Delhi's air quality this November was the worst for the month in seven years with the city witnessing severe pollution on 11 days and not a single "good" air quality day, according to Central Pollution Control Board (CPCB) data.
Experts attributed this to the shifting of the peak stubble burning period by around a week due to a prolonged monsoon season.
The 30-day average of the capital's air quality index (AQI) stood at 376. It was 328 in 2020, 312 in 2019, 335 in 2018, 361 in 2017, 374 in 2016 and 358 in 2015, according to the CPCB.
An AQI between zero and 50 is considered "good", 51 and 100 "satisfactory", 101 and 200 "moderate", 201 and 300 "poor", 301 and 400 "very poor", and 401 and 500 "severe".
Delhi saw 11 "severe" air quality days in November this year, the highest in the month since the CPCB started maintaining air quality data in 2015.
Seven "severe" air quality days were recorded in the first half of the month, which saw rampant cracker bursting on Diwali and a sharp increase in farm fires. The share of stubble burning in Delhi's PM2.5 pollution stood at 41 per cent on November 6 and 48 per cent on November 7.
The city witnessed nine "severe" air quality days last year, seven in 2019, five in 2018, seven in 2017, 10 in 2016 and six in 2015.
The data also showed that the capital did not record even a single "good", "satisfactory" or "moderate" air quality day this month, while there were two "poor" and 17 "very poor" air quality days.
Gufran Beig, founder project director of air quality forecast agency SAFAR, said extreme pollution events -- Diwali and peak stubble burning period -- got shifted towards November this year due to the delayed withdrawal of the monsoon.
"This is the major reason why November saw poorer air quality this year as compared to the last few years," he said.
Dipankar Saha, former head of the CPCB's air laboratory said, "The emission sources for any defined area more or less remain the same throughout the year. However, the concentration varies with the season. November being the transition phase, the air quality in northern India is always problematic because of lowering of atmospheric boundary layer, wind speed, temperature and local action plan's implementation."
Anumita Roychowdhury, executive director (research and advocacy), Centre for Science and Environment, said stubble burning, which otherwise would have been done in October, got deferred due to prolonged rains.
"So, a lot of things happened together in the first week of November. Around Diwali, the meteorological conditions turned adverse and trapped local pollutants and farm fires peaked," she said.
This year, the southwest monsoon started receding from west Rajasthan and adjoining Gujarat on October 6, making it the second-most delayed withdrawal since 1975.
The withdrawal of the southwest monsoon from northwest India usually begins on September 17.
The extended monsoon season and record-breaking rainfall in October had given Delhi its best air quality for the month since 2015.