A recent study by González-Pleiter et al. showed that microplastics, specifically polymer fragments and artificial fibers, can accumulate to concentrations reaching up to 0.032 L -1 in the PBL (planetary boundary layer) above urban areas. The identified fibers exhibited an average length and width of 662 µm and 25.4 µm, respectively, similar to PP-fibers b in our study. To provide an initial assessment of the potential impact of microplastics on clouds, we combined the upper limit of microplastics concentrations reported by González-Pleiter et al. (0.032 L -1 ) and our freezing results for the PP-fibers b . First, we calculated the number of ice nucleation sites per fiber at a freezing temperature of -20°C using the freezing data for PP-fibers b and the following equation: