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Pristine and Aged Microplastics Can Nucleate Ice through Immersion Freezing

Summary

Laboratory-prepared microplastics (LDPE, PP, PVC, PET) are tested as immersion freezing nuclei before and after simulated aging (UV, ozone, sulfuric acid, ammonium sulfate); ATR-FTIR tracks surface chemistry changes alongside ice nucleation activity.

Methods

Materials: Commercial polymers (PP isotactic, LDPE, PVC, PET with glass fiber) were cryo-milled and sieved into size classes (25-53, 53-75, 75-106, >106 micrometers); a combined <106 micrometer fraction was used for aging and most freezing assays. PET was density-separated in chloroform (sonication 20 min, settling >=48 h, repeated) to remove glass fibers. Aging: UV (broad metal-halide lamp, quartz lid; exposure converted to Harrisburg PA equivalent days), ozone (Poseidon 200, chamber exposure; ppm-h converted to equivalent days using 0.030 ppm surface ozone), pH 2 H2SO4 and matched (NH4)2SO4 (72 h, room temperature, rinse). Ice nucleation: immersion freezing chamber (Alstadt-type protocol cited), ~0.1 wt% LDPE/PP and ~1 wt% PVC/PET in UHPLC water, 2.0 microL droplets on siliconized slides, cooling -3 C/min, K-type thermocouple, LabView imaging 0.5 or 0.1 C steps, ImageJ contrast, MATLAB frozen fraction F(T) from at least 100 droplets total; active site density K(T) with background water subtraction; n_m(T) normalized per mass. ATR-FTIR (Nicolet 6700 Smart iTX) on dry powders; UV-vis on THF solutions checked for chromophores (300-800 nm).

Findings

All four polymers act as immersion ice nuclei. Aging generally reduces ice nucleation activity for LDPE, PP, and PET (median freezing temperatures shift colder or n_m drops depending on treatment), while PVC often shows increased activity after aging, attributed to cleaning chemical defects from stock material. ATR-FTIR: growth of 1650-1800 cm^-1 absorbance correlates with decreased ice activity in several cases; loss of an existing peak in that region correlates with increased activity. Particle size (<106 micrometer classes) shows little systematic trend of n_m with size for PVC, LDPE, and PP in the range studied, supporting use of the combined size class for aging comparisons. Ammonium sulfate on LDPE produces a large shift to warmer median freezing (~-19.5 C vs ~-23.7 C pristine for 0.1 wt% LDPE in one comparison). The authors argue that, at plausible atmospheric loadings, MP could be a non-negligible INP source.

Limitations

Despite beam blanking during heating, in situ microscopy still couples temperature with finite electron dose; aging protocols simplify atmospheric multiphase chemistry (acid, ozone, UV) relative to full environmental matrices. FTIR peak assignments for surface oxidation are interpretive; INP activity is measured on laboratory suspensions and droplets, so atmospheric relevance requires scaling arguments beyond this study’s scope.

Relevance to group

Experimental ice nucleation on polymers and aging chemistry—useful cross-reference for water–interface and material aging themes; not a ReaxFF simulation paper.

Citations and evidence anchors