Atmospheric transport is a major pathway of microplastics to remote regions

The study addresses the largely overlooked role of the atmosphere in transporting microplastics, specifically road-traffic-derived tyre wear particles (TWPs) and brake wear particles (BWPs), to remote regions. While global plastic production and pollution impacts on marine, freshwater, terrestrial ecosystems, and human health are well documented, knowledge gaps persist regarding atmospheric dispersion, long-range transport, and deposition of road microplastics. TWPs and BWPs, generated by mechanical abrasion and corrosion during driving and braking, can include particles smaller than 10 µm that remain airborne for extended periods. Prior observations have detected microplastics in remote areas, suggesting atmospheric pathways may be important. The purpose of this study is to quantify global emissions of road microplastics, simulate their atmospheric transport and deposition (including to oceans and cryospheric surfaces), and evaluate their potential climatic relevance due to light absorption and albedo reduction. The work is significant as it quantifies a major pathway delivering microplastics to remote regions and the world’s oceans, potentially rivaling or exceeding riverine inputs, and highlights the Arctic as a sensitive receptor.

The paper synthesizes prior research showing: (1) substantial global plastic production with environmental fragmentation into micro- and nanoplastics; (2) road traffic as an important source of microplastics via tyre and brake wear; (3) studied pathways of TWPs/BWPs to aquatic systems via runoff and washout; (4) limited understanding of atmospheric dispersion and deposition despite observed microplastics in remote environments (mountains, Arctic snow, sea ice); (5) potential health impacts on animals and humans and enhanced toxicity via adsorbed organics/metals; (6) microplastics as light-absorbing, potentially altering snow/ice albedo similar to black carbon. These gaps motivate a global-scale atmospheric transport and deposition assessment.

Emissions: TWP emissions were estimated using two approaches for cross-validation: (a) a CO2 ratio method based on detailed national data of tyre lifetime mass loss from Norway, Sweden, and Germany, deriving a global TWP/CO2 emission ratio of 0.49 mg TWP per g CO2 applied to 2014 CMIP6 road-transport CO2 emissions at 0.5°×0.5°; and (b) the GAINS integrated assessment model (nearly 200 global regions) with size-resolved non-exhaust PM emissions (PM1, PM2.5, PM10, total PM) incorporating vehicle-type-specific activity (distance driven) and emission factors, distributed on 0.5°×0.5° using road networks and population. Due to lack of analogous brake statistics, BWP emissions were taken from GAINS only. Size fractions: Large uncertainties in airborne fractions led to ensemble assumptions. For TWPs, five fractions were assumed for PM10 (2.5, 5, 10, 20, 40%) and PM2.5 (0.25, 0.5, 1, 2, 4%) of total TWPs. For BWPs, assumed airborne fractions spanned PM2.5 (30–70%) and PM10 (60–100%). Atmospheric transport and deposition: The FLEXPART v10.4 Lagrangian particle dispersion model was used with ECMWF operational meteorology to simulate global transport, wet (in-cloud and below-cloud) and dry deposition, and particle removal. Road microplastics were treated as rather hydrophobic, yielding small in-cloud scavenging coefficients; sensitivity to CCN/IN efficiency was included via three wet scavenging coefficient sets. Particle densities differed for TWPs and BWPs per literature, affecting deposition velocities. Ensemble design: For each species and size (PM2.5, PM10), 120-member ensembles combined uncertainties in airborne fraction (5), particle size distribution (8), and scavenging coefficients (3). Emissions for TWPs used both CO2-ratio-derived and GAINS inventories; BWPs used GAINS. Results were summarized by geometric mean and geometric standard deviation (log-normal variability). Diagnostics: Surface concentrations, atmospheric lifetimes, and annual total wet+dry deposition were computed globally. Snow concentrations in the Arctic were calculated by combining daily modelled deposition with ECMWF daily snowfall and sea-ice fraction fields, restricting to pixels with substantial snowfall. Regional deposition and transport efficiencies were computed using land–sea, ocean basin, continental, and polar masks. Transport efficiency was defined as the ratio of mass deposited in a receptor region to total global emissions. Uncertainty was expressed as geometric standard deviation factors (up to ~3), mapped spatially for deposition.

