New Delhi, January 20
Delhi's air quality recorded a marginal improvement on Wednesday, moving to the 'poor' category from 'severe' the day before, due to a favourable wind speed, India Meteorological Department (IMD) officials said here.
The city's 24-hour average air quality index (AQI) stood at 283 on Wednesday. It was 404 on Tuesday, 372 on Monday and 347 on Sunday.
Before that, Delhi's air quality remained in the 'severe' zone for three consecutive days.
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 between 401 and 500 'severe'.
The maximum wind speed was 15 kmph on Wednesday.
The central government's Air Quality Early Warning System for Delhi said the city's ventilation index—a product of mixing depth and average wind speed—was 9,000 square metres/sec on Wednesday. It is likely to be 11,000 square metres/sec on Thursday.
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/sec, with an average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants.
Delhi has recorded six 'severe' air quality days this month so far, while four 'severe' air days were registered in December. — PTI
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