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Accurate spectral identification of weathered plastics and analyses that provide insight into environmental degradation and age are desirable for source tracking and understanding hazards. The objectives of this study were to (1) evaluate the kinetics of spectral changes for lab-weathered polymers and compare to spectra from environmental microplastics (MPs), and (2) assess the accuracy of spectral databases in identifying weathered polymers. For objective 1, polyethylene (PE) and polypropylene (PP) fragments were exposed to simulated solar radiation in water for 90 days. FTIR spectra were collected periodically and degradation was quantified using carbonyl and hydroxyl bond indices. Significant linear increases in carbonyl indices for PP, but not PE, were observed as a function of exposure time. Spectra (via principal component analysis) and bond indices from lab-weathered polymers were then compared to environmental MPs collected from urban stormwater and the Delaware Bay estuary. Estuarine PP carbonyl and hydroxyl indices varied as a function of spectral collection mode (i.e., ATR vs. transmission) and by sampling site, potentially indicating the bond indices provide insight into sources/fate/transport of PP and are worthy of further study. In contrast, no significant differences were observed for PP in stormwater samples, possibly due to the close proximity of collection locations. PE exhibited non-linear trends in bond indices in the laboratory study and showed no significant association with sampling location in environmental samples, suggesting these indices may be less useful for PE degradation analysis. For objective 2, 14 different polymers, eight of which were polymer blends, were exposed to simulated solar radiation for up to 90 days, in dry and wet conditions. FTIR spectra were collected periodically and analyzed with two spectral identification software. OpenSpecy achieved an 88 % true positive rate compared to siMPle's 57 % at a 70 % hit quality threshold. Expanding reference libraries, to include weathered polymers and polymer blends, could improve spectral identification accuracy, and manual interpretation of FTIR spectra is recommended for low-confidence matches. 

Stormwater runoff is a pathway of entry for microplastics (MPs, plastics <5 mm) into aquatic ecosystems. The objectives of this study were to determine MP size, morphology, chemical composition, and loading across urban storm events. Particles were extracted from stormwater samples collected at outfall locations using wet peroxide oxidation and cellulose digestion followed by analysis via attenuated total reflectance (ATR) FTIR. Concentrations observed were 0.99 ± 1.10 MP/L for 500–1000 μm and 0.41 ± 0.30 MP/L for the 1000–5000 μm size ranges. Seventeen different polymer types were observed. MP particle sizes measured using a FTIR-microscope camera indicated non-target size particles based on sieve-size classification, highlighting a potential source of error in studies reporting concentration by size class. A maximum MP load of 38.3 MP/m2 of upstream catchment was calculated. MP loadings had moderate correlations with both rainfall accumulation and intensity (Kendall τ = 0.54 and 0.42, respectively, both p ≤ 0.005). First flush (i.e. rapid wash-off of pollutants from watershed surfaces during rainfall early stages) was not always observed, and antecedent dry days were not correlated with MP abundance, likely due to the short dry periods between sampling events. Overall, the results presented provide data for risk assessment and mitigation strategies.