Delhi's AQI surged from 117 on Saturday to 261 on Sunday. By Monday’s 4 PM bulletin, the index hit 324—crossing the 300-mark threshold for “very poor” air for the first time this season. The primary driver pushing Delhi’s air quality into the “very poor” category on Monday was smoke from evening farm fires in Punjab and Haryana, according to an analysis of weather, air-quality, and satellite data by HT.