The difference between the viscosities of guaiacol-NO compared to guaiacol-OH SOA. Water has a very low viscosity (10 -3 Pa s) and lowers the overall viscosity when added to a high viscosity material, by acting as a plasticizer. A lower hygroscopicity means less water uptake, and hence less reduction in viscosity as relative humidity increases. Recently, Gregson et al. showed that primary biomass burning organic aerosols generated by smoldering of pine wood contain two phases, a hydrophobic phase and a hydrophilic phase. The viscosity of the hydrophobic phase was largely independent of the RH up to ∼95%. The trend we observed for guaiacol-NO 3 SOA is similar to the trend Gregson et al. observed for their hydrophobic phase and can be attributed to the relatively hydrophobic nature of the material.