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Effects of Shape and Size on Microplastic Atmospheric Settling Velocity
Abstract
Microplastics (MPs) have been found in all terrestrial, marine, and riparian environments, including remote regions. This implies that atmospheric transport is an important pathway when considering MP sources and global budgets. However, limited empirical data exist to aid in effective development and parameterization of MP atmospheric transport models. This study measured the atmospheric settling and horizontal drift velocities of various sizes and shapes of MPs in two specially designed settling columns using a laser Doppler anemometer. The settling velocities were generally lower than modeled values, while shape exerted the most significant influence on the rate of settling. Rather than conforming to well-established, power-law models, each class of MP exhibited a linear but different relationship between MP size and settling velocity, with markedly higher slopes for the spheres and cylinders as compared to the films and fibers. Shape also had a substantial influence on particle drift, with the fibers and films exhibiting the greatest horizontal motion, as suggestive of their changing orientation in response to particle interactions and fluid drag. As a consequence, microplastic particles identified within atmospheric deposition samples collected at a single point may derive from entirely different sources representing a wide range in transport distance.
... The red ellipse represents the 95 % confidence level. and shape of microplastics are major factors influencing the duration a particle can remain suspended in the air (Preston et al., 2023). Smaller MFs may remain suspended for a longer time and, therefore, travel longer distances than MFs of greater length. ...
... Similar to Aeolian deposition (Bullard et al., 2016), even large fibers with relatively low densities can be resuspended by wind or anthropogenic activities from various surfaces on which they settle, a process that has received limited attention in microplastic studies. Additionally, particles may experience multiple uplifting while being suspended, which can lengthen their residence time (Preston et al., 2023). Therefore, the estimated maximum distance of 25 km travelled by MF in this study might indicate the point where MF settled after the last resuspension event rather than its actual source. ...
... Strong ocean winds can transport these MNPs further away to more remote areas in oceans and terrestrial areas [72]. A recent study by Preston et al. demonstrated that flat fibers are efficiently transported over long distances compared to spherical counterparts, which also suggested that oceans could be major sources of MNPs in the atmospheric [73]. ...
... spherical counterparts, which also suggested that oceans could be major sources of MNPs in the atmospheric [73]. ...
Plastics, due to their varied properties, find use in different sectors such as agriculture, packaging, pharmaceuticals, textiles, and construction, to mention a few. Excessive use of plastics results in a lot of plastic waste buildup. Poorly managed plastic waste (as shown by heaps of plastic waste on dumpsites, in free spaces, along roads, and in marine systems) and the plastic in landfills, are just a fraction of the plastic waste in the environment. A complete picture should include the micro and nano-plastics (MNPs) in the hydrosphere, biosphere, lithosphere, and atmosphere, as the current extreme weather conditions (which are effects of climate change), wear and tear, and other factors promote MNP formation. MNPs pose a threat to the environment more than their pristine counterparts. This review highlights the entry and occurrence of primary and secondary MNPs in the soil, water and air, together with their aging. Furthermore, the uptake and internalization, by plants, animals, and humans are discussed, together with their toxicity effects. Finally, the future perspective and conclusion are given. The material utilized in this work was acquired from published articles and the internet using keywords such as plastic waste, degradation, microplastic, aging, internalization, and toxicity.
... They also proposed empirical formulations to correct the drag coefficients for each particle shape. Preston et al. (2023) measured settling velocities for MPs characterised by varied densities and four idealised shapes -spheres, cylinders, fibres and films -performing release experiments into an isolated fall column. They assessed the implications of the MPs shape, size and aerodynamic characteristics on modelling their atmospheric transport, highlighting that applying Stokes' law in numerical models leads to a substantial overestimation of the MPs settling velocity values. ...
... The presence of MPs in wildlife feces indicates that the particles were present in the animals' intestines and therefore were ingested in food and/or water. In the protected environment of Komchén de los Pájaros, it is likely that the majority of MPs arrived through the air, which is possibly the most common transport mechanism for these pollutants (Gupta et al., 2023;Martina & Castelli, 2023;Preston et al., 2023). The potential health effects resulting from the ingestion of MPs by aquatic animals have garnered significant attention (Li et al., 2022;Nelms et al., 2019;Osman et al., 2023;Wang et al., 2023). ...
transform infrared (FT-IR) coupled to attenuated total reflection (ATR) to characterize the microplastics present. Results showed microplastics with granular, fibrous, amorphous, and granulated morphology in the four water supply sites and in animal stools. Fibers were the main shapes. ATR FT-IR identified five different types of microplastics: elastomers, polystyrene , high-density polyethylene (HDPE), cellulolytic polymers, and polyurethane. HDPE was only identified in the animal feces. This is the first report of the presence of MPs in water wells and animal feces in the conservation area Komchén de los Pájaros. Abstract Komchén de los Pájaros is a 740-acre Area Voluntarily Dedicated for Conservation (AVDC) located in the municipality of Dzemul, in the north of the state of Yucatán, Mexico. This area is important because it contributes to the conservation of flora and wildlife. Although AVDCs are protected, little is known about the presence, distribution and prevalence of anthropogenic nano and micro pollutants such as metal nanoparticles and microplastics. We carried out an initial study to identify microplas-tics in Komchén de los Pájaros. Samples of water supply sites and feces from wildlife were analyzed using Nile red staining, optical microscopy, and Fourier
... The presence of MPs in wildlife feces indicates that the particles were present in the animals' intestines and therefore were ingested in food and/or water. In the protected environment of Komchén de los Pájaros, it is likely that the majority of MPs arrived through the air, which is possibly the most common transport mechanism for these pollutants (Gupta et al., 2023;Martina & Castelli, 2023;Preston et al., 2023). The potential health effects resulting from the ingestion of MPs by aquatic animals have garnered significant attention (Li et al., 2022;Nelms et al., 2019;Osman et al., 2023;Wang et al., 2023). ...
