To help put the freezing ability of microplastics into atmospheric context, the ice nucleation active surface-site densities, 𝑛 𝑠 (𝑇), of the PP-needles c , PP-fibers b , and PET-fibers b were compared to those for volcanic ash , fungal spores , NX-illite , K-feldspar , and a natural mineral dust parameterizations (see Figure ). The ice nucleation properties of mineral dust and volcanic ash have been the focus of many studies, and both represent important types of INPs in the atmosphere.