Mapping the tire supply chain and its microplastics emissions using a multi-stakeholder approach

The important contribution of tires to microplastics found in the environment raises the question how to effectively mitigate the environmental release of tire microplastics. The aim of this study is to map the tire supply chain and quantify the resulting tire microplastics emissions in close collaboration with stakeholders, thereby providing a solid foundation for the development of a mitigation strategy. To this end, the supply chain was mapped and used to identify stakeholders and to quantify microplastics emissions using a material system analysis for the Netherlands, 2021. Stakeholder involvement was integrated for data gathering and to build trust for future collaboration. The total initial tire microplastics emissions were estimated at 19,580 t/y (85 % by tire wear and 15 % by infill). Approximately 9,450 t/y of tire microplastics were estimated to reach the environment, 80 % to soils and 20 % to surface waters. This is the first step towards an effective mitigation strategy.


Introduction
Microplastics from tires are responsible for an important part of the microplastics detected in the environment (Boucher and Friot, 2017;Kole et al., 2017), e.g., in the form of tire and road wear particles (TRWP).TRWP, or tire wear particles (TWP), are the result of mechanical forces caused by the contact between tires and the road surface while driving (Panko et al., 2013;Sieber et al., 2020;Wagner et al., 2018).Microplastics are also released during other stages of the tire's lifecycle, e.g., granulated tires that are used as infill for artificial turfs (Schneider et al., 2020).In the present paper, we use the term "tire microplastics" to refer to all microplastics that originate from tires 1 (Luo et al., 2021).
The important contribution of tires to microplastics found in the environment triggers the question how the release of tire microplastics to the environment can be reduced in an effective and efficient way.Answering this question is not straightforward as many interventions are possible.These vary from source-oriented measures, such as improving the wear resistance of tires, to end-of-pipe solutions, such as capturing and treating road runoff (Furuseth and Rødland, 2020;OECD, 2021).Whether an intervention is feasible depends on factors such as its effectiveness in reducing microplastics emissions, its costs and (upstream) implications for important stakeholders, such as the tire industry.Moreover, the implementation of interventions requires sufficient acceptance from stakeholders (de Kraker et al., 2021).Therefore, planning such interventions does not only require detailed insight into the environmental sources, pathways and effects of tire microplastics (Mitrano and Wohlleben, 2020), but also an integrative multi-stakeholder approach (Mian et al., 2022).Stakeholder involvement is critical as it provides a way to better understand the complexity of a problem and the diversity of perspectives around it, and it builds trust for the exploration of, in this case, robust and realistic mitigation measures (Chua, 2016;Pohl and Hirsch Hadorn, 2007).Additionally, it facilitates data gathering and the validation of results (Diedrich et al., 2011;Oreskes, 2004).The first step towards a multi-stakeholder mitigation strategy is to create a systematic overview of the processes associated to the tire's lifecycle, such as manufacturing processes and recycling processes, how these relate to microplastics emissions, but also of the stakeholders involved and their interests.
Several studies already provide an extensive overview of the release of tire microplastics, but most focus exclusively on emissions during the user phase, i.e., TRWP and/or TWP (e.g., Baensch-Baltruschat et al., 2020;Knight et al., 2020;Prenner et al., 2021).The studies by Wang et al. (2019), Sieber et al. (2020), Luo et al. (2021) and Zhou et al. (2023) are the only studies that included tire microplastics emission sources other than TRWP and/or TWP.Sieber et al. (2020) performed a probabilistic material flow analysis.That study quantified solely the mass of rubber, excluding additives.However, as the tire vulcanization process results in an inseparable mixture of -partly synthetic -rubber and additives (ECHA, 2017;Wik and Dave, 2009), this results in an underestimation of the actual mass of tire microplastics released to the environment.The study by Luo et al. (2021) entails a literature study describing up-to-date literature on different types of tire microplastics.Unfortunately, no quantification of the emissions is provided.Zhou et al. (2023) and Wang et al. (2019) both estimated the release of tire microplastics through modelling.However, both studies focus more on the geographical distribution of the emissions, rather than where in the supply chain emissions occur.Moreover, none of the studies mentioned above involved stakeholders, which makes it uncertain whether all intricacies of the tire supply chain were captured and shared by stakeholders, providing a suboptimal basis for developing a comprehensive mitigation strategy.In conclusion, we argue that, up to this date, no systematic overview exists on the release of tire microplastics from the entire supply chain and the stakeholders involved that provide a sufficient basis for developing an effective mitigation strategy.
The aim of the present study is to map the tire supply chain and quantify the resulting tire microplastics emissions in close collaboration with stakeholders, thus providing a solid foundation for the development of an emission mitigation strategy.The supply chain was mapped using the Supply Chain Operations Reference (SCOR) model methodology (SSC, 2012).Stakeholders and their interests were identified and connected to the supply chain.A material system analysis was performed as a case study to estimate tire microplastics emissions for a specific country and year, i.e., the Netherlands in 2021.Input from stakeholders was used to validate the supply chain map, identify (other) stakeholders and to support the estimation of tire microplastics emissions to the environment.

