Delhi air sees fastest post-Diwali improvement in a decade, barring pandemic year

While the Capital on Friday recorded a AQI of 275 (Poor), it was 305 (Very Poor) on Thursday, 353 (Very Poor) on Wednesday and 350 (Very Poor) on Tuesday. Delhi had celebrated Diwali on Monday, when the AQI stood at 345 (Very Poor).

This is the fastest improvement in post-Diwali AQI, barring the Covid-19 pandemic year in 2020, as per an analysis of Central Pollution Control Board (CPCB) data since 2015 from when records are available. The AQI in 2020 was recorded at 414 on Diwali and improved to 221 in two days.

The only time earlier when a similar situation was observed, barring 2020, was in 2017, when Diwali was celebrated on October 19 — the earliest in the last decade. The AQI had then improved by more than 70 points in two days after post-Diwali AQI was reported at 350 in the ‘Very Poor’ category.

With Diwali this year celebrated amid warmer conditions, the average minimum temperature in Delhi between October 1 and 24 this year was recorded at 20.6 degree Celsius, as per India Meteorological Department (IMD) — higher than most years since 2015. The night temperature was 21.7 degree Celsius in 2024, 19.1 degree Celsius in 2023, 19.5 degree Celsius in 2022 and 22 degree Celsius in 2021.

The long-term average for this period since 2011 is around 20.6 degree Celsius, making this October among the warmest in recent years.

Warmer nights help prevent pollutants from getting trapped close to the surface, allowing them to disperse more easily. In cooler conditions, a layer of cold air forms near the ground after sunset, capping pollutants and leading to sharp overnight spikes in PM2.5 levels. The absence of such strong temperature inversion this week, along with steady winds from the northwest blowing at up to 14 kmph, has supported better vertical mixing and ventilation.

Between September 15 and October 23, 2,017 instances of stubble burning were reported from Punjab, Haryana, Uttar Pradesh, Madhya Pradesh, Rajasthan and Delhi — lowest for this period in over a decade. This was largely driven by a steep drop in number in Punjab and Haryana, the two major contributors to stubble burning in the region.

This year, with 512 fires, Punjab’s count in regard to farm fires is over 90% lower than the average of 5,627 for this period since 2020, Indian Agricultural Research Institute (IARI) data shows. Similarly, with 58 farm fire incidents, Haryana’s count is around 95% lower than the average of 1,164 for this period. The Uttar Pradesh tally, however, with 717 incidents, is slightly higher than its average for this period of 602.

The dip in the number is due to delayed harvesting caused by flooding in late August to early September and standing water in large tracts of Punjab’s paddy belt, which has pushed back the crop cycle by nearly two weeks. The contribution of stubble burning to Delhi’s pollution remains negligible at less than 2%, as per the Decision Support System (DSS) for air quality management.

Also, the predominant wind direction over the past few days has been northwesterly, as per the IMD. Had easterly winds been the predominant wind direction instead, smoke from Uttar Pradesh — which has reported the highest number of residue burning events this season — could have been transported towards Delhi, worsening the AQI.