A high-resolution time-of-flight Aerosol Mass Spectrometer (HR-ToF-AMS) (Aerodyne, USA) was used to measure the mass spectra of the BBOA extracts. The working principle and operation of the HR-ToF-AMS are described in detail elsewhere; briefly, the AMS measures electron-ionization mass spectra of aerosol particles sampled from ambient air and can report the bulk aerosol quantities of organics, nitrate, sulfate, ammonium, and chloride mass concentrations. The vaporizer current was set to 1 A. To prepare the sample in a form that could be sampled by the AMS, extracts were nebulized using the same syringe pump pneumatic nebulizer used for the phase behaviour measurements (Figure ). After nebulization, samples were diluted and dried with a sheath gas flow of 10 to 15 L min -1 . The aerosol then passed through a home-built diffusion dryer packed with 30 cm of molecular sieves (Type 4A beads 8-12 mesh, Supelco, Germany) and 30 cm of activated charcoal (Sigma-Aldrich, USA) to remove any residual water and methanol, respectively, and then sampled by the AMS (Figure ). Highresolution AMS data were processed using Squirrel v1.66 and PIKA 1.26 in the Igor Pro software environment (WaveMetrics, USA). Elemental ratios were estimated using the Improved Ambient method using peaks up to m/z 180, since over 93% of the mass was contained in m/z below 180 for all samples. For each sample, we report the mean oxygen-to-carbon ratio (O:C), hydrogen-tocarbon ratio (H:C), and carbon oxidation state (OS ���� 𝐶𝐶 , ≈ 2 × O: C -H: C).