## 3 Results and Discussion
### Identification of MP Particles in seawater Samples LUsing ATR-FTIR and Raman Spectroscopy  
Figure 5 shows the relative % contribution of each shape and polymer typology for larger (>1 mm-5 mm) MPs. Most of the MPs were fragments (30%) and films (30%), followed by foams (20%) and fibers (20%). ATR-FTIR analysis, in combination with micro-Raman spectroscopy, revealed the presence of six polymer types in water samples (Figures 6 and 7). More examples of representative examples of Raman spectra for MPs detected in seawater can be found in the Supplementary Materials. PE was the predominant polymer, with a frequency of 40% in all analyzed samples. PS was the second one together with EAA in terms of frequency with 15% and 25%, respectively, while EVA, ethylene oxide (PEO), and PP were detected sporadically in samples (5%, 5%, and 10%, respectively). In fact, despite the focus of this study was not a full evaluation of MPs in seawater, data on a number of polymer types and shapes of MPs in water samples obtained were comparable to those observed by other researchers [4; 6; 11; 22; 46] confirming the occurrence of the above MPs in the area of the Ionian Sea. Furthermore, results obtained from ATR-FTIR (Figure 7) revealed that the spectra of plastic fragments PP and PE often exhibit addition or increase the intensity of carbonyl absorption band (C=O) located around 1700 cm\({}^{-1}\), at 1027 cm\({}^{-1}\) (C=O) and 874 cm\({}^{-1}\) and 911 cm\({}^{-1}\) (C=C) which associated with weathering process of plastics. Absorption bands also present in polymers (EVA, EAA, and PEO) correspond to a functional group including hydroxyl or hydroperoxide, O-H (3400 cm\({}^{-1}\)), carbonyl, C-O (1595 cm\({}^{-1}\)), and ether, C-O-C (1045 cm\({}^{-1}\)). In addition to the identification of polymers, ATR-FTIR was also used to determine the carbonyl index (CI), a parameter to measure the degree of oxidation in commonly used polymers. In this study, the specified area under band (SAUB) technique was applied to the IR spectra. Accordingly, the CI of PE, PS, and PP MPs was calculated from the ratio of the integrated band absorbance of the carbonyl (C=O) peak from 1650 to 1850 cm\({}^{-1}\) to that of the methylene (CH\({}_{2}\)) scissoring peak from 1420 to 1500 cm\({}^{-1}\), as expressed with the following equation [72]:  
\[Carbonyl\ index=\frac{Area\ under\ the\ band\ from\ 1650\text{--}1850\text{ cm}^{-1}}{Area\ under\ the\ band\ from\ 1420\text{--}1500\text{ cm}^{-1}}\]  
The area under the band from 1420 to 1500 cm\({}^{-1}\) that is used to calculate CI includes the reference peaks for the common polymers: PE at 1471 cm\({}^{-1}\), PP at 1460 cm\({}^{-1}\), and PS at 1452 cm\({}^{-1}\) since all have C\({}_{2}\)H\({}_{2}\) as the main composition. Thus, a change in absorption because of oxidation of the carbonyl group was evident [73]. The relative level of surface oxidation was expressed as a low surface oxidation rate (with CI between 0 and 0.15), medium (CI between 0.16 and 0.30), and high (with CI \(\geq\) 0.31) [74]. In this study, the CI  
Figure 5: Relative contribution (%) of each polymer (**a**) and shape (**b**) on total MPs extracted from seawater samples. calculated for PS, PE, and PP polymers varied from 0.32-0.34, 0.28-0.31, and 0.34-0.40, respectively. High values of CI correspond to a higher content of the carbonyl group in MPs caused by an increasing duration of exposure to UV radiation and high temperature and generally throughout the progression of deterioration [72]. The results obtained in this study are consistent with the research of [PERSON] et al. (2021) [75] about the seasonal variation of diversity, weathering, and inventory of MPs in coast and harbor sediments. The average \(\pm\) standard deviation CI values of PP and PE MPs in seawater were 0.55 \(\pm\) 0.17 and 0.75 \(\pm\) 0.21, respectively, which appeared unrelated to spatiotemporal variations. In addition, [PERSON] et al.