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

Microplastics (MPs) can be transported into clouds, where they may act as ice nucleating particles (INPs). However, MPs have not been considered as contributors to INP concentrations. Here, we quantify road traffic-related MP number concentrations, and estimate their contribution to total INP concentrations using the atmospheric transport model FLEXPART. We find that under a high emission scenario ice-active MPs can account from about 0.1% to more than 40% of the total INP number in immersion freezing conditions in the tropics, whereas for cirrus conditions, their contribution can be up to about 7% over the tropical Pacific and up to about 20% over East Antarctica. Thus, in regions where other effective INPs are rare, ice-active MP concentrations may be sufficient to trigger heterogeneous nucleation of ice crystals in mixed-phase clouds or cirrus. This suggests that MP may affect cloud formation and highlights the need to reduce uncertainty in MP emissions and their fate in the atmosphere as plastic use continues to grow.