Current state of research on microplastics in the marine-atmosphere environment of the Arctic region

GONG Tianjiao, XU Guojie*, CHEN Liqi & ZHANG Miming

Review

Current state of research on microplastics in the marine-atmosphere environment of the Arctic region

GONG Tianjiao1, XU Guojie1*, CHEN Liqi2& ZHANG Miming3

1China Meteorological Administration Aerosol-Cloud and Precipitation Key Laboratory, Nanjing University of Information Science & Technology, Nanjing 210044, China;2Polar and Marine Research Institute, College of Harbor and Coastal Engineering, Jimei University, Xiamen 361021, China;3Key Laboratory of Global Change and Marine-Atmospheric Chemistry (GCMAC) of Ministry of Natural Resources (MNR), Third Institute of Oceanography (TIO), Xiamen 361005, China

Microplastics, plastic particles smaller than 5 mm in size, are a growing source of environmental pollution. Microplastic pollution has been observed in situ in the remote Arctic, where it has been found in the land, sea, cryosphere, and atmosphere. This review summarizes the sample pretreatment techniques and analytical methods commonly used in microplastic research, as well as the pollution status of microplastics in the Arctic, their sources, and their effects on the environment.In the Arctic, the size distribution of microplastics is more inclined to small-scale aggregation, the shape of microplastics is mostly fibrous, with the proportion of fibers often accounting for more than 70%. There are marked differences among studies in terms of abundance and polymer composition, but polyester is generally dominant in seawater. Many microplastic particles are transported to the Arctic by ocean currents and rivers, but atmospheric transport and deposition are slowly being recognized as an important transport route. Sea ice is particularly important for the temporary storage, transport, and release of Arctic microplastics. The average storage of microplastics in sea ice was estimated to be approximately 6.1×108items. Given their properties, microplastics can affect glacier melting, sea surface temperature changes, and even the carbon cycle. Urgent measures must be taken to improve research standards and overcome sampling difficulties in the Arctic region to achieve time continuity and large-scale distribution patterns of Arctic microplastics.

microplastics, Arctic, atmosphere, environmental effects, sea ice, seawater, snow

1 Introduction

The production of plastics has grown rapidly since the 1950s, reaching 3.9×108t globally in 2021 alone (Plastics Europe, 2022), and widespread plastic use has resulted in the dissemination of large amounts of plastic waste into the environment (Geyer et al., 2017). During the long and slow decomposition process, plastic products undergo differentiation by physical, chemical, and biological processes, ultimately giving rise to microplastics. Microplastics can generally be defined as plastic particles with a particle size of less than 5 mm and were first described in the literature by Thompson et al. in 2004. Primary microplastics are plastic particles that measure 1mm to 5 mm in size and enter the environment as small particles through personal care products, industrial abrasives, and so on, whereas secondary microplastics are those formed after large pieces of plastic are broken down in the environment (Andrady, 2011). As an emerging pollutant, microplastics have received attention from many researchers worldwide owing to their small size, widespread distribution, and toxicological hazards that differ from those of large plastic products.

Over the past half-century, the Arctic has warmed three times faster than the global average (Farmen et al., 2021). The melting of the Arctic cryosphere, including reduced sea-ice extension, the intensification of land-ice melting, and the acceleration of permafrost melting, has a critical impact on the deposition and transport of pollutants in the region (Natali et al., 2021). Research has reported that microplastics have interactions with zooplankton, fish, and other organisms, including reducing feeding, organ damage, and reproductive toxicity (Cole et al., 2013; Lusher et al., 2013). Clearly, more research on microplastics is needed. Despite decreased human activity in the Arctic, microplastic pollution is still present and cannot be ignored. The environmental conditions in the Arctic are harsh and have different effects on plastic transport and degradation compared with lower latitudes. Excessively low temperatures, prolonged UV exposure, and the digestive process of zooplankton will lead to the breakdown of plastic and, eventually, the formation of microplastic particles (Lambert and Wagner, 2016; Dawson et al., 2018). The decomposition of large plastic particles has, to some extent, exacerbated microplastic pollution in the Arctic. Due to the Arctic region being surrounded by many countries in northern Asia, Europe and North America, as well as various transport mechanisms, microplastics from other regions deserve more attention. Ocean currents are an important driver of the distribution of floating microplastic particles in the marine environment, and large amounts of microplastics are transported to the Arctic from other regions via ocean currents. High levels of microplastics have also been detected in Arctic snow, suggesting that atmospheric transport may be an important route for microplastics to enter the Arctic (Bergmann et al., 2019; Evangeliou et al., 2020). Arctic sea ice has been identified as a temporal sink and means of transport for microplastics (Peeken et al., 2018), and rivers are thought to be freshwater sources of microplastics in the Arctic Ocean (Rochman and Hoellein, 2020; Frank et al., 2021).

