While smoke from farm fires in Punjab and Haryana is believed to be driving the spike in Delhi's pollution, an unusually high number of stubble burning incidents in Pakistan is aggravating the crisis, satellite data shows. We explain the Pakistan factor behind the rise in Delhi's AQI. Delhi's annual pollution season appears to have arrived earlier than usual, with the air quality (AQI) deteriorating sharply to 'very poor' days before Diwali.