Delhi is witnessing an "unusual" condition and no quick recovery is predicted from the "severe" air pollution, a central government forecasting agency said on Tuesday.
The Ministry of Earth Sciences' air quality monitor, SAFAR, said three major factors are responsible for this situation -- secondary particle formation, extremely calm local surface winds and stubble burning-related intrusion.
"Delhi's overall AQI is in the higher end of the "severe" category, which is an unusual condition," it said.
"Due to a high moisture content, humidity has touched a new high and the air holding capacity has increased under such cool conditions, triggering rapid secondary fine particulate formation -- a scientific process when gas-to-particle conversion happens and in-situ chemical production takes place on available surfaces and multiplies PM2.5," the SAFAR added.
The extremely calm local surface winds continued to arrest all old and new accumulated pollutants, it said.
"Significant stubble burning-related intrusion took place until Tuesday morning due to a favourable, transport-level wind direction and speed," the SAFAR said.
However, the transport-level wind direction has changed now, which is likely to reduce stubble intrusion.
"But due to the first two factors, no quick recovery is predicted unless the moisture content declines with radiative heating," the SAFAR said.
It said the air quality index (AQI) of the national capital is likely to improve to the lower end of the "severe" category to the higher end of the "very poor" category on Wednesday owing to a change in the transport-level wind direction and marginally better ventilation conditions.
The SAFAR said the farm fire count in Punjab, Haryana, Uttar Pradesh, Uttarakhand and neighbouring areas stood at 2,247 on Monday.
"The number has reduced marginally but is still significantly high," it said.
The share of stubble burning in Delhi's PM2.5 pollution was 22 per cent on Tuesday.
An apocalyptic smog enveloped Delhi in grey on Tuesday, blotting out the sun from the sky and smudging landmarks from view as the air quality hit "emergency" levels.
The city recorded an overall AQI of 476 at 4 pm, which falls in the "severe" category. The neighbouring cities of Faridabad (474), Ghaziabad (476), Noida (490), Greater Noida (467) and Gurugram (469) also recorded "severe" air quality.
This is the sixth "severe" air day on the trot in Delhi. The city witnessed seven "severe" air days in November last year.
The levels of PM2.5 -- which is about three per cent the diameter of a human hair and can lead to premature deaths from heart and lung diseases -- were 644 micrograms per cubic metre at 12 noon -- around 11 times higher than the safe limit of 60 micrograms per cubic metre.
The PM10 levels rose to 777 micrograms per cubic metre at 8 am, the highest since June 14, 2018, when it peaked to 847 micrograms per cubic metre, according to Central Pollution Control Board (CPCB) data. PM10 levels below 100 micrograms per cubic metre are considered safe in India.
If the PM2.5 and PM10 levels are more than 300 micrograms per cubic metre and 500 micrograms per cubic metre respectively for over 48 hours, emergency measures such as a ban on construction activities and entry of trucks, and a car-rationing scheme can be implemented under the Graded Response Action Plan (GRAP) for Delhi-NCR, notified by the Ministry of Environment and Forests in 2017.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)