New particle formation (NPF) is commonly characterized by particle formation rates (FR) and growth rates (GR) derived from fixed-site Eulerian observations. However, particles detected at different times in Eulerian observations do not necessarily represent the evolution of the same air parcel, so the observational time intervals used to derive FR and GR may differ from the corresponding in-situ formation and growth timescales. Here, we examine the representativeness of the observational time interval (Δt_observe) in the calculation and interpretation of FR and GR using synchronous high-time-resolution measurements from multiple particle instruments in a semi-enclosed indoor environment and ambient air in southeastern Hainan, China. Nine short-term NPF cases were analyzed to determine the observational intervals associated with particle number increase and modal-size evolution. The results show that substantial increases in newly formed particle number concentration and particle growth to tens of nanometers can occur within several to tens of minutes, and that the derived FR of 2.7–21 cm^-3 s^-1 and GR of 22–186 nm h^-1 were highly sensitive to the selected Δt_observe. Comparison of the FR-NMINP (net maximum increase in the nucleation-mode particle number concentration) and GR-D_pgmax (maximum geometric median diameter of the grown new particles) relationships between the NPF events in this study and previously reported regional NPF events further revealed substantially lower slopes (by approximately 1–2 orders of magnitude) for events characterized by longer Δt_observe, suggesting that such intervals may not adequately represent the true formation and growth timescales of newly formed particles.