24 Aug 2023
 | 24 Aug 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

The interhemispheric gradient of SF6 in the upper troposphere

Tanja J. Schuck, Johannes Degen, Eric Hintsa, Peter Hoor, Markus Jesswein, Timo Keber, Daniel Kunkel, Fred Moore, Florian Obersteiner, Matt Rigby, Thomas Wagenhäuser, Luke M. Western, Andreas Zahn, and Andreas Engel

Abstract. Anthropogenic trace gases often exhibit interhemispheric gradients because of larger emissions in the northern hemisphere. Depending on a tracer's emission pattern and sink processes, trace gas observations can thus be used to investigate interhemispheric transport in the atmosphere. Vice versa, understanding interhemispheric transport is important for interpreting spatial tracer distributions and for inferring emissions. We combine several datasets from the upper troposphere (UT) to investigate the interhemispheric gradient of sulfur hexafluoride (SF6) covering latitudes from ~80° N to~60° S: canister sampling based measurements from the IAGOS-CARIBIC infrastructure and data from the in-flight gas chromatography instruments GhOST and UCATS. The interhemispheric gradient of SF6 in the UT is found to be weaker than near the surface. Using the concept of a lag time removes the increasing trend from the time series. At the most southern latitudes, a lag time of over 1 year with respect to the northern mid-latitude surface is derived, and lag times decrease over the period 2006–2020 in the tropics and in the southern hemisphere. Observations are compared to results from the two-dimensional AGAGE 12-box model. Based on EDGAR 7 emissions, fair agreement of lag times is obtained for the northern hemisphere, but southern hemispheric air appears too "old". This is consistent with earlier findings that transport from the northern extra-tropics into the tropics is too slow in many models. The influence of the emission scenario and the model transport scheme are evaluated in sensitivity runs. It is found that EDGAR 7 underestimates emissions of SF6 globally and in the southern hemisphere, whereas northern extra-tropical emissions seem overestimated. Faster southward transport from the northern extra-tropics would be needed in the model, but transport from the southern tropics into the southern extra-tropics appears too fast.

Tanja J. Schuck et al.

Status: open (until 05 Oct 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1824', Anonymous Referee #1, 03 Sep 2023 reply
  • RC2: 'Comment on egusphere-2023-1824', Anonymous Referee #2, 18 Sep 2023 reply
  • RC3: 'Comment on egusphere-2023-1824', Anonymous Referee #3, 04 Oct 2023 reply

Tanja J. Schuck et al.

Model code and software

mrghg/py12box: v0.2.1 Matt Rigby and Luke Western

Tanja J. Schuck et al.


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Short summary
We study the interhemispheric gradient of sulfur hexafluoride (SF6), a strong long-lived greenhouse gas. Its emissions are stronger in the northern hemisphere, therefore mixing ratios in the southern hemisphere lag behind. Comparing the observations to results from a box model, the model predicts air in the southern hemisphere to be older. For a better agreement, the emissions used as model input need to be increased, their spatial pattern changed, and we need to modify north-south transport.