Delhi's air quality recorded a marginal improvement but remained in the "very poor" category on Saturday morning, while a government forecasting agency said it is likely to get better due to a favourable wind speed. The city recorded an air quality index (AQI) of 369 at 9.30 am. The 24-hour average AQI was 374 on Friday, 395 on Thursday, 297 on Wednesday, 312 on Tuesday and 353 on Monday.
Jahangirpuri (412), Mundka (407) and Anand Vihar (457) recorded the air quality in the "severe" category. An AQI between 0 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".
On Thursday, Delhi's AQI touched the "severe" levels for a brief period, before slipping back to the "very poor" category. According to the Ministry of Earth Sciences' air quality monitoring agency, SAFAR, the share of stubble-burning in Delhi's PM2.5 pollution was 19 per cent on Friday.
It was 36 per cent on Thursday, the maximum so far this season, 18 per cent on Wednesday, 23 per cent on Tuesday, 16 per cent on Monday, 19 per cent on Sunday and nine per cent on Saturday. The number of farm fires has again increased in Punjab (around 3,000), Haryana and Uttar Pradesh and is likely to impact Delhi-NCR's air quality.
The wind speed has picked up. A significant improvement is predicted by Monday and the air quality is likely to slip back to the "poor" category, the SAFAR said. According to the India Meteorological Department (IMD), the predominant wind direction was northwesterly and the maximum wind speed 15 kilometres per hour. The minimum temperature was recorded at 13 degrees Celsius.
Calm winds and low temperatures trap pollutants close to the ground, while a favourable wind speed helps in their dispersal. The city's ventilation index — a product of the mixing depth and the average wind speed — is likely to be around 8,500 square metre per second on Saturday — favourable for dispersal of pollutants.
The mixing depth is the vertical height in which pollutants are suspended in the air. It reduces on cold days with a calm wind speed. A ventilation index lower than 6,000 square metre per second, with an average wind speed of less than 10 kmph, is unfavourable for dispersal of pollutants.
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