As Delhi and its neighbours spar over stubble burning, a Centre-run monitoring agency has identified a West Asian dust storm as the chief trigger behind the recent smog episode in the region.
On November 8, the contribution of the dust storm was 40 per cent, eclipsing the role of emissions from stubble burning, which stood at 25 per cent, the Pune-based System of Air Quality And Weather Forecasting And Research (SAFAR) said.
That was the day pollution levels peaked with PM2.5 concentration reaching 640 micrograms per cubic metre (ug/m3), according to the SAFAR scientific assessment report of the week-long pollution crisis accessed by PTI.
"Rest was made up of emissions from local sources such vehicular combustion. If external sources did not have any role, levels of PM2.5 during this period could have been around 200 g/m3," the report stated.
The subsequent imposition of emergency measures such as a ban on the entry of trucks and construction activities yielded positive results, the agency said, putting the gains at around 15 per cent in terms of percentage.
PM2.5 are ultrafine particulates, up to 30 times finer than a width of a human hair, which can lodge deep in the lungs and enter the bloodstream, causing irreparable harm to living beings.
The 24-hour prescribed standard of this variety of suspended particulate matter is 60 ug/m3.
SAFAR, an arm of the Indian Institute of Tropical Meteorology, said particulates from the dust storm, which swept across Iraq, Kuwait, and Saudi Arabia between the last week of October and November 4, entered the upper atmospheric layer of Delhi and the larger region.
Moreover, stubble burning at Punjab, UP and Haryana were very high on November 6 and as upper air winds became North Westerly (towards Delhi), pollutants were strongly pumped in, exacerbating the situation.
"As per SAFAR forecasting model, the pollution contribution of Gulf Dust Storm on peak day (8th Nov 2017) was around 40 per cent and 25 per cent from stubble burning," the report said.
Till about November 6, the mean PM2.5 level remained between 140-190 ug/m3, usual for this time of the year, SAFAR, the only agency which puts out pollution forecast bulletins in India, said.
Subsequently, from around November 10, there was no pumping of dust from West Asia. Influence of stubble burning also came down as upper air winds slowed down and changed direction, resulting in a brief respite from pollution.
"However, localized weather took over by the evening of November 11. Before pollutants could be flushed, inversion layer (beyond which pollutants cannot escape) fell down from 1600 metres to just 45 metres in eight hours and Delhi entered the severe zone again which delayed full recovery by two days," SAFAR said.