Carbonyl index of miniaturized microplastics at the sea surface.

We investigated the carbonyl index (CI) of small microplastics (SMPs; 50-350 μm) and large microplastics (LMPs; >350 μm) in the surface waters of the Tokai region and Tokyo Bay in Japan. The degradation was evaluated by determining the CI from the infrared spectrum of microplastics (MPs) adopting the specified area under the band technique. Average CIs of SMPs and LMPs in polyethylene were 2.47 and 0.67 off Tokai and 2.71 and 0.76 in Tokyo Bay, respectively. No significant differences in the CI were found between polyethylene and polypropylene. However, significant differences in CIs were found between SMPs and LMPs. The relationship between the major length of MPs and CI was significantly negative for both polyethylene and polypropylene (p < 0.05). The results indicate that as MPs degrade, they become finer and exhibit a higher CI at the sea surface.