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Measurements of particle formation following the gas phase oxidation ofvolatile organic compounds (VOCs) emitted by Scots pine (Pinus sylvestris L.) seedlings arereported. Particle formation and condensational growth both from ozone(O3) 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 theconcentrations of O3 and OH. Particle formation and condensationalgrowth rates were interpreted with a chemical kinetic model. Scots pineemissions mainly included α-pinene, β-pinene, Δ3-carene,limonene, myrcene and β-phellandrene, composing morethan 95% of total emissions. Modeled OH concentrations in the O3-and OH-induced experiments were on the order of ~106 molecules cm−3.Our results demonstrate that OH-initiated oxidation of VOCs playsan important role in the nucleation process during the initial new particleformation stage. The highest average particle formation rate of 360 cm−3 s−1was observed for the OH-dominated nucleation events andthe lowest formation rate of less than 0.5 cm−3 s−1 was observedfor the case with only O3 present as an oxidant. In contrast to theparticle formation process, ozonolysis of monoterpenes appears to be muchmore efficient to the aerosol growth process following nucleation. Highercontributions of more oxygenated products to the SOA mass loadings fromOH-dominated oxidation systems were found as compared to the ozonolysissystems. Comparison of mass and volume distributions from the aerosol massspectrometer and differential mobility analyzer yields estimated SOAeffective densities of 1.34±0.06 g cm−3 for the OH+O3oxidation systems and 1.38±0.03 g cm−3 for the O3 dominatedchemistry

New particle formation from the oxidation of direct emissions of pine seedlings