A case study of gas-to-particle conversion in an eastern Canadian forest

Aerosol and trace gas measurements were made at Kejimkujik National Park, N ova Scotia, Canada, during the summer of 1996. A case study from July 7-8 p rovides 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 var iations in isoprene. Late in the evening the alpha- and beta-pinene mixing ratios and the aerosol particle volume increased. Soon after, there was a s harp 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 relati vely steady, and the alpha- and beta-pinene mixing ratios continually decre ased. The decay of alpha-pinene is explained through reaction with O-3. How ever, the addition of OH radicals from the reaction of terpenes with O-3 is necessary to explain the observed rate of decay of beta-pinene. During the same time, the aerosol volume increased with the decrease in alpha- and be ta-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 c oncentrations of the measured inorganic ions (sulfate, nitrate, chloride, a mmonium, sodium, acid calcium) and organic ions (oxalate, formate, acetate, pyruvate, propionate) account for 25-30% of the total aerosol volume durin g the period (2.7 mu m(3) cm(-3)) indicating that the aerosol volume increa se was due to unidentified species. Assuming that the increase in the aeros ol was the result of products from the oxidation of alpha- and beta-pinene, an aerosol mass yield of 13% is estimated. The concentrations of cloud con densation nuclei active at 0.2% supersaturation were enhanced by the appear ance of the 80-90 nm mode pointing to at least some of these forest-generat ed particles as being able to serve as nuclei for cloud droplets at common atmospheric supersaturations.