Do Microplastics Contribute to the Total Number Concentration of Ice Nucleating Particles?

Microplastics (MPs) can be transported into clouds, where they can affect formation and properties of clouds by acting as ice-nucleating particles (INPs), indirectly influencing the global climate.However, MPs have not been considered contributors to total INP concentrations.In this study, we quantify road traffic-related (i.e. from tire wear, brake wear, road markings and polymer-modified bitumen) MP number concentrations and estimate their contribution to total INP concentrations using the atmospheric transport model FLEXPART.To do this, we provide two possible global road traffic-related MP emissions scenarios.We find that MPs can disperse throughout the entire troposphere and reach regions with low natural ice nucleating particle concentrations.Under a high emissions scenario, ice-active MPs can contribute from about 0.1% to more than 40% of the total INP number under immersion freezing conditions in some areas of the tropics, while under cirrus conditions, their contribution can be up to about 7% over the tropical Pacific and up to about 20% over East Antarctica.Our results suggest that in regions where other effective INPs are rare, ice-active MP concentrations may be sufficient to trigger heterogeneous ice nucleation of ice crystals in mixed-phase or cirrus clouds, especially when concentrations of other effective INPs (mineral dust, marine particles, or bioaerosols) are low or absent.These results underscore the potential role of MPs in cloud formation and highlight the need to reduce uncertainties in MP emissions and their fate in the atmosphere as plastic production and use continue to grow.