Dust storm in West Asia, not stubble burning, main trigger for Delhi toxic smog
On November 8, the contribution of the dust storm was 40%, eclipsing the role of emissions from stubble burning, which stood at 25%, the Pune-based System of Air Quality And Weather Forecasting And Research said.
A dust storm from the Gulf was the main reason behind the recent weeklong smog episode in Delhi and adjoining areas, a System of Air Quality And Weather Forecasting And Research (SAFAR) report has said. (LIVE UPDATES)
On November 8, when the AQI was an alarming 478, SAFAR said the contribution of the dust storm from Iraq, Kuwait, and Saudi Arabia was as much as 40%, whereas that of stubble burning was 25%.
PM2.5 concentration on that day was 640ug/m3, which is nearly 11 times higher than the 24-hour prescribed standard, 60ug/m3.
“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 µg/m3,” the report stated.
The emergency measures, put in place under the graded response action plan, such as ban on entry of trucks and construction activities had a positive effect, it said, adding that the impact was around 15%.
The report said that 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.
Stubble burning in Punjab, Uttar Pradesh 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.
“Dust storm is also a reason behind smog in Delhi and its neighbouring areas. But we cannot ignore the contribution of stubble burning as smoke is also adding to the problem,” S Narayanan, member secretary, Haryana Pollution Control Board, told HT.
Punab differed. “We have been already been saying that stubble burning was not only the reason behind smog. In Punjab, cases of straw burning declined by 40%. Against 70,000 straw burning cases last year, only 40,000 cases have been reported so far,” said Charanjeet Singh, a scientist with the Punjab Pollution Control Board.
“As per SAFAR forecasting model, the pollution contribution of Gulf dust storm on peak day (Nov 8 2017) was around 40% and 25% from stubble burning,” the report said.
Till November 6, the mean PM2.5 level remained between 140 and 190 ug/m3, usual for this time of the year, SAFAR, the only agency which puts out pollution forecast bulletins in India, said.
From 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, localised weather took over by the evening of November 11. Before the pollutants could be flushed, the inversion layer (beyond which pollutants cannot escape) fell down from 1,600 metres to just 45 metres in eight hours and Delhi entered the severe zone again which delayed full recovery by two days,” SAFAR said.
From 354 on November 6, the air quality index reached the “severe” level at 448 on November 7. On 8th, it reached 478, 486 on 9, 468 on 10, 403 on 11, and 460 on both 12 and 13. On November 14, after a week, the AQI came down substantially to 308, while on 15, it reached 361.On Thursday, November 16, it reached 363.