Despite growing attention to microplastics in the Arctic, most published research on microplastics has focused on a single environment (e.g., seawater, rivers, snow, and so on) (Peeken et al., 2018; Huang et al., 2022), and some studies have been limited to a single geographic region. A comprehensive research paper by Zhang et al. (2023) summarized the distributions of microplastics in different sample media in the Arctic. They also calculated the microplastic deposition flux from the Arctic land snow cover and the river discharge flux to illustrate microplastic’s potential sources and sinks (Zhang et al., 2023). To track advances in microplastic research in the Arctic, we collected recent publications to assess the current state of research. We examine topics such as sample pretreatment and analytical methods, as well as the size distribution, abundance, shape, and polymer composition of microplastics, and mainly focus on the marine-atmosphere environment. We summarize microplastic research methods, including sample processing methods and identification techniques, and their advantages and disadvantages. We discuss the size distribution and abundance characteristics of microplastics in the Arctic, and analyze the sources of microplastics in the Arctic environment and their transport mechanisms. We also examine the potential environmental effects of microplastics, which are causing serious changes in the Arctic and even around the world. Overall, our aim is to organize and analyze the research methods, pollution status, transport mechanisms, and potential impacts of microplastics from the perspective of the Arctic region as a whole, to provide a baseline reference for future research and environmental management.

2 Analytical methods in microplastic research

There are many different methods for sampling microplastics in various environmental media, such as seawater, atmosphere, snow, and ice. But most studies share similarities with regard to how samples are handled and microplastics are identified. Research methods for microplastics can be divided into two simple steps: the pretreatment of samples and the identification of microplastics.

2.1 Sample pretreatment methods

Sample pretreatment methods generally include density separation and purification. Density separation is widely used to separate low-density microplastic particles from other high-density sample media (such as sediment). A highly concentrated saturated salt solution (e.g., saturated sodium chloride solution) is mixed with the sample for a certain amount of time. Microplastic particles are separated from impurities using the density differences of different substances. Purification is the removal of non-plastic substances, such as biofilm, sand, and other substances, from the sample, while retaining the microplastic particles and avoiding the production of artificial secondary microplastics (Wang et al., 2018). Currently, 30% H2O2is the most commonly used reagent for the purification of microplastics (Renner et al., 2018).

Microplastic research in the Arctic also requires that some unique sample types should be processed, such as snow, sea-ice, and biological samples. For the extracted snow samples, different types of filters are used to filter the snow melt prior to analysis, with the most common being PTFE filters or alumina filters with different apertures. Moreover, saturated sodium chloride solution is typically used for the density separation of microplastics in snow samples (Bergmann et al., 2019; Kim et al., 2021). For sea-ice samples, a stainless-steel corer can be used to drill sea-ice cores, which should be stored in pre-cleaned plastic bags. After that, a ceramic knife may be utilized to take a portion of sea-ice samples from the outer surface of the sea-ice core, and then they are melted and filtered for analysis (Obbard et al., 2014; Peeken et al., 2018). For fish samples, the fish should be delivered intact and rinsed with filtered water before the tissue samples are dissected and prepared in a clean laboratory. Depending on the composition of the substrate, alkali and enzyme solutions can be used to remove the tissue of the fish to avoid contamination (K?gel et al., 2023). Zooplankton samples should be covered in a 20% KOH solution in reverse osmosis water, capped, and held at room temperature for 7–14 d or until the zooplankton are fully digested (Huntington et al., 2020).

2.2 Identification methods for microplastics

Many studies use visual identification, which involves initial examination of the shape, color, and scale of microplastics with the naked eye or an optical microscope. Visual identification can sort out residual non-microplastic materials (e.g., organic debris, metal coating, glass) in the sample and is suitable for the identification of microplastic particles larger than 500 microns. Although this method is advantageous in terms of its simple operation, low cost, and lack of sample destruction, the potential error that can arise because of subjective factors are large. Thus, it is generally not used as the main identification method.

