Delhi's air quality was recorded in the "poor" category on Saturday and the share of stubble burning in the city's PM2.5 pollution stood at 19 per cent, according to a central government agency. The share of stubble burning was 18 per cent on Friday, around one per cent on Wednesday and around 3 per cent on Tuesday, Monday and Sunday.
The city recorded a 24-hour average air quality index (AQI) of 286. It was 239 on Friday and 315 on Thursday, the worst since February 12 when the AQI was 320.
An AQI between 0 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'.
Winds were blowing from the northwest, bringing pollutants from farm fires, according to an India Meteorological Department official.
The Ministry of Earth Sciences' air quality monitor, SAFAR, said the farm fire count around Haryana, Punjab and its nearby regions across the International Border on Saturday was 882.
"Since transport-level wind direction is favourable for intrusion, stubble contribution in PM2.5 is around 19 per cent on Saturday," it said.
The Ministry of Earth Sciences' Air Quality Early Warning System for Delhi said the ventilation index -- a product of mixing depth and average wind speed -- was 10,000 metre square per second on Saturday -- favourable for dispersion of pollutants.
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 average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants.
The impact of stubble burning on the national capital's air quality is likely to "increase significantly" by Monday, the Ministry of Earth Sciences' Air Quality Early Warning System for Delhi said.
Punjab and Haryana have recorded more incidents of stubble burning this season so far compared to last year, largely due to early harvesting of paddy and unavailability of farm labour due to the coronavirus pandemic, officials said on Saturday.
According to the Punjab Pollution Control Board, the state has recorded 4,585 farm fires this season so far compared to 1,631 such incidents during the corresponding period last year.
Haryana has also recorded an increase in farm fires -- from around 1,200 incidents till October 16 last year to 2,016 this year.
The Central Pollution Control Board (CPCB) had on Friday said meteorological conditions in Delhi have been "extremely unfavourable" for dispersion of pollutants since this September as compared to last year.
With less area under non-basmati paddy cultivation this time, CPCB Member Secretary Prashant Gargava hoped the number of stubble burning incidents will be fewer this year compared to 2019.
Non-basmati paddy straw is considered useless as fodder because of its high silica content and so farmers burn it.
Gargava also said stubble burning peak might not coincide with the peak of adverse meteorological conditions this year due to early harvesting of paddy.
(With inputs from PTI)