Heterogeneous Ice Nucleation on Simulated Sea-Spray Aerosol Using Raman Microscopy

Abstract:

The deliquescence and heterogeneous ice nucleation behavior of simulated sea-spray aerosol was probed from 215 to 235 K using Raman microscopy coupled to an environmental cell. Water uptake prior to deliquescence was also probed on sea-salt particles along deliquescence/ice nucleation experimental trajectories. Synthetic sea-salt particles were generated from solutions of aquarium salt, which emulates all major and minor ions that are found in saltwater. Our results indicate that, under cirrus cloud conditions, sea-salt particles will be in an internally mixed liquid-solid phase, with a brine layer surrounding a crystalline core. Above 230 K, the core of the internally mixed liquid-solid particle fully deliquesces prior to ice nucleation. Below 225 K, however, the crystalline core can act as efficient immersion-mode ice nuclei prior to full liquefaction. To simulate the presence of carbohydrate-like organics in sea spray aerosol, sea-salt particles were mixed with the disaccharide sucrose 1:1 by mass. At low relative humidity, the sea-salt particles effloresced into an amorphous organic-salt matrix. Above 225 K, sea-salt/sucrose particles fully deliquesce prior to ice nucleation; however, at lower temperatures, the organic-salt matrix was found to be glassy and could act as an efficient depositional ice nucleus. This work suggests the possible role of sea-salt aerosol in low-temperature ice nucleation. Consistent with this, data from aircraft campaigns suggests sea-salt aerosol is enhanced in ice residuals from low-temperature anvil cirrus whose convective inflow regions are primarily marine environments.