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Zusammenfassung:
Diffusion of organic vapours to the surface of aerosol or cloud particles is an important step for the formation and transformation of atmospheric particles. So far, however, a database of gas phase diffusion coefficients for organic compounds of atmospheric interest has not been available. In this work we have compiled and evaluated gas phase diffusivities (pressure-independent diffusion coefficients) of organic compounds reported by previous experimental studies, and we compare the measurement data to estimates obtained with Fuller's semi-empirical method. The difference between measured and estimated diffusivities are mostly <10 %. With regard to gas-particle interactions, different gas molecules, including both organic and inorganic compounds, exhibit similar Knudsen numbers (Kn) although their gas phase diffusivities may vary over a wide range. This is because different trace gas molecules have similar mean free paths in air at a given pressure. Thus, we introduce the pressure-normalised mean free path, lambda(P) approximate to 100 nm atm, as a near-constant generic parameter that can be used for approximate calculation of Knudsen numbers as a simple function of gas pressure and particle diameter to characterise the influence of gas phase diffusion on the uptake of gases by aerosol or cloud particles. We use a kinetic multilayer model of gas-particle interaction to illustrate the effects of gas phase diffusion on the condensation of organic compounds with different volatilities. The results show that gas phase diffusion can play a major role in determining the growth of secondary organic aerosol particles by condensation of low-volatility organic vapours.
