HALO

Images by DLR

HALO (High Altitude and Long range research aircraft) is a unique research aircraft designed for flying long distances (over 8000 km), high altitudes (up to 15.5 km) and for carrying large payloads (up to 3 tons). HALO is based on a modified Gulfstream G550. The German Aerospace Center (DLR) is the owner and operator of HALO. HALO is stationed at DLR in Oberpfaffenhofen. Numerous missions have already been successfully completed with the aim of earth system research.

The university part of HALO is funded by the priority program 1294 "Atmosphere and Earth System Research with the HALO Research Aircraft" of the German Research Foundation (DFG). The program is  coordinated by  Prof. Manfred Wendisch (University of Leipzig) and Prof. Joachim Curtius (Goethe University Frankfurt am Main).

In recent years we have been involved in the missions CAFE-Pacific (2024, Australia), PHILEAS (2023, Alaska), CAFE-Brazil (2022/23, Brazil), CAFE-EU/BLUESKY (2020, Europe), SouthTRAC (2019, Argentina) and WISE (2017, Ireland). The HALO measurement campaigns are funded by the Priority Program 1294 of the German Research Foundation DFG.

Further information on HALO can be found on the → official HALO website, at → DLR and on the website of the → DFG Priority Program.

The CAFE-Pacific campaign (Chemistry of the Atmosphere: Field Experiment over the Pacific) with the HALO  aircraft is the direct successor of the CAFE-EU and CAFE-Brazil campaigns and will take place in January and February 2024 in Australia. The aircraft will be stationed in Cairns in northeast Australia, and flights over the Indo-Pacific as well as along the Australian coast are planned.

The formation of aerosols in the free troposphere above the Pacific has been observed in multiple measurement campaigns (Perry and Hobbs, 1993; Brock et al., 1995; Clarke et al., 1998; Clarke and Kapustin, 2002; Williamson et al., 2019). However, the chemical mechanisms and involved gases are still mostly unclear. The planned measurement flights with the extensive instrumentation of HALO offer the opportunity to conduct direct measurements in this region of the atmosphere. The objective is to achieve an improved understanding of the photochemical processes and the new particle formation in the tropical troposphere. The oxidation mechanisms as well as the nucleation and growth of particles will be investigated. The Indo-Pacific is especially interesting due to the high efficiency of oxidation processes and the frequent deep convective events which provide a direct link to the stratosphere. Additionally, there is little research on this region compared to the extra tropics.

Further research topics include the comparison between marine conditions and industrially influenced air masses and the direct comparison with the results from CAFE-Brazil over the Amazon rainforest.

The CI-APi-TOF operated by our group has a central role in this project by providing measurements of sulfuric acid, methane sulfonic acid (MSA), highly oxygenated organics (HOMs), as well as Amines and small clusters. Sulfuric acid and MSA could be produced from Dimethylsulfide (DMS), a compound emitted by the ocean and possibly transported upwards by deep convection.

As in previous HALO campaigns, there is a strong cooperation with other research groups that conduct measurements of substances like DMS, carbon monoxide, ozone, or nitrogen oxides.

One special circumstance of this campaign is the climate phenomenon El Nino, which influences the weather in the whole South Pacific. It could lead to decreased precipitation and stronger forest fires in the measurement region, offering the opportunity for additional interesting findings.

With our measurements, we expect to contribute to the understanding of the global aerosol budget and answer many of the open questions revolving around this topic.

CAFE-Brazil (Chemistry of the Atmosphere: Field Experiment in Brazil) is a HALO  mission that took place in winter 2022/23 above the Amazon rainforest. Researching the upper tropical troposphere is of high importance because previous studies (e.g., Andreae et al., 2018) show elevated aerosol particle concentration in this area compared to the middle and lower troposphere. These newly formed particles be transported over large distances and potentially form an important source of cloud condensation nuclei in the tropical troposphere.

Various research flights during the CAFE-Brazil campaign contributed to the generation of altitude profiles for different gases. Additionally, it was possible to measure air masses that had been transported to the upper troposphere by deep convective events. The differences between day and nighttime processes were studied.

Physical parameters like pressure and temperature, as well as trace gases and aerosol particles, were measured continuously to understand the processes in the atmosphere. The CI-APi-TOF operated by our group could measure sulfuric acid, methanesulfonic acid, and also highly oxygenated organic compounds. Especially interesting was the detection of organonitrates, which could be directly linked to the production of particles. It was subsequently possible to establish a detailed chemical mechanism for the nucleation of aerosols in the upper troposphere above the Amazon. This proposed process will be tested under laboratory conditions in the CLOUD chamber at CERN.

The conducted measurements can be very helpful in the future to improve the representation of aerosols in climate models. Cooperation with modeling groups both nationally and internationally is crucial to achieve this goal.