The Capital continued to choke on Wednesday, with its average air quality index (AQI) remaining ‘Severe’ for the second consecutive day at 418 and stubble burning in the neighbouring states contributing most to Delhi’s bad air. This is the first time this season that the share of stubble burning as a pollution source has surpassed other emissions.Delhi had recorded its first ‘Severe’ air quality day of the season on Tuesday, with the overall AQI reaching 428 – the worst since December 19 last year.According to data from the Decision Support System (DSS) of the Indian Institute of Tropical Meteorology (IITM), stubble burning contributed 22.47% to Delhi’s PM2.5 concentration on Wednesday – the highest share recorded so far this season. It surpassed transport emissions recorded at 15.59%, the data showed.For Thursday, stubble burning is projected to contribute around 10.1% to Delhi’s PM2.5 levels, while transport-related emissions are expected to increase to 19.3%, according to DSS projections.Data from the Indian Agricultural Research Institute (IARI) showed 1,477 stubble burning incidents on Monday – 155 in Punjab, 29 in Haryana, 266 in Uttar Pradesh, 159 in Rajasthan, and 868 in Madhya Pradesh.Between September 15 and November 12 this year, the six states had recorded 13,793 farm fires – 4,662 in Punjab, 464 in Haryana, 2,490 in Uttar Pradesh, four in Delhi, 1,736 in Rajasthan and 4,437 in Madhya Pradesh, according to IARI’s bulletin.The contribution of stubble burning had been gradually increasing since late October – it rose from about 2-4% in between October 26 and November 5 to around 10% on November 6. It then rose to 13.68% on November 10, 15.45% the next day, and finally peaked at 22.47% on Wednesday.Story continues below this adThe sharp deterioration in Delhi’s AQI this week, which had been ‘very poor’ for several days, was due to unfavourable local weather conditions and transport emissions. The Capital has been witnessing calm surface winds, lower temperatures, and strong nighttime inversion, all of which can trap pollutants close to the surface and prevent their dispersion. While Westerly winds, blowing at 10-17 kmph, have been carrying smoke from Punjab and Haryana towards Delhi, local sources such as vehicular emissions and road dust continue to add to the pollutant load.The IITM forecast on Wednesday said the AQI is “very likely to remain in the Very Poor category from November 13 to 15”, with a similar trend expected for the following six days.According to India Meteorological Department (IMD), the night temperature on Wednesday was recorded at 10.4 degrees Celsius, nearly three degrees below normal, while the maximum settled at 27.7 degrees Celsisus, about a degree below normal.Since Monday, night temperatures have hovered around 10 degrees Celsius – the lowest for this season so far. The IMD forecast mainly clear skies with mist or shallow fog during the morning until November 18.Story continues below this adWith the AQI slipping into the ‘Severe’ range, the Commission for Air Quality Management (CAQM) had on Tuesday invoked Stage III of the Graded Response Action Plan (GRAP). This entails a ban on non-essential construction and demolition activities, a switch to hybrid mode for primary classes, and prohibits entry of BS-III petrol and BS-IV diesel light motor vehicles into the Capital.