Remedy 1: Operationalise the Aircore technique at a range of sites also measuring using FTIR
Remedy 2: Enhance the airborne infrastructure in Europe.
Remedy 3: Create a database of in-situ vertical profiles of CO2, CH4, and CO with sufficient spatiotemporal coverage, possibly as part of the ICOS RI.
For the ECVs temperature, ozone, water vapour and aerosol, vertical-profile information with relatively high vertical resolution (100m to a few km) in the troposphere is available from sonde and/or lidar measurements. For greenhouse gases (CO, CH4, CO2), the non-satellite observing system does not have sufficient capabilities. The FTIR measurements of these greenhouse gases have a low vertical resolution of the order of 5 to 8 km, if any, and this vertical information is difficult to validate. For example, measurements analysed so far within the GAIA-CLIM project have shown that the CH4 retrieval can be improved under polar vortex conditions as a result of applying new profile data. Also, the modelling component in GAIA-CLIM has highlighted the deficiencies of the FTIR vertical profile information and the resulting needs for better in-situ vertical profiles.
One option to obtain in-situ vertical profiles is the use of the Aircore technique. This technique has been under development since 2000 and has the capability to obtain vertical profiles up to the middle stratosphere. Several Aircore sites exist in Europe, but the system is not yet a fully operational system. It is necessary to make the Aircore measurements easier for the users. Moreover, the Aircore cannot be launched at all sites, due to air traffic limitations and the fact that the Aircore must be recovered upon landing. The landing site cannot be pre-determined as long as the Aircore is launched with a balloon and descends with a simple parachute, thereby drifting with the wind and landing at a location which is not always suitable for retrieving the payload for performing the post-flight analysis of the air sample.
To solve the latter issue, some projects have investigated the design of a steered system to bring the Aircore down. A second option to obtain in-situ vertical profiles of greenhouse gases is to make use of aircraft spiral flights. The aircraft capacity in Europe is too limited to perform regular aircraft campaigns. Europe has no capability similar to the HIPPO campaigns in the USA. In any case, aircraft campaigns cannot cover vertical profiles higher than 12 km (a better calibration is possible if the profiles cover an altitude range from the ground up to the middle stratosphere), are very expensive, and are also difficult to organise above remote locations that are not situated on the European continent. High-altitude UAV or Aircore are required to cover higher altitudes. At present, high-altitude UAV are still largely in proof-of-concept stage.
However, although expensive, in-situ calibration of CH4, CO2 (and CO) columns/profiles measured by FTIR remote sensing instruments can be performed by aircraft overpasses equipped with in-situ instruments that are calibrated relative to the WMO standards. Such campaigns have been undertaken in the past, for example in Europe as part of the EU project IMECC. But, as mentioned above, new flight campaigns in Europe are currently not planned, the flights cover only an altitude up to about 12 km, and calibration flights are very costly and difficult over stations that are not situated in the European continent, like islands, S. America, Africa, Asia. Hence more regular verification of the calibration of the instruments is desirable, to ensure long-term and network-wide consistency with the standards, as well as to ensure a better understanding and minimization of the biases across the networks when studying fluxes from e.g. hot spot regions.
Current and future satellite missions, which have the capability to measure greenhouse gases from space include GOSAT, IASI, OCO-2, Tansat, S5P, GOSAT-2, Merlin, MicroCarb, OCO-3, Sentinel-5.
New Aircore in-situ vertical profile data will be made available outside of GAIA-CLIM that can serve as calibration of FTIR greenhouse gas measurements and in support of modelling activities. However, they are limited to only one site (Sodankyla) and with limited temporal coverage.
This gap addresses the need for sustained calibration of the FTIR remote sensing data (essentially columns with some vertical information that enables to separate partial columns) for CO2, CH4 (and CO). This can be done by comparing the FTIR data with co-located or nearby in-situ soundings of the same species that are calibrated to community standards, in this case the WMO standards. At present, however, there is not enough capacity to provide such in-situ data.
This gap also addresses the need for a European infrastructure for vertical greenhouse gas profiling in the troposphere for CO2 and CH4 There is a need for vertical profile information about these ECVs in the troposphere, among others, to verify model results, and to validate remote sensing total and partial column data. The capabilities of the ground-based remote-sensing observing systems are limited when it comes to vertical-profile information, and are not sufficiently validated. Options for filling this gap are the facilitation of access to airborne in-situ measurement systems, like aircraft or UAV, or Aircore for greenhouse gases.