The woodsmoke aerosol was subjected to 10:1 dilution with HEPA-filtered air into a sampling chamber (4.75 m 3 ) out of which the aerosol was sampled onto glass slides. For microscopy experiments (e.g. Fig. and), BBOA was sampled onto 12 mm glass slides using a MOUDI-II impactor (120R using stage 7, TSI, Minnesota USA), cut size 0.32 μm, serviced by a pump (Varian, Lexington USA) running at 30 l min -1 for ~ 5 minutes. Collection times were long enough so that submicron BBOA particles deposited and agglomerated on the slides to form super-micron BBOA particles. Slides were pre-coated with FluoroPel-800 (Cytonix, Beltsville USA) to have a hydrophobic and oleophobic surface for higher particle contact angles and improved imaging. Experimental details about the microscopy imaging of particles is given in Section S3.1. Samples for rFRAP measurements were collected onto 22 mm siliconized slides with a slit impactor (Sioutas, SKC, Pennsylvania USA) with cut size 0.25 μm serviced by the pump running at 9 l min -1 . For rFRAP, thin films of sample are required, so the glass slides were not coated with Fluoropel-800 to allow for a lower contact angle. Samples were sealed in a Petri dish and stored in a freezer until use. A detailed schematic of the woodsmoke generation system is presented in Fig. .