AAAR 33rd Annual Conference
October 20 - October 24, 2014
Rosen Shingle Creek
Orlando, Florida, USA
Abstract View
Effect of Precursor Molecular Structure on the Volatility, Viscosity and Oligomer Content of SOA Particles Formed by Ozonolysis of Cycloalkenes
JACQUELINE WILSON, Alla Zelenyuk, Dan Imre, ManishKumar Shrivastava, Pacific Northwest National Laboratory
Abstract Number: 360 Working Group: Aerosol Chemistry
Abstract The formation, properties and temporal evolution of secondary organic aerosol (SOA) particles strongly depend on their phase and volatility. These properties affect the ability of SOA particles to maintain equilibrium with rapidly changing gas phase concentrations and temperature, as experienced in the mixed planetary boundary layer, by internal mixing and evaporation/condensation.
Studies by our group, and others, have shown that alpha-pinene SOA particles are highly viscous semi-solids with viscosity characteristic of tars, and their evaporation rates are orders of magnitude slower than previously assumed. We have also shown that when hydrophobic organic vapors, like polycyclic aromatic hydrocarbons (PAHs), are present during SOA formation their presence further reduces SOA volatility. Evaporation kinetics studies reveal that these semi-volatile hydrophobic compounds are trapped within the SOA and that their evaporation rates are determined by their diffusion through the SOA and can therefore be used to obtain the SOA viscosity. Application of this approach to freshly made alpha-pinene SOA doped with various PAHs yielded a viscosity of 10$^8 Pa s, and 3∙10$^8 Pa s for the same particles after aging.
These findings are consistent with the fact that SOA particles contain significant amounts of high molecular weight organic compounds (oligomers), which have the potential to severely retard diffusion, mixing, and thus evaporation of smaller molecules.
We present the results of recent studies that explore the relationship between cycloalkenes with differing molecular weight and ring structure (C$_5 – C$_8 cycloalkenes, including C$_7 isomeric cycloalkene systems (1-methyl-1-cyclohexene, 3-methyl-1-cyclohexene, methylenecyclohexane and cycloheptene) used as SOA precursors – in the absence and presence of hydrophobic organics – and SOA volatility and viscosity. We will show that differences are mostly related to differing SOA oligomer content, which depends on the precursor, particle age, and how much, if any, hydrophobic organic vapors are incorporated.