
==== Front
Environ Sci Technol
Environ Sci Technol
es
esthag
Environmental Science & Technology
0013-936X
1520-5851
American Chemical Society

39172764
10.1021/acs.est.4c02639
Article
Microplastic Particles Contain Ice Nucleation Sites That Can Be Inhibited by Atmospheric Aging
https://orcid.org/0000-0001-5971-2502
Seifried Teresa M. †
https://orcid.org/0000-0001-6183-6889
Nikkho Sepehr †
https://orcid.org/0009-0009-3707-9385
Morales Murillo Aurelio ‡
https://orcid.org/0000-0003-4222-1701
Andrew Lucas J. †
https://orcid.org/0000-0001-5264-2644
Grant Edward R. †
https://orcid.org/0000-0002-5621-2323
Bertram Allan K. *†
† Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
‡ Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada
* Email: bertram@chem.ubc.ca.
22 08 2024
03 09 2024
58 35 1571115721
14 03 2024
01 08 2024
31 07 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

Recent research has shown that microplastics are widespread in the atmosphere. However, we know little about their ability to nucleate ice and their impact on ice formation in clouds. Ice nucleation by microplastics could also limit their long-range transport and global distribution. The present study explores the heterogeneous ice-nucleating ability of seven microplastic samples in immersion freezing mode. Two polypropylene samples and one polyethylene terephthalate sample froze heterogeneously with median freezing temperatures of −20.9, −23.2, and −21.9 °C, respectively. The number of ice nucleation sites per surface area, ns(T), ranged from 10–1 to 104 cm–2 in a temperature interval of −15 to −25 °C, which is comparable to that of volcanic ash and fungal spores. After exposure to ozone or a combination of UV light and ozone, simulating atmospheric aging, the ice nucleation activity decreased in some cases and remained unchanged in others. Our freezing data suggest that microplastics may promote ice formation in cloud droplets. In addition, based on a comparison of our freezing results and previous simulations using a global transport model, ice nucleation by microplastics will impact their long-range transport to faraway locations and global distribution.

Minimal research exists on microplastics’ impact on atmospheric ice nucleation. This study reports on the ability of microplastics to nucleate ice heterogeneously and how atmospheric aging influences this ability with implications for ice formation in clouds and the long-range transport of microplastics.

microplastics
ice nucleation
ozone
atmospheric oxidation
Natural Sciences and Engineering Research Council of Canada 10.13039/501100000038 RGPIN-2019-04242 Austrian Science Fund 10.13039/501100002428 J 4752-N Natural Sciences and Engineering Research Council of Canada 10.13039/501100000038 RGPIN-2023-05333 document-id-old-9es4c02639
document-id-new-14es4c02639
ccc-price
==== Body
pmcIntroduction

In the absence of ice-nucleating particles (INPs) in the atmosphere, cloud droplets freeze homogeneously at a temperature of approximately −35 °C.1,2 In the presence of INPs, cloud droplets can freeze at higher temperatures. After INPs initiate the formation of ice in cloud droplets, the resulting ice crystals can grow at the expense of liquid droplets since the saturation water vapor pressure is lower over ice than over water.3 In addition, ice crystals may further grow by collisions until they attain a size that causes them to fall out of the cloud, ultimately contributing to the formation of precipitation. Notably, a substantial fraction of global precipitation proceeds via these processes.4,5 INPs also influence the optical properties and lifetimes of clouds, with implications for the cloud albedo effect. Atmospheric ice nucleation thus influences the hydrological cycle and radiative budget, underlining the importance of gaining a comprehensive understanding of the properties, concentration, and sources of INPs in the atmosphere.

The ability of atmospheric particles to nucleate ice also influences their long-range transport and concentrations in the atmosphere. When aerosol particles trigger ice nucleation, followed by precipitation, the atmospheric concentration and long-range transport of the aerosols can be reduced. For example, a study by Haga et al.6 showed that ice nucleation on fungal spores, which have modest freezing efficiencies (freezing range −20 to −30 °C), leads to a decrease in the annual mean concentrations of fungal spores in remote regions and the upper troposphere.

Natural sources of INPs include mineral dust,7,8 volcanic ash,9−11 and biological particles (e.g., pollen12,13 and archaea14). Possible anthropogenic sources of INPs include soot15 and coal fly ash.16,17 With the advent of large-scale plastic production and consumption, microplastics have emerged as a potential new class of anthropogenic INPs.18,19

In 2015, Dris et al.20 first detected microplastic aerosols in the atmosphere. Since then, researchers have tried to identify the sources of these aerosols.21,22 Tire wear particles from roads are thought to be one of the most significant sources of plastics to the atmosphere, followed by microplastics ejected to the atmosphere with sea spray aerosol and agricultural soil dust.23 Additionally, microplastics originating from the breakdown of frequently used commercial products, such as polyester clothing, can contribute to the release of plastic fibers into the atmosphere.24,25

Recent studies showed the pivotal role of the atmosphere as a transport system for microplastics connecting natural and urban systems.26,27 As microplastics get transported throughout the atmosphere, aging is induced by exposure to ultraviolet (UV) radiation and oxidants such as ozone (O3). This aging process can oxidize the surface and bulk of microplastics, changing the hydrophobicity/hydrophilicity and chemical properties of the particles. Such changes will likely modify the ability of microplastics to nucleate ice.