Methodology
Fig. 1 shows the conceptual model of the research methodology.As a starting point, the tire supply chain was mapped.Stakeholders were identified by linking them to processes in the supply chain.In return, stakeholder collaboration was used to refine the supply chain map and to validate and acquire data for the material flow analysis.The material flow analysis, with the supply chain map as a basis, was used to quantify the tire material streams for the Netherlands in the year 2021, including the emissions to receiving soil and surface water.

Supply chain mapping
The Closed Loop Supply Chain (CLSC) of a tire was mapped using SCOR.The CLSC refers to all forward logistics in the chain, such as extraction, while at the same time including the reverse logistic network, which focuses on the collection and processing of returned products (Bloemhof-Ruwaard et al., 2004).SCOR is a process reference model for supply chain management that aims to facilitate the mapping, assessment and evaluation of processes and activities in the supply chain in order to improve and streamline processes (SSC, 2012).
Firstly, the different lifecycle stages present in the supply chain were distinguished.Then the processes in each lifecycle stage were identified and mapped.Only processes related to (tire) material streams were included.Data was obtained from peer-reviewed and grey literature.Literature was found using Scopus with keywords including: 'tires/ tyres', 'supply chain', 'manufacturing', 'collection', 'End-of-Life'.A tire manufacturing plant and R&D department were visited to acquire indepth insight into the tire manufacturing process.Open interviews were conducted with RecyBEM, VACO & Rijkswaterstaat to acquire additional knowledge and to validate information found in literature.The supply chain map was discussed for validation during the stakeholder workshop (see Section 2.2).

Identifying stakeholders and stakeholder involvement
Stakeholders were identified by linking them to processes in the CLSC.To identify additional stakeholders snowball mapping was used by conducting semi-structured interviews with a number of stakeholders.These interviews were also used to refine the supply chain map, creating a more detailed description from where more stakeholders could be identified.The identified stakeholders were categorized into subsets.The respective interest of each stakeholder subset was described as well as the relation of the subset to tire microplastics.
Individual contact, consisting of interviews and meetings, was established and maintained during the study with the aim to obtain data to acquire insight into stakeholder interests, to build trust and to expand the stakeholder network.A stakeholder workshop was organized to validate the initial supply chain map and the identified tire streams and emission flows, and to acquire more insights into the stakeholder network and its dynamics.The workshop was performed in the interactive online platform MIRO with thirteen particpants attending the workshop.The workshop started with an introduction to the research project and by the participants, followed by a brief discussion on microplastics related challenges observed in the different sectors.Most time was spent on discussing the supply chain map and the quantified emission streams.The workshop ended with participants sharing possible mitigation measures and potential obstacles.Information with supporting figures were sent to the attendees beforehand.A short questionnaire (Supplementary Information, Section 2.2) was sent before and after the workshop eliciting expectations and experiences regarding the workshop.

