Transport and mixing of pollutants into the Arctic LMS derived from HAGAR-V in situ observations of a wide range of trace gases during the HALO ASCCI mission

Transport and mixing strongly determine the trace gas composition of the Arctic upper troposphere / lower stratosphere (UTLS), but spatial and temporal variability of the relevant processes are still not well quantified. The Arctic lowermost stratosphere (LMS) is fed via, and thus controlled by, various transport paths from regions with very differing chemical composition - young tropospheric air from the subtropics or even directly from the boundary layer, to photochemically very old air descending slowly from the mid- and high-latitude stratosphere. The HALO aircraft mission ASCCI conducted from Kiruna, Sweden in late winter and early spring 2025 aimed at investigating transport and mixing processes and time scales in the Arctic UTLS region, especially stratosphere-troposphere exchange, and the role of chemistry and tropospheric pollution for ozone in the Arctic UTLS. For answering these questions, measuring a wide range of trace gases with different source regions and chemical lifetimes ranging from days to many years is crucial.

We present a survey of first results of in situ tracer measurements made with the High Altitude Gas AnalyseR - five channel version (HAGAR-V) instrument. HAGAR-V measured a suite of more than 30 trace gases including very short-lived NMHCs (e.g. Benzene, C₂H₂, C₄H₁₀), halogenated VOC (e.g. CH₂Cl₂, CHCl₃, C₂Cl₄, CH₂Br₂), as well as longer-lived halocarbons (e.g. CH₃Cl, CH₃Br, CCl₄, Halons, HCFCs, and HFCs) every 120 s using in-flight gas chromatography and mass spectrometry. Further very long-lived species, including the age-of-air tracer SF₆, were measured every 40 s (CFC-12, SF₆) and every 80 s (CFC-11, CFC-113, H1211) using electron capture detection. Additionally, very precise CO₂ measurements by a NDIR analyser were conducted at high time resolution (5 s).

Using tracer-tracer relations of short-lived pollutants with long-lived tracers, we can distinguish between different transport and mixing processes in the Arctic UTLS. Besides observing mixing of fresh tropospheric air with older stratospheric air at the extratropical tropopause, we also identified rather young (several months) air transported from the tropical tropopause layer (TTL) to the high latitude stratosphere and mixing with old polar air at potential temperatures about 380 K. We also observed pollution by short-lived chlorinated substances such as CH₂Cl₂, CHCl₃, C₂Cl₄ and 1,2-dichloroethanein the Arctic, likely from both regional and remote sources. Besides the analysis of transport processes, we also derived the mean age of air both from SF₆ and CO₂. Using both species independently increases the reliability of the calculated ages significantly.