Abstract
Microplastics are being increasingly detected in the atmosphere at altitudes relevant to mixed-phase cloud formation. However, the extent to which microplastics, along with their dynamic surface properties resulting from environmental weathering, could influence atmospheric microphysical processes remains largely unexplored. Here, through a series of ice nucleation experiments and droplet freezing assays, we highlight the capability of model polyethylene microplastics to induce heterogeneous ice nucleation via immersion freezing under atmospherically relevant conditions. We find that sunlight-induced weathering of the microplastic surface influences the structure of surface-bound water molecules and dictates the ice nucleation activity of the microplastics. Using polyethylene, polypropylene, polystyrene, and polyethylene terephthalate as models, we demonstrate that the ice nucleation ability of microplastics is intrinsically linked to their underlying chemistry. Our findings underscore the need to establish a connection between microplastics and atmospheric processes, as the behavior of microplastic pollutants in the atmosphere holds the potential to influence their environmental transport as well as atmospheric microphysical processes.
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Introduction
Microplastics (MPs) are plastic particles of diverse chemistries and morphologies in sizes ranging from 1 to 1000 µm1. These MPs can enter the environment either in their microscopic state or gradually form upon disintegration of their parent macroscopic plastic. These plastic particles are an emerging class of pollutants increasingly detected across the planet2,3,4,5, including geographically secluded regions with minimal human activity6,7. For instance, MPs have been found to be atmospherically transported and subsequently deposited in remote locations such as the Pyrenees mountains8 and Antarctica9. The atmospheric transport and eventual deposition of MPs are well documented10,11; however, their potential to impact fundamental atmospheric processes, such as cloud condensation and ice nucleation remain largely unknown. Although natural and anthropogenic airborne particulate matter, such as dust12, pollen13, and carbon soot14,15,16,17 currently present a greater atmospheric burden than MPs, the atmospheric fate of MPs remains largely unclear. Specific questions regarding atmospheric MPs are echoed across the research field, such as: “Do atmospheric MPs hold the potential to exert influence on atmospheric microphysical processes?” and “how does the atmospheric cycling of MPs influence their environmental transport and fate?” Considering the low mass density and extended residence time of MPs in the atmosphere11, coupled with their dynamic and constantly evolving surface characteristics18,19, it becomes imperative to identify the relation between the physicochemical properties of MPs and their potential to influence atmospheric water condensation and ice nucleation.
Ice nucleation is defined as the first appearance of a thermodynamically stable ice phase. The ice nucleation process can be categorized as either homogenous or heterogeneous. Homogeneous ice nucleation occurs in pure water droplets at a temperature limit of approximately −38 °C, with variations depending on droplet size20,21. In heterogeneous ice nucleation, the freezing temperature of water is increased by reducing the barrier for nucleation, stabilizing the ice clusters, and facilitating the nucleation process to occur onto the active sites of an ice nucleating particle (INP)22. In the atmosphere, a predominant mechanism of heterogeneous ice nucleation is via immersion freezing23,24. Here, a micro or nanosized INP immersed in a supercooled water droplet can catalyze the freezing process by reducing the free energy barrier for ice nucleation. The temperature at which immersion freezing occurs depends on the characteristics of the INPs, such as porosity, chemical composition, and crystallographic features25,26,27. Typical INPs include minerals with lattice parameters that epitaxially match ice crystals, surfaces with hydroxyl groups that favor hydrogen bonding with water molecules, or rough surfaces with high defect densities that drastically reduce the barrier for heterogeneous nucleation21,28,29. Despite recent advances, the relative impact of these properties of INPs on the heterogenous ice nucleation is yet to be resolved, beyond predictions based on empirical results and chemical intuition30,31. This challenge is further compounded for MPs, which display a wide range of particle morphology and surface chemistry. For example, the presence of various additives will further diversify the weathering process and corresponding surface characteristics of MPs in the environment32. One recent study suggested that low-density polyethylene nanoplastics could nucleate ice at relatively high temperatures33. However, the effect of continuous exposure to atmospheric conditions on the surface properties of MPs and its corresponding influence on heterogenous ice nucleation is yet to be identified, which is the central theme of this article.
This article demonstrates the impacts of the dynamic surface chemistry of MPs in the environment on their atmospheric ice nucleation ability. We specifically focus on the effects of sunlight-induced weathering/photooxidation of MPs on their ability to uptake atmospheric water and nucleate ice. We identify effects of sunlight exposure on the ice nucleation activity of model spherical polyethylene (PE) MPs of diameter ~ 70 µm (Fig. S1). The low mass density of MPs enables particles of this relatively large size to be advected into the troposphere, as documented in the literature8,11,34,35,36. The use of model MPs enables us to delineate the specific role of MP surface oxidation (from sunlight) on the atmospheric ice nucleation, avoiding any confounding from other parameters such as MP shape and size. By combining x-ray photoelectron spectroscopy (XPS) and three-dimensional atomic force microscopy (3D AFM) with ice nucleation experiments and droplet freezing assays, we establish a direct link between the surface chemistry of MPs and their efficacy in catalyzing ice nucleation through immersion freezing. We further identify the role of chemical structure of plastics on ice nucleation by using MPs derived from commodity plastics, including polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET), all of which have been identified in the atmosphere8,37. All tested MPs showed water condensation and ice nucleation respectively occurring at lower relative humidity and warmer temperatures in comparison to vapors condensing onto a hydrophobic surface and subsequent ice nucleation. The observed change in the condensation and ice nucleation behavior onto MPs is directly linked to their chemical properties. In this work, we present laboratory experiments that elucidate the role of environmentally-driven changes in the surface chemistry of MPs and their influence on atmospheric microphysical processes. By isolating the effects of surface photooxidation on MPs, we directly connect the dynamic alterations in their surface chemistry to their behavior under atmospheric conditions, ultimately impacting their transport fate and eventual deposition.
