This talk is motivated by the slower-than-expected recovery of the ozone layer at mid-latitudes due to the long-term decline of ozone depleting substances (ODSs), which is more acute for the Northern Hemisphere (NH) compared to the Southern Hemisphere (SH). Trends in Cly (HCl+ClONO2) and N2O observed by ACE-FTS in the NH and SH midlatitude stratosphere from 2004 to 2025 will be compared to trends simulated by eight chemistry-climate models (CCMs) that submitted output for the CCMI-2022 effort. These comparisons reveal a slower observed decline of stratospheric Cly than simulated by the CCMs, particularly for the NH midlatitude lower stratosphere (LS). The impact on trends in Cly of very short-lived chlorine compounds (VSL-Cl) (which are produced by industrial processes, are not yet regulated by the Montreal Protocol, and are also not considered by these CCMs) will be evaluated. Finally, we will also analyze CCMI-2022 modeled and ACE-FTS measured trends in N2O over the past two decades that appear to show a major change in stratospheric transport of the NH, mid-latitude stratosphere that is not represented in any of these models.
