SOA Aging and Oligomer Content and their Effect on the Volatility and Viscosity of SOA Particles Generated from Different Precursors

Formation, properties, transformations and temporal evolution of secondary organic aerosol (SOA) particles strongly depend on particle phase and volatility. Our recent studies indicate that laboratory-generated 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. This is not surprising given that numerous studies provide evidence that SOA particles contain significant amounts of high molecular weight organic compounds (oligomers), which affect SOA phase and volatility. It is well known that oligomers can severely retard diffusion, mixing, and thus evaporation of smaller molecules. One of the most intriguing findings is that SOA fractional evaporation rates are nearly size independent. We begin by presenting our results of evaporation studies of particles composed of hexaethylene glycol (HEG), polyethylene glycols (PEGs) of different polymer chain length, and their mixtures. The data indicate that HEG particles exhibit the size-dependent evaporation expected for liquid droplets, while particles containing polymers with different chain lengths exhibit size-independent evaporation kinetics similar to those of SOA. We will then present the results of evaporation studies of SOA particles generated by oxidation of several different precursors, including alpha-pinene, isoprene, limonene, n-alkenes and cyclo-alkenes, from which we explore the relationship between SOA oligomer content and SOA volatility and viscosity. We, and others, also find that oligomer content in SOA increases with time, and with it we expect corresponding changes in viscosity and volatility. We will present the results of studies aimed at characterizing evaporation kinetics and the viscosity of SOA particles as a function of particle age. We will also present our findings on the effect of hydrophobic organics on SOA oligomer content, its volatility and viscosity.