Weathering influences the ice nucleation activity of microplastics

The study investigates whether atmospheric microplastics (MPs) can influence fundamental atmospheric microphysical processes, particularly water condensation and heterogeneous ice nucleation relevant to mixed-phase clouds. MPs are now detected in remote regions and can be atmospherically transported, but their effects on cloud processes remain unclear. Key questions include whether MPs act as ice-nucleating particles (INPs) and how atmospheric cycling and weathering (e.g., sunlight-induced photooxidation) alter their surface properties and thus their nucleation activity. The paper posits that MPs’ low density, long atmospheric residence time, and dynamic surface chemistry from environmental weathering could change their interactions with water, potentially shifting onset conditions for condensation and freezing.

Heterogeneous ice nucleation in the atmosphere often occurs via immersion freezing, with onset depending on INP properties such as porosity, chemistry, crystallography, hydroxylated surfaces, and surface defects. Minerals with lattice matches to ice, hydroxylated surfaces, and rough substrates are known INPs, yet the relative contribution of these properties is unresolved beyond empirical correlations. MPs exhibit broad variability in morphology, additives, and surface chemistry, complicating predictions. A recent report suggested low-density polyethylene nanoplastics can nucleate ice at relatively high temperatures, but the impacts of continuous atmospheric weathering on MPs’ surface properties and their heterogeneous ice nucleation activity are largely unknown. Existing literature also notes discrepancies on whether oxidation enhances or inhibits MPs’ ice nucleation activity, potentially due to differences in materials and aging protocols.

Model systems and materials: Spherical polyethylene (PE) microspheres (~70 µm, Cospheric; density ~1.0 g cm−3) dyed blue/magenta were used to decouple weathering-driven surface chemistry changes from size/shape effects. Additional microplastics (PE, polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET)) were produced by in-house milling of macroscopic sheets. Arizona Test Dust (ATD) served as a model mineral dust INP.

Accelerated weathering: MPs were exposed to a xenon arc lamp in a Xe-1 chamber (Q-Labs) with a 340 nm filter (primarily UVA), irradiance 0.35 W m−2, chamber temperature 63 °C, per ASTM D5071. MPs were floated as a monolayer at the air–water interface in glass containers; water was replenished to prevent drying. The protocol accelerates photooxidation by a factor of ~10–30 relative to the environment. Weathering times studied included 0, 2, 5, 10, 90, and 180 days (reported as chamber days).

Surface and bulk characterization: X-ray photoelectron spectroscopy (XPS; Al Kα, 1486.7 eV) assessed surface chemistry (O 1s, C 1s; Na peaks at extended aging). Atomic force microscopy (AFM, tapping mode) quantified nanoscale roughness and provided 3D topography; effective specific surface area was estimated from 5 µm × 5 µm scans and corroborated by N2 adsorption BET at −196 °C. Three-dimensional AFM force mapping (amplitude-modulated, high-speed Cypher VRS) in water resolved hydration structure normal to PE surfaces (3 × 3 × 5 nm3 volumes). ATR-FTIR probed chemical functional groups for milled MPs before/after 1-day weathering. X-ray diffraction (XRD, Bruker D8) measured crystallinity indices of bulk sheets used for milling.

Ice nucleation and water uptake experiments: MPs were immobilized on hydrophobic glass (HMDS vapor treatment). In a closed cell on a temperature-controlled stage (Linkam THMS600), relative humidity (RH) was ramped by decreasing temperature at 0.1 °C min−1 under constant water vapor pressure maintained by N2 flow. RH was computed from vapor pressure using the Magnus–Tetens relation; water uptake was recorded at first observed condensed droplet (>1 µm) on an MP. The experiment was terminated after condensation (RH thereafter compromised). Ice nucleation temperature TIN was defined as first observable ice (>1 µm) upon further cooling. Control runs without particles quantified condensation/freezing on the hydrophobic substrate.

Droplet freezing assays: Six droplets (5 µL each) of ultrapure water, with or without INPs (PE MPs or ATD, 10 mg mL−1), were deposited on hydrophobic glass, submerged in squalene oil to suppress extraneous condensation and WBF effects, and cooled at 1 °C min−1 on a Linkam THMS600 stage. Freezing was scored when droplets turned opaque/white. Fraction frozen (fice) vs temperature was measured over ≥5 repeats. The heterogeneous ice nucleation rate coefficient Jice(T) was computed as Jice = −ln(1 − fice)/(α m t), where α is INP specific surface area, m is INP mass per droplet, and t is liquid dwell time at each temperature.

Polymer-chemistry contrast experiments: Milled MPs of PE, PP, PS, PET were tested unweathered (D0) and after 1 day of weathering (D1) to isolate early-stage photooxidation effects without morphological changes. XPS and ATR-FTIR corroborated the extent of oxidation; XRD assessed crystallinity (order: PE > PP > PS > PET; unchanged after 1 day).

