In search of an optimal moss transplant biomonitor for airborne microplastics: Moss cubes

There is growing interest in the use of moss transplant samplers to assess atmospheric microplastics. Here we explored the influence of sampler design and exposure duration on the accumulation of microplastics. We evaluated three samplers, two bag samplers (5 cm × 8 cm) each containing 1 g of moss, but with different mesh sizes (1 mm versus 6 mm), and a 6 cm cube made from 6 mm mesh containing 3 g of moss. The cube samplers had the highest microplastic particle accumulation (2.92 mp/g), followed by 1 mm mesh bags (2.57 mp/g) and 6 mm mesh bags (1.97 mp/g). Further, cube samplers had the lowest variation between replicates, with all of them above the limit of detection. All sampler designs were dominated by fibres ranging from 80 % in cubes to 94 % in 6 mm mesh bags, suggesting that cubes captured a higher diversity of particle morphologies. The net loss in moss mass during sampler deployment was lowest for the cube samplers (3.9 %) and highest in 1 mm mesh bags (10.5 %). Transplant samplers exposed for six-weeks suggested higher particle accumulation than four-week exposures; however, mass concentration was lower in the six-week exposures, suggesting that heavier microplastics were lost. In general, the results suggest that effective moss transplant samplers should incorporate sufficient moss mass to ensure particle retention, larger mesh size (≥5 mm) to maximize exposure to atmospheric microplastics, and a three-dimensional shape to allow equal exposure from all sides. In this respect, we recommend the 'moss cube' sampler with a 6 mm mesh size containing at least 3 g of moss depending on cube dimensions.