The most commonly used qualitative and quantitative analytical methods in microplastic research are Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and pyrolysis gas chromatography-mass spectrometry (GC-MS) technology. First, FTIR is molecular spectroscopy, which can be used to separate and identify microplastics and other contaminants based on the absorption of light at different wavelengths according to the functional groups and chemical bond vibrations. It can be combined with different spectrophotometers for analysis, which can better display the spectral information. Second, Raman spectroscopy is a kind of scattering spectra, which can be analyzed according to the scattering spectra of different frequencies of incident light to obtain information on molecular vibration and rotation and applied to the study of molecular structure. Raman spectroscopy is very similar to FTIR, and combining these two can be suitable for the complementary analysis and identification of microplastics (Suaria and Aliani, 2014). Raman spectroscopy can detect smaller microplastic particles than FTIR, even for 1 μm microplastics, but the measurement time for Raman spectroscopy imaging is much longer compared with FTIR. Tagg et al. (2015) found that some other substances in microplastics, such as additives and colored chemicals, can interfere with the identification of microplastic polymers when being analyzed by Raman spectroscopy. Finally, the GC-MS technique can be useful for obtaining structural information about macromolecules through the analysis of their thermal cleavage products. This microplastic analysis technique enables the cleavage of microplastic polymers at specific temperatures, and the mass-to-charge ratio of the released small molecules is determined to infer the polymer type. Although it does not require pretreatment and is sensitive and reliable for qualitative and quantitative analyses of samples, it is experimentally demanding and can destroy samples.

3 Microplastic properties in the Arctic

3.1 Size distribution and abundance of microplastics

Until now, many researchers have focused on microplastics in media such as Arctic surface snow, seawater, sea ice, and sediments. The size and abundance of microplastics in different regions and media vary greatly. This paper briefly summarizes the recently published literature (Table 1). The size distribution of microplastics obtained in different studies depends more on the sampling and analysis methods used in the study, so it may not be appropriate to directly compare the results of different studies.

Table 1 Characteristics of microplastics in the Arctic

Continued

Notes: PA: polyamide; PE:polyethylene; PET:polyethylene terephthalate; PU: polyurethane; PP: polypropylene; PS: polystyrene.

Owing to limitations in sampling methods and detection techniques in microplastic research, the minimum size for microplastic identification in some studies is 11 μm (Peeken et al., 2018; Bergmann et al., 2019), which may limit our understanding on smaller-size microplastic particles. Most studies show that the size distribution of microplastics usually follows a similar trend; that is, the number of microplastic particles typically increases with decreases in particle size (Peeken et al., 2018; Bergmann et al., 2019; Kanhai et al., 2020).

The ocean is one of the major aggregation sites for microplastic particles. In Chukchi Sea seawater, 52.5% of microplastics were less than 1000 μm, and the abundance reached 0.23±0.07 items·m?3(Mu et al., 2019). The microplastics in Svalbard seawater samples collected by von Friesen et al. (2020) were small in size, mainly in the range of 100 to 300 μm, and the abundance reached 0.7±0.5×103items·m?3, which is quite different from the result (0.34±0.31 items·m?3) of Lusher et al. (2015), indicating the possibility of increased microplastic pollution. However, the abundance of microplastics in groundwater was slightly higher than that in surface water, reaching 2.6±2.95 items·m?3, and the size was within a narrow range of 0.2 to 43.2 mm (Lusher et al., 2015). Approximately 98% microplastics of the snow samples collected by Bergmann et al. (2019) in the Arctic were smaller than 100 μm, of which 80% were smaller than 25 μm, and the abundance of microplastics was 1.76×106items·m?3. However, most of the microplastics found in the snow of the western Arctic Ocean were larger than 100 μm, and the abundance was only 0.87×103items·m?3(Kim et al., 2021). Approximately 67% of the microplastic particles in Arctic sea-ice cores collected by Peeken et al. (2018) were smaller than the current minimum detectable size (11 μm or smaller). The microplastic abundance in sea ice reached 1.1×106to 1.2×107items (Peeken et al., 2018), which is much higher than the microplastic abundance in seawater. However, the microplastic concentration in sea ice varies greatly in different environments, ranging from a few items to hundreds or thousands of particles per liter, and can thus vary by 2–3 orders of magnitude. The microplastic abundance in floating ice under the surface seawater (2×103–1.7×104items·m?3) was several orders of magnitude lower than that in sea ice (Kanhai et al., 2020). Bergmann et al. (2017), reporting microplastic pollution in sea-ice cores in the western and eastern Fram Strait, found that the average microplastic concentration in inland ice (6×105items·m?3) was smaller than that in drifting sea ice (2×106items·m?3). For sediment samples collected in the Hudson Bay and the Canadian Arctic, fiber abundance was 205–380 μm and debris abundance was 0.28–300 μm (Huntington et al., 2020). This was similar to the results obtained in Svalbard (55–381 μm) (Ramasamy et al., 2021). Clearly, the differences in the detection of microplastics in seawater and marine sediments may be the result of differences in sampling, filtration, and processing methods (Bergmann et al., 2022). In summary, numerous studies have shown that microplastic pollution is prevalent in Arctic ice, snow, sediment, and especially in seawater. Furthermore, the Arctic may be a particularly sensitive receptor region for atmospheric microplastics (Evangeliou et al., 2020). Therefore, the observation and simulation of microplastic transport and distribution in the Arctic is urgently needed.

