Aerosol and trace gas measurements were made at Kejimkujik National Park, Nova Scotia, Canada, during the summer of 1996. A case study from July 7–8 provides evidence of nucleation and condensation of products related to the oxidation of different biogenic emissions. Particles from 5 nm to 50 nm in diameter evolved during the afternoon and early evening associated with variations in isoprene. Late in the evening the α‐ and β‐pinene mixing ratios and the aerosol particle volume increased. Soon after, there was a sharp increase in RO2H/H2O2 that persisted until about 0100 LT. The initial increases in the pinenes and aerosols were strong and influenced by changes in winds. After 2200 LT, and into the early morning, the winds were relatively steady, and the α‐ and β‐pinene mixing ratios continually decreased. The decay of α‐pinene is explained through reaction with O3. However, the addition of OH radicals from the reaction of terpenes with O3 is necessary to explain the observed rate of decay of β‐pinene. During the same time, the aerosol volume increased with the decrease in α‐ and β‐pinene. The volume increase was distributed 40∶60 between particles in a mode centered at 80–90 nm and particles > 150 nm. The fine particle mass concentrations of the measured inorganic ions (sulfate, nitrate, chloride, ammonium, sodium, and calcium) and organic ions (oxalate, formate, acetate, pyruvate, propionate) account for 25–30% of the total aerosol volume during the period (2.7 μm3 cm−3) indicating that the aerosol volume increase was due to unidentified species. Assuming that the increase in the aerosol was the result of products from the oxidation of α‐ and β‐pinene, an aerosol mass yield of 13% is estimated. The concentrations of cloud condensation nuclei active at 0.2% supersaturation were enhanced by the appearance of the 80–90 nm mode pointing to at least some of these forest‐generated particles as being able to serve as nuclei for cloud droplets at common atmospheric supersaturations.