HALO Circle

Circumpolar survey of greenhouse gas fluxes

Mission status: No information available yet.

Persons in Charge


  •  Andreas Fix (DLR-IPA)

Scientific Co-Leadership


Mission coordinator​


Contact point at DLR-FX for this mission:


HALO Deployment Base

Time Period

Not scheduled, yet.


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Project description

Short Summary

The main objectives of HALO Circle are to improve our present understanding of the carbon cycle in the Arctic region, with focus on the large natural wetlands, identify hemispheric differences, and validate the French-German methane mission MERLIN.

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1. Introduction

Climate Change is one of the greatest societal challenges of the 21 st century. The dominant source of Global Warming is the increase of anthropogenic greenhouse gases (GHGs) in the Earth`s atmosphere. The two most important of those species are carbon dioxide (CO 2 ) and methane (CH 4 ). Together they account for ~81% of the anthropogenic radiative forcing (WMO, 2018).

Further increase in the atmospheric abundance of these gases is predicted to result in a warmer climate (IPCC, 2013). However, uncertainties in our knowledge of the budgets of these gases, which are determined by their sources, sinks, and inadequately understood feedback mechanisms, limit the accuracy of current climate change projections from the local to the global scale. In order to reliably predict the climate of our planet, and to guide political conventions on greenhouse gas avoidance, adequate knowledge of the sources and sinks of these greenhouse gases, their feedbacks, and the quantification of natural versus anthropogenic sources is mandatory. In spite of the recognized importance of this issue, our current understanding of sources and sinks of the gases CO 2 and CH 4 is still poor and not
sufficient to address the needs of science and policymakers (IPCC, 2013).

The HALO Circle mission intends to address these deficiencies with a multi-disciplinary approach combining aircraft measurements, models, and satellite data with a focus on the Arctic.

There are manifold reasons to focus on the Arctic region and perform measurements of CO 2 and CH 4 using the unique CoMet payload. The Arctic is warming twice as fast as the global average, making climate change’s polar effects more intense than anywhere else in the world (AMAP, 2015). The Arctic accounts for nearly 50% of all organic carbon stored in the Planet`s soil, but rising temperatures and thawing permafrost threatens its stability. Wetland emissions of methane constitute the largest single source of methane to the atmosphere, even when considering all anthropogenic emissions, and are the most uncertain part of the budget (Kirschke et al., 2013; Saunois et al., 2019). After the tropics, the largest distribution of wetlands is in the Arctic. Fires in boreal forests or tundra peatlands (as seen in Greenland in 2017) are both sources of methane and CO 2 and also accelerate the thawing of permafrost which leads to the release of carbon. There is increasing, but divergent, evidence that changing climate in the modern period has shifted these ecosystems from sinks into net carbon sources (IPCC 2019).

Another important consequence of global warming is the reduced sea-ice extent which effectively „unlocks“ the Arctic Ocean and leads to an expansion of human activities such as oil and gas production. Current emission estimates from major oil and gas fields show substantial differences in estimated CH 4 emissions and their distribution (Scarpelli et al. 2019). These oil and gas fields overlap with the natural sources making the quantification of both natural and anthropogenic sources particularly difficult. Flying around the Arctic cycle will unravel interhemispheric (east-west) differences of GHG fluxes.

In general, passive remote sensors have difficulties accurately measuring GHGs at high latitudes due to the low solar zenith angle and low surface reflectance which compromises their signal-to noise ratio. Nevertheless, new passive remote sensors like TROPOMI on Sentinel-5 Precursor deliver XCH 4 in the Arctic data during summer (Schneising et al., 2019). Active remote sensors using lidar such as the upcoming joint French-German MERLIN mission (Ehret et al., 2017), currently scheduled for launch in 2024, promise to provide high quality methane data at all latitudes and seasons. Therefore, validation of MERLIN in this area of the world, for which the CoMet payload provides unrivalled capacities, are of high importance for this mission. Together, highly-resolved aircraft data, lower-resolution MERLIN data, all passive satellites available during the campaign period, and models will provide a unique combination to constrain the Arctic carbon cycle.


