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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 …