Viscous organic aerosol particles in the upper troposphere: diffusivity-controlled water uptake and ice nucleation?

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/NH 4 HSO 4 , 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.

The effect of viscosity and diffusion on the HO<sub>2</sub> uptake by sucrose and secondary organic aerosol particles

“…By running the KM-SUB model, it can be estimated that the diffusion coefficient of HO 2 within the particles would need to be approximately 1 × 10 −10 cm 2 s −1 for TMB-derived aerosol particles and < 5 × 10 −12 cm 2 s −1 for α-pinene-derived aerosol particles. This range of values seems to be consistent with the diffusion coefficients estimated by Berkemeier et al (2014) and Lienhard et al (2015) for water diffusion in low and medium O : C SOA. Thornton et al (2003) previously suggested that for malonic acid aerosol particles, the liquid water content could be Figure 6.…”

Diffusivity measurements of volatile organics in levitated viscous aerosol particles

“…Point (iii) is of real concern, as it is clear from our measurements of the diffusivity of water molecules in various organic substances (Lienhard et al, 2015) that sucrose has particularly low water diffusivity and high E act compared to other organics. Assuming that diffusivities of organic species in various hosts scale like the diffusivity of water, from Table A1 for water diffusivity of Lienhard et al (2015) we would need to reduce E act = 300 kJ mol −1 for PEG-4 in sucrose to ∼ 250 kJ mol −1 in levoglucosan, ∼ 225 kJ mol −1 in levoglucosan/NH 4 HSO 4 and ∼ 100 kJ mol −1 in α-pinenebased SOA, but increase to 325 kJ mol −1 in shikimic acid.…”

“…Assuming that diffusivities of organic species in various hosts scale like the diffusivity of water, from Table A1 for water diffusivity of Lienhard et al (2015) we would need to reduce E act = 300 kJ mol −1 for PEG-4 in sucrose to ∼ 250 kJ mol −1 in levoglucosan, ∼ 225 kJ mol −1 in levoglucosan/NH 4 HSO 4 and ∼ 100 kJ mol −1 in α-pinenebased SOA, but increase to 325 kJ mol −1 in shikimic acid. Thus, diffusing molecules will typically have a lower E act in organic aerosols than in sucrose.…”

“…Measured water diffusivities may exceed Stokes-Einstein predictions by several orders of magnitude (Power et al, 2013;Price et al, 2014). Several bulk and levitated aerosol particle techniques, based on isotopic tracers, fluorescence and mass transport measurements, have emerged to measure water diffusivity in highly viscous systems (Zobrist et al, 2011;Price et al, 2014;Lienhard et al, 2015;Davies and Wilson, 2016;Marshall et al, 2016). Few studies have addressed the diffusion of larger molecules (Champion et al, 1997;Price et al, 2016;Chenyakin et al, 2017) such as VOCs.…”

The rate of equilibration of viscous aerosol particles

“…To our knowledge however, no detailed comparison of all three approaches, including cases of D i dependent on composition, has yet been published. Despite this, a recent study by Lienhard et al (2015) linked the impact of particulate viscosity on ice nucleation using a composition dependent D i . A critical review of these models is intended to guide those with an interest in simulating particle evolution inside instruments, chamber experiments, and the ambient atmosphere.…”