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Airborne hydrophilic microplastics in cloud water at high altitudes and their role in cloud formation [1]

['Wang', 'Wangyizeqqcs Fuji.Waseda.Jp', 'Graduate School Of Creative Science', 'Engineering', 'Waseda University', 'Tokyo', 'Okochi', 'Hokochi Waseda.Jp', 'Tani', 'Hayami']

Date: 2023-12-29

Airborne microplastics in cloud water

Nine polymers were detected in cloud water: polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyamide 6, polycarbonate, ethylene–propylene copolymer or polyethylene–polypropylene alloy, polyurethane, epoxy resin, and one rubber were detected. For rubber specific identification method, please refer to Tables S2.

Among the identified airborne microplastics, polycarbonate, polymethyl methacrylate, polyurethane, polyethylene terephthalate, and polyamide 6 exhibited C=O stretching vibrations at 1770 cm−1, 1725 cm−1, 1720 cm−1, 1710 cm−1, and 1630 cm−1, respectively (Perkin Elmer, FTIR Blog). Airborne microplastics other than polymethyl methacrylate and polycarbonate were identified by screening for C–H stretching vibrations; however, 94% of polyethylene terephthalate and 50% of polycarbonate were identified by screening for C=O stretching vibrations (1740–1710 cm−1). A wider screening range for the C=O stretching vibrations could improve the detection of these types of polymers. The degradation level, carbonyl index, of polypropylene was calculated as the ratio of the intensity of the peak at 1715 cm–1 (carbonyl groups formed during degradation) to the peak at 2920 cm−1 (CH 2 asymmetric stretching). Five of the 14 polypropylene in cloud water were degraded, with medium and high degradation levels of 21.4% (Fig. S6). The hydroxyl index calculated from the ratio of the peak at 3467 cm–1 (hydroxyl formation due to degradation) to the peak at 2920 cm–1 suggested that most polypropylene (85.7 %) formed hydroxyl groups and became hydrophilic.

Figure 1 shows polyethylene terephthalate (fragment, 75.2 µm) in cloud water at Mt. Oyama, polymethyl methacrylate (fragment, 80.3 µm) and polypropylene (fragment, 27.2 µm) in cloud water at Tarobo, and polyamide 6 (fragment, 15.5 µm) in cloud water at Mt. Fuji as some examples.

Fig. 1 Fourier transform infrared spectroscopy (FTIR) spectra of airborne microplastics detected in cloud water, including a polyethylene terephthalate; PET, b polymethyl methacrylate; PMMA, c polypropylene; PP, and d polyamide 6, as well as reference spectra and the Feret diameter of airborne microplastics Full size image

Figure 2 shows the number of airborne microplastics detected in cloud water and their proportions of polymer types. There were 20 pieces at Mt. Oyama (n = 9), 13 pieces at Mt. Fuji (n = 19), and 37 pieces at Tarobo (n = 16). There were number concentrations of 6.8 pieces L−1, 6.7 pieces L−1, and 13.9 pieces L−1, respectively. The number concentrations in cloud water obtained in this study were very low compared with those in snow cover and ice sheets in the Arctic (10,700 pieces L−1; Bergmann et al. 2019), Europe (1434 pieces L−1; Bergmann et al. 2019), Everest (30 pieces L−1; Napper et al. 2020), and Antarctica (29 pieces L−1; Aves et al. 2022). The analytical methods used in each study were different; therefore, caution should be exercised when comparing the data. Using the same analytical method as that used in this study, we recently reported a number concentration of 119 pieces L−1 in snow cover at Mt. Fuji in 2022 (Tani et al. 2023), indicating that the number concentration of airborne microplastics in cloud water was low.

Fig. 2 Material composition of airborne microplastics (AMPs) in cloud water at the summit of Mt. Oyama, at the summit of Mt. Fuji, and at the foot of Mt. Fuji-Tarobo. Twenty pieces AMPs (sample numbers = 9) were detected in Mt. Oyama, 13 pieces (sample numbers = 19) at Mt. Fuji, and 37 pieces (sample numbers = 16) at Tarobo. The full names of the abbreviations in the legend are as follows: polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), polyamide 6 (PA), polycarbonate (PC), ethylene–propylene copolymer or polyethylene–polypropylene alloy (PP/PE), polyurethane (PUR), epoxy resin (EP), and rubber Full size image

Polypropylene and polyethylene terephthalate were predominantly detected at Mt. Oyama and Tarobo in atmospheric boundary layer. Other polymers detected in the cloud water were polyurethane at Mt. Oyama, polymethyl methacrylate at Tarobo, and ethylene–propylene copolymer or polyethylene–polypropylene alloy at both locations. However, many types of polymers were detected in the cloud water at Mt. Fuji, with polycarbonate being the major polymer. The number concentration and polymer types of airborne microplastics in cloud water likely depend on the air mass. The origin of the air masses with airborne microplastics detected in the cloud water during the observation period was analyzed using backward trajectory analysis (Fig. S7). Air masses from northern China (NC), southern China (SC), peripheral maritime (PM), and northwestern (NW) directions prevailed at Mt. Oyama and Tarobo, where the types of airborne microplastics were similar. However, the air masses at Mt. Fuji in summer originated not only from the NC and SC directions but also from the southern maritime (SM), eastern maritime (EM), and southeastern (SE) directions. This explains why the similarity in the types of airborne microplastics in cloud water was higher than that in the other two locations. In Southeast Asia, the concentration of airborne microplastics is high and many types of polymers have been detected (Onozuka et al. 2023).