Komchén de los Pájaros is a 740- acre Area Voluntarily Dedicated for Conservation (AVDC) located in the municipality of Dzemul, in the north of the state of Yucatán, Mexico. This area is important because it contributes to the conservation of flora and wildlife. Although AVDCs are protected, little is known about the presence, distribution and prevalence of anthropogenic nano and micro pollut- ants such as metal nanoparticles and microplastics. We carried out an initial study to identify microplas- tics in Komchén de los Pájaros. Samples of water sup- ply sites and feces from wildlife were analyzed using Nile red staining, optical microscopy, and Fourier transform infrared (FT-IR) coupled to attenuated total reflection (ATR) to characterize the microplastics present. Results showed microplastics with granular, fibrous, amorphous, and granulated morphology in the four water supply sites and in animal stools. Fib- ers were the main shapes. ATR FT-IR identified five different types of microplastics: elastomers, polysty- rene, high-density polyethylene (HDPE), cellulolytic polymers, and polyurethane. HDPE was only identi- fied in the animal feces. This is the first report of the presence of MPs in water wells and animal feces in the conservation area Komchén de los Pájaros.
The presence of microplastics (MPs) is increasing at a dramatic rate globally, posing risks for exposure and subsequent potential adverse effects on human health. Apart from being physical objects, MP particles contain thousands of plastic-associated chemicals (i.e., monomers, chemical additives, and non-intentionally added substances) captured within the polymer matrix. These chemicals are often migrating from MPs and can be found in various environmental matrices and human food chains; increasing the risks for exposure and health effects. In addition to the physical and chemical attributes of MPs, plastic surfaces effectively bind exogenous chemicals, including environmental pollutants (e.g., heavy metals, persistent organic pollutants). Therefore, MPs can act as vectors of environmental pollution across air, drinking water, and food, further amplifying health risks posed by MP exposure. Critically, fragmentation of plastics in the environment increases the risk for interactions with cells, increases the presence of available surfaces to leach plastic-associated chemicals, and adsorb and transfer environmental pollutants. This review proposes the so-called triple exposure nexus approach to comprehensively map existing knowledge on interconnected health effects of MP particles, plastic-associated chemicals, and environmental pollutants. Based on the available data, there is a large knowledge gap in regard to the interactions and cumulative health effects of the triple exposure nexus. Each component of the triple nexus is known to induce genotoxicity, inflammation, and endocrine disruption, but knowledge about long-term and inter-individual health effects is lacking. Furthermore, MPs are not readily excreted from organisms after ingestion and they have been found accumulated in human blood, cardiac tissue, placenta, etc. Even though the number of studies on MPs-associated health impacts is increasing rapidly, this review underscores that there is a pressing necessity to achieve an integrated assessment of MPs’ effects on human health in order to address existing and future knowledge gaps.
Recent studies have highlighted the importance of the atmosphere in the long-range transport of microplastic fibers (MPFs). However, both dry deposition processes and sources of MPFs are poorly understood due to their complexity in size and shape, which can be 100s $\mu m$ long, possessing round or flat cross-sections with dimensions of $O(1)\,\mu m$ thickness, and $O(10)\,\mu m$ width. Here, we develop a theory-based settling velocity model for MPFs in the atmosphere, predicting a much smaller aerodynamic size than a volumetrically equivalent spherical particle. Incorrect identification of flat fibers as cylindrical ones due to uncertainty in the thickness of sampled MPFs overestimates their dry deposition rate. Accounting for fiber thickness in sampled MPFs leads to a mean residence-time enhancement above $450\%$ compared to spherical-shaped particles, suggesting a much more efficient long-range transport of flat fibers than previously thought and that the ocean might be a major source of atmospheric plastics.
Plastic pollution accumulating in an area of the environment is considered “poorly reversible” if natural mineralization processes occurring there are slow and engineered remediation solutions are improbable. Should negative outcomes in these areas arise as a consequence of plastic pollution, they will be practically irreversible. Potential impacts from poorly reversible plastic pollution include changes to carbon and nutrient cycles; habitat changes within soils, sediments, and aquatic ecosystems; co-occurring biological impacts on endangered or keystone species; ecotoxicity; and related societal impacts. The rational response to the global threat posed by accumulating and poorly reversible plastic pollution is to rapidly reduce plastic emissions through reductions in consumption of virgin plastic materials, along with internationally coordinated strategies for waste management.
We described in 2017 how weathering plastic litter in the marine environment fulfils two of three criteria to impose a planetary boundary threat related to “chemical pollution and the release of novel entities”: (1) planetary-scale exposure, which (2) is not readily reversible. Whether marine plastics meet the third criterion, (3) eliciting a disruptive impact on vital earth system processes, was uncertain. Since then, several important discoveries have been made to motivate a re-evaluation. A key issue is if weathering macroplastics, microplastics, nanoplastics, and their leachates have an inherently higher potential to elicit adverse effects than natural particles of the same size. We summarize novel findings related to weathering plastic in the context of the planetary boundary threat criteria that demonstrate (1) increasing exposure, (2) fate processes leading to poorly reversible pollution, and (3) (eco)toxicological hazards and their thresholds. We provide evidence that the third criterion could be fulfilled for weathering plastics in sensitive environments and therefore conclude that weathering plastics pose a planetary boundary threat. We suggest future research priorities to better understand (eco)toxicological hazards modulated by increasing exposure and continuous weathering processes, to better parametrize the planetary boundary threshold for plastic pollution.
Iceland contains some of the largest and most active dust sources in the Arctic, with the frequency of major emission events increasing in the wake of catastrophic volcanic eruptions, such as the 2010 Eyjafjallajökull eruption. Relatively juvenile Icelandic volcaniclastic dust particles are porous and highly angular, and they frequently bear ultrafine nanoscale dustcoats. Their low density is speculated to affect deposition rates in the atmosphere, while substantial amounts of water adsorbed onto their expansive surface area may affect nucleation, aggregation, and settling. Using a 2‐D laser Doppler anemometer, the present paper reports on a set of highly controlled fall column experiments designed to evaluate the dependency of the settling velocity upon particle morphology (e.g., size, shape, density, and surface area) and relative humidity (RH). Measurements for samples obtained from four Icelandic field sites are compared to those for solid glass spheres of similar diameter. The results confirm that the amount of adsorbed water increases with RH and the particle specific surface area. Higher dust concentrations are found to favor particle interaction and the formation of aggregates, especially at higher RH. Under dry conditions (20 % < RH < 30 % ), the horizontal drift velocity measured for the angular, porous Icelandic particles exceeds that observed for solid glass spheres, although these measurements are an order of magnitude smaller than the vertical velocity. As compared to particle diameter, shape does not appear to influence either the settling rate or drag coefficient at Reynolds numbers ≤10⁻², owing to the dominance of viscous effects over form drag.
