvement could not be attributed solely to missing data, pointing to favourable meteorological conditions that had helped disperse pollutants across Delhi-NCR.
“Actually, this morning we had very good wind conditions. Better wind conditions support the dispersion of pollutants, and that’s why we had better or cleaner air, not just in Delhi but across the NCR,” he said.
He emphasised that favourable winds and missing monitoring-station data were two separate factors that needed to be examined independently while assessing the city’s air quality.
Despite the subsequent improvement, Diwali 2026 was the fourth most polluted day of the year in Delhi, with the daily average PM2.5 concentration reaching 257 micrograms per cubic metre (µg/m³), according to CREA.
The pollution levels in the days preceding the festival were also among the highest recorded this year. November 5, 6 and 7 ranked sixth, seventh and eighth, respectively, in terms of daily average PM2.5 concentrations.
Diwali 2026 also emerged as the fourth most polluted Diwali in Delhi since 2016, with an average PM2.5 concentration of 257 µg/m³.
Meanwhile, Siri Fort recorded a PM2.5 concentration of 1,000 µg/m³, the maximum value displayed on the CPCB’s Central Control Room public portal.
The portal does not display readings beyond the 1,000 µg/m³ mark, raising concerns over whether the actual intensity of extreme pollution episodes is being fully captured and represented in the publicly available data.
