Viscosity, glass formation, and mixing times within secondary organic aerosol from biomass burning phenolics

Biomass burning events emit large amounts of phenolic compounds, which are oxidized in the atmosphere and form secondary organic aerosol (SOA). Using the poke-flow technique, we measured relative humidity (RH)-dependent viscosities of SOA generated by the oxidation of three biomass burning phenolic compounds: catechol, guaiacol, and syringol. All systems had viscosity < 3 × 10³ Pa s at RH ≳ 40% and > 2 × 10⁸ Pa s at RH ≲ 3% at room temperature. At RH values of 0–10%, the viscosities of these SOA were at least 2 orders of magnitude higher than the viscosity of primary organic aerosol from biomass burning. We also developed a parameterization for predicting the viscosity of phenolic biomass burning SOA as a function of RH and temperature. Based on this parameterization, the viscosity of phenolic biomass burning SOA is strongly dependent on both RH and temperature. Under dry conditions, phenolic biomass burning SOA is highly viscous at room temperature (∼10⁹ Pa s) and becomes a glass (viscosity > 10¹² Pa s) when the temperature is < 280 K. For tropospheric temperature and RH values, phenolic biomass burning SOA is often in a liquid state (η < 10² Pa s) below ∼2 km altitude, a semi-solid state (10² < η < 10¹² Pa s) between ∼2 and ∼9 km, and a glassy state (η > 10¹² Pa s) above ∼9 km. Furthermore, the mixing time of organic molecules in a 200 nm phenolic biomass burning SOA particle exceeds 1 h above 3 km in the troposphere.