In recent years, marine, freshwater and terrestrial pollution with microplastics has been discussed extensively, whereas atmospheric microplastic transport has been largely overlooked. Here, we present global simulations of atmospheric transport of microplastic particles produced by road traffic (TWPs – tire wear particles and BWPs – brake wear particles), a major source that can be quantified relatively well. We find a high transport efficiencies of these particles to remote regions. About 34% of the emitted coarse TWPs and 30% of the emitted coarse BWPs (100 kt yr−1 and 40 kt yr−1 respectively) were deposited in the World Ocean. These amounts are of similar magnitude as the total estimated direct and riverine transport of TWPs and fibres to the ocean (64 kt yr−1). We suggest that the Arctic may be a particularly sensitive receptor region, where the light-absorbing properties of TWPs and BWPs may also cause accelerated warming and melting of the cryosphere. Plastic pollution is a critical concern across diverse ecosystems, yet most research has focused on terrestrial and aquatic transport, neglecting other mechanisms. Here the authors show that atmospheric transport is a major pathway for road plastic pollution over remote regions.
Plastic waste is currently entering the natural environment at a rate of over 300 Mt year⁻¹. Coupled with the long lifetimes of common plastics, this input is already resulting in rapid accumulation, yet our understanding of plastic persistence in the environment is very limited. This review summarizes the existing literature on environmental degradation rates and pathways for the major types of thermoplastic polymers. It also highlights the need for better experimental studies of polymer degradation under well-defined reaction conditions, and standardized reporting of rates. A metric to harmonize disparate types of measurements, the specific surface degradation rate (SSDR), is implemented and used to extrapolate half-lives. SSDR values cover a very wide range, with some variability due to degradation studies conducted in different natural environments. For example, SSDRs for HDPE in the marine environment range from essentially 0 to ca. 11 μm year⁻¹. Using a mean SSDR for HDPE in the marine environment and linear extrapolation of the extent of degradation with time leads to estimated half-lives ranging from 58 years (bottles) to 1,200 years (pipes). SSDRs for HDPE and PLA are surprisingly similar in the marine environment, although PLA degrades ca. 20x faster than HDPE on land.
Microplastics are a global environmental issue contaminating aquatic and terrestrial environments. They have been reported in atmospheric deposition, and indoor and outdoor air, raising concern for public health due to the potential for exposure. Moreover, the atmosphere presents a new vehicle for microplastics to enter the wider environment, yet our knowledge of the quantities, characteristics and pathways of airborne microplastics is sparse. Here we show microplastics in atmospheric deposition in a major population centre, central London. Microplastics were found in all samples, with deposition rates ranging from 575 to 1008 microplastics/m2/d. They were found in various shapes, of which fibrous microplastics accounted for the great majority (92%). Across all samples, 15 different petrochemical-based polymers were identified. Bivariate polar plots indicated dependency on wind, with different source areas for fibrous and non-fibrous airborne microplastics. This is the first evidence of airborne microplastics in London and confirms the need to include airborne pathways when consolidating microplastic impacts on the wider environment and human health.
Microplastics (MPs) are ubiquitous, and considerable quantities prevail even in the Arctic; however, there are large knowledge gaps regarding pathways to the North. To assess whether atmospheric transport plays a role, we analyzed snow samples from ice floes in Fram Strait. For comparison, we investigated snow samples from remote (Swiss Alps) and populated (Bremen, Bavaria) European sites. MPs were identified by Fourier transform infrared imaging in 20 of 21 samples. The MP concentration of Arctic snow was significantly lower (0 to 14.4 × 10 3 N liter −1) than European snow (0.19 × 10 3 to 154 × 10 3 N liter −1) but still substantial. Polymer composition varied strongly, but varnish, rubber, polyethylene, and polyamide dominated overall. Most particles were in the smallest size range indicating large numbers of particles below the detection limit of 11 µm. Our data highlight that atmospheric transport and deposition can be notable pathways for MPs meriting more research.
Because of their diverse sizes, shapes and densities, environmental microplastics are often perceived as complex. Many studies struggle with this complexity, and either address only a part of this diversity, or present data using discrete classifications for sizes, shapes and densities. We argue that such classifications will never be fully satisfactory, as any definition using classes does not capture the essentially continuous nature of environmental microplastic. Therefore, we propose to simplify microplastics by fully defining them through a 3D probability distribution, with size, shape and density as dimensions. Besides introducing the concept, we parameterize these probability distributions, using empirical data. This parameterization results in an approximate yet realistic representation of ‘true’ environmental microplastic. This approach to simplify microplastic could be applicable to exposure measurements, effect studies and fate modelling. Furthermore, it allows for easy comparison between studies, irrespective of sampling or laboratory setup. We demonstrate how the 3D probability distribution of environmental versus ingested microplastic can be helpful in understanding bioavailability of and exposure to microplastic. We argue that the concept of simplified microplastic will also be helpful in probabilistic risk modelling, which would greatly enhance the understanding of the risk that microplastics pose to the environment.
Plastic litter is an ever-increasing global issue and one of this generation’s key environmental challenges. Microplastics have
reached oceans via river transport on a global scale. With the exception of two megacities, Paris (France) and Dongguan
(China), there is a lack of information on atmospheric microplastic deposition or transport. Here we present the observations
of atmospheric microplastic deposition in a remote, pristine mountain catchment (French Pyrenees). We analysed samples,
taken over five months, that represent atmospheric wet and dry deposition and identified fibres up to ~750 µm long and frag-
ments ≤300 µm as microplastics. We document relative daily counts of 249 fragments, 73 films and 44 fibres per square
metre that deposited on the catchment. An air mass trajectory analysis shows microplastic transport through the atmosphere
over a distance of up to 95 km. We suggest that microplastics can reach and affect remote, sparsely inhabited areas through
atmospheric transport.
