The severe Delhi SMOG of 2016: A case of delayed crop residue burning, coincident firecracker emissions, and atypical meteorology

Rising levels of air-borne suspended particulate matter with aerodynamic diameter <= 2.5 mu m (PM2.5) in the Asian mega cities is a serious cause of concern. A severe SMOG (smoke + fog) episode with air-visibility of about 5 km or shorter and average PM2.5 concentration of 793 mu g M-3 occurred in Delhi and the National Capital Territory (NCT) just after the Diwali-festival and spanned between 30 October (Oct) and 07 November (Nov.) 2016. Using daily variations in chemical and isotopic signatures of PM2.5 in tandem with meteorological parameters, we deduce here primary contributing factors responsible for development of Delhi-SMOG-2016 event and dominant transformation pathways of carbon, sulphur, and nitrogen compounds. Our multi-tracer analyses revealed there could be three major factors contributing to the SMOG event (i) transport of carbonaceous material from North-Western India (mainly Punjab-Haryana) due to open-field agricultural-waste burning that rose from 26 Oct through 11 Nov. 2016, (ii) weaker northerly winds, shallower boundary layer, cooler air temperatures, and enhanced humidity enforcing 'atypical' air-stagnation, and (iii) direct emissions from fire-cracker bursting on 30 Oct 2016 (Diwali night). Investigating timing of agricultural waste burning in northwester states of Delhi, there appears to be a delay in the timing of agricultural-waste burning in the Punjab area since 2010; that could possibly causing a gradual increase in severity of SMOG events in the recent years.