UHPLC-HESI-HRMS analysis was conducted on unheated catechol SOA samples and catechol SOA heated for ~2 h at either 36 or 52 °C. Fig. shows the integrated mass spectra (from 2-16 min of chromatographic elution), and Table presents a summary of the most abundant peaks from the mass spectra as well as possible molecular assignments. Some of the peaks were identified as monoterpene and sesquiterpene oxidation products, likely from SOA experiments conducted in the same environmental chamber prior to catechol SOA production. See Section S4 for further discussion. The viscosity of catechol SOA was previously measured to be > 1 × 10 9 Pa s at 20 °C (293 K) under dry conditions, i.e., ~0% RH. We used these results and the Vogel-Fulcher-Tammann (VFT) equation to predict the viscosity of the catechol SOA at temperatures > 20 °C. The VFT equation describes the temperature dependence of viscosity: ln 𝜂( 𝑇) = ln 𝜂 0 +