UC San Diego

Aerosols are ubiquitous in the atmosphere and play a major role in balancing the incoming solar radiation directly through scattering and absorption, and indirectly by acting as seeds to nucleate clouds. The ability of an aerosol to uptake water is fundamental to the condensational growth necessary for cloud formation, and additionally alters aerosol properties such as concentration, density, refractive index, and viscosity. The growth rate under particular relative humidity is dependent on the mobility of water molecules in the aerosol, reported as the water diffusion coefficient, and becomes extremely slow at high viscosity.

Ocean-derived sea spray aerosol (SSA) is one of the most abundant natural sources of atmospheric aerosol and exhibits complex chemical composition. SSA contains both inorganic and organic compounds, including inorganic salts (NaCl and CaCl2) and saccharides. A number of recent studies revealed that model SSA organic compounds can achieve high-viscosity, semi-solid phase states at low humidity. The relationship between viscosity and diffusion based on the Stokes-Einstein (S-E) relation is used to estimate water diffusion coefficients in aerosol because of the supersaturated concentrations not accessible to bulk diffusion techniques. However, the S-E relation has been shown to under-predict water mobility by several orders of magnitude using recently developed single aerosol diffusion techniques. Only a few methods currently exist for measuring diffusion in single aerosol, limiting the database of concentration-dependent water diffusion coefficients necessary for improving the parameterization between viscosity and diffusion. In this thesis, a new method is presented for directly measuring the diffusion coefficient of water that applies the Raman spectroscopic isotope tracing method to single, charged viscous aerosols trapped in a newly built electrodynamic balance (EDB) under controlled humidity. The EDB Raman isotope tracing method is validated by comparing results of the concentration-dependent water diffusion coefficient using sucrose as a model system. We then apply the method to directly measure the water diffusion coefficient in ternary sucrose-NaCl and sucroseCaCl2 mixtures for the first time, and model the results with a Vignes-type diffusion model. Further application of the EDB isotope tracing method will continue to clarify the role of chemical composition on aerosol physical properties.