The environmental chamber was a 1.8 m 3 Teflon bag housed within an aluminum enclosure. The flow rate through the chamber was 18 L min -1 , resulting in a mean residence time in the chamber of ~1.7 h. Both the VOCs and O3 were carried into the chamber via pure air streams. For the VOC feed line, a 2 wt% solution of catechol or α-pinene in 2-butanol was injected into a heated round bottom flask using a syringe pump. The injection rate for both catechol and αpinene was 30 μL h -1 . 2-butanol was used as a scavenger for OH radicals which can be generated as a by-product of ozonolysis reactions. For the O3 feed line, pure air flowed through an ozone generator (UV-pen lamp; Jelight, model 610) and into the chamber. Both the ozone concentration and aerosol mass loading were measured at the outlet of the chamber. Ozone was in excess and was measured to be ~325-370 ppb using an O3 monitor (49i, Thermo Scientific). The mass loading in the chamber for catechol SOA ranged from 13-70 μg m -3 during collection. For α-pinene SOA, the mass loading was ~30-55 μg m -3 , as measured using an optical particle counter (GRIMM, 11-S OPC). The resulting SOA particles were collected on hydrophobic glass slides or silicon wafers at the outlet of the chamber using a multi-orifice single-stage impactor with a 50% cut-off diameter of ~0.18 μm (MOSSI, MSP Corporation), operated at a flow rate of 12 L min -1 . For imaging experiments, proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) analysis, and ultra-high-performance liquid chromatography heated electrospray ionization high-resolution mass spectrometry (UHPLC-HESI-HRMS) analysis, the particles were collected on hydrophobic glass slides (22 mm, Hampton Research Corp., HR3-215). For electron impact mass spectrometry (EI-MS) analysis, the particles were collected on silicon wafers with a native oxide layer (p-type, 111, Wacker Siltronic Inc., Germany). SOA collection times were ~16-27 h.