Drivers of the 2016 particulate matter pollution episode over northern India

Intense episodes of fine particulate matter (PM_2.5) pollution often overwhelm large areas of the  Indo-Gangetic Plain (IGP) in northern India during the post-monsoon season, a time when crop residue burning is at its peak. We conduct idealised emission sensitivity experiments using the WRF-Chem model to investigate the leading causes and spatiotemporal extent of one such extreme episode from 31 0ct - 8 Nov 2016, when hourly PM_2.5 levels exceeded 500 µg m^–3across much of the IGP on several days. We utilise the anthropogenic emissions from EDGARv5.0 and the latest FINNv2.5 for fire emissions and evaluate modelled and observed ambient PM_2.5 and black carbon  (BC) concentrations across the IGP. The model captured the PM_2.5 and BC peaks during the latter half of the episode and underestimated on other days. We find that biomass burning (BB) emissions during this episode have the strongest effect across the source regions in the upper (NW) IGP, followed by Delhi (middle IGP), where it contributes 50 - 80 % to 24-h mean PM_2.5. Complete elimination of BB emissions decreases PM_2.5 concentrations by 400 µg m^–3  (80 - 90 %) in the upper IGP and by 280 µg m^–3  (40 - 80 %) across the middle IGP during this episode. Contributions from the BB source to daily varying BC concentrations are 80-90 %, 40 - 85 % and 10 - 60 % across upper, middle and lower IGP, respectively. BB emissions dominantly contribute to daily mean secondary organic aerosols (80 %), primary organic aerosols (90 %), dust (60 %), and nitrate (50 %) components of PM_2.5 across the upper and middle IGP. In comparison, the anthropogenic share of these compounds was nearly one-third everywhere except across the lower IGP. The buildup of the episode across the middle IGP was facilitated by prolonged atmospheric stratification and stagnation, causing BB-derived BC and PM_2.5 to be trapped in the lowest 1 km. Our work emphasises the need for rigorous policy interventions during post-monsoon to reduce agricultural crop burning, together with targeted anthropogenic emissions control across the IGP, to minimise such extreme episodes in the future.