This paper describes a set of studies which has attempted to make a first estimate of the contribution of biogenic emissions to secondary organic aerosol (SOA) formation over the Nordic countries. The studies comprise first simulations of smog-chamber experiments and second atmospheric modeling. The α-pinene chemical mechanism suggested by Kamens et al. <1999> has been extended for use in regional oxidant models. Inclusion of dimer-formation in this scheme substantially improved performance for some experimental conditions. Although the modified scheme is necessarily tentative, it fits with experimental findings over a wide range of conditions, covering α-pinene concentrations of between 20 and 900 ppb and NOx concentrations of between zero and 240 ppb. The EMEP oxidant model has been used to estimate SOA formation in northern Europe using this chemical scheme and modified gas/particle partitioning methods. The results suggest that biogenic volatile organic compounds (VOCs) contribute far more than anthropogenic VOC to SOA formation. SOA is calculated to be a very variable fraction, between 2 and 50%, of total organic carbon mass in aerosols over the Nordic countries. This fraction is least at coastal sites in southern Norway and Denmark, and much greater for inland sites and sites further north, reflecting the relative distribution of biogenic precursors. There are great uncertainties in these figures, however, and we have had to neglect important primary sources of organic carbon (OC), notably plant waxes, which probably also contribute significantly to total OC. Further progress will be difficult until more measurements are available to characterize the ambient organic aerosol. ¿ 2001 American Geophysical Union