Effect of Hydrophobic Gas-phase Organics on the Nucleation, Mass Loading, Volatility, Viscosity, and Oligomer Content of SOA Particles

Description

Studies by our group, and others, have shown that, contrary to the long-standing assumptions, ambient and a number of laboratory-generated SOA particles are highly viscous, semi-solids and evaporate orders of magnitude slower than assumed. 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 evaporation of smaller molecules. Low volatility and high viscosity makes it impossible for SOA particles to maintain equilibrium with changes in the gas-phase. We have also shown that when hydrophobic organic vapors, including polyaromatic hydrocarbons (PAHs), are present during SOA formation, their presence further reduces SOA volatility and increases SOA viscosity and oligomer content. Evaporation kinetic studies reveal that these semi-volatile hydrophobic compounds become trapped within the viscous SOA and their evaporation rates are determined by their diffusion through SOA and can therefore be used to calculate the SOA viscosity. Application of this approach to freshly made α -pinene SOA doped with pyrene yielded a viscosity of 108 Pa s, and 3∙108 Pa s for the same particles after aging. We will present the results of recent studies that explore the volatility and viscosity of SOA particles generated by ozonolysis of C6 – C8 cycloalkenes in the absence and presence of hydrophobic organics. We find that the evaporation rates of C6 – C8 cycloalkene SOA particles are directly related to their oligomer content. SOA particles generated from the precursors with higher molecular weight evaporate slower. However, once vapors of hydrophobic organics are introduced, particles’ oligomer content and viscosity increase, their volatility significantly decreases and becomes nearly independent on the precursor. In all cases, aging farther decreases SOA evaporation rates. Moreover, we find that the presence of PAHs significantly increases particles number concentrations and mass loadings. For example, pyrene increases cyclohexene SOA formation yield by a factor of 3 and particles number concentration by a factor of 100. Similarly, we find that pyrene increases α-pinene SOA formation yield by a factor of 1.7. These findings provide direct evidence related to field data indicating that biogenic-anthropogenic interactions could be responsible for an increase in SOA loadings.