Estimation of the age of polyethylene microplastics collected from oceans: Application to the western North Pacific Ocean

The knowledge of microplastic (MP) age could aid the deduction of the origin and fate of a fragment carried by ocean currents over long time periods and distances. The present study developed a novel method to estimate the age of MPs (i.e., UV radiation exposure time) using the oxidation level of polyethylene (i.e., carbonyl index) from infrared spectrometry, ultraviolet erythemal radiation (UVER) data, and ambient seawater temperatures. Accelerated and outdoor exposure experiments were conducted to establish relationships among the temporally integrated UVER, ambient temperature, and carbonyl index. Thereafter, the age of MPs was computed, with Miyakojima Island serving as the reference location. The estimated ages of MPs collected from the western North Pacific Ocean ranged from 1 to 3 years, and those MPs from nearshore waters ranged from 0 to 5 years. •A novel method to estimate the age (i.e., UV radiation exposure time) of polyethylene microplastics(MPs) was developed.•The oxidation level of polyethylene from FT-IR, UV data, and ambient temperatures were factors in determining age of MPs.•The age of MPs collected from the western North Pacific Ocean was computed.•Both “fresh” and “mature” MPs were present in the nearshore areas. MP ages in the offshore were ranged from 1 to 3 years.