Aerosol and Cloud Residual Particle Measurements during HALO-South 2025: Overview and first Results

Aerosol-cloud-radiation interactions remain one of the largest sources of uncertainty in climate projections. The Southern Ocean is one of the cloudiest regions on Earth and among the most pristine atmospheric environments. It is characterized by persistent low-level stratocumulus clouds that frequently occur in mixed-phase state, owing to exceptionally low concentrations of cloud condensation nuclei and, in particular, ice-nucleating particles. This combination makes the Southern Ocean a unique natural laboratory for investigating the coupling between aerosols, cloud microphysics, and radiation.

Understanding the formation and persistence of mixed-phase clouds in this region critically depends on the availability and chemical nature of aerosol particles acting as cloud condensation nuclei and ice-nucleating particles. Here, we present measurements of aerosol chemical composition in the Southern Ocean obtained during the HALO-South aircraft campaign (https://halo-research.de/sience/previous-missions/halo-south). The HALO-South campaign took place in September and October 2025 with the HALO aircraft operating from Christchurch, New Zealand, and comprised 19 research flights over the Southern Ocean and 8 transfer flights from and to Germany (spanning the globe).

Combined with observations of cloud and radiation properties, we aim to identify particle sources and investigate how aerosol chemistry influences cloud microphysics.

During the campaign we operated a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS, Schulz et al., 2018) to measure the composition of the non-refractory aerosol particles (organics, nitrate and sulphate) in a size range of 40-800 nm. The C-ToF-AMS was operated behind the HALO aerosol sampling inlet HASI and the cloud residual inlet HALO-CVI (Counterflow Virtual Impactor). The measurement altitudes ranged between 150 m to 12.4 km, thus including liquid, mixed-phase and ice cloud conditions.

First results indicate that residues from ice clouds contain more organic compounds, while liquid cloud residuals contain mainly sulphate and nitrate. The aerosol mass concentrations in the troposphere over the Southern Ocean were generally low, however we observed occasionally aerosol layers from biomass burning over Australia, from volcanic plumes, and from long-range transport.

 

Schulz, C., et al.: Aircraft-based observations of isoprene-epoxydiol-derived secondary organic aerosol (IEPOX-SOA) in the tropical upper troposphere over the Amazon region, Atmos. Chem. Phys., 18, 14979-15001, 2018.