Only a few studies have investigated the ice-nucleating ability of microplastics. Ganguly and Ariya18 synthesized three microplastics types (polypropylene, low-density polyethylene, and high-density polyethylene) in the laboratory and quantified their freezing properties using a droplet freezing technique. Teska et al.29 investigated the ice nucleation ability of fibers from casual apparel consisting of acrylic, polyester, nylon, and a combination of cotton, polyester, viscose, and elastane. Since microplastics can vary widely in terms of particle composition and morphology, additional studies are still needed to better understand the ice nucleation ability of microplastics. Studies on the ice nucleation ability of microplastics formed by the breakdown of frequently used products are especially necessary, as these materials may contribute significantly to the microplastic burden of the atmosphere. Furthermore, the effect of atmospheric aging by O3 or UV radiation on the ice nucleation properties of microplastics remains unstudied.

Here, we investigate the ice nucleation ability of seven different microplastic substances in the immersion freezing mode. We focus on polymers prevalent in the atmosphere:21 polypropylene (PP), polyethylene terephthalate (PET), and polyethylene (PE). For each polymer type, we included at least one commercial sample and one sample generated from the breakdown of a frequently used commercial product. We further tested if ice nucleation changes after exposure to O3 and a combination of UV light and O3.

Materials and Methods

Samples

Table 1 provides an overview of all samples used. We procured commercial samples PP-needlesc, PET-spheresc, and PE-spheresc from Nanochemazone (Leduc, AB, Canada) and PP-spheresc from Goonvean Fibers (Cullompton, U.K.). In addition to commercially sourced samples, we generated samples by the breakdown of each polymer type in the laboratory: PP-fibersb originating from a PP rope (The Home Depot, Everbilt Light-Duty Diamond Braid Rope, Model #301831), PE-flakesb from a PE cup (The Home Depot, LESSO Test Cap 3, Model #RLN132-030), and PET-fibersb originating from PET textile. To obtain PP-fibersb and PE-flakesb, we first cut the rope and the cup into small pieces (approximately 1 cm long), then dipped them into liquid nitrogen, broke them apart using a mortar and pestle, and then passed the material through a filter sieve. PET-fibersb were made by Cotton Incorporated (North Carolina), which produces standard fiber textiles for environmental assessments and toxicity studies. We cut the PET fabric into small pieces of approximately 1 cm2 and then ground them using a Wiley Mill 134.

Table 1 Detailed Information About the Microplastic Samplesa

polymer type	origin	shape	size	sample-ID	
polypropylene (PP)	Nanochemazone (CAS: 9003-07-0)	needles	SD < 30 μm	PP-needlesc	
LD < 150 μm	
Goonvean Fibres Ltd (HM20/70P)	spherical	d ≈ 60 μm	PP-spheresc	
Rope (The Home Depot, Inc.)	fibers	SD ≈ 35 μm	PP-fibersb	
LD ≈ 800 μm	
polyethylene terephthalate (PET)	Nanochemazone (CAS: 25038-59-9)	spherical	d ≈ 80 μm	PET-spheresc	
Textile (Cotton Inc.)	fibers	SD ≈ 20 μm	PET-fibersb	
LD ≈ 300 μm	
polyethylene (PE)	Nanochemazone (CAS: 9002-88-4)	spherical	d ≈ 55 μm	PE-spheresc	
Cup (The Home Depot, Inc.)	flakes	d ≈ 250 μm	PE-flakesb	
a The superscript c in the sample-ID indicates samples of commercial origin; b denotes samples obtained from the breakdown of larger plastic pieces. For nonspherical particles, SD refers to the short dimension and LD to the long dimension. For spherical and flake particles, d refers to diameter.

Sample Characterization (SEM, FTIR)

We used scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy to determine the morphology and chemistry of each microplastic sample.

SEM imaging was performed using a scanning electron microscope SU3500 (Hitachi) with a secondary electron detector. All images, unless noted, were collected using a low 1.5 kV acceleration voltage to ensure real surface imaging at high and low magnifications to characterize morphology and size, respectively. More details on the SEM methodology are provided in the Supporting Information.

IR spectra were recorded at room temperature on a PerkinElmer FTIR spectrometer equipped with an ATR accessory (ZeSe crystal). For each measurement, 16 scans were accumulated at a resolution of 4 cm–1.

Figure 1 shows SEM images of the unaged samples and the corresponding chemical structure; Figure S1 summarizes the FTIR spectra of the unaged samples, and Table S1 lists the corresponding FTIR peak assignments.

