Volatility and Viscosity Are Correlated in Terpene Secondary Organic Aerosol Formed in a Flow Reactor

Abstract

Secondary organic aerosol (SOA) is a complex mixture of largely unspeciated compounds. The volatility and viscosity of the bulk organic aerosol influence new particle formation, processing, and lifetime in the atmosphere. Relationships between these properties are well-defined for pure compounds but currently unavailable for bulk organic aerosol. In this survey study, we characterized SOA formed from a range of biogenic precursors and conditions in an oxidation flow reactor for volatility (thermodenuder), viscosity (dimer coagulation, isolation, and coalescence), and oxidation state (aerosol chemical speciation monitor). We find linear trends in log–linear and log–log plots of single-parameter representations of volatility and viscosity, with higher condensed-phase fractions of extremely low and low volatility material associated with an increased viscosity (R = 0.69). Per this relationship, an increase in the contribution of these fractions (i.e., lower volatility) by 0.1 results in an increase in viscosity of approximately 200%. The viscosity (at 30 °C) of SOA fell between 6.2 × 105 and 8.0 × 108 Pa s and thus in the semisolid range. The SOA oxidation state ranged from −1.0 and 0.1 and was weakly anticorrelated with volatility (but not viscosity). We found larger SOA mass loadings generally associated with increased volatility and decreased viscosity. The results of this preliminary study are consistent with “molecular corridor”-style frameworks employing molecular mass and volatility to estimate the viscosity of bulk real-world SOA.