Mass-based hygroscopic growth factors (mass growth factors), the ratio of the particle mass at a certain RH to the particle mass under dry conditions (RH < 1%), were determined with a quartz crystal microbalance for both guaiacol-NO 3 and guaiacol-OH SOA (Fig. ). The mass-based hygroscopic growth factor for guaiacol-NO 3 SOA particles was 1.01-1.05 at RH ≤ 80%. The guaiacol-OH SOA hygroscopic growth factor was 1.03-1.25, significantly higher over the same RH range. Above 95% RH, the hygroscopic growth factor for guaiacol-NO 3 SOA drastically increased. This drastic increase may have been due to liquid-liquid phase separation occurring within the SOA at RH values above 95%. Previous thermodynamic calculations have shown that a low-polarity SOA can exist as an organic-rich phase below approximately 95% and undergo liquid-liquid phase separation at high RH values to form an organic-rich and water-rich phase, concurrent with a drastic increase in the SOA hygroscopic growth factor. Mass-based hygroscopicity parameters (k m , Eq. 1) were also determined with QCM measurements (Fig. ). Guaiacol Pa s. Our results are consistent with previous studies that suggested SOA generated with NO 3 may be highly viscous. For comparison purposes, in Fig. we have included the viscosities of guaiacol-OH SOA reported by Kiland et al. The viscosity of guaiacol-NO 3 SOA is significantly higher than guaiacol-OH SOA at ≳ 20% RH (Fig. ). For instance, at 40-50% RH, the viscosity of guaiacol-NO 3 SOA particles is > 5 orders of magnitude higher than that of guaiacol-OH SOA particles.