Fig. S1.

Mass spectra of fog water OA and aqSOA at SPC 2011. (A) High-resolution mass spectrum of OA in fog water sample, and (B–E) mass spectra of SOA averaged over 2 h after fog dissipation (B for event 1, C for event 2, D for event 3, and E for event 9). aqSOA is defined here as the total OA minus the primary OA. O:C indicates the oxygen-to-carbon ratio. Marker colors indicate fragment type (C_xH_y C_xH_yO₁, C_xH_yO_z > 1). The aqSOA mass spectra are dominated by oxygen-containing fragments (purple and pink markers) typical of highly oxygenated OA, and specifically by signals at m/z 29 (CHO⁺), m/z 43 (C₂H₃O⁺), and m/z 44 (CO₂⁺).