For boiling to occur in the SOA particles, two conditions must be fulfilled: (1) chemical reactions resulting in a formation of molecules with vapor pressures > 1 atm must occur when SOA is heated; (2) the production rate of these high vapor pressure molecules must be larger than the loss rate of these molecules by molecular diffusion of the molecules to the surface of the SOA particles followed by evaporation. The vapor pressure of acetone is 1 atm at 36 and > 1 atm at 52 °C. The vapor pressure of CO2 is >> 1 atm at these temperatures. Hence, the formation of acetone or CO2 upon heating the SOA would satisfy the first criterion. The slow mixing time of acetone and CO2 in the particles calculated above (Fig. ) also suggests that the second criterion would be satisfied in our experiments. Taken together, the formation of acetone and CO2 during mild heating and the slow mixing time of acetone and CO2 within the supermicron SOA particles explains the observed particle boiling behavior. This is the first study that has directly observed boiling in SOA material due to heating.