Abstract
Organic halogens are of great environmental and climatic concern. In this work, we have compiled their gas phase diffusivities (pressure-normalized diffusion coefficients) in a variety of bath gases experimentally measured by previous studies. It is found that diffusivities estimated using Fuller's semi-empirical method agree very well with measured values for organic halogens. In addition, we find that at a given temperature and pressure, different molecules exhibit very similar mean free paths in the same bath gas, and then propose a method to estimate mean free paths in different bath gases. For example, the pressure-normalized mean free paths are estimated to be 90, 350, 90, 80, 120 nm atm in air (and N2/O2), He, argon, CO2 and CH4, respectively, with estimated errors of around ±25%. A generic method, which requires less input parameter than Fuller's method, is proposed to calculate gas phase diffusivities. We find that gas phase diffusivities in He (and air as well) calculated using our method show fairly good agreement with those measured experimentally and estimated using Fuller's method. Our method is particularly useful for the estimation of gas phase diffusivities when the trace gas contains atoms whose diffusion volumes are not known.
Highlights
Gas–surface interactions play important roles in many aspects of atmospheric chemistry and physics, including heterogeneous and multiphase reactions of atmospheric aerosol particles, cloud and rain droplets, and ice particles [1,2,3,4]
An interesting question has been raised: can we estimate its diffusivity if the molecule contains atoms with diffusion volumes being unknown? In our previous work [24], it has been found that for a given particle diameter, the Knudsen number (Kn), which is the ratio of the mean free path of gas molecules to the particle radius, is identical for different trace gases with very different diffusion coefficients
It is found that estimated diffusivities using Fuller’s semiempirical method agree very well with experimental values reported in the literature, suggesting that Fuller’s method can reliably estimate gas phase diffusivities of organic halogens
Summary
Gas–surface interactions play important roles in many aspects of atmospheric chemistry and physics, including heterogeneous and multiphase reactions of atmospheric aerosol particles, cloud and rain droplets, and ice particles [1,2,3,4]. Atmosphere–land exchanges of gas molecules, such as dry deposition of trace gases, can 2 be considered as gas–surface interactions [5,6,7]. Interactions with surfaces of human-made structures have been proposed as potentially important sources/sinks for several reactive trace gases [8,9,10,11]. The importance of gas–surface interactions has been widely recognized in other fundamental and applied research areas, such as heterogeneous catalysis [12,13,14]
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