Measurement report: Greenhouse gas profiles and age of air from the 2021 HEMERA-TWIN balloon launch

Within the HEMERA balloon infrastructure project, a stratospheric balloon carrying a multi-instrument payload to a maximum altitude of 31.2 km was launched on 12 August 2021. On board the openly constructed gondola, several types of instruments were used for simultaneous air sampling and in-flight measurements to characterize climate-relevant trace gases in the stratosphere and troposphere, as well as to compare and evaluate different instrumental approaches and sampling techniques. For observations of the main greenhouse gases carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and sulfur hexafluoride (SF6), flask with AirCore sampling and in-flight spectrometry were deployed. Overall, results from different methods agree well. While better precision was achieved for the post-flight measurements of AirCore devices and flask sampling, in situ spectrometry provided a higher degree of detail on the vertical structure of the CH4 profile. Age of air was derived from mixing ratios of CO2 and SF6. As seen in previous studies, higher values were obtained from SF6 than from CO2. Correcting for chemical losses, maximum values of 4.4-5.1 years were derived from SF6 mixing ratios at altitudes above 20 km compared to 4.2-5.0 years from CO2 mixing ratios. The resulting dataset should be well suited for multi-tracer approaches to derive age of air, particularly in combination with a large suite of halocarbons measured from flask and AirCore sampling and one more AirCore sample which was reported in a companion publication (Laube et al., 2025).