Secondary organic aerosol from biomass burning phenolic compounds and nitrate radicals can be highly viscous over a wide relative humidity range

Biomass burning events, including wildfires, can emit large amounts of phenolic compounds such as guaiacol. These phenolic compounds can undergo oxidation by nitrate radicals (NO₃) to form secondary organic aerosol (SOA). Viscosity and hygroscopicity are key properties that affect SOA’s role in atmospheric chemistry, air quality, and climate. However, these properties have not been quantified for SOA formed from the reaction of phenolic compounds with NO₃. We used the poke-flow technique and a quartz crystal microbalance (QCM) to measure the viscosity and hygroscopicity of SOA particles generated from the reaction of NO₃ with guaiacol, termed guaiacol-NO₃ SOA. The viscosity of this SOA is extremely high ($gtrsim 5times10^{7}$ Pa s) at RH $lesssim 70%$ and drastically higher than previously reported for other SOA types investigated with the poke-flow technique at RH $gtrsim 40%$. The high viscosity for guaiacol-NO₃ SOA can be attributed, at least in part, to the low hygroscopicity measured by the QCM. From the viscosity results, we calculated the mixing times of organic molecules within guaiacol-NO₃ SOA. The results suggest that mixing times within guaiacol-NO₃ SOA particles with atmospherically relevant sizes exceed 1 hour for most tropospheric conditions, with possible implications for predicting the size, mass, and long-range transport of pollutants in phenolic SOA.