To understand which molecules were participating in the boiling behavior of catechol SOA, we built a setup to flow the evolving gas phase molecules into a Vocus. We measured the background vapors from the experimental setup by heating a flow cell to 36 or 52 °C (Fig. and). Next, we added the SOA sample to the heated flow cell and observed the vapors that evolved from the heated SOA sample and flow cell combined (Fig. and). Difference mass spectra between the sample and heated flow cell show the enhancement of m/z C3H7O + and C3H9O2 + assigned to acetone and an acetone-water cluster, respectively (Fig. ). There was a strong correlation between the signals of C3H7O + and C3H9O2 + (R 2 = 0.99, Fig. ). These results indicated that acetone was a dominant species that evolved from the SOA material when heated to 36 or 52 °C.