Separation of tropopause mixing from long-term stratospheric variability using in-situ measurements during PHILEAS and ASCCI

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Mixing between the upper troposphere (UT) and the lower stratosphere (LS) occurs on short timescales compared to dynamic processes within the stratosphere. These mixing processes form the Extratropical Transition Layer (ExTL). Due to the nature of the tropopause as transport barrier, tracers exhibit strong vertical gradients within the ExTL. The ExTL is often identified based on the correlations of airborne trace gas measurements. However, inner-stratospheric variability on longer time scales can also lead to enhanced variability and thus might cause false identification of the ExTL.Our goal is to distinguish the ExTL from stratospheric variability on longer timescales. Therefore, the choice of tracers is crucial, particularly for species with stratospheric sources like CO or H2O. To circumvent this problem, comparisons with tracers that have only tropospheric sources such as C2H6 are necessary. For this purpose, simultaneous measurements of C2H6 and CO have been conducted during the PHILEAS (Probing High Latitude Export of air from the Asian Summer Monsoon) campaign and the ASCCI (Arctic Springtime Chemistry Climate Investigations) campaign using the University of Mainz QCL-based Spectrometer (UMAQS).Our results show that similarly to CO, stratospheric variability of C2H6 is also non-zero up to potential temperatures of 400 K. Therefore, dynamic processes rather than chemical sources most likely are the origin of this variability. By using tracer-tracer correlations, we are able to account for the longer-term variability and to separate cross-tropopause mixing from transport and mixing on longer timescales.When applying this method to PHILEAS data (autumn of 2023) and ASCCI data (spring of 2025) from the northern lowermost stratosphere (LMS), the ExTL can be isolated in vertical tracer profiles, showing a similar extent in autumn and spring. Further, the LMS structure in winter shows a surprisingly well separation from the overworld, indicating two different transport timescales in the background lower stratosphere.

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