Microplastic pollution in marine, freshwater, and terrestrial ecosystems has received considerable attention. However, atmospheric transport of microplastics has been largely overlooked. Road traffic is a significant source of microplastics, primarily through tire wear particles (TWPs) and brake wear particles (BWPs). TWPs are generated by the mechanical abrasion of tires, composed of rubber, carbon black, steel cord, fibers, and other components. BWPs are produced by brake linings, a complex mixture of binders, fibers, fillers, and abrasives. While runoff and washout of TWPs and BWPs have been studied, atmospheric dispersion and deposition remain poorly understood, despite potential health impacts and contribution to climate change through light absorption. This study aims to address this knowledge gap by simulating the global atmospheric transport and deposition of TWPs and BWPs.

Previous research has documented significant global plastic production, resulting in substantial pollution. Studies highlight the presence of microplastics (1 µm to 5 mm) and nanoplastics (<1 µm) in various environments, caused by the fragmentation of larger plastics through various processes. The impact on marine and terrestrial ecosystems, including coral reefs and animals, and human health, is a growing concern. Road traffic has been identified as an important source of microplastics, with studies estimating significant emissions of TWPs and BWPs. Research on the transport of these particles via runoff and washout exists, but understanding their atmospheric dispersion and deposition is limited. The study draws on existing data on TWP and BWP emissions, including estimations from vehicle wear, and utilizes existing atmospheric models to simulate their transport and deposition.

The study utilizes two methods to estimate global annual emissions of TWPs: (a) a CO₂ ratio method, extrapolating from detailed Northern European data using a CO₂ emission ratio; and (b) the Greenhouse gas-Air pollution Interactions and Synergies (GAINS) model. For BWPs, only the GAINS model was used due to data limitations. The FLEXPART Lagrangian particle dispersion model was employed to simulate atmospheric transport and deposition globally. The model considered various factors, including emissions, particle size distribution (PM2.5 and PM10), wet and dry deposition, and scavenging coefficients. Multiple sensitivity scenarios were run, varying airborne fractions, particle size distributions, and cloud condensation nuclei/ice nuclei (CCN/IN) efficiency to assess uncertainty. The study also defined transport efficiency as the ratio of microplastic mass deposited in a remote area to the total globally emitted mass, enabling analysis of susceptibility in different remote regions. The study used the European Centre for Medium-Range Weather Forecasts (ECMWF) data for meteorological conditions.

Global annual TWP emissions were estimated at 2907 kt/year, while BWP emissions were 175 kt/year. Emissions were concentrated in regions with high vehicle densities (eastern US, Northern Europe, and Eastern China). Surface concentrations ranged from a few ng/m³ to 50 ng/m³ for both TWPs and BWPs, below air quality limits. Modelled lifetimes varied significantly depending on particle size; PM2.5 particles exhibited longer lifetimes than PM10 particles. Deposition maps revealed that smaller particles (PM2.5) were dispersed more widely than larger ones (PM10). A substantial portion of both TWPs and BWPs (approximately 30-34%) were deposited into the world's oceans. The Arctic was identified as a significant receptor region, with modeled concentrations ranging from 1 to 10 ng/kg of snow for PM2.5 TWPs and from 2 to 70 ng/kg for PM10 BWPs. High uncertainties in deposition were found, primarily due to uncertainties in emitted particle size distributions and airborne fractions, leading to variations in deposition, particularly in remote regions. Analysis of transport efficiency showed that a considerable fraction of PM2.5 microplastics were transported to the Atlantic and Pacific Oceans, while the Arctic region demonstrated significant transport efficiency as well.

The findings highlight the significant role of atmospheric transport in the distribution of road-derived microplastics to remote regions, including oceans and the Arctic. The substantial deposition of TWPs and BWPs in the oceans suggests this pathway is comparable to, or possibly greater than, riverine transport. The high transport efficiency to the Arctic, particularly during winter and spring, is concerning due to the potential for light absorption causing accelerated warming and ice melt. While the modeled concentrations are lower than black carbon, the cumulative effect of all microplastics, considering their proportion of total plastic production, should be considered. The uncertainties in the model highlight the need for further research and measurement data to refine estimates and validate the results.

This study demonstrates the significant contribution of atmospheric transport to microplastic pollution in remote regions. Road-derived microplastics, particularly TWPs and BWPs, are efficiently transported long distances, reaching oceans and the Arctic. The Arctic's vulnerability to these particles is noteworthy, posing a threat to climate stability and ecosystems. Future research should focus on refining emission estimates, improving model accuracy, and conducting measurements of atmospheric concentrations and deposition to validate the model results. The impact of non-road vehicles and re-suspension of particles should also be explored.

The study acknowledges several limitations. The model does not account for potential re-suspension of deposited particles, which could enhance transport to remote areas. Emission inventories do not include non-road vehicles, potentially underestimating total emissions. Furthermore, the lack of comprehensive measurement data limits the validation of model results. Uncertainties in particle size distribution and airborne fraction significantly affect deposition estimates, particularly in remote regions.