Secondary organic aerosol (SOA) particles, which are abundant in the troposphere, experience a wide range of water activity (aw) and temperature. Several properties of SOA depend on diffusion rates of organic molecules within SOA. Diffusion rates of a fluorescent organic dye, rhodamine-6G (R6G), were measured as a function of temperature and aw in a solution of sucrose-water, previously used as a proxy for SOA. The diffusion rates were measured using rectangular area fluorescence recovery after photobleaching (rFRAP). Diffusion rates depended strongly on both aw and temperature. For example, at a aw of 0.43, the diffusion rates decreased by approximately one order of magnitude as the temperature decreased by 10 K. The effect of temperature on diffusion rates diminished as aw increased. The apparent activation energies for diffusion of R6G in sucrose-water ranged from 51.8 to 152.6 kJ/mol for aw values of 0.85 to 0.43. If SOA have diffusion properties similar to sucrose-water particles, then mixing times within SOA particles will be longer than 1 hour at temperatures less than 275 K at a water activity of 0.43, or less than 240 K at an aw of 0.59, based on our measurements and the apparent activation energies.

Diffusion of organics in sucrose-water solutions as a function of temperature: Measurements and atmospheric relevance

2019 Fall National Meeting

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