Delhi's air quality remained "poor" on Sunday and government agencies said it is likely to deteriorate further due to unfavourable meteorological conditions. The city's 24-hour average AQI was 274 on Sunday It was 251 on Saturday, 296 on Friday, 283 on Thursday and 211 on Wednesday.
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". The central government's Air Quality Early Warning System for Delhi said Delhi-NCR's air quality is likely to deteriorate to the "very poor" category on Monday due to unfavourable meteorological conditions.
The predominant surface wind direction was northwesterly and the maximum wind speed 12 kmph on Sunday, it said, adding that the wind speed is likely to drop to eight kmph on Monday. The central agency said the AQI is likely to deteriorate to the upper end of the "very poor" category between Tuesday and Friday.
As many as 649 farm fire counts were observed over Punjab, Haryana and adjoining regions on Saturday, according to the Ministry of Earth Sciences' air quality monitor, SAFAR. Stubble burning accounted for 12 per cent of Delhi's PM2.5 pollution on Sunday. It was 13 per cent on Saturday, 15 per cent on Friday, 20 per cent on Thursday and eight per cent on Wednesday.
The minimum temperature settled at 6.9 degrees Celsius on Sunday, the lowest in the month of November in 17 years, according to IMD. Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion.
Delhi's ventilation index — a product of mixing depth and average wind speed was around 6,500 m2/s on Sunday and is likely to drop to 1,500 m2/s on Monday and Tuesday. Mixing depth is the vertical height in which pollutants are suspended in the air. It reduces on cold days with calm wind speed.
A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants. Earlier in the day, Delhi Environment Minister Gopal Rai said the Public Works Department has installed 23 anti-smog guns and deployed 150 tankers for water sprinkling at key intersections and construction sites to reduce dust pollution.