Figure 1 Chemical structures of various polymers tested for ice nucleation activity: Polypropylene (PP), polyethylene terephthalate (PET), and polyethylene (PE). Corresponding scanning microscopy images of the unaged microplastic samples used in the study: (a) PP-needlesc, (b) PP-spheresc, (c) PP-fibersb, (d) PET-spheresc, (e), PET-fibersb, (f) PE-spheresc, and (g) PE-flakesb.

Droplet Freezing Experiments

We assessed the ice nucleation activity of the polymers using the droplet freezing technique in the immersion freezing mode.30,31 First, we placed 20 mg of each polymer sample into a clean glass vial, followed by the addition of 10 mL of ultrapure water (Nuclease-Free Water for Molecular Biology; Sigma-Aldrich, Burlington, MA) to create a stock suspension. Next, we sonicated the samples for 5 min to disperse the microparticles in the ultrapure water. For ice nucleation measurements, we first cleaned three siliconized glass slides (18 mm, HR3-239; Hampton Research, Aliso Viejo, CA) with Milli-Q water (specific resistivity ≥17.5 MΩ/cm, 25 °C; produced with Millipore SAS SIMSV001, Merck Millipore, Burlington, MA) and then dried them with purified air before placing them on a cooling stage (Grant Asymptote EF600 freezer; Grant Instruments (Cambridge) Ltd., Durham, U.K.). As previously reported,32 the uncertainty of the temperature measurements with this droplet freezing technique is ±0.25 °C. For each ice nucleation measurement, we pipetted approximately 60 droplets with a volume of 1 μL onto the glass slides. A chamber sealed the cooling stage, and a nitrogen flow of 0.2 L/min prevented condensation during cooling. The nitrogen flow exerts no influence on the freezing temperature.31 The cold stage gradually cooled from 20 to −30 °C at a rate of 3 °C/min. A camera located on top of the chamber allowed for the real-time monitoring of the freezing process. A schematic image of the droplet freezing technique used in this study is shown in Figure S2. Employing a MATLAB script,33 we determined the freezing temperature of each droplet from the recorded video, which we then used to calculate the fraction of frozen droplets (fice(T), eq 1):1

where Nf(T) represents the number of frozen droplets in an experiment at temperature T and N0 is the total number of droplets in an experiment.

To normalize the freezing data to the mass or surface area of the microplastics and facilitate comparison with existing literature, we calculated the number of nucleation sites per unit mass, nm(T) (eq 2), and per surface area, ns(T) (eq 3):2

3

where Vd depicts the volume of an individual droplet, gives the reciprocal of the mass concentration of the sample suspension, and s represents the surface area of the particles.

To obtain the specific surface area of the unaged particles, we analyzed the SEM images and utilized ImageJ for all samples, except for PP-needlesc. By using the ImageJ software, we measured the diameter (and in the case of fibers also the length) of at least 100 particles per sample and calculated the corresponding surface area. For these calculations, we approximated particles as spheres and fibers as cylinders. For PP-needlesc, characterized by dense, hay-like features (see Figure 1a), we employed N2 sorption measurements on an ASAP 2020 gas sorption instrument (Micromeritics) to obtain the Brunauer–Emmett–Teller (BET) specific surface area,34 since obtaining identifiable, individual particles from SEM images was challenging. Before BET analysis, the sample was degassed for 9 h at 90 °C. We collected N2 sorption data at 77 K in the relative pressure range of 0–1 (saturation pressure 1 atm) and calculated the BET surface area using the BETSI software package.35

Oxidation Experiments

Microplastic samples underwent exposure to O3 in a rotating-wall flow-tube reactor (see Supplement Information, Figure S3).36 A few milligrams of each microplastic sample were placed within a glass tube that was inserted inside the flow-tube reactor. The glass tube was then rotated at a fixed rate with a motor to ensure constant mixing. An ozone generator (Jelight, model: 600) produced O3 outside the flow cell. The output from the ozone generator (2 L/min) flowed through the flow-tube reactor containing the microplastic samples. At the outlet, an O3 monitor (Thermo Scientific, 49i) continuously monitored the O3 concentration. We set the exposure duration for the microplastic samples at 2 h and used an O3 concentration of 2.8 ppmv. Background concentrations of O3 in the planetary boundary layer (PBL) are around 30 ppbv.37 Assuming that the aging of microplastic surfaces is proportional to both concentration and time, the exposures in our experiment correspond to 7.7 days at 30 ppbv.