Results and discussion
Water uptake and immersion freezing with MPs
MP pollutants in the atmosphere can induce the heterogeneous nucleation of water and subsequent ice formation via condensation freezing (Fig. 1). MPs can enter the atmosphere through many pathways, including droplet ejection from marine surfaces38 and wind erosion39. Suspended in the atmosphere, MPs can then be advected to altitudes where lower temperatures favor the condensation of water on their surface. Subsequently, the condensed water on the MP surface can freeze at sufficiently low temperatures. We demonstrate such ability of MPs through well-established ice nucleation experiments12,14,40,41,42. In a typical experiment, dry INPs are immobilized on a hydrophobic substrate which is cooled at a desired rate using a temperature-controlled stage (see Methods). The ice nucleation experiments enable identification of characteristic temperatures and relative humidities (RH) of water uptake and ice nucleation (TIN) on an individual INP. Lowering the temperature and raising the humidity within the ice nucleation cell help pinpoint these critical conditions, as colder temperatures and higher humidity levels favor ice formation and water condensation, respectively. For a weathered MP immobilized on a gradually cooled substrate, we observe the condensation and uptake of water from the atmosphere on the MP surface (Fig. 1a-b). The water uptake is driven by the saturation of water vapors in the vicinity of the MP, which leads to the observed condensation. The water droplets condensed on the MP surface coalesce and lead to the formation of a continuous hydro shell which completely engulfs the MP. Upon further cooling the MP, the hydro shell undergoes immersion freezing, forming stable ice (Fig. 1b). In our experiments with weathered MPs, micron-sized water droplets formed on the particle surface via condensation at RH ~ 50% and −11 °C, and subsequently froze around −25 °C. In our control experiments with water condensation on a hydrophobic surface (no MPs), droplets of similar size appeared at RH = 95% and −19 °C, freezing at −32 °C.
a A schematic representation of how MPs could promote atmospheric ice nucleation process via condensation freezing. Beginning with the water uptake by the MP surface due to cooling in the atmosphere, the hydro shell is formed. A gradual decrease in the atmospheric temperature will lead to the ice nucleation and freezing of the hydro shell. b Experimental images of a weathered MP on a substrate gradually cooled showing the water uptake, hydro shell formation and the immersion freezing process. The scalebar in (b) is 20 µm.
Weathering driven changes in MPs and structure of surface-bound water
Chemical changes to the MP surface precede any physical deterioration to the plastic subjected to sunlight induced weathering (Fig. 2). In the laboratory, we simulate the sunlight exposure for PE MPs using an accelerated weathering chamber (ASTM Standard D507143) and monitor corresponding changes in their chemical and physical properties at specified time intervals (Fig. 2; and Methods). XPS is used to identify chemical changes, while AFM is utilized to examine physical alterations in the MPs upon weathering. After 10 days of accelerated weathering, the XPS spectra of MPs shows a peak in the O 1 s region, signifying the formation of carbon-oxygen bonds (C = O, O = C-O and C-O) and indicative of plastic oxidation (Figs. 2d, S2–S4). However, no significant changes in MP morphology are observed via AFM during this period (Fig. 2a, b and e, f). Extended weathering periods result in both the presence of the O 1 s peak amplitude in XPS and increase in roughness of the MP surface, indicating simultaneous chemical and physical alterations (Fig. 2a–f). Interestingly, the XPS spectra shows the sodium contamination in the MPs aged for 90 and 180 days, indicated by the appearance of a Na 1 s peak and a KLL auger peak (Figs. 2d, S3). This observation is expected, given the ubiquity of atmospheric salts like NaCl, and will be further discussed in the context of our experiments in following section.
a–c Three-dimensional (3D) surface plots generated using atomic force microscopy (AFM) data for the polyethylene (PE) MPs which are weathered for 0, 10 and 180 days. d X-ray photoelectron spectroscopy (XPS) spectra of PE MPs weathered for 0, 10, and 180 days. The emerging peaks fall within the O 1 s region of the XPS spectra. e Change in the height profile of the PE MPs with increasing weathering time. f The average (left y-axis) and maximum (right y-axis) height of the roughness profile of the PE MP surface as a function of weathering time. The black squares represent the average surface roughness (left y-axis) and the green circles represent the maximum height of the surface roughness (right y-axis). The error bars represent the standard error. g-i Slices of the force gradient map normal to the PE plastic surface at (g) Day 0, (h) Day 5 and (i) Day 10 of accelerated weathering. The force gradient map suggests local ordering of the water molecules at the weathered plastic surface (indicated by the arrows). j Average force gradient curves for PE weathered for 0, 5 and 10 days extracted from 3D AFM data. The data reported in (g–j) was obtained for PE sheets instead of MPs, because of the need for smooth surface for identifying the hydration structure using 3D AFM. Source data are provided as a Source Data file.
