Delhi's air quality slipped into the 'severe' category on Sunday due to unfavourable meteorological conditions slowing down dispersion of pollutants, authorities said.
The Central Pollution Control Board (CPCB) data showed the overall air quality index (AQI) of Delhi at 412, which falls in the 'severe category', while the Centre-run System of Air Quality and Weather Forecasting (SAFAR) recorded an AQI of 388, which comes under the 'very poor' category.
The SAFAR also pointed out that a consistent day-night trend is observed for the past one week.
"During night, combination of calm wind and colder conditions is elevating the pollution level, where in many locations, 24-hour rolling average (AQI) is touching 'severe' levels for a few hours, whereas during the day, wind speed is picking up slightly and temperature is increasing to keep the level in the 'very poor' range," it said.
The air quality slipped into the 'severe' category on Sunday morning for this fourth time in 10 days.
Delhi's air quality has oscillated between the 'very poor' and the 'severe' categories during this period.
According to the CPCB data, 27 areas recorded 'severe' pollution, while eight areas witnessed 'very poor' air quality.
In the NCR, Ghaziabad, Gurgaon and Noida recorded 'severe' air quality, while Faridabad recorded 'very poor' quality air.
The overall PM2.5 level (fine particulate matter in the air with a diameter of less than 2.5 micrometer) was recorded at 339 and the PM10 level at 506 in Delhi, the CPCB said.
The national capital recorded its second highest pollution level of the year on Sunday with an AQI of 450. The air quality remained 'severe' Monday and Tuesday. There was a slight drop in the pollution level and the air quality had moved to the 'very poor' category Wednesday.
On Thursday, the air quality had again worsened and turned 'severe'.
According to the SAFAR, the air quality over Delhi has deteriorated slightly and touched the upper side of the 'very poor' category.
"Air quality over Delhi is very poor, which may improve slightly as winds are relatively high during the day but predicted to deteriorate by tomorrow as wind speed is unlikely to remain the same and other meteorological conditions are already not favourable. Cold wave conditions are likely to persist at least for the next two days," the SAFAR said.
"The air quality index will continue to fluctuate between 360-380 for the next three days, which comes under the 'very poor' category," it added.