The city's air quality dropped to "very poor" in at least one location on Friday night after a relatively cleaner Diwali on Thursday.

A stronger-than-usual wind speed on Thursday evening had ensured the dispersion of the pollutants, experts had said.

On Friday, the wind speed dropped.

A scientist at the Alipore Met office said the wind speed recorded on Friday evening was "calm", which means there was negligible wind.

On Thursday evening, the wind speed recorded at Alipore was 1.5kmph, said the scientist with the Met office.

"Winds disperse pollutants. The absence of winds helps the pollutants hang around in the air for a long period. If there is no wind or negligible wind, then the pollutants remain in the air close to the ground that we breathe," said a senior scientist with the state pollution control board (PCB).

At 10pm on Friday, the air quality reported by the air quality monitoring station at Rabindra Bharati University was "very poor".

The air quality was "moderate" in Bidhannagar, Ballygunge, Fort William and Jadavpur. It was "satisfactory" in Rabindra Sarobar. The station at Victoria had "insufficient data".

On Diwali night, the air quality recorded at 10pm was "satisfactory" in six stations and "good" in one. There are seven air quality monitoring stations in Calcutta and Bidhannagar.

"The air quality monitoring station at Rabindra Bhara- ti University is located on the BT Road campus. It has high vehicular traffic, including a lot of trucks. It seems that the vehicular pollution contributed to the "very poor" recording," said the PCB scientist.

According to the National Air Quality Index, "very poor" air quality can lead to "respiratory illness on pro- longed exposure; "moderate" air quality can cause "breathing discomfort to the people with lungs, asthma and heart diseases" and "satisfactory" air quality can cause "minor breathing discomfort to sensitive people".

"Friday was cloudy and there was hardly any wind speed. Both these meteorolog- ical factors contributed to a drop in air quality," said the scientist.

He said because of the overcast sky on Friday, the air did not heat up as much as Thursday.

"If the air close to the ground is warm, it rises up and carries with it the pollutants. Cooler air is heavier and hangs close to the ground. The pollutants, too, keep floating in the air," the scientist said.

"The wind plays a similar role. On a windy day, the pollutants are dispersed faster. On a windless day, the pollutants remain concentrated," he said.

A man in his 30s who was travelling from Chandi Chowk to Kasba around 10pm on Friday said there was a burning smell in the air around Maidan and Loudon Street.

A woman, a resident of Kalighat, said she, too, felt a similar smell on Friday night.

Residents across the city said they heard sounds of indiscriminate bursting of crackers both on Thursday and Friday evening.

The Telegraph reported on Saturday that Calcutta experienced a cleaner Diwali this year than 2023.

In 2023, six air quality monitoring stations recorded "poor" air quality at 10pm. One station -- at Rabindra Bharati University -- recorded "moderate" air quality.