The evolving chemistry of MP surfaces is expected to alter the solution structure of surface-bound water, which in turn can impact the ice nucleation behavior. To test this hypothesis, we performed 3D AFM force mapping on model PE sheets subjected to photooxidation and placed in contact with water (Fig. 2g-i). This technique records the forces experienced by a nanosized probe as it navigates the interfacial region, which enables the visualization of solid-liquid interfaces at the molecular scale44,45. Note that 3D AFM measurements for mapping the hydration structure on a surface are complex and cannot be readily performed on surfaces that are curved or rough. The presence of curvature or roughness hinders the ability to distinguish the hydration structure from other morphological features, which is why we chose a sheet over a MP for the 3D AFM mapping. We acknowledge that the weathering behavior of a sheet versus a MP may not be identical due to slight differences in the form factor, light exposure, and potential differences in additives. However, since both the sheet and particles are primarily made of polyethylene, the fundamental chemical degradation mechanisms are expected to be similar18. Prior to sunlight exposure (Day 0), the force maps are dominated by interactions between the probe and the underlying substrate, with minimal oscillatory features that would indicate the presence of hydration layers (Fig. 2g, j). Upon accelerated weathering of the PE substrate for 5 days, we observed the occurrence of vertical oscillatory features in force gradient with a spacing of 0.3–0.4 nm near the PE substrate (Fig. 2h) In principle, these features correspond to the presence of multiple layers of water molecules that assemble at the interface (Fig. 2j)46,47. Following this trend, the 3D force maps of the PE substrates after 10 days of weathering also show pronounced hydration layers (Figs. 2i, S6). Previous studies have attempted to develop a theoretical framework that connects the 3D force gradient maps to water distributions, particularly at crystalline surfaces, but such models are beyond the scope of the current study47. The presence of the force gradient oscillations in weathered plastics is only a qualitative indication of the hydration layer formation and should not be quantitively compared as the underlying plastic surface is not atomically smooth (Fig. 2g–j). For example, the hydration layers organize around local roughness features by tilting from the normal direction to the surface, which breaks the longer-range ordering of the hydration layers. Accordingly, the magnitude of the oscillatory features in the average force gradient curves should not be interpreted quantitatively as it is influenced by local surface roughness, colloidal forces, as well as nanoscale homogeneity of the simulated sunlight within the weathering chamber, among other factors. The emergence of ordered hydration layers at the PE substrate suggests an augmentation in the hydrophilic nature of the PE substrate following weathering process48,49. This finding is in agreement with the previously reported decrease in the water contact angle of the PE substrates19, and points to the potential impact of the state of adsorbed water molecules on the ice nucleation process.
Effects of MP weathering on water uptake and freezing
Heterogeneous water and ice nucleation are both localized phenomena, governed by the presence of active sites on a surface. Any changes in the active site density due to chemical and/or physical transformations in the MPs will have a direct influence on the relative humidity at which water condenses on the MP surface i.e. RH for water uptake, and the temperature at which the condensed water subsequently freezes i.e. TIN. We identify the effects of weathering on RH for water uptake and TIN of the MPs with ice nucleation experiments (see Methods). We perform ice nucleation experiments on a mixture of unweathered (blue) and 10 day weathered (magenta) MPs, as shown in Fig. 3. Before any weathering, the two MPs differ only in color (added dye) and were similar in their chemical composition (Fig. S5). The distinct colors of the PE MPs enable us to identify the water uptake and ice nucleation behavior among co-existing unweathered and weathered MPs. We find that the RH for water uptake and TIN of the weathered MPs is ~40% lower and 5°C warmer, respectively, when compared to the unweathered MPs (Fig. 3). As the temperature continues to decrease, the hydro shell formed on the weathered MPs freezes prior to the ice formation on the unweathered MPs (Fig. 3a–c). The MPs weathered for 90 and 180 days show a further decrease in RH for water uptake and increase in TIN (Fig. 3d, e), which is attributed to the increase in specific surface area due to the amplified roughness and the adsorption of hygroscopic salts during the weathering process (discussed below) in addition to the photooxidation of the MPs.
a–c Batch ice nucleation experiments performed on co-existing unweathered (magenta) and weathered (cyan) MPs. The uptake of water by the MPs weathered for 10 days (cyan) is observed prior to the unweathered MPs (Magenta). Here the temperature was decreased at a rate of 0.1 °C min-1. Scale bar in (a) is 50 µm. d, e Box plots representing the change in relative humidity (RH) and temperature at which water uptake and ice nucleation was observed, respectively, for model polyethylene (PE) MPs with respect to the weathering time. The box represents the interquartile range, error bars represent one standard deviation, and the centerlines represent the mean. Note that the RH can no longer be accurately estimated after the first appearance of condensed water and is not reported in (e). Source data are provided as a Source Data file.