Researcher and media alarms have caused plastic debris to be perceived as a major threat to humans and animals. However, although the waste of plastic in the environment is clearly undesirable for aesthetic and economic reasons, the actual environmental risks of different plastics and their associated chemicals remain largely unknown. Here we show how a systematic assessment of adverse outcome pathways based on ecologically relevant metrics for exposure and effect can bring risk assessment within reach. Results of such an assessment will help to respond to the current public worry in a balanced way and allow policy makers to take measures for scientifically sound reasons.
Microplastic pollution has exhibited a global distribution, including seas, lakes, rivers, and terrestrial environment in recent years. However, little attention was paid on the atmospheric environment, though the fact that plastic debris can escape as wind-blown debris was previously reported. Thus, characteristics of microplastics in the atmospheric fallout from Dongguan city were preliminarily studied. Microplastics of three different polymers, i.e., PE, PP, and PS, were identified. Diverse shapes of microplastics including fiber, foam, fragment, and film were found, and fiber was the dominant shape of the microplastics. SEM images illustrated that adhering particles, grooves, pits, fractures, and flakes were the common patterns of degradation. The concentrations of non-fibrous microplastics and fibers ranged from 175 to 313 particles/m²/day in the atmospheric fallout. Thus, dust emission and deposition between atmosphere, land surface, and aquatic environment were associated with the transportation of microplastics.
Plastic debris is an environmentally persistent and complex contaminant of increasing concern. Understanding the sources, abundance and composition of microplastics present in the environment is a huge challenge due to the fact that hundreds of millions of tonnes of plastic material is manufactured for societal use annually, some of which is released to the environment. The majority of microplastics research to date has focussed on the marine environment. Although freshwater and terrestrial environments are recognised as origins and transport pathways of plastics to the oceans, there is still a comparative lack of knowledge about these environmental compartments. It is highly likely that microplastics will accumulate within continental environments, especially in areas of high anthropogenic influence such as agricultural or urban areas. This review critically evaluates the current literature on the presence, behaviour and fate of microplastics in freshwater and terrestrial environments and, where appropriate, also draws on relevant studies from other fields including nanotechnology, agriculture and waste management. Furthermore, we evaluate the relevant biological and chemical information from the substantial body of marine microplastic literature, determining the applicability and comparability of this data to freshwater and terrestrial systems. With the evidence presented, the authors have set out the current state of the knowledge, and identified the key gaps. These include the volume and composition of microplastics entering the environment, behaviour and fate of microplastics under a variety of environmental conditions and how characteristics of microplastics influence their toxicity. Given the technical challenges surrounding microplastics research, it is especially important that future studies develop standardised techniques to allow for comparability of data. The identification of these research needs will help inform the design of future studies, to determine both the extent and potential ecological impacts of microplastic pollution in freshwater and terrestrial environments.
We present a new general model for the prediction of the drag coefficient of non-spherical solid particles of regular and irregular shapes falling in gas or liquid valid for sub-critical particle Reynolds numbers (i.e. _Re_ < 3 × 105). Results are obtained from experimental measurements on 300 regular and irregular particles in the air and analytical solutions for ellipsoids. Depending on their size, irregular particles are accurately characterized with a 3D laser scanner or SEM micro-CT method. The experiments are carried out in settling columns with height of 0.45 to 3.60 m and in a 4 m-high vertical wind tunnel. In addition, 881 additional experimental data points are also considered that are compiled from the literature for particles of regular shapes falling in liquids. New correlation is based on the particle Reynolds number and two new shape descriptors defined as a function of particle flatness, elongation and diameter. New shape descriptors are easy-to-measure and can be more easily characterized than sphericity. The new correlation has an average error of ~ 10%, which is significantly lower than errors associated with existing correlations. Additional aspects of particle sedimentation are also investigated. First, it is found that particles falling in dense liquids, in particular at _Re_ > 1000, tend to fall with their maximum projection area perpendicular to their falling direction, whereas in gases their orientation is random. Second, effects of small-scale surface vesicularity and roughness on the drag coefficient of non-spherical particles found to be < 10%. Finally, the effect of particle orientation on the drag coefficient is discussed and additional correlations are presented to predict the end members of drag coefficient due to change in the particle orientation.
Simplified physical models and geometrical considerations reveal general physical and dynamical properties of microplastic particles (0.5–5 mm) of different density, shape and size in marine environment. Windage of extremely light foamed particles, surface area and fouling rate of slightly positively buoyant microplastic spheres, films and fibres and settling velocities of negatively buoyant particles are analysed. For the Baltic Sea dimensions and under the considered idealised external conditions, (i) only one day is required for a foamed polystyrene particle to cross the sea (ca. 250 km); (ii) polyethylene fibres should spend about 6–8 months in the euphotic zone before sinking due to bio-fouling, whilst spherical particles can be retained on the surface up to 10–15 years; (iii) for heavy microplastic particles, the time of settling through the water column in the central Gotland basin (ca. 250 m) is less than 18 h. Proper physical setting of the problem of microplastics transport and developing of physically-based parameterisations are seen as applications.
Explicit equations are developed for the drag coefficient and for the terminal velocity of falling spherical and nonspherical particles. The goodness of fit of these equations to the reported experimental data is evaluated and is compared with that of other recently proposed equations.Accurate design charts for CD and ut are prepared and displayed for all particle sphericities.
The transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols. This paper presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars. Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices. We also discuss the physics of wind-blown sand and dune formation on Venus and Titan.
Solid particles of cylindrical shape play a significant role in many separations processes. Explicit equations for the drag coefficient and the terminal velocity of free-falling cylindrical particles have been developed in this work. The developed equations are based on available experimental data for falling cylindrical particles in all flow regimes. The aspect ratio (i.e., length-over-diameter ratio) has been used to account for the particle shape. Comparisons with correlations proposed by other researchers using different parameters to account for the geometry are presented. Good agreement is found for small aspect ratios, and increasing differences appear when the aspect ratio increases. The aspect ratio of cylindrical particles satisfactorily accounts for the geometrical influence on fluid flow of settling particles.Graphical abstractThe drag coefficient and terminal velocity of a cylinder falling freely in a liquid column of similar density is predicted as a function of the Galileo number and the cylinder aspect ratio.