Material system analysis
In order to estimate the tire microplastics emissions, a material system analysis was performed with the supply chain map as a basis.By doing this, the (global) supply chain map was narrowed down to a select region, or case study area, where all relevant tire streams could be quantified.The Netherlands and the year 2021 were chosen as a case study.A material system analysis focuses on the flows of an individual material or natural resource within a system, including lifecycle-wide inputs and outputs (OECD, 2008).For each process in the tire supply chain, the tire material streams were tracked and quantified.External flows into the system were defined by mass inflow values, whereas internal flows, including the initial lost and environmentally released microplastics, were defined by transfer coefficients.A sensitivity analysis was performed to determine the variation and importance of the input data and transfer coefficients.Lastly, a visual representation was made.The translation of the supply chain map to the material system analysis based on the specifications of the case study area is described in detail in Section 3.

Case study and model description
The system boundaries for the material system analysis were set to the Netherlands and the year 2021.This decision was based on data availability and the good contact with key stakeholders in this region.The types of tires included were passenger car tires (including van and caravan tires) and bus-and truck tires.

Tire streams
The tire's lifecycle starts with the resource extraction and processing of raw materials.The subsequent tire manufacturing consists of mixing, component manufacturing, tire building, tire curing and final inspection (Continental, 2021;Shanbag and Manjare, 2020).Afterwards, tires are distributed as part of a new vehicle or via the numerous tire distributers on the replacement market (RecyBem, personal contact, 11 April 2022).After a tire is disposed of, it is collected and distributed based on its quality.Tires with more than the legal minimum tread depth can be sold on the international used tire market (RecyBEM, personal communication, 11 April 2022).Retreading is possible for bus and truck tires of which the carcass is still of sufficient quality (VACO, personal communication, 23 September 2022).The remaining tires are End-of-Life Tires, of which the main portion is mechanically recycled (RecyBEM, 2022).The rubber granulate that is produced can be used in a number of applications, such as infill on artificial turfs and molded objects (Recy-BEM, 2022).
Data from RecyBEM, the Dutch organization responsible for used tire collection, and the European Tire Manufacturers Association (ETRMA) was used to determine the number of tires entering the model's system (ETRMA, 2022;RecyBEM, 2022).To calculate the weight, an average weight of 8 kg for passenger car tires and 80 kg for bus-and truck tires was used (Continental, 2013).The passenger car tire stock was based on the number of vehicles in use in 2021 (CBS, 2021).The bus and truck tire stock was estimated by multiplying the input of tires by four representing the average lifespan of a tire, i.e., 4 years (Hillenbrand et al., 2005).From the collected passenger car tires, 71 % was mechanically recycled, 26 % was sold on the reused tire market, 1.1 % was retreaded, 1.7 % was incinerated and 0.2 % was used for alternative purposes (RecyBEM, 2022).From the collected bus and truck tires, 35 % was retreaded (VACO, 23 September 2022).The share of bus and truck tires that is reused was unknown.Therefore, a reuse percentage of 26 %, similar to passenger car tires, was assumed.The remaining share of collected bus and truck tires is mechanically recycled to rubber granulate (VACO, 23 September 2022), which is then 39 %.Rubber granulate was, subsequently, used in products, such as molded objects, safety and roof tiles and infill for artificial turf (RecyBEM, 2022).Details regarding the tire streams can be found in Supplementary Information, Sections 3.1, 3.2 and 3.3.

Initial generation of tire wear particles
To calculate the mass of TWP generated during the user phase, the Emission Factors (EF) (in mg/km) from ADAC (2019) , 2 Luhana et al.  (2004) and Gebbe & Hartung (1997) were used, as these are considered the most trustworthy studies (Mennekes and Nowack, 2022).The average weight loss percentage per tire type was determined using Eqs.
(1) and ( 2).An average mileage per tire of 55,000 and 80,000 km for passenger car tires and bus-and truck tires was used, respectively.The average mileage was based on Boulter (2005 andGRPE (2013) and validated by stakeholders.Details regarding the formula and the calculations can be found in Supplementary Information, Section 4.1.