In interpreting the results of this study, it should be noted that the cloud water collectors used at the three sites were not identical. In particular, an active-type collector that can collect more cloud water was used at Tarobo. In addition, the string-type collectors used at all sites are used for the analysis of major ions in cloud water and are unsuitable for the analysis of suspended particles such as airborne microplastics, as some of them may become trapped in strings and flow channels. This suggests that the number concentrations of airborne microplastics reported in this study were likely underestimated.

The Feret diameter distributions of airborne microplastics in cloud water at three sites, are ranging from 7.1to 94.6 µm (identified airborne microplastics number, n = 70) in Fig.S8. The mean Feret diameter was found to be 32.0 ± 20.6 μm at Mt. Oyama (n = 9), 29.9 ± 17.5 μm at Mt. Fuji (n = 19), and 38.3 ± 23.8 μm at Tarobo (n = 16), respectively. The proportion of Feret diameters between 10 and 20 μm in cloud water at Mt. Fuji was approximately twice that of the other two sites in this study. These results suggest that smaller airborne microplastics are more likely to diffuse (Evangeliou et al. 2020) and advect at cloud-forming altitudes (Aeschlimann et al. 2022). The shape of airborne microplastics in cloud water was nearly fragmented (more than 87%) at three sites, similar to the trend of airborne microplastics in wet and dry depositions measured in mountainous areas (Allen et al. 2019). In particular, the proportion of fragmented airborne microplastics in the cloud water was higher at Mt. Fuji than at the other two sites. Mt. Fuji is located in the free troposphere, whereas Mt. Oyama and Tarobo are located at similar altitudes in the atmospheric boundary layer. Our results suggest that small and fragmented airborne microplastics are more likely to be transported to higher altitudes than large and fibrous airborne microplastics because of their size. This indicates that the physical properties of airborne microplastics, specifically their size, play a role in their atmospheric transport.

Sources of airborne microplastics in cloud water

Figure 3 shows time-course change in the number concentrations of airborne microplastics and their composition in cloud water at Mt. Fuji during the sampling period between July 21–22 and 26–27, 2022. Backward trajectory analysis classifies air masses into three categories: northern (Continental), southwestern (Continental), and maritime (Pacific Ocean). Polyethylene terephthalate and ethylene–propylene copolymer or polyethylene–polypropylene alloy were detected in the cloud water from 16:40 to 21:12 on July 21 and from 21:12 on July 21 to 6:27 on July 22, respectively. At this time, the air mass came from the cities of Zhejiang, Fujian, and around the coastal sea of Hainan prefecture of China at a low height, gradually climbing to Mt. Fuji (Fig. S11 & S12), suggesting long-range transportation of air pollutants from the atmospheric boundary layer. The concentrations of NO 3 − and non-sea-salt sulfate (nss–SO 4 2−) as indicators of anthropogenic emissions and Na+ as an indicator of oceanic origin were high during this period in Fig S10. This indicates that the airborne microplastics in cloud water originate from both anthropogenic and oceanic sources. The size of airborne microplastics was 26.3 µm for polyethylene terephthalate and 28.7 µm for ethylene–propylene copolymer or polyethylene–polypropylene alloy, respectively. Both were fragments. Although the air masses originated from the continental area in the sample from 6:27 to 12:00 on July 22, airborne microplastics were not detected. This is because the air mass originated only from within the free troposphere and did not pass through the atmospheric boundary layer, which was strongly influenced by the ground (Fig. S13).

Fig. 3 Microplastics changes over time in cloud water at the summit of Mt. Fuji (Jul. 21 21–22, 2022; Jul. 26–27, 2022), the direction of air mass, the number concentration of airborne microplastics (AMPs) in cloud water, and their compositions. The full names of the abbreviations in the legend are as follows: polyethylene terephthalate (PET), ethylene–propylene copolymer or polyethylene–polypropylene alloy (PP/PE), polycarbonate (PC), rubber, and polyethylene (PE) Full size image

Airborne microplastics were detected in the cloud water collected at 12:10–15:00, 15:00–18:05, and 21:05 on July 26 to 0:00 on July 27, 2022, when the starting point of the air mass was close to sea level, particularly from 12:10 to 18:05 on July 26 (Fig. S15, S16, and S17). During the studied period at Mt. Fuji, the highest number concentration of airborne microplastics was observed in cloud water collected from 12:10 to 15:00 (polycarbonate: 32.7 pieces/L, size: 10.7–17.8 μm, fragment), followed by in cloud water collected from 15:00 to 18:05 (rubber: 14.2 pieces/L, size: 28.7 μm, fragment). Polyethylene was detected in cloud water collected between 21:05 on July 26 to 0:00 on July 27, when the air mass came from below 850 hPa (approximately 2000 m a.s.l.) in the atmospheric boundary layer (8.2 pieces/L, size: 61.0 μm, fragment). Pan et al. (2019) reported that polyethylene is the predominant component of surface seawater in the Northwest Pacific Ocean. Moreover, polyethylene has been detected in cloud water transported by air masses originating near the sea surface, suggesting that it may be derived from microplastics in the ocean. Although ocean air was present during these periods, the sodium concentration in the cloud water was low. Sea salt particles are typically found in coarse particle regions and are easily washed away by precipitation. Rainfall was observed around Mt. Fuji during the study period (Japan Meteorological Agency 2023). This is likely the reason for the reduced sodium concentrations.

An air mass passed through the free troposphere over the Pacific Ocean from 0:00 to 4:00 on July 26 and from 0:00 to 4:30 on July 27, without descending below 850 hPa (approximately 2000 m a.s.l.) within the atmospheric boundary layer. This is likely why airborne microplastics were not detected in cloud water (Fig. S14 & S18).

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[1] Url: https://link.springer.com/article/10.1007/s10311-023-01626-x

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