NEW DELHI: Delhi's air quality improved marginally on Saturday (November 21) due to favourable wind speed, but remained in the 'poor' category.
Government agencies said that the Air Quality Index (AQI) is likely to remain in the 'moderate' to 'poor' category on Sunday. The city's 24-hour average AQI was 251 on Saturday. It was 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 that Delhi-NCR air quality is likely to remain in the 'moderate' to 'poor' category on Sunday.
The predominant surface wind direction is likely to be northwesterly and the maximum wind speed 12 kmph on Sunday, it said.
The central agency said that the AQI is likely to deteriorate to the upper end of the "very poor" category between Tuesday and Friday as "unfavourable meteorological conditions" are predicted during that period.
Around 1,264 farm fire counts were observed in Punjab, Haryana and adjoining regions on Friday, according to the Ministry of Earth Sciences' air quality monitor, SAFAR.
Stubble burning accounted for 13 per cent of Delhi's PM2.5 pollution on Saturday. It was 15 per cent on Friday, 20 per cent on Thursday and eight per cent on Wednesday.
The System of Air Quality and Weather Forecasting and Research (SAFAR) said the share of stubble burning in Delhi's pollution could have been much higher on Saturday had there been calm winds.
The minimum temperature settled at 8.5 degrees Celsius on Saturday. It was 7.5 degrees Celsius on Friday ? the lowest in the month of November in 14 years, according to the India Meteorological Department (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 13,000 m2/s on Saturday and is likely to be 6,000 m2/s on Sunday.
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.
