Abstract View
Particle Size Distribution Dynamics Can Help Constrain the Phase State of Secondary Organic Aerosol
CHARLES HE, Ali Akherati, Theodora Nah, Jeffrey R. Pierce, Rahul Zaveri, Christopher Cappa, Lauren A. Garofalo, Nga Lee Ng, Delphine K. Farmer, Shantanu Jathar, Colorado State University
Abstract Number: 212
Working Group: Aerosol Physics
Abstract
Particle phase state is an important property of atmospheric aerosols that has implications for the formation, evolution, and gas/particle partitioning of secondary organic aerosol (SOA). Yet, the phase state of SOA and its relation to the VOC precursor, SOA molecular composition, and environmental variables remain largely uncertain. In this work, we use a size-resolved chemistry and microphysics model (i.e., SOM-TOMAS), updated to include an explicit treatment of particle phase state, to constrain the bulk diffusion coefficient (Db) of SOA produced from alpha-pinene ozonolysis. By leveraging laboratory experiments performed in the absence of a seed and under dry conditions, we find that the Db for SOA can be constrained (~4×10-19 m2 s-1 in these experiments) by simultaneously reproducing the time-varying SOA mass concentration, O:C ratio, and, most importantly, the evolution of the number size distribution. The Db could not be constrained with the seeded experiments because the model-predicted evolution of the particle size distribution was nearly identical over a large range of Db values (10-16 to 10-23 m2 s-1) due to the SOA coating thickness being relatively thin and similar across all seed diameters. An updated version of our model that used the model-predicted SOA composition to calculate the glass transition temperature, viscosity, and, ultimately, Db (~10-19 m2 s-1) of the SOA was able to reproduce the measurements when we also included rapid oligomer formation that accounted for more than half of the SOA mass. Our work highlights the potential of a size-resolved SOA model to constrain the particle phase state of SOA by leveraging historical measurements of the evolution of the particle size distribution.