Vertical dynamics of microplastics (MPs) in the water column are complex and not fully understood due to the diversity of environmental MPs and the impact of weathering and biofouling on their dynamical properties. In this study, we investigate the effects of the particle properties and biofilm on the vertical (settling or rising) velocity of microplastic sheets and fibers under laboratory conditions. The experiments focus on three types of MPs (polyester PES fibers, polyethylene terephthalate PET sheets, and polypropylene PP sheets) of nine sizes and two degrees of biological colonization. Even though pristine PES fibers and PET sheets had a similar density, the sinking velocity of fibers was much smaller and independent of their length. The settling or rising velocity of sheets increased with the particle size up to a threshold and then decreased in the wake of horizontal oscillations in large particles. Biofilms had unexpected effects on vertical velocities. Irregular biofilm distributions can trigger motion instabilities that decrease settling velocities of sheets despite the increase in density. Biofilms can also modify the orientation of fibers, which may increase their settling velocity. Finally, we selected the most performant theoretical formulation for each type of particle and proposed modifications to consider the effect of biofilm distribution.
In this study, the quantitative and qualitative compositions of microplastics (MPs) deposited from the atmosphere in the coastal zone were analysed. Moreover, links between MP deposition and meteorological factors (air humidity, wind speed, precipitation height, and air mass trajectories) were investigated. MP deposition samples were collected in the southern Baltic area in 2017 and 2018 for 286 days in total. The morphological features of MPs (shape and size) were analysed using a digital microscope. Qualitative analysis was performed using micro-attenuated total reflectance Fourier-transform infrared spectroscopy (μATR FT-IR). The size of the deposited MPs ranged from 5 μm to 5000 μm, and smaller size classes (<720 μm) were predominant (70%). Fibres were of the dominant type (60%). Films and fragments constituted 26% and 14% of the total MPs, respectively, while 55% of the MPs found in the study were polymers often used in textiles, packaging materials, and fishing gear (polyesters and polypropylene). The atmospheric MP deposition ranged from 0 m−2·d−1 to 30 m−2·d−1 (average: 10 ± 8 m−2·d−1; median: 8 m−2·d−1). Based on the air mass trajectory cluster analysis, MPs deposited in Gdynia mainly originated from local sources (<100 km). Furthermore, higher MP deposition occurred when the inflowing air masses were terrestrial, and lower deposition occurred when they were marine. On average, during wet periods twice the amount of MPs was deposited in comparison with dry periods. During dry periods deposition increased with increasing wind speed and was intensified by high relative humidity.
Microfibres (mf) are the most common type of microplastic in the environment. Few studies have focused on their abundance in atmospheric deposition in background environments. In the current study, we collected wet-only and bulk rainfall from four precipitation chemistry monitoring stations, primarily located in coastal areas around Ireland. Anthropogenic mf were observed in all samples; the average deposition across the four study sites was 80 mf m–2 day–1. Wet-only deposition of mf was 70 mf m–2 day–1 compared with bulk deposition of 100 mf m–2 day–1. The wet-only collectors were estimated to capture ~70% of the bulk collectors, suggesting that dry deposition makes up at least 30% of total deposition. Meteorological variables, i.e., relative humidity, rainfall volume, wind speed, and wind direction, were significantly related to mf abundance, suggesting that rainfall washout and air mass movements are important predictors of mf deposition in background regions. In total, 15% of all anthropogenic mf were identified as plastic. The most abundant polymer type was polyester or polyethylene terephthalate at 71%, followed by polyacrylonitrile at 11%, polyethylene at 11% and polypropylene at 4%. The average deposition of plastic mf was 12 mf m–2 day–1.
Microplastics are ubiquitous pollutants within the marine environment, predominantly (> 90 %) accumulating in sediments worldwide. Despite the increasing global concern regarding these anthropogenic pollutants, research into the remediation of microplastics is lacking. Here we examined those characteristics of microplastics that are essential to adequately evaluate potential remediation techniques such as sedimentation and (air) flotation techniques. We analyzed the sinking behavior of typical microplastics originating from acutal plastic waste products and identified the best-available drag model to quantitatively describe their sinking behavior, namely the drag model derived by Dioguardi et al. (2018). Particle shape is found to be an important parameter strongly affecting the sinking behavior of microplastics. Various common shape descriptors were experimentally evaluated on their ability to appropriately characterize frequently occuring particle shapes of typical microplastics such as spheres, films and fibers. Circularity χ and sphericity Φ are found to be appropriate shape descriptors in this context. We also investigated the effect of biofouling on the polarity of marine plastics and estimated its potential contribution to the settling motion of initially floating microplastics based on density-modification. It is found that biofouling alters the polarity of plastics significantly, this is from (near) hydrophobic (i.e. water contact angles from 70 to 100°) to strong hydrophilic surfaces (i.e. water contact angles from 30 to 40 °) rendering them more difficult to separate from sediment based on polarity as primary separation factor. Thus, next to providing a better understanding of the fate and behavior of typical marine microplastics, these findings serve as a fundamental stepping stone to the development of the first large-scale sediment remediation techniques for microplastics to answer the global microplastic accumulation issue.
Here, there, and everywhere
No place is safe from plastic pollution. Brahney et al. show that even the most isolated areas in the United States—national parks and national wilderness areas—accumulate microplastic particles after they are transported there by wind and rain (see the Perspective by Rochman and Hoellein). They estimate that more than 1000 metric tons per year fall within south and central western U.S. protected areas. Most of these plastic particles are synthetic microfibers used for making clothing. These findings should underline the importance of reducing pollution from such materials.
