Air quality ‘poor’, Delhi AQI falls below 300-mark after a month

According to the Indian Institute of Tropical Meteorology forecast, the air quality will be in the ‘very poor’ category on Monday and Tuesday, after which it will be in the ‘poor’ category.

CPCB data revealed that particulate matter of size less than 2.5 microns (PM2.5) and less than 10 microns (PM10) were the prominent pollutants observed Sunday. The particulate matter pollution had been on the decline since Saturday noon, owing to slightly stronger, sustained winds.

As per data from the Delhi Pollution Control Committee (DPCC), the wind speed touched above 10 kmph at a few stations and aided in better dispersal of pollutants. For instance, Rohini, a pollution hotspot, recorded a high wind speed of 6-12 kmph. Other pollution hotspots such as Bawana, Wazirpur and Ashok Vihar also reported similarly high wind speeds.

Delhi’s air has turned cleaner as Easterlies, the moisture-carrying winds that cause fog as the humidity condenses around the particulate matter, have stayed away from the city.

“Winds have been generally Northwesterly/Northerly/NNWly. There have been no easterlies established for more than a week. A fog spell which was expected from November 28 was also not realised because Easterlies didn’t set up over Delhi. The last two dense fog spells occurred due to Westerlies but that was due to the advection of fog layer from Pakistan but even this is not happening now as there is no fog layer established over Pakistan and adjoining areas,” said IMD scientist Krishna Mishra. Advection fog forms when warm, moist air passes over a cool surface causing water vapour to condense and occurs mostly when warm, tropical air meets cooler ocean water.

Mishra also said that a good western disturbance (WD) is needed to bring down the humidity levels. Though a WD has been felt over the last two days, he added that it is in the far north impacting only Jammu and Kashmir.

The minimum temperature on Sunday was 11.4 degrees Celsius, a notch above normal. The IMD marks a ‘cold day’ or a ‘severe cold day’ when the maximum temperature is below the normal by 4.5 degrees or more.