• MPs act as immersion-mode INPs under atmospherically relevant conditions. Weathering enhances their activity via surface oxidation and hydration structure formation. • Direct observation: Weathered PE MPs showed condensation at RH ≈ 50% and T = −11 °C; subsequent immersion freezing at ~−25 °C. On a particle-free hydrophobic substrate, condensation occurred at RH ≈ 95% and −19 °C, freezing at ~−32 °C. • Side-by-side tests of unweathered vs 10-day weathered PE MPs revealed ~40% lower RH for water uptake and ~5 °C warmer TIN for weathered MPs. MPs weathered 90 and 180 days further decreased RH for uptake and increased TIN, attributed to increased surface oxidation/roughness and adsorption of hygroscopic salts (NaCl) evidenced by Na 1s and KLL peaks in XPS. • 3D AFM in water detected emergent hydration layering near weathered PE surfaces, with oscillatory force gradient spacing of 0.3–0.4 nm after 5–10 days, indicating enhanced interfacial water structuring consistent with increased hydrophilicity and decreased contact angles. • Droplet freezing assays: Without INPs, droplets began freezing at ~−28 °C and were fully frozen by ~−36 °C (homogeneous regime). Addition of MPs shifted freezing to warmer temperatures; Jice increased with weathering time. MPs weathered 90–180 days exhibited ice nucleation activity comparable to mineral dust below −24 °C when normalized by surface area. • Polymer dependence: For unweathered milled MPs (PE, PP, PS, PET), RH for water uptake and TIN were similar across polymers and higher/colder than the substrate alone. After 1 day weathering, PP and PS exhibited lower RH for uptake and warmer TIN, while PE and PET showed no statistically significant change. XPS showed emergent O 1s features (carbonyl/oxygen incorporation) for PP and PS, minimal changes for PE and PET over 1 day; FTIR corroborated. Differences reflect polymer-specific photooxidation kinetics (stabilized radicals in PP/PS vs PE; PET’s slower early-stage changes). Crystallinity did not change after 1 day, indicating surface chemistry as the driver.

The results directly link environmentally driven changes in MP surface chemistry to their immersion freezing activity. Photooxidation increases hydrophilicity and organizes interfacial hydration layers, lowering barriers to water uptake and heterogeneous nucleation. These findings align with theoretical descriptors that favor surfaces allowing lateral water mobility over rigid lattice matching, suggesting oxidized polymer surfaces can be efficient INAs by stabilizing interfacial water while permitting configurational sampling. Although the current atmospheric abundance of MP fragments is much lower than mineral dust, their increasing presence and altered nucleation efficiencies under weathering imply potential impacts on cloud microphysics, precipitation initiation, and, critically, on MP atmospheric residence time and deposition patterns via enhanced water uptake and freezing. Discrepancies with studies reporting null or negative effects of oxidation likely arise from differing polymers, additives, and aging protocols. The study underscores that both chemical (oxidation, salt adsorption) and physical (roughness) transformations jointly modulate MP nucleation behavior.

This work establishes that microplastics, particularly after sunlight-induced weathering, can function as atmospheric INPs via immersion freezing. Weathering drives surface oxidation, enhances wettability, forms interfacial hydration layers, and can introduce hygroscopic salts and roughness, collectively lowering RH thresholds for condensation and increasing freezing temperatures. For sufficiently weathered MPs, nucleation activities can approach those of mineral dust below −24 °C when normalized by surface area. Polymer chemistry governs early-stage susceptibility to photooxidation and consequent nucleation shifts (PP, PS > PE, PET). These insights connect molecular-scale surface changes to macroscopic atmospheric behavior, with implications for both climate-relevant microphysics and MP transport/deposition. Future research should quantify real-atmosphere MP loading, aging histories (UV spectrum, oxidants), additive effects, mixed-phase cloud interactions, and coupling to cloud-resolving models to assess climatic significance.

• Atmospheric MP concentrations are currently much lower than mineral dust, limiting immediate climatic impact, though increasing trends warrant attention. • 3D AFM hydration mapping required flat PE sheets rather than curved MPs; sheet–particle differences in form factor, light exposure, and additives may lead to nonidentical weathering, so hydration-layer observations are qualitative proxies for MPs. • After initial condensation during RH ramps, RH in the cell is perturbed and cannot be accurately tracked further, constraining precise RH reporting post-onset. • Extended weathering (90–180 days) introduced Na/K signals indicating salt adsorption; hygroscopic salts likely contributed to lower RH for uptake and enhanced freezing, complicating isolation of pure photooxidation effects. • Accelerated weathering emphasized UVA and elevated temperature; real environmental spectra and temperatures vary, and polymer-specific UV sensitivities may lead to different kinetics. • Morphological roughening at long weathering times co-varies with chemistry, making it challenging to deconvolve their individual contributions.