The instrument

Principle

The AirCore system stems from a concept 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 AirCore tube 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. 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.

The AirCore-Fr instruments

Three kinds of AirCore have been developed by LMD and are now operated by the French AirCore Program:

• the AirCore-HR, characterized by a higher resolution than any other AirCores, 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.
• the AirCore-extralight, which is the lightest.

The following figure shows the vertical resolutions of air samples taken at different altitude for AirCore-HR (red), AirCore-light (blue) and the original NOAA AirCore (black).