The second type of experiment involved linearly heating the sample from -45 °C to 95 °C at a rate of 0.1 °C s -1 . During heating, the mass spectrometer was set for single ion monitoring at m/z 44, identified during the first experimental approach. The CO2 signal at m/z 44 was observed to increase starting at 40 °C and reach a maximum at 95 °C (Fig. ). The temperature was then held at 95 °C and the signal decreased back to the background level within 5 min (Fig. ). The onset temperature for the evolution of CO2 here is consistent with the onset observed by imaging the particle's cross-sectional area (Fig. ). When the same procedure was carried out with α-pinene SOA, the m/z 44 signal was much lower (Fig. and).