The viscosity of the super-micrometer SOA particles was determined using the poke-flow technique described in detail previously. 102-104 A hydrophobic slide containing SOA particles was placed inside a RH-controlled flow cell that was coupled to an inverted optical microscope (AmScope, ME1400TC-INF, Fig. ). To control the RH within the flow cell, dry and/or humidified zero-air was continuously flowed through the cell. The dew point of the air was measured by a chilled mirror hygrometer (General Eastern, 1311DR-SR-115V) mounted downstream of the flow cell. The temperature of the cell was monitored by a thermocouple (OMEGA, HH-200a) probe connected to the flow cell, and was approximately 294 K for all measurements reported here. The RH within the cell was calculated from the dew point of the air and temperature of the cell.