Atmospheric aerosols can exist in amorphous semi-solid or glassy phase states whose viscosity varies with atmospheric temperature and relative humidity. The particle phase state has important implications for intraparticle mixing time, chemical reactivity, and deposition mode ice nucleation in the upper free troposphere.  One method to study the viscosity is to prepare coagulated dimer particles using the Dual Tandem DMA (DTDMA) technique, followed by probing the thermodynamic states and time required to induce the dimer particles to relax into spheres. Here we show experimental data characterizing the relationship between humidity, volatility, and viscosity of terpene-derived secondary organic aerosol from flow reactor experiments. We show that there is a robust inverse relationship between viscosity at 30 °C derived from DTDMA measurements and the effective saturation concentration derived from thermodenuder measurements. We also show that there is an inverse relationship between viscosity and oxidation state derived from aerosol mass spectrometry, which suggests that molecular weight is the dominant controlling factor of dry particle viscosity. Finally, we investigate whether glassy organic materials delay the condensation time scale required for the particles to equilibrate with the water in sub- and supersaturated environments. Observed condensation timescales indicate that water equilibration at subsaturated humidities occurs rapidly at temperatures between −7.5 and 20 °C, with only a marginal delay observed for particles starting as a glass. No effect was observed for hydrophobic organic coatings on inorganic seeds in the supersaturated regime.

Influences of temperature, humidity, composition, volatility, and time on the amorphous phase transitions of organic aerosols

2019 Fall National Meeting

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