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

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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.