Twenty‐One Years of Global Atmospheric Chlorine Inventories From Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE‐FTS) Measurements

We present atmospheric chlorine inventories over 21 years (2004–2024) and five latitude bands (82–60°N, 60–30°N, 30°N–30°S, 30–60°S, 60–82°S) across altitudes from the surface up to 61 km. These inventories were calculated using the Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer (ACE‐FTS) version 5.3 retrievals of the volume mixing ratios (VMRs) of 13 chlorine‐containing species. Of these, five are product gases: HCl, HOCl, , COClF, , and eight are source gases: , , CFC‐11 (), CFC‐12 (), CFC‐113 (), HCFC‐22 (), HCFC‐141b (), and HCFC‐142b (). Where necessary, ACE‐FTS data were supplemented with data from the TOMCAT 3‐D chemical transport model, the Aura Microwave Limb Sounder (MLS) instrument, and ground‐based measurements from the National Oceanic and Atmospheric Administration (NOAA) and the Advanced Global Atmospheric Gases Experiment (AGAGE). Total chlorine () profiles are dominated by source gas contributions in the troposphere and lower stratosphere. At lower altitudes, the primary contributors are chlorofluorocarbons (CFCs), chlorocarbons, and, increasingly in recent years, chlorine‐containing very short‐lived substances (Cl‐VSLS). At higher altitudes, becomes the dominant contributor comprising up to 99% of by 61 km. The relative contribution of individual species to showed that Cl‐VSLS and HCFC‐22 have partially slowed the reduction in atmospheric chlorine achieved by the phase‐out of CFC production and consumption. Between 2004 and 2024, the global mean time series decreased by ppt/year (%/year), with values approaching ppb in 2024. These findings demonstrate the significant impact of the Montreal Protocol in reducing emissions of substances that are both ozone‐depleting and greenhouse gases.