Suaria et al. (2020) collected seawater samples from six ocean basins and found that fiber lengths in surface ocean water showed peak abundance in the range of 800–900 μm. Comparing the microplastic particles found in the Arctic with those found in other oceans, Arctic microplastic particles are significantly smaller in size. At the same time, Arctic microplastic particles are generally slightly smaller than urban atmospheric microplastics (Wright et al., 2020), possibly because microplastics break down as they travel long distances to the Arctic. However, specific effects of microplastic size on their atmospheric retention time and atmospheric transport distance remain unknown, and differences in deposition rates of different types of microplastics are still poorly understood.

3.2 Shape and polymer composition of microplastics

Research on microplastics in the Arctic region shows that the most common shapes are usually fibers or fragments. Fibers and fragments larger than 50 μm were detected in Svalbard seawater samples, and 71% of the microplastics were fiber (von Friesen et al., 2020). A study in the central Arctic found similar results, with fibers accounting for 79% and fragments for 21% of the total (Kanhai et al., 2020). One reason for the high fiber abundance is the presence of certain marine activities (e.g., fishing, tourism) in the Arctic, where fibrous microplastics are mainly derived from fishing gear and domestic sewage. Another possibility is that the fiber has less mass and surface area, so it is more likely to be suspended in the atmosphere by winds and transported to the Arctic (Pakhomova et al., 2022). Based on the downward transport mechanism of microplastics, the deep sea may be an important sink for fragmented microplastics (Cózar et al., 2017).

There are natural polymers (e.g., cotton fibers from plants) and synthetic polymers. Whether environmental microplastic pollution should include natural polymers remains controversial. Lusher et al. (2015) reported that rayon accounted for 30% of the microplastics in seawater samples collected in Svalbard. In a study of the Arctic Ocean, rayon accounted for 54%, followed by polyester (21%) and polyamide (abbreviated PA, 16%) (Obbard et al., 2014). Polyester generally refers to polyethylene terephthalate (PET), which also includes polybutylene terephthalate and polyarylester. Ross et al. (2021) found that in near-surface seawater in the Arctic regions of Europe and North America (including the Arctic), polyester accounted for 73% of the total synthetic fiber. Huang et al. (2022) studied near-surface waters from East Asian waters to the Central Arctic Basin and found that polyester accounted for 71.3% of the total microplastics. The dominance of polyester in seawater has also been reported in the Central Arctic Basin (Kanhai et al., 2018), Svalbard (von Friesen et al., 2020), and the Chukchi Sea (Mu et al., 2019). PET is a popular plastic that is often used in the manufacture of synthetic fibers and plastic bottles, and it is relatively lightweight (1.3×103–1.4×103kg·m?3) (Zhang et al., 2022). This property makes it conducive to atmospheric suspension and long-distance transport compared with other polymers; however, this point requires further study.

4 Sources and transport of microplastics in the Arctic environment

While some microplastic pollution comes from the breaking of large pieces of plastic and local household waste, offshore fishing activities, and industrial activities, inputs from distant sources are also crucial. This section will examine the transport routes of microplastics from distant sources into the Arctic (Figure 1).

Figure 1 Overview of pathways for plastic pollutants entering the Arctic region.

