In the recent years, a large attention has been given to pollution from plastic products as a major environmental problem. Plastics degrade into smaller particles in the environment via photodegradation, physical abrasion, hydrolysis and biodegradation. Microplastics (1 um to 5 mm size particles) have been reported to affect coral reefs, marine and terrestrial animals, as well as humans. It has been reported that about 30% of microplastics in freshwater and oceanic ecosystems are tire wear particles (TWPs), while brake wear particle (BWP) emissions constitute 55% of all non-exhaust traffic-related particle emissions and 21% of all traffic-related PM emissions. There is a general conviction that the relative contribution of TWP and BWP emissions to total transport-related emissions will grow in the near future, due to the continuous reduction of exhaust traffic-related emissions. Although transport of TWPs and BWPs via runoff and wash-out processes to the marine and/or freshwater ecosystem has been studied extensively, very little is known about how these particles are dispersed in the atmosphere and where they are deposited. This is important due to the aforementioned impact in animals and humans. They also have an environmental impact as they are derived by materials made from fossil fuels such as ethylene and propylene. Thus, larger needs of plastics result in larger emissions of greenhouse gases. Since TWPs and BWPs can become airborne and have been detected already in remote areas, they may absorbe light decreasing surface albedo and accelerating ice melting.
Here, we present for the first time the results of the atmospheric dispersion and deposition of traffic-related microplastics (TWPs and BWPs). We assess the suggested emissions using two methods, one indirect based on CO2 country ratios with road microplastics and extrapolation, and another employing an emissions model, which has been extensively used to determine global emissions of various substances by IIASA-International Institute for Applied Systems Analysis (GAINS model). We calculate that 34.4–290 kt y^-1 (mean: 100 kt y^-1), out of 102–787 kt y^-1 (mean: 284 kt y^-1) of PM10 TWPs emitted, were deposited in the World Ocean, while the respective annual terrestrial and riverine discharges are about 64 kt y-1. This shows that direct deposition of airborne road microplastics is likely the most important source for the ocean and marine biota. The calculated transport of PM10 road microplastics shows a relatively high efficiency over remote regions such as the Arctic Ocean (14%). High latitudes and the Arctic are highlighted as an important receptor of mid-latitude microplastic emissions, which may imply a future climatic risk taking into account that TWPs and BWPs constitute a small portion of the total plastic emissions. As of now, snow concentrations of road microplastics are 100 times lower than those of black carbon or polymers of larger usage (e.g., PVC or PPC). Around 15% of the PM2.5 road microplastic emissions were deposited in the Atlantic Ocean, whereas coarse particles were less efficiently deposited there (10-11%). The efficiency of PM2.5 deposition (TWPs: 19% - BWPs: 18%) over the Pacific Ocean was even more strongly enhanced relative to PM10 deposition (TWPs: 12% - BWPs: 11%), due to their smaller size. Transport efficiencies of coarse particles were up to twice of those for the fine particles in areas surrounded by microplastic emissions sources (e.g., Alps, Mediterranean, Baltic and South China Seas).

1. Tire wear particle (TWP) emission calculations based on a CO2 ratio method. Top-down estimates of total annual TWP emissions reported for Norway, Sweden and Germany, based on measurements of lifetime weight loss of returned tires, were used here as a proxy to extrapolate globally. Assuming a constant ratio of TWP emissions to CO₂ emissions from the road transport sector (0.49 mg TWP/g CO₂) from the CMIP6 (Coupled Model Intercomparison Project phase 6) inventory (0.5°×0.5° resolution), we created global TWP emissions for the year 2014. 2. TWP and BWP (brake wear particle) emission calculations with the GAINS model. We also used the emissions model GAINS (Greenhouse gas – Air pollution Interactions and Synergies; http://gains.iiasa.ac.at) to create global emissions for both TWPs and BWPs. The model gets emissions of air pollutants and Kyoto gases for nearly two hundred regions globally considering key economic activities, environmental regulation policies, and regionally specific emission factors and provides size speciated PM (particulate matter), as well as carbonaceous particles (BC, OC) for different emission sectors (transport, energy, industry, domestic and waste combustion, etc.). 3. Flexpart model outputs. The Lagrangian particle dispersion model FLEXPART (FLEXible PARTicle Dispersion Model) version 10.4 (https://www.flexpart.eu) was set to run in forward mode for year 2014 with a spin-up period of 3 months (September 2013) in 0.5°×0.5° resolution globally with a daily temporal resolution using 3-hourly 1°×1° operational analyses from the European Centre for Medium Range Weather Forecast (ECMWF).

SupInfoMetaData
Metadata explaining emissions and model simulations.

TWP_CO2_ratio_EMI
One (1) file with global TWP emissions calculated with the CO2 ratio method.

TWP_BWP_GAINS_EMI
One (1) file with global emissions of both TWPs and BWPs.

1.flexout_CO2_ratio_TWP_0.5um, 2.flexout_CO2_ratio_TWP_1.0um, 3.flexout_CO2_ratio_TWP_2.1um, 4.flexout_CO2_ratio_TWP_3.2um, 5.flexout_CO2_ratio_TWP_6.0um, 6.flexout_CO2_ratio_TWP_9.5um
Six (6) files with results from FLEXPART model simulations for TWP with emissions calculated using the CO2 ratio method and different particle diameter.

1.flexout_GAINS_TWP_0.5um, 2.flexout_GAINS_TWP_1.0um,  3.flexout_GAINS_TWP_2.1um, 4.flexout_GAINS_TWP_3.2um, 5.flexout_GAINS_TWP_6.0um, 6.flexout_GAINS_TWP_9.5um.nc

Six (6) files with results from FLEXPART model simulations for TWPs using emissions calculated with the GAINS model and different particle diameter.

1.flexout_GAINS_BWP_0.5um, 2.flexout_GAINS_BWP_1.0um, 3.flexout_GAINS_BWP_2.1um, 4.flexout_GAINS_BWP_3.2um, 5.flexout_GAINS_BWP_6.0um, 6.flexout_GAINS_BWP_9.5um.nc

Six (6) files with results from FLEXPART model simulations for BWPs using emissions calculated with the GAINS model and different particle diameter.

ECMWF_lsm_si_720x180_2014_daily

Sea-ice area fraction (siconc) and snow depth (sd) masks from ECMWF.

ECMWF_precip_720x180_2014_daily

Snowfall (sf) and total precipitation (tp) from ECMWF.

lsm_mask, ocean_mask

Land-sea mask

Mask for oceanic regions