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