Microplastic particles contain ice nucleation sites that can be deactivated by atmospheric aging

Recent research has shown that microplastics are widespread in the atmosphere. However, we know little about their ability to nucleate ice and impact cloud glaciation. Ice nucleation by microplastics could also limit their long-range transport and global distributions. The present study explores the heterogeneous ice nucleation activity of seven microplastic samples in the immersion-freezing mode. Two polypropylene and one polyethylene terephthalate sample froze heterogeneously with median freezing temperatures of -20.9°C, -23.2°C and -21.9°C, respectively. The number of ice nucleation sites per surface area, n_s (T), ranged from 10-1 to 104 cm-2 in a temperature interval of -15 to -25°C, which is comparable to that of volcanic ash and fungal spores. After exposure to ozone or a combination of UV light and ozone, simulating atmospheric aging, the ice nucleation activity decreased in some cases and remained unchanged in others. An initial assessment using microplastic concentrations reported in the literature and our freezing data suggest that microplastics may influence cloud glaciation in some situations, but more studies are needed to confirm this initial assessment. Ice nucleation by microplastics may also impact their long-range transport and global distribution of microplastics based on a comparison of our freezing results and those of fungal spores.