Biomass burning events, including wildfires, can emit large amounts of phenolic compounds, such as guaiacol, catechol, and syringol into the atmosphere. Once in the atmosphere, phenolic compounds can be oxidized by hydroxyl radicals (OH), ozone (O 3 ), and nitrate radicals (NO 3 ) to form lower volatility products, which can partition to the particle phase, forming secondary organic aerosol (SOA). OH and O 3 are the main oxidants during the day, while NO 3 and O 3 are the main oxidants during the night or during the day in light-limited environments, such as within dense biomass burning plumes. A recent study suggests that oxidation of biomass burning plumes by NO 3 during the night can lead to rapid production of SOA in the atmosphere. SOA, including phenolic SOA can impact climate indirectly by acting as cloud condensation nuclei and possibly ice nucleating particles, thereby changing the reflectivity and lifetime of clouds. In addition, SOA can contribute to poor air quality and negatively impact human health. Viscosity is a physiochemical property of SOA that is important for predicting its impact on climate and air quality. Viscosity is inversely related to molecular diffusion rates within SOA particles, based on the Stokes-Einstein equation. As a result, viscosity can impact gas-particle partitioning and the mass and size distributions of SOA in the atmosphere. In addition, viscosity can affect the rates and mechanisms of multiphase reactions within SOA particles and the long-range transport of particle-borne pollutants like polycyclic aromatic hydrocarbons. Some studies have also suggested that highly viscous SOA can act as heterogeneous nuclei for ice formation in the atmosphere, , although the importance of the glassy phase state for ice nucleation is still a matter of debate. Many studies have quantified the viscosity of SOA generated with OH or O 3 . In addition, two studies investigated the viscosity of phenolic SOA generated using OH or O 3 as the oxidants. In contrast, much less is known on the viscosity of SOA, either phenolic or other types, formed with NO 3 . Kasparoglu et al. suggested that SOA generated from the oxidation of α-pinene by NO 3 was viscous or hydrophobic or both, based on ice nucleation measurements. In addition, experiments by Perraud et al. suggested that SOA generated from the oxidation of α-pinene by a combination of O 3 and NO 3 is highly viscous, based on the non-equilibrium partitioning of organic nitrates. Field measurements have also suggested that atmospheric aerosol particles can be more viscous at night compared to the day. Slade et al. found this to be true for aerosol particles in a forest environment. Differences were attributed to terpene-derived, higher molecular weight SOA at night and isoprene-derived, lower molecular weight SOA during the day. Bateman et al. studied anthropogenic emissions in the Amazon and observed an increase in particle rebound fraction in an impactor at night, which is associated with more viscous particles. Hygroscopicity is another key physiochemical property of SOA that is important for predicting its climate impact. Hygroscopicity determines the water content of the particles, and hence their light scattering properties and ability to act as nuclei for cloud droplets. Hygroscopicity also impacts the viscosity of SOA. Consider for example an SOA type that has a high hygroscopicity and a high viscosity under dry conditions, such as SOA generated by OH oxidation of toluene. For this system, as the relative humidity (RH) increases, the SOA particles will take up water and the overall viscosity will decrease since water has a lower viscosity than dry SOA. The hygroscopicity of SOA generated from O 3 or OH has been extensively studied. In addition, there have been some studies on the hygroscopicity of phenolic SOA and proxies for phenolic SOA. In comparison, there have been fewer studies on the hygroscopicity of SOA generated by NO 3 , and we are not aware of any research that has quantified the hygroscopicity of phenolic SOA generated with NO 3 as the oxidant.