The air quality in Delhi remained in the ‘poor’ category on Sunday when government agencies said it might witness a further decline as a result of unfavourable meteorological conditions.
The 24-hour average Air Quality Index of the national capital was recorded at 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 Centre’s Air Quality Early Warning System for Delhi informed that the air quality of the Delhi-NCR is likely to slip into the “very poor” category on Monday because of unfavourable meteorological conditions.
“The predominant surface wind direction was northwesterly and the maximum wind speed 12 kmph on Sunday,” adding that the wind speed may drop to 8 kmph on Monday.
It said that the AQI may remain at the upper ends of the “very poor” category on Tuesday and Friday.
The Ministry of Earth Sciences' air quality monitor, SAFAR, said on Saturday, 649 farm fire counts were recorded over Punjab, Haryana and adjoining regions.
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.
