Secondary organic aerosol particles (SOA) are important climate forcers, especially in otherwise clean environments such as the boreal forest. There are, however, major uncertainties in the mechanisms behind the formation of SOA, and in order to predict the growth and abundance of SOA at different conditions, process-based understanding is needed. In this study, the processes behind new particle formation (NPF) events and subsequent growth of these particles in the northern Europe sub-Arctic forest region are explored with the one-dimensional column trajectory model ADCHEM. The results from the model are compared with particle number size distribution measurements from Pallas Atmosphere-Ecosystem Supersite in Northern Finland. The model was able to reproduce the observed growth of the newly formed particles if a small fraction of the emitted monoterpenes that are oxidized by O 3 and OH undergo autoxidation and form highly oxidized multifunctional organic molecules (HOMs) with low or extremely low volatility. The modeled particles originating from the NPF events (diameter