New-particle formation (NPF) events are frequently observed in urban environments, yet their occurrence and climate effects are still in question. We analyzed the physical and chemical processes during the new-particle growth (NPG) at the ground and 260 m based on measurements on the 325-m tower in the megacity of Beijing, China. Our results provide evidence for stronger NPG aloft in city, mainly due to the higher production of sulfuric acid aloft, and the downwards transport of newly formed particles from the upper atmospheric boundary layer. The particle growth aloft is promoted by the higher relative humidity, facilitating gas-to-particle partitioning, especially of nitrate. Therefore, higher particle concentrations accompanied by stronger hygroscopicity lead to >20% higher NPG-induced cloud condensation nuclei aloft, demonstrating the importance of vertical differences in NPF for a better understanding of the climate effect in urban areas.