Oxygen-containing functional groups on microplastic surfaces may also promote ice nucleation through hydrogen bonding. FTIR analysis (Figure ) revealed that PP-needles c had such groups (i.e. O-H and O-O-H stretching vibrations between 3300 and 3100 cm -1 , C=O stretching between 1550 and 1700 cm -1 ), potentially explaining their higher nucleation temperatures. However, a clear relationship between oxygen content and ice nucleation ability was not observed across all samples. For instance, PET-spheres c had high oxygen content (see chemical structure in Figure and FTIR spectrum in Figure ) but did not induce ice nucleation.