A Computationally Efficient Chemistry, Thermodynamics, and Microphysics Model to Study Atmospheric Evolution of Organic Aerosol

 

Authors

Wayne Chuang — Colorado State University
Shantanu Jathar — Colorado State University
Jeffrey Robert Pierce — Colorado State University
Chris Cappa — University of California, Davis
Rahul Zaveri — Pacific Northwest National Laboratory
John E Shilling — Pacific Northwest National Laboratory

Thornton Joel — University of Washington

Category

Secondary organic aerosol

Description

We have developed a model that integrates organic chemistry with particle microphysics—the simpleSOM-MOSAIC model—and applied this framework to the DOE-supported HI-SCALE campaign. The simpleSOM consists of a framework of volatility bins each separated by an order of magnitude, and compounds are distributed throughout this space based on their volatilities. The initial oxidation of high-volatility precursors produce a distribution of lower volatility compounds. Subsequent reactions result in further functionalization or fragmentation that distribute products to lower or higher volatility bins, respectively. These distributions of products are different for each organic aerosol precursor and reaction environment, such as NOx levels. Given organic aerosol production and O-to-C ratios from chamber experiments, simpleSOM determines parameters for the extent of oxidation and the degree of functionalization vs. fragmentation that best fits the data. By creating precursor-specific parameters, simpleSOM captures the unique products and potential for aerosol formation from different precursors. MOSAIC handles the microphysics that describe how particles form and grow through nucleation, coagulation, and condensation. Our work expands upon the condensation module by including heterogeneous reactions and the effects of particle phase state. We examine the diffusion coefficient of aerosol particles and the accommodation coefficient of hydroxyl radicals at the particle surface, and how changing these properties may explain low volatility products and aerosol formation. By combining these two models, we derive new simpleSOM parameters that include how phase state and particle-phase reactions affect aerosol growth. We apply simpleSOM-MOSAIC to ambient observations from the HI-SCALE campaign that examined the interactions between land-use, aerosols, and clouds. Using our newly developed precursor-specific parameters, we can constrain the contribution to organic aerosols from urban outflows and season- and land use-resolved biogenic sources, and can predict how changes to sources may affect organic aerosol and cloud formation.