In addition, we performed aging experiments using a commercial UV-ozone cleaning device (ProCleaner, Bioforce Nanosciences). We placed a thin layer of each microplastic sample onto the sample stage within the oxidation chamber. The cleaning device generates UVC light (185 and 254 nm) using a mercury lamp. Figure S4 in the Supporting Information depicts the irradiance spectrum of the mercury lamp used in the chamber. Also, 185 nm light generates O atoms and O3 by reactions (1a) and (2a):381a

2a

In addition, 254 nm light generates hydroxyl radicals (·OH) in the presence of water vapor by reactions (3a) and (4a):38,393a

4a

For logistical reasons, we exposed the microplastics to these aging conditions for 2 h. To ensure uniform treatment of light and O3 throughout the entire sample, we opened the chamber approximately every 15 min and thoroughly mixed the samples manually. O3 concentrations generated in these experiments reached 5.9 ppmv. The levels of exposure to O3 in these experiments correspond to 16.3 days at 30 ppbv (background PBL concentrations). The exposure to 185 and 254 nm light in these experiments are upper limits to exposure in the troposphere, since most of these wavelengths are filtered out by the ozone layer in the stratosphere. The ·OH concentrations in these experiments were not measured. However, using the same wavelengths and a similar configuration, ·OH concentrations of 2 or 3 orders of magnitude higher than those observed in the atmosphere can be produced.40 Assuming ·OH concentrations 2 orders of magnitude higher than ambient, the ·OH exposures correspond to roughly 200 h or 8.3 days under ambient conditions. Until additional data is available, these aging experiments with UVC and O3 should be considered as upper limits to aging in the troposphere. Additional studies are needed with a combination of UV and O3 exposures, typical of the troposphere.

Following both oxidation experiments, we transferred 20 mg from each treated microplastics sample into a sterile glass tube, added 10 mL of ultrapure water, sonicated for a duration of 5 min, and then measured ice nucleation activity (triplicates at minimum).

Results and Discussion

Freezing Results for Unaged Microplastics

Freezing curves in Figure 2 reveal that three suspensions of microplastic samples (PP-needlesc, PP-fibersb, and PET-fibersb) froze at temperatures above the water blank, indicating a heterogeneous freezing process. The T50 (i.e., the temperature at which 50% of the droplets were frozen) values for these three suspensions were −20.9 ± 0.4 °C for PP-needlesc, −23.2 ± 0.2 °C for PP-fibersb, and −21.9 ± 0.5 °C for PET-fibersb, compared to −25.8 ± 1.0 °C for the water blank. Table 2 summarizes all T50-values including 83% confidence intervals. Two data sets are statistically distinct with a 95% confidence level if their respective 83% confidence bands do not overlap.41

Figure 2 Fraction of frozen droplets in dependence of temperature (fice(T)) for seven microplastic suspensions (2.0 × 10–2 wt %) and the water blank. (a) Polypropylene (PP) needlesc, fibersb and spheresc, (b) polyethylene terephthalate (PET) spheresc and fibersb, and (c) polyethylene (PE) spheresc and flakesb. In all plots, the water blank is depicted in gray. The error bars of the samples and blank represent the 83% confidence interval. Two data sets are statistically distinct with a 95% confidence level if their respective 83% confidence bands do not overlap.41 Experiments were performed in triplicate at a minimum.

Table 2 T50 Values of Microplastic Suspension Samples and Water Blank with 83% Confidence Intervalsa

 	unaged	after O3	after UVC + O3	
sample	T50 (°C)	CI83% (°C)	T50 (°C)	CI83% (°C)	T50 (°C)	CI83% (°C)	
PP-needlesc	–20.9	0.4	–21.7	0.1	–22.8	0.2	
PP-spheresc	–25.2	0.5	–25.6	0.3	–24.8	0.6	
PP-fibersb	–23.2	0.2	–23.5	0.1	–23.9	0.7	
PET-spheresc	–25.5	1.2	–25.7	0.4	–24.5	0.3	
PET-fibersb	–21.9	0.5	–22.6	0.2	–24.8	0.9	
PE-spheresc	–24.8	0.6	–24.9	1.0	–24.7	0.3	
PE-flakesb	–24.9	0.6	–24.3	0.5	–25.1	0.5	
blank	–25.8	1.0	 	 	 	 	
a Two data sets are statistically distinct with a 95% confidence level if their respective 83% confidence bands do not overlap.41

The freezing curves of PP-spheresc, PET-spheresc, PE-spheresc, and PE-flakesb fall within the confidence interval of the water blank freezing curve and are therefore not distinguishable from the background. However, the freezing curves of these plastic samples deviate from the mean of the blank at fraction frozen values close to 1, which could indicate that some heterogeneous activity may exist. Given that freezing of the water background in μL-sized droplet freezing assays, as used in this study, is typically around −25 °C,31 evaluating particle-induced freezing below this temperature would require an alternative assay using droplets in the nL range, where background freezing occurs around −35 °C.42

To normalize the freezing data to the mass and surface area and compare our data with the literature, we determined the number of nucleation sites per unit mass, nm(T), and surface area, ns(T) (Figure 3). The nm(T) values of the different ice-active microplastics samples agree within about 1 order of magnitude and increase roughly exponentially as temperature decreases (Figure 3a). At −18 °C, the values range from 7.2 × 104 to 1.5 × 105 g–1, and at −22 °C, the values range from 9.0 × 105 to 1.3 × 107 g–1. The following order was observed at a freezing temperature of −22 °C: PP-needlesc > PET-fibersb > PP-fibersb. The ns(T) values of all ice-active microplastics agree within about 2 orders of magnitude and increase roughly exponentially with decreasing temperature (Figure 3b). At −18 °C, the ns(T) values range from 4.5 × 10–1 to 8.7 × 101 cm–2, and at −22 °C, the values range from 8.0 × 101 to 2.1 × 103 cm–2. In general, the following order was observed: PET-fibersb ≅ PP-fibersb > PP-needlesc.

