A Centre-run monitoring agency has identified a West Asian dust storm as the chief trigger behind the recent smog episode in the region as Delhi and its neighbours' spar over stubble burning. 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 or 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. 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. 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.