Initial loss of infill granules from artificial turfs
To determine the microplastics emissions from artificial turfs, the mass balance by Kole et al. (2023) was used.Other authors also aimed to quantify the initial loss of infill granules, such as Løkkegaard et al. (2019) and Verschoor et al. (2021).Kole et al. (2023) builds further onto the mass balances derived from these studies, while also incorporating additional studies.The losses as a result of snow clearing were omitted, due to the limited snow days in the Netherlands.The yearly weight loss per m2 of turf was determined using the absolute weight loss per field per year and dividing it by the m 2 per field, as provided by Kole et al.To extrapolate this to the entire Netherlands, it was estimated there were 2688 large turfs and 5052 small turfs in the Netherlands in 2021 (Hann et al., 2018;Oldenkotte, 2017), translating to a total of 2326 ha of artificial turf.Details regarding the calculations of the infill stock and loss of infill granules can be found in Supplementary Information, Sections 3.4 and 4.2.

Environmental pathways
Insights in how tire microplastics are released to the environment and to which environmental compartment help to identify and prioritize mitigation measures based on their expected impact.Tire microplastic emissions were modelled up to the first receiving environmental compartment, i.e. soils and (fresh) surface waters.Emissions to air and to the marine environment were considered of minor importance and therefore not included in the current study; neither was transport between environmental compartments such as transport from (fresh) surface waters to the marine environment.The transfer coefficients underlying the environmental pathways can be found in the results (4.3.) and are also summarized in Supplementary Information, Table 18.

Release of tire wear emissions from driving
TRWP can be captured in asphalt, particularly very open asphalt (ZOAB) which covers the majority of the Dutch highways (Geilenkirchen et al., 2022).As the ZOAB asphalt roads are cleaned twice a year and the wastewater is disposed of properly, the captured TRWP do no contribute to environmental emissions (Rijkswaterstaat, personal communication, 2 May 2022; Geilenkirchen et al., 2022).The part of the TRWP that is not captured in the asphalt will either be transported to the roadside or to the sewer system or surface waters (Baensch-Baltruschat et al., 2020;Verschoor et al., 2016).Another share of the emitted TRWP is airborne.This fraction likely deposits outside the system's borders (Evangeliou et al., 2020;Kole et al., 2017) and is therefore excluded from the total emission calculations.As there are important differences in pathways between different types of roads, e.g.due to differences in asphalt and connectedness to the sewer, a distinction was made between urban roads, rural roads and highways.

Release of infill granules from artificial turfs
Part of the emitted infill particles will be prevented from reaching the environment, e.g., by collecting infill granules from clothes and shoes or by cleaning maintenance equipment (Løkkegaard et al., 2019).Emitted infill particles that are not captured will either be transported to adjacent soils or paved areas or to the sewer system (Løkkegaard et al., 2019;Regnell, 2019).The distribution of environmental pathways was based on Løkkegaard et al. (2019).

Transport and removal in the sewer system
The sewer system in the Netherlands consists for 66 % of combined sewers and 34 % of (improved) separate sewers (Liefting and de Man, 2017).In the combined sewer system, the household and industry wastewater is collected together with road runoff and treated in a central wastewater treatment plant (WWTP).In separate sewer systems, the road runoff is collected separately from the household and industry wastewater.The untreated road runoff is then often discharged directly to surface waters (Kole et al., 2017).In an improved separate sewer system the road runoff it transported to a WWTP as long as there is capacity (Liefting and de Man, 2017).

Supply chain map
Fig. 2 shows the supply chain map, in which both the processes per lifecycle stage and the tire material streams between them are presented.The tire's lifecycle stages are shown in relation to the primary stakeholder group, i.e., suppliers of raw ingredients, tire manufacturers, distributers, consumers and processors of used tires, which includes used tire collectors.It was determined that only tire usage and rubber inill granules contribute to the environmental release of tire microplastics.Microplastic release from other processes were considered to be negligible or without release to the environment.The assessment underlying this conclusion and a brief explanation of the different lifecycle stages can be found in the Supplementary Information, Section 1.

