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 &agr;- and &bgr;-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. ¿ 1999 American Geophysical Union