• Emissions and spatial patterns: Annual global TWP emissions were 2907 kt yr^-1 (3434 kt yr^-1 from the CO2 ratio method; 2380 kt yr^-1 from GAINS). Annual global BWP emissions were 174.6–175 kt yr^-1. Emissions concentrate in the eastern USA, Northern Europe, and urbanized regions of Eastern China, the Middle East, and Latin America. Size-resolved emissions: TWP PM2.5 = 29 kt yr^-1 (12–75); TWP PM10 = 288 kt yr^-1 (113–826). BWP PM2.5 = 98.2 kt yr^-1 (63.4–152); BWP PM10 = 146 kt yr^-1 (85.8–248). - Atmospheric concentrations and lifetimes: Surface concentrations reach up to ~20 ng m^-3 (TWP PM2.5) and ~50 ng m^-3 (TWP PM10/BWP). All concentrations are far below WHO PM limits. Modelled lifetimes: TWP PM2.5 = 28 ± 2.7 days (18–37); TWP PM10 = 8.3 ± 1.0 days (5.5–11). BWP PM2.5 = 28 ± 2.8 days (17–37); BWP PM10 = 1.3 ± 0.16 days (0.94–1.6). - Deposition totals and partitioning: TWP PM2.5 deposition total = 28.4–28.5 kt yr^-1 (reported mean 28.4 kt), with ~43% to land (mean 12 kt) and ~57% to ocean (mean 16 kt); ~8.1 kt yr^-1 to snow/ice. TWP PM10 deposition total = 284 kt yr^-1 (102–787), with ~65% to land (mean 184 kt), ~35% to ocean (mean 100 kt), and ~30 kt yr^-1 to snow/ice. BWP PM2.5 deposition total = 97.1 kt yr^-1 (59.2–162), with ~46% to land (45 kt) and ~54% to ocean (52 kt); ~30 kt yr^-1 (31%) to snow/ice. BWP PM10 deposition total = 142.3 kt yr^-1 (102–787 reported table; mean land 102 kt (72%), ocean 40 kt (28%)); ~30 kt yr^-1 (14%) to snow/ice. - Ocean deposition significance: About 34% of emitted coarse TWPs and 30% of emitted coarse BWPs (≈100 kt yr^-1 and 40 kt yr^-1, respectively) deposit to the world ocean via the atmosphere. Scaling riverine export estimates indicates ~64 kt yr^-1 of TWP may reach the ocean via washout/runoff, suggesting direct atmospheric deposition of road microplastics is likely the dominant source to the oceans. - Polar and snow impacts: Arctic snow concentrations are modelled at 1–10 ng kg^-1 (TWP PM2.5), 4–80 ng kg^-1 (TWP PM10), 2–30 ng kg^-1 (BWP PM2.5), and 2–70 ng kg^-1 (BWP PM10), with higher values in Northern Europe and North America. - Transport efficiencies to remote regions: PM2.5 transport to oceans is more efficient than PM10. Approximate efficiencies include Atlantic Ocean ~15% (PM2.5), Pacific enhanced for PM2.5 relative to PM10; South China Sea up to ~2%. For PM10 over Greenland: TWP 1.7%, BWP 2.3%; over the Arctic Ocean: TWP 6.8%, BWP 4.3%; Southern Ocean small (TWP 1.4%, BWP 0.5%); negligible to Antarctica. The Arctic (excluding Greenland) shows ~3.6% transport efficiency, similar magnitude for Greenland. - Health-relevant regional contrast: In regions surrounding major sources (e.g., Alps, Mediterranean, Baltic, South China Sea), PM10 deposition efficiencies exceed PM2.5, implying higher short-term exposure in populated areas. In remote regions (Arctic, Greenland), fine particle deposition dominates. - Uncertainty: Deposition uncertainties are high (geometric SD up to ~3), driven mainly by uncertainties in emitted size distributions and wet scavenging, with PM2.5 uncertainties larger near sources and PM10 uncertainties larger in remote regions.

The simulations demonstrate that road-traffic-derived microplastics (TWPs and BWPs) can remain airborne long enough—especially as PM2.5—to be transported globally and deposited efficiently in remote receptor regions, including the world’s oceans and the Arctic. This addresses the core question by quantifying an atmospheric pathway that is comparable to, or larger than, riverine inputs for certain sources (e.g., TWPs). The spatial partitioning shows that oceans receive a substantial fraction of airborne deposition, making the atmosphere a major conduit for global ocean microplastic loading. The Arctic’s notable transport efficiencies, combined with deposition on snow and ice, indicate potential climatic implications due to light absorption and reduced albedo, possibly accelerating melting. Regionally, higher PM10 deposition efficiencies near sources highlight potential local health concerns, while PM2.5 dominance in remote deposition underscores the role of long-range transport processes. The findings align with prior observations of microplastics in remote environments and quantify the atmospheric contribution, providing a basis for policy and monitoring strategies. However, high uncertainties and limited observational constraints necessitate targeted measurements for validation and refinement.

This work provides the first global-scale, ensemble-based quantification of atmospheric emissions, transport, and deposition of road microplastics (TWPs and BWPs). It shows that atmospheric pathways deliver significant amounts of microplastics to the world’s oceans—likely surpassing riverine inputs for certain sources—and to snow and ice surfaces in the Arctic and other cryospheric regions. The results suggest potential climate-relevant impacts via albedo reduction and point to the Arctic as a sensitive receptor. The study also highlights regional differences in deposition by particle size with implications for human exposure near sources. Future research should: (1) develop and deploy coordinated atmospheric and deposition (including snow/ice) measurement campaigns for microplastics to validate models; (2) better constrain emission factors, especially airborne fractions and size distributions for TWPs/BWPs across vehicle types and regions; (3) include re-suspension processes and non-road vehicle/machinery sources in emission inventories; (4) refine microplastic physicochemical assumptions (hydrophobicity, CCN/IN activity, density) affecting scavenging and deposition; and (5) assess radiative and cryospheric impacts of microplastics relative to black carbon and other impurities.

• Emission inventory gaps: Lack of comprehensive, globally consistent data for brake wear; non-road vehicle and machinery emissions excluded. - Airborne fraction and size distribution uncertainties: Large, poorly constrained fractions for PM10 and PM2.5 emissions drive major variability in results. - Microphysical assumptions: Hydrophobic treatment and simplified CCN/IN efficiency and wet scavenging may bias lifetimes and deposition. - Re-suspension not modelled: Potential secondary entrainment from land and ocean surfaces could enhance long-range transport (grasshopper-like effects). - Observational scarcity: Limited atmospheric and deposition (e.g., snow) measurements of microplastics preclude robust validation; reported uncertainties (geometric SD up to ~3) remain high. - Model structural uncertainties: Sensitivity of PM10 long-range transport to gravitational settling and below-cloud scavenging; regional meteorological variability (e.g., NAO phase) not explicitly assimilated beyond the meteorological forcing used.