Poke-flow measurements. The poke-flow technique was used to constrain the viscosities of the inner and outer phases of the BBOA particles. Poke flow measurements were done on the same prepared samples used for the phase behaviour measurements. The setup was similar to that used previously. The samples were put in a humidified flow cell which has a window on the bottom to allow viewing by an inverted microscope (ME1400TC-INF, AmScope, USA). The cell also has a hole in the top, sealed by a flexible latex membrane, through which an ultrafine needle (13561-10, Ted Pella Inc., USA) is inserted. The needle is moved using a micromanipulator to poke BBOA particles. Experiments were conducted at 30%, 40%, 50%, and 60% RH, and the particles were conditioned in the flow cell for 3 hours before being poked. Poking caused the particles to crack if they were highly viscous, or else resulted in the formation of a hole in the particle which would recover over time to reduce the surface free energy of the system. Images of the particles were recorded for 2 hours after poking. Each measurement was taken at least twice. At 30%, 40%, and 50% RH, the particles cracked and did not flow during the 2 hour observation time (see below). For these cases, the viscosity is assigned ≳ 2.5×10 8 Pa s based on fluid dynamics simulations in COMSOL Multiphysics 5.4 (COMSOL, Sweden) with a quarter sphere model of a cracked particle. In short, the viscosity in the simulation was adjusted until the edge of the cracked particles in the simulation moved by 0.5 µm in 2 hours. In the simulations we assumed that the surface tension was 30 mN m -1 and the slip length was 5 nm, as conservative lower limits. The viscosity that gave 0.5 µm of movement in 2 hours was 2.5×10 8 Pa s. Since the movement in our experiments was less than 0.5 µm (based on the resolution of our microscope), and the surface tension and slip length used in the simulations were conservative lower limits, a viscosity of 2.5×10 8 should be consider a lower limit to the viscosities in the experiments when cracking and no flow was observed over 2 hours. At 60% RH, the particles did not crack and rather a hole formed and slowly recovered over time (see below). Fluid dynamics simulations based on a half-torus model have been developed to determine the viscosity of single-phase particles that formed a hole after poking and slowly recovered. However, in the current experiments, the BBOA particles contain two phases (see below). Since the previously developed half-torus model is based on single-phase particles, we did not attempt to apply this model to the current results. Developing and testing a half-torus model for two-phase particles is beyond the scope of the current study. Along with designing a new model, we would need information about the interactions between the two phases, e.g. the interfacial tension, which is not information we have knowledge of. As a result, we were not able to assign a viscosity to the poke flow experiments at 60% RH. Our approach of only assigning a viscosity to multiphase particles that cracked and did not flow is similar to the approach used by others when constraining the viscosity of field-collected multiphase samples. Samples for the poke-flow experiments were prepared by nebulizing 1/1 (v/v) solutions of methanol and water containing BBOA extracts. Even though the BBOA particles were exposed to a methanol-free carrier gas for an extended time after formation, we cannot rule out trace amounts of residual methanol remaining in the BBOA particles from the extraction process. For RH values of 0-50 %, we report a lower limit to the viscosity of the field-collected BBOA samples based on the observations that the particles shatter and did not flow (see below). The further removal of any residual methanol from the BBOA particles would not change the reported lower limits to viscosity, since methanol has a low viscosity and removal of any residual methanol would only increase the viscosity of our particles.