1Institute for Atmospheric and Climate Science, ETH Zürich, 8092 Zürich, Switzerland
2Department of Environmental Sciences, Weizmann Institute, Rehovot 76100, Israel
3Marcolli Chemistry and Physics Consulting GmbH, 8092 Zürich, Switzerland
4Physikalisch-Meteorologisches Observatorium Davos and World Radiation Center PMOD/WRC, 7260 Davos, Switzerland
5School of Chemistry, University of Bristol, BS8 1TS Bristol, UK
6Chemistry Department, Boston College, Chestnut Hill, MA 02467, USA
7Aerodyne Research Inc., Billerica, MA 01821, USA
8Laboratory of Radiochemistry and Environmental Chemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland
9Faculty of Chemistry, Bielefeld University, 33615 Bielefeld, Germany
apresent address: Department of Chemistry, University of Cambridge, Cambridge, UK
bpresent address: Chemistry Department, Union College, Schenectady, NY, USA
cpresent address: School of Chemistry, University of Leeds, Leeds, UK
Received: 20 Aug 2015 – Discussion started: 09 Sep 2015
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.
Revised: 19 Nov 2015 – Accepted: 25 Nov 2015 – Published: 09 Dec 2015
Lienhard, D. M., Huisman, A. J., Krieger, U. K., Rudich, Y., Marcolli, C., Luo, B. P., Bones, D. L., Reid, J. P., Lambe, A. T., Canagaratna, M. R., Davidovits, P., Onasch, T. B., Worsnop, D. R., Steimer, S. S., Koop, T., and Peter, T.: Viscous organic aerosol particles in the upper troposphere: diffusivity-controlled water uptake and ice nucleation?, Atmos. Chem. Phys., 15, 13599-13613, doi:10.5194/acp-15-13599-2015, 2015.