𝐹𝑖𝑏𝑒𝑟𝑠/𝑑𝑟𝑜𝑝𝑙𝑒𝑡 was determined based on the average volume of the individual PP-fibers b and a density of 0.90 g/cm 3 . The calculation using Equation 4 gave 0.15 ice nucleation sites per fiber at -20°C. Next, we used this value along with the microplastic concentrations reported by González-Pleiter et al. (0.032 L -1 ) to estimate an INP concentration of 0.005 L -1 of air at a freezing temperature of -20 °C. Previous studies have shown that an INP concentration of 0.01 L - 1 of air at -20°C can be sufficient to influence cloud glaciation. Hence, we conclude that in some situations the concentrations of microplastics may be high enough to cause cloud glaciation.