The largest ice desert has the fewest ice nuclei worldwide

Leipzig. There are fewer ice nuclei in the air above the large ice surfaces of Antarctica than anywhere else in the world. This is the conclusion reached by an international research team led by the Leibniz Institute for Tropospheric Research (TROPOS) based on filter measurements of cloud particles at three locations in Antarctica. These are the first of their kind on the continent. The data now published fills a knowledge gap and could explain the large proportion of supercooled liquid water in the clouds of the southern polar region. Clouds containing liquid water droplets reflect sunlight more strongly than clouds containing ice. Fewer ice nuclei and less ice in the clouds could contribute to the southern hemisphere not warming as much as the northern hemisphere, the researchers write in the journal Geophysical Research Letters.

It has long been known that the clouds over the Southern Ocean around Antarctica contain more water and less ice than comparable clouds in the Northern Hemisphere. However, without details on the causes and measurement series, climate models based on data from the Northern Hemisphere cannot be adjusted. The measurements of ice nuclei now provide an important detail for this. Further data will be provided by flights of the German research aircraft HALO, whose HALO-South mission ended in New Zealand in mid-October, as well as a series of Antarctic expeditions planned for 2026-2030 as part of the major international research project "Antarctica InSync".

Clouds continue to be the greatest source of uncertainty in climate models. Ice in clouds plays a major role in this, as ice formation processes influence radiation properties, precipitation formation and, consequently, the lifespan of clouds. Ice formation is made possible by so-called ice nucleating particles (INPs). INPs act as catalysts, because without these particles, cloud droplets only freeze below -38°C. Particularly over the Southern Ocean around Antarctica, where concentrations of ice nuclei are low in the clean atmosphere, large differences in radiation effects between models and measurements have been observed. For this reason, ice nuclei have become the focus of cloud research.

Globally, mineral dust particles make up the majority of ice nuclei at low temperatures. At higher temperatures, however, ice nuclei are mostly of biological origin and contain proteins or polysaccharides. Since there is more biological activity in summer than in winter, a clear annual cycle with a maximum in summer and a minimum in winter can be observed in many regions. Researchers have been able to observe these seasonal fluctuations even in the Arctic – but not in Antarctica.

Until now, there has been no solid data on Antarctica. For the recently published study, aerosol particles were collected using filters in Antarctica, stored at -20°C and finally examined for ice nuclei in the TROPOS laboratory in Leipzig. To do this, the researchers used two devices, LINA (Leipzig Ice Nucleation Array) and INDA (Ice Nucleation Droplet Array), which optically count the number of ice nuclei in the atmosphere at different temperatures. This standardised method makes it possible to determine where there are more ice nuclei floating in the atmosphere and where there are fewer.

Most of the samples examined came from the German Antarctic station Neumayer III, where data from two complete years was obtained between December 2019 and 2021. Neumayer III is located on the Eckström Ice Shelf, about 20 kilometres from the ice edge. The time series obtained there is unique to date and particularly valuable due to measurements taken during the southern winter. The researchers were also able to analyse filter samples taken during the southern summers of 2020/21 and 2021/22 at the Belgian Antarctic station Princess Elisabeth, which is located on a mountain range at an altitude of about 1400 metres and 200 kilometres from the sea. The analysis also included filter samples from the Spanish PI-ICE expedition, which had studied the atmosphere above the Antarctic Peninsula and the Spanish Antarctic station Juan Carlos I on Livingston Island during the southern summer of 2018/19. "To our knowledge, there has never been such a long time series of filters from which INPs have been determined on the Antarctic mainland. In 2023, Chinese researchers collected snow samples on a sled tour, but these only allow indirect conclusions to be drawn about ice nuclei. Our direct and extended measurements are therefore a first for the Antarctic continent," says Dr Heike Wex from TROPOS, explaining the significance of the study.

The number of ice nuclei above the sea on the Antarctic Peninsula was comparable to previous measurements at other locations in the Southern Ocean. However, the two Antarctic stations Neumayer III and Princess Elisabeth showed lower values than ever before. What was particularly striking about the 24-month measurements from the German Antarctic station Neumayer III was that there were no seasonal variations in the number of ice nuclei, nor were there any heat-sensitive ice nuclei in the samples. "This generally indicates very few biogenic protein-containing ice nuclei, which is probably related to the low level of biological activity on the Antarctic continent, which – if at all – is only found near the coast in summer," explains Dr Heike Wex. Since Antarctica releases few ice nuclei into the atmosphere due to a lack of dust sources and biological activity, the number of ice nuclei above the Southern Ocean around Antarctica is also relatively low. This could explain the large proportion of supercooled droplets in the clouds there, which remain liquid and do not freeze due to the lack of ice nuclei. The proportion of water and ice in the clouds, in turn, influences the radiation properties and could contribute to the Southern Hemisphere warming less than the Northern Hemisphere.

"Our results provide important data that can help improve understanding and thus also global climate models. In addition, the concentration of ice nuclei in Antarctica could increase due to global warming, as retreating glaciers expose more land to vegetation and the biosphere could become more active. Therefore, determining the current state can be helpful in assessing the potential impacts of future changes," reports Dr Silvia Henning from TROPOS. From the measurements taken at Neumayer III, the team was able to derive a parameterization that could be used to predict ice nuclei at Princess Elisabeth and which can therefore be used for modelling, at least for this part of Antarctica. Future studies in 2027-2030 as part of the large international research project "Antarctica InSync" will show whether this also applies to other regions of the Antarctic continent.

Tilo Arnhold