Global atmospheric chlorine declined over 2004–2024

What the study found: Global atmospheric chlorine inventories decreased from 2004 to 2024, and by 2024 the global mean total chlorine values were approaching parts per billion. The inventories show that source gases dominate total chlorine in the lower atmosphere, while at higher altitudes chlorine gas becomes the main contributor.
Why the authors say this matters: The authors conclude that these findings demonstrate the significant impact of the Montreal Protocol in reducing emissions of substances that are both ozone-depleting and greenhouse gases.
What the researchers tested: The researchers calculated atmospheric chlorine inventories over 21 years and across five latitude bands, using Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) version 5.3 retrievals of volume mixing ratios for 13 chlorine-containing species from the surface to 61 km. Where needed, they supplemented ACE-FTS with the TOMCAT 3-D chemical transport model, Aura Microwave Limb Sounder (MLS) data, and ground-based measurements from NOAA and AGAGE.
What worked and what didn't: Total chlorine profiles were dominated by source gas contributions in the troposphere and lower stratosphere. At lower altitudes, chlorofluorocarbons, chlorocarbons, and increasingly in recent years chlorine-containing very short-lived substances were the primary contributors; at higher altitudes, chlorine gas became dominant, reaching up to 99% of total chlorine by 61 km. The relative contributions also showed that chlorine-containing very short-lived substances and HCFC-22 have partially slowed the reduction in atmospheric chlorine achieved by the phase-out of CFC production and consumption.
What to keep in mind: The abstract does not describe specific limitations beyond the use of combined satellite, model, and ground-based datasets. Some species names are listed in the abstract with missing symbols, so the provided summary is limited to the information explicitly stated.

Key points

• Atmospheric chlorine inventories decreased over the 2004–2024 period.
• Source gases dominated total chlorine in the troposphere and lower stratosphere.
• Chlorine gas became the dominant contributor at higher altitudes, reaching up to 99% by 61 km.
• Chlorine-containing very short-lived substances and HCFC-22 partially slowed the decline in atmospheric chlorine.
• The authors say the results show the impact of the Montreal Protocol on reducing emissions.