AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Phase, Viscosity, Morphology, and Room Temperature Evaporation Rates of SOA Particles generated from different Precursors, at Low and High Relative Humidities, and their Interaction with Hydrophobic Organics
ALLA ZELENYUK, Dan Imre, Josef Beranek, Jacqueline Wilson, Evan Abramson, Pacific Northwest National Laboratory
Abstract Number: 589 Working Group: Carbonaceous Aerosols in the Atmosphere
Abstract Formation, properties, transformations, and temporal evolution of secondary organic aerosols (SOA) particles strongly depend on particle phase. Recent studies by our group indicate that laboratory-generated alpha-pinene SOA particles and fresh ambient SOA characterized in a recent field campaign are in semi-solid phase and their evaporation rates are orders of magnitude slower than predicted. These results demonstrate that assumptions used by all current models that SOA form solution that can be modeled with Raoult’s law, and that the particles maintain equilibrium with the gas-phase at all times need to be significantly changed.
We extend these studies to include SOA particles generated by oxidation of a number of different precursors that include limonene, n-alkenes, cyclo-alkenes, and isoprene and characterized their phase, morphology, and room temperature evaporation rates. To examine the effect of relative humidity on SOA properties and evaporation rates we conducted the experiments at low and high (~90%) relative humidities.
In addition, we examine the interaction between SOA particles and different hydrophobic organics that represent typical anthropogenic emissions. We find that when hydrophobic organic vapors are present during SOA formation, they become incorporated into SOA particles and significantly slow the particle evaporation rates. We demonstrate that it is possible to directly measure the diffusion rates of these molecules in SOA, and use them to calculate a reasonably accurate value for the SOA viscosity, from which particle coalescence rates are also calculated.
Similar measurements were also conducted on aged SOA particles, including those doped with hydrophobic organics. The data indicate that aging further slows the rates of evaporation and results in decreased diffusion rates and increased viscosity, indicating hardening occurs with time, which is consistent with observed decrease in water uptake.