Balloon-based trace gas observations reveal Asian Summer Monsoon impact in the UTLS over Germany

Abstract. For many important trace gases, precise observations in the upper troposphere and lower stratosphere (UTLS) are spatially and/or temporally sparse and/or inhomogeneous. This is particularly limiting in the UTLS as a highly variable transition region between the troposphere and stratosphere. We here present a new and unique dataset obtained from weather balloon-based sensors across the summer of 2025. Between 27^th May and 5^th November, 46 balloons were launched on an almost weekly basis from near Jülich, Germany, typically reaching altitudes of around 30 km. Of those flights, 37 focused on trace gases, with 18 near-simultaneous double launches quantifying the vertical distribution of water vapour and ozone as well as multiple halogenated species; thus, enabling access to the temporal evolution of many key UTLS components over a period of about five months. We here focus mainly on ozone-depleting chlorinated very short-lived substances (Cl-VSLSs) with model-based surface-origin tracers to investigate the source regions of the air masses. In particular, we investigate the influence of the Asian Summer Monsoon region – a source of Cl-VSLSs such as CH₂Cl₂ – as the upper-level anticyclone exports air masses into the global northern hemispheric UTLS during the monsoon season. We find that the “background” lower stratospheric composition in summer and autumn is clearly impacted by the Asian monsoon-related stratospheric entrance pathway, and for Cl-VSLSs much more so than estimated in previous studies.