Microplastics have long been known to accumulate in wastewaters, rivers, and the the worlds' oceans. More recently, U.S.-based scientists found them in rain collected from 11 national parks. Now, a new study showed for the first time that microplastics are also present in the free troposphere - the lowest layer of the atmosphere of Earth - and the atmospheric boundary level, the bottom layer of the troposphere that is in contact with the surface of the earth.
Scientists from Waseda University in Japan collected cloud water from the summit of Mount Fuji, south-eastern foothills of Mt. Fuji (Tarobo), and the summit of Mt. Oyama – regions at altitudes ranging between 1300 meters to 3776 meters. Backward trajectory analysis at the summit of Mount Fuji suggested that the airborne microplastic in cloud water originated primarily from the ocean.
Using advanced imaging techniques that capture chemical images of various materials, like attenuated total reflection imaging and micro-Fourier transform infrared spectroscopy (µFTIR ATR imaging), they observed nine microplastics including polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), polyamide 6 (PA6), polycarbonate (PC), ethylene–propylene copolymer or polyethylene–polypropylene alloy, polyurethane (PU), and epoxy resin.
Most of the PP that was detected in the samples was degraded and had carbonyl (C=O) and/or hydroxyl (OH) groups. The Feret diameters of the microplastics ranged between 7.1 – 94.6 µm, the smallest of which were seen in the free troposphere. Moreover, the presence of hydrophilic (water loving) polymers in the cloud water was abundant, suggesting that they were removed as cloud condensation nuclei, also known as cloud seeds, according to the researchers. These small particles are a unique subset of aerosols in the atmosphere on which water vapour condenses, which can affect the radiative properties of clouds and the overall atmosphere. These findings suggest that microplastics play a key role in rapid cloud formation, which may influence the overall climate.
“[Microplastics] are degraded much faster in the upper atmosphere than on the ground due to strong ultraviolet radiation, and this degradation releases greenhouse gases and contributes to global warming,” said Hiroshi Okochi, Professor at Waseda University. “As a result, the findings of this study can be used to account for the effects of [microplastics] in future global warming projections.”
The team shared their findings in “Airborne hydrophilic microplastics in cloud water at high altitudes and their role in cloud formation,” recently published in Environmental Chemistry Letters.