Stakeholder identification
Table 1 shows the identified stakeholder subsets, including their interest and relation to tire microplastics emissions.A brief description of these stakeholders can be found in the Supplementary Information, Section 2.1.Not all of the identified stakeholders were involved in the stakeholder participation process.The key stakeholders involved consisted of a tire manufacturer, End-of-Life industry organizations (Recy-BEM and VACO), two research institutes, an asphalt expert, a water sector expert, an environmental organization and individual researchers.Furthermore, efforts were made to engage with representatives from government bodies, the product reuse market, and a consumer interest organization.Unfortunately, aside from a brief discussion, further involvement with these stakeholders did not materialize at this stage.

Material system analysis
The processes identified in the supply chain were translated to a MSA, as shown in Fig. 3. Here, the material streams within the supply chain map are quantified together with environmental pathways.
The MSA (Fig. 3) shows that in 2021 112,600 t/y of tires entered the Dutch system, of which 67 % are passenger car tires.The tire stock counted around 469,500 t/y in that year.Driving resulted in the generation of approximately 16,650 t/y of tire wear.The loss of infill granules from artificial turf was estimated to be approximately 2930 t/y.Hence, 85 % of the initial emissions are caused by the user phase.From the generated tire microplastic particles, approximately 50 % was captured before release to the environment.
The estimated weight loss over a tire's lifetime was 14 %, which is in line with the 10 % to 30 % weight loss found in literature (Grigoratos and Martini, 2014;Wik and Dave, 2009).On highways, it was estimated that 80 % of the generated TWP is captured by the ZOAB asphalt (Geilenkirchen et al., 2022), 18 % ends up at the roadside and 2 % is emitted Fig. 2. Tire supply chain map.The supply chain is a global system, yet the presented tire collection and End-of-Life processes are Dutch-specific.The rectangular boxes represent the processes, with the diamond shapes representing check processes.The boxes on the right side represent the different products that result from the processes on the left.The red arrows show the intercontinental or international streams, the yellow are regional or local streams and the blue are undefined or on-site.The shapes in bold are the processes from which microplastics emissions emerge.

S. Hoeke et al.
directly to surface waters (Verschoor et al., 2016).On urban and rural roads, 40 % of the generated TWP was estimated to end up at roadside and 60 % to be transported to the sewer system (Verschoor et al., 2016).For emitted infill particles it was estimated that 52 % ends up in adjacent soils or paved areas, and the remaining share flows into the sewer system (Løkkegaard et al., 2019).From the tire microplastics that enter the sewer system, 65 % is captured by the combined sewer system and 35 % by the (improved) separated sewer system (Liefting and de Man, 2017).From the combined sewer system 8 % is discharged directly to surface waters without further treatment, due to incorrect connections or overflows (Liefting and de Man, 2017).From the (improved) separated sewer system, 82 % is released to surface waters without treatment.The remaining 18 % joins the sewage from the combined sewer to the WWTP (Liefting E and de Man, 2017).Here, 88 % is removed and captured in the sludge, after which it is incinerated.The remaining 12 % is released to surface waters as effluent (Iyare et al., 2020;Liefting and de Man, 2017;van Egmond et al., 2021).
Table 2 shows both the total mass flow of the estimated emission streams and the emission streams in kg per capita for the year 2021.As shown in the table, the majority of the tire microplastics end up in the soil (81 %) and 19 % is received by surface waters.

