Phase, viscosity, morphology, and room temperature evaporation rates of SOA particles at low and high relative humidities, and their interaction with hydrophobic organics

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Description

Formation, properties, transformations, and temporal evolution of secondary organic aerosol (SOA) particles strongly depend on particle phase. Semi-volatile molecules that comprise SOA particles were assumed to form a low-viscosity solution that maintains equilibrium with the evolving gas phase by rapid evaporation condensation. However, recent studies by our and other groups indicate that SOA particles are in a semi-solid, highly viscous phase, and their evaporation rates are orders of magnitude slower than predicted.

Given that atmospheric relative humidity (RH) can change particle phase, it is important to investigate the effect of RH on the phase and evaporation kinetics of SOA particles. To this end, SOA particles were generated at low and high (~90%) RH, and their evaporation kinetics and phase were characterized as a function of RH.

In the ambient atmosphere, SOA particles form in the presence of a mixture of different organic compounds, which are present at or below their equilibrium vapor pressure and thus have been ignored. However, our data show that these compounds can adsorb to the surface of particles during SOA formation, becoming trapped in the highly viscous SOA, and affect particle properties.

We examine the interaction between SOA particles and different hydrophobic organics representing typical anthropogenic emissions by making SOA in the presence of the vapors of these hydrophobic organics and characterizing their properties. We find that the interaction between SOA and hydrophobic organics leads to a symbiotic relation, in which trapped hydrophobic organics are protected from evaporation and the oxidizing atmosphere, and the presence of hydrophobic organics virtually stops SOA evaporation.

We also demonstrate that it is possible to directly measure the diffusion rates of these molecules in SOA and use them to calculate the SOA viscosity of 10^8 Pa s. Such a high viscosity is characteristic of tars and is consistent with published measurements of SOA particle bounce, evaporation kinetics, and the stability of two reverse-layered morphologies.

Measurements on aged SOA particles doped with pyrene yield an estimated diffusivity ~3 times smaller, indicating that hardening occurs with time, which is consistent with the increase in SOA oligomer content, decrease in water uptake, and decrease in evaporation rate previously observed with aging.