Delhi's air quality improves marginally

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New Delhi, Dec 28 (PTI) Delhi's air quality improved slightly on Monday due to favourable wind speed, weather department officials said. The city's 24-hour average air quality index (AQI) was 253 on Monday. It was 396 on Sunday, 337 on Saturday, 357 on Friday, 423 on Thursday, 433 on Wednesday and 418 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'.

Officials at the India Meteorological Department (IMD) said favourable wind speed, up to 15 kmph, aided in the dispersion of pollutants. On Sunday night, Delhi's air quality had entered the 'severe' category briefly as light rainfall in neighbouring areas increased moisture content in the air. 'Water droplets suspended in the air and made pollutants heavier which do not get dispersed easily even with a moderate wind speed,” an IMD official said. 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 5,000 m2/s on Monday.

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. PTI GVS DPB