Bulk phase water diffusion is significantly inhibited by inhomogeneity of single non-crystal particle at low relative humidity

Chen Cai, Stephen Ingram, Chunsheng Zhao

It has been suggested by recent studies that atmospheric particles adopt non-crystalline states which significantly impact aerosol-cloud interactions and atmospheric chemistry. In this study, the effect of non-crystalline states on water diffusion is detailed investigated from single multi-component particles levitated in aerosol optical tweezers. We infer the time-dependent particle size from Raman spectra using Mie fitting, thus derive the water diffusion coefficient (Dwater) from particle radius changes during evaporation or condensation processes. In both glassy states (in saccharide particles) and gel states (in MgSO4 particles), the bulk phase water diffusion is shown to be severely restricted, thus limiting the gas-particle water partitioning on the particle surface. The Dwater of glassy particles generally gradually decreases as the RH decreases, while the relative humidity (RH) - Dwater relationship of particle in gel state is complicated and brings huge deviation of Dwater determination. We therefore present the time dependent water content at different location (radial coordinate) of the particle. The time scale required for particle to get equilibrium to environmental RH is vastly extended by the kinetic inhibition of bulk phase water transfer. This can give direct and quantitative indication of water diffusion within single non-crystalline particle and its effect on gas-particle partitioning and equilibrium.

DOI