Abstract
Abstract. Organic aerosol particles play a key role in climate by serving as nuclei for clouds and precipitation. Their sources and composition are highly variable, and their phase state ranges from liquid to solid under atmospheric conditions, affecting the pathway of activation to cloud droplets and ice crystals. Due to slow diffusion of water in the particle phase, organic particles may deviate in phase and morphology from their thermodynamic equilibrium state, hampering the prediction of their influence on cloud formation. We overcome this problem by combining a novel semi-empirical method for estimation of water diffusivity with a kinetic flux model that explicitly treats water diffusion. We estimate timescales for particle deliquescence as well as various ice nucleation pathways for a wide variety of organic substances, including secondary organic aerosol (SOA) from the oxidation of isoprene, α-pinene, naphthalene, and dodecane. The simulations show that, in typical atmospheric updrafts, glassy states and solid/liquid core-shell morphologies can persist for long enough that heterogeneous ice nucleation in the deposition and immersion mode can dominate over homogeneous ice nucleation. Such competition depends strongly on ambient temperature and relative humidity as well as humidification rate and particle size. Due to differences in glass transition temperature, hygroscopicity and atomic O / C ratio of the different SOA, naphthalene SOA particles have the highest potential to act as heterogeneous ice nuclei. Our findings demonstrate that kinetic limitations of water diffusion into organic aerosol particles are likely to be encountered under atmospheric conditions and can strongly affect ice nucleation pathways. For the incorporation of ice nucleation by organic aerosol particles into atmospheric models, our results demonstrate a demand for model formalisms that account for the effects of molecular diffusion and not only describe ice nucleation onsets as a function of temperature and relative humidity but also include updraft velocity, particle size and composition.
Highlights
Atmospheric aerosol particles influence climate through affecting the earth’s radiation budget directly by scattering and absorbing light, and indirectly by acting as nuclei for cloud droplets and ice crystals (Yu et al, 2006; Andreae and Rosenfeld, 2008; IPCC, 2013)
We investigate ice nucleation in glassy organic aerosols induced by changing ambient conditions during the updraft of an air parcel
Adiabatic cooling leads to a decrease in temperature and a corresponding increase of relative humidity (RH)
Summary
Atmospheric aerosol particles influence climate through affecting the earth’s radiation budget directly by scattering and absorbing light, and indirectly by acting as nuclei for cloud droplets and ice crystals (Yu et al, 2006; Andreae and Rosenfeld, 2008; IPCC, 2013). It has been proposed recently that formation of highly porous structures upon atmospheric freezedrying could enhance the IN ability of organic aerosol particles (Adler et al, 2013) These observations suggest a connection between particle phase state and the resulting predominant ice nucleation pathway (Murray et al, 2010). Water diffusivity in SOA materials from various biogenic and anthropogenic precursors are deduced from water diffusivity parameterizations of model compounds using a semi-empirical physico-chemical model of water diffusion in glass-forming aqueous organics
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