The Indo-Gangetic Plain is one of the largest sources of air pollution worldwide, and throughout winter strong fluctuations in the planetary boundary layer (PBL) height, driven by a strong radiative thermal inversion, affect the dispersion of this pollution. To date, tie-ins into aerosol-water vapour interactions, especially cloud condensation nuclei (CCN) activity, and the associated implications for aerosol indirect effects and hence on regional and global climate have been little studied. We present the results of a one-month field campaign (February-March 2018) in the polluted megacity of Delhi. The composition of fine particulate matter (PM1) and size-resolved CCN properties were measured over a wide range of water vapour supersaturations. PBL modelling, backward trajectories, and fire spots were included in the analysis to elucidate the influence of PBL and air mass origins on the aerosols. The aerosol properties depended strongly on the PBL height, with enhanced PM1 concentrations, high mass fractions of organic matter and BC, and low aerosol hygroscopicity during time periods of low PBL height (<100m). The observed correlations of PM1, aerosol particle number and CCN number with PBL height were parameterized by simple power law fit. Changes in PBL height induced changes in aerosol accumulation and aging processes, as manifested in aerosol composition and hygroscopicity. In contrast, aerosol properties did not depend strongly on air mass origins or wind direction, implying that the observed aerosol and CCN represented local emissions. The relationship between CCN number and supersaturation was well described by an error function parameterization.