Discussion
The supply chain approach provided a solid base to systematically map the tire's lifecycle, including stakeholders, processes and material streams.In doing so, a framework was created to prioritize specific emission sources and to map potential interventions along the supply chain.Using the supply chain map as a common language was valuable to build trust between primary stakeholders and the researchers and for discussing concepts, such as lifecycle stages, processes and material streams.
The effectiveness of stakeholder participation is influenced by the willingness of stakeholders to participate, the characteristics of the stakeholder group, e.g.interests, concerns and needs (Basco-Carrera et al., 2017), and by the trust between stakeholders among each other and between stakeholders and researchers (Voinov et al., 2016).Most stakeholders showed enthusiasm to share their expertise and to collaborate with other actors.Actors from the industry were especially interested in participating, partly due to their ambition to mitigate tire microplastics emissions and their interest in preventing factual inaccuracies from causing reputational damage.Consistent with research findings in the literature, stakeholder participation has proven to be valuable for the collection of data (e.g.Jolibert and Wesselink, 2012;Voinov et al., 2016).Information supplied by stakeholders could at times be used to compensate for a lack of data found in literature, e.g. the year reports from the RecyBEM and expert knowledge to estimate the End-of-Life flows for bus-and truck tires.Assumptions were checked or validated using expert judgment wherever possible.Stakeholder knowledge was specifically indispensable for evaluating the relevance of supply chain processes in terms of tire microplastics emissions.Next to validating the mapping results, the stakeholder workshop helped to identify stakeholder interests.Up to now, stakeholder participation consisted mainly of consultation (Arnstein, 2007) or extractive use (Voinov et al., 2016).In future research steps, we aim to broaden and deepen stakeholder interaction.
The generated TWP in 2021 was estimated at 16,650 t/y.These findings are in line with literature data, where the annual mass of generated TWP in the Netherlands was estimated at approximately 15,000 t/y (Kole et al., 2017) and 17,300 t/y (Verschoor et al., 2016), both for the year 2012.Furthermore, the release of TWP to the environment was estimated at 7,800 t/y, of which the majority is emitted to the soil (77 %) and 23 % to surface waters.Similar results can be found in literature, where release of TWP to soil range from 66 % to 78 % and emissions to surface waters from 12 % to 26 % (Baensch-Baltruschat et al., 2021;Sieber et al., 2020;Verschoor et al., 2016;Wagner et al., 2018).The capturing capacity of ZOAB asphalt has a strong impact on the TWP emissions to the environment, as this directly avoids environmental emissions.
The initial loss of infill granules from artificial turfs was estimated at 2930 t/y.The initial loss was based on a loss percentage of 1.0 % per year.A similar loss percentage was used by Sieber et al. ( 2020), which

Consumers
Being able to buy safe and durable tires for a good price.Some will value sustainability more than others.
With current tire labels it is not possible to compare tire in terms of TRWP generation and additional information is not always available.Consumer might change (driving) behavior or purchases to limit TRWP release.

End-of-Life industry
Maintain a stable inflow of tires from a certain quality in order to make a living from the produced product.
Mechanical recycling is considered a circular option by the industry and is the most common route.This results in the need for a sales market for granulate.

Science
Provide scientific-sound knowledge to citizens, policymakers and the industry.
Distinguishing facts from beliefs by filling in the data gaps around tire microplastics.

Authorities
Provide and execute guidelines and legislations to protect citizens, the environment and the economy.
Balancing economic and environmental needs based on the information available.

Builders & directors of roads
Provide safe roads that require minimal maintenance.
No strong focus on the role of asphalt on TRWP/TWP release.Innovating asphalt involves financial risks as builders are responsible for maintenance for the first number of years.

NGOs
Protect the environment and the health of citizens and inform society.
Aims to limit the amount of tire microplastic release as much as possible, though none have tire microplastics as their main focus.

Product reuse market
Being able to buy safe and durable reuse products, such as granulate.
Need to manage new regulations and public opinions on the reuse product.