4.1 Ocean current transport

The schematic diagram of ocean currents in the Arctic region is shown in Figure 2. Ocean currents are an important driver of the distribution of floating microplastic particles in the marine environment. Buoyant microplastics can be transported by ocean currents from lower latitudes to the Arctic (Lusher et al., 2015). Most Arctic microplastics are transferred from the North Atlantic to the Arctic by thermohaline circulation, and some microplastics make their way to the Arctic through the Bering Strait (Cózar et al., 2017). Generally, the main sources of microplastics in the Arctic are global thermohaline circulation and riverine transport in coastal areas. The circulation and interaction of different seas, resulting from inconsistencies between the borders of the seas and water masses, likely control the transport of microplastics (Yakushev et al., 2021). The convergence zones of five subtropical oceanic circulations are reported to be plastic debris aggregation zones (Lebreton et al., 2012), in which the transport of easterly currents in the tropics and the Ekman circulation in the mid-latitudes occurred. Onink et al. (2019) conducted simulations and found that wave-driven Stokes drift can enable microplastic migration to remote Arctic regions. Although microplastic abundance is low in most Arctic waters, high abundances have been found in the Greenland and Barents Seas. Cózar et al. (2017) found that the polar branch of the thermohaline circulation transports floating microplastics from the North Atlantic to the Greenland and Barents Seas, in which they are accumulated. The amount of plastic in the Eurasian basin may be even higher owing to the transport of microplastic pollution by North Atlantic currents and transpolar drift. Additionally, Brach et al. (2018) suggest that Pacific water does not spread throughout the Arctic basin because it circulates mostly around the Beaufort Gyre. This seems to be supported by the fact that microplastic abundance in the eastern Arctic is almost three times higher than that in the western Arctic (Ross et al., 2021).

Figure 2 A schematic diagram of ocean currents in the Arctic region.

Meanwhile, marine microplastic particles can leave the ocean along with other materials (e.g., sea salt, bacteria) through a process called bubble-bursting jets and wave action (Allen et al., 2020). Under normal conditions, bubbles of trapped air rise to the surface and burst when the waves break, and salt particles of micro or nano size are ejected from the ocean. As the surface bubbles disappear, water tries to fill the gap left by the bubbles, and the collision of water in all directions leads to secondary jets, which eject larger micro-particles. This suggests that microplastics can reach more distant places through the combined action of the ocean and atmosphere. It also reveals the uncertainty in these marine-atmospheric fluxes. Limitations in data and the lack of studies are urgent issues that must be addressed in the future.

4.2 Atmospheric transport

The physical properties of microplastics (e.g., size, shape, density) may affect the aerodynamics and atmospheric deposition of microplastics. For example, heavy microplastics fall quickly from the air and may not be far from their sources. In contrast, microplastics with lighter or larger surface volumes (e.g., lines, films, debris) may settle more slowly, which allows them to be transported over longer distances in the atmosphere. High microplastic abundance was found in snow samples collected from the Fram Strait, despite being more than 100 km from the mainland (Bergmann et al., 2019). Microplastics have also been found in remote areas beyond the poles (Zhang et al., 2019), suggesting that atmospheric transport may be the main mode of transport to remote areas. A simulation of the microplastics produced by road traffic (Evangeliou et al., 2020) shows that the transport of road microplastics to the Arctic is highly efficient, and the microplastic concentration in Arctic snow is much higher in winter and spring than in summer. It is speculated that reduced wet deposition in the dry Arctic winter troposphere greatly increases the efficiency of microplastic transport through the atmosphere into the Arctic.

Atmospheric deposition may be an important means of microplastic transport in the Arctic. During precipitation, rain or snow particles can make microplastic particles enter the marine or terrestrial environment from the atmosphere through wet deposition. Compared with rainfall, freezing rain and snow (approximately-10 ℃) can be more effect- ive in removing atmospheric microplastics (Dris et al., 2016). Additionally, high fiber abundance and fiber ratios were found in most studies, which suggests that atmospheric transport may have different transport effects depending on the shape of the particles. Pakhomova et al. (2022) found a high correlation between fiber abundance and latitude, which suggests that warm air rising in the tropics prevents microplastic particles from being deposited into the ocean, while cold sinking polar air drives the deposition of airborne microplastic particles. It further demonstrates that fibers are more easily transported than other shapes over long distances through the atmosphere to reach the Arctic.

