The air quality index (AQI) stood at 414, in the “severe” category, on Diwali day. But rain and high-velocity winds under the influence of a Wester Disturbance brought relief from high pollution levels.
The city’s 24-hour average AQI improved from 435 on Sunday to 221 on Monday. On Tuesday, it was 171.
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 Delhi-NCR’s air quality is likely to remain in the “moderate” and the lower end of the “poor” category on Thursday and Friday.
On Wednesday, the prominent surface wind direction will be northwesterly and the maximum wind speed will be 10 kmph, it said.
The wind speed will continue to be favourable for dispersion of pollutants over the next three to four days, according to IMD.
Delhi’s ventilation index — a product of mixing depth and average wind speed — will be around 5,000 m2/s on Wednesday and 7,500 m2/s 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/second, with the average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants.
The Air Quality Early Warning System for Delhi also said very few farm fire were observed over Punjab and Haryana on Tuesday and “their impact on air quality of Delhi is likely to be negligible”.
The IMD said the minimum temperature in Delhi is predicted to drop to 9 degrees Celsius by Saturday, as cold winds have started blowing from hilly regions, which have witnessed a fresh bout of snowfall.
The minimum temperature this month, barring on November 16, has remained 2-3 degree Celsius below normal in the absence of a cloud cover, according to IMD officials.