Science , this issue p. 1257 ; see also p. 1184
Microplastics, which are plastic particles < 5 mm, have been found throughout the environment. However, few studies have focused on their transport via atmospheric deposition. Bryophytes have been used as biomonitors for the atmospheric deposition of trace elements, persistent organic pollutants and particulate matter, and may potentially be used to monitor the atmospheric deposition of microplastics or anthropogenic microfibres (mf). Hylocomium splendens was collected from three lake catchments, which are remote from anthropogenic disturbance and emissions sources. Anthropogenic mf were observed in all moss samples; the average number was 24 mf g-1 dry weight (range: 15-30 mf g-1) across the three study sites. The average length of mf was 1.02 mm (range: 0.83-1.20 mm). Plastic mf were identified using five rigorous visual criteria; 27% of the observed mf passed four criteria and 13% passed all five, suggesting at most a quarter of the mf may be plastic. Annual average atmospheric deposition of anthropogenic mf across the three lake catchments was estimated at ~47,700 mf m-2 (~12,000 plastic mf m-2), based on a moss biomass of 2 kg dry weight m-2. These preliminary findings suggest that moss may be a useful biomonitor for the atmospheric deposition of mf (and microplastics).
At present, microplastic (MP) is pervasive globally and has a regional difference. Recent studies have identified MP in the terrestrial atmospheric environment. However, the connection between terrigenous atmospheric MP emissions and impacts over the ocean is not well known. Here, we present the distribution of atmospheric MP abundance over the ocean based on a transoceanic survey conducted across 21 sampling transects from the Pearl River Estuary (PRE) to the South China Sea (SCS) and then to the East Indian Ocean (EIO). The abundance of atmospheric MP over the PRE (4.2 ± 2.5 items/100 m3) was significantly higher than that over the EIO (0.4 ± 0.6 items/100 m3). However, the abundance of atmospheric MP in the SCS (0.8 ± 1.3 items/100 m3) was not significantly different from the EIO and PRE. This result revealed that MP undergoes long-range transport, more than 1000 km away, through the atmosphere, but atmospheric MP transmission as the main source of oceanic MP based on transoceanic studies is not a plausible assumption. Furthermore, backward trajectory model analysis of 21 sampling transects preliminary showed the potential sources of atmospheric MP over the PRE, SCS, and EIO.
Microplastics (plastic particles <5 mm) are a contaminant of increasing ecotoxicological concern in aquatic environments, as well as for human health. Although microplastic pollution is widespread across the land, water, and air, these environments are commonly considered independently; however, in reality are closely linked. This study aims to review the scientific literature related microplastic research in different environmental compartments and to identify the research gaps for the assessment of future research priorities. Over 200 papers involving microplastic pollution, published between 2006 and 2018, are identified in the Web of Science database. The original research articles in ‘Environmental Sciences’, ‘Marine/Freshwater Biology’, ‘Toxicology’, ‘Multidisciplinary Sciences’, ‘Environmental Studies’, ‘Oceanography’, ‘Limnology’ and ‘Ecology’ categories of Web of Science are selected to investigate microplastic research in seas, estuaries, rivers, lakes, soil and atmosphere. The papers identified for seas, estuaries, rivers and lakes are further classified according to (i) occurrence and characterization (ii) uptake by and effects in organisms, and (iii) fate and transport issues. The results reveal that whilst marine microplastics have received substantial scientific research, the extent of microplastic pollution in continental environments, such as rivers, lakes, soil and air, and environmental interactions, remains poorly understood.
https://authors.elsevier.com/c/1ZXP3zLNSRpcN
Atmospheric transport is an important pathway for the deposition of micro- and nano-plastics in remote areas. However, the sources and fate of atmospheric microplastics remain poorly understood. A study on atmospheric transport and deposition in the Pyrenean Mountains highlights the movement of microplastics away from known sources (cities, agriculture, and industry) into remote areas. Following this first evidence of atmospheric microplastic deposition in a pristine location, it is necessary to reconsider previous studies on atmospheric microplastic deposition and behavior in remote areas.
Only few studies investigated the input of microplastic particles via the atmosphere, so far. Here, we present results on microplastic concentrations in the atmospheric deposition in the metropolitan region of Hamburg. In total, six investigation sites were equipped with three bulk precipitation samplers each and sampled biweekly over 12
weeks (12/17-03/18). Three sites were located in a rural area south of Hamburg comprising one open field site and two throughfall sites under beech/oak and Douglas fir forest canopy, respectively. Three further sites were selected within the city following a transect from north to south representing urban sites of varying degrees concerning population, traffic and industrial pressures. Particles and fibers were counted under UV light within a photo box and under a fluorescence
microscope.
Results show that microplastic particles are ubiquitous at all sites. A median abundance between 136.5 and 512.0 microplastic particles per m²/day has been found over the sampling period. This equals a mean microplastic abundance of 275 particles/m²/day. μRaman spectroscopy showed that polyethylenes/ethylvinyl acetate copolymers are dominating significantly (48.8 and 22.0%, espectively), 16 particles analyzed (14.6%) were identified as contamination from PE (polyethylene) samplers.
In contrast to other studies, fragments were significantly dominating compared to fibers. The spatial distribution comparing the urban sites concentrations followed in the order from high to low: “north” (Henstedt-Ulzburg, low population density, suburb) – “center (University; large population density) – “south” (Wilhelmsburg, middle population density, port and industrial facilities) with highly varying concentrations within the time series. Surprisingly, the rural sites in the southern part of Hamburg showed highest concentrations (Douglas fir > open field > beech/oak). This finding is most likely a result of factors such as the comb out capacity of the different forest types and/or direct input pathways from the agricultural areas and the nearby highway.
Empirical correlations relating the drag coefficient of spheres to the Reynolds number are qualitatively reviewed with the focus on the presumed mathematical expressions used in the regression procedure to determine the parameter constants. By putting the shape factors to one in general correlations valid for spheres and non-spheres indirect empirical correlations for spheres are obtained and these are reviewed likewise. It is concluded that the total drag experienced by a sphere in an infinite flowing medium is exactly the sum of Stokes's law and Newton's law. The resulting Stokes-Newton equation is proposed to be the new landmark for the relation between the drag coefficient and the Reynolds number. The values calculated using this new landmark correlation are cross-compared with the estimates of the direct and indirect correlations and with the data set of the historical Standard Drag Curve. The retained Stokes-Newton correlation is confirmed by the fluid dynamics of diluted fluidization at the extreme point of entrainment of an individual particle.
