Abstract
Abstract. 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, λP ≈ 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.
Highlights
Organic aerosols are ubiquitous in the atmosphere and can account for a dominant fraction of submicron aerosol particles (Jimenez et al, 2009)
Organic aerosol particles can be directly emitted into the atmosphere, and they can be produced by condensation of semi- and low-volatility organic vapours which are directly emitted or formed by gas phase reactions between atmospheric oxidants like OH, O3, and NO3 with volatile organic compounds (VOCs) of biogenic and anthropogenic precursors (Robinson et al, 2007; Kroll and Seinfeld, 2008)
We find that different gas molecules, including both organic and inorganic compounds, have very similar Knudsen numbers, and propose a simple equation to calculate Knudsen numbers for all gas molecules
Summary
Organic aerosols are ubiquitous in the atmosphere and can account for a dominant fraction of submicron aerosol particles (Jimenez et al, 2009). Many important heterogeneous processes, such as the formation and transformation of organic aerosols, are still not well quantified, and our current knowledge is not sufficient to develop processbased modules to be included in air quality, cloud, or climate models. The gas phase diffusion coefficient is an important parameter in kinetic models of SOA (secondary organic aerosol) formation and growth (Riipinen et al, 2011; Roldin et al, 2014; Shiraiwa et al, 2014). To illustrate the effects of gas phase diffusion on organic aerosol formation, the condensation of two organic compounds with distinctive volatility onto seed aerosol particles is simulated using a kinetic multilayer model for gas-particle interactions in aerosols and clouds (Shiraiwa et al, 2012; Shiraiwa and Seinfeld, 2012)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
