What is the role of gel particles in the formation of clouds?

- Joint press release of Leibniz Institute for Tropospheric Research (TROPOS) and GEOMAR Helmholtz Centre for Ocean Research Kiel -

In marine ecosystems, polymeric gels and gel-like material play an important role in the biochemical cycle of organic matter. One type of gel-like particles, Transparent Exopolymer Particles (TEPs), have gained increasing attention. Chemically, they are composed of polysaccharides linked by calcium ions that store water. TEPs have been shown to be extremely important for sedimentation processes and carbon cycling in the ocean, and are widely distributed in the sea surface microlayer (SML). They could therefore be of great importance for the exchange processes between the ocean and the atmosphere. Transparent exopolymer particles are very sticky and provide surfaces for other molecules and for bacterial colonisation. Up to a quarter of marine bacteria can be bound to TEPs in this way. Depending on how much material is bound, these gels sink to the depths and contribute to carbon storage in the oceans or rise to the surface and react with the atmosphere in the surface film. From the ocean surface, TEPs can be transferred to the air. Wind and breaking waves create aerosol particles from ocean spray, which could be a transfer mechanism for TEPs from the ocean to the atmosphere.

In addition to transfer from the ocean to the atmosphere, organic material such as the gel particles could also be formed in the atmosphere through biological processes. These formation processes in the atmosphere and their importance have hardly been studied so far, and there have been no studies on the in situ formation of TEP gels in the atmosphere. Yet the tiny gels could play a major role in the formation of clouds as cloud condensation nuclei (CCN), especially in the relatively clean air of the polar regions, but also in other marine regions, and thus influence the climate there. It is also suspected that these biopolymers, due to their structure, play a role in the formation of ice in clouds. They could act as ice nuclei (INP) because they form a three-dimensional network to which water molecules can attach, creating a structured surface for ice formation. However, a direct link between TEPs and INPs has not yet been experimentally proven in field studies.

Therefore, researchers investigated the number and size distribution of these gel particles in the ambient atmosphere of the tropical Atlantic. "Our goal was to determine the TEP number concentrations in the aerosol particles and cloud water of the atmosphere and to derive correlations with oceanic transfer and possible in situ formation mechanisms. To our knowledge, this is the first study with detailed measurements of TEP number size distribution in different atmospheric marine compartments in the tropical Atlantic," reports Dr Manuela van Pinxteren from TROPOS, who led the measurements.

The samples were taken during the campaign "Marine biological production, organic aerosol particles and marine clouds: a process chain" (MarParCloud), which took place from 13 September to 13 October 2017 on the island of Sao Vicente in the Cabo Verde archipelago in the eastern tropical North Atlantic. The particle measurements were taken at the Cape Verde Atmospheric Observatory (CVAO). The CVAO is located directly on the coastline at the north-eastern tip of the island of São Vicente at 10 metres above sea level. Thanks to the trade winds, this location is free of local island pollution and provides reference conditions for studying ocean-atmosphere interactions, as a constant northwest wind blows from the open ocean to the observatory. The cloud water was collected on Mt Verde, the highest point on the island at 744 metres.

In order to be able to compare the samples from the air as well as possible with the samples from the ocean water, the team also conducted a special experiment: 1400 litres of ocean water from the Atlantic were transported into a tank installed on site and the release of the gel particles by waves and sea spray was simulated under controlled conditions. When the bubbles burst on the sea surface, organic material from the surface film is also thrown into the air. This so-called "bubble bursting" ensures that substances from the ocean water pass through the surface film into the atmosphere. The tank experiment provided important comparative values: In the real atmosphere in the tropical Atlantic, the concentrations of TEP gels were up to two orders of magnitude higher than in the recreated atmosphere above the water tank in the laboratory.

"We think that the high concentrations of TEPs in the particles and cloud water are not only due to transfer from the ocean, but that there must be other formation processes in the atmosphere. We suspect that similar formation mechanisms as reported for TEP formation in the oceans could also take place in the atmosphere: In situ chemical formation could have resulted from aqueous reactions of dissolved organic precursors that were present in the particle and cloud water samples. The biotic formation is particularly exciting and could be due to bacteria, which were abundant in the cloud water samples," explains Manuela von Pinxteren. Conclusion: The formation of the TEP gel particles in the atmosphere is probably also related to biogenic processes and to bacteria in the air.
 

Original publication:

van Pinxteren, M., Robinson, T.-B., Zeppenfeld, S., Gong, X., Bahlmann, E., Fomba, K. W., Triesch, N., Stratmann, F., Wurl, O., Engel, A., Wex, H., and Herrmann, H.: High number concentrations of transparent exopolymer particles in ambient aerosol particles and cloud water – a case study at the tropical Atlantic Ocean, Atmos. Chem. Phys., 22, 5725–5742, https://doi.org/10.5194/acp-22-5725-2022 , 2022.

Projekt funding:

The study was funded by the Leibniz Association (project "Marine biological production, organic aerosol particles and marine clouds: a Process Chain (MarParCloud)" (SAW-2016-TROPOS-2)), the European Union (Research and Innovation Staff Exchange EU project MARSU (69089)) and the German Research Foundation (DFG, project number 268020496 - TRR 172).