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

Abstract

Rising levels of air-borne suspended particulate matter with aerodynamic diameter ≤ 2.5 μm (PM_2.5) 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 PM_2.5 concentration of 793 μg 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 PM_2.5 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.