The freezing curves of PP-spheres c , PET-spheres c , PE-spheres c , and PE-flakes b fall within the confidence interval of the water blank freezing curve and are therefore not distinguishable from the background. However, the freezing curves of these plastic samples deviate from the mean of the blank at fraction frozen values close to 1, which could indicate that some heterogeneous activity may exist. Given that freezing of the water background in µL-sized droplet freezing assays, as used in this study, is typically around -25°C , evaluating particle-induced freezing below this temperature would require an alternative assay using droplets in the nL range, where background freezing occurs around -35°C. To normalize the freezing data to the mass and surface area and compare our data with the literature, we determined the number of nucleation sites per unit mass, 𝑛 𝑚 (𝑇), and surface area, 𝑛 𝑠 (𝑇), (Figure ). The 𝑛 𝑚 (𝑇) values of the different ice-active microplastics samples agree within about one order of magnitude and increase roughly exponentially as temperature decreases (Figure ). At -18°C the values range from 7.