Particle formation and growth from ozonolysis of alpha-pinene

Observations of particle nucleation and growth during ozonolysis of cc-pine ne were carried out in Calspan's 600 m(3) environmental chamber utilizing r elatively low concentrations of cc-pinene (15 ppb) and ozone (100 ppb). Mod el simulations with a comprehensive sectional aerosol model which incorpora ted the relevant gas-phase chemistry show that the observed evolution of th e size distribution could be simulated within the accuracy of the experimen t by assuming only one condensable product produced with a molar yield of 5 % to 6% and a saturation vapor pressure (SVP) of about 0.01 ppb or less. Wh ile only one component was required to simulate the data, more than one pro duct may have been involved, in which case the one component must be viewed as a surrogate having an effective SVP of 0.01 ppb or less. Adding trace a mounts Of SO2 greatly increased the nucleation rate while having negligible effect on the overall aerosol yield. We are unable to explain the observed nucleation in the alpha -pinene/ozone system in terms of classical nucleat ion theory. The nucleation rate and, more importantly, the slope of the nuc leation rate versus the vapor pressure of the nucleating species would sugg est that the nucleation rate in the a-pinene/ozone system may be limited by the initial nucleation steps (i.e., dimer, trimer, or adduct formation).