Abstract
Abstract. New measurements of water diffusion in secondary organic aerosol (SOA) material produced by oxidation of α-pinene and in a number of organic/inorganic model mixtures (3-methylbutane-1,2,3-tricarboxylic acid (3-MBTCA), levoglucosan, levoglucosan/NH4HSO4, raffinose) are presented. These indicate that water diffusion coefficients are determined by several properties of the aerosol substance and cannot be inferred from the glass transition temperature or bouncing properties. Our results suggest that water diffusion in SOA particles is faster than often assumed and imposes no significant kinetic limitation on water uptake and release at temperatures above 220 K. The fast diffusion of water suggests that heterogeneous ice nucleation on a glassy core is very unlikely in these systems. At temperatures below 220 K, model simulations of SOA particles suggest that heterogeneous ice nucleation may occur in the immersion mode on glassy cores which remain embedded in a liquid shell when experiencing fast updraft velocities. The particles absorb significant quantities of water during these updrafts which plasticize their outer layers such that these layers equilibrate readily with the gas phase humidity before the homogeneous ice nucleation threshold is reached. Glass formation is thus unlikely to restrict homogeneous ice nucleation. Only under most extreme conditions near the very high tropical tropopause may the homogeneous ice nucleation rate coefficient be reduced as a consequence of slow condensed-phase water diffusion. Since the differences between the behavior limited or non limited by diffusion are small even at the very high tropical tropopause, condensed-phase water diffusivity is unlikely to have significant consequences on the direct climatic effects of SOA particles under tropospheric conditions.
Highlights
Recent field measurements showed that secondary organic aerosol (SOA) particles are often amorphous glasses under dry and/or cold conditions (Virtanen et al, 2010)
Our measurements suggest that water diffusion coefficients in secondary organic material are sufficiently high such that SOA particles can be considered as equilibrated with the surrounding water vapor at temperatures above 220 K, even when the particles are in the amorphous solid state
The model simulations of realistic atmospheric water uptake scenarios in the upper troposphere show that homogeneous ice nucleation is only suppressed for a short period of time for the model runs starting at 195 K, i.e. under conditions found only near the tropical tropopause
Summary
Recent field measurements showed that secondary organic aerosol (SOA) particles are often amorphous glasses under dry and/or cold conditions (Virtanen et al, 2010). A much reduced water diffusivity slows down the growth of droplets compared with that predicted by equilibrium thermodynamics, but its significance for the ability of the droplets to act as ice nuclei or provide a medium for multi-phase chemistry is still poorly quantified (Murray et al, 2010; Shiraiwa et al, 2011; Koop et al, 2011; Price et al, 2015) This is mainly because viscosity, bounce factors and glass transition temperatures only provide indirect evidence for kinetic limitations of water diffusivity, whereas diffusion coefficients in the amorphous state span many orders of magnitude. The relationship between the viscosity of mixtures such as aqueous solutions of SOA materials and the diffusion coefficients of the different components present in the solution is not well characterized
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