Delhi AQI crosses 400 on Diwali: The air quality deteriorated while Stage III of the Graded Response Action Plan (GRAP) is in force across Delhi-NCR. Delhi experienced alarmingly high pollution levels on Monday, the day after Diwali, with its Air Quality Index (AQI) reaching 'very poor' and 'severe' categories. Delhi’s AQI reached 373 on Diwali night, while several areas recorded severe pollution above 400.