High Frequency of Urban New Particle Formation on the Tibetan Plateau: Quantifying Formation Rates, Growth Rates, and CCN Production

Abstract. New particle formation (NPF) strongly influences aerosol number size distributions and cloud condensation nuclei (CCN), yet its characteristics and climatic relevance in high-altitude urban environments over the Tibetan Plateau (TP) remain poorly constrained. We conducted in situ observations at Tibet University in Lhasa during the monsoon and pre-monsoon seasons. NPF event frequencies were 86.2 % during the monsoon season and 55.1 % during the pre-monsoon period. Observation days were classified into four types (Class I, Class II, Undefined, and Non-Event) based on the evolution of particle number size distribution (PNSD) and modal development. Seasonal comparisons indicate clear differences in the factors associated with NPF: monsoon season NPF was mainly associated with sulfur precursor availability, whereas no single clear controlling factor was evident during the pre-monsoon period. NPF substantially enhanced CCN at TU, with mean concentrations of 3351 ± 1919 cm⁻³ on Class I days, 1540.2 ± 872 cm⁻³ on Class II days, 880.5 ± 374 cm⁻³ on Undefined days, and 1278.1 ± 667 cm⁻³ on Non-Event days. The impact of NPF on CCN depends not only on nucleation, but also on the continued growth and survival of newly formed particles. This study provides new insights into urban NPF characteristics and the contribution of NPF to CCN on the TP, improving our understanding of aerosol–climate interactions in high-altitude urban environments.