Despite rain and fewer farm fires, why Delhi’s October air was second worst in 5 years

This year marked the first festive season since the Supreme Court eased the 2020 blanket ban on firecrackers in Delhi-NCR, allowing the use of government-approved green crackers. The relaxation, however, coincided with this being the most polluted Diwali since 2021. According to Central Pollution Control Board data, Delhi’s AQI on October 20, Diwali day was at 345, worse than last year’s 328 recorded on the same day. The AQI was also higher than what was recorded on Diwali day in 2023 (218) and 2022 (312).

Delhi spent nearly two-thirds of October breathing ‘Poor’ air or worse — 11 days in the ‘Poor’ category, seven in ‘Very Poor’ and none in the ‘Good’ category.

The Capital saw its worst air on October 30, when the AQI touched 373, the highest for any October day in three years. The late-month spike was linked to weak winds, high humidity and a sharp night-time drop in temperature that created an inversion layer and trapped pollutants near the ground. On Thursday, Delhi’s day temperature dipped to 27 degrees Celsius, four notches below normal and the lowest for the month in two years.

Satellite data from NASA’s Terra MODIS sensor showed a clear link between weather conditions and the late-October haze. Aerosol Optical Depth (AOD), which is a measure of how dense is the haze, rose sharply over Delhi on October 28 and 29, before dropping by the end of the month as wind speed improved. The data confirmed that the brief improvement in AQI was driven by meteorological factors rather than emission control.

The particulate matter (PM) pollution spiked sharply by Friday night, as per data from the Delhi Pollution Control Committee, due to calm winds. At Jahangirpuri, a pollution hotspot, PM10 levels rose to 418 microgrammes per cubic metre at 8 pm after staying below 200 microgrammmes per cubic metre through most of the day. Similar night spikes were observed in other hotspots like Anand Vihar and Bhawana, too. Delhi recorded an AQI of 218 on Friday.

As per the Indian Agricultural Research Institute (IARI), satellites detected 259 farm fires across six Northern states on October 31 — 224 in Punjab, six in Haryana, four in Uttar Pradesh, 25 in Rajasthan and none in Delhi or Madhya Pradesh. Between September 15 and October 31, 3,845 farm fires were recorded — 1,642 in Punjab, 99 in Haryana, 977 in Uttar Pradesh, 641 in Rajasthan, 483 in Madhya Pradesh and three in Delhi.

No increase in the trend was observed in any of these states in the recent days, making this the lowest farm-fire activity in five years.

The Decision Support System (DSS) for air quality management showed that the contribution of farm fires to Delhi’s PM2.5 load dipped to 0.5% on October 30. In contrast, local sources dominated: the transport sector accounted for 13.44% of the emissions, followed by neighbouring Ghaziabad (13%) and Gautam Buddha Nagar (11.7%).

Despite the heavy rain, the weather offered little relief in October.

Delhi received about 90 mm of rainfall this month, almost six times the usual 15.1 mm, making it the third-wettest October since 2011. The average day temperature stood at 31.5 degrees Celsius, slightly above the 31.4 degrees Celsius recorded in 2022, making it the second-coolest October day temperature since 2011. The average night temperature of 20.1 degrees Celsius was among the warmest in recent years.

The combination of warm nights, high humidity and light winds prevented pollutants from dispersing, keeping the air stagnant and hazy.

“Between 2015 and 2025, Delhi’s average October AQI has fluctuated between 173 and 285,” said Dr Manoj Kumar, pollution analyst at the Centre for Research on Energy and Clean Air (CREA). “Yet the overall trend shows stagnation, not progress. The sharp dip in 2021 (AQI 173) was an anomaly caused by Covid-19 lockdown.”

“Since then, October averages have stabilised around 210 to 234, signalling a return to business-as-usual… This October’s average AQI of 223 again places Delhi in the ‘Poor’ category, showing that mitigation efforts have failed to deliver lasting emission reduction,” he added.