NEW DELHI: The city Wednesday logged its first poor air-quality day in 100 days. More remarkably, this run of relatively clean air had extended this year till Oct 20 — the farthest into the post-monsoon period since Air Quality Index (AQI) records began in 2015 — shielding Delhiites from toxic air for longer during the festive season than ever in recent years.However, the clean-air party appears to have finally ended. Projections for the next few days and expert assessments of the outlook in the weeks ahead suggest that Delhi’s air quality is set to deteriorate. Late Oct to late Nov is also the period when a storm of factors — dipping temperatures, still air, stubble burning in neighbouring states and Diwali firecrackers — come together to keep the region’s air quality at severe levels for days on end.“Air quality is likely to dip substantially by the end of Oct. The week following Diwali will be particularly challenging for residents of Delhi-NCR, going by indications from a number of factors,” said Gufran Beig, atmospheric scientist and chair professor at National Institute of Advanced Studies, Bengaluru.But first, a look back at the exceptional run of relatively clean air that Delhi has experienced. The 99-day period of better-than-poor AQI from July 14 to Oct 20 isn’t the longest such streak in the city. Last year, for instance, the streak ran for 124 days, the longest on record. Due to rain at regular intervals, the monsoon period has the cleanest air of the year in Delhi.This year’s clean-air spell was unprecedented on two counts. One, the period from June 1 (start of the monsoon season) till Oct 20 recorded just three poor-air days. The previous lowest for this period was 10 days, seen in 2021 and 2023. So, the city this year was largely free of poor air quality for nearly five months. Two, this extended run was possible because Oct remained relatively clean during the first 20 days. This is unprecedented in AQI records. In 2021, which recorded the cleanest Oct till date, the first poor-air day was logged on the 16th of the month.Weather appears to have played a major role in keeping Delhi’s air clean during this period, with rain spells at regular intervals. The monsoon arrived in Delhi on July 2. Even before that, the month of June saw very light-to-light rain on at least eight days, spaced more or less equally across the month. Delhi govt’s efforts to keep dust levels down would also have played a role.In Oct, a dip in temperatures and low wind conditions usually lead to the accumulation of pollutants. This year, however, temperatures remained higher than normal in the initial days of the month, followed by two spells of rain on Oct 12 and 16.Conditions have since changed. Temperatures have fallen below normal over the past week, wind speeds are low and so are the chances of rain. Plus, around the corner are the additional sources of pollution the region usually gets during this season — stubble burning and firecrackers.A relatively late Diwali this year on Nov 8, when temperatures are likely to have further dipped, doesn’t bode well for the region’s air quality. The outlook becomes even bleaker when you consider that Diwali coincides with the peak stubble-burning period of Nov 1 to 10.Research has shown that while the burning of paddy residue in Punjab and Haryana has reduced in the past two years, the reduction is nowhere close to what the fire counts from satellite data suggest.“The satellite data used by Indian agencies has been shown to miss around 90% of stubble-burning incidents. Plus, this year, the paddy harvest has been delayed. I expect a spurt in burning during the week following Diwali, when the air quality in Delhi-NCR is likely to worsen,” Beig said.
