Delhi's air quality inched closer to the "very poor" category on Monday morning. Around 9 am, the overall Air Quality Index (AQI) across Delhi stood at 276, according to SAFAR, a forecasting agency under the Union Ministry of Earth Science.

The drop in air quality was driven by unfavourable meteorological conditions, which allowed accumulation of pollutants. Also, emissions from firecrackers and stubble burning compounded the situation.

An AQI between zero and 50 is considered good, 51 and 100 satisfactory, 101 and 200 moderate, 201 and 300 poor, 301 and 400 very poor, and 401 and 500 severe.

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On Sunday evening, the city reported a 24-hour average air quality index (AQI) of 259, the lowest for the day before Diwali in seven years. However, pollution levels crept up at night amid a drop in temperature and wind speed as people burst firecrackers in several parts of the capital, news agency PTI reported.

The number of farm fires rose to 1,318, the highest this season so far. Delhi's AQI stood at 298 at 6 am on Monday.

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Nineteen of the 35 monitoring stations in the city recorded air quality in the "very poor" category, while one (Anand Vihar) reported "severe" pollution levels. The neighbouring cities of Ghaziabad (300), Noida (299), Greater Noida (282), Gurugram (249) and Faridabad (248) reported poor air quality.

The air quality will continue to remain "very poor" if no firecrackers are burst, SAFAR said. In case firecrackers are burst like last year, the air quality may plunge to "severe" levels on the night of Diwali and continue to remain in the "red" zone for another day, it predicted.

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Diwali is being celebrated across the country on Monday. The contribution of stubble burning to Delhi's PM 2.5 pollution has so far remained low (up to 5 per cent) due to a slow transport-level wind speed. But, it is likely to increase to 8 per cent on Monday.

Gufran Beig, founder project director, SAFAR, said the transport-level wind direction and speed is likely to become "very favourable" for transport of smoke from stubble burning from Monday afternoon.

"It will increase the share of stubble burning in Delhi's PM 2.5 pollution to 15-18 per cent on October 25 and push the air quality into the 'severe' category," he said. The Indian Agricultural Research Institute reported 902 farm fires in Punjab, 217 in Haryana and 109 in Uttar Pradesh on Sunday evening.