The observed differences in RH for water uptake and TIN between the unweathered and weathered MPs is due to the sunlight-induced photooxidation, adsorption of atmospheric salts and changes in the morphology of MPs. The photooxidation of PE MPs upon 10 days of weathering leads to the formation of hydrophilic moieties on its surface, as shown previously19, and drives an increase in water wettability. The increase in water wettability of the MP upon weathering is confirmed by the measured decrease in the contact angle of water on the PE (Fig. S7). The increase in water wettability of MPs reduces the free energy barrier for water uptake, which can be estimated through Volmer’s classical nucleation theory as50 \(\triangle G={{{\rm{\pi }}}}{\gamma }_{{{{\rm{lv}}}}}{r}_{c}^{2}\left(2-3\cos \theta+{\cos }^{3}\theta \right)/3\); where \(\theta\) is the water contact angle on the PE surface, \({\gamma }_{{{{\rm{lv}}}}}\) is the liquid-vapor surface energy, and \({r}_{c}\) is the critical radius given by Kelvin’s equation (~2 nm). The corresponding nucleation rate is given as50 \({J}_{{water}}=K\exp (-\triangle G/{k}_{{{{\rm{B}}}}}T)\), where K is the kinetic constant, \({k}_{{{{\rm{B}}}}}\) is the Boltzmann constant and \(T\) is the temperature. The nucleation rate of water condensation on MPs nearly doubles upon 10 days of weathering (Fig. S8).
The MPs weathered for 90 and 180 days show a further decrease in RH for water uptake and an increase temperature of ice nucleation (Fig. 3d-e), which is partially attributed to the increase in specific surface area due to the amplified roughness in addition to the photooxidation of the MPs. However as mentioned in previous section, the presence of Na on the surface of Day 90 and Day 180 MPs is a significant factor to consider when interpreting these results. The inherent hygroscopicity of atmospheric salts such as NaCl bound to the MP surfaces likely contributes to the lower observed RH for water uptake. In real atmospheric conditions, MPs re-emitted to the atmosphere from the ocean are expected to carry salts, making this factor particularly relevant51. This influence, combined with the increased surface roughness from extended weathering, suggests the likelihood that both the physical and chemical transformations of the MPs play roles in their water uptake behavior. Therefore, while the presence of salts is not the sole contributor, it is an important factor that must be considered in the analysis and interpretation.
Immersion freezing of MPs compared to mineral dust
Through heterogeneous ice nucleation, immersed MPs lower the free energy barrier for ice nucleation, while providing a surface for stable ice crystals to form within the aqueous droplet. We experimentally identify the impacts of MP weathering on their ability to nucleate ice and compare it with the mineral dust, a known INP, using the droplet freezing assay21,52,53 (Fig. 4a–c). In a typical experiment six 5 µL droplets of ultrapure water containing 10 mg mL-1 model PE MPs or mineral dust are placed on a hydrophobic glass substrate. The glass substrate containing the droplets are then submerged in squalene oil and the temperature is decreased at a rate of 1 °C min-1. The experiments were performed using MPs in their unweathered and weathered states (Days 0, 2, 10, 90, and 180), and the fraction of droplets frozen at a given temperature (\({f}_{{ice}}\)) was determined, as shown in Fig. 4d.
a–c Experimental images of the aqueous droplet (ultrapure water) freezing in the absence of any added INPs (a), containing PE MPs weathered for 10 days (b), and containing mineral dust (c). Once the droplets turn white/opaque, they are counted as frozen. The scale bar in a is 5 mm. d The fraction of droplets frozen, fice, as a function of temperature. Each point represents the fraction calculated from at least 36 droplets. e The change in heterogeneous ice nucleation rate coefficient Jice upon decrease in temperature. The Jice is calculated from fice using Eq. 1. The error bars in (e) represent one standard deviation and are equal in both the positive and negative direction. d, e Share the abscissa and use the same legend. Source data are provided as a Source Data file.
The ice nucleation activity of MPs weathered for 90 and 180 days, particularly at temperatures below -24°C, is comparable to that of mineral dust when normalized using the surface area (Fig. 4d, e). In the absence of any MPs the water droplets begin to freeze at -28°C with all droplets being frozen at -36°C. These temperatures align with previously reported values for freezing of water droplets entering the homogeneous ice nucleation regime54. Upon the addition of MPs, the droplets freeze at warmer temperatures, which is dependent on their degree of photooxidation and can be quantified using the heterogeneous ice nucleation rate coefficient (\({J}_{{ice}}\)). Here we determine the \({J}_{{ice}}\) from the droplet freezing assay experiments using the relation21,52,53
Where a is the specific surface area of the INPs within the aqueous droplet, m is the mass of INPs per droplet, and t is the time that each droplet remains in liquid form. The specific surface area of the MPs was estimated from AFM measurements and corroborated with BET analysis of nitrogen gas adsorption (Fig. S8), while the specific surface area of the model mineral dust was obtained from literature55. The addition of any INPs i.e. unweathered or weathered MPs or mineral dust increase the ice nucleation rate in comparison to homogenous ice formation (Fig. 4e). However, the ice nucleation activity of MPs is linked to their weathering time. An increase in \({J}_{{ice}}\) is observed upon the weathering of MPs. This is attributed to the increase in surface wettability and the formation of the hydration layers on the surface of MPs (Figs. S6, S7 and 2g–j). We report a greater increase in \({J}_{{ice}}\) of droplets containing MPs weathered beyond 10 days, where the MP surface chemistry, adsorbed salts as well as roughness contribute to the ice nucleation process.
