Microplastics – tiny particles of plastic most often formed from the degradation of bulkier plastic waste – are known to be widespread in the environment, including in oceans around the world. However, a new study in Japan has become the first to confirm their presence in clouds.
The study, led by Hiroshi Okochi, a professor at Waseda University in Tokyo, explored the path of airborne microplastics (AMPs) as they circulate in the biosphere. Through their analysis of cloud water samples taken from high-altitude mountains in Japan, the research team was also able to shed light on the influence of AMPs on cloud formation and their potential negative impact on the environment, particularly their contribution to climate change. MTI spoke to Prof. Okochi about his work and the technologies required for the research.
Why are your findings important?
Prior to our research there was no conclusive evidence to confirm the presence of AMPs in cloud water. Our findings are therefore groundbreaking. We successfully collected and analyzed samples of cloud water, revealing the presence of AMPs. While these findings are important, they represent only preliminary evidence. We must undertake further research to thoroughly understand the role of AMPs as cloud condensation nuclei and ice nuclei.
What field work did you conduct for this study?
Our study on AMPs in cloud water commenced in 2021 and we have been actively engaged in this research since. Our collection strategy involved a variety of tools and locations. Our study sites included the summit of Mt Fuji, the southeast foothills of Mt Fuji, and the summit of Mt Oyama [a 1,251m peak in the Kanagawa region of Japan]. The summit of Mt Fuji is in the free troposphere, while the southeast foothills of Mt Fuji and the summit of Mt Oyama in the Tanzawa mountain range are in the upper layer of the atmospheric boundary layer.
We used passive string-type cloud-water collectors at the summits of Mt Oyama and Mt Fuji. Additionally, for year-round data collection, an active string-type automatic cloud-water collector was sited at the base of Mt Fuji.
Cloud-water observation at the summit of Mt Fuji was conducted only during the two summer months when the Mt Fuji Research Station was accessible. During this intensive observation period (one week in July and one week in August), cloud water was automatically collected with each cloud event. Outside this period, cloud water collected by the automatic device was retrieved every two weeks. At the summit of Mt Oyama, observations were carried out throughout the year.
Why did you use passive cloud-water collection devices in your research?
In general, commercial power sources are not available at mountain summits and even if they are, there is a risk of lightning strikes traveling through power cables, making it advisable to avoid using outdoor power sources as much as possible. If there are no obstructions such as trees, the strong winds at mountain summits mean that passive samplers, which do not require power, can collect a sufficient volume of cloud water.
Did you notice any seasonal variation in the volume or types of microplastics you found in the clouds?
As this work has been ongoing only since 2021, we have a relatively small number of samples collected to date. Therefore specific details regarding the seasonal variation in the number concentration and polymer composition of AMPs in cloud water remain indeterminate. In our future work we aim to persist with our observations of AMPs in cloud water, with the goal of gaining a deeper understanding of their seasonal and annual variation, as well as identifying any distinctive regional features.
How do you ensure that your water samples contain cloud water and not rainwater?
The key distinction between cloud droplets and raindrops lies in their sizes. Raindrops are large and therefore have a high gravitational settling velocity, making them easily collectible in containers such as buckets. On the other hand, cloud droplets are small and have a slow gravitational settling velocity, making them impossible to collect in containers like buckets.
There are several devices for collecting cloud droplets, but the fine wire collection device used in this study, which uses the inertial collision of cloud droplets transported by the wind, is widely used. The disadvantage of passive collection devices that use natural wind is that they are constantly open and cannot clearly distinguish between clouds and rain during storms. However, at the summit of Mt Fuji rain rarely falls vertically and the summit is usually covered in clouds during bad weather.
Why should we be concerned about the presence of microplastics in clouds?
AMPs may significantly influence cloud formation, potentially causing major shifts in the global hydrological cycle by affecting rainfall patterns. Additionally, their role in altering the solar radiation budget through indirect radiative effects caused by cloud formation could lead to a global cooling phenomenon.
On the other hand, since AMPs are primarily composed of carbon and hydrogen, they are prone to emitting greenhouse gases such as CO2 and CH4 on degradation. This degradation is accelerated by the stronger ultraviolet radiation in the upper atmosphere compared with ground level. Moreover, the incorporation of AMPs into clouds suggests their subsequent transport to Earth’s surface via rainfall, risking contamination of water sources with microplastics. This is particularly concerning for mountain water sources that contribute to our drinking water supply, raising the issue of increased microplastic consumption through drinking water.
In essence, the presence of airborne microplastics and nanoplastics in cloud water suggests the potential for widespread dispersion across land through precipitation. It could lead to the contamination of forests and soils with microplastics and nanoplastics, and ultimately contribute to the destruction of marine ecosystems via river systems.
Are you hoping to carry out follow-up studies?
To verify whether AMPs promote cloud formation, it is necessary to experimentally clarify the cloud-forming ability of AMPs in atmospheric aerosols. Our current results are only preliminary.
To measure AMPs we need to develop new cloud-water collection devices and conduct cloud-water sampling in mountainous regions domestically and internationally, using aircraft and drones, to elucidate the global presence of AMPs in cloud water. As the analysis methods for submicron-sized AMPs (0.1-1μm) and airborne nanoplastics smaller than 0.1μm are not yet established, the development of these methodologies is essential.
Moreover, from the perspective of public concern I believe the primary interest lies in health risks. Currently we are conducting cell and animal experiments to assess the impact on the respiratory system, but many aspects remain unclear. We intend to continue our research to elucidate the health and environmental risks of airborne microplastics and nanoplastics.