4.3 Sea-ice transport

Arctic sea ice can be considered a temporary sink, source, and important means of transport for microplastics. High levels of microplastic particles are retained by Arctic sea ice and are released into the ocean when the ice melts. In relation to sea ice-growth areas and sea ice-drift routes, microplastic pollution in the offshore North Atlantic waters will follow the drift of sea ice to the Arctic Ocean and eventually be carried to the Fram Strait (Lusher et al., 2014; Peeken et al., 2018). Recent increases in shipping, fishing, and tourism in the Arctic are expected to increase the amount of microplastics released directly into the Arctic Ocean, which may lead to changes in microplastic concentrations in sea ice (Cole et al., 2013). Additionally, there are some similarities between the western Arctic Ocean and the Central Arctic Basin in terms of microplastic polymers and shape composition, suggesting that transpolar drift may transport microplastics from the western Arctic Ocean to the central Arctic (Kim et al., 2021). The average storage of microplastics in sea ice was estimated to be approximately 6.1×1018items, with an annual release of about 5.1×1018items (Zhang et al., 2023).

With amplified warming and sea-ice retreat owing to global climate change, increased maritime activities in the Arctic will also lead to increased inputs of anthropogenic microplastics. Notably, the Arctic sea ice melting will result in microplastic particles being released from the sea ice into the seawater. This process has a significant impact on microplastic distribution within the Arctic region and the transport of microplastics out of the Arctic region (von Friesen et al., 2020).

4.4 River transport

Freshwater rivers are considered important transport routes for microplastics (Rochman and Hoellein, 2020). The Arctic Ocean receives river flows from the six great Arctic rivers (Mackenzie, Yukon, Kolyma, Lena, Yenisei, and Ob rivers). The results of von Friesen et al. (2020) in Svalbard emphasize that the contribution of local wastewater to microplastic pollution is equally important for coastal fjord systems. A single garment can release more than 1900 fibers in a single wash (Browne et al., 2011), and these microplastics, which cannot be effectively filtered by sewage treatment, may be released into rivers and eventually into the Arctic environment. To study microplastic transport from the Western Siberian region to the Arctic Ocean, Frank et al. (2021)found high microplastic abundances (44.2–51.2 items·m?3) in the middle reaches of the Ob River system. The concentration of microplastics in the discharge plumes of great rivers in Siberia is high, and river discharge has been identified by Yakushev et al. (2021) as the second largest source of microplastic pollution in the Eurasian Arctic. The simulation results of microplastic buoyancy by Lagrange particle advection show that riverine European microplastics can reach the North Pole along the Eurasian continental shelf; meanwhile, the ocean gyres of the Nansen Basin, Laptev Sea, and Nordic Seas are other areas of microplastic accumulation (Huserbr?ten et al., 2022). Based on large-scale annual flows from large Arctic rivers, Zhang et al. (2023) estimated that riverine microplastic input into the Arctic Ocean is approximately 180 t·a?1(roughly 1.1× 1014items·a?1). Additionally, rivers have freezing periods, and when river ice melts in the spring, the flow discharged to the ocean becomes greater and microplastic pollution increases, which indicates a clear seasonality of riverine transport of microplastics (von Friesen et al., 2020).

5 Environmental effects of microplastics

5.1 Potential climatic risk of microplastics

There is convincing evidence that plastics have an impact on various organisms and even human health, but the influence of microplastics on the environment cannot be ignored. The interaction between climate change and microplastic pollution in the Arctic is illustrated in Figure 3. Numerous studies (Obbard et al., 2014; Peeken et al., 2018; von Friesen et al., 2020) have shown that the abundance of microplastics frozen in sea ice is of a much higher magnitude than that in seawater. The retreat of glaciers and ice caps, reduced snowpack, and reduced sea-ice area in the Arctic owing to global warming may result in microplastic particles being released from their frozen state. The potential ecological pollution is caused by the release of microplastic particles from the frozen state. Some researchers (Evangeliou et al., 2020; Zhang et al., 2022) have suggested that microplastic particles deposited in snow and ice have light absorption properties and may affect surface albedo and accelerate the warming and melting of the cryosphere, while glacier melting may release large amounts of frozen microplastics into the marine environment. However, most of these studies have focused on the abundance, size, and shape of microplastics, while few have studied their light absorption properties in snow and ice, which may be related to the difficulty of measuring the optical properties of microplastics. Additionally, atmospheric microplastic particles can act as effective ice nucleating particles that may affect cloud lifetime and albedo and, thus, ice melting (Allen et al., 2022). On the other side, plastic particles in the marine environment can contribute to the warming or cooling of the water column by scattering and attenuating incident solar radiation (VishnuRadhan et al., 2019), leading to potential changes in the optical and other physicochemical properties of the water column. Other potential changes in the physicochemical properties of the water column may induce climate feedback loops at the ocean surface and near-surface layers. Furthermore, the warming of surface waters will lead to more frequent and intense storms and may accelerate the melting of sea ice to some extent (Peng et al., 2021).

