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NEW DELHI: Dust travelling thousands of kilometre from a severe storm in the Gulf contributed significantly to the weeklong killer smog that choked Delhi-NCR and much of north India from November 7 onwards, an analysis by the government’s air quality research body , SAFAR, has concluded.SAFAR said dust coming in from the Gulf constituted nearly 40% of pollutants in the smog while stubble-burning in Punjab and Haryana contributed around 25%. Making up the remaining 35% was pollution produced locally in Delhi-NCR .In a nightmarish confluence of factors, favourable upper winds carried the dust from the Gulf and smoke from crop burning into Delhi-NCR while an anti-cyclonic wind circulation over the region pushed these pollutants towards the surface and trapped them there as surface conditions were calm.The incursion of these pollutants began on the night of November 6 and continued till November 10, said Gufran Beig who heads SAFAR. “By 5pm on November 7, the air quality had dropped to severe levels,” he said.PM 2.5 concentration was recorded at 537 micrograms (µg)/m3 on November 7, nine times the 24-hour average standard. It rose to a peak of 640 g/m3 the next day. According to the SAFAR analysis , if external sources of pollution had not played a role, Delhi’s air quality on November 8 would have been closer to 200 g/m3.More significantly, the analysis said that the measures initiated by the environment pollution control authority (EPCA) under the graded response action plan (GRAP) may have helped reduce pollution by as much as 15%. That was the observed offset in SAFAR’s offline model between known anthropogenic and weather induced natural sources and observations from the ground.The incursion had ceased by November 10, SAFAR said. “There was no pumping and influence of stubble burning and Gulf storm dust after November 10 night, owing to slowing down of upper air winds and change in wind direction,” the analysis stated.The storm had hit Saudi Arabia, Iraq and neighbouring countries on October 29, sending a massive cloud of sand and dust into the air that was visible in NASA satellite images. SAFAR said the storm continued up to November 4. This dust rose 1.5 to 3km into the atmosphere “where winds became very strong (15-20kmph) and direction became towards India (westerly, north-westerly) and dust affected larger region of Delhi.”READ ALSO: Why coal plants will continue to spit fireWhile local weather conditions had taken over by November 11 evening, it took another couple of days for pollutants to get flushed out of the region, it said. This was because the inversion layer (the height at which a warmer layer of air exists) fell from 1,600m to just 45m in eight hours on November 11, trapping pollutants close to the ground and delaying recovery of air quality by another two days.