New Delhi, Nov 20 (PTI) Delhi's air quality was recorded in the 'poor' category on Friday while central government agencies said it is likely to improve marginally due to favourable wind speed. The city's 24-hour average air quality index (AQI) was 296. It was 283 on Thursday, 211 on Wednesday and 171 on Tuesday.
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 air quality is likely to remain in the 'poor' to 'moderate' category on Saturday.
The predominant surface wind direction is likely to be west-northwesterly and the maximum wind speed 15 kmph on Saturday, it said.
Around 600 farm fire counts were observed over Punjab, Haryana and adjoining Pakistan on Thursday, it said.
According to the Ministry of Earth Sciences' air quality monitor, SAFAR, stubble burning accounted for 15 per cent of Delhi's PM2.5 pollution on Friday. It was 20 per cent on Thursday, eight per cent on Wednesday and three per cent on Tuesday.
SAFAR said 'energetic' surface winds are likely to influence AQI positively in the next two days.
The minimum temperature settled at 7.5 degrees Celsius on Friday – the lowest in the month of November in 14 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 12,500 m2/s on Friday and is likely to be 13,000 m2/s on Saturday.
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.
The IMD said the minimum temperature in Delhi will drop to 7 degrees Celsius by Monday, as cold winds have started blowing from hilly regions, which have witnessed a fresh bout of snowfall.
The minimum temperature this month, barring on November 16, has remained 2-3 degree Celsius below normal in the absence of a cloud cover, according to IMD officials. PTI GVS TDS TDS