Abstract
Abstract. An original and innovative sampling system called AirCore was presented by NOAA in 2010 Karion et al.(2010). It consists of a long ( > 100 m) and narrow ( < 1 cm) stainless steel tube that can retain a profile of atmospheric air. The captured air sample has then to be analyzed with a gas analyzer for trace mole fraction. In this study, we introduce a new AirCore aiming to improve resolution along the vertical with the objectives to (i) better capture the vertical distribution of CO2 and CH4, (ii) provide a tool to compare AirCores and validate the estimated vertical resolution achieved by AirCores. This (high-resolution) AirCore-HR consists of a 300 m tube, combining 200 m of 0.125 in. (3.175 mm) tube and a 100 m of 0.25 in. (6.35 mm) tube. This new configuration allows us to achieve a vertical resolution of 300 m up to 15 km and better than 500 m up to 22 km (if analysis of the retained sample is performed within 3 h). The AirCore-HR was flown for the first time during the annual StratoScience campaign from CNES in August 2014 from Timmins (Ontario, Canada). High-resolution vertical profiles of CO2 and CH4 up to 25 km were successfully retrieved. These profiles revealed well-defined transport structures in the troposphere (also seen in CAMS-ECMWF high-resolution forecasts of CO2 and CH4 profiles) and captured the decrease of CO2 and CH4 in the stratosphere. The multi-instrument gondola also carried two other low-resolution AirCore-GUF that allowed us to perform direct comparisons and study the underlying processing method used to convert the sample of air to greenhouse gases vertical profiles. In particular, degrading the AirCore-HR derived profiles to the low resolution of AirCore-GUF yields an excellent match between both sets of CH4 profiles and shows a good consistency in terms of vertical structures. This fully validates the theoretical vertical resolution achievable by AirCores. Concerning CO2 although a good agreement is found in terms of vertical structure, the comparison between the various AirCores yields a large and variable bias (up to almost 3 ppm in some parts of the profiles). The reasons of this bias, possibly related to the drying agent used to dry the air, are still being investigated. Finally, the uncertainties associated with the measurements are assessed, yielding an average uncertainty below 3 ppb for CH4 and 0.25 ppm for CO2 with the major source of uncertainty coming from the potential loss of air sample on the ground and the choice of the starting and ending point of the collected air sample inside the tube. In an ideal case where the sample would be fully retained, it would be possible to know precisely the pressure at which air was sampled last and thus to improve the overall uncertainty to about 0.1 ppm for CO2 and 2 ppb for CH4.
Highlights
Understanding the global atmospheric budget of the two major greenhouse gases (GHG) emitted by human activities, carbon dioxide (CO2) and methane (CH4), is essential for predicting their future concentration levels
This paper focuses on the development of an AirCore that allows the retrieval of profiles of GHG with a higher resolution along the vertical, with the following objectives: (i) to better capture the vertical distribution of atmospheric CO2 and CH4 in the troposphere, upper troposphere and low stratosphere (UTLS) and stratospheric regions; and (ii) to provide a tool to compare low-resolution AirCores and validate the theoretical resolution achievable by AirCores
Thanks to the combination of two tubes, it allows retaining air samples with a vertical resolution better than 500 m up to 20 km when the analysis is performed within 6 h after landing of the instruments
Summary
Understanding the global atmospheric budget of the two major greenhouse gases (GHG) emitted by human activities, carbon dioxide (CO2) and methane (CH4), is essential for predicting their future concentration levels. Used techniques include FTS measurements such as the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS; Oelhaf et al, 1991), cryogenic samplers (e.g., Schmidt and Khedim, 1991; Engel et al, 2008) to capture air in flasks at different altitudes along the balloon flight to be analyzed at a later stage, and laser-diode spectrometers such as the Spectromètre Infra Rouge pour l’Étude de l’Atmosphère par Diode Laser Embarquées (SPIRALE; Moreau et al, 2005) or Pico-SDLA instruments (Durry et al, 2004; Ghysels et al, 2011; Joly et al, 2007). This paper focuses on the development of an AirCore that allows the retrieval of profiles of GHG with a higher resolution along the vertical, with the following objectives: (i) to better capture the vertical distribution of atmospheric CO2 and CH4 in the troposphere, UTLS and stratospheric regions; and (ii) to provide a tool to compare low-resolution AirCores and validate the theoretical resolution achievable by AirCores The design of this new high-resolution AirCore, named AirCore-HR, is presented in Sect.
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