New particle formation (NPF) in the polluted boundary layer arises from the clustering of electrically neutral compounds. Newly formed particles may affect concentrations of cloud condensation nuclei (CCN), thereby affecting cloud formation and global climate. The fundamental mechanism of NPF, especially the initial stages, remains largely unknown. However, it is commonly believed that neutral molecular clusters play an important role in processes that lead to boundary layer NPF. Knowledge of chemical composition, concentrations, and evolution of neutral molecular clusters is essential to better elucidating the mechanisms behind NPF and to better assessing the contributions of new particles to CCN formation. It also reduces the uncertainties for the nucleation rates used in global climate model.