The AQI is likely drop to the lower end of ''severe'' category by Friday.
When rains and high-velocity winds washed away hazardous smog from the national capital around 10 days ago, many believed that bad air days were over for this season. But that was not the case.
With Delhi registering the season's lowest minimum temperature at 7.6 degrees Celsius on Thursday, smog returned to choke Delhi-NCR again, with pollution levels entering the ''severe'' zone in many parts of the national capital and its suburbs.
The overall air quality index (AQI) in the national capital read 382 at 4 pm. The satellite towns of Ghaziabad (432), Greater Noida (417) and Noida (414) also breathed extremely polluted air on Thursday.
An AQI between 0-50 is considered ''good'', 51-100 ''satisfactory'', 101-200 ''moderate'', 201-300 ''poor'', 301-400 ''very poor'' and 401-500 ''severe''. An AQI above 500 falls in the ''severe plus'' category.
The levels of PM2.5 -- particulate matter so small that they can enter the lungs and even the bloodstream -- shot up to 245 micrograms per cubic metre, six times the safe limit of 0-60, by 8 pm. The last time such levels were reported was on November 16.
Weather experts said though stubble burning period had almost ended, a dip in the air quality was expected due to slow wind speed and falling temperatures.
"It shows that meteorological conditions play a very important role in keeping the air clean. Farm fires have come to an end. The smog that shrouded the city on Thursday was mostly local pollutants and moisture," said Mahesh Palwat, a scientist at private forecaster Skymet Weather.
The government's air quality monitoring and forecasting centre, System of Air Quality and Weather Forecasting And Research (SAFAR), said smoke from stubble burning accounted for 9 per cent of Delhi''s PM2.5 pollution on Thursday and it is likely to drop to 3 per cent on Friday.
Kuldeep Srivastava, the head of the regional forecasting centre of the India Meteorological Department, said falling temperatures and calm winds led to accumulation of pollutants and high humidity made the situation worse.
Senior IMD scientist VK Soni said the region is likely to witness slow wind speed in the next five days. The wind direction is expected to shift from north to northeast Friday onwards which may increase pollution further, he said.
The AQI is likely drop to the lower end of ''severe'' category by Friday. Rains and thunderstorms on December 11 may bring some relief, VK Soni said.
After the first episode of smog in November, the Supreme Court banned construction activities in the region till further orders.
VK Shukla, the head of air quality management at CPCB, said the ban imposed on hot mix plants, stone crushers and diesel generator sets by the Environment Pollution (Prevention and Control) Authority also continues.
The EPCA had, however, lifted the restrictions on the coal-based industries in NCR and non-PNG one in Delhi on November 16.
Meanwhile, a CPCB-led task force on Thursday asked all agencies in Delhi-NCR to remain on high alert and to take stringent measures to control air pollution. It asked them to intensify enforcement activities in hot spots and industrial areas and recommended people to minimise use of private vehicles.
During a review meeting with the implementing agencies on Tuesday, the CPCB said December and January have seen a number of days in the "very severe" category in the last three years.
"Similar situation may emerge... unless timely and adequate field-level preventive action is taken by implementing agencies," it said.
NK Gupta, Additional Director of CPCB, pointed out that a large number of incidents of construction and demolition activities and open dumping of garbage were being reported from some parts of Delhi and neighbouring towns such as Ghaziabad and Faridabad.