Viscosity, Glass Formation, and Mixing Times within Secondary Organic Aerosol from Biomass Burning Phenolics

Abstract

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.