Figure 3 The interaction between climate change and microplastic pollution in the Arctic (modified from Bergmann et al., 2022).

5.2 Possible impact on the carbon cycle

Fossil fuels are used as raw materials in the production of most plastics, which contributes to global warming and affects the terrestrial and marine carbon cycles in the Arctic. At the elemental level, plastics are predominantly carbon, and one study reported that homogenized microplastics from North Pacific Gyre surface waters contained 83% carbon by mass (Zhu et al., 2020). Plastics stocks and fluxes of carbon in some ecosystems are approaching the levels of natural organic carbon (Stubbins et al., 2021).When exposed to solar radiation, some common plastic polymers degrade to release greenhouse gases such as methane, ethylene, and ethane, even throughout the life cycle of plastic particles (Royer et al., 2018). The surface of microplastics is a new niche for aquatic microorganisms, and increasing microplastic pollution has the potential to have a global impact on the carbon dynamics of pelagic environments by altering heterotrophic activities (Arias- Andres et al., 2018). As the Arctic is warming, large quantities of organic carbon (including microplastics), stored in glaciers from local or remote sources, are transported downstream and affect terrestrial and aquatic carbon fluxes (Li et al., 2018).

6 Summary and future perspectives

This paper introduces the latest progress in the study of microplastics in the Arctic environment. Microplastics have been found in the Arctic and their abundance and size distribution in various studies are described. The types of plastic polymers detected in various studies mainly include PET, PA, and PE. The Arctic is considered to be distant from direct sources of microplastic emissions, so the presence of these microplastics is likely the combined result of transport from ocean currents, atmosphere, sea ice, and rivers. Microplastics may exacerbate climate change in the Arctic region, alter the regional carbon cycle, and have potential impacts on the Arctic environment.

Despite recent advances in research, the abundance and distribution of microplastics and the importance of different transport routes in the Arctic remain poorly understood. Additionally, there is no estimated data on the relative contributions of local and long-distance sources to Arctic microplastic pollutions. Current research shows that, in addition to marine transport, another major input is related to atmospheric transport. Microplastics can penetrate into the Arctic through atmospheric transport and deposition. However, there are few studies on sampling microplastics in the Arctic atmosphere, which may be a consequence of difficulties in sampling the Arctic atmosphere. Microplastic pollution clearly exacerbates the effects of climate change, and these changes will in turn affect the transport and distribution of microplastics in the Arctic environment (Bergmann et al., 2022). Plastic pollution research is particularly challenging in the Arctic because of its remoteness, freezing temperatures, and poor infrastructure. Thus, microplastics studies in the Arctic are usually conducted at individual sites during summer, leading to limitations in temporal and spatial scales. Meanwhile, Arctic rivers, which pass through more local settlements, are conduits for land-based sources of plastic pollution into the sea. Strengthening relationships and cooperation with the local residents may be important for researching and responding to microplastics pollution in the Arctic. Along with the difficulties in conducting research in the Arctic, there is a lack of standardized sampling and analysis methods for microplastics, thus affecting comparison and analysis. However, recent research and monitoring recommendations from the Arctic Monitoring and Assessment Programme will improve the current situation (Farmen et al., 2021), and more studies should be conducted in the future.

This work was supported by the National Natural Science Foundation of China (Grant no. 42006190), the Chinese Polar Environmental Comprehensive Investigation and Assessment Programs (Grant no. CHINARE2010-2020), and the Chinese International Coopera- tion Projects (Grant no. 2009DFA22920) from the Ministry of Science and Technology. We thank the Chinese Arctic and Antarctic Administration and the Third Institute of Oceanography of MNR for their support. We appreciate two anonymous reviewers and Associate Editor Dr. Liyang Zhan for constructive comments that helped us improve the manuscript.

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10.12429/j.advps.2023.0005

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