Aerosol Formation from the Reaction of α-Pinene and Ozone Using a Gas-Phase Kinetics-Aerosol Partitioning Model

Abstract

As a result of new aerosol compositional information, we have implemented an exploratory model for predicting aerosol yields from the reaction of α-pinene with ozone in the atmosphere. This new approach has the ability to embrace a range of different atmospheric chemical conditions, which bring about biogenic aerosol formation. A kinetic mechanism was used to describe the gas-phase reactions of α-pinene with ozone. This reaction scheme produces low vapor pressure reaction products that distribute between gas and particle phases. Some of the products have subcooled liquid vapor pressures which are low enough to initiate self-nucleation. More volatile products such as pinonic acid and pinonaldehyde will not self-nucleate but will partition onto existing particle surfaces. Partitioning was treated as an equilibrium between the rate of particle uptake and rate of particle loss of semivolatile terpene reaction products. Given estimated liquid vapor pressures and activation energies of desorption, it was possible to calculate gas-particle equilibrium constants and aerosol desorption rate constants at different temperatures. This permitted an estimate of the rate of absorption from the gas phase. Gas- and aerosol-phase reactions were linked together in one chemical mechanism, and a chemical kinetics solver was used to predict reactant and product concentrations over time. Aerosol formation from the model was then compared with aerosol production observed from outdoor chamber experiments. Approximately 20−40% of the reacted α-pinene carbon appeared in the aerosol phase. Models vs experimental aerosol yields are shown in Figure 2 and illustrate that reasonable predictions of secondary aerosol formation are possible. The majority of the aerosol mass came from the mass transfer of gas-phase products to the aerosol phase. An important observation from the product data and the model was that as temperatures and aerosol mass changed from experiment to experiment, the composition of the aerosol changed.