The Montreal Protocol is an international treaty that aims to protect the ozone layer by phasing out the production of substances that are responsible for ozone depletion. Certain chlorofluorocarbons (CFCs), for example, were banned by 2010 for most uses if they would end up in the atmosphere. They can, however, still be used as intermediates in synthesis.

Luke M. Western, Global Monitoring Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, USA, and University of Bristol, UK, and colleagues have used measurements from 14 sites worldwide, combined with an atmospheric transport model, to investigate how atmospheric abundances and emissions of five such banned CFCs developed between 2010 and 2020. The team considered the CFCs CFC-13, CFC-112a, CFC-113a, CFC-114a, and CFC-115 (CFC-13 (chlorotrifluoromethane, CClF₃), CFC-112a (tetrachlorodifluoroethane, CClF₂CCl₃), CFC-113a (trichlorotrifluoroethane, CF₃CCl₃), CFC-114a (dichlorotetrafluoroethane, CF₃CCl₂F) and CFC-115 (chloropentafluoroethane, CF₃CClF₂)).

The researchers observed an increase in emissions, against the goals of the treaty. The combined emissions of these five CFCs increased from ca. 1.6 to ca. 4.2 ODP-Gg yr^–1 (ODP = a CFC-11-equivalent ozone-depleting potential) in the studied timeframe.

The observed CFCs have few to no known applications left and an atmospheric lifetime ranging from 52 to 640 years. CFC-13 and CFC-112a were once used as refrigerants but are now banned. The remaining three CFCs are manufactured as intermediates during the production of other chemicals, but they are also forbidden for use as refrigerants, propellants, or in other dispersal applications. According to the researchers, the emissions of CFC-113a, CFC-114a, and CFC-115 probably stem in part from leakage during the synthesis of hydrofluorocarbons (HFCs), which are used as alternatives to CFCs. Specific source locations could not be identified. The current emission levels of these CFCs do not significantly threaten the recovery of the ozone layer. However, ongoing emissions of these ozone-depleting chemicals may pose a problem. In addition, they have a negative impact on climate change.

• Global increase of ozone-depleting chlorofluorocarbons from 2010 to 2020,
  Luke M. Western, Martin K. Vollmer, Paul B. Krummel, Karina E. Adcock, Paul J. Fraser, Christina M. Harth, Ray L. Langenfelds, Stephen A. Montzka, Jens Mühle, Simon O’Doherty, David E. Oram, Stefan Reimann, Matt Rigby, Isaac Vimont, Ray F. Weiss, Dickon Young, Johannes C. Laube,
  Nat. Geosci. 2023.
  https://doi.org/10.1038/s41561-023-01147-w