We applied two tandem differential mobility analyzer methods to shed light on the evolution of the nucleation- and Aitken-mode-particle compositions at a virgin boreal-forest site during nucleation events in varying conditions. The overall results show a clear anthropogenic influence on the nucleation- and Aitken-mode-particle compositions during the events. The SO₂/MTOP and NO_x/MTOP (monoterpene oxidation products) ratios best explain the variation in the nucleation mode composition during clean and pollution-affected events, suggesting also the importance of organic sulfur compounds, in addition to other sulfur, nitrogen and organic compounds, in particle formation, composition and properties. During the cleanest events, MTOP explain well the time behaviour of the 10-nm particle composition with an estimated organic fraction of over 95%.