For the EI-MS experiments, SOA was impacted onto silicon wafers. The mass on each wafer ranged from 0.28 to 0.45 mg for catechol SOA and 0.47 to 0.65 mg for α-pinene SOA. For each experiment, a wafer was inserted in the vacuum chamber at 25 °C, which was then cooled to -45 °C while purging the chamber with helium gas. After reaching -45 °C, the chamber was pumped down to 1×10 -4 Torr.