Night of lights, morning of smoke: How Diwali turned Delhi’s air heavy once again

The air in Delhi always starts to feel different by mid-October — a little cooler, a little stiller and a little heavier. This year was no exception. As the city prepared for Diwali, the familiar haze began to creep in. It wasn’t sudden; the numbers had already started whispering the story.

On October 10, the 24-hour average PM2.5 levels stood at 64 microgrammes per cubic metre (µg / m3), safely within limits. But over the following 10 days, the numbers climbed nearly four times, reaching 237 µg / m3 by October 20, the night of Diwali.

Each day, from October 17 to 19, pollution levels inched upward — 113, 125, 152 µg / m3 — until Diwali evening, when the curve steepened sharply. For a few hours around noon, there was a brief pause. PM2.5 dropped to around 94 µg / m3, helped by sunlight and movement in the air. But as dusk fell and diyas lit up balconies, the numbers began to rise again — slowly at first, then all at once. 

By evening, the PM2.5 levels had spiked nearly sevenfold from noon, a familiar peak that Delhiites have almost learnt to expect. The pollution levels climbed 49 per cent higher than the previous day.

Hourly PM2.5 concentrations in Delhi, showing the sharp Diwali night spike

Interestingly, the midnight peak of 2024 was about 10 per cent lower than that of Diwali 2023. Yet, the night-time average pollution remained the same — around 497 µg / m3 for both years.

The comparison is telling: In 2023, Diwali came later in November, a time when cold air traps pollutants closer to the ground. This year, the festival arrived earlier, in mid-October, when meteorological conditions were relatively better and winds more forgiving. Still, Delhi’s air darkened almost the same way.

So, if the weather wasn’t the villain, what was? 

According to data from the Indian Agricultural Research Institute for Haryana, Punjab and Uttar Pradesh, on Diwali day, UP led with 57 per cent of farm fires, Punjab followed with 33 per cent, and Haryana with 10 per cent. While stubble burning significantly contributes to the seasonal haze, Indian Institute of Tropical Meteorology data showed that only 0.8 per cent of the PM2.5 particles in the air were directly linked to these farm fires. In other words, the smoke that filled Delhi’s lungs this Diwali came mostly from within the city.

Despite such a negligible contribution from external sources, Delhi’s AQI reached 345 at 4 pm on Diwali day, firmly in the 'Very Poor' category. Out of 35 monitoring stations, readings from four were in the 'Severe' category, 28 'Very Poor' and only three — Sirifort, DTU and Aurobindo Marg managed to stay under Poor category.

Every year, Diwali tells the same story in slightly different words. The outcome remains painfully consistent even when the festival comes earlier, with fewer farm fires and milder weather conditions.