PNNL

Abstract

We present measurements of the isothermal evaporation of a-pinene ozonolysis secondary organic aerosol (SOA). Using a novel, filter-based method, we replicate previous findings of the volume fraction remaining, presented here as carbon number weighted signal fraction remaining (SFR), as a function of evaporation time. We apply two detailed physical models to interpret the evaporative behavior of both the bulk SOA and individual components. Both models find that a combination of effectively non-volatile products, together with reversible oligomers (or otherwise reactive monomers) having a decomposition lifetime of 9 to 28 hours, best explains the evolution of composition and volatility as particles age in the absence of both organic vapors and oxidants, even under an assumption of relatively viscous (soft wax-like) particles. We find that the residence time in the SOA formation chamber, and time spent undergoing isothermal evaporation, both indicative of the physical age of the aerosol, are the most important experimental parameter determining the evaporation rate. The evolution of volatility observed in these experiments is compared to field measurements in a boreal forest site. The ambient monoterpene-dominated SOA volatility is only reproduced in the laboratory after 24 hours of extended aging in a dilute, dark, oxidant-free environment.