By using adequate sampling techniques we studied the chemical pathways, the gas‐to‐particle partition and the conversion processes leading to the formation of organic aerosols via the photo‐oxidation of biogenic hydrocarbons in the atmosphere over a conifer forest. Photo‐oxidation products of monoterpenes such as pinonic acid, nor‐pinonic acid, pinic acid, pinonaldehyde and nopinone were detected in the gas and particulate phases of all forest aerosol samples. Considering the diurnal concentration pattern of the photo‐oxidation products of α‐ and β‐pinene and Aitken nuclei concentration measured during the same periods, we observed that the acidic photo‐oxidation products of monoterpenes play a more important role in the formation of new particles than the corresponding carbonyl compounds.