Delhi's air quality turns 'severe' again

Delhi's air quality turned 'severe' again on Sunday as high moisture content due to light rains in neighbouring areas made pollutants heavier, weather department officials said.

The city's 24-hour averge air quality index (AQI) was 396, which falls in the "very poor" category. However, it rose to 406 by 10 pm.

The 24-hour average AQI was 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".

Kuldeep Srivastava, the head of the India Meteorological Department's regional forecasting centre, said light rainfall in neighbouring areas due to a Western Disturbance 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," he said.

A marginal improvement is likely as wind speed picks up during the day, Srivastava said.

He 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 3,500 metre square per second.

Mixing 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.