The oxygen-to-carbon (O/C) ratios of each BB-SOA type were determined with a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne Inc.) which directly sampled BB-SOA-containing air masses from the environmental chamber. SOA particles were impacted onto a heated (~600 °C) filament, causing the organic particles to vaporize. The vapors were ionized by electron impact ionization (~70 eV). The bulk chemical composition was determined by collecting mass spectra using the MS mode. Air interferences were corrected by particle-filtered air from the chamber, which was sampled with the AMS. To determine the average elemental O/C ratios, mass spectra were collected in V-mode (V-shaped ion flight paths, single ion reflection; resolution of ~2400 m/Δm) and processed by the improved ambient method 69 using the PIKA v.1.25 and SQUIRREL 1.65 software packages in Igor Pro 8 (Wavemetrics Inc.). 200