Delhi pollution worsens as winds slow down again

Delhi’s air quality, which briefly improved on Monday morning to enter the “poor” category for the first time in nearly a month, deteriorated sharply by the afternoon, returning to the “very poor” zone as wind speeds dropped significantly, trapping pollutants within the city again.
Strong surface winds (at speeds of 15km/hr) that had helped disperse pollutants on Sunday, dropped to near-calm levels of 4-6km/hr by Monday morning, leading to a haze engulfing the city again.
Delhi’s 24-hour average air quality index (AQI) was recorded at 285 (“poor”) at 9am on Monday, but by 4pm, when the Central Pollution Control Board (CPCB) releases the day’s national bulletin, it had dropped to 349 (“very poor”) and further to 391 by 10pm.
This was much higher than the 4pm reading of 318 (“very poor”) on Sunday. The last time Delhi’s AQI was below 300 was on almost a month ago — October 29, when it was 268.
Forecasts from the Centre’s Early Warning System (EWS) predict further deterioration in the coming days. AQI is expected to remain “very poor” through Tuesday and Wednesday, with Thursday likely to breach the “severe” category, which signifies an AQI over 400.
CPCB classifies AQI between 0-50 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”.
Out of 40 monitoring stations in the city, 14 reported severe AQI levels by 4pm, with Mundka and Bawana recording 436 and 422, respectively.
Weather impact on pollution
The drastic change in air quality underscores the dependence of Delhi’s air pollution levels on weather patterns.
A day after Delhi residents woke up to clear blue skies on Sunday, a rare sight this time of the year, Monday gave them a stark contest and an all-too familiar grey, hazy smog-filled morning – indicating the impact of wind speeds slowing down and thus trapping local pollutants over the city.
Meteorological experts said winds touched a speed of 15 km/hr during the day on Sunday, but began dipping after 6pm. “They mostly remained calm overnight and even after picking up slightly during the day, were between 4-6 km/hr on Monday,” said an India Meteorological Department (IMD) official.
CPCB’s real-time data showed Delhi’s average hourly PM 2.5 concentration was 223 micrograms per cubic metre (µg/m3) on Sunday at 6am, but an increase in wind speeds saw this drop down to 69µg/m3 by 5pm – almost close to the national PM2.5 standard of 60µg/m3. However, from 6pm onwards, this began to gradually rise, touching 116µg/m3 by 8 pm, 184µg/m3 at midnight and a peak of 394µg/m3 at noon on Monday – over six times the safe standard.
Sunil Dahiya, lead analyst at the think-tank Envirocatalysts said this shows the impact meteorological conditions can have on Delhi’s air quality. “We know Delhi has enough local sources which can lead to a spike in pollution every time conditions become calm. At the same time, you also have cross-boundary pollution in play, which can create a haze over Delhi-NCR,” he said.
Delhi’s air quality has been grim throughout November, with Saturday being Delhi’s eighth severe day this winter season. The first severe day was recorded on November 13, with the AQI touching 494 on November 18 — the joint second highest ever in the capital.
Monday also brought shallow fog, reducing visibility to 900 metres at Safdarjung in the morning. While the maximum temperature dropped to 25.8°C, a degree below normal, the minimum rose to 14°C, two degrees above average. Tuesday’s forecast predicts a high of 26°C and a low of 12°C.
Delhi reported shallow fog on Monday morning, with the lowest visibility of 900 metres recorded at Safdarjung. In terms of temperature, Delhi saw a dip in maximum, but the minimum temperature rose. Delhi’s maximum was 25.8°C on Monday, a degree below normal. It was 29.1°C a day earlier. The minimum meanwhile stood at 14°C on Monday, two notches above normal. It was 12°C a day earlier.