Why north India’s air gets worst in the days after Deepavali

Every year, the air in Delhi and much of north India turns “very poor” or “severe” around Deepavali, and stays bad for days afterwards. On the eve of the festival this year, Delhi’s air quality index (AQI) was 369, its worst Diwali-eve reading since 2016, and 23 areas crossed 400 on the morning of the festival itself. Here is why.

What the crackers add

Firecrackers release a burst of fine particles (PM2.5) and gases such as sulphur dioxide over a few hours, mostly between evening and midnight. On a still night, those particles have nowhere to go.

Why the weather makes it worse

In early November, nights get cooler and winds drop. A layer of warmer air often sits above the cooler air near the ground, an inversion, acting like a lid. Smoke from crackers, vehicles, industry, waste burning and dust collects under it. Any one of these sources would be manageable in windy weather; together, under the lid, they push the AQI past 400.

What about stubble burning?

Farm fires in Punjab and Haryana are usually blamed, and in some years they add a third or more of Delhi’s PM2.5 on peak days. This year their share has been small, 1.7% on Deepavali eve. That shows how much of the problem is Delhi’s own.

What the government can do

The Graded Response Action Plan (GRAP) adds curbs in stages as the AQI rises: Stage III, now in force, bars older petrol and diesel cars and halts non-essential construction; Stage IV, above 450, can bring more trucks off the road and more schools online.

What you can do

Limit time outdoors on the worst days, especially in the morning, use an N95 mask if you must go out, and keep children, older people and anyone with asthma or heart disease indoors.