Air pollution can affect cloud formation in more than one way. When the pollutant gases are condensable (e.g. oxides of sulphur), then the process of aerosol activation is eased to a certain extent aiding cloud formation. However, polluted days are often characterised by low updraught speeds which inhibit aerosol growth. In this study, we have critically examined the aerosol activation process in a polluted coastal environment where both effects are present. We have concentrated on the Chennai region (one of the largest cities in the world) of the Indian sub-continent because its pace of industrialisation is increasing rapidly, adding to increasing SO2 pollution over the years. Air masses over Chennai contain a mixture of aerosol particles including NaCl, because of its proximity to the Bay of Bengal, along with ammonium sulphate. We have used observational data along with a detailed microphysical chemical parcel model to study cloud activation effects. We find that over Chennai, often the presence of the condensable pollutant vapour (SO2) more than compensates for the low updraught speeds by lowering the level of maximum super saturation significantly. This latter effect favours the activation of ammonium sulphate as well as NaCl aerosol particles. We have undertaken a systematic analysis to quantify the relative strengths of these two competing effects and find that even with low updraught speeds, oxides of sulphur can perturb the activation domain comprising a mixture of aerosol particles to such an extent that aerosol particles in polluted environments often grow efficiently. This effect is non-intuitive in the sense that one associates smaller cloud droplet sizes with polluted air masses. This is the first microphysical modelling study for the Indian sub-continent where National Environmental Engineering Research Institute (NEERI) observations have been applied to cloud microphysical processes. Copyright © 2009 Royal Meteorological Society.