Effects of reversible adsorption and Langmuir–Hinshelwood surface reactions on gas uptake by atmospheric particles

Abstract

An expression for the reactive uptake coefficient (γ) of gas phase species due to surface reaction with particle-bound reactants via a Langmuir–Hinshelwood mechanism is derived from first principles. The new parameterization separates the processes of adsorption and chemical reaction, and it implies that γ for a surface reaction limited system will depend on the gas-phase concentration of the reacting species and decrease with reaction time, unless the particle-bound surface reactants are replenished. It also implies that γ will scale linearly with the concentration of the particle-bound reactants, in contrast to the square root dependence typical of diffusion-limited reactive uptake by liquids. The presented formulae enable calculation of γ from basic physico-chemical parameters for relevant atmospheric conditions, and they extend the existing resistor models of gas uptake by particles. The implications of this modified parameterisation are discussed by numerically integrating a reactive system with parameters of relevance to the atmosphere and by addressing a few recent laboratory studies.