Our findings indicate that the ice nucleation activity of weathered MPs at temperatures below -24°C can be comparable to that of mineral dust (Fig. 4e). However, the current atmospheric loading of MP fragments is much lower than that of mineral dust. For instance, Chen and coworkers found ~0.03–0.04 MP fibers and fragments per cubic meter of air in the atmosphere11. Although the atmospheric burden of MPs is currently much less than that of mineral dust, it remains crucial to understand the potential influence MPs might have on atmospheric processes. This comes as the environmental deposition of MPs continues to increase at an alarming rate. Understanding the influence of the surface chemistry of MPs on their interaction atmospheric water will shed light on how such interactions may impact transport and deposition patterns of MPs in the environment.
Ice nucleation activity of common plastic polymers
The ice nucleation ability of MPs not only depend on the sunlight-induced weathering but also influenced by the chemical structure of the underlying plastic (Fig. 5). We perform ice nucleation experiments on MPs made via milling macroscopic sheets of PE, PP, PS, and PET (Fig. 5a, b and Fig. S9). The ice nucleation behavior of the milled MPs was identified in either their unweathered state or after just 1 day of weathering. The short duration of weathering ensured that all observed changes in the RH for water uptake and TIN were solely due to photooxidation of the MP surface and not changes in morphology. Additionally, by comparing the unweathered and 1 day weathered samples, we highlight the varying susceptibilities of different plastics to photooxidation. This approach allows us to differentiate between early-stage chemical transformations and more extensive weathering effects, providing insights into how photooxidation affects the onset conditions for water uptake and ice nucleation of different polymers. The presence of unweathered MPs of all tested polymer types leads to an increase in the RH for water uptake and TIN in comparison to the hydrophobic substrate in the absence of particles. However, there is no statistically significant difference in RH for water uptake or TIN among the tested unweathered MP polymers. After 1 day of accelerated weathering, the RH for water uptake and TIN for PP and PS shift to lower relative humidities and warmer temperatures, respectively, whereas no statistically significant difference was observed for PE and PET (Fig. 5d, e).
a, b scanning electron microscopy (SEM) images of a milled polyethylene (PE) MP. The scale bars in (a), and (b) are 50 µm and 5 µm, respectively. c X-ray photoelectron spectroscopy (XPS) spectra in the O 1 s region of the milled MPs. The solid lines are the XPS spectra of each unweathered milled MP, and the dashed lines are the XPS spectra of the weathered (1 day) milled MPs. d Box plot representation of the relative humidity (RH) and temperature at which water uptake was observed for unweathered (D0) and weathered (D1) PE, polypropene (PP), polystyrene (PS) and polyethylene terephthalate (PET) MPs immobilized on a hydrophobic substrate. The water condensation on the hydrophobic substrate (Sub.) was observed at RH ~ 95%. e The temperature of ice nucleation of the water condensed onto unweathered (D0) and weathered (D1) PE, PP, PS or PET MPs and the hydrophobic substrate (Sub.). The data in (d) and (e) represent an average of at least 5 experimental runs represented by individual points. The box represents the interquartile range, error bars represent one standard deviation, and the line represents the mean. Source data are provided as a Source Data file.
The alteration in the RH for water uptake and freezing temperatures for PP and PS, but not PE and PET, is attributed to variations in susceptibility of the plastics to photooxidation. The XPS spectra of weathered PP shows the emergence of the O 1 s peak, indicative of the formation of surface carbonyl groups, likely leading to increase in the hydrophilicity (Fig. 5c). The XPS for the PS MPs shows the emergence of additional peaks in O 1 s region, indicating the incorporation of oxygen atoms at the PS surface. In contrast, the O 1 s region for the PE and PET MPs do not show the appearance of new peaks upon 1 day of accelerated weathering (Fig. 5c). In case of PET, the oxygen is integrated within the polymer chain network, and the observed shift of the peak in O 1 s region to lower binding energy upon weathering can be attributed to the surface reorganization of molecules as well as partial photooxidation of the PET56. The XPS measurements are further corroborated with Fourier Transform Infrared Spectroscopy (FTIR), where no detectable differences are observed between unweathered and weathered PET and PE MPs (Figs. S10 and S11). These observations are in good agreement with the existing literature56,57,58,59, where differing kinetics of photooxidation of the plastics originate from their underlying molecular structure. Here, the polyolefins with functional groups on the alpha carbon atoms (PP and PS) make the free radical generated during photooxidation of PP and PS more stable than for PE57,58,59. The photooxidation of polyesters (PET) follows a different pathway60, where chemical changes to the PET surface are not typically observed after only 24 h of exposure to environmentally relevant ultraviolet radiations56. Note that crystallinity of the milled MPs, inferred from the crystallinity index61 as measured via X-ray diffraction (XRD), follows the order PE > PP > PS > PET (Figs. S12 and S13). No change in the crystallinity was observed for any of the plastics after 1 day of weathering. Hence any alterations to the RH for water uptake and TIN upon weathering of MPs can be ascribed to variances in the surface physicochemical properties instead of the changes in the crystallinity of the plastics.
