Modeling Size-dependent Processes in Multiphase Reactive Systems

Sandhya Sethuraman, Zifeng Tang, Kedong Gong, Adriane Tam, Vicki Grassian, V. FAYE MCNEILL, Columbia University

     Abstract Number: 635
     Working Group: Aerosol Chemistry

Abstract
Many atmospherically relevant multiphase reactive systems exhibit behavior in laboratory studies that have led to those reactions being categorized as 'interfacial'. In this study, we analyze data from several such systems from an additive resistance perspective, considering various processes that could be taking place simultaneously, and their relative timescales. These include diffusion in the gas and aqueous phases, mass accommodation, reactions in the bulk, and reactions at the surface.

Typically, these systems exhibit a pattern involving a slow bulk reaction, and an efficient (although not necessarily fast) reaction at the surface. Above a critical radius, which varies among systems, the reaction is size-independent, and is governed by kinetics in the bulk. Below the tipping point, surface kinetics dominate and reaction rate is inversely proportional to droplet size.

We have developed a physicochemical modeling framework which can describe observed kinetic data from the literature from a range of systems involving S(IV) to S(VI) conversion, including: uncatalyzed O₂ oxidation, TMI-catalyzed oxidation, and H₂O₂, NO₂, and O₃ oxidation at various experimental conditions (RH, pH, initial oxidant concentration). This model accurately captures size-dependent kinetics and the critical radius, where applicable, for each system.

Using GAMMA 6.0, which contains updated S(IV) to S(VI) chemistry, we use a lab-to-environment approach to understand the implications of these interfacial processes under atmospherically relevant conditions, and offer recommendations based on this analysis for representing these systems in numerical models of atmospheric chemistry.