On the synergetic relation between SOA and hydrophobic organics: SOA formation yield, evaporation kinetics, phase, morphology, diffusivity, viscosity, and coalescence rates

 

Authors

Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
Dan Imre — Imre Consulting
Josef Beranek — Pacific Northwest National Laboratory
Evan Abramson — University of Washington
Manishkumar Shrivastava — Pacific Northwest National Laboratory

Category

Aerosol Properties

Description

Secondary organic aerosols (SOA) comprise the largest and least understood fraction of atmospheric aerosols. Recent studies by our group demonstrated that these particles are in quasi-solid phase and that their evaporation rates are orders of magnitude slower than predicted. Similar measurements on ambient SOA particles mixed with small amount of sulfate reveal nearly identical results, providing direct evidence that SOA evaporation in the ambient atmosphere is negligible. These results demonstrate that assumptions used by current models that SOA can be treated as liquid and modeled with parameterized Raoult’s law, while assuming gas-particle equilibrium at all times, need to be significantly changed. In the atmosphere, SOA often forms in the presence of hydrophobic organics. We explore the interaction between hydrophobic organics and SOA by condensing SOA on particles composed of hydrophobic organics and coating SOA particles with the same, and find that hydrophobic organics and SOA form separate phases. When SOA is condensed on liquid hydrophobic organic particles, the SOA coating prevents the core from evaporating. When the same hydrophobic liquid organic is used to coat SOA particles, it evaporated rapidly. In both cases, particles remain spherical throughout the evaporation process. When solid hydrophobic organics, like polyaromatic hydrocarbons, are deposited on top of pure SOA particles, we find that the coated particles are aspherical and the hydrophobic organic rapidly evaporates, regenerating spherical SOA particles and leaving no trace of hydrophobic organics behind. However, SOA particles formed in the presence of the vapors of hydrophobic organics incorporate these organics into their bulk, where they become trapped for days. Independent of the hydrophobic organic, these particles are spherical and contain significant amounts of hydrophobic organics. Vapors of hydrophobic organics increase SOA formation yield by a factor of 1.6. The evaporation rates of SOA particles with trapped hydrophobic organics are significantly lower, and slow down more with aging. We have successfully generated SOA particles from a number of precursors in the presence of different hydrophobic organics, measured evaporation rates of fresh and aged SOA particles, characterized the morphological distribution of the organics trapped inside the SOA particles, and measured their diffusion rates through SOA, from which we calculate SOA viscosity and particle coalescence rates.