Mechanism of microplastic and nanoplastic emission from tire wear

Tire and brake-wear emissions, in particular nanoparticulate aerosols, can potentially impact human health and the environment adversely. While there is considerable phenomenological data on tire wear, the creation and environmental persistence of particulate pollutants is not well understood. Here, we unequivocally show that normal mechanical tire wear results in two distinct micro and nanoplastic (MNP) populations: a smaller, aerosolized fraction (<10 μm), and larger microplastics. Nanoplastic emissions follow a power law distribution that we show is consistent with the classical arguments of Archard, and Griffiths. Nanoplastic pollution increases dramatically with vehicle speed and weight, as the power law distribution characterizing these gets steeper. Charge stabilization of the tire wear nanoparticles keeps them suspended, while microplastics settle due to gravity. Larger microplastics are formed by sequential wear processes and show a log-normal distribution, as anticipated by Kolmogorov. Thus, the particle size distribution provides mechanistic insights to tire fragmentation: the aerosolized fraction is determined by power input to the tire while the larger microplastics are determined by sequential wear processes due to tire-road surface interactions, independent of vehicle weight and speed.