Aerosol-cloud interaction remains a major source of uncertainty in climate forcing estimate. Our knowledge about the aerosol-cloud interaction is particularly weak in heavily polluted conditions. In this study, cloud residual (cloud RES) and cloud interstitial (cloud INT) particles were collected during cloud events under different pollution levels from 22 July to 1 August, 2014 at Mt. Tai (1532 m above sea level) located in the North China Plain (NCP). Transmission electron microscopy (TEM) was used to investigate size, composition, and mixing state of individual cloud RES and INT particles. Our results show that S-rich particles were predominant (78 %) during clean periods (PM 2.5 < 15 μg m −3 ), but a large amount of anthropogenic refractory particles (e.g., soot, fly ash, and metal) and their mixture with S-rich particles (named as S-refractory) were observed during polluted periods. Cloud droplets collected during polluted periods were found to become an extremely complicated mixture by scavenging of abundant refractory particles. We found that 76 % of cloud RES were S-refractory particles and that 26 % of cloud RES contained two or more types of refractory particles. Soot-containing particles (i.e., S-soot and S-fly ash/metal-soot) were the most abundant (62 %) among cloud RES, followed by fly ash/metal-containing particles (i.e., S-fly ash/metal and S-fly ash/metal-soot, 37 %). The complicated cloud droplets have not been reported in clean continental or marine air before. Our findings provide an insight into the potential impacts on cloud radiative forcing from black carbon, atmospheric biogeochemical cycle and metal catalysis reactions of SO 2 in micro-cloud droplets from soluble metals releasing from fly ash and metals in cloud droplets over polluted air.