The share of stubble burning in Delhi's PM2.5 concentration was 6 per cent on Thursday, according to a central government agency.
It was only around one per cent on Wednesday and around 3 per cent on Tuesday, Monday and Sunday, according to the Ministry of Earth Sciences' air quality monitor, SAFAR.
PM2.5 refers to fine particulate matter less than 2.5 microns in diameter which can penetrate deep into the lungs, impairing their function, and even the bloodstream.
A layer of smoky haze lingered over Delhi-NCR on Thursday with air quality in the region hitting 'very poor' levels, even as stricter anti-air pollution measures, including a ban on electricity generators, came into force under the Graded Response Action Plan (GRAP).
"Delhi's overall AQI was recorded in the very poor category. The calm and variable surface wind condition continues and the AQI is likely to remain very poor on Friday," it said.
According to SAFAR data, the contribution of farm fires to Delhi's PM2.5 concentration was around 6 per cent on Thursday.
An increase was observed in stubble burning incidents around Haryana, Punjab, and neighbouring border regions on Wednesday. The farm fire count was 740, it said.
The wind direction is partly favourable for transport of smoke from farm fires and "hence, an increase in contribution in Delhi's PM2.5 is expected", SAFAR said.
NASA's satellite imagery also showed a large cluster of farm fires near Amritsar, Patiala, Tarn Taran, and Firozpur in Punjab, and Ambala and Rajpura in Haryana.
The Union Environment Ministry said the share of stubble burning changes every day.
"Last year, between 08 Oct-09 Dec, the share of Stubble burning in #AirPollution in Delhi (as per SAFAR data), was greater than15% on six days, while on a single day it was greater than 40%, it tweeted.
The Punjab Pollution Control Board has reported 3,517 farm fires so far this season as compared to 1,217 in the corresponding period last year.
The Haryana Pollution Control Board has reported 1,710 stubble burning incidents against 1,072 in the corresponding period last year.