Secondary organic aerosols (SOA) constitute a major fraction of the submicron aerosol mass, and may therefore exert a significant influence on Earth’s climate via aerosol direct and indirect radiative effects. Since aerosol optical and cloud condensation nuclei activation properties depend on both aerosol size and number concentration, it is critical that climate-chemistry models are able to reliably predict the aerosol number size distribution due to SOA formation. We first briefly review the new theoretical framework in the MOSAIC aerosol box model that performs kinetic gas-particle partitioning of SOA and takes into account diffusion and chemical reaction within the particle phase, thus capturing the complex interplay between chemistry, aerosol phase state (i.e., bulk diffusivity), and aerosol size distribution dynamics. We then apply the model to analyze the observed growth dynamics of SOA formed from isoprene photooxidation in: a) ambient, isoprene-rich air during the CARES field campaign, and b) laboratory environmental chamber. In each case, SOA formation took place in the presence of pre-existing Aitken and accumulation mode aerosols. The model was initialized with the observed aerosol size distribution and composition at the beginning of the experiment, and the total amount of SOA formed in the model at any given time was constrained by the observed total amount of SOA formed. Sensitivity simulations were performed by varying the number of condensing organic species, their gas-phase formation rates, their effective volatilities, and their bulk diffusivities in the Aitken and accumulation modes. The objective of the constrained modeling exercise was to determine which model configuration was able to best reproduce the observed size distribution evolution, thus providing valuable insights into the possible mechanisms of SOA formation. We also examine the evaporation kinetics of size-selected particles formed in the environmental chamber to provide additional constraints on the effective volatility and bulk diffusivity of the organic species. Our results suggest that SOA formed from isoprene photooxidation is semi-volatile, and the resulting size distribution evolution is highly sensitive to the phase state of the SOA, and that it could not be reproduced with the traditional equilibrium SOA partitioning approach. Implications of these findings on further SOA model development and evaluation strategy will be discussed.

Interplay between secondary organic aerosol chemistry, phase state, and growth dynamics

249th National Meeting (2015)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.