A Model to Derive Langmuir-Hinshelwood Parameters from Heterogeneous Uptake Experiments in a Fast Flow Reactor.

Heterogeneous interactions between reactive trace gases and aerosol particles alter both the atmospheric trace gas composition and the chemical properties of the participating particles. Incorporating these interactions into aerosol models requires kinetic parameters, which are often derived from uptake experiments in flow reactors and are typically limited to uptake coefficients. We present a framework that enables the extraction of a more fundamental set of kinetic parameters, including adsorption, desorption, and reaction rate constants, as well as the concentrations of adsorptive and reactive sites, directly from such uptake data. The framework is currently formulated for the Langmuir-Hinshelwood mechanism on a solid surface but can be readily extended to other mechanisms. The model for time-dependent gas-phase mass transfer and surface chemistry that this framework is based on was developed procedurally, starting from the most rigorous implementation and undergoing two major simplifications, enabling computationally efficient fitting to experimental data. Application of the framework was demonstrated by fitting the model to experimental uptake curves, deriving uptake coefficients and partitioning constants from fitted parameters at both experimental and atmospheric conditions. Finally, an analytical approach for predicting partitioning through reactive Langmuir-Hinshelwood uptake was derived that can improve the accuracy of treating partitioning in aerosol models at no additional computational cost.