Figure 3 Number of ice nucleation sites per unit (a) mass and (b) surface of polypropylene (PP) needlesc, fibersb, and polyethylene terephthalate (PET) fibersb. Error bars denote the 83% confidence intervals of nm(T) and ns(T) at temperature T calculated using Student’s t-distribution. Two data sets are statistically distinct with a 95% confidence level if their respective 83% confidence bands do not overlap.41 Literature data of nm(T) values from PP nanoparticles are shown from Ganguly and Ariya18 in black (dash-dot-dot). Literature values of ns(T) parametrizations are shown for comparison: K-feldspar from Harrison et al.43 (orange dash-dot-dot), natural dust from Niemand et al.44 (black dots), volcanic ash from Murray et al.45 (turquoise dash), and NX-Illite from Hiranuma et al.46 (purple dash-dot). ns(T) values of PP nanoparticles (PPN) were calculated from data obtained from Ganguly and Ariya18 (black dash-dot-dot). The gray area depicts ns(T) values from fungal spores by Haga et al.47 and Haga et al.6

Comparison of the Freezing Results and Physicochemical Properties

Several physical and chemical properties can influence the freezing behaviour. These include polymer type, crystallinity, morphology, the presence of oxygen-containing functional groups, and mechanical grinding. Below, we discuss each of these properties in detail.

Polymer type alone is insufficient to predict the freezing results. For example, PP-fibersb had higher ns(T) values than PP-needlesc. On the other hand, crystallinity may be important, as previous studies have shown that porous organic polymers of higher crystallinity exhibit greater ice nucleation efficiency.28 PP can exist in different structural forms (atactic, syndiotactic, and isotactic), which affect crystallinity. Thus, differences in crystallinity could explain variations in the ice nucleation behavior among PP samples. The morphology of the microplastics also appears to be important. Both fiber samples (PP-fibersb and PET-fibersb) showed the highest ns(T) values, suggesting morphology may have a role in ice nucleation. However, PP-needlesc also nucleated ice, indicating that morphology alone is not the sole predictor.

Oxygen-containing functional groups on microplastic surfaces may also promote ice nucleation through hydrogen bonding. FTIR analysis (Figure S1) revealed that PP-needlesc had such groups (i.e., O–H and O–O–H stretching vibrations between 3300 and 3100 cm–1, C=O stretching between 1550 and 1700 cm–1), potentially explaining their higher nucleation temperatures. However, a clear relationship between the oxygen content and ice nucleation ability was not observed across all samples. For instance, PET-spheresc had high oxygen content (see chemical structure in Figure 1 and FTIR spectrum in Figure S1) but did not induce ice nucleation.

Mechanical grinding can also influence ice nucleation by altering the surface functional groups. Two of the three ice-active microplastics were from commercial product breakdown (PP-fibersb and PET-fibersb), suggesting breakdown and grinding may have generated nucleation sites in these samples. However, PE-flakesb did not show ice activity, indicating that grinding is not the sole factor. In summary, the freezing properties of the microplastics studied appear to be influenced by a combination of factors, possibly including crystallinity, morphology, oxygen-containing functional groups, and mechanical grinding.

Comparison of the Freezing Results for Unaged Microplastics with Literature Data

Ganguly and Ariya18 synthesized three microplastics types (PP, low-density PE, and high-density PE) in the laboratory and quantified their freezing properties using a droplet freezing technique. In Figure 3a, we compared our freezing results with their PP microplastics. Our observed nm(T) data are lower than the literature values by 1–4 orders of magnitude for freezing temperatures between −16 and −22 °C. In addition, we calculated ns(T) values by computing them from the nm(T) values reported by Ganguly and Ariya18 using an average diameter of 193 nm for their microplastics, which corresponds with their own measurements. Using this approach, the calculated ns(T) values based on the data from Ganguly and Ariya18 closely match our microplastics data (Figure 3b). This suggests that the difference in nm(T) values between our study and Ganguly and Ariya18 can be attributed to differences in the surface area used in the experiments.

Teska et al.29 showed that microplastic fibers from clothing textiles are effective ice nuclei with ice nucleation onset temperatures between −6 and −9 °C and nm(T) values between 0.2 and 3 mg–1. Their measurements also showed that ice nucleation was biological in origin, which may explain the higher onset temperatures compared to the current study.

