Particle formation and growth from ozonolysis of α-pinene

Observations of particle nucleation and growth during ozonolysis of cc-pinene were carried out in Calspan's 600 m(3) environmental chamber utilizing relatively low concentrations of cc-pinene (15 ppb) and ozone (100 ppb). Model simulations with a comprehensive sectional aerosol model which incorporated the relevant gas-phase chemistry show that the observed evolution of the size distribution could be simulated within the accuracy of the experiment 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. While only one component was required to simulate the data, more than one product 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 amounts 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 nucleation theory. The nucleation rate and, more importantly, the slope of the nucleation rate versus the vapor pressure of the nucleating species would suggest 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).