Delhi air at season’s worst as winds slow down dispersion

After better-than-expected air quality levels recorded post-Diwali — air quality indices (AQIs) of 339 and 316 (both “very poor”) on Friday and Saturday, respectively, led to the second-best air quality post-Diwali in nine years — Delhi’s air quality rapidly deteriorated over the past 24 hours, clocking the season’s worst AQI reading of 382, on the cusp of “severe” air level, on Sunday, according to the Central Pollution Control Board’s (CPCB’s) daily bulletin.
Calm winds and dipping mercury levels led to a deterioration of over 60 points over the past day, experts said, as the minimum temperature on the day was 16.5 degrees Celsius (°C), which dipped gradually from 21.1°C recorded on October 31.
Experts said that the temperature dip, coupled with wind direction transitioning from northwesterly to southeasterly, led to a calm over Delhi. “We expected a spike in pollution on Sunday as whenever wind direction changes, you see a brief period where winds are almost calm. Wind direction was variable in the last 24 hours, predominantly northwesterly, but will switch to southeasterly by Monday. We can expect wind speeds to be calm overnight and below 10 km/hr during the day till Tuesday,” said Mahesh Palawat, vice president at Skymet meteorology.
He forecasted a slight increase in wind speed from Wednesday.
In its forecast for Monday, the India Meteorological Department (IMD) said there were chances of “smog/mist” in the Capital towards nighttime, with winds to remain below 10 km/hr. Delhi is yet to see shallow fog this winter — when visibility dips below 1,000 metres. The lowest visibility in the Capital over the past few days has been between 1,000 and 1,500 metres, officials said.
“The wind speed will gradually increase going up to 8 km/hr during the afternoon. It will decrease thereafter, becoming less than 5 km/hr during evening and night-time. Smog/mist is likely in the night,” the IMD said in its daily forecast for Monday.
Winds impact air quality
The last time Delhi recorded a worse AQI reading this year was on January 31, when it was 392 (“very poor”).
Delhi’s average AQI on Sunday was calculated based on data from 39 of Delhi’s 40 ambient air quality monitoring stations, with both PM 2.5 (particulate matter having diameter less than 2.5 microns) and PM 10 dominant pollutants. At 4pm, 14 of these stations clocked “severe” AQIs, including Anand Vihar (436) and Rohini (435). The lowest average AQI was 275 (“poor”) at DTU and Dilshad Garden.
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”.
The Commission for Air Quality Management (CAQM), meanwhile, said it was keeping a tab on Delhi’s air quality.
Currently, stages 1 and 2 of the Graded Response Action Plan (Grap) are in place in Delhi-NCR, and Stage 3 may be invoked if the AQI touches “severe”. “No meeting on Grap was held on Sunday, but officials are keeping a tab on the AQI,” a CAQM official said.
Before the past weekend, strong winds of 10-15 km/hr, even at nighttime, were favourably impacting Delhi’s AQI, so much so that a major chunk of firecracker emissions, released on both Thursday and Friday, was almost simultaneously dispersed.
Delhi’s AQI on Diwali (Thursday) was 328 (“very poor”), and increased marginally to 339 (“very poor”) the next day. This was the second lowest AQI on the day after Diwali in the past nine years, behind 2022, when the AQI was 302.
Realtime data shows that the AQI improved briefly to “poor” level on Saturday morning, before deteriorating sharply due to wind speeds dropping.
“With winds transitioning from northwesterly to easterly, they became almost calm in the second half of Saturday. The same condition prevailed in the early hours of Sunday,” the IMD official said.
Temperature’s role
Delhi’s minimum temperature has dipped by nearly four degrees in the past four days. The minimum was 21.1°C, five degrees above the normal on October 31, and it fell to 16.5°C on Sunday, which was a degree above the normal. It was 17.1°C a day earlier.
Low temperature impacts air quality negatively, slowing down the dispersion of pollutants and bringing down the mixing height — an invisible layer of the atmosphere within which pollutants get trapped.
The Early Warning System for Delhi (EWS), under the ministry of earth sciences, said that the mixing height on Sunday was around 1,100 metres, down from around 2,100 metres on Friday. “Delhi’s air quality is likely to be in the “very poor” category from Sunday till Wednesday. Meteorological conditions are likely to be extremely unfavourable for dispersion of pollutants in the coming days,” EWS said on Sunday.
Despite the deterioration, the contribution of stubble burning to Delhi’s air dropped in the past 24 hours. The contribution of farm fires to Delhi’s PM 2.5 on Saturday was 14.96%, down from a season-high of 35.1% on Friday, according to data from the Centre’s Decision Support System (DSS).
It meanwhile estimated vehicular pollution accounted for 12.24% of Delhi’s pollution on Sunday, followed by 11.36% from neighbouring Gautam Budh Nagar in Uttar Pradesh.