Measurements of particle formation following the gas phase oxidation of volatile organic compounds (VOCs) emitted by Scots pine ( Pinus sylvestris L.) seedlings are reported. Particle formation and condensational growth both from ozone (O 3 ) and hydroxyl radical (OH) initiated oxidation of pine emissions (about 20-120 ppb) were investigated in a smog chamber. During experiments, tetramethylethylene (TME) and 2-butanol were added to control the concentrations of O 3 and OH. Particle formation and condensational growth rates were interpreted with a chemical kinetic model. Scots pine emissions mainly included α-pinene, β-pinene, Δ 3 -carene, limonene, myrcene and β-phellandrene, composing more than 95% of total emissions. Modeled OH concentrations in the O 3 - and OH-induced experiments were on the order of ~10 6 molecules cm −3 . Our results demonstrate that OH-initiated oxidation of VOCs plays an important role in the nucleation process during the initial new particle formation stage. The highest average particle formation rate of 360 cm −3 s −1 was observed for the OH-dominated nucleation events and the lowest formation rate of less than 0.5 cm −3 s −1 was observed for the case with only O 3 present as an oxidant. In contrast to the particle formation process, ozonolysis of monoterpenes appears to be much more efficient to the aerosol growth process following nucleation. Higher contributions of more oxygenated products to the SOA mass loadings from OH-dominated oxidation systems were found as compared to the ozonolysis systems. Comparison of mass and volume distributions from the aerosol mass spectrometer and differential mobility analyzer yields estimated SOA effective densities of 1.34±0.06 g cm −3 for the OH+O 3 oxidation systems and 1.38±0.03 g cm −3 for the O 3 dominated chemistry.