HALO South is part of a series of internationally significant measurement campaigns using the German research aircraft HALO (High Altitude and Long Range Research Aircraft). The objective of HALO South is to investigate the interactions between aerosols, clouds, and radiation in pristine and hard-to-access regions of the Southern Ocean. HALO South took place during the period of maximum sea ice extent in the Southern Hemisphere, a time for which no high-resolution aircraft measurement data from this region had previously been available. A particular focus is on clarifying which microphysical processes contribute to cloud formation under these pristine atmospheric conditions. Measurements in these regions help to improve weather and climate models.

The HALO aircraft is a heavily modified Gulfstream G550 that has been specifically adapted for atmospheric research. With a range of more than 10,000 kilometers, a service ceiling of over 15 kilometers, and a payload capacity of up to three tons, HALO is currently the most capable airborne research platform in Europe.

The development of HALO originated from an initiative of the German atmospheric research community. The Max Planck Society (MPG) and the German Aerospace Center (DLR), together with more than 30 scientific institutions, were jointly responsible for the conceptual design and proposal. Following a positive evaluation by the German Council of Science and Humanities, the project was approved by the Federal Ministry of Education and Research (BMBF) in 2004 and implemented with the support of the Helmholtz Association.

The scientific work conducted with HALO focuses on five main thematic areas:

• Aerosols: sources, properties, and transformation processes of atmospheric particles
• Clouds and precipitation: formation, development, and radiatively relevant effects
• Dynamics: transport and circulation processes in the troposphere and lower stratosphere
• Chemistry: composition and chemical transformations of atmospheric trace gases
• Coupling mechanisms: interactions between different altitude levels and climate zones of the atmosphere