New Delhi: As people in the national capital started celebrating Diwali by defying the ban on firecrackers in several areas, the air quality in Delhi plunged to ‘Severe’ category and a thick smog reduced visibility across the national capital region. Also Read - Delhi Air Quality Improves Marginally But Likely to Remain 'Very Poor' Till End of Year

Weather officials said stubble burning accounted for 32 per cent of the city’s PM2.5 pollution and calm winds worsening the situation as they allowed accumulation of pollutants. Also Read - Stubble Now Keeping Protesting Farmers Warm Amid Intensified Protests

The Ministry of Earth Sciences’ air quality monitor, SAFAR, said, “Even a small increase in local additional emissions is likely to have significant deterioration impact on Sunday and Monday.” It said peak levels of PM10 and PM2.5 are expected between 1am and 6am in case of additional internal emissions. Also Read - Amid Rising Air Pollution, Delhi's Ghazipur Landfill Catches Fire

Earlier, it had said that the PM2.5 concentration in Delhi on Diwali is likely to be the “lowest” in the last four years if no firecrackers are burnt. The city recorded an overall AQI of 414 on Saturday, which falls in the “severe” category.

On Saturday morning, Delhi’s air quality was very poor. SAFAR had said that the pollution levels in Delhi during the Diwali period are likely to remain in the higher end of the very poor category in the absence of emissions due to fireworks.

But two factors smoke from farm fires and stagnant surface winds will keep the air quality in the higher end of the very poor category to the lower end of the severe on Diwali night, it said.

The central government’s Air Quality Early Warning System for Delhi said that the air quality in Delhi-NCR is likely to deteriorate further and enter the ‘severe’ category by evening.

SAFAR said transport-level winds have become northwesterly and the stubble burning-induced impact on air quality index (AQI) is expected to increase.

The National Green Tribunal (NGT) had on Monday imposed a total ban on sale or use of all kinds of firecrackers in the National Capital Region (NCR) from November 9 midnight to November 30 midnight, saying “celebration by crackers is for happiness and not to celebrate deaths and diseases”.

The Central Pollution Control Board (CPCB) had on Wednesday ordered closure of hot mix plants and stone crushers in Delhi-NCR till November 17 in view of a likely increase in pollution levels during the festive season.

(With inputs from PTI)