Supplemental Information: Microplastic Entrainment in Hail: Uncovering New Pathways for Atmospheric Pollution

Microplastics are recognized as a ubiquitous component of atmospheric particulate pollution, yet the inflow wind distribution, transport and accretion on hailstones within severe convective storm systems remain poorly understood. This study provides the first comprehensive analysis of microplastics preserved in hailstones from two major supercell storms in Texas: Del Rio (11 April 2020) and Uvalde (28 April 2021). Twenty-one hailstones were randomly chosen, melted, filtered through Whatman 0.2µm membranes, and retained particles analyzed using Fourier-Transform Infrared (FTIR) microscopy for polymer identification and morphological characterization. Microplastics were detected in every hailstone (N = 283 particles), confirming their ubiquity in the lower troposphere. Fragments were dominant morphology (69.4%), followed by fibers (23.6%), films (6.6%), and spheres (0.4%). Fifty-eight distinct polymer species were identified, with polycarbonate (n = 87), epoxy (n = 23), and polybutadiene (n = 14) comprising the most abundant chemical classes. A Kruskal–Wallis H test (p = 0.0176) revealed statistically significant heterogeneity among hailstones, implying multiple emission sources and atmospheric transport histories. HYSPLIT back-trajectory and land-use analyses revealed that low-level inflow air masses traversed manufacturing zones along the U.S. Mexico border and urban population centers that contributed to microplastic loading within the storm systems. These findings demonstrate that convective storms effectively scavenge and archive airborne microplastics, offering a novel approach for assessing inflow air mass transport and emission patterns in the atmosphere. Hailstones represent an underutilized natural archive for evaluating emission sources, inflow air mass transport, monitoring microplastic pollution and contributing to the atmospheric microplastic body of knowledge.