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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. [ABSTRACT FROM AUTHOR]

Urban sprawl comprising densely populated slums over South Asian cities yields copious amounts of soot and black carbon from archaic cooking methods involving cow dung cakes and firewood, which remain afloat for over 10–12 h, enabling them to age in a sulphur rich environment. Not only are there toxicological concerns arising out of improper ventilation mechanisms, but there are also other concerns impacting the local microclimate. These emissions mix with other aerosol particles and, when conditions are favourable, are rendered partially soluble, enabling them to activate into cloud condensation nuclei. This study first yields a quantification of the soluble mass fraction and subsequently shows how aerosols from this local area source mix with background aerosol modes to perturb the local cloud microphysics over Chennai, a megacity in Southern India. On‐site sampling was undertaken to find the mass concentrations of the collected deposits separately from cow dung and firewood fuel. Additional micro‐physical attributes, including the morphological indentations that served as a receptacle to contain the accreted sulphate along with the particle size distribution were ascertained through Scanning Electron Microscopy. It is shown that accreted sulphate on carbonaceous particles facilitates CCN activation over the city. We show through large‐eddy simulations (LES) that extensive slum emissions over the study region contribute to the observed local cloud cover and enhanced rain amounts over a densely built‐up area housing the city's most vulnerable citizens. [ABSTRACT FROM AUTHOR]

The characteristic features of inter-seasonal variability of precipitation microphysics for the year 2019 over Mumbai and Chennai – the two urban-coastal cities of Indian sub-continent have been studied in this paper. The importance of choosing the regions lies in the fact that these two cities are well-known for the occurrences of severe rainfall events due to the effect of south-west and north-east monsoon during the summer and winter season of Indian sub-continent respectively. It has been observed that raindrops of diameter 2‐–4 mm dominate the summer rainfall over Chennai with respect to winter rainfall. Correspondingly, while comparing the two cities together, the impact of the south-west and north-east monsoon in conjunction with the interplay of the continental and maritime effect on the microphysics of precipitation over these cities is strongly visible in the analysis. The study reveals that the rainfall evolving from the continental clouds along with contrasting surface temperature produces distinct diurnal variation for the summer (winter) rainfall over Chennai (Mumbai) while such variations are not visible for the maritime clouds. It has been observed that the drops of larger diameter along with higher radar reflectivity dominated the summer rainfall of Chennai with respect to Mumbai. While for the winter rainfall, such distinct variations are visible only for higher rainfall regimes from 16 mm/h and above where the domination of larger drops is visible for the Mumbai rainfall. Thus the difference in surface temperature which in turn escalates the convective environment in the atmosphere can be termed as one of the important factors responsible for this inter-seasonal variability of precipitation microphysics. • Typical characteristics of rainfall and wind direction during inter-seasonal phases • Contrasting features of precipitation microphysics during summer and winter monsoon • Strong impact of continental and maritime clouds on precipitation microphysics • Important role of surface heating for diurnal variation of precipitation [ABSTRACT FROM AUTHOR]