HALO inlet systems
Many measurement systems aboard HALO use external air to analyse particle compositions, size distributions, or trace gas concentrations in the ambient air. Those measurements can only work properly, when the inlets, through which exterior air, which is moving relative to HALO at aircraft speed, is collected ina way, that the flow through the inlet itself, the tubing connecting them to the measurement systems, and the airflow in front of the inlet do not interfere with the measurement. Thus, a number of different inlet systems have been developed over time, to serve the different needs of the measurement systems on HALO.
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Trace Gas Inlet (TGI) for HALO
The standard inlet for gas phase measurements was developed and build by enviscope and certified by DLR. This intake is part of the basic equiment provided by the DLR for scientific users working with in-situ equipment aboard the HALO aircraft.
The design of the intake allows different tubing configurations to be integrated into the inlet structure, thus providing a high flexibility to the scientific requirement. To achieve this, the tubing and ducting system is built in a modular way enabling an individual sampling equipment – according to the respective demands – to be integrated by simply exchanging the complete tubing assembly.
General mounting options:
- The Inlet can be mounted on each of the prepared aperture plates
- Two different heights are available: The smaller one is intended to be operated in the forward positions, the longer version for rearward mounting positions of the aircraft
- The inlet can be mounted with one forward directed sampling tube and 3 backward directed tubes or vice versa
- Each of the tubes can have a diameter up to 1/2 inch, e.g. 1/2″ or 3/8″ or 1/4″. Any combination of those diameters is possible
- The material of the tubes can be chosen according to the requirements, e.g. stainless steel, PTFE or PFA
- Different feed-throughs are possible. The pressure flange can be exchanged with the assembly
- Special configurations are possible as long as the assembly fits to the body
The moving image shows the stepwise disassembly of the intake to mount different tubing configurations.
At present, two basic configurations for the tubing assemblies are suggested: One with a flat plate and an alternative layout with a chamfered surface which leaves more room for larger fittings. The electrical feed-throughs are meant to connect the anti-ice provisions and an optional heating of the sample tubes.
To allow these tubing assemblies to be built by the scientific users, an „interface control document“ will be prepared to provide all information necessary to meet the requirements of compatibility to the TGI structure.
(Images: enviscope, DLR)
Total Water and Ice Crystal Sampler (TWICS)
(below the fuselage)
The TWICS is a modified TGI designed for installation on the bottom of the fuselage. The TGI is slightly tilted to compensate for the angle of attack of HALO and to enable sampling parallel to the direction of flight. This installation position, particularly on the bottom, ensures undisturbed sampling of droplets and ice crystals and thus allows for the determination of liquid water and ice water content in clouds .
Reference: Afchine, A., et al., (2018): Ice particle sampling from aircraft – influence of the probing position on the ice water content, Atmos. Meas. Tech., 11, 4015–4031, doi: 10.5194/amt-11-4015-2018.
(Image: TWICS installed below HALO during NAWDIC, 2026 || credit: C. Rolf / FZ Jülich)
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Aerosol & cloud particle inlets for HALO
Inlets allowing air sampling for particle measurements are designed for specific size ranges of the particles in question. As a result, there are multiple inlets used on HALO, depending on the mission goals. Those are also developed and certified by various institutions in collaboration with enviscope.
A common obstacle for all those systems is the fact that the aerosol particles have to be sampled isoaxial to the intake tube and afterwards being deccelerated in the counterflow or diffusor arrangements. To achieve this, a shroud assembly has been attached to parallelize the flow around the sample inlet tip.
HALO Submicrometer Aerosol Inlet (HASI)
(sampling range: <= 2-3µm)
HASI samples the air on the top of the fuselage outside of the aircraft boundary layer. The air stream is aligned in the inlet and de-accelerated in three stages by roughly a factor of 40 using a front shroud, a main diffusor and smaller diffusers at the tips of four sample tubes which protrude into the deaccelerated air stream. In addition, there is a backward-facing sample tube installed for optional use. The design concept is to allow regulating the sample air flow in each of the four forward-facing sample lines to achieve isokinetic sampling conditions according to the actual speed of the aircraft. The geometric design of the inlet should prevent large cloud droplets and ice crystals from entering the sample lines directly.
Reference: Minikin, A., et al., (2017): The HALO Submicrometer Aerosol Inlet (HASI): Design concept and first characterization. 1st HALO symposium: Airborne Research with HALO: Achievements and Prospects, 2017-03-14 – 2017-03-16, Oberpfaffenhofen, Deutschland. (not published)
(Image: enviscope)
Micrometer Aerosol Inlet (MAI) (planned project)
(sampling range: supermicron particles)
The MAI („Micrometer Aerosol Inlet“) Inlet will be developed to enable super-micron particles. This will lead to a larger range of possible measurements, inlcuding larger aerosols.
The concept is based on an inlet already developed in 2014, which was not further persued. For MAI, this development will be re-evaluated, and hardware adjustments will be made.
(Image: enviscope)
HALO-CVI
(sampling range: 5 – 60 μm)
This inlet is for collecting cloud particles and is thus used when flying through couds. It is mounted underneath HALO, as the measurements of the relatively large cloud particles (in comparison to aerosols) is more representative due to the airflow around the inlet. Also, this inlet does not use the shroud assembly, to avoid ice particle shattering in front of the inlet.
The HALO Counterflow Virtual Impactor (CVI) has been developed and built for the in-situ collection of cloud droplets and ice particles and simultaneous separation of non-activated particles. The functional principle of the CVI is based on the virtual impaction of the cloud particles on an artificially generated counterflow, which emanates from the inlet tip against the collection direction. This prevents the entire gas phase and the smaller interstitial particles from entering the CVI, while the larger cloud particles (droplets, ice particles) overcome the counterflow due to their higher inertia and are collected. The cloud particles enter a dry and particle-free carrier air which leads to a complete evaporation/sublimation of the water or ice phase. In this way, dry droplet or ice particle residues are released, which can be interpreted as cloud condensation or ice nuclei and analyzed microphysically and chemically. [cited from TROPOS webpage: Counterflow Virtual Impactor (CVI)]
Reference:
(ground-based system): Mertes, S., et al., (2007): Counterflow Virtual Impactor Based Collection of Small Ice Particles in Mixed-Phase Clouds for the Physico-Chemical Characterization of Tropospheric Ice Nuclei: Sampler Description and First Case Study. Aerosol Science and Technology, 41(9), 848–864. doi: 10.1080/02786820701501881
(Image: S. Mertes, TROPOS)
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