Secondary Organic\nAerosol from Biomass Burning Phenolic\nCompounds and Nitrate Radicals can be Highly Viscous over a Wide Relative\nHumidity Range

Biomass burning events, including wildfires, can emit\nlarge amounts\nof phenolic compounds such as guaiacol. These phenolic compounds can\nundergo oxidation by nitrate radicals (NO<sub>3</sub>) to form secondary\norganic aerosol (SOA). Viscosity and hygroscopicity are key properties\nthat affect SOA’s role in atmospheric chemistry, air quality,\nclimate and public health. However, these properties have not been\nquantified for SOA formed from the reaction of phenolic compounds\nwith NO<sub>3</sub>. We used the poke-flow technique and a quartz\ncrystal microbalance (QCM) to measure the viscosity and hygroscopicity\nof SOA particles generated from the reaction of NO<sub>3</sub> with\nguaiacol, termed guaiacol-NO<sub>3</sub> SOA. The viscosity of this\nSOA is extremely high (≳5 × 10<sup>7</sup> Pa s) at RH\n≲ 70% and drastically higher than other SOA types previously\n investigated with the poke-flow technique at RH ≳ 40%. The\nhigh viscosity for guaiacol-NO<sub>3</sub> SOA can be attributed,\nat least in part, to the low hygroscopicity measured via the QCM.\nFrom the viscosity results, we calculated the mixing times of organic\nmolecules within guaiacol-NO<sub>3</sub> SOA. The results suggest\nthat mixing times within this type of SOA exceed 1 h for most tropospheric\nconditions, with possible implications for predicting the size, mass,\nand long-range transport of pollutants in phenolic SOA.