This may be Delhi’s worst air quality streak

New Delhi
Weaker fog and surface winds of around 10km/hr failed to provide respite from air pollution, as Delhi recorded a 24-hour average air quality index (AQI) of 419 and was classified to have “severe” levels of pollution for the fifth day in a row and for a seventh time in the past eight days—effectively marking one of the worst pollution spells experienced by the Capital—according to the Central Pollution Control Board’s (CPCB’s) daily bulletin.This was, however, an improvement from the AQI reading of 460 (“severe”) recorded on Tuesday.
The five-day streak of “severe” air was also the longest since December 2021, when Delhi recorded six consecutive days between December 21 and 26. But it comes with a caveat: the AQIs preceding and succeeding the six-day period, i.e., on December 20 and December 27 of 2021 were 332 (“very poor”) and 283 (“poor”), respectively. In contrast, in the eight-day period from November 13 to 20 this year, the AQI on November 15 was 396 (“very poor”), which was on the cusp of “severe” air, which it duly deteriorated to.
While there was little cause for celebration on Wednesday, the AQI nevertheless improved in the evening hours, recording a reading of 408 (“severe”) at 7pm and 400at 10pm.
The CPCB classifies AQI between 0 and 50 as “good”, between 51 and 100 as “satisfactory”, between 101 and 200 as “moderate”, between 201 and 300 as “poor”, between 301 and 400 as “very poor”, and over 400 as “severe”.
Respite on the cards?
According to Centre’s Early Warning System (EWS) forecasts, the AQI is expected to stay in the “severe” range on Thursday too, but may improve to “very poor” on Friday as meteorological conditions gradually improve.
Experts said that with fog intensity reducing in the region, the Capital clocked bright sunshine during the day, a factor which was missing since Monday, when Delhi’s AQI touched 494 — its joint second-highest ever. Delhi recorded shallow fog on Wednesday, with sunshine recorded in parts of the city after 10am, leading to a gradual improvement in the AQI through the day.
“Once again, shallow fog was reported from the Capital. The thick layer of fog over northwest India has weakened,” an official with the India Meteorological Department (IMD) said.
The minimum visibility on Wednesday was 500 metres at the Safdarjung observatory and 600 metres at the Palam weather station.
Mahesh Palawat, vice president at Skymet meteorology, said fog did not lift on Monday, ensuring sunlight could not penetrate through. With wind speeds picking up from Tuesday, a significant portion of fog began to clear from northwest India, he said.
“These are northwesterly winds, which are cold and dry, so moisture is reducing. These winds have also pushed fog away from Punjab, Haryana and Delhi, thus reducing its intensity. Once that happens, you see sunshine and this leads to an improvement in AQI. We expect similar shallow fog on Thursday and Friday, meaning sunshine is likely and the air quality should improve further,” he said, adding surface winds were also up to a speed of 10km/hr during the day, helping disperse pollution.
But northwesterly winds continued to bring stubble intrusion to the Capital as well. Data from the Decision Support System (DSS) showed the contribution of farm fires to Delhi’s PM 2.5 was 19.82% on November 18 and 22.10% on November 19.
HT reported on Wednesday how DSS revised its stubble contribution percentage for November 13-15 by assuming firecrackers were burst at weddings in Delhi on the basis of news reports. DSS estimated 50 firecrackers per wedding, before re-running its model. The website did not mention if the same had been done for November 18 or 19.
Delhi, a mixed bag
According to CPCB data, the average AQI for the day was calculated on inputs from 37 of 40 weather stations in the Capital. PM2.5 (particulate matter with a diameter of 2.5 microns or less) was the lead pollutant. At 4pm, 35 of the 40 stations had AQIs in the “severe” zone, with Major Dhyanchand station, at 353, and NSIT Dwarka, at 396, being the only exceptions. The highest AQI, 461, was recorded at the Wazipur station.
Prior to this streak of deteriorating air quality, Delhi recorded a six-day streak of “severe” air in December 2021, and a a seven-day streak in each of November 2016 (from 5 to 11) and November 2017 (from 7 to 13). However, Delhi only had 17 AQI stations in 2017 and nine in 2016.
IMD data showed that although day-time temperature is rising and getting warmer, nights are getting cooler. Delhi recorded its lowest minimum of the season on Wednesday, at 11.2°C, which was a degree below the normal and down from 12.3°C recorded a day earlier. The maximum temperature was 26.8°C, a degree below the normal but 1.4°C higher than that of Tuesday’s.
With shallow fog, no diversions were reported at Delhi’s Indira Gandhi International Airport on Wednesday, according to flight tracking website Flightradar24. However, over 300 flights were delayed and eight were cancelled. Northern Railways, meanwhile, reported late arrival of 13 trains due to fog.