Molecular origin of ice nucleation on microplastics and its climatic ramifications
In the present study, we have experimentally demonstrated that MPs possess the capability to act as atmospheric INPs. Furthermore, we have revealed that the onset conditions for water uptake and heterogeneous ice nucleation on MPs is dynamic, influenced by plastic chemistry and environmental stressors such as sunlight exposure. Predicting the atmospheric residence time of MPs is inherently challenging due to the variability in the environmental conditions and MP properties62. Despite this uncertainty, it is clear that MPs will undergo weathering from sunlight exposure, regardless of whether they are in the atmosphere, aquatic, or terrestrial environments11. This continuous environmental degradation, including photooxidation, will directly influence the surface chemistry of MPs and their interaction with atmospheric water. The weathering process leads to changes in surface oxidation state, wettability, and roughness of the MPs, all of which can influence the nucleation efficiency of MPs in the atmosphere. Furthermore, our work directly links the molecular-scale nature of surface chemistry to macroscopic surface hydrophilicity. Using 3D AFM, we resolved the assembly of hydration layers with 0.3 nm spacing at the oxidized plastic interface, validating the observed decrease in water contact angles and increase in surface wettability. The significance of molecular surface chemistry in ice nucleation has been emphasized in a recent study by Fitzner et al., who used molecular dynamics simulations to sample thousands of surfaces and identify key descriptors for effective ice nucleating agents63. One key result was that surfaces with low barriers for lateral water displacement promoted ice nucleation. The rationale was that water molecules that are not rigidly bound to specific lattice sites can efficiently sample multiple configurations, which leads to a stochastic nucleation event. The current work demonstrates that MPs are an excellent candidate that validates this hypothesis. In contrast to crystalline minerals with localized water adsorption sites, hydrophilic MPs can adsorb and retain water molecules, but also allow for sufficient water mobility in the interfacial region.
Our work has quantified that, following sufficient exposure to environmental stress, the onset conditions for MPs to uptake water and induce heterogeneous ice nucleation can be significantly altered. Similar research finds conflicting results, claiming that oxidation does not impact or even inhibits the ice nucleation activity of MPs64,65. This discrepancy likely originates from different aging techniques and the composition of the plastic materials used in the studies. It should also be noted that current MP concentrations in the atmosphere are relatively low in comparison to other atmospheric INPs such as dust and soot. A substantial increase in the atmospheric loading of MP pollutants could impact cloud formation and local climate patterns. Alternatively, the process of water uptake and ice nucleation on to MPs will impact the transport and cycling of these pollutants through the total environment. The specific atmospheric conditions under which MPs uptake water and nucleate ice will influence their residence time in the atmosphere and potential deposition locations. However, more research is needed to confirm these hypotheses. The increasing presence of MPs in the environment underscores the need for further research into their atmospheric dynamics and the development of effective mitigation and remediation strategies.
Methods
Materials
Model microplastics
We use commercially available PE microspheres (Cospheric LLC) of diameter ∼70 μm as model MPs to obtain general findings that correlate time of accelerated weathering with changes in ice nucleation activity. The model PE MPs had a mass density of 1.0 g cm–3, and the particles were labeled with a blue or magenta dye for improved visualization and analysis purposes.
Model mineral dust
Arizona test dust (ATD) obtained from Powder Technology Inc. was used as a surrogate for atmospheric mineral dust. In literature, ATD is often used as a model mineral dust, due to its well-established composition and size distribution15,66,67,68. Note that ATD does not represent a homogenous material as it contains a variety of different species commonly found in dust, including quartz, illite, kaolinite and montmorillonite66. The size distribution of ATD is from 0.97 to 170 µm.
Milled microplastics
To access a variety of plastic chemistries, polydisperse MPs were generated by in-house milling of macroscopic plastic obtained from McMaster Carr (PE, PP, PS, PET). Here, an abrasive bur (cylinder ball end) was rotated on the macroplastic plate at a standardized rate (1530 RPM) to generate MPs. The physical properties of the generated MPs (size and surface chemistry) are characterized using SEM, FTIR and XPS (Figs. S9-S11).