To help put the freezing ability of microplastics into atmospheric context, the number of ice nucleation sites per surface, ns(T), of the PP-needlesc, PP-fibersb, and PET-fibersb were compared to those for volcanic ash,45 fungal spores,6,47 NX-Illite,46 K-feldspar,43 and a natural mineral dust44 parametrization (see Figure 3b). The ice nucleation properties of mineral dust and volcanic ash have been the focus of many studies, and both represent important types of INPs in the atmosphere. NX-Illite consists of Illite (main component with 74 wt %48,49), kaolinite, quartz, calcite, and feldspars and has been used as a proxy for mineral dust INPs.46 K-feldspar is one of the most effective types of mineral dust INPs and has been incorporated into models of atmospheric INPs.50 Fungal spores have been found in atmospheric aerosol and cloudwater samples and are moderately effective INPs.51 In addition, fungal spores and microplastics can have similar sizes, and the impact of ice nucleation by fungal spores on their long-range transport and atmospheric concentrations and distributions has been investigated.6Figure 3b shows that ns(T) values for PP-needlesc, PP-fibersb, and PET-fibersb are comparable to those for volcanic ash, NX-Illite, and fungal spores. On the other hand, ns(T) values for the same microplastics are approximately 2–3 orders of magnitude lower than those for K-feldspar and a parametrization for natural mineral dust. K-feldspar and natural mineral dust are considered highly effective INPs.

The relative importance of different INP types in the atmosphere will depend on both the ns(T) values (e.g., Figure 3b) and their atmospheric concentrations. For most conditions in the atmosphere, the concentrations of microplastics are likely far less than the concentrations of mineral dust particles (see Aeschlimann et al.19 and references therein). Regions where microplastic INPs will be important will be limited to regions where microplastic concentrations are very high or when concentrations of mineral dust particles are low, such as over the Southern Ocean.19 The concentrations of microplastics in the atmosphere may also increase in the future, as their atmospheric sources increase. The ns(T) values included here provide information for calculating the relative importance of microplastic INPs in the atmosphere, now and in the future.

Possible Effects of Unaged Microplastics on Clouds and Long-Range Transport

A recent study by González-Pleiter et al.52 showed that microplastics, specifically polymer fragments and artificial fibers, can accumulate to concentrations reaching up to 0.032 L–1 in the planetary boundary layer (PBL) above urban areas. The identified fibers exhibited an average length and width of 662 and 25.4 μm, respectively, similar to PP-fibersb in our study. To provide an initial assessment of the potential impact of microplastics on clouds, we combined the upper limit of microplastics concentrations reported by González-Pleiter et al.52 (0.032 L–1) and our freezing results for the PP-fibersb.53 First, we calculated the number of ice nucleation sites per fiber at a freezing temperature of −20 °C using the freezing data for PP-fibersb and the following equation:4

Fibers/droplet was determined based on the average volume of the individual PP-fibersb and a density of 0.90 g cm–3.54 The calculation using eq 4 gave 0.15 ice nucleation sites per fiber at −20 °C. Next, we used this value along with the microplastic concentrations reported by González-Pleiter et al.52 (0.032 L–1) to estimate an INP concentration of 0.005 L–1 of air at a freezing temperature of −20 °C. Previous studies have shown that an INP concentration of 0.01 L–1 of air at −20 °C can be sufficient to influence cloud glaciation.55 Hence, we conclude that in some situations, the concentrations of microplastics may be high enough to cause cloud glaciation. However, this analysis did not consider the presence of mineral dust INPs or biological INPs in the atmosphere. The concentrations of mineral dust and biological particles are likely higher than the concentrations of microplastics for most atmospheric conditions, and mineral dust and biological INPs can cause ice nucleation at temperatures warmer than those of microplastic INPs. These combined effects will likely limit the effect of microplastic INPs on cloud glaciation for most atmospheric conditions. Nevertheless, additional studies are needed to consider these competing effects.

If the microplastics nucleate ice (as shown here for certain types of microplastics), then ice nucleation followed by precipitation can also affect their long-range transport to faraway regions and their atmospheric concentrations and distributions. Recent simulations using a global chemistry-climate transport model showed that ice nucleation by fungal spores followed by precipitation significantly impacted their long-range transport and concentrations in remote regions and the upper troposphere.6 The size of fungal spores ranges from 2 to 50 μm.56 Since some microplastics have similar ns(T) values to fungal spores (Figure 3b), and may have similar sizes, ice nucleation by microplastics may influence their long-range transport and concentrations in remote regions and the upper troposphere under some atmospheric conditions, such as when mineral dust and biological INP concentrations are low. The freezing data included here provide the necessary information for including this phenomenon into atmospheric models.

Freezing Results for Aged Microplastics

When suspended in the air, microplastics undergo atmospheric aging due to exposure to oxidants (e.g., O3) and UV light, which can change their chemical and physical properties, including ice-nucleating properties. To investigate the effect of atmospheric aging on the ice nucleation activity of microplastics, we exposed the microplastics to O3 using a flow-tube reactor and measured the change in the ice activity. The number of days of exposure to O3 in our experiments corresponded to 16.3 days at 30 ppbv (background PBL conditions). We also conducted UVC + O3 experiments, which represent an upper limit to photooxidation in the troposphere. Figure 4 shows the freezing curves in the form of boxplots before and after the aging experiments, and Table 2 summarizes all T50 values, including 83% confidence intervals before and after oxidation experiments.

