Effective density of ambient black carbon aerosols: implications for morphological transformation and dry deposition

he effective density (ρeff) is a key parameter of black carbon-containing (BCc) particles and is related to their morphologies, deposition processes, and optical properties. In this study, a tandem system was established and used to determine the ρeff of ambient BCc particles. The results showed that the ρeff distribution of ambient BCc particles exhibited a bimodal pattern with a left peak located at 0.69 g/cm3 and a right peak located at 1.45 g/cm3. The average ρeff of BCc particles over the entire observation period was 1.38 g/cm3. The ρeff of BCc particles showed a clear diurnal pattern with a relatively stable distribution at night and large variations during the daytime. The ρeff value was demonstrated to be a good indicator of BCc particle morphology. BCc particles became more regular with increasing ρeff related to the increasing coating thickness. More coating led to morphological restructuring of BCc particles. The restructuring could be more efficient under high relative humidity conditions. The observed data was further used in a dry deposition scheme, it’s found the dry deposition velocity of fresh emitted BCc could be largely influenced by its irregular shape. This study reveals the presence of a significant amount of low-density/irregularly shaped black carbon in the environment with rapid morphological changes occurring during the daytime and highlights the necessity of considering morphological influences in future research on the physicochemical properties of BCc.