Accelerated weathering
Environmental photooxidation processes was simulated using a Xe-1 weathering chamber (Q-Labs) with a 340 nm wavelength filter and irradiance set at 0.35 W m–2, calibrated according to the ASTM D5071 standard43. In a typical experiment, 100 mg of MPs were added to 100 mL of deionized water in a clean glass container, such that the air-water interface was covered with a single layer of MPs. The glass containers with the MPs were then placed in an accelerated weathering chamber and water level in the jars was carefully observed on a bi- daily basis, and continuously refilled before the water could entirely evaporate out of the experimental setup. In the accelerated weathering chamber, MPs are exposed to a xenon arc lamp fitted with a filter designed to impede UVB and UVC radiation. By selectively restricting UVB and UVC radiation and promoting UVA radiation (~340 nm), the conditions in the weathering chamber aligned with those of natural sunlight. However, it should be noted that different polymers will have unique susceptibility to different wavelengths of UV light, depending on the chemical bonds present69. In this study we specifically focus on the weathering of PE using the UVA spectrum of the light due the environmental abundance of the PE MPs and ability of UVA to represent the sunlight-induced degradation process. The maintenance of a constant temperature of 63°C and irradiance of 0.35 W m-2 within the weathering chamber contributes to the acceleration of the photooxidation process. In our experimental setup, the weathering was accelerated by a factor of 10–30 (TenCate Geosynthetics70). The exact factor is contingent upon various external parameters, including latitude, altitude and local climate conditions. This implies that one 24 h cycle in the accelerated weathering chamber corresponds to between 10 and 30 days in the real environment. However, for ease of understanding, we report experimental results as a function of the days spent in the accelerated weathering chamber (weathering time).
Water uptake and ice nucleation experiments
Ice nucleation experiments were conducted according to well-established methodology12,14,40,41,42. We immobilize MPs on a hydrophobic glass slide. Here, the immobilization technique involves the wet deposition of model MPs onto the glass slide, followed by a 24 h drying period. The surface chemistry of the MPs remains invariant even after multiple dispersing-drying cycles as shown by the FTIR measurements (Fig. S5). The hydrophobic coating on the glass was synthesized via the adsorption (and subsequent interlinking) of hexamethyldisilazane vapors to a glass substrate71. The MPs on the hydrophobic slide are then placed into a closed cell situated on a temperature control stage (Linkam THMS600). We then increase the relative humidity by reducing the temperature at a rate of 0.1 °C min−1. Throughout the experiments, we maintain a constant vapor pressure in the cell via the flow of N2. The relative humidity was calculated by dividing the actual vapor pressure of water (\(e\)) by saturation vapor pressure (\({e}_{s}\)), at a given temperature (T):
However, since we are applying a temperature ramp to our experimental system, an expression forthe dependence of \({e}_{s}\) on temperature is required. We use the empirical Magnus-Tetens expression to estimate the saturation vapor pressure at a given temperature72,73:
This calculation uses well-established coefficients: \({A}_{1}\) = 17.625, \({B}_{1}\) = 243.04°C and\({C}_{1}\) = 610.94 Pa. These coefficients provide values for \({e}_{s}\) with 0.4% error in the temperature range of -40°C ≤ t ≤ 50°C74. In the experiments, water uptake is recorded as the first appearance of a liquid water droplet (> 1 µm) condensed onto the MP. After water condenses, the vapor pressure and relative humidity within the cell are compromised and the experiment is then terminated. The cell is then returned to room temperature before repeating the experiment. The same criterion was applied to define the TIN variable, marking the temperature at which ice first appeared (crystal size > 1 µm). For comparison, experiments were conducted in the same manner without the presence of particles. Here, RH for water uptake and TIN are defined as the relative humidity and temperature at which water condensed onto the hydrophobic glass substrate and subsequently froze.
Droplet freezing assays
Well-established methodology was used to quantify the freezing temperatures of aqueous microdroplets with differing PE MPs and mineral dust33,54,75,76,77,78. For each experiment, 5 µL of six aqueous droplets containing PE MPs (and control droplets without PE MPs) were pipetted onto a hydrophobic glass slide. The slide with the aqueous droplets was then placed in an aluminum pan and submerged in squalene, an oil matrix, in order to prevent condensation and Wegener–Bergeron–Findeisen effects54. The experimental system was then placed on a cooling stage (Linkam THMS600) and cooled at a rate of 1 °C min-1. The described experiments were repeated at least five times to generate the experimental results (Fig. 3).
Atomic force microscopy
The nanoscale surface roughness of both unweathered and weathered PE MPs was assessed using AFM (Horiba SmartSPM AIST) in tapping mode (Fig. 2a–f). The experiments were performed using AFM cantilevers (MikroMasch) with a resonance frequency of ~ 160 kHz and a spring constant of 5 N m−1. Particles were affixed to a silicon wafer for analysis and placed within the scanning area. The images were obtained at a scan rate of 1.0 Hz (Fig. S14). Three-dimensional topographical images and the height profiles were generated using Gwyddion software version 2.6279. For the surface roughness measurements, we scan five individual particles. For each particle, we obtain ten different roughness profiles to calculate the maximum and average surface roughness. All collected data was processed in Gwyddion following a standardized methodology. The data processing steps included the removal of a second-degree polynomial background from the AFM scans to account for the curvature of the PE MPs, thus enabling the accurate measurement of maximum and average surface roughness. Additionally, 5 µm ✕ 5 µm area scans are obtained from the AFM to estimate the effective specific surface area of a single particle. This measurement is repeated for five different particles to ensure accuracy and limited variation in particles surface area within the bulk sample.