Microplastic (MP) contaminates terrestrial, aquatic and atmospheric environments. Although the number of river sampling studies with regard to MP concentrations is increasing, comprehension of the predominant transport processes of MP in the watercourse is still very limited. In order to gain a better process understanding, around 500 physical experiments are conducted to shed more light on the effects of particle shape, size and density on the rise and settling velocities of MP. The determined velocities ranged between 0.39 cm/s for polyamide fibres (settling) and 31.4 cm/s for expanded polystyrene pellets (rise). Subsequently, the determined velocities were compared with formulae from sediment transport and, as there were large differences between theoretically and experimentally determined velocities, own formulae were developed to describe settling and rise velocities of MP particles with a large variety of shapes, sizes and densities. This study shows that MP differs significantly from sediment in its behavior and that a transfer of common sediment transport formulae should be treated with caution. Furthermore, the established formulae can now be used in numerical simulations to describe the settling and rising of MP more precisely.
The objective of this review is to summarize information on microfibers in seawater and sediments from available scientific information. Microfibers were found in all reviewed documents. An heterogeneous approach is observed, with regard to sampling methodologies and units. Microfibers in sediments range from 1.4 to 40 items per 50 mL or 13.15 to 39.48 items per 250 g dry weight. In the case of water, microfibers values ranges from 0 to 450 items·m − 3 or from 503 to 459,681 items·km − 2. Blue is the most common color in seawater and sediments, followed by transparent and black in the case of seawater, and black and colorful in sediments. Related with polymer type, polypropylene is the most common in water and sediments, followed by poly-ethylene in water and polyester in water and sediments. Some polymers were described only in water samples: high-density polyethylene, low-density polyethylene and cellophane, whilst only rayon was reported in sediments .
This new edition of the near-legendary textbook by Schlichting and revised by Gersten presents a comprehensive overview of boundary-layer theory and its application to all areas of fluid mechanics, with particular emphasis on the flow past bodies (e.g. aircraft aerodynamics). The new edition features an updated reference list and over 100 additional changes throughout the book, reflecting the latest advances on the subject.
Terminal settling velocity of around 600 microplastic particles, ranging from 0.5 to 5 mm, of three regular shapes was measured in a series of sink experiments: Polycaprolactone (material density 1131 kg m− 3) spheres and short cylinders with equal dimensions, and long cylinders cut from fishing lines (1130–1168 kg m− 3) of different diameters (0.15–0.71 mm). Settling velocities ranging from 5 to 127 mm s− 1 were compared with several semi-empirical predictions developed for natural sediments showing reasonable consistency with observations except for the case of long cylinders, for which the new approximation is proposed. The effect of particle's shape on its settling velocity is highlighted, indicating the need of further experiments with real marine microplastics of different shapes and the necessity of the development of reasonable parameterization of microplastics settling for proper modeling of their transport in the water column.
The settling characteristics of large non-spherical particles in the form of discs have been studied. The coefficient of resistance has been determined for cylinders of aspect ratio from 0. 25:1 to 5. 0:1. Cylinders of aspect ratio less than 1:1 showed agreement with previous results, when the coefficient of resistance was plotted against sphericity. Cylinders with aspect ratio greater than 1:1 were found to have a constant coefficient of resistance for increasing aspect ratio. When plotted against sphericity, the value of this coefficient remained constant until the particle was large enough for the wall effect to be observed. The coefficient of resistance was found to increase for aspect ratios decreasing below 1:1 and again for aspect ratios increasing above 4:1. A relationship between the Reynolds and Galileo numbers has been established. Ga equals 5. 50 Re**1**. **7**8.
Environmental context Plastics production has increased considerably in recent years, leading to pollution by plastics, including microplastics (comprising particles smaller than 5mm). This work addresses the issue of microplastics from urban sources and in receiving waters in Greater Paris. Microplastics were found in all urban compartments investigated, namely atmospheric fallout, waste- and treated water, and surface water. Abstract This study investigates the microplastic contamination of both urban compartments (wastewater and total atmospheric fallout) and surface water in a continental environment. These first investigations on an urban environment confirm the presence of microplastics in sewage, fresh water and total atmospheric fallout and provide knowledge on the type and size distribution of microplastics in the 100-5000-μm range. For the first time, the presence of microplastics, mostly fibres, is highlighted in total atmospheric fallout (29-280particlesm-2day-1). High levels of fibres were found in wastewater (260-320×103particlesm-3). In treated effluent, the contamination significantly decreased to 14-50×103particlesm-3. In the River Seine, two sampling devices were used to collect both large and small microplastic particles: (i) a plankton net (80-μm mesh), and (ii) a manta trawl (330-μm mesh). Sampling with the plankton net showed a predominance of fibres, with concentrations ranging from 3 to 108particlesm-3. A greater diversity of both microplastic shapes and types was found during manta trawl sampling but at much lower concentrations (0.28-0.47particlesm-3). This combined approach could be relevant and implemented in future studies to provide an accurate overview of microplastic distribution in freshwater.
The fall velocity of natural sand grains is a fundamental attribute of sediment transport in fluid environments where particles may become partially or fully suspended. Several formulae have been proposed to calculate the fall velocity of particles in air, but there is considerable uncertainty about which is the most accurate or appropriate for a given set of environmental conditions. Five experiments that reported observations of fall velocity of different types of particles in air are described, evaluated, and compared. The experiment data were quality-controlled using four criteria: (1) particles had to have sufficient drop heights to attain their terminal fall velocity; (2) particles had to be in the range of sand sizes; (3) data identified as being problematic by the original authors were removed; and (4) particles comprise natural, irregular shaped sediments. The quality-controlled data were aggregated and analyzed using linear regression to obtain a relationship between grain size (d, in mm) and fall velocity (w0, in ms-1): . This is a statistically strong relationship with a coefficient of determination of 0.89 (p < 0.001). This relationship can be regarded as a universal fall velocity model for natural, sand-sized particles falling through a static column of air. In terms of predictive analyses, our heuristic method outperforms alternative formulae and yields a better fit to the experimental data over the full range of sand sizes.
