Chlorine Chemistry and Partitioning in the arctic UT/LS during Spring 2025 from in-situ measurements during ASCCI 

Chemically active chlorine species (ClOx​) play a central role in the catalytic depletion of ozone in the polar winter and spring stratosphere. Together with the reservoir species HCl and ClONO2​, they form inorganic chlorine (Cly​). The amount and partitioning of Cly strongly influences the magnitude of polar ozone loss. Trends and variability in the upper troposphere lower stratosphere region (UTLS) are of particular importance, as this region is a large contributor to lower-stratospheric ozone change. We present new in-situ measurements from the HALO campaign ASCCI (Arctic Springtime Chemistry and Climate Investigations), obtained in spring 2025. The total organic chlorine (CCly) is directly derived from the major chlorine species measured by the Gas Chromatograph for Observational Studies using Tracers (GhOST). From these measurements, inorganic chlorine is derived, allowing an observational assessment of the chlorine budget in the Arctic UTLS. Additional in situ observations of the reservoir species HCl and ClONO2​ from the Airborne Chemical Ionisation Mass Spectrometer (AIMS) are used to constrain the abundance of chemically active chlorine (ClOx) and to investigate chlorine activation and deactivation processes during spring 2025. The new ASCCI data are compared with observations from previous aircraft campaigns, including PGS (Arctic measurements in 2015) and SOUTHTRAC (Antarctic measurements from 2019) both performed during hemispheric springtime, allowing for an assessment of hemispheric differences between Arctic and Antarctic conditions as well as temporal changes in chlorine loading and partitioning. These comparisons place the new measurements in the context of declining stratospheric chlorine and ozone recovery.