A thick plume consisting of sulphur dioxide (SO2) and volcanic ash touched upper levels of the atmosphere over Delhi at around 11pm on Monday night, but did not impact Delhi’s air quality index (AQI) significantly, even as air quality stations reported minor spikes in PM 2.5 and SO2 levels around midnight.
The impact of this spell over Delhi — at upper levels of the atmosphere between heights of 35,000-45,000 feet — was visible for a couple of hours, before moving further eastwards – towards the hills.
Delhi’s AQI stood at 362 at 9am on Tuesday. It was 370 at 11pm, when the outer limit of the ash cloud reached over the national Capital — indicating there was no significant impact on surface level pollution.
HT also assessed real-time air quality data from stations across the city, observing only minor spikes at some places.
For instance, at Mandir Marg, the PM 2.5 levels were recorded at 173µg/m3 (micrograms per cubic metre) at 11pm, which rose to 185µg/m3 at midnight, before marginally dipping to 184µg/m3 at 1am. A second spike – to 218µg/m3 came at 2am. The safe standard for PM 2.5 is 60µg/m3.
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SO2 levels also shot up marginally here. The concentration was 41.3µg/m3 at 11pm, rising to 61.8µg/m3 at midnight and 61µg/m3 at 1am, before dipping. The SO2 standard is 80µg/m3, which was not breached.
At Anand Vihar, the PM 2.5 levels shot up from 328µg/m3 at 11pm to 382µg/m3 at midnight but dipping again to 367 at 1am. The SO2 levels there increased from only 9.2µg/m3 at 11pm, to 10.6µg/m3 at midnight and 12.9µg/m3 at 1am.
At Punjabi Bagh, the PM 2.5 levels shot up from 306µg/m3 at 11pm to 345µg/m3 at midnight, DPCC data showed. The SO2 levels there in fact marginally dipped – from 27.6µg/m3 at 11pm, to 21.2µg/m3 at midnight, indicating no fixed trend in the pollution spikes, or direct correlation to the ash plume advancing.
Experts said this was largely down to the height at which the pollutants reached Delhi-NCR, moving further rapidly at speeds of 100-120 km/hr, which made surface level impact more difficult.
“The ash plume mostly consisted of SO2 with low to moderate concentrations of volcanic ash. It did not impact AQI levels in Delhi, but it will impact the SO2 levels in the mountains of Nepal, Himalayas and adjoining Terai belt of Uttar Pradesh,” said Ashwary Tiwari, who runs the handle IndiaMetSky on X, stating the plume moved at a height between 30,000-45,000 feet — far above the surface to cause accumulation of pollutants at lower levels.
He added while isolated ash particles were observed at the surface, the height at which this event occurred – made its impact more prominent for flights.
Airlines have begun cancelling some flights post noon on Monday, with the Directorate General of Civil Aviation (DGCA) also issuing an advisory for airlines on Monday evening.
Airlines IndiGo, Air India, and Akasa Air have assured passengers that safety remains their top priority following the eruption of Ethiopia’s Hayli Gubbi volcano, whose ash clouds continue to drift towards parts of India.
The plume of ash touched parts of Rajasthan around 6:30pm, advancing further through more parts of northwest India.