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

Biomass burning events emit large amounts of phenoliccompounds,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 threebiomass burning phenolic compounds: catechol, guaiacol, and syringol.All systems had viscosity < 3 x 10(3) Pa s at RH greater than or similar to 40% and > 2 x 10(8) Pa s at RH less than or similar to 3%at room temperature. At RH values of 0-10%, the viscositiesof these SOA were at least 2 orders of magnitude higher than the viscosityof primary organic aerosol from biomass burning. We also developeda parameterization for predicting the viscosity of phenolic biomassburning SOA as a function of RH and temperature. Based on this parameterization,the viscosity of phenolic biomass burning SOA is strongly dependenton both RH and temperature. Under dry conditions, phenolic biomassburning SOA is highly viscous at room temperature (similar to 10(9) Pa s) and becomes a glass (viscosity > 10(12) Pas) when the temperature is < 280 K. For tropospheric temperatureand RH values, phenolic biomass burning SOA is often in a liquid state(eta < 10(2) Pa s) below similar to 2 km altitude, asemi-solid state (10(2) < eta < 10(12) Pa s) between similar to 2 and similar to 9 km, and a glassy state (eta> 10(12) Pa s) above similar to 9 km. Furthermore, the mixingtime of organic molecules in a 200 nm phenolic biomass burning SOAparticle exceeds 1 h above 3 km in the troposphere.