Modeling Effects of Aerosol Phase State and Particle-phase Chemical Reactions on Secondary Organic Aerosol Partitioning Dynamics

 
Poster PDF

Authors

Rahul Zaveri — Pacific Northwest National Laboratory
Dick C Easter — Pacific Northwest National Laboratory
John E Shilling — Pacific Northwest National Laboratory
Mikhail S. Pekour — Pacific Northwest National Laboratory
Chen Song — Pacific Northwest National Laboratory
John Seinfeld — California Institute of Technology

Category

Secondary Organic Aerosol

Description

There is increasing evidence that particle phase state and particle-phase chemical reactions play an important role in controlling the number-diameter distribution of the resulting secondary organic aerosol (SOA). A proper representation of these physicochemical processes are therefore needed in the next generation models to reliably predict not only the total SOA mass, but also its composition and number size distribution, all of which together determine its overall optical and cloud-nucleating properties. This poster describes a new mathematical formulation for modeling kinetic gas-particle partitioning of SOA that takes into account diffusion and chemical reaction within the particle phase. The new formulation uses a combination of: (a) an analytical quasi-steady-state treatment for the diffusion-reaction process within the particle phase for fast-reacting organic solutes, and (b) a two-film theory approach for slow- and non-reacting solutes. The formulation is amenable for use in regional and global atmospheric models, although it currently awaits specification of the actual species and particle-phase reactions that are important for SOA formation. We apply the formulation within the framework of the computationally efficient Model for Simulating Aerosol Interactions and Chemistry (MOSAIC) to investigate the competitive growth dynamics of the Aitken and accumulation mode particles. Results show that the timescale of SOA partitioning and the associated size distribution dynamics depend on the complex interplay between organic solute volatility, particle-phase bulk diffusivity, and particle-phase reactivity, each of which can vary over several orders of magnitude. In general, the timescale of SOA partitioning increases with increase in volatility and decrease in bulk diffusivity and rate constant. At the same time, the shape of the aerosol size distribution displays appreciable narrowing with decrease in volatility and bulk diffusivity and increase in rate constant. We also apply the new formulation to interpret the observed evolution of size distribution due to SOA formation from photooxidation of a-pinene in an environmental chamber.

Lead PI

Rahul Zaveri — Pacific Northwest National Laboratory