Contrasting features of winter-time PM pollution and PM-toxicity based on oxidative potential: A long-term (2016-2023) study over Kolkata megacity at eastern Indo-Gangetic Plain.

The present study is an attempt to understand the level of PM pollution and its toxicity based on the oxidative potential (OP) during the winter-time pollution period over Kolkata, a megacity at the eastern most parts of Indo-Gangetic Plain (IGP) during the period of 2016-2023. We have assessed the effectiveness of the Government of India's national mission, the National Clean Air Program (NCAP) in PM reduction over this city, and the study revealed that the mission has been efficacious in lessening the PM load by 28 % from pre-NCAP (2016-2019) to post-NCAP (2021-2023) periods. Several policy interventions reduced the contributions from various anthropogenic sources; however, biomass/solid waste burning remained a major concern with no significant reduction. The results revealed that the volume-weighted OP (OPv) remains mass-independent and the same when PM remains within 70 μg m (OPv range between 2.7 and 3.1 nmol DTT min m). With the rise in PM mass from 70 μg m, OPv boosts up sharply and reaches its peak (at ∼145 μg m during pre-NCAP and ∼105 μg m during post-NCAP) followed by an insignificant change with the further rise in PM. We observed that biomass/solid waste burning is the major concern over Kolkata in the current scenario (post-NCAP) even after NCAP policy interventions. Such high OP-based toxicity of PM during post-NCAP periods could be minimized if actions are taken against this particular source.