Phase behaviour. Samples for phase behaviour experiments were prepared by nebulizing extracts of the BBOA filters onto 13 mm diameter glass slides coated with FluoroPel 800 (Cytonix, USA), a hydrophobic coating, to promote spherical droplet formation. Extracts were nebulized using a syringe pump and a pneumatic nebulizer (Part #G1946-67098, Agilent, USA) with a sheath gas flow of 15 L min -1 to dry the particles (Figure ). The particles were then collected on the glass slides using a micro-orifice uniform deposit impactor (MOUDI-II 120, TSI, USA) (Figure ). After impaction, the slides were conditioned at 95% relative humidity (RH) overnight above a saturated KNO3 solution in a sealed jar. The high RH values allowed for the impacted particles to take up water and grow and take on more spherical shapes, which are better for imaging. To observe the phase behaviour, the slides were imaged at relative humidity values ranging from 0-100% within an RH-controlled flow cell with glass windows coupled to a microscope (Axiotech 100 HD reflected-light microscope, Zeiss, Germany). The RH was ramped from 0 to 100% over the course of 2 hours, then decreased back to 0% RH over 2 hours. Results are only shown for decreasing RH experiments since the same behavior was observed when the RH was increased and decreased.