This paper delineates the process of activation of soot or black carbon particles into cloud droplets over Chennai, India (an Asian megacity). Archaic wood-fired cooking stove emissions from crowded shantytown settlements dispense copious amounts of carbonaceous particles into the city’s atmosphere during morning and evening cooking periods. Traffic emissions also add considerably to the city’s particulate matter loads. At other times, when these sources are reduced (e.g. during non-cooking periods), a pall of residual pollution hovers above the city. This study assesses this problem definitively through process modelling studies undertaken during a typical pre-monsoonal month (October) when the levels of pollution are very high. The novelty of this paper includes a clear quantification of the percentage of activated nuclei resulting from these sources using a sophisticated cloud chemical parcel model. The results show that 85% of the soot and black carbon particles are activated into very small low-lying cloud droplets within Chennai’s boundary layer. We further show for the first time that the presence of these particles results in a slowing down of the auto-conversion process (cloud water to rain droplets). This study could serve as a pointer for other Asian Cities with a sizeable shantytown population in India and elsewhere. © 2019, Springer-Verlag GmbH Austria, part of Springer Nature.