3D force mapping
3D Atomic Force Mapping measurements (Fig. 2g–j) were performed using an Asylum Research Cypher Video Rate Scanning (VRS) atomic force microscope, which was operated in amplitude modulated mode using ARROW UHFAuD probes (NanoWorld). The experiments utilized ARROW UHFAuD probes (NanoWorld) that were initially cleaned with UV-Ozone treatment for 10 min, followed by rinsing in water and ethanol solutions. The cantilever was photothermally driven at resonance using the BlueDrive system, with drive amplitudes in the bulk solution maintained at <0.2 nm. For a typical dataset, the xyz scan volume was set to 3 × 3 × 5 nm3, which corresponds to 128 × 128 × 2500 pix3. The tip-sample force gradient \({k}_{ < z > }\) is calculated using the equations by Söngen et al. based on a harmonic oscillator approximation for the cantilever80
where φ, A, A0, k, and Q denote the phase shift, measured amplitude, drive amplitude, cantilever spring constant, and cantilever quality factor, respectively.
X-ray photoelectron spectroscopy
Chemical changes to the surface of PE MPs weathered for 0, 2, 10, 90 and 180 days were detected using XPS (Fig. 2d). Approximately 50 mg of MPs were situated in the sample area such that they completely covered the detector. The measurements were performed at least three times. Survey scans of the model MP surfaces were collected from 1200 – 0 eV using a Scienta Omicron ESCA 2SR X-ray photoelectron spectroscope equipped with a monochromatic Al Kα X-ray source (1486.7 eV). The individual components of the O 1 s and C 1 s regions of the XPS spectra were identified and fitted using the CasaXPS software (Figs. S3–S4)81.
ATR-FTIR
To identify chemical changes to the milled MPs after 1 day of weathering, we performed ATR-FTIR experiments (Fig. 5c). Here again, ~50 mg of MPs were placed on the monolithic diamond crystal accessory, ensuring total coverage during scanning. The measurements were performed at least three times. The vibrational spectra were obtained using a monolithic diamond crystal ATR accessory on a Bruker Alpha FTIR instrument. Measurements are obtained by collecting 32 scans per spectrum at a 4 cm−1 resolution, after blanking against the air.
BET analysis
The surface area of the MPs was measured by N2 gas adsorption onto the model MPs at −196 °C using the Quantachrome AS-1 instrument (Fig. S8). In a typical experiment, 300 mg of model PE MPs were degassed under vacuum at 60 °C for 12 h. The surface areas were calculated using a BET plot of the adsorption isotherm.
X-Ray diffraction
X-ray diffraction experiments were conducted using a Bruker D8 X-ray diffractometer. For the measurements, the macroscopic sheets from which the milled MPs were generated were utilized. Here, the XRD measurements were repeated three times for each plastic sample, yielding consistent results. Following background subtraction, the crystalline peaks were fitted assuming a gaussian function. The crystallinity index (CI) was employed to estimate the quantity of crystalline features within the milled MP samples. It is calculated by comparing the intensity of the crystalline peaks in the X-ray diffraction pattern to the total intensity of all peaks61. A higher CI value indicates a greater proportion of crystalline regions within the material, while a lower CI suggests a higher proportion of non-crystalline or amorphous regions. All the data is shown in Figs. S12–S13.
Data availability
Source data are provided with this paper.
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Acknowledgements
Authors acknowledge Prof. K.T. Valsaraj for useful discussions, Ms. Titilayo D. Oluwole and Prof. J. Lawrence for providing liquid N2 for the experiments, and Prof. K. M. Dooley for assistance with gas adsorption measurements. P.J.B and B.B. thank LSU’s Nano Fabrication Facility (NFF) for atomic force microscopy measurements shown in Figure 2a-c and 2e. P.J.B. and B.B. acknowledge the Division of Chemistry at the National Science Foundation (MPS-2032497) for partial financial support. BB also acknowledges the financial support by LSU via Anding Endowed Professorship. The 3D atomic force microscopy measurements and data analysis were supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Materials Science and Engineering, Synthesis, Biomolecular Materials Program at Pacific Northwest National Laboratory (PNNL), FWP 81642. PNNL is a multiprogram national laboratory operated for DOE by Battelle under contract no. DE-AC05-76RL01830.
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B.B. and P.J.B. conceived the project and planned the experiments. P.J.B. performed the experiments under the guidance of B.B. B.B. acquired the funding for the project. P.J.B. performed experimental data analysis. E.N. and M.Z. performed the 3D AFM experiments, the corresponding data analysis and interpretation of the force curves and hydration structures. B.B. and P.J.B. wrote the manuscript. All authors read and commented on the manuscript.
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Brahana, P., Zhang, M., Nakouzi, E. et al. Weathering influences the ice nucleation activity of microplastics. Nat Commun 15, 9579 (2024). https://doi.org/10.1038/s41467-024-53987-8
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DOI: https://doi.org/10.1038/s41467-024-53987-8
