A rapid scan vacuum FTIR method for determining diffusion coefficients in viscous and glassy aerosol particles

Area of Science:

• Atmospheric Chemistry
• Physical Chemistry
• Aerosol Science

Background:

• Understanding water transport in aerosols is crucial for climate and air quality.
• Previous methods lacked the time resolution to capture rapid water dynamics.
• Aerosol particle phase and viscosity significantly influence water uptake and release.

Purpose of the Study:

• To develop and apply a novel method for investigating water transport kinetics in aerosol particles.
• To differentiate water transport mechanisms in crystalline versus amorphous aerosol particles.
• To quantify the impact of particle viscosity on water evaporation and condensation timescales.

Main Methods:

• Utilized rapid scan Fourier-transform infrared (FTIR) spectroscopy.
• Employed a custom-built system for rapid pulse changes in relative humidity (RH).
• Achieved high time resolution (0.12 s/spectrum) and fast RH changes (up to 60%/s) for ~3 μm particles.

Main Results:

• Dissolution of crystalline inorganic salt particles upon increasing RH is prompt (<1 s), comparable to RH change speed.
• Water evaporation and condensation in amorphous sucrose particles exhibit significantly delayed timescales (orders of magnitude) dependent on viscosity (10^1 to 10^9 Pa s).
• Kinetics for amorphous particles align with previous larger particle studies, suggesting simplified modeling may suffice for equilibration timescales.

Conclusions:

• The new FTIR method effectively probes water transport kinetics in aerosols.
• Particle phase (crystalline vs. amorphous) and viscosity critically control water equilibration timescales.
• Gas and condensed phase diffusion interplay determines overall water mass transport rates in aerosols.