Delhi's air quality deteriorated sharply over the weekend, with the Air Quality Index (AQI) entering the 'very poor' category on Monday. A major reason behind the sudden spike appears to be smoke from stubble-burning fires in Punjab and Haryana, which was carried towards the national capital by north-westerly winds.
An analysis by Hindustan Times, based on air-quality readings, weather observations and satellite data, found that several farm fires were started during the evening, potentially escaping detection by NASA's satellite during its routine afternoon pass.
Delhi's AQI rose from 117 on Saturday to 261 on Sunday. By Monday, the Central Pollution Control Board (CPCB) recorded an AQI of 324 at 4 pm, marking the first 'very poor' reading of the season.
Why weather conditions alone cannot explain the spike
Weak winds and calm nights had already created conditions favourable for pollution to accumulate near the ground. However, similar weather conditions were recorded on October 18 and October 28, when pollution levels were considerably lower.
Between 10 pm on November 1 and 6 am on Monday, Delhi's average PM2.5 concentration reached 194 micrograms per cubic metre, more than three times India's prescribed daily limit of 60 micrograms per cubic metre.
Unlike the earlier dates, pollution levels did not fall during the afternoon on November 1. PM2.5 concentrations remained slightly higher than morning levels, suggesting that polluted air was entering the city or that pollution at higher altitudes was also contributing to the spike.
The composition of particulate matter offered another clue. Fine particles accounted for nearly 60% of the total particulate matter on the night of November 1, compared with just over 40% on October 18 and October 28. Smoke largely consists of fine particles.
Why NASA satellite data may have missed farm fires
NASA's satellite generally passes over Punjab at around 1.30 pm, potentially missing fires ignited later in the day.
According to iFOREST's Stubble Burning Status Report 2025, more than 90% of large fires in Punjab during 2024 and 2025 were started after 3 pm.
India's INSAT-3DS satellite, which monitors the same geographical area throughout the day, recorded a rise in evening fires from 17 on October 29 to 90 on November 1. NASA's count also reached 171 fires on November 1, its highest this season.
How winds carried smoke towards Delhi
Weather balloon observations showed that transport-level winds came from the north-west on October 31 and November 1, carrying air from the stubble-burning regions of Punjab and Haryana towards Delhi.
In contrast, winds on October 28 came from the south, keeping smoke from the farm-fire belt away from the capital.
The analysis, however, could not establish the exact contribution of stubble burning to Delhi's pollution. High humidity following rainfall may also have contributed to the increase in fine particulate matter. Humidity crossed 90% on the nights of October 31 and November 1.