Media
Draw attention to topics that are considered societal relevant and/or that the public is interested in.
Broadcast specific journalistic items and highlight new research developments related to tire microplastics.
applied a loss percentage of 2 % based on literature.In contrast, the study by Verschoor et al. (2021) found lower initial losses per field due to different assumed maintenance frequencies and refill quantities which results in lower values per pathway compared to Kole et al. (2023).The difference in pathways between these studies points towards the lack of consistency in housekeeping between artificial turfs fields and, consequently, the potential variation in initial infill release between fields.In the present paper, the release of infill granules to the environment was estimated at 1650 t/y, of which 96 % is received by soilsas a result of processes such as runoff, movement and housekeeping activities (Løkkegaard et al., 2019;Regnell, 2019), and 4 % is received by surface waters.Over the last years, owners and users of artificial turf fields have taken several measures to limit the loss and release of infill granules from artificial turf fields (VACO, personal communication, 14 June 2022).These measures are expected to lead to lower loss and dispersion values.As the exact effect of these measures could not be determined, these measures were not included in the analysis.Furthermore, rubber infill alternatives and new design (4G) turfs are receiving interest as an European ban on rubber infill material might lie on the horizon (ECHA, 2017(ECHA, , 2020)).These future developments will likely affect the tire microplastics emissions from artificial turfs.The sensitivity analysis, shown in Supplementary Information, Section 5, showed that both the total surface area of artificial turfs and the distribution of the infill-related environmental pathways have a large potential influence on the total tire microplastics emissions.Several assumptions had to be made here due to a lack in data.More research to verify these figures is recommended.As the material system analysis is based on a case study, some of the data is location-specific.The ZOAB asphalt is used widely on the Dutch highways, but is not found in many other countries.Landfilling of Endof-Life Tires is prohibited in Europe, but is still a practiced End-of-Life option in other regions (Valentini and Pegoretti, 2022).Management of artificial turfs can differ between countries, but also per fields within the same country (Kole et al., 2023).When translating this approach to other regions, it is important to consider these regional differences, such as variations in asphalt types and the specific transfer coefficients of environmental pathways.Furthermore, increasing volumes of electric vehicles, which tend to be heavier than conventional vehicles, could lead to higher tire wear emissions (Prenner et al., 2021).Lastly, the current study does not consider nanoplastics particles.It is possible this study only presents part of the full picture.To strengthen the validity of the results presented in this paper, comparison to field data is recommended.For this, analysis and detection methods for tire microplastics should be improved, as these are currently lacking (Eisentraut et al., 2018;Goßmann et al., 2021).Data quality could be enhanced by data from monitoring studies measuring microplastics, e.g., the removal efficiency in WWTPs and the magnitude of separate environmental pathways.
This being the groundwork, additional steps are needed to reach an effective multi-stakeholder mitigation strategy.These steps are aimed to shed light on the environmental fate of microplastics and the implementability, effectiveness and acceptance of potential interventions.Future research steps include further integration of environmental modelling, system mapping and stakeholder participation to test interventions in terms of microplastic release, societal impact, acceptance and implementability.A stakeholder network analysis could facilitate stakeholder participation by identifying potential collaboration possibilities and obstacles, as it helps to shed light on the influence of individual actors, their interrelations and the connectedness and clustering within the network.

Conclusion
The present paper presents a transdisciplinary approach to map the supply chain, its emissions, the stakeholders involved, their potential roles and interdependencies with the aim to lay the foundation for an emission mitigation strategy for tire microplastics.The user phase and artificial turf were found to contribute most to environmental tire microplastics emissions, thus, interventions should be aimed at reducing these emission sources.The presented results provide insight in the

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 1 .
Fig. 1.Conceptual model of the research methodology, showing the different research steps and how they are connected.

Fig. 3 .
Fig. 3. Material system analysis for the Netherlands, 2021.Streams are shown in tonnes (kg) per year and are rounded.Other 1 includes retreading, incineration and repurposing.Other 2 includes agricultural mats, rubber tiles, molded objects, bedding construction for artificial turfs, rubber powder, rubber in asphalt and additional purposes.

Table 1
Identified stakes regarding the project per stakeholder subset.

Table 2
(Worldbank, 2022)tire microplastic streams for the Netherlands with 2021 as year of interest.The emissions per capita were based on a number of 17,533,405 inhabitants(Worldbank, 2022). of the problem, the underlying mechanisms and the stakeholder network.It thereby provides a solid foundation for the further development of a mitigation strategy in collaboration with stakeholders.Conceptualization, Methodology, Investigation, Writing review & editing, Visualization.Jikke van Wijnen: Conceptualization, Writingreview & editing.Harold Krikke: Conceptualization, Writingreview & editing, Funding acquisition.Ansje Löhr: Conceptualization, Writingreview & editing.Ad M.J. Ragas: Conceptualization, Writingreview & editing, Funding acquisition. scope