The AirCore system stems from an idea originally developed by Pieter Tans of NOAA/ESRL (Tans, 2009, Karion et al., 2010): that a long tube descending from a high altitude with one end open and the other closed can sample and retain a mole fraction profile of a gas to be analyzed at a later date. The current AirCore evacuates to ambient pressure as it ascends to approximately 30 km (~12 hPa). As the AirCore descends through the atmosphere under a balloon or a parachute, surrounding air flows into the AirCore tube to maintain equal pressure with the air outside. At each pressure level, an equal mass of air enters the tube (assuming a constant temperature is maintained throughout the descent), collecting a profile of the atmospheric column. The open end of the AirCore is sealed upon landing, preserving the atmospheric sample inside. The inside air is then pushed through a gas analyser (such as a Picarro based on the CRDS technique – Crosson et al., 2010). Using the temperature and pressure data measured by an electronic package flying under the balloon, it is possible to capture a continuous profile (in contrast with discrete flask sampling).
The vertical resolution of the profile is directly linked to both the molecular diffusion and the Taylor dispersion that happen inside the tube. It thus depends on: (i) the length and diameter of the tube itself (the longer the tube, the highest number of independent measurements available and thus the highest the resolution, but also the heaviest the AirCore); (ii) the time between the landing and the analysis.
Two kinds of AirCore have been developed and are now operated by LMD:
- the AirCore-HR (Fig.1), characterized by a higher resolution than any other AirCores that have been developed by NOAA or other institutions, in order to better capture the vertical distribution of atmospheric CO2 and CH4, and better analyze the profiles measured by the ’lower resolution’ AirCores;
- the AirCore-light that can fly under meteorological balloons (BLD-type) for easier deployment.
Figure 2 shows the vertical resolution of air samples taken at different altitude for AirCore-HR (red), AirCore-light (blue) and the original NOAA AirCore (black).
Figure 1. Two AirCore-light from LMD.
Figure 2. Theoretical vertical resolution against altitude of the air sample for a 3 hour delay between landing and analysis for AirCore-HR from LMD (red), AirCore-light from LMD (purple), AirCore-GUF from Frankfurt University (blue) and original NOAA AirCore (black).
The AirCore may contribute to several research topics concerning the observation of greenhouse gases and, more generally, carbon cycle studies. The two main scientific objectives of the AirCore are:
1) Cal/val activities for greenhouse gas space missions (SWIR and TIR)
In order to fully validate the retrieved total column (both from space with Merlin, Sentinel 5P/5 or MicroCarb, or from on-ground FTS) at the level of precision required by carbon cycle studies, it is necessary to use independent in-situ measurements of GHG profiles, especially in the Upper Troposphere-Lower Stratosphere (UTLS) region and above. This is the goal of the AirCore. As it provides a full recovery of the atmospheric profiles of CO2 and CH4, the AirCore will also be very useful in the validation of mid-tropospheric columns of GHG derived from IASI, AIRS or TES, observations.
In addition to providing a primary calibration for retrieved total columns, AirCore profiles will benefit forward models used to create a priori profiles for the retrievals. These improvements are important because the shape of the a priori profile used on the retrieval process is retained in the gas retrievals, and any errors in the stratospheric part of the retrieved profile will result in corresponding biases in the troposphere, which may eventually translate into biases in the estimates of surface carbon fluxes.
2) Understanding of carbon exchanges along the atmospheric column
By providing a full recovery of the atmospheric profiles of CO2 and CH4 from the surface up to an altitude of 30-40 km, AirCores provide a means to better understand carbon exchanges along the atmospheric column, as well as the chemistry processes that yield the strong decreasing slope of CH4 in the stratosphere.
Results from StratoScience 2014 and 2015 campaigns
The AirCore-HR was flown for the first time during the stratospheric balloon campaign operated by the French space agency CNES and the Canadian space agency CSA in Timmins (Ontario, Canada) in August 2014 on the “EdS-Stratéole” flight. On this flight, the multi-instrument Gondola carried several instruments that measured CO2 and CH4 in-situ profiles: Pico-SDLA (based on laser spectrometry) from GSMA, University of Reims and INSU Division Technique and two other AirCores with lower vertical resolution from Frankfurt University. The same instruments, together with 2 LMD Aircores-light, were flown during the CNES/CSA stratospheric balloon campaign in Timmins (Ontario, Canada) in August 2015 on the "CLIMAT" flight.
The vertical profiles of CO2 and CH4 measured simultaneously by the 3 instruments are shown in Fig. 3. For CO2, very thin structures as well as airmass vertical transport signatures are clearly seen on the profiles. The strong CO2 uptake by vegetation near the surface (August) is also well captured by the AirCore-HR. For CH4, the agreement is excellent between the 3 datasets, with layer-to-layre baises of less than 1ppb. This is particularly the case for the slope of CH4 in the stratosphere and the structures along the vertical.
|Figure 3. Vertical profiles of CO2 (left) and CH4 (right) measured by the 3 instruments of the "CLIMAT" flight of StratoScience 2015: AirCore-HR (blue), AirCore-light (cyan and dark blue) and AirCore-GUF (green and violet). Data of AirCore-GUF (courtesy of A. Engle and H. Boenish from Frankfurt University) have been processed at LMD.
The comparison between the 3 AirCores also illustrates the difference in vertical resolution of both instruments (see Fig. 1) and the smearing effect of AirCore-GUF and LMD AirCore-light low resolution. The good agreement between all AirCores gives confidence in the repeatability of AirCore measurements. Also, the artificial degradation of AirCore-HR measurements to the vertical resolution of AirCore-GUF (not shown) yields a perfect match of the 2 sets of data, which completely validate the expected theoretical resolution.
A video summarizing the "CLIMAT" flight performed during the StratoSciences2015 CNES/CSA campaign may be found here.
Membrive O., Crevoisier C.,Hertzog A., Danis F., Sourgen D., Barbé B., Amarouche N., Samaké J.-C., Frérot F., Durry G., Joly L., Decarpenterie T. , Cousin J., Engel A., Bönisch H., Louvel S., Renard J.-B., Des ballons pour le climat, La Météorologie, 91, 2015, doi : 10.4267/2042/57848.