The yield of CO2 produced during the heating process was calculated by calibrating the signal at m/z 44 using known pressures of CO2. The CO2 signals in the chromatogram (Fig. ) were integrated. The integrated signal represents the total CO2 evolved from the SOA sample (Table ) since all the CO2 molecules evolved eventually reach the detector. The total number of SOA molecules initially present was calculated by using the estimated total mass of SOA impacted on the silicon wafer substrates and by assuming an average molar mass of 200 g mol -1 . By dividing the number of CO2 molecules evolved by the total number of initial molecules we estimate a CO2 yield to be 12 ± 2.9% for catechol SOA. For comparison, we estimated a CO2 yield of 0.7 ± 0.1% for α-pinene SOA.