The room-temperature viscosity of the SOA was determined with the poke-flow technique (Fig. ) that has been described in detail elsewhere. Briefly, the sample slides were placed in a flow cell mounted above an inverted optical microscope. The RH within the flow cell was controlled by adjusting the RH of the carrier gas, using a bubbler system. The RH was calculated from measurements of the temperature and the dewpoint temperature of the carrier gas. After RHconditioning, an ultra-fine tungsten needle with an oleophobic coating was used to poke one of the phenolic BB-SOA particles while recording images with a digital camera coupled to the microscope. At RH values of approximately 20% and greater, poking the particle changed the geometry of the particle from a spherical cap to a half-torus (Fig. , top row). Following the poke, the particle material flowed and returned to its energetically preferred spherical cap geometry. The experimental flow time (exp,flow) was assigned as the time for the area of the hole (A0) to reach ¼A0. The experimental flow time has previously been shown to be proportional to the particle's viscosity. In some experiments with very long recovery times, the experiment was stopped before the inner area could reach 1/4 th its initial value. In these cases, the time from the initial poke until the last image captured was used as a lower limit to exp,flow. In experiments with RH > 40%, exp,flow was shorter than the camera's frame rate (3 fps) and the time required to remove the needle from the center of the particle. When this fast flow occurred, we used a time of 0.5 s (the approximate time to remove the needle from the particle) as an upper limit to exp,flow. In experiments with RH ~ 0%, poking caused the particle to crack as opposed to forming a half-torus geometry (Fig. , bottom row). In this case, no flow was observed even after ~20 h.