Figure 4 Freezing curves (fice(T)), represented by boxplots, of the seven microplastic samples (polypropylene (PP) needlesc, spheresc, and fibersb; polyethylene terephthalate (PET) spheresc and fibersb; polyethylene (PE) spheresc and flakesb) before and after oxidation experiments. Within each sample type, the first box represents the unaged sample (gray box), followed by postozone (O3) exposure (depicted in purple), and post UVC + O3 (i.e., photooxidation) exposure (highlighted in red). Each box contains cumulative data from at least three replicates. The upper and lower limits of the box represent the interquartile range (25th and 75th percentiles), the median (T50) is displayed as a horizontal line within the box, and the black circle signifies the mean value. Whiskers extend to the 10th and 90th percentiles. The light gray boxplot to the right represents the water blank. The dashed line throughout the graph indicates theT50 value of the blank, the dark gray area denotes the corresponding standard error of T50, and the light gray area illustrates the interquartile range.

Overall, exposure to O3 or UVC + O3 (referred to as photooxidation hereafter) either caused no change (i.e., the change was less than the uncertainties of the measurements) or a decrease in ice nucleation activity. For the non-ice nucleation active plastics (PP-spheresc, PET-spheresc, PE-spheresc, PE-flakesb), no statistically significant change in ice nucleation activity was observed after exposure to O3 or photooxidation. A similar result was observed for the ice-active sample PP-fibersb (i.e., no change was observed). On the other hand, for the ice nucleation active samples PP-needlesc and PET-fibersb, nucleation temperatures decreased slightly after exposure to O3 and more strongly after photooxidation.

We conclude that atmospheric aging due to O3 exposure and photooxidation can lead to a reduction in the ice nucleation ability of some microplastics but not all. These findings align with previous studies demonstrating that exposure of some atmospherically relevant organic particles and materials, such as lignin57 and pollen,58 to O3 or other oxidants leads to either no change or a decrease in ice nucleation ability. Furthermore, some studies also showed that soot particles aged with O3 also did not show enhanced ice nucleation ability.59,60 However, Gorbunov et al.61 and Gao and Kanji62 reported contradictory results. Additionally, a study on inorganic mineral dusts (Arizona test dust and kaolinite) has revealed an enhancement in ice nucleation ability after exposure to O3.63

We employed FTIR and SEM measurements to track chemical and morphological alterations due to exposure to O3 and photooxidation. FTIR spectra with peak assignments and SEM images for all samples before and after oxidation experiments are provided in the Supporting Information (Figures S5–S7 and Tables S2 and S3 for FTIR; Figures S8 and S10 for SEM). Table 3 summarizes the observed changes in the chemical and physical properties of the ice-active microplastics (PP-fibersb, PP-needlesc, and PET-fibersb) due to exposure to O3 and photooxidation. The change in ice nucleation activity with O3 exposure and photooxidation are also included in Table 3 for comparison purposes.

Table 3 Summary of the Effect of O3 Exposure and Photooxidation on Ice Nucleation Activity, Chemistry, and Morphology of Ice-Active Microplastic Samplesa

 	O3 exposure	photooxidation (UVC + O3)	
sample	ΔIN activity	Δchemistry	Δmorphology	ΔIN activity	Δchemistry	Δmorphology	
PP-needlesc	decrease	no detectable change	no detectable change	decrease	surface and/or bulk oxidation	no detectable change	
PP-fibersb	no detectable change	no detectable change	no detectable change	no detectable change	surface and/or bulk oxidation	formation of cracks	
PET-fibersb	decrease	no detectable change	no detectable change	decrease	no detectable change	no detectable change	
a Text highlighted in bold indicates that a significant change was observed.

O3 exposure did not cause a detectable change in the chemical or morphological properties of the ice-active microplastics (PP-fibersb, PP-needlesc, and PET-fibersb) (Table 3, column 3 and column 4). On the other hand, O3 exposure caused a decrease in ice nucleation ability of the PP-needlesc and PET-fibersb (Table 3, column 2). We conclude that the exposure of O3 must have caused some chemical and/or physical changes at the surface of the PP-needlesc and PET-fibersb that were not detected with FTIR or SEM measurements. Future studies that use surface-focused analytical techniques, such as X-ray photoelectron spectroscopy, for example, are needed to understand the relationship between O3 exposure and ice nucleation ability of microplastics.

