New insights into the chemical composition of organic aerosols from field studies in the boreal and mixed forest environment will be presented. The observed molecular patterns by chemical ionization mass spectrometry in the ambient particle phase during a new particle formation event will be discussed, and compared to the molecular pattern from oxidation experiments of biogenic volatile organic compounds at the CLOUD simulation chamber. Furthermore, the effect of the ambient pollutants NO_x and SO₂ on new particle formation and aerosol hygroscopicity will be discussed.