Higher than expected ice-nucleating particle concentrations in the Southern Ocean: Preliminary findings from the SOC 2024 cruise

Biases in surface radiation and sea surface temperature in climate models are larger over the Southern Ocean than anywhere else in the world, severely impacting on our ability to predict global climate. These biases are thought to be caused by the poor representation of mixed-phase clouds in the region, including aerosol-cloud interactions such as the role of atmospheric ice-nucleating particles (INPs). INPs can trigger the freezing of supercooled liquid cloud droplets, greatly influencing the lifetime and radiative properties of mixed-phase clouds. To better understand the role of INPs in the Southern Ocean, it is crucial to know their sources and concentrations, but there are relatively few INP measurements from the region, particularly around the Antarctic Peninsula. Further, discrepancies have been noted between INP measurements from traditional polycarbonate filter analysis techniques and other methodologies during recent field campaigns

We have collected the first ever set of combined real-time and offline measurements of INPs around the Antarctic Peninsula, South Sandwich Islands, and South Georgia during the Southern Ocean Clouds (SOC) research cruise during the austral summer of 2024. The cruise took place aboard the RRS Sir David Attenborough over a period of 5 weeks in November/December, and covered an area from 50° S to 67° S and 70° W to 25° W. Online INP measurements were collected every 6 min using a Portable Ice Nucleation Experiment (PINE) chamber, which uses adiabatic expansion to generate a cloud and then detects the INP concentrations within the cloud. Even with such a short time resolution, INP concentrations >0.5 INP L⁻¹ were measured throughout the cruise at temperatures of −25 to −28 °C.

These measurements were supported by offline filter measurements, with INP concentrations measured using a traditional droplet freezing assay. Importantly, two types of filter were used to collect and analyse the samples: polycarbonate filters prepared using a traditional “wash off” procedure, and Teflon filters using a “drop on” droplet freezing technique. The traditional polycarbonate method yielded very low INP concentrations, consistent with recent literature data for the Southern Ocean region, while the Teflon filters showed much higher concentrations, including when the two filter types were run side-by-side. This suggests, in line with our recent lab-based studies, that the traditional polycarbonate washing technique employed in INP analysis may be missing a fraction of INPs during measurements, and that INP concentrations may be undercounted in some scenarios including in the Southern Ocean

Here, we present initial findings of the INP concentrations collected throughout the SOC cruise, including discussion of the inconsistencies between filter techniques.