Sample Preparation. BBOA filters were extracted with a 1/1 (v/v) solution of methanol (HPLC grade, Sigma-Aldrich)/water (HPLC grade, Thermo Scientific). Filters were cut into quarters and placed in glass vials into which 1.5 mL of solvent was added, and then shaken on a platform shaker at 200 rpm for 2 hours. Afterwards, the solutions were passed through 13 mm diameter, 0.45 µm pore Fluoropore membrane syringe filters (Millipore, USA) to remove soot and other insoluble components. Filtered extracts were stored in glass vials at 4°C and wrapped in aluminum foil to limit their exposure to light and avoid any photochemistry. Hems et al. and Trofimova et al. showed that water extracts 70-75% of unaged BBOA, while organic solvents, including methanol, are able to extract nearly 100% of unaged BBOA mass. As BBOA is aged, the amount that can be extracted with water is expected to increase and the amount extracted with methanol may decrease as oxidation will increase BBOA's polarity. We therefore expect our extract to have recovered between 70 to 100% of the organic aerosol material from the filters.