At 50% RH, poking the center of the Kamloops sample caused cracks in the outer phase but only a hole in the inner phase, without cracks. For the Kelowna sample at the same relative humidity, a poke in the center caused cracks from the center to the outer edge. Again, there were no signs of recovery in any of the cracked particles at 40 or 50% RH. We conclude that our samples had at least one phase that was semi-solid or amorphous solid with a viscosity ≥ 2. At 60% RH, both samples were poked once in the inner phase and once on the outer phase (Figure , fourth row), forming holes in the particles with no signs of cracking. After 2 hours, the hole in the middle almost fully recovered while the outer phase did not move. The unmoving outer phase may have been due to a difference in the total restorative forces acting on the outer edge of the droplet compared to the middle and the thinness of the outer phase, or, alternatively, the outer phase may have been more viscous than the inner phase. Without fluid dynamics simulations that include the geometry and interfacial tensions of the two phases, it is not possible to conclusively distinguish between these cases. Experiments were not conducted at > 60% RH, because a higher RH can only lead to lower viscosities as the particles take up water. The Kelowna and Kamloops samples comprised 88% and 96% wildfire BBOA, respectively (Table ). We estimated the influence of background non-BBOA on our measured viscosities using calculations based on the Arrhenius-type mixing rule (Supplementary Information Section S1). The calculations show that the background non-BBOA does not have a large influence on the lower limits to viscosities reported here for the BBOA. E.g. the presence of background non-BBOA could, at most, decrease our reported lower limit by a factor of 3 for the Kelowna sample.