Photooxidation caused an increase in oxygen-containing functional groups for PP-needlesc and PP-fibersb, but not PET-fibersb (Table 3, column 6). Postphotooxidation FTIR data revealed a new band at 1720 cm–1, corresponding to C=O stretching vibrations in PP-needlesc. Additionally, the FTIR data for PP-fibersb showed C=O stretching vibrations between 1500 and 1720 cm–1, O–H and O–O–H stretching vibrations between 3300 and 3100 cm–1, and peaks in the 1300 to 650 cm–1 region, attributed to C–O–C or O–O stretches.64 Photooxidation caused a decrease in ice nucleation activity for PP-needlesc and PET-fibersb, but not PP-fibersb (Table 3, column 2). We conclude that the decrease in ice nucleation ability caused by photooxidation cannot be explained just by the increase in oxygen-containing functional groups observed with FTIR spectroscopy. Photooxidation also caused a change in morphology (formation of cracks) for PP-fibersb, but not PP-needlesc and PET-fibersb (Table 3, column 7). We conclude that the decrease in ice nucleation ability caused by photooxidation cannot be explained by just a change in morphology observed with SEM as there was no observed relationship between the change in ice nucleation activity and the change in morphology. These results also underscore the need for surface-sensitive analytical techniques to explain the relationship between photooxidation and ice nucleation ability of microplastics. Related, Teska et al.29 showed that fibers from clothing textiles (including PET-fibers) can carry biological ice-nucleating material. If our samples also possess such material, then photooxidation may have degraded this film, potentially leading to the observed decrease in nucleation temperature.

The influence of microplastics on atmospheric processes, notably their potential influence on ice nucleation in clouds, remains a subject of uncertainty. Our results demonstrate that certain microplastic samples, specifically PP-needlesc, PP-fibersb, and PET-fibersb heterogeneously nucleate ice in the immersion freezing mode at temperatures between −15 and −25 °C and have ns(T) values similar to fungal spores and volcanic ash. The ability of microplastics to freeze at these relatively warm temperatures suggests that they can be removed from the atmosphere by ice nucleation followed by precipitation, which may modify their long-range transport to faraway regions and their distributions throughout the atmosphere. Our results are consistent with field measurements by Kozjek et al.65 who observed fibrous microplastics inside hailstones.

Exposure to O3 and UVC + O3 led to a reduction in the nucleation temperature for two (PP-needlesc and PP-fibersb) out of the three ice-active microplastics.

Since the mid-20th century, our environment has been increasingly inundated with plastics. Considering the time lag from the release of plastics into the environment to their gradual transformation into microplastics, it is likely that the concentration of microplastics will continue to rise in the future and play a more significant role in ice formation in the atmosphere.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.4c02639.It contains additional experimental details and results: (i) Description of the method used to obtain SEM microscopy images; (ii) FTIR spectra of unaged microplastic samples; (iii) Schematic illustration of the cold stage; (iv) Schematic illustration of the oxidation flow-tube reactor; (v) Spectrum of the mercury lamp used for the photooxidation experiment; (vi) FTIR spectra of aged microplastic samples; (vii) SEM images of unaged and aged microplastic samples (PDF)

Supplementary Material

es4c02639_si_001.pdf

Author Contributions

T.M.S. prepared the manuscript with contributions from all authors. T.M.S. and A.K.B. designed the project. T.M.S. performed the ice nucleation experiments. T.M.S. and S.N. performed oxidation experiments. A.M.M. conducted SEM measurements. L.J.A. collected BET data. E.R.G. and A.K.B. supervised the project.

The authors acknowledge the support of the Natural Sciences and Engineering Research Council of Canada (RGPIN-2023-05333). T.M.S. and E.R.G. gratefully acknowledge support by a Discovery Grant from The Natural Sciences and Engineering Research Council of Canada (RGPIN-2019-04242). T.M.S. acknowledges funding by the Austrian Science Fund (FWF) under project number J 4752-N.

The authors declare no competing financial interest.

Acknowledgments

The authors thank Feng Jiang (UBC Vancouver, Department of Forestry) for providing the UV-ozone cleaner and Marcus A. Johns (UBC Vancouver, Department of Wood Science) for his support during the photooxidation experiments; Oceanwise Microfiber Partnership and Cotton Incorporated for providing PET-fibers; Lori-jon Waugh and Yuanji Sun (UBC Vancouver, Department of Earth, Ocean and Atmospheric Sciences) for their support in fabricating and sharing PP-fibers and PE-flakes; and Natasha Klasios (UBC Vancouver, Department of Zoology) for providing PP-spheres. SEM characterization was performed in the Centre for High-Throughput Phenogenomics at the University of British Columbia, a facility supported by the Canada Foundation for Innovation, British Columbia Knowledge Development Foundation, and the UBC Faculty of Dentistry. This manuscript is based on work that has been previously submitted as a preprint: Seifried T.M., Nikkho S., Morales Murillo A., Andrew L.J., Grant E.R., Bertram A.K. Microplastic particles contain ice nucleation sites that can be inhibited by atmospheric aging. ChemRxiv 2024; 10.26434/chemrxiv-2024-k0sgz-v2 (accessed 2024-07-29).

Abbreviations

BET Brunauer–Emmett–Teller

INPs ice-nucleating particles

O3 ozone

PBL planetary boundary layer

PE polyethylene

PET polyethylene terephthalate

PP polypropylene

SEM scanning electron microscopy

UV ultraviolet
==== Refs
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