The national capital's air quality was recorded in the "severe" category on Saturday morning as farmers in Punjab and nearby regions continued to set their fields on fire to clear crop residue.
The central government's Air Quality Early Warning System for Delhi said the city's air quality is likely to remain "severe" on Diwali as well.
Experts said while meteorological conditions were "moderately" favourable for dispersion of pollutants, a "very high" number of farm fires in Punjab was the primary reason for "severe" air quality.
According to the ministry of earth sciences' air quality monitor, SAFAR, Delhi's overall air quality index was 443 this morning.
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".
PM10 levels in Delhi-NCR stood at 486 microgram per cubic meter (µg/m3) at 9 am, according to Central Pollution Control Board data.
PM10 is particulate matter with a diameter of 10 micrometers and inhaled into the lungs.
These particles include dust, pollen and mold spores.
PM10 levels below 100 µg/m3 are considered safe in India.
The levels of PM2.5 -- finer particles which can even enter the bloodstream -- were 292 µg/m3 at 9 am.
PM2.5 levels up to 60 µg/m3 are considered safe.
Air Quality Early Warning System for Delhi said the number of farm fires in Punjab remains very high (around 4,000) which is likely to impact the air quality in Delhi-NCR and other parts of northwest India.
It said the AQI is likely to remain in the "upper end of 'very poor' category on November 13 and 'severe' category on November 14 (Diwali)".
According to the India Meteorological Department, the maximum wind speed was 14 kilometres per hour on Friday and the minimum temperature 11.8 degrees Celsius.
Calm winds and low temperatures trap pollutants close to the ground, while favourable wind speed helps in their dispersion.
V K Soni, head of IMD's environment monitoring research centre, said, "A very large number of farm fires over Punjab is the primary reason for the severe air quality in the region."
SAFAR said the share of stubble burning in Delhi's PM2.5 pollution was estimated at 21 per cent on Friday. It was 42 per cent on Thursday, the maximum so far this season.
Last year, the contribution of stubble burning to Delhi's pollution had peaked to 44 per cent on November 1, according to SAFAR data.