A meta-analysis was conducted to inform the epistemology, or theory of knowledge, of contaminants of emerging concern (CECs). The CEC terminology acknowledges the existence of harmful environmental agents whose identities, occurrences, hazards, and effects are not sufficiently understood. Here, data on publishing activity were analyzed for 12 CECs, revealing a common pattern of emergence, suitable for identifying past years of peak concern and forecasting future ones: dichlorodiphenyltrichloroethane (DDT; 1972, 2008), trichloroacetic acid (TCAA; 1972, 2009), nitrosodimethylamine (1984), methyl tert-butyl ether (2001), trichloroethylene (2005), perchlorate (2006), 1,4-dioxane (2009), prions (2009), triclocarban (2010), triclosan (2012), nanomaterials (by 2016), and microplastics (2022±4). CECs were found to emerge from obscurity to the height of concern in 14.1±3.6 years, and subside to a new baseline level of concern in 14.5±4.5 years. CECs can emerge more than once (e.g., TCAA, DDT) and the multifactorial process of emergence may be driven by inception of novel scientific methods (e.g., ion chromatography, mass spectrometry and nanometrology), scientific paradigm shifts (discovery of infectious proteins), and the development, marketing and mass consumption of novel products (antimicrobial personal care products, microplastics and nanomaterials). Publishing activity and U.S. regulatory actions were correlated for several CECs investigated.
The most commonly encountered particles in the areas of sediment transport, fluidization, sedimentation, etc. are nonspherical. The role of these particles can be ascertained only through the knowledge of drag coefficient and fall velocity. Graphical relationships for the determination of drag coefficient and fall velocity are available in the literature. The graphical relationships are less accurate because they are subject to errors of judgment. Furthermore, these relationships cannot be used for any analytical purposes. In 1954, Schulz et al. collected extensive data for the drag coefficient and fall velocity of nonspherical particles. Based on these data, empirical equations for the drag coefficient and fall velocity of nonspherical particles of natural and mechanical origin are derived from curve fittings and presented. It is hoped that these equations will find use in the analytical modeling of various engineering problems involving fluid‐nonspherical particles interaction.
In this work, a selection of widely used correlations have been critically evaluated for estimating the drag coefficient of non-spherical particles in incompressible viscous fluids. Experimental results have been culled from 19 independent studies embracing wide ranging particle shapes including cylinders, needles, cones, prisms, discs, rectangular, parallelepiped and cubes. The resulting data base consisting of 1900 data points encompasses wide ranges of physical and kinematics conditions as: sphericity, 0.09 to 1 and the Reynolds number ranging from 10−4 to 5×105. In particular, the performance of five methods has been critically examined. The best method appears to be that of Ganser which uses the equal volume sphere diameter and the sphericity of particle. The resulting overall mean error is about 16%, though maximum error can be as large as ∼100%. In general, the lower the sphericity, the poorer is the prediction.
In the past, general formulas for calculating the drag coefficient for both spherical and nonspherical particles have involved a dependence on a Reynolds number (Re) and one or more shape descriptors resulting in complicated functions of at least two variables. This paper shows that both the Stokes' shape factor (K1) and the Newton's shape factor or scruple (K2) are important for the prediction of drag. However, instead of requiring a function of three variables (Re, K1, K2) to predict drag, a function only of the generalized Reynolds number ReK1K2 is needed. The shape factors K1 and K2 are then modeled as functions of the geometric shape descriptors' sphericity and the projected area in the direction of motion. Goodness of fit to experimental data is calculated and compared with a recently determined formula.
Particle rotation plays an important role on several aspects in gas–solid two-phase flow. However, it has not been paid much attention due to a lack of appropriate measurement methods. An attempt has been made in the present paper on the experimental study of particle rotation characteristics in a cold pilot-scale Circulating Fluidized Bed (CFB) riser, by using a high-speed digital imaging measurement system. It is found that one can measure rotation speeds manually for particles with special speckles on their surfaces or irregular shapes by observing particle image sequences. A dual-frequency imaging method was presented to enlarge the maximal measurable rotation speed at finite frame frequency and the measured rotation speeds are validated theoretically. Furthermore, particle rotation characteristics in a cross-section in upper dilute-phase zone were analyzed statistically. The results show that the average particle rotation speed is about 300 rev/s with the top speed of 2000 rev/s, when the superficial gas velocity Ug, external solids mass flux Gs and average particle diameter are 5 m/s, 1.5 kg/(m2 s) and 0.5 mm, separately. The average particle rotation speed near the wall area is higher than that in the center area at the testing cross-section. Those particles, with either smaller size or higher radial component of translational speed, may have higher average rotation speed. The average rotation speed of irregular particles is apparently higher than that of the spherical ones.
The aerodynamic behaviour of long aspect ratio nylon fibrous particles has been investigated experimentally while settling in air under super dilute conditions without any influence of secondary flows and at fibre Reynolds numbers of 0.5–2 based on fibre diameter. A method for laser-based measurement of the orientations and velocities of fibrous particles is also presented. The experimental apparatus employs a two-dimensional Particle Tracking Velocimetry (PTV) to calculate orientation and velocity based on the two end-points. The controlling length scale in the relationship between Reynolds number and drag coefficient was investigated and the equivalent diameter of settling fibre in air was reported. Finally the influence of volume fraction and fibre straightness were assessed.
High-speed motion pictures (2000 frames/s) of saltating spherical glass microbeads (of diameter 350–710 μm and density 2·5 g/cm3) were taken in an environmental wind tunnel to simulate the planetary boundary layer. Analysis of the experimental particle trajectories show the presence of a substantial lifting force in the intermediate stages of the trajectories. Numerical integration of the equations of motion including a Magnus lifting force produced good agreement with experiment. Typical spin rates were of the order of several hundred revolutions per second and some limited experimental proof of this is presented. Average values and frequency distributions for liftoff and impact angles are also presented. The average lift-off and impact angles for the experiments were 50° and 14° respectively. A semi-empirical procedure for determining the average trajectory associated with given conditions is developed.
Aeolian processes, involving the entrainment, transport, and deposition of sediment by the wind, are important geomorphic
processes operating in arid regions. This chapter, in association with Chapters 18, 19, and 20 form an integrated unit that
discusses the fundamentals of aeolian sediment entrainment and transport, dune morphology and dynamics, wind erosion processes
and aeolian landforms, and the significance of dust transport.