The HALO Circle instrumentation will consist of the same suite of instruments envisaged for the CoMet 2.0 campaign. The HALO cabin layout (floor plan variant 12) for CoMet 2.0 has already been prepared during the CoMet certification and consists of a suite of the most advanced remote sensing and in-situ instruments for greenhouse gases plus ancillary data already deployed within the first CoMet 2018 campaign. An important replacement will be MAMAP2D which is an improved 2-dimensional imaging spectrometer system currently in development for CoMet 2.0. The core payload will be complemented by a quantum cascade laser spectrometer to measure ethane (C 2 H 6 ), a tracer for emissions from oil and gas emissions. Possible synergetic additions could include instruments for in-situ GHG and isotope monitoring, or instruments able to address the OH sink of methane. The CoMet payload still provides some available rack space.

The data collected will be used within regional inverse models and regionally nested global chemistry climate models in combination with wetland models. In addition to the CH 4 data products from MERLIN, the CH 4 and CO 2 data products from ongoing missions expected for this timeframe (Sentinel-5, OCO-2/-3, GOSAT-2/-3, Microcarb … ) will be validated with HALO Circle data.

The timing of the HALO Circle campaign has to be compatible with the MERLIN launch and its subsequent commissioning phase. Given he current launch date of MERLIN, which is foreseen for 2024, and the requirement that this mission has to be performed in northern hemispheric summer, a campaign period in late summer 2025 at the earliest, or 2026 is envisaged.

The campaign shall comprise a period of ~6 weeks with a total of ~120-140 flight hours. This should be adequate to circle the Arctic at least twice (preferably three times) to investigate interhemispheric differences, intrahemispheric variations, and also provide an adequate number of in-situ profiles and surveys over areas of particular interest such as oil and gas fields, profiles extending as close to the ground as possible, and dedicated flight patterns for MERLIN validation.

Flight patterns will consist of large-scale flights loosely following the course of the Arctic Circle at 66°N. Doing so, we can achieve measurements in all important boreal wetlands (Western Siberia, Mackenzie Delta, Hudson Bay Lowlands, etc.) and also in the vicinity of important gas and oil fields (e.g. Tyumen, Alaskan North Slope, Athabascan oil sands, etc.). The detailed flight routing and decision on the stop-over bases will be based on a feasibility study.

It is obvious that flying into Russia is a huge endeavour, both politically and logistically, and requires a tight cooperation with Russian entities. As an example, the V.E. Zuev Institute of Atmospheric Optics of the Siberian Branch of the Russian Academy of Sciences in Tomsk will support the mission. Furthermore, international co-operation with Canadian, American (e.g. NASA), Scandinavian, British, Japanese, and – as part of the MERLIN validation efforts – French partners (e.g. CNES) will be established or reinvigorated.


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Scientific instruments and payload configuration

  • List of scientific instruments for the mission:

DescriptionPrincipal investigatorInstitution
CHARM-FIntegrated path differential absorption lidar (IPDA)NameDLR-IPA
MAMAP2DImaging NIR/SWIR spectrometerNameUniversity of Bremen
Mini-DOASUV/VIS/NIR spectrometerNameMax-Planck-Institute for Biogeochemistry
JIGCavity-Ringdown Spectroscopy (CRDS)NameMax-Planck-Institute for Biogeochemistry
JASAir samplerNameMax-Planck-Institute for Biogeochemistry
MIRACLEQuantum Cascade Laser Spectrometer (QCLS)NameDLR-IPA
BAHAMASBasic aircraft measurement systemNameDLR-FX
Others?To be determinedNameDLR-IPA

Cabin and exterior configuration of HALO for the mission